Hole-making and Drilling Technology for Composites: Advantages, Limitations and Potential [1 ed.] 9780081023976, 0081023976

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Hole-Making and Drilling Technology for Composites

Woodhead Publishing Series in Composites Science and Engineering

Hole-Making and Drilling Technology for Composites Advantages, Limitations and Potential

Edited by

A.B. Abdullah S.M. Sapuan

An imprint of Elsevier

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom © 2019 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-08-102397-6 For information on all Woodhead publications visit our website at https://www.elsevier.com/books-and-journals

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List of contributors

A.B. Abdullah School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia J. Abdullah  School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Malaysia M.S. Abdullah  School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia H.Y. Chan  School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia K. Debnath Department of Mechanical Engineering, National Institute of Technology Meghalaya, Shillong, India M.H. Hassan  School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Malaysia N. Ishak  School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia S. Kachhap National Institute of Technology Patna, Patna, India A. Krishnamoorthy  School of Mechanical Engineering, Sathyabama Institute of Science and Technology, Chennai, India R. Kumar Indian Institute of Technology Roorkee, Roorkee, India J. Lilly Mercy School of Mechanical Engineering, Sathyabama Institute of Science and Technology, Chennai, India S. Norisam  School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia S. Prakash School of Mechanical Engineering, Sathyabama Institute of Science and Technology, Chennai, India

x

List of contributors

S. Ramesh  Department of Mechanical Engineering, KCG College of Technology, Chennai, India Z. Samad  School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia S.M. Sapuan Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, Serdang, Malaysia N.A. Sheikh  Department of Mechanical Engineering, International Islamic University, Islamabad, Pakistan A. Singh National Institute of Technology Patna, Patna, India K.F. Tamrin Department of Mechanical and Manufacturing Engineering, Universiti Malaysia Sarawak (UNIMAS), Kota Samarahan, Malaysia P.V. Siva Teja  Department of Mechanical Engineering, Dhanekula Institute of Engineering and Technology, Vijayawada, India M.S.M. Zain School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia

Review of hole-making technology for composites

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M.S. Abdullah, A.B. Abdullah, Z. Samad School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia

1.1 Introduction Composite material can be used for a great number of applications in modern structures because of its high modulus, strength, light weight, durability, corrosion resistance, design flexibility, and so on. However, due its nonhomogeneity and high-­ abrasivity, machining composite material is a major problem [1–4]. In many manufacturing industries, conventional machining processes such as drilling remain the primary method of hole-making in composites. Generally in the aviation manufacturing industry, most aircraft structures, such as stabilizers, wings and fuselage, consist of a great number of varying types of holes (e.g., round, counterboring, countersinking, honing, reaming, lapping, sanding, etc.) with different diameters, depths and surface finishes [1]. Commonly these aircraft structures require assembly and therefore a number of holes need to be drilled. Most of the joints involved in the assembly are mechanical joints, such as bolted connections, rivet connections and pin connections, therefore mechanical joint efficiency is highly dependent on the quality of the holes [5]. With advances in processing techniques, the development of new hole-making technology, which is now integrated with the digital computer, has improved the efficiency and productivity of hole-making in terms of cutting time and hole quality. This chapter reviews the existing technologies in hole-making for composite panels, and the advantages and limitations of these technologies. The chapter begins with a brief introduction to hole-making technology, followed by a discussion of machining and nonmachining technologies.

1.2 Hole-making for composite laminates Hole-making technology can be divided into machining and nonmachining technology. The machining technology can be further divided into traditional and nontraditional machining. Drilling and milling are the traditional machining methods used for composites. The difference between the two methods is the approaching mechanism; for example, in drilling, the drill bit is rotated and fed the stationary workpiece and the volume of the workpiece is removed to produce a circular hole. Since drilling is a primary method of making holes [6], we will focus on this first. Many types of drilling technologies have evolved to meet the demand for composite materials. Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00001-5 © 2019 Elsevier Ltd. All rights reserved.

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Hole-Making and Drilling Technology for Composites

Hole-Making Technology

Machining

Traditional machining

Milling

Drilling

AWJM

Nonmachining

Nontraditional machining

Punching

EDM

Laser

Fig. 1.1  Technologies in hole-making.

In nontraditional machining, methods such as Wire EDM, laser and Abrasive Water Jet Machining (AWJM) are being used. In nonmachining, to date only punching technology is being used for hole-making in composite materials. Fig. 1.1 summarizes the hole-making technologies that have been developed to date.

1.2.1 Machining Machining (i.e., drilling) remains the preferred method of hole-making in composite materials across industries, although it is obvious that drilling in composite material is not the same as drilling in metallic material [7]. Composite material is very abrasive and since the mechanical drilling operation involves direct contact between tool and workpiece, the tool suffers extreme wear and creates a great amount of heat, which induces residual stress that leads to degradation of both tool life and the quality of workpiece [2]. The mechanical drilling operation can be divided into five types: conventional drilling, high-speed drilling (HSD), grinding drilling, vibration-assisted twist drilling (VATD), and orbital drilling (OD). As an alternative to drilling, punching also shows potential for producing holes in composites.

1.2.1.1 Conventional drilling Most drilling operations use twist drill bits and other special drill bits (step drill bit, center drill bit and dagger drill bit) as the cutting tools. However, the twist drill bit is the most widely used [8]. In conventional drilling, multiple stages of drilling need to be executed before the hole reaches its specified size or diameter. If the diameter of the hole is relatively large, a pilot hole with a small diameter may have to be drilled first and then enlarged to the final size with a larger tool [9]. This is to avoid a concentration

Review of hole-making technology for composites3

of high stress at the hole boundary on the workpiece material and to keep the damage to a minimum [10, 11]. The research on conventional drilling in composite laminates includes a number of experiments studying input variables, such as drilling parameters (spindle speed and feed), drill bit geometry, drill bit material, type of composite material and diameter size, and output variables, such as hole quality (delamination, surface roughness and roundness), thrust force and bearing strength of the hole [12].

1.2.1.2 High-speed drilling In a single aircraft there are thousands of holes that need to be drilled, mostly for mechanical fasteners like rivets and bolts [13]. The continuous development of cutting tools (material and geometry) has reduced cutting time and improved productivity in hole-making [12]. There are several basic requirements to take into account in HSD, namely, concentricity (related to tool material behavior when operating at different cutting speeds), tool material, coating material, flute geometry and coolant delivery [14]. HSD technology has been widely studied and employed in many areas of composite drilling. It is supposed to produce less delamination damage in a short time with single-shot drilling [15]. Similar to conventional drilling, HSD is the most promising drilling operation that leads to better performance and improves the quality of the hole. Unlike other drilling operations, HSD is carried out at very high spindle speed and results in reduction of delamination [16, 17]. However, increasing the speed literally increases the power consumption of the machine operation as well as the tooling cost due to excessive tool wear, and causes the total machining cost to become very expensive [18]. As the speed goes higher, usually 5–10 times more than conventional drilling, the rate of temperature increases making the composite laminates prone to thermal damage [19]. At the higher temperature, which exceeds the epoxy melting temperature, the heat from the friction contact between the tool and the workpiece softens the epoxy matrix making it evaporate, known as matrix burnout, and causes only the fiber to be left. This results in interlaminar delamination [20]. Not only that, it is also shortens the tool life span. Because the temperature significantly affects the hole quality and tools, coolant is used to combat temperature issues. Nowadays, most applications of HSD incorporate a high-pressure coolant flow system to avoid catastrophic thermal damage to the workpiece instead of removing chips. Yet the aerospace manufacturing industry is moving toward HSD under dry conditions with optimum cutting parameters due to economic and environmental reasons [21]. Dry drilling conditions might be a better choice for thin composite laminates because the short engagement time may limit heat buildup.

1.2.1.3 Grinding drilling Grinding drilling, also known as core drilling, is one of the drilling operations that best reduces delamination damage. Grinding drilling focuses on the periphery of the hole. There is no chisel edge acting on the predrilling like with a normal drill bit since the center of the drill bit is hollow. The absence of chisel edge reduces thrust force and hence delamination [22, 23]. It was found that increasing the number of teeth of the cutting tool can reduce the cutting force. The easiest way to achieve an unlimited

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Hole-Making and Drilling Technology for Composites

number of teeth is by coating the cutting tool with a certain grit size of material. Tsai and Hocheng [24] proposed coating it with diamond material. Diamond material has an extremely high thermal conductivity, which can remove heat from the cutting edge and extend tool life. The research suggests that diamond material is preferable because it provides high abrasive wear resistance. The different grain size of coated material also influences the surface quality of the hole and the heat distribution over the matrix of the hole boundary. The result of the investigation shows that increasing the grain size results in lower thermal load and allows the heat to dissipate more efficiently. However, it was shown that finer grains result in better surface quality of the hole [25]. There are different types of geometry for core drill bits and each of them serves different purposes. Fig. 1.2 shows different designs of core drill bits. An improvement of the cutting mechanism in micro-core drilling has been made by introducing a shear mechanism at the cutting edge of the core drills using novel tool design (defined cutting edge using polycrystalline diamond (PCD)). The conventional core drills (randomly distributed micro grains) use an electroplated diamond abrasive micro-core drill that produces an abrasive/rubbing action that results in random cutting edge geometry (negative rake angle, protrusions, densities). This deficiency of random cutting edge geometry is not a good solution for machining parameters. According to the research, a novel micro-core drill reduces drilling force and temperature by 36% and 11%, respectively, compared with conventional core drills. In addition to these findings, the evaluation of the shearing action of the novel micro-core drill found it produces holes with superior edge definition and surface quality [26]. Fig. 1.3 shows a conceptual image of a laser-generated PCD core drill.

1.2.1.4 Vibration-assisted twist drilling VATD is another branch of vibration cutting that uses vibration in the drilling process. There are three directional modes of vibration that occur in the drilling process, namely, axial, lateral and torsional. The drill moves in these three directions when it is run on the surface of the workpiece. When it comes to composite laminates, the typical damage that has been recognized as the major damage when drilling is d­ elamination [27].

Fig. 1.2  Different design types of core drill bits (A) abrasive tools [25] and (B) hollow grinding [12].

Review of hole-making technology for composites5

Fig. 1.3  (A) Conceptual design of laser generated PCD core drill and (B) SEM image of laser generated tooth profile in PCD and profilometric measurement image of its geometry [26].

Most of the aviation industry uses conventional drilling, which employs continuous axial motion of the drill bit toward the workpiece. This continuous motion of the drill generates heat by the friction between the tool and workpiece, and the temperature rises as the drilling progresses. The VATD process uses a small-amplitude, low/high-­ frequency tool superimposed with conventional feed in an axial motion with controllable intermittent intervals. This allows frequent separation and contact between the tool and the chips, which reduces the contact area and leads to a decrease in frictional force [28]. Compared with conventional drilling, VATD has unique characteristics such as impacting, separating, changing speed and changing angle during the drilling process [29]. In general, this process interrupts continuous contact between the tool and workpiece and exhibits great potential in improving the cutting ability of a chisel edge and restraining the skidding of a chisel edge, reducing surface roughness, thrust force, and delamination, and extending the tool life while maintaining process productivity [30, 31]. It has been reported in some research that thrust force reduces by around 40% in VATD compared to conventional drilling. Therefore VATD is used to reduce delamination in drilling composite laminates [32].

1.2.1.5 Orbital drilling OD has great potential as an alternative to conventional drilling for minimizing damage associated with the drilling of composite laminates. OD is particularly effective for hole-making operations in laminate materials such as carbon fiber reinforced polymers (CFRPs) used in aerospace applications, which need precise dimensional accuracy and tight tolerances [33, 34]. The working principles of OD can be described in three basic motions: spindle rotation, feed and orbital rotation. Combining these motions creates spiral/helical rotation of the cutting tool. As the cutting tool spins in

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Hole-Making and Drilling Technology for Composites

its own axial direction, it simultaneously moves offset (in a lateral direction) to the desired hole diameter. By calculating the desired offset, a single cutting tool can be used to drill any diameter larger than the tool’s diameter [35]. Thus it essentially reduces the need for multiple tools to drill a single hole and eliminates the time needed for tool changeover. Moreover, since the tool diameter is substantially smaller than the diameter of the hole, it reduces the risk of the tool sticking out during the drilling process. During the OD process the tool is in partial contact with the workpiece, and this action enables the performance of heat extraction to become more efficient. Additionally, in the normal mechanism of other mechanical drilling the tool move straight concentric in an axial motion to the laminate, which results in high pressure at the center of the machine hole. However, in the OD process the tool moves in a helical feed motion, which reduces the pressure in the center of the hole while machining. This, in turn, reduces the matrix resin from burnout in the heat-affected zone [36]. These advantages of OD significantly decrease total cycle time thus increasing productivity and profitability [37]. Fig. 1.4 shows the working mechanism of the OD. The cutting tool material commonly used in OD is PCD and carbide end mills.

1.2.1.6 Milling Milling potential is well known and there are a number of publications describing its advantages. Use of circular milling began in 1995 by Park et al. [39]. Usually, circular milling will combine with other machining processes to achieve better results. For example, Schulze et al. [40] combined circular milling and spiral milling and studied the parameters of cutting velocity, tool feed, depth of cut and tool inclination angle. Regarding all parameters, feed rate is the most influential on delamination, where increases in feed rate will result in worse delamination or greater damage. Also in their research, they introduced wobble milling (Fig.  1.5) and found that this innovation will result in less damage compared to the combined milling process. Prior to that, Ali et al. [41] found that milling performed better compared to drilling in terms of

Fig. 1.4  (A) Tool motion in orbital drilling; (B) and (C) deflection of the last ply in conventional drilling (CD) and orbital drilling (OD), respectively [38].

Review of hole-making technology for composites7

Fig. 1.5  Concept of wobble milling for hole-making in composites [40].

minimum surface roughness and minimum difference between exit and entrance hole diameters. This can be achieved at low feed rate and high cutting speed. Milling also performed on composite-metal stacking panel. For example Rahim et al. [42] found that feed rate and cutting speed is not significant to the roughness of the hole surface produced via milling on CFRP/Al stacks. In other work, Yagishita and Osawa [43] studied the CFRP/TiAl6V4 stack from the perspective of roundness and hole diameter. They found that newly developed hole-making machine according to double eccentric mechanism improved the quality of the hole. Similarly, Wang et al. [44] compared the hole-making process using helical milling of CFRP/Ti stacks and they found that tool wear will affect the cutting performance. Liu et al. [45] investigated the temperature variation of helical milling.

1.2.2 Nontraditional machining Moving toward future technology, which involves the machining of complex job profiles, hard/brittle materials, and the abrasive nature of the reinforcing phases, mechanical drilling might prove impractical for several reasons. These include extreme tool wear, incapability of machining a complex profile, poor surface finish quality and prohibitive economic costs. Thus unconventional machining methods have a huge potential over conventional/mechanical drilling for composite drilling. Generally, the unconventional machining technique is defined as a group of processes that removes excess material by various methods involving mechanical, thermal, electrical or chemical energy, or combinations of these energies, but does not use a sharp cutting tool as is necessary in traditional manufacturing processes. Among the many unconventional machining techniques, there are a few we will discuss in this chapter, namely, water jet machining (WJM), electro-discharge machining (EDM) and laser beam machining (LBM).

1.2.2.1 Abrasive water jet machining WJM has been used across industries and provides some advantages over conventional/mechanical drilling operations. Some of these advantages include no thermal

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Hole-Making and Drilling Technology for Composites

Pressurized water, p

Orifice, do

Traverse feed rate, v

Abrasive ma

Stand-off distance, s Focusing tube, df

Fig. 1.6  Process parameters in abrasive water jet machining (AWJM) [46].

effect, high machining versatility, high flexibility, small cutting force and high productivity. Also, WJM makes issues like burr formation and delamination in hole making on CFRPs almost negligible [46]. AWJM uses an erosion mechanism; a water jet of high pressure, high velocity and abrasive slurry cuts the target material by means of erosion [47]. Fig.  1.6 shows the schematic diagram of AWJM for hole-making on composite materials. There are few process parameters that affect the quality of the workpiece surface cut by AWJM, namely, hydraulic pressure, standoff distance, abrasive flow rate and types of abrasive [48]. Important quality parameters in AWJM are material removal rate (MMR), surface roughness, kerf width and tapering of the kerf. The abrasive particle used for AWJM is usually silicon carbide and aluminum oxide. The abrasive particle is embedded to the water jet purposely to increase the MMR of the process. Conventional/mechanical drilling uses friction and shearing to cut the workpiece. Alternatively, AWJM uses erosive action to cut the workpiece leaving a smooth, finished edge, less burr and no delamination. Since there is no physical contact between the cutting tool and workpiece, AWJM eliminates heat-affected zones and expensive cutting tools like diamond and others [49]. In research investigating the quality of the cutting hole in glass fiber reinforced plastic (GFRP), the author found that the quality of the hole in GFRP is highly dependent on the right choice of the cutting parameter in the cutting process [50]. Another researcher carried out the experiment using AWJM on composite laminates and found the delamination of the laminates can be reduced by reducing the jet speed. However, this also caused the piercing to deteriorate [51].

Review of hole-making technology for composites9

1.2.2.2 Electro-discharge machining EDM is also known as spark erosion machining. The process basically removes the workpiece material using electric spark. There are three types of EDM machining, namely, die sinking EDM, wire EDM and hole drilling EDM. Here we will cover basic concepts of hole drilling EDM, which is also called electro-discharge drilling (EDD) [52]. EDD is a hole-making process for electrically conductive workpiece materials, which harden to the machine. In addition to CFRPs, metal matrix composites (MMCs) are also growing rapidly in the aircraft manufacturing industry. Machining of MMCs using EDD is inevitable due to the material’s high hardness and wear-resistance properties. Conventional/mechanical drilling might not be a suitable choice for making a substantial number of holes in an MMC due to rapid tool wear. MMCs are composite materials with at least two constituent parts; one being a metal necessarily, the other material may be a different metal or another material, such as a ceramic or organic compound. When at least three materials are present, it is called a hybrid composite [53]. This makes drilling MMCs more difficult because each material has different properties. The working principles of this EDD can be explained clearly based on the schematic diagram shown in Fig. 1.7. The tool (metal electrode as a cathode) and the workpiece (acting as an anode) are placed very close together with a small gap (around 0.01–0.5 mm), separated by nonconducting liquid known as dielectric. As a pulse of DC electricity reaches the electrode and the part, an intense electrical field develops in the gap and microscopic contaminants suspended in the dielectric fluid are attracted by the field and concentrate at the field’s strongest point and build a high conductivity across the gap. As the field voltage increases, the material in the conductive bridge heats up and forms the spark channel between the tool and the workpiece [54]. As a

Fig. 1.7  Schematic experimental set-up of electro-discharge drilling (EDD) [52].

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Hole-Making and Drilling Technology for Composites

result, the workpiece erodes. Drilling process set-up is attached with servo system provide tool feed to keep the gap constant. The electrode used is usually copper, graphite, tungsten and brass. For dielectric fluid, kerosene is commonly used.

1.2.2.3 Laser beam cutting technology LBM is a form of machining in which a laser is directed toward the workpiece. LBM is one of the advanced manufacturing processes capable of machining all ranges of material from metallic alloys to nonmetals [55, 56]. Therefore LBM provides a solution to a critical problem that conventional/mechanical drilling is not capable of solving. Material removal in LBM is a thermal material removal process that utilizes a high-­ energy, coherent light beam to melt and vaporize particles on the workpiece in the focus point [57]. The advantages of LBM, such as improved end product quality, short processing time, noncontact process, cost reduction and small heat affected zone, have led to its use in many manufacturing industries [58]. For cutting composites, the general types of lasers used are CO2 and neodymium yttrium aluminum garnet (Nd:YAG) [59]. Fig. 1.8 shows the schematic diagram of the LBM removal mechanism. Composite laminates have more than one constituent material, which makes them difficult to laser machine because the components of the composites have varied thermal conductivity [59]. This means, in fiber-reinforced plastics, the power needed for a laser to vaporize (cut) the fiber (CFRP or GFRP) is much higher than the power needed for a polymer (epoxy). For this reason, it is important to set the laser cutting parameters (laser power, cutting speed, gas pressure, etc.) on the cut quality parameters (heat-affected zone, surface roughness, kerf width, taper angle etc.) carefully before starting LBM.

1.2.3 Nonmachining methods Other than machining, punching is another hole-making technology used to produce holes in composite panels.

Fig. 1.8  Laser cutting material removal mechanism [55].

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Puncher Composite panel

Die

Slug

Fig. 1.9  Principles of the punching process on composite panel [61].

1.2.3.1 Punching Based on preliminary study, the punching technique shows potential for hole-making on laminated composites [5, 60–63]. Punching is a forming process that uses punch and die to form holes via shearing. In composites punching, the mechanism for cutting holes is different from that of metal material since the composite is brittle. There are several cutting parameters affecting the punching of composites, namely, clearance, tool geometry, speed or stroke rate, blank holder force, sheet thickness, blank layout, material type, punch-die alignment and friction [64]. Punching in composites might result in poor surface quality due to the nonhomogeneity, multiphase structure and anisotropic nature of the composite [65]. Lambiase and Durante [66] investigated the influence of process parameters (punch-die clearance) on quality of punched holes (delamination factor, bearing strength) compared to drilled holes on thin GFRP laminates. The results showed that the punching force increases when punch-die clearance is decreased. They also noticed that the delamination factor increases when the punchdie clearance increases. They concluded that punching results in a higher delamination factor and lower bearing strength. One of the main advantages of punching is that it does not produce peel-up edge; in drilling, both peel-up and push-down edges occur. Furthermore the process is faster and produces less wear compared to drilling. The basic principles of punching are illustrated in Fig. 1.9.

1.3 Conclusions Hole-making technology can be divided into two main technologies: machining and nonmachining methods. The machining method can be further divided into traditional and nontraditional machining. Capability of the technologies is evaluated based on several parameters, for example, spindle speed and feed, drill bit diameter, drill bit geometry, drill bit material and type of composite material. In punching, for example, die clearance and puncher profiles are among the most influential parameters. All these parameters are important for ensuring high quality of the produced holes.

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Hole-Making and Drilling Technology for Composites

In hole-making, quality depends on the cutting method or tools used. In drilling, for example, wear is the main problem. In addition, other important factors such as productivity and technology cost also need to be taken into consideration.

Acknowledgments The authors want to acknowledge Universiti Sains Malaysia for their sponsorship through Research University Grant (Acc. No. 1001/PMEKANIK/814270) and to Mr. Fakruruzi Fadzil who helps in conducting the experiments.

References [1] Federal Aviation Administration (FAA), Chapter 7, Advanced Composite Materials, 2015. [2] R.L.  Peng, J.  Zhou, S.  Johansson, A.  Billenius, V.  Bushlya, J.E.  Stahl, Surface integrity and the influence of tool wear in high speed machining of Inconel 718, in: 13th International Conference of Fracture, 2013, pp. 1–10. [3]. John, D. 2018 Non Traditional Machining & Material Addition Process. Available: https://www.slideshare.net/dennyjohn9279/non-traditional-machining-material-addition-process-as-per-mgu-syllabus (Accessed 1 January 2018) [4] S. Arul, D.S. Raj, L. Vijayaraghavan, S.K. Malhotra, R. Krishnamurthy, Modeling and optimization of process parameters for defect toleranced drilling of GFRP composites, Mater. Manuf. Process. 21 (4) (2006) 357–365. [5] M.S.M. Zain, A.B. Abdullah, M.S. Abdullah, Z. Samad, Delamination measurement of a laminates composite panel due to hole punching based on the focus variation technique, AIP Conf. Proc. 1865 (2017) 080002. [6] C.P. Rana, P.D. Pandey, A.Y. Parmar, P.A. Parmar, Advance types of drill bit—a review, Int. J. Adv. Res. Innov. Ideas Educ. 6 (2017) 543–551. [7] M. Ariffin, M.I.M. Ali, S.M. Sapuan, N. Ismail, An optimise drilling process for an aircraft composite structure using design of experiments, Sci. Res. Essays 4 (10) (2009) 1109–1116. [8] Y.Y. Wei, J.Y. Xu, X.J. Cai, Q.L. An, M. Chen, Effect of drills with different drill bits on delamination in drilling composite materials, Key Eng. Mater. 589–590 (2013) 173–178. [9] A.M. Dalavi, P.J. Pawar, T.P. Singh, A.S. Warke, P.D. Paliwal, Review on optimization of hole-making operations for injection mould using non-traditional algorithms, Int. J. Ind. Eng. Manag. 7 (1) (2016) 9–14. [10] C.C. Tsao, H. Hocheng, The effect of chisel length and associated pilot hole on delamination when drilling composite materials, Int. J. Mach. Tools Manuf. 43 (11) (2003) 1087–1092. [11] C.C. Tsao, The effect of pilot hole on delamination when core drill drilling composite materials, Int. J. Mach. Tools Manuf. 46 (12−13) (2006) 1653–1661. [12] D. Liu, Y. Tang, W.L. Cong, A review of mechanical drilling for composite laminates, Compos. Struct. 94 (4) (2012) 1265–1279. [13] A. Pramanik, A.K. Basak, Y. Dong, P.K. Sarker, M.S. Uddin, G. Littlefair, A.R. Dixit, S. Chattopadhyaya, Joining of carbon fibre reinforced polymer (CFRP) composites and aluminium alloys—a review, Compos. A: Appl. Sci. Manuf. 101 (2017) 1–29.

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[14] P. Zelinski, The Fast Track To High Speed Drilling, Available: Modern Machine Shop vol. 71, 1998. https://www.mmsonline.com/articles/the-fast-track-to-high-speed-drilling, (Accessed January 1, 2018). [15] E. Uhlmann, S. Richarz, F. Sammler, R. Hufschmied, High speed cutting of carbon fibre reinforced plastics, Proc. Manuf. 6 (2016) 113–123. [16] V.N. Gaitonde, S.R. Karnik, J.C. Rubio, A.E. Correia, A.M. Abrão, J.P. Davim, Analysis of parametric influence on delamination in high-speed drilling of carbon fiber reinforced plastic composites, J. Mater. Process. Technol. 203 (1–3) (2008) 431–438. [17] J.C. Campos-Rubio, A.M. Abrão, P.E. Faria, A.E. Correia, J.P. Davim, Delamination in high speed drilling of carbon Fiber reinforced plastic (CFRP), J. Compos. Mater. 42 (15) (2008) 1523–1532. [18] P. Sitek, A. Katunin, Analysis of drilling process of composite structures—part I: evaluation of thermal condition, Modelowanie Inżynierskie (Model. Eng.) 55 (2015) 88–94. [19] N. Hosono, T. Miwa, Y. Mukai, S. Takenaka, T. Makino, T. Fuji, Potential risk of thermal damage to cervical nerve roots by a high-speed drill, Bone Joint J. 91 (11) (2009) 1541–1544. [20] U.A.  Khashaba, A.A.  El-Keran, Drilling analysis of thin woven glass-fiber reinforced epoxy composites, J. Mater. Process. Technol. 249 (2017) 415–425. [21] J.S.  Dureja, M.S.  Bhatti, A review of near dry machining/minimum quantity lubrication machining of difficult to machine alloys, Int. J. Mach. Mach. Mater. 18 (3) (2016) 213–251. [22] S. Jain, D.C.H. Yang, Delamination-free drilling of composite laminates, J. Eng. Ind. 116 (4) (1994) 475–481. [23] S. Jain, D.C.H. Yang, Effects of Feedrate and chisel edge on delamination in composites drilling, J. Eng. Ind. 115 (4) (1993) 398–405. [24] C.C. Tsao, H. Hocheng, Parametric study on thrust force of core drill, J. Mater. Process. Technol. 192–193 (2007) 37–40. [25] D. Biermann, T. Bathe, C. Rautert, Core drilling of fiber reinforced materials using abrasive tools, Proc. CIRP 66 (2017) 175–180. [26] P.W.  Butler-Smith, D.A.  Axinte, M.  Daine, A.R.  Kennedy, L.T.  Harper, J.F.  Bucourt, R.  Ragueneau, A study of an improved cutting mechanism of composite materials using novel design of diamond micro-core drills, Int. J. Mach. Tools Manuf. 88 (2015) 175–183. [27] R. Krishnamurthy, J. Ramkumar, S. Aravindan, S.K. Malhotra, An enhancement of the machining performance of GFRP by oscillatory assisted drilling, Int. J. Adv. Manuf. Technol. 23 (3–4) (2004) 240–244. [28] X.  Wang, L.J.  Wang, J.P.  Tao, Investigation on thrust in vibration drilling of fiber-­ reinforced plastics, J. Mater. Process. Technol. 148 (2) (2004) 239–244. [29] L. Wang, J. Zhao, Q. Tan, Kinematics of ultrasonic vibration cutting and a study of the resulting surface quality, Binggong Xuebao 2 (3) (1987) 24–31. [30] L.B. Zhang, L.J. Wang, X.Y. Liu, H.W. Zhao, X. Wang, H.Y. Luo, Mechanical model for predicting thrust and torque in vibration drilling fibre-reinforced composite materials, Int. J. Mach. Tools Manuf. 41 (5) (2001) 641–657. [31] V.I.  Babitsky, A.V.  Mitrofanov, V.V.  Silberschmidt, Ultrasonically assisted turning of aviation materials: simulations and experimental study, Ultrasonics 42 (1–9) (2004) 81–86. [32] S.  Arul, L.  Vijayaraghavan, S.K.  Malhotra, R.  Krishnamurthy, The effect of vibratory drilling on hole quality in polymeric composites, Int. J. Mach. Tools Manuf. 46 (3–4) (2006) 252–259.

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[33] Q. Zhao, X. Qin, C. Ji, Y. Li, D. Sun, Y. Jin, Tool life and hole surface integrity studies for hole-making of Ti6Al4V alloy, Int. J. Adv. Manuf. Technol. 79 (5–8) (2015) 1017–1026. [34] H. Yagishita, J. Osawa, Highly accurate hole making technology of Ti6Al4V by orbital drilling: effect of oil mist, Proc. Manuf. 5 (2016) 195–204. [35] R.  Iyer, P.  Koshy, E.  Ng, Helical milling: an enabling technology for hard machining precision holes in AISI D2 tool steel, Int. J. Mach. Tools Manuf. 47 (2) (2007) 205–210. [36]. Vanysek, P., Us, I.L., Ricco, H.S. United States Patent, US 2008/0141517 A1, 2008. [37] I. Sultana, Z. Shi, M.H. Attia, V. Thomson, Surface integrity of holes machined by orbital drilling of composites with single layer diamond tools, Proc. CIRP 45 (2016) 23–26. [38] A. Sadek, M. Meshreki, M.H. Attia, Characterization and optimization of orbital drilling of woven carbon fiber reinforced epoxy laminates, CIRP Ann. Manuf. Technol. 61 (1) (2012) 123–126. [39] K.Y. Park, J.H. Choi, D.G. Lee, Delamination-free and high efficiency drilling of carbon fiber reinforced plastics, J. Compos. Mater. 29 (1995) 1988–2002. [40] V. Schulze, C. Becke, K. Weidenmann, S. Dietrich, Machining strategies for hole making in composites with minimal workpiece damage by directing the process forces inwards, J. Mater. Process. Technol. 211 (2011) 329–338. [41] H.M. Ali, A. Iqbal, L. Liang, A comparative study on the use of drilling and milling processes in hole making of GFRP composite, Sadhana 38 (4) (2013) 743–760. [42] E.A.  Rahim, Z.  Mohid, M.R.  Hamzah, A.F.  Yusuf, N.A.  Rahman, Helical milling of CFRP with aluminum stack, Appl. Mech. Mater. 465–466 (2014) 1075–1079. [43] H. Yagishita, J. Osawa, Hole making machine based on double eccentric mechanism for CFRP/TiAl6V4 stacks, Proc. Manuf. 1 (2015) 747–755. [44] H. Wang, X. Qin, H. Li, Y. Tan, A comparative study on helical milling of CFRP/Ti stacks and its individual layers, Int. J. Adv. Manuf. Technol. 86 (2016) 1973–1983. [45] J. Liu, G. Chen, C.H. Ji, X.D. Qin, H. Li, C.Z. Ren, An investigation of workpiece temperature variation of helical milling for carbon fiber reinforced plastics (CFRP), Int. J. Mach. Tools Manuf. 86 (2014) 89–103. [46] A. Alberdi, A. Suárez, T. Artaza, G.A. Escobar-Palafox, K. Ridgway, Composite cutting with abrasive water jet, Proc. Eng. 63 (2013) 421–429. [47] P.D. Unde, M.D. Gayakwad, R.S. Ghadge, Abrasive water jet machining of composite materials—a review, Int. J. Innov. Res. Sci. Eng. Technol. 3 (4) (2014) 6–8. [48] H. Ali, A. Iqbal, M. Hashemipour, Experimental analysis of hole making in GFRP composite using abrasive water jet cutting technology, Appl. Mech. Mater. 325–326 (2013) 1392–1398. [49] P.D. Unde, M.D. Gayakwad, N.G. Patil, R.S. Pawade, D.G. Thakur, P.K. Brahmankar, Experimental investigations into abrasive waterjet machining of carbon fiber reinforced plastic, J. Compos. (2015) 971596. [50] K.  Thongkaew, J.  Wang, G.H.  Yeoh, An investigation of hole machining process on a carbon-fiber reinforced plastic sheet by abrasive waterjet, Adv. Mater. Res. 1136 (2016) 113–118. [51] H.M.A. Ibraheem, A. Iqbal, M. Hashemipour, Numerical optimization of hole making in GFRP composite using abrasive water jet machining process, J. Chin. Inst. Eng. 38 (1) (2015) 66–76. [52] A.  Singh, P.  Kumar, I.  Singh, Electric discharge drilling of metal matrix composites with different tool geometries, Proc. Inst. Mech. Eng. B J. Eng. Manuf. 227 (8) (2013) 1245–1249. [53] A. Singh, P. Kumar, I. Singh, Process optimization for electro-discharge drilling of metal matrix composites, Proc. Eng. 64 (2013) 1157–1165.

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[54] H.  Hocheng, W.T.  Lei, A.S.  Hsu, Preliminary study of material removal in electrical-­ discharge machining of SiC/AI, J. Mater. Process. Technol. 63 (1997) 813–818. [55] P. Parandoush, A. Hossain, A review of modeling and simulation of laser beam machining, Int. J. Mach. Tools Manuf. 85 (2014) 135–145. [56] P. Patel, P. Gohil, S. Rajpurohit, Laser machining of polymer matrix composites: scope, limitation and application, Int. J. Eng. Trends Technol. 4 (6) (2013) 2391–2399. [57] M.H. Industries, High-speed & high-quality laser drilling technology using a prism rotator, Mitsubishi Heavy Ind. Tech. Rev. 52 (1) (2015) 106–109. [58] A.K. Dubey, V. Yadava, Laser beam machining—a review, Int. J. Mach. Tools Manuf. 48 (6) (2008) 609–628. [59] J. Meijer, Laser beam machining (LBM), state of the art and new opportunities, J. Mater. Process. Technol. 149 (1–3) (2004) 2–17. [60] H.Y.  Chan, A.B.  Abdullah, Z.  Samad, M.S.M.  Zain, Precision punching on laminates composite panel: effect of dual-stages puncher, Int. J. Mater. Eng. Innov. 6 (4) (2015) 288–296. [61] M.S.M.  Zain, A.B.  Abdullah, Z.  Samad, Effect of puncher profile on the precision of punched holes on composite panels, Int. J. Adv. Manuf. Technol. 89 (9–12) (2017) 3331–3336. [62] A.B. Abdullah, M.S.M. Zain, Z. Samad, Delamination assessment of punched holes on laminated composite panels based on the profile measurement technique, Int. J. Adv. Manuf. Technol. 93 (1–4) (2017) 993–1000. [63] N.A. Ghaffar, A.B. Abdullah, Z. Samad, Precision hole making on laminate composite: a tool Wear analysis and comparison between drilling and punching, Int. J. Mater. Eng. Innov. 7 (2) (2016) 143–157. [64] E. Al-momani, I. Rawabdeh, An application of finite element method and design of experiments in the optimization of sheet metal blanking process, Jordan J. Mech. Indust. Eng. 2 (1) (2008) 53–63. [65] K.K.  Panchagnula, K.  Palaniyandi, Drilling on fiber reinforced polymer/nanopolymer composite laminates: a review. J. Mater. Res. Technol. (2018). https://doi.org/10.1016/j. jmrt.2017.06.003. [66] F. Lambiase, M. Durante, Mechanical behavior of punched holes produced on thin glass fiber reinforced plastic laminates, Compos. Struct. 173 (2017) 25–34.

Defects in holes-making on composite panels: A review on delamination

2

M.S. Abdullah, A.B. Abdullah, Z. Samad School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia

2.1 Introduction Hole-making is one of the most costly and time-consuming processes in industry. For some applications, the measurement quality of holes is crucial, as they are expected to have precise tolerance. Composite material is widely used nowadays in aerospace and automotive industry because of its strength-to-weight ratio [1]. In the past, most aircraft structures were manufactured using conventional metallic materials, which have greater tensile strength and higher elastic modulus than composite materials [2]. In recent years, however, composites have begun to succeed metallic material in many structures of modern aircraft, commonly making up 50%–70% of structures, which results in total weight reduction by approximately 20% [3]. Composite structures are inevitably assembled as structural materials with other metal material structures; this cannot be avoided. The mechanical joint efficiency and quality of joining are dependent on hole quality [4]. Since composite material is characterized by heterogeneity and high abrasiveness, hole-making has become a problem for traditional machining processes, such as mechanical drilling, cutting and trimming, which lead to tool wear and reduced quality of the machining part [5]. The quality of holes produced is a critical criterion in all machining methods whether traditional or modern. Both drilling geometry and cutting parameters directly affect hole quality. Hole quality can be characterized on the basis of a few criteria, including delamination factor, out-of-roundness, cut neatness, surface roughness, damage surface layer, fiber fracture, burr formation and crack [6, 7]. However, in composite laminates, damage due to delamination is a big concern in drilling. For example, It was reported that the rejection of composite parts of the final assembly in aircraft industry is as high as 60% due to drilling-induced delamination damages [8]. Thus, in this chapter, we focus on delamination in drilling laminated composites.

2.2 Delamination Delamination is one of the most major failure modes of hole-making in composite laminates. Since composite is made up of layers of fibrous composite material joined together with liquid resin to form laminate stacks, the presence of repeated Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00002-7 © 2019 Elsevier Ltd. All rights reserved.

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Hole-Making and Drilling Technology for Composites

Fz

Fz Peel-up delamination

Push-down delamination

(A)

(B)

Fig. 2.1  (A) Peel-up delamination (B) Push-down delamination delamination [10].

cyclic stresses, impact and applied pressure causes the layers to separate, significantly degrading the material’s performance in bearing strength and stiffness as well as structural integrity. This type of damage can occur on both the top and bottom of the composite’s surface and is almost invisible. Therefore identifying the damage is integral to avoiding catastrophic structural failure [9]. Fig. 2.1 shows types of delamination that frequently occur during drilling of laminated composites.

2.3 Delamination mechanism Delamination is a major concern when drilling laminated composites and several studies have examined ways of keeping damage to a minimum. Delamination can be divided into two types: peel-up delamination and push-down delamination, as shown in Fig. 2.1. a) Peel-up delamination. Peel-up delamination occurs instantly when the drill comes in contact with the surface of the laminate. The cutting force acts peripherally (tangential to the drill bit’s outer surface) and induces a peeling force in the axial direction through the slope of the drill flute. The drill flute then pulls up the upper laminas and the material is removed in a spiral way. This action is carried out by downward acting thrust force and forms a peel-up delamination zone at the top surface of the laminate [11]. b) Push-down delamination. Push-down delamination occurs at the exit of the material. As the drill move down to the last laminas, the thrust force exceeds the interlaminar bond strength. At this point, the drill acts as a punch and pierces the laminate without cutting it properly. As a result, the laminate cannot hold the excessive thrust force and the tool pierces the exit side. In order to minimize push-down delamination, many researchers have suggested reducing the thrust force before the drill approaches the exit [11].

In the aerospace manufacturing industry, hole quality is crucial. Substantial progress in drilling has been made to achieve optimal hole quality on laminate composites. In composite drilling, there are a number of parameters that need to be taken into

Defects in holes-making on composite panels

19

a­ ccount in order to obtain standard hole quality, namely, cutting parameter, cutting tool geometry and material, specification of machine and so on [7]. Incorrect selection of the parameters may lead to catastrophic failure in the long run.

2.4 Causes of delamination In order to improve the quality of machined holes, many researchers [8–11] (Srinivasan et al.; Sasikumar et al.; Sonkar et al.; Shunmugesh et al.) have investigated the behaviors of the drilling process and the damages induced therein by conducting experiments to determine optimal parameters. A study of the effects of variable feed rate and lay-up configuration on surface roughness and integrity when drilling carbon fiber reinforced plastic (CFRP) composites found that different lay-up configurations yield a different value of surface roughness under constant cutting speed and feed. [12] However, increasing the feed rate (from 0.2–0.4 mm/rev) decreases the surface roughness value. Another study of the formation of exit defects in carbon fiber reinforced plates [13] found that spalling and fuzzing are the major exit defect mechanisms, but spalling contributes to more severe damage. The author states that spalling damage is dependent on drilling speed, feed, and diameter, while cutting speed has no effect. It was also found that under the same cutting conditions the spalling damage in a unidirectional CFRP is larger than in a multidirectional CFRP. The relationship between drilling parameters, such as point angle, spindle speed, and feed (input response), and the response variables of thrust force and torque is important in drilling composite laminates because delamination damage always depends on these two response variables. Some authors have been researching the influence of drill geometry on delamination and found that thrust force changes with drill geometry, and less delamination is caused at higher feed rates if proper drill geometry is selected [14]. An investigation on thrust force and torque in drilling found that the effect of the cutting speed on the cutting force is minor for the same drill material. However, the cutting force was found to decrease with decreased feed rate. The author concluded that to minimize push-down delamination, the feed rate needs to decrease at the hole exit. The result of reducing the feed rate before exit was proven effective in another study as well [15]. In that study, the author found that almost 80% of the surface roughness value was lower at hole exit due to the lower feed rate. Won and Dharan [16] studied the effect of the chisel edge and pilot hole in drilling CFRP laminates. The result of the experiment showed that the chisel edge contributes to most of the total thrust force and can be reduced significantly through the use of pilot holes thus minimizing delamination. In other work, Langella et  al. [17] suggested a mechanistic model of prediction thrust force and torque for drilling fiberglass reinforced composite (FRP). The author used traditional twist drills in his model and found that the difference between theoretical and experimental does not exceed 8% except for few cases and average error is below 5%. Results show that during FRP drilling, the damage of the laminates is proportional to the thrust values and increases as the feed and point angle increase. Khashaba et al. [18] investigated the effect of drilling parameters (feed, speed, and drill prewear) on machinability parameters in glass fiber reinforced epoxy (GFRE).

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Hole-Making and Drilling Technology for Composites

The authors found that drill prewear plays an important role in thrust force. It becomes more noticeable at high-cutting speeds and feed rates, resulting in increased delamination. Further, drilling at high speeds and a high level of drill prewear increases the temperature in GFRE composites, which have poor thermal conductivity, and therefore increases surface roughness. In addition, the GFRE specimen tested experienced bearing failure due to the interlaminar shear failure contributed by delamination. Similar behavior was observed by Hamzeh et al. [19]. They studied the effect of machining parameters and tool geometry (spindle speed, feed, tool point angle) during the drilling of CFRP laminates on the output response of surface roughness, delamination factor and thrust force. They used full factorial design and ANOVA to decrease the cost of experiment as well as to find the correlation between the parameters. The results revealed that surface roughness increases with feed rate, and decreases with spindle speed. Moreover, thrust force increases with feed, and increases in cutting speed lead to decreases in thrust force. In addition, the delamination factor increases when both feed and tool angle point increase, but decreases as spindle speed decreases. The authors further concluded that feed rate is the most important factor in determining thrust force, delamination and surface roughness. Davim et  al. [20] studied the cutting characteristics of GFRP by using two different drill geometries (i.e., Stub Length drill and Brad & Spur drill of cemented tungsten carbide). The results showed that thrust force increases with feed, however, Brad & Spur drill geometry showed lower values than Stub Length drill geometry under constant cutting parameters (cutting speed and feed) as shown in Fig. 2.2.

B D 2

Fc

Fc

f 2

D

(A)

Point angle, q

D 2

f 2

(B)

D 2

Fig. 2.2  Schematic of the chip section of (A) “Stub Length” drill and (B) “Brad & Spur” drill. Where, f is the feed rate in mm/rev, d the diameter of the drill in mm [20].

Defects in holes-making on composite panels

21

Heisel et al. [21] carried out an investigation of the influence of the point angle and drilling parameters on drill hole quality and drilling forces on CFRP. The authors used cemented carbide cutting material with four different types of cutting edge and point angle. The results showed that drilling torque is highly affected by the point angle. It showed that the delamination factor at hole entrance is smaller when the point angle is greater than 180 degrees, however, the opposite is true at hole exit. Tsao and Chiu [22] conducted an experiment to evaluate the thrust force by varying drilling parameters (cutting velocity ratio, feed rate, stretch, inner drill type, and inner drill diameter) using compound core drills on CFRP laminates. Fig. 2.3 shows the image of a compound drill. A compound drill consists of two drills that are combined together: the outer drill, which is the core drill, and the inner drill (twist, saw or candlestick). Their findings revealed that cutting velocity ratio, feed, and inner drill type are the drilling parameters that have the most effect on thrust force. Stretch and inner drill diameter have only a minor effect on thrust force. It was concluded that the use of compound core drills results in better hole quality, that is, lower thrust force, lower delamination, lower chip clogging and higher chip removal. Campos Rubio et  al. [23] conducted an experiment comparing high speed drilling (HSD) on CFRP to conventional cutting techniques. The experiment aimed to evaluate the delamination induced in both cutting methods. The author used three cemented carbide drills with different geometry, namely, a helical drill (115-degree and 85-degree point angle) and a Brad & Spur drill. The results showed that delamination decreases as spindle speed increases. At spindle speeds of 4000 and 8000 rpm

Fig. 2.3  Special type of core drills (A) Core-twist drill, (B) Core-saw drill, (C) Corecandlestick drill, (D) Step-core-twist drill, (E) Step-core-saw drill and (F) Step-corecandlestick drill [22].

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Hole-Making and Drilling Technology for Composites

delamination increases with feed rate, however, at spindle speeds of 40,000 rpm, increased feed rate does not increase damage. Among the three drill geometries, the Brad & Spur drill and the helical drill with an 85-degree point angle produced less delamination. Rameshet et al. [24] investigated drilling of pultruded and liquid composite molded glass/epoxy thick composites to find the best drill geometry and corresponding optimal process parameters. They used six different types of drill geometry with different coating materials. The results reveal that spindle speed is a significant parameter in reducing the thrust force due to thermal softening of the material. Moreover, the damage induced by the drilling is influenced by drill geometry and feed rate. However, the surface roughness of the hole depends on fiber orientation of the composite. The author concluded that the diamond-electroplated HSS twist drill is cost-effective for drilling pultruded composite. Durão et al. [25] found that damage onset and propagation is a function of feed. Increased feed rate results in higher delamination extension on whatever criterion was used. They stated that in order to obtain minimum delamination of a drilled hole, an adequate combination of feed and proper tool geometry is needed. They also noticed that the bearing strength of the drilled hole decreased almost 16% by changing from tungsten carbide to HSS considering the same cutting geometry (twist drill). In another study on drilling damage of CFRP [26], researchers compared the damaged pattern between data extracted from radiographic image with mechanical test result. The results demonstrate that high feed results in higher delamination and lower bearing strength of the hole. They also found the expected outcome that in the delamination onset test, as testing speed increased, the delamination onset load also increased. Fernandes and Cook [27, 28] proposed a mathematical model of drilling CFRP using a one-shot drill bit. This model is an extension of their earlier research on one-shot drilling and focuses on behavior of force and torque. The results provided a good estimation of maximum thrust force and torque produced during drilling of CFRP using a one-shot drill bit. Furthermore, the finding is useful in determining critical thrust force, which then lead to result in minimum delamination. Their earlier research also showed a correlation between tool wear and thickness of the workpiece to thrust force and torque. Throughout a large number of experiments, they found that thrust force increases as the number of holes drilled increases, while torque is not affected. The results also showed that the thinner the workpiece, the more thrust force increases due to wear. Thrust force is a function of feed, drill bit, thickness of the workpiece and tool wear. An experimental study on surface quality and damage induced when drilling CFRP used a polycrystalline diamond (PCD)-tipped eight-facet drill as the cutting tool [29]. The investigation characterized the influence of cutting parameters (cutting speed and feed) to the output response of thrust force and quality of the hole, including surface roughness, delamination and fiber pullout. The study concluded that thrust force increases as feed increases, and decreases slightly as cutting speed increases. In addition, better hole quality was attained at higher cutting speeds (4500–6000 rpm) and lower feed rate (64 μm/rev). Caggiano et  al. [30] assessed damage of drilled holes in of CFRP using an image analysis focusing on peel-up and push-down delamination. The results showed that delamination at hole exit is poor as tool wear progresses

Defects in holes-making on composite panels

23

during drilling. However, at the hole entry, more stable delamination was attained. Brinksmeier et al. [31] studied orbital drilling of Al/CFRP/Ti laminated composites (as shown in Fig. 2.4) and compared the results with conventional drilling. The aim the study was to investigate the influence of process parameters (cutting speed, feed, cutting temperature) on the surface integrity (surface crack, thermal damage) of the borehole. The results revealed that orbital drilling yields better surface quality of the boreholes in Al/CFRP/Ti compared to conventional drilling. Since conventional drilling is a continuous in-contact action with the hole surface, the influence of the titanium chip at high temperature results in thermal damage on the CFRP layer in the middle of the composite laminate as shown in Fig. 2.5. The author concluded that the use of orbital drilling was advantageous assuring low cutting temperatures that lead to the best bore hole surfaces.

Aluminum

25 µm

10 mm CFRP 10 mm

25 µm

10 mm

Titanium

Fan 0701_eng

25 µm

Fig. 2.4  Laminated composite made of aluminum, CFRP, and titanium [31].

Fig. 2.5  Micrographs of the borehole surface layer of the Al/CFRP/Ti composite cut by orbital and conventional drilling [31].

24

Hole-Making and Drilling Technology for Composites

Palanikumar et  al. [32] investigated the thrust force in drilling GFP/PP composites. The idea of the study was to reduce the thrust action during drilling and suppress delamination by identifying the parameters that significantly contribute to the thrust. They used Brad & Spur drill geometry with different diameter sizes and drilling parameters. The results show that feed and drill diameter are significant factors in the cutting process. The thrust force increases with an increase in feed rate and diameter, while an increase in spindle speed does not show any significant effect. Karimi et al. [33] studied the effect of drilling parameters on thrust force, adjusted delamination factor and compressive residual strength of unidirectional GFRP using a twist drill. The results showed that feed and drill point angle are major contributors to adjusted delamination factor. However, compressive residual stress is strongly dependent on feed and the best result is attained at lower feed rates. Cutting speed and drill point angle parameters were shown to be insignificant. In studies by Abdullah et al. [34] and Zain et al. [35], delamination level due to punching in hole-making is considerably low compared to delamination produced via drilling using an HSS drill bit. In the study, parameters studied included puncher profiles and die clearance.

2.5 Delamination measurement Delamination in general is a measure of damage area. There are a number of published works that focus on the development of reliable and effective delamination measurement [26, 36–40]. Principally at least two diameters need to be determined: original, D and damage area, E as shown in Fig. 2.6. The ratio of these diameters depict the level of damage (i.e., delamination) [41]. New methods have been developed to assist in measuring delamination. One example is 3D surface measurement based on the profile variation technique [34, 35]. In this method the 2D profiles of the holes are scanned and the maximum radius of delamination area is determined as shown in Fig. 2.7. Then the delaminated value is computed by dividing the maximum delamination or damage area, E, to the initial diameter of the hole, D. Ideal delamination value should be closest to one. Sohn et  al. [42] investigated hidden delamination and disbond defects in multilayer composites and found that these defects can be detected using a scanning laser vibrometer. In this study, a 1D scanning laser vibrometer was used to acquire outof-plane velocity field information across the scanned surface. Then, graphic tools in MATLAB were used to create wave propagation videos. The experiment setup is shown in Fig. 2.8. Both delamination and disbond areas were found to exhibit high ultrasonic activity, which was manifested especially in the images of cumulative energy field. In other work, Albuquerque et al. [43] developed a computational methodology system based on image-processing techniques and analysis and a backpropagation artificial neural network in order to evaluate the delamination damage in laminated plates caused by drilling operations. The delaminated regions were obtained by radiographic digitalization as shown in Fig. 2.9.

Defects in holes-making on composite panels

E

D

(cm)

Depth - z

1.85

1.84

1.83

1.82 (mm) 0

1

2

3

4

5

6

7

8

9

10

11

12

Path length - I

Fig. 2.6  Definition of delamination [36].

25

26

Hole-Making and Drilling Technology for Composites

Fig. 2.7  2D profile of hole to measure the delamination [34].

Fig. 2.8  Experiment setup of delamination detection [42].

Defects in holes-making on composite panels

27

Maximum delaminated diameter

Holes nominal diameter

Fig. 2.9  Image of the delaminated area obtained from focus variation technique [35].

Fig. 2.10  (A) the captured image, (B) delaminated area [30].

Image processing is also utilized in the measurement of delamination. Hafeez and Almaskari [44] used image processing and capability to conduct in situ measurements. Similarly Silva et al. [45] developed an algorithm for systematic measurement of delamination. They also compared few other developed algorithms. Caggiano et al. [30] developed an approach utilizing digital image processing of a few parameters related to hole quality, such as hole geometries. Fig. 2.10 illustrates the image processing stages.

2.6 Conclusions This reviewed of the main defects found in hole-making, that is, delamination. Understanding factors that contribute to delamination is very important as the level of delamination may affect the strength of the assembled panel. Based on this review, it is concluded that in drilling operations, input parameters such as cutting speed, feed rate, drill point angle, tool geometry, tool materials and type of drilling operation greatly affect delamination. The use of high cutting speeds and low feed rates can minimize delamination in laminated composites. In punching operations, die clearance and puncher profile play important roles.

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Hole-Making and Drilling Technology for Composites

Acknowledgments The authors want to acknowledge Universiti Sains Malaysia for their sponsorship through Research University Grant (Acc. No. 1001/PMEKANIK/814270). For Mr. Fakururuzi Fadzil, who helps in conducting the experiments.

References [1] R.T.  Katsumata, Y.  Mizutani, A.  Todoroki, R.  Matsuzaki, A study on failure behavior of CFRP bolted joints with cone washers by using AE monitoring, in: EWGAE 2010, Vienna, 8–10 September 2010, 2010. [2]. Spampinato, A. The Materials Used in the Design of Aircraft Wings, Editorial Features, AZO Materials, 2015. https://www.azom.com/article.aspx?ArticleID=12117 (Accessed 2 February 2018) [3] A.N. Amir, L. Ye, L. Chang, Drilling conditions on hole quality for CFRP laminates, in: American Society of Composites—31st Technical Conference, Williamsburg, Virginia, 19–22 September, 2016. [4] M.S.M. Zain, A.B. Abdullah, Z. Samad, Delamination measurement of a laminates composite panel due to hole punching based on the focus variation technique, Int. J. Adv. Manuf. Technol. 93 (1–4) (2017) 993–1000. [5] H. Xu, J. Hu, Modeling of the material removal and heat affected zone formation in CFRP short pulsed laser processing, Appl. Math. Model. 46 (2017) 354–364. [6] M. Khoran, P. Ghabezi, M. Frahani, M.K. Besharati, Investigation of drilling composite sandwich structures, Int. J. Adv. Manuf. Technol. 76 (9–12) (2014) 1927–1936. [7] S. Ragunath, C. Velmurugan, T. Kannan, A review of influential parameters in drilling delamination on fiber reinforced polymer composites, Int. J. Chem. Technol. Res. 10 (7) (2017) 298–303. [8] T.  Srinivasan, K.  Palanikumar, K.  Rajagopal, B.  Latha, Optimization of delamination factor in drilling GFR–polypropylene composites, Mater. Manuf. Process. 32 (2) (2017) 226–233. [9] K.S.K. Sasikumar, Optimization of drilling parameters on delamination based on Taguchi method in drilling of natural fiber reinforced (Agave) composite, Int. J. Recent Technol. Eng. 4 (1) (2015) 53–55. [10] V. Sonkar, K. Abhishek, S. Datta, S.S. Mahapatra, Multi-objective optimization in drilling of GFRP composites: a degree of similarity approach, Procedia Mater. Sci. 6 (2014) 538–543. [11] K. Shunmugesh, R. Rajasekar, C. Moganapriya, V. Karthik, Optimization of machining force and delamination factor of GFRP in dry drilling process using Taguchi method, Adv. Nat. Appl. Sci. 8 (2017) 220–230. [12] P. Mehbudi, V. Baghlani, J. Akbari, A.R. Bushroa, N.A. Mardi, Applying ultrasonic vibration to decrease drilling-induced delamination in GFRP laminates, Procedia CIRP 6 (2013) 577–582. [13] M.  Ghrib, L.  Berthe, N.  Mechbal, M.  Rébillat, M.  Guskov, R.  Ecault, N.  Bedreddine, Generation of controlled delaminations in composites using symmetrical laser shock configuration, Compos. Struct. 171 (2017) 286–297. [14] H. Hocheng, C.C. Tsao, Effects of special drill bits on drilling-induced delamination of composite materials, Int. J. Mach. Tools Manuf. 46 (12) (2006) 1403–1416.

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29

[15] M.J.  Li, S.L.  Soo, D.K.  Aspinwall, D.  Pearson, W.  Leahy, Influence of lay-up configuration and feed rate on surface integrity when drilling carbon fibre reinforced plastic (CFRP) composites, Procedia CIRP 13 (2014) 399–404. [16] M.S. Won, C.K.H. Dharan, Chisel edge and pilot hole effects in drilling composite laminates, J. Manuf. Sci. Eng. 124 (2) (2002) 242. [17] A. Langella, L. Nele, A. Maio, A torque and thrust prediction model for drilling of composite materials, Compos. A: Appl. Sci. Manuf. 36 (1) (2005) 83–93. [18] U.A.I.  Khashaba, A.I.  Selmy, A.A.  Megahed, Drilling analysis of woven glass fiber-­ reinforced/epoxy composites, J. Compos. Mater. 47 (2) (2013) 191A. [19] H.  Shahrajabian, M.  Hadi, M.  Farahnakian, Experimental investigation of machining, parameters on machinability of carbon fiber/epoxy composites, Int. J. Eng. Innovative Technol. 2 (3) (2012) 30–36. [20] J.P. Davim, P. Reis, C.C. António, Experimental study of drilling glass fiber reinforced plastics (GFRP) manufactured by hand lay-out, Compos. Sci. Technol. 64 (2) (2004) 289–297. [21] U. Heisel, T. Pfeifroth, Influence of point angle on drill hole quality and machining forces when drilling CFRP, Procedia CIRP 1 (1) (2012) 471–476. [22] C.C. Tsao, Y.C. Chiu, Evaluation of drilling parameters on thrust force in drilling carbon fiber reinforced plastic (CFRP) composite laminates using compound core-special drills, Int. J. Mach. Tools Manuf. 51 (9) (2011) 740–744. [23] J.C. Campos Rubio, A.M. Abrão, P.E. Faria, A.E. Correia, J.P. Davim, Delamination in high speed drilling of carbon Fiber reinforced plastic (CFRP), J. Compos. Mater. 42 (15) (2008) 1523–1532. [24] B. Ramesh, A. Elayaperumal, S. Satishkumar, A. Kumar, Drilling of pultruded and liquid composite moulded glass/epoxy thick composites: experimental and statistical investigation, Measurement 114 (2018) 109–121. [25] L.M.P. Durão, J. Tavares, M. R, S. V, H.C. de Albuquerque, D.J.S. Gonçalves, Damage evaluation of drilled carbon/epoxy laminates based on area assessment methods, Compos. Struct. 96 (2013) 576–583. [26] L.  Durão, J.  Tavares, V.  de Albuquerque, J.  Marques, O.  Andrade, Drilling damage in composite material, Materials 7 (5) (2014) 3802–3819. [27] M. Fernandes, C. Cook, Drilling of carbon composites using a one shot drill bit. Part II: empirical modeling of maximum thrust force, Int. J. Mach. Tools Manuf. 46 (1) (2006) 76–79. [28] M. Fernandes, C. Cook, Drilling of carbon composites using a one shot drill bit. Part I: five stage representation of drilling and factors affecting maximum force and torque, Int. J. Mach. Tools Manuf. 46 (1) (2006) 70–75. [29] E.D. Eneyew, M. Ramulu, Experimental study of surface quality and damage when drilling unidirectional CFRP composites, J. Mater. Res. Technol. 3 (4) (2014) 354–362. [30] A. Caggiano, R. Angelone, R. Teti, Image analysis for CFRP drilled hole quality assessment, Procedia CIRP 62 (2017) 440–445. [31] E. Brinksmeier, S. Fangmann, R. Rentsch, Drilling of composites and resulting surface integrity, CIRP Ann. Manuf. Technol. 60 (1) (2011) 57–60. [32] K.  Palanikumar, T.  Srinivasan, K.  Rajagopal, B.  Latha, Thrust force analysis in drilling glass fiber reinforced/polypropylene (GFR/PP) composites, Mater. Manuf. Process. 31 (5) (2016) 581–586. [33] N. Zarif Karimi, H. Heidary, G. Minyak, M. Ahmadi, Effect of the drilling process on the compression behavior of glass/epoxy laminates, Compos. Struct. 98 (2013) 59–68.

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[34] A.B. Abdullah, M.S.M. Zain, Z. Samad, Delamination assessment of punched holes on laminated composite panels based on the profile measurement technique, Int. J. Adv. Manuf. Technol. 93 (1–4) (2017) 993–1000. [35] M.S.M.  Zain, A.B.  Abdullah, Z.  Samad, Effect of puncher profile on the precision of punched holes on composite panel, Int. J. Adv. Manuf. Technol. 89 (9–12) (2017) 3331–3336. [36] M.A.J. Bosco, K. Palanikumar, B.A. Velayudham, Influence of machining parameters on delamination in drilling of GFRP-Armour steel sandwich composites, Procedia Eng. 51 (2013) 758–763. [37] J.Y. Sheikh-Ahmad, M. Dhuttargaon, H. Cheraghi, New tool life criterion for delamination free milling of CFRP, Int. J. Adv. Manuf. Technol. 92 (2017) 2131–2143. [38] J. Xu, Q. An, M. Chen, An experimental investigation on cutting-induced damage when drilling high-strength T800S/250F carbon fiber–reinforced polymer, Proc. IMechE B J. Eng. Manuf. (2015) 1–10. [39] M.R.V. Sereshk, H.M. Bidhendi, The contribution of different fracture modes on drilling delamination crack propagation, J. Manuf. Sci. Eng. 139 (1) (2016) 011013. [40] K. Giasin, S. Ayvar-Soberanis, S. French, V. Phadnis, 3D finite element modelling of cutting forces in drilling fibre metal laminates and experimental hole quality analysis, Appl. Compos. Mater. 24 (1) (2017) 113–137. [41] W.C. Chen, Some experimental investigations in the drilling of carbon fiber-reinforced plastic (CFRP) composite laminates, Int. J. Mach. Tools Manuf. 37 (1997) 1097–1108. [42] H. Sohn, D. Dutta, J.Y. Yang, H.J. Park, M. DeSimio, S. Olson, E. Swenson, Delamination detection in composites through guided wave field image processing, Combust. Sci. Technol. 71 (9) (2011) 1250–1256. [43] V.H.C. Albuquerque, J.M.R.S. Traves, L.M.P. Durao, Evaluation of delamination damages on composite plates using techniques of image processing and analysis and a back propagation artificial neural network, J. Compos. Mater. 44 (9) (2010) 1139–1159. [44] F. Hafeez, F. Almaskari, Image processing for measuring damage and delamination in glass reinforced epoxy, J. Test. Eval. 44 (2) (2016) 995–1008. [45] J.P.T.  Duarte Silva, C.M.  Machado, Methodology analysis for evaluation of drilling-­ induced damage in composites, Int. J. Adv. Manuf. Technol. 17 (2014) 1919–1928.

Further reading [46] A.P. Singh, M. Sharma, I. Singh, A review of modeling and control during drilling of fiber reinforced plastic composites, Compos. Pt. B 47 (2013) 118–125. [47] U.A. Khashaba, Drilling of polymer matrix composites: a review, J. Compos. Mater. 47 (15) (2013) 1817–1832. [48] H.J. Zhang, W.Y. Chen, D.C. Chen, L.C. Zhang, Assessment of the exit defects in carbon fibre-reinforced plastic plates caused by drilling, Key Eng. Mater. 196 (2001) 43–52, 2001. [49] U.N.S. Jayabal, Influence of cutting parameters on thrust force and torque in drilling of E-glass/polyester coposites, Indian J. Eng. Mater. Sci. 17 (2010) 463–470.

Structural integrity assessment of a composite joint: A review

3

M.S. Abdullah, A.B. Abdullah, Z. Samad School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia

3.1 Introduction Composite materials comprise two or more constituent materials that are combined together to develop a material with properties that are far superior to those of its individual constituents. Fiber-reinforced polymers (FRPs) usually comprise a fiber set that includes fiber glasses, carbon, and aramid that are bonded to a matrix resin (either thermoplastic or thermoset). The fiber provides strength and stiffness, while the matrix resin acts as a medium to bind, protect, and facilitate load transfer among the fibers. FRPs have been recently examined and used in many industries, especially in aircraft construction, due to their numerous advantages. For instance, those structures that are built by using FRPs can be easily molded to their final shape with custom-made properties. However, joints need to be used to assemble the composite structures into the final product, especially when the product design is too complex to be molded into a single structure or when the structure needs frequent inspection and replacement for maintenance. However, not all joining methods (either bolted or adhesive) can be used to assemble composite structures, such as aircraft wings. Given that aircrafts are commonly designed and built to fly at a fast rate and high altitude, the temperature range over which the composite joint will be exposed also increases. Therefore, the joint must be strong enough to withstand these conditions. Many researchers have examined several circumstances, such as composite bearing fatigue [1], fastener fatigue [2], composite bearing creep [3], and bearing strength [4], all of which depend on connection details [5] and material and fastener specifications, such as geometry [6], fiber reinforcement architecture [7, 8], bolt type [6], clearance hole size [9], and bolt loading and tightening [10]. For instance, Thoppul et al. [1] comprehensively reviewed the mechanically fastened joint of a polymer-based composite structure. Hole integrity can be assessed based on many parameters, such as the bearing strength of holes. This chapter reviews the bearing strength and the recently applied methods for assessing hole integrity as well as evaluates those joints that are commonly used in composite panel assembly. It also reviews recent studies on three types of joints and examines the standard test procedures, the parameters to be considered when evaluating joint performance, and the failure modes. This chapter ends by presenting the conclusion and highlighting those issues in hole integrity that need to be examined in future work.

Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00003-9 © 2019 Elsevier Ltd. All rights reserved.

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Hole-Making and Drilling Technology for Composites

3.2 Types of joints Two types of joints, namely, mechanical joints and adhesively bonded joints, are generally used in engineering composites. These two types may also be combined to create a hybrid bolted/adhesive joint that has been examined by researchers for >30 years. Fig. 3.1 summarizes the types of joints that are commonly used in composite structure assembly.

3.2.1 Mechanical joints Mechanical joints are always used in composite structure assembly because of their cost efficiency and accessibility. However, the use of mechanical joints involves the drilling of holes in which fasteners can be installed. The capability of mechanical joints greatly depends on hole quality because the load that is transmitted via fasteners can lead to stress concentration around the hole fastener boundary and may lead to the premature failure of structures. Unlike ductile materials, composite laminates are brittle, non-homogenous, multi-phase, and anisotropic in nature. Therefore, unlike in metals, the stress concentration in composite laminates cannot be relieved by localized yielding [11]. Most of the critical structures in the aircraft industry, such as wings, pressurized fuselages, and empennages, install mechanical joints over bonded joints to meet safety certification requirements [9]. However, due to the complex nature of composites and the various factors that influence mechanical joints (e.g., stacking sequence, joint geometry, geometric properties, clearance between the hole and pin, types of fasteners, clamping pressure, and lateral constraint), many studies have been extensively conducted to advance the current understanding about the influence of such factors on the strength of mechanical joints [6, 12]. Kolks and Tserpes [13] studied the bolted joint of titanium-lamella-reinforced CFRP by using a simulation Bolted Mechanical

Rivet Pin Butt joint

Types of joint

Scarf joint Adhesive

Single lap joint

Hybrid

Single strap joint Double strap joint

Fig. 3.1  Types of joint.

Structural integrity assessment of a composite joint: A review33

model. Similarly, a simulation model was utilized to study the static and dynamic failure behavior [14], bearing failure [15, 16], influence of bolt clamping force, friction coefficient and bolt-hole clearance [17], and effects of joint thickness, laminate taper, and secondary bending of countersunk-type fasteners [18]. Another aspect recently studied is on hole perpendicularity error, which is applied for single-lap, single-bolt [19] and single-lap, and double-bolt [16] tests and assessed based on their effect on the mechanical joint performance. Several types of fasteners, such as bolted, rivet, and pin joints, are often used in mechanical joins as shown in Fig. 3.1. Among these fasteners, bolted joints are considered the best fasteners for joining primary structures in a mechanical joint [1]. These fasteners are constructed by using either a double or single-lap joint arrangement. Many studies in this area have used experimental data and numerical models to determine the optimum joint strength, predict the failure mode, and construct highly reliable structures. Mechanical joints are being widely used in primary structures with a high load-­ carrying capability and excellent detectability, reparability, and replaceability. However, composite materials are very complex and numerous factors (e.g., joint geometry, material parameters, fastener types, and lateral constraint) can influence the mechanical joint behaviors [1]. Ascione et al. [7] studied the effects of the fiber inclination angle and laminate stacking sequence on the bearing failure load of composite bolted joints. Their experimental results reveal that the bearing load failure significantly depends on the fiber inclination angle to the external load direction and that the stacking sequence is not significantly related to the bearing failure.

3.2.2 Adhesive joints Adhesive or adhesive-bonded joints have been widely used in many applications that require moderate-strength joint structures [20]. The increasing acceptance of adhesive-­ bonded joints can be ascribed to the weaknesses or limitations of mechanical joints. Adhesive-bonded joints are major contributors to the total weight assembly and to joints that are exposed to cumulative mechanical fatigue, thermal fatigue, and environmental conditions [21, 22]. Adhesive joining is a permanent joining technique with certain disadvantages related to the dismantlement and replacement of certain components for maintenance, thereby limiting the application of this technique in the construction of primary joint structures [6]. Meanwhile, adhesive-bonded joints have high potential in reducing the stress concentration and uniform stress distribution around the joining part (adherent) as well as demonstrate a high load-carrying potential and resistance to corrosion and fatigue [23]. The joint strength of adhesive-bonded joints greatly depends on the types and properties of adhesives, the properties of adherents, and the bond between the adhesive and adherent. Meanwhile, the adherent greatly depends on the types of joints, types of adherent interfaces, wettability of the adhesive, surface properties of the joint surface, types of loadings, geometry configuration, and joint overlap area [24]. Adhesive-bonded joints are known for their high specific stiffness and continuity of structures yet are limited by their lack of dismantlement and detectability, which remarkably limits their application [1].

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Hole-Making and Drilling Technology for Composites

Mechanical and adhesive joints both have their advantages and disadvantages. However, adhesive joints have received more demand than mechanical joints because of their very high strength under tension. Nevertheless, when applying adhesive joints, part of the structure cannot be dismantled for repair, thereby degrading the strength of adhesive joints over time. Engineers have several options when deciding which of these joints they must use to connect composite structures. Accordingly, researchers have examined the parameters that affect the performance of bonded joints, such as surface treatment, joint configuration, geometric and material parameters, and failure mode [24–26].

3.2.3 Hybrid (bolted and adhesive) joint Combining bolted and adhesive joints will produce a hybrid joint. Adhesively bonded and mechanically fastened composite joints have been widely studied since the 1980s starting from the work of Hart-Smith [27]. Fig. 3.4 illustrates a hybrid joint. Bodjona and Lessad [29] comprehensively examined the application of a hybrid bonded-­fastened joint in a composite structure, compared bonding-bolting with bonding-­pinning, and discussed the advantages and potential of both types of joints. Lopez-Cruz et al. [9] reported that these two types of joints outperformed the commonly used joints, such as mechanical and adhesive joints. Paroissien et al. [30] developed a model that can optimize the mechanical performance of hybrid bolted/bonded joints. Another study revealed that hybrid joints are more efficient than riveted and bonded joints [31]. Majid et al. [32] conducted experimental works on hybrid joints and compared the mechanical properties of several materials through a tensile test. Esmaeili et al. [33] studied the effects of bolt clamping force on the fatigue life of bolted and hybrid joints and observed that clamping could improve fatigue life by producing compressive stress in the adherents and adhesive underneath the bolt or washer. Bois et al. [34] developed an analytical model to predict the strength of hybrid bonded composite joints. Few other studies, such as Bodjona and Lessard [35] and Bodjona et al. [36], have examined the load sharing among hybrid joints. After assembling composite panels together by using several types of joints, the strength of the assembly needs to be tested. The following section discusses the standard procedure for evaluating composite panel assembly.

3.3 Standard test procedure The presence of holes in mechanical joints is inevitable during structure assembly. Accordingly, the expanded use of polymer-matrix composites in aerospace, marine, automotive, energy, and civil infrastructure structural applications underscores the necessity of conducting mechanical standard tests, which aim to ensure that mechanically fastened joints are cost-efficient and have sufficient safety margins and reduced weight. Only few organizations have established standard test procedures, with the American Society for Testing and Materials (ASTM) being one of the most famous of these organizations.

Structural integrity assessment of a composite joint: A review35

3.3.1 Bearing test Bearing strength refers to the bearing stress that is produced when the bolt-hole is deformed by 4% of the initial bolt-hole diameter. As illustrated in Fig. 3.2, for a given load, the P displacement, D curve, ABC, and a tangent are drawn at point B in such a way that when the tangent is projected to intersect the zero load axis at O, the distance between O and D must be 4% of the bolt-hole diameter. The bearing strength that corresponds to the bearing load, P, is calculated as Sb = P / ( td )

(3.1)

where t denotes the thickness of the specimen and d denotes the diameter of the bolthole. The compression test yields a higher bearing strength than the tension test. The bearing test aims to determine the bearing strength in tension (Procedure A) and compression loading (Procedure B). Several test standards have been developed for polymer-matrix composites, with ASTM D5961 being one of the most common standards [37] that provides useful data of bearing response on how the specimen exhibits under load. The bearing strength of the hole needs to be examined to determine the limit that can be achieved before failure. The ASTM test standard can be divided into the ASTM D5961 Procedures A, B, C, and D, which will be further discussed in the following sub-sections. Each of these procedures is designed for a specific purpose. In most standard tests, the test specimen must be configured according to the standard. Five of the tested specimens must also be nominally identical to one another to achieve repeatability. Fukada [38] proposed a new failure criteria instead of setting a 4% load, and experiments showed that by providing >0.1 mm clearance for a 6.35-mm hole could influence the failure load but not the maximum load. The surface roughness of the plate leads to the non-linearity of thickness-wise stiffness, which may explain why thick plates show a high kink point.

P

C

Bearing load B

A D

O 4 % of hole diameter (d)

Fig. 3.2  Definition of the bearing strength [1].

D

36

Hole-Making and Drilling Technology for Composites

3.3.1.1 Procedure A Procedure A is a standard test method in ASTM D 5961/D 5961M-05 [37] that determines the bearing response of polymer-matrix composite laminates in double-shear tensile loading. In Procedure A, the bearing load is applied by pulling the specimen through a lightly torqued (2.2 nm to 3.4 nm) fastener or a pin with D = 6 mm as shown in Fig. 3.3. The effective bearing strain, ɛbr, and the effective bearing stress, αbr, are calculated by monitoring the applied load and the bolt-hole deformation during the test. The test is stopped when the maximum load is reached and when both the load carried by the specimen at failure, Pf, and the maximum load carried by the test specimen prior to failure, Pmax, are recorded. Afterward, the bearing stress versus bearing strain is plotted and the ultimate bearing strength, Pbru = Pmax/(k*d*h), of the specimen is calculated, where k denotes the load per bolt-hole factor (1.0 for a single fastener or pin and 2.0 for a double-fastener test or pin). In this procedure, the double-shear with single fastener configurations can be used to determine the fatigue response. Jadee and Othman [40] studied the effect of the defense hole system on the failure load and bearing strength of GFRP by using double-lap and single-bolt joints. The laminates with W/D ≥ 3 and E/D ≥ 3 have failed in the bearing failure mode. By contrast, the laminates with small W/D ratios, except for those with a small edge distance ratio (E/D ≤ 2), have failed in the net-tension mode. All laminates with small edge distance ratios (E/D ≤ 2) have shown signs of shear out failure. Jadee [41] simulated the test by using the Hashin failure criteria to determine the failure load, failure mode, and bearing strength. Yilmaz and Sinmazcelik [42] found that E/D is critical at 4, where increasing the E/D ratio will increase the bearing strength. Similarly, the observed ultimate bearing failure load increases along with the E/D ratio but not with the W/D ratio. Aktas and Dirikolu [8] studied the effect of stacking sequence on bearing strength and found that the [90°/45°/−45°/0°]S sequence is stronger than the [0°/45°/−45°/90°]S

Pin Specimen

Fig. 3.3  Test setup for the Procedure A. [39].

Structural integrity assessment of a composite joint: A review37

sequence by up to 12% and 20% in terms of safe and maximum bearing strengths, respectively. Sayman and Esendemir [39] found that the bearing load decreased along with an increasing rainwater immersing duration. Kim et  al. [43] studied how the interference fit of the pin influences the bearing strength of GFRP by conducting a simulation and experiment. Soykok et al. [44] found that thermal conditions are significantly related to the tensile strength of the joint.

3.3.1.2 Procedure B Procedure B is part of the standard test method ASTM D 5961/D 5961M-05 [37] that determines the bearing response of polymer-matrix composite laminate specimens in single-shear tensile or compressive loading. In the single-shear test, two specimens that are identical to the specimen used for the double-shear test are fastened together through one or two holes that are located centrally near one end for a single-shear, single-fastener test or a single-shear, double-fastener test, respectively. The test is performed by pulling the lightly torqued specimens as explained in Procedure A. In Procedure B, a single-shear with single- or double-fastener configurations can be used to determine the fatigue response. The setup of this test and the geometries for the specimen as recommended by [28] are presented in Fig. 3.4. Several researchers have conducted their studies based on Procedure B. For example, Lawlor et al. [28] found that the joints with the largest clearance show the greatest degree of damage. Sivakumar et al. [31] studied the geometrical effect of two serial bolted joints on the bearing strength of a hybrid composite and found that the bearing strength increases along with the K/D and E/D ratios.

3.3.1.3 Procedure C Procedure C involves pulling a specimen that has been prepared according to Procedure B. In this test, the specimen or coupon is mounted on a fixture by a f­ astener as shown in

155 24

Grip area

48

Grip area

90° 45°

32

75

0° –45° 5.2 8

Fig. 3.4  Geometry of the coupon for Procedure B. [28].

38

Hole-Making and Drilling Technology for Composites M = 0.3 and 6 nm Bolt Composite specimen

q = pressure Washer

D = 15 mm

Fig. 3.5  Setup of the fixture for Procedure C [10].

Fig. 3.5. This procedure has been used by many researchers in testing their ­composites. For example, Pakdil [10] experimentally studied the effect of the geometry of joints and the stacking sequence of laminated composites on bearing strength and failure mode. This experimental work focused on the E/D and W/D parameters. Pakdil found that failure mode and bearing strength were significantly related to stacking sequence, geometrical parameters, and bold tension. Meanwhile, Palwankar [45] performed a finite element simulation on the bolted joint bearing test of the composites and compared ASTM D5961 Procedure A, modified Procedure A, and Procedure C with one another. For Procedure C, Palwankar used countersunk holes and observed a very high bearing stress at the bottom edge of the hole. The effect of t­hickness–­diameter ratio suggests that the possibility for a delamination to occur decreases along with the T/D ratio. Moreover, the transverse normal stresses increase along with a decreasing T/D ratio. Khashaba et al. [46] studied the effect of stacking sequences on the failure and reliability of pinned-joint composite laminates.

3.3.1.4 Procedure D Procedure D is not as preferred as the aforementioned procedures. As shown in Fig. 3.6., the test fixture consists of two plates to sandwich and fix the specimen with a bolt or a pin. Load is applied from the top at one end, and the other end can be fixed on the machine table. Khashaba et al. [46] applied Procedure D to examine the effect of stacking sequence on mean bearing strength, mean ultimate failure stress, failure displacement, and bearing stiffness. They also developed some models to predict the characteristic bearing strength, lower bound bearing strength, and safe design bearing strength at different reliability levels. Abd-Elhady and Sallam [47] numerically and experimentally examined the effect of cracks at the fastener hole surface on the ultimate strength.

Structural integrity assessment of a composite joint: A review39

Specimen t = 4 or 2

Fixture bolt, D = 10 mm

Specimen fixed end

6.5

8

Pin applied load

Bolt, D = 6 mm Washer All dimensions in mm

Fig. 3.6  Experimental setup for Procedure D [47].

3.3.2 Open hole and filled hole test The open hole and filled hole test follows ASTM D 5766/5766M-02a [48]. The results for ultimate strength are affected by several factors, including the bolt-hole preparation, specimen geometry, and thickness scaling. The test specimen geometry must have a width/bolt-hole diameter ratio of W/D = 6, edge distance/bolt-hole diameter ratio of E/D = 3, and bolt-hole diameter-thickness ratio of D/t = 1.5–3.0 unless the experiment focuses on the effects of these ratios. The recommended centrally located hole/notch diameter is D = 6 mm, while the recommended specimen length is L = 200–300 mm as shown in Fig. 3.7. The ultimate open hole test strength can be calculated as Fx OHTu = P max / A

(3.2)

where A refers to the area that is calculated based on the gross cross-sectional area (A = wt) without considering the bolt-hole dimensions, while Pmax denotes the maximum load before the occurrence of failure in the composite. For the open hole comprehensive test, the ASTM D 6484/6484M-04 [49] standard can be used. Meanwhile, ASTM D 6742/6742M-02 [50] may be used to determine the filled-hole tensile and compressive strengths of continuous fiber reinforced polymer-matrix composites. Sudarsono and Ogi [51] studied the fatigue behavior and damage progress of openhole ICF CFRP laminates with toughened interlayers.

40

Hole-Making and Drilling Technology for Composites Open hole with w/d = 6 mm w

L/2 L

Fig. 3.7  Geometry of the specimen for open hole test [1].

3.3.3 Pull-through test Pull-through strength measures the maximum load that can be withstood by a mechanically fastened composite plate when the composite plates are pulled apart perpendicular to the plane of the plates. This test follows the ASTM D 7332/D 7332M-07 [52] standard, and the fastener pull-through strength test method [29] may be used to determine the pull-through strength of a composite plate/fastener combination. This test can also be used to evaluate different components of the fastener, such as bolts/ nuts, pins/collars, or washers. The test can be divided into. (a) Procedure A, which is used for screening and developing the fastener, and (b) Procedure B, which is used for developing the design variables.

Both of these procedures require flat plate specimens with rectangular cross-­ sections and a circular hole at the center of the fastener. Procedure A requires two plates and four additional holes to be drilled on the periphery of the specimen to accommodate the test fixture. In Procedure A, the two plates are joined by a fastener and one plate is rotated by 45 degree with respect to the other plate. The plates are then pried apart to generate a tensile load on the fastener. In Procedure B, the load is applied on a composite plate through a yoke as shown in Fig. 3.8. The thickness, t, of the composite plate must be 1.5 times greater than the normal fastener shank diameter, d. The pull-through strength is calculated according to the first peak load observed in the load-displacement curve. Load

t

Typical steel frame and test fixture

Fastener

Test specimen

Fig. 3.8  Experimental setup for pull through test [1].

Structural integrity assessment of a composite joint: A review41

3.3.4 Pin-bearing strength test Pin contact tests the strength of the composite panel [53], while pin-bearing strength tests are used for the condition in which no lateral restraint is observed due to tightening of the bolting [4]. Fig. 3.9 presents the pin-bearing strength test setup. The strength for this distinct failure mode is calculated as [5]. Rbr = tdFθ br

(3.3)

br which requires the specific pin-bearing strength, Fθ , to be measured with respect to the direction of pultrusion. The longitudinal or lengthwise orientation, θ, is 0 degree when the connection force is parallel to the direction of pultrusion, while the transverse or crosswise orientation is 90 degree when the connection force is orthogonal. The projected area for bearing is calculated by multiplying the thickness of the material (t) by the diameter of the bolt or pin (d). The specimen is semi-notched at the middle. Given that the bearing strength decreases along with an increasing hole diameter, the maximum clearance must be calculated with respect to the nominal hole clearance of 1.6 mm [54]. Matharu and Mottram [4] characterized the laterally unrestrained pin-bearing strength of a bolted pultruded FRP material with and without a thread and found that the thread in bearing significantly influences the pin-bearing strength, load-stroke behavior, and bearing failure mechanisms regardless of the thread profile. Coelho and Mottram [55] extended this work by performing FE modeling on pin-bearing strength and found that such strength increases along with a decreasing hole clearance and is significantly higher (at 48% and 39% for the two pin diameters) when no clearance is observed. Marino et  al. [56] used various models to simulate a pin-plate system comprising a mono-directional, fiber-reinforced laminated plate to study the bearing failure load.

Load

Pin Specimen

Free span

Clamping

Fig. 3.9  Schematic diagram of pin contact test [53].

42

Hole-Making and Drilling Technology for Composites

(A)

(B)

Shearout

(D)

Cleavage

Tearout

(E)

(C)

Bearing

Net tension

Fig. 3.10  Failure mode (A) shear-out, (B) tear-out, (C) bearing, (D) cleavage, and (E) net tension [14].

3.4 Failure mode Determining the failure mode is a crucial step in evaluating the bearing response of materials. Five failure modes, including shear-out, tear-out, bearing, net tension, and cleavage, are frequently observed in the bearing test as specified in ASTM D 5961/D 5961 M-05 and as shown in Fig. 3.10. (a) Shear-out (Fig. 3.10A) and tear-out (Fig. 3.10B) refer to the separation of materials in front of the traversing bolt. These failure modes are most dominant when the E/D ratio is small [10, 39]. (b) Bearing failure refers to the local compression in front of the hole and often results in the out-of-plane buckling of materials around the hole as shown in Fig. 3.10C. Bearing failure typically occurs at large E/D and W/D ratios, where W denotes the width of the tested sample and D denotes the diameter of the rivet hole. (c) Net tension failure refers to the lateral fracture perpendicular to the traversing direction and often takes place when the E/D ratio is high and the W/D ratio is low as shown in Fig. 3.10E. Heimbs et al. [14] found that the failure mode might change due to speed from net tension to extensive bearing and pull-through failure at highest test speed of 10 m/s. (d) Cleavage refers to the breakage in both lateral and vertical directions as shown in Fig. 3.10D. This failure mode often occurs in anisotropic materials. Similar to shear-out, cleavage is frequently observed at low E/D ratio [10].

3.5 Conclusions This chapter reviews the structural joint of composite panels and focuses on mechanical joints, adhesive joints, or hybrid joints that use both mechanical fasteners (e.g., bolt/nut) and adhesion materials. All these joints have their own advantages and

Structural integrity assessment of a composite joint: A review43

d­ isadvantages and strongly depend on many factors, including the space required for the joint and application. This chapter also discusses the standard tests for evaluating these joints as well as the criteria followed by these tests. The tests are usually performed via computer simulations and experiments. The failure modes that result from these tests are also outlined. The following issues and challenges need to be addressed in future work: 1. The mechanical joint requires more space and the structure needs additional weight. Therefore a strong and light fastener that consumes a small amount of space must be used. 2. Adhesive and hybrid joints are considered permanent. Therefore the assembly error must be minimized. 3. The experiment inevitably requires a large amount of materials and consumes much time. In addition, the composite material may have numerous variants and unpredictable properties. Therefore a reliable and flexible computer software must be developed to perform the test.

Acknowledgment The authors want to acknowledge Universiti Sains Malaysia for their sponsorship through Bridging Grant 2018 (304/PMEKANIK/6316086).

References [1] S.D.  Thoppul, J.  Finegan, R.F.  Gibson, Mechanics of mechanically fastened joints in polymer-matrix composite structures—a review, Compos. Sci. Technol. 69 (3–4) (2009) 301–329. [2] W.A. Counts, W.S. Johnson, O. Jin, Assessing life prediction methodologies of fasteners under bending loads, in: P.M. Toor (Ed.), Structural Integrity of Fasteners: Second Volume, ASTM STP 1391, American Society for Testing and Materials, West Conshohocken, PA, 2000, pp. 3–15. [3] V. Mara, R. Haghani, M. Al-Emrani, Improving the performance of bolted joints in composite structures using metal inserts, J. Compos. Mater. 50 (21) (2016) 3001–3018. [4] S. Navroop, J. Matharu, T. Mottram, Plain and threaded bearing strengths for the design of bolted connections with pultruded FRP material, Eng. Struct. 152 (2017) 878–887. [5] ASCE, Pre-Standard for Load and Resistance Factor Design (LRFD) of Pultruded Fiber Reinforced Polymer (FRP) Structures, American Society of Civil Engineers, 2010. [6] T. Qin, L. Zhao, J. Zhang, Fastener effects on mechanical behaviors of double-lap composite joints, Compos. Struct. 100 (2013) 413–423. [7] F. Ascione, L. Feo, F. Maceri, An experimental investigation on the bearing failure load of glass fibre/epoxy laminates, Compos. Part B 40 (2009) 197–205. [8] A.  Aktas, M.H.  Dirikolu, The effect of stacking sequence of carbon epoxy composite laminates on pinned-joint strength, Compos. Struct. 62 (2003) 107–111. [9] P. Lopez-Cruz, J. Laliberté, L. Lessard, Investigation of bolted/bonded composite joint behaviour using design of experiments, Compos. Struct. 170 (2017) 192–201. [10] M. Pakdil, Failure analysis of composite single bolted-joints subjected to bolt pretension, Ind. J. Eng. Mater. Sci. 16 (2009) 79–85.

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[11] P.P.  Camanho, F.L.  Matthews, A progressive damage model for mechanically fastened joints in composite laminates, J. Compos. Mater. 33 (24) (1999) 2248–2280. [12] B. Okutan, Z. Aslan, R. Karakuzu, A study of the effects of various geometric parameters on the failure strength of pin-loaded woven-glass-fiber reinforced epoxy laminate, Compos. Sci. Technol. 61 (10) (2001) 1491–1497. [13] G. Kolks, K.I. Tserpes, Efficient progressive damage modeling of hybrid composite/titanium bolted joints, Compos. A: Appl. Sci. Manuf. 56 (2014) 51–63. [14] S. Heimbs, S. Schmeer, J. Blaurock, S. Steeger, Static and dynamic failure behaviour of bolted joints in carbon fibre composites, Compos. A: Appl. Sci. Manuf. 47 (1) (2013) 91–101. [15] B.  Egan, M.  Mccarthy, R.  Frizzell, P.  Gray, C.  Mccarthy, Modeling bearing failure in countersunk composite joints under quasi-static loading using 3D explicit finite element analysis, Compos. Struct. 108 (2014) 963–977. [16] P. Liu, X. Cheng, S. Wang, S. Liu, Y. Cheng, Numerical analysis of bearing failure in countersunk composite joints using 3D explicit simulation method, Compos. Struct. 138 (2016) 30–39. [17] C.  Stocchi, P.  Robinson, S.  Pinho, A detailed finite element investigation of composite bolted joints with countersunk fasteners, Compos. A: Appl. Sci. Manuf. 52 (2013) 143–150. [18] P.J. Gray, R.M. O’Higgins, C.T. McCarthy, Effect of thickness and laminate taper on the stiffness, strength and secondary bending of single-lap, single-bolt countersunk composite joints, Compos. Struct. 107 (2014) 315–324. [19] H. Gao, J. Wang, Y. Yang, X. Liu, L. Chen, R. Li, Effect of hole perpendicularity error on mechanical behavior of single-lap composite joints, Astronaut. Sin. 2 (2017) 285–293. [20] P.A. Gustafson, A.M. Waas, The influence of adhesive constitutive parameters in cohesive zone finite element models of adhesively bonded joints, Int. J. Solids Struct. 46 (10) (2009) 2201–2215. [21] R. Sachse, A.K. Pickett, W. Essig, P. Middendorf, Experimental and numerical investigation of the influence of rivetless nut plate joints on fatigue crack growth in adhesively bonded composite joints, Int. J. Fatigue 105 (2017) 262–275. [22] H. Aglan, S. Shroff, Z. Abdo, T. Ahmed, L. Wang, L.D. Favro, R.L. Thomas, Cumulative fatigue disbond of adhesive joints and its detection using thermal wave imaging, in: D.O. Thompson, D.E. Chimenti (Eds.), Review of Progress in Quantitative Nondestructive Evaluation, vol. 14, Springer US, Boston, MA, 1995, pp. 431–438. [23] J.H. Song, J.K. Lim, Bonding strength in structural adhesive bonded joint, Met. Mater. Int. 7 (5) (2001) 467–470. [24] S. Budhe, M.D. Banea, S. de Barros, L.F.M. da Silva, An updated review of adhesively bonded joints in composite materials, Int. J. Adhes. Adhes. 72 (2017) 30–42. [25] R. Hazimeh, R. Othman, K. Khalil, G. Challita, Influence of plies’ orientations on the stress distribution in adhesively bonded laminate composite joints subjected to impact loadings, Compos. Struct. 152 (2016) 654–664. [26] T.E.A.  Ribeiro, R.D.S.G.  Campilho, L.F.M.  da Silva, L.  Goglio, Damage analysis of composite-aluminium adhesively bonded single-lap joints, Compos. Struct. 136 (2016) 25–33. [27] L.J. Hart-Smith, Bonded-bolted composite joints, J. Aircr. 22 (11) (1985) 993–1000. [28] V.P. Lawlor, W.F. Stanley, M.A. McCarthy, Characterization of damage development in single shear bolted composite joints, Plast. Rubber Compos. 31 (3) (2002) 126–133. [29] K. Bodjona, L. Lessard, Hybrid bonded-fastened joints and their application in composite structures: a general review, J. Reinf. Plast. Compos. 35 (9) (2016) 764–781.

Structural integrity assessment of a composite joint: A review45

[30] E. Paroissien, F. Lachaud, S. Schwartz, A. Da Veiga, P. Barrière, Simplified stress analysis of hybrid (bolted/bonded) joints, Int. J. Adhes. Adhes. 77 (2017) 183–197. [31] D. Sivakumar, L.F. Ng, N.S. Salmi, Eco-hybrid composite failure behavior of two serial bolted joint holes, J. Eng. Technol. 7 (1) (2016) 114–124. [32] M. Majid, M. Afendi, W.W. Lieh, K. Hafizan, Strength of composites hybrid joint, ARPN J. Eng. Appl. Sci. 11 (1) (2016) 216–221. [33] F. Esmaeili, T.N. Chakherlou, M. Zehsaz, Investigation of bolt clamping force on the fatigue life of double lap simple bolted and hybrid (bolted/bonded) joints via experimental and numerical analysis, Eng. Fail. Anal. 45 (2014) 406–420. [34] C. Bois, H. Wargnier, J.C. Wahl, E. Le Goff, An analytical model for the strength prediction of hybrid (bolted/bonded) composite joints, Compos. Struct. 97 (2013) 252–260. [35] K. Bodjona, L. Lessard, Load sharing in single-lap bonded/bolted composite joints. Part II: global sensitivity analysis, Compos. Struct. 129 (2015) 276–283. [36] K. Bodjona, K.G. Raju, H. Lim, L. Lessard, Load sharing in single-lap bonded/bolted composite joints. Part I: model development and validation, Compos. Struct. 129 (2015) 268–275. [37] ASTM D 5961-10, Standard Test Method for Bearing Response of Polymer Matrix Composite Laminates, American Society for Testing of Materials, 2010. [38] Y. Fukada, Bearing strength of carbon fibre/epoxy laminate with direct measurement of hole deformation, Adv. Compos. Mater. 22 (5) (2013) 311–325. [39] O. Sayman, U. Esendemir, Rainwater effect on bearing strength of glass–epoxy laminated composite pinned joints, J. Compos. Mater. 50 (30) (2016) 4269–4278. [40] K.J. Jadee, A.R. Othman, The effect of defence hole system on the failure load and bearing strength of GFRP bolted joint, Am. J. Mech. Eng. 3 (4) (2015) 135–141. [41] K.J. Jadee, Progressive failure analysis and failure map into plain weave glass fibre reinforced polymer bolted joint, Am. J. Mater. Sci. Eng. 3 (2) (2015) 21–28. [42] T. Yilmaz, T. Sinmazcelik, Effects of geometric parameters on the pin-bearing strength of glass/polyphenylenesulphide Composites, J. Compos. Mater. 43 (20) (2009) 2239–2253. [43] S.Y. Kim, B. He, D. Kim, C.S. Shim, H.C. Song, Bearing strength of interference-fit pin joined glass fiber reinforced plastic composites, J. Compos. Mater. (2016) 1–13. [44] I.F. Soykok, O. Sayman, M. Ozen, B. Korkmaz, Failure analysis of mechanically fastened glass fiber/epoxy composite joints under thermal effects, Compos. Part B 45 (1) (2013) 192–199. [45] Palwankar, M.P. Evaluation of a Modified Fixture for Testing Composite Bolted Joints With Countersunk Fasteners Under Bearing Loads (Master thesis), San Diego State University, 2016. [46] U.A. Khashaba, T.A. Sebaey, K.A. Alnefaie, Failure and reliability analysis of pinned-joints composite laminates: effects of stacking sequences, Compos. Part B 45 (2013) 1694–1703. [47] A.A.  Abd-Elhady, H.E.M.  Sallam, Crack sensitivity of bolted metallic and polymeric, Eng. Fract. Mech. 147 (2015) 55–71. [48] ASTM D5766/D5766M-02a, Standard Test Method for Open Hole Tensile Strength of Polymer–Matrix Composite Laminates. Vol. 15.03. Composite Materials, ASTM International, West Conshohocken, PA, 2007. [49] ASTM D6484/D6484M-04, Standard Test Method for Open-Hole Compressive Strength of Polymer–Matrix Composite Laminates, Vol. 15.03. Composite Materials, ASTM International, West Conshohocken, PA, 2004. [50] ASTM D6742/D6742M-02, Standard Practice for Filled-Hole Tension and Compression Testing of Polymer–Matrix Composite Laminates. Vol. 15.03. Composite Materials, ASTM International, West Conshohocken, PA, 2007.

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[51] S. Sudarsono, K. Ogi, Fatigue behavior of open-holed CFRP laminates with initially cut fibers, Open J. Compos. Mater. 7 (2017) 49–62. [52] ASTM D7332/D7332M-07, Standard Test Method for Measuring the Fastener PullThrough Resistance of a Fiber-Reinforced Polymer–Matrix Composite. Vol. 5.03. Composite Materials, ASTM International, West Conshohocken, PA, 2007. [53] P.S. Wu, C.T. Sun, Modeling bearing failure initiation in pin-contact of composite laminates, Mech. Mater. 29 (1998) 325–335. [54] Matharu N.S. Aspects of Bolted Connections for Fibre Reinforced Polymer Structures (PhD thesis), University of Warwick, 2014. [55] A.M.G. Coelho, J.T. Mottram, numerical evaluation of pin-bearing strength for the design of bolted connections of pultruded FRP material, J. Compos. Constr. 21 (5) (2017) 04017027. [56] M. Marino, F. Nerilli, G. Vairo, A finite-element approach for the analysis of pin-bearing failure of composite laminates, Fratturaed Integrità Strutturale (29) (2014) 241–250.

Further reading [57] US Department of Defense, Military Handbook—MIL-HDBK-17-1F: Composite Materials Handbook. Vol. 1. Polymer–Matrix Composites Guidelines for Characterization of Structural Materials. US. Department of Defense.

Drilling of fiber-reinforced composites: An innovative tool design

4

M.H. Hassan, J. Abdullah School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Malaysia

4.1 Introduction The superior properties of composite materials in comparison to those of their conventional counterparts have generated a lot of interest in utilizing the materials in various applications ranging from automotive to aircraft structures. Carbon fiber reinforced composite (CFRC), in particular, has been used to a great extent in structural applications, as the material provides higher strength than steel and is stiffer than titanium while still retaining its lighter weight. The use of composites in aircraft structures has resulted in improved fuel economy and reduced emissions as well as increased load-carrying capacity. Although composite structures are usually made to near-net shape products, more intricate components require secondary machining processes for the required accuracy in assembly. For that, drilling provides the most significant machining to allow the application of screws and rivets in the assembly of parts. Nonetheless, unlike working on metallic material, drilling of fiber reinforced composite (FRC) material presents unique challenges as its machining behavior differs in various aspects. The material is not only inhomogeneous and anisotropic, but also its behavior is dependent on the constituents’ properties, fiber orientation, and fabric constructions [1]. Inappropriate choice of cutting parameters or drill bit geometry could cause different types of damages, including delamination, fiber pull-out, fiber-matrix debonding, splintering thermal alterations, and geometrical defects [2–4]. Among all, delamination is the most critical damage; it occurs when the thrust force exerted by the drill exceeds the interlaminar fracture toughness of the layers, resulting in poor assembly tolerance and reduced structural integrity [5]. Drillinginduced delamination occurs both at the entrance and the exit planes of the workpiece. To some extent, Davim and Reis [6] clearly distinguished two different mechanisms of delamination, referred to as peel-up and push-down, both of which are primarily influenced by the thrust force developed during drilling [3]. In the peel-up delamination mechanism, as the drill progresses, the upper layers of the material tend to be pushed through the cutting faces of the drill instead of being cut. Push-down delamination results from the indentation effect, acting over the uncut layers of the laminate. It was found that delamination associated with push-down is more severe than that of a Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00004-0 © 2019 Elsevier Ltd. All rights reserved.

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Hole-Making and Drilling Technology for Composites

peel-up [3, 5]. However, the most established method to reduce this particular delamination is to use a support plate under the workpiece [7]. The drill bits should be designed to generate as little heat as possible to avoid degradation of material, which could subsequently influence the extent of delamination. It was established that different designs of the drill produced different severities of delamination and hole quality. Piquet et al. [8] has highlighted that the use of a helical twist drill and special geometry tools with a straight flute in a drilling of thin CFRC. In their research, the application of special geometry tools is significantly reduce the CFRC delamination at the entrance and exit. A similar work in the drilling of CFRP with a helical flute carbide drill produced lesser delamination in comparison to the drilling using a four-flute carbide drill [6]. In addition, the use of a spur drill could produce a higher rate of production without the occurrence of delamination, obtained with a feed rate of 2025 mm/min and a spindle speed of 6750 rpm [9]. An extensive study has been further carried out by Davim and Reis [6] to relate the cutting parameters on the power, specific cutting pressure, and delamination in drilling of CFRC. As a result, the use of a “Brad and Spur” drill was found to produce less delamination on the laminate (entrance and exit) in comparison to its “Straight Shank” counterpart [10]. Furthermore, on a similar type of laminate, Tsao and Hocheng [11] found that the candlestick drill and saw drill cause a smaller delamination factor than those of the twist drill. When drilling a composite panel, tool wear is an unwanted phenomenon whereby the tool loses an amount of matter. It affects the quality of the drilled surfaces (holes) and geometry of the material workpiece. The chip produced during the drilling of carbon composites is abrasive dry powder. The ineffective extraction of these chips is one of the major reasons for high tool wear rates. The most common type of wear, crater wear, occurred to a major extent as a result of discontinuous chip formation and caused flagging of the tool, which propped up cutting edge chipping [12]. The cutting force increased with the workpiece-drill tool interface temperature, followed by the increased drill wear and resulted in workpiece deflection and drill bit breakage [9]. The flank wear decreased near the corner of the drill chisel edge, while the maximum flank was common at the drill outer corner. An increase in thrust force, torque on the drill bit, and the number of drilled holes caused a proportional increase in the drill tool wear [6].

4.2 Types of FRC laminates materials in aircraft manufacturing 4.2.1 CFRC and glass fiber reinforced composite (GFRC) laminates CFRC and GFRC composite laminates are by far the most common FRC materials used in many industries in view of their high mechanical strength. They are formed by combination of fiber (carbon, glass, or Kevlar) and polymer matrix. Fibers are lightweight, stiff, and strong, which provides most of the composite laminate’s stiffness and strength.

Drilling of fiber-reinforced composites

49

Polymer matrix Fibers Transverse direction

Ply No. Longitudinal direction

(A)

1–6 7–12 13–18 19–24 25–34 31–36 37–42 43–48

Orientation angle 0 45 90 –45 –45 90 45 0

(C)

(B) Fig. 4.1  (A) Unidirectional fiber orientation ply, (B) bidirectional fiber orientations ply (woven-ply), (C) a typical quasi-isotropic laying-up sequence of a unidirectional-plies FRP composite laminate.

The polymer matrix binds the fiber together thus transferring load to reinforced fibers and providing protection from environmental attack. Fig. 4.1 shows the example of fiber construction normally used in composite manufacturing. The unidirectional type is normally used for the flat panel application, while the woven type is used at the complex shape, which needs higher flexibility of fabric that can follow the desired shape.

4.2.2 Fiber metal composite (FMC) laminates FMC laminates are made as an alternative material that consist of a thin layer of alloy and FRC that is bonded and cured into a single laminate using the autoclave technique. The main objective of FMC is to introduce fatigue crack resistance since conventional material, like aluminum sheet, is unable to withstand immense pressure in parts like aircraft fuselage. Aluminum-based FMC can be grouped into three categories depending on the bonded FRC, which are Glass Reinforced Aluminum Laminate (GLARE) [13–16], carbon reinforced aluminum laminate [17], and aramid reinforced aluminum laminate [18]. These FMCs take advantages of metal and FRCs, providing superior mechanical properties to the conventional laminate consisting only of FRC ply or monolithic metal (mostly aluminum or titanium) sheet. They offer several advantages, such as better damage tolerance to fatigue crack growth and impact damage especially for aircraft structural applications [19]. The major aircraft manufacturers, Boeing and Airbus, are shifting from traditional aerospace alloys to FMCs for use in their new aircraft design. Fig. 4.2 shows the lay-up configuration for the GLARE material at different directions of the fiber prepregs’ orientation.

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Hole-Making and Drilling Technology for Composites

Fig. 4.2  Example of GLARE material, (A) consist of aluminum sheet and prepreg layer with 0 degree and 90 degree orientation, (B) consist of aluminum sheet and prepreg layer with 45 degree and −45 degree, (C) laminate of GLARE material after cure) [13].

4.3 Hole-making procedures in drilling FRC laminate materials In hole making of FRC laminate, there are several steps that need to be fulfilled before the rivet or bolt installation process can be done. For holes of larger diameter, two steps in the drilling process are needed to prevent the laminate from delamination and to produce a good quality hole. Fig. 4.3 shows the detail stages for producing holes in

Drilling of fiber-reinforced composites

Stage 1

Stage 2

51

Stage 3

Fig. 4.3  Stage of drilling a composite laminate before bolt and rivet installation.

Fig. 4.4  (A) Pilot hole; (B) reamer hole/full size; (C) counter sink hole; (D) counter bore hole.

FRC laminate. The process starts with drilling, then reaming, and finally countersinking or counterboring. Fig. 4.4 shows the hole for each stage of the process.

4.3.1 Predrilling process (stage 1) A predrilled hole is the first stage of the drilling process where a small diameter hole is drilled on the blank panel. This predrilled hole is used as a guide before it reaches the final size of the hole; normally, hole size ranges from 3.2 to 4.5 mm. With a predrilled pilot hole, the drilling-induced delamination can be reduced significantly as demonstrated in theoretical analysis and experiments conducted by Tsao and Hocheng [20, 21] using a twist drill bit and core drill bit [22], respectively. Based on their experiments, they observed that chisel edges of a twist drill bit and chip removal for a core drill bit are major contributions to thrust force. Won and Dharan [23] also observed that the thrust force due to chisel edge is 40% from the total thrust force when feed rate is low and 60% when the feed rate is high during drilling of composite laminates.

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Hole-Making and Drilling Technology for Composites

Hence a pilot hole is predrilled to eliminate the thrust force caused by the chisel edge [21] or by chip removal [22]. The diameter of the predrilled pilot hole is set equal to the chisel edge length of a twist drill bit or the inner diameter of a core drill bit. This technique can provide a useful approach to drilling composite laminates at higher feed rates without causing delamination [22, 24].

4.3.2 Reaming (stage 2) Reaming is the operation of finishing a hole to bring it to accurate size and a fine surface finish [25]. This operation is performed by means of a multitooth tool called a reamer. Normally the reamer has four or more teeth in order to improve cutting efficiency. The greater the number of teeth, the more efficient the cutting. There are many different types of reamers and they may be designed for use as hand tools or in machine tools. High surface finish, superb hole quality, and close dimensional tolerance are achieved at high penetration rates and small depths of cut. Solid reamers do almost all their cutting with the 45-degree chamfered front end. The flutes guide the reamer and slightly improve the finish. Therefore reamers should not be used for heavy stock removal. In reaming, speed and feed are important; stock removal and alignment must be considered in order to produce chatter-free holes. Reaming speeds for machine reaming may vary considerably depending in part on the material to be reamed, type of machine, and required finish and accuracy.

4.3.3 Countersinking or counterboring (stage 3) Countersinking is the process [26] of enlarging the top end of a hole to the shape of a cone to accommodate the conical-shaped head of fasteners. The head of the fastener will be flush with or below the surface. They are available in several angles: 60, 90, 100, 110, and 120 degrees. Counterboring is used for enlarging only a limited portion of the hole. The difference between counterboring and countersinking is the angle of the head; it is 180 ­degree in counterboring. A counterbore tool enlarges the top portion of an existing hole to the diameter of the tool. Counterboring is often performed after drilling in order to provide space for the head of a fastener, such as a bolt to sit flush with the workpiece surface. The counterboring tool has a pilot on the end to guide it straight into the existing hole [27].

4.4 Drill tool material and tool design geometry 4.4.1 Drill tool material The life of a drill depends mainly on hardness, toughness, wear, and thermal resistance. A good drill must possess the ability to resist wear, fracture, and quick rupture and retain hardness at the state of hot hardness. The major properties of different tool materials are shown in Fig. 4.5. From these figures, the hardest tooling material,

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53

PCD DLC PCBN Si2N4

Coated cermet

Ceramics

Coated carbide

HARDNESS

Al2O3

Coated micrograin carbide

Cermet Micrograin carbide Carbide

Cobalt HSS

PM HSS HSS

TOUGHNESS

(A)

Ceramics

Hardness HRC

90

PCD

70

Carbides 50 HSS Carbon tool steels 300

(B)

500

700

900

1100

Temperature, °C

Fig. 4.5  (A) Hardness and toughness of tool materials; (B) hardness of tool materials versus temperature [28].

polycrystalline diamond (PCD), possesses the least toughness property as its sharp deformation occurs around a temperature of 600°C unlike high speed steel (HSS) with the best toughness, but which deforms around 700°C when compared with other tooling materials [28]. The important criteria in tool material selection are hardness and toughness. Theoretically, the hardness of the cutting tool must be greater than the workpiece so that the tool may cut the workpiece efficiently. Hardness is defined as the resistance to indenter penetration. The ability to maintain high hardness at elevated temperatures is called hot hardness. During the drilling process, the temperature may raise up to 200°C. For Toughness is defined as the ability of a material to absorb energy before fracture. The greater fracture toughness of a tool material, the better it resists shock

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Hole-Making and Drilling Technology for Composites

load, chipping and fracturing during vibration, misalignments, runouts, and other imperfections in the machining system. Hardness and toughness change in opposite directions for different tool materials. A major trend in the development of tool materials is to increase their toughness while maintaining their hardness.

4.4.1.1 High speed steel In a drilling application, HSS material is normally used due to its cheaper price from other materials like tungsten carbide, ceramic or PCD. Liu et al. [29] reported in their review study of drilling composite laminates that HSS or carbide drill bits have primary attraction based on better performance at high cutting speed compared with other drill bits. Some studies [30–32] used HSS drills frequently, making it the most widely used tooling material due to its availability, low cost, and high toughness. For the composite drilling application, HSS is not advisable due to the low wear resistance while drilling high abrasive material like CFRP.

4.4.1.2 Tungsten carbide Carbide materials consist of carbide particles (carbides of tungsten, titanium, tantalum, or some combination of these) bound together in a cobalt matrix by sintering. Normally, the size of the carbide particles is less than 0.8 μm for micro grains, 0.8–1.0 μm for fine grains, 1–4 μm for medium grains, and more than 4 μm for coarse-grain cutting inserts. The amount of cobalt significantly affects the properties of carbide inserts. Normally, the cobalt content is 3%–20%, depending upon the desired combination of toughness and hardness. As the cobalt content increases, the toughness of a cutting insert increases, while its hardness and strength decrease. However, the correct combination of carbide insert composition (grade), coating materials, and layer sequence and the selection of the appropriate coating technology makes it possible to increase metal cutting productivity substantially without sacrificing insert wear resistance. Tungsten carbide drills performed better in terms of wear resistance, delamination effect, and surface finish when compared with HSS under comparative low speed and feed at high temperatures when drilling the same composite materials [12]. After the drilling process, when the radius apart from the corner was measured, almost null wear land was shown in the flank surface of carbide drills, while the HSS drill had considerable wear [7].

4.4.1.3 Ceramic/zirconia The prime benefit of ceramics is high hardness (and thus abrasive wear resistance) at elevated temperatures, as seen in Fig. 4.5. All tool materials soften as they become hotter, but ceramics do so at a much slower rate. Among the major advantages of ceramic cutting tools is their chemical stability [33]. In practical terms this means that the ceramic does not react with the material it is cutting, that is, there is no diffusion wear, which is the weakest spot of carbides in high-speed machining applications. Ceramics are suitable for machining the majority of ferrous materials, including superalloys.

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Fig. 4.6  Example of Si-AlONs material at different design of tool geometry; (A) G-1 type, (B) G-2 type, (C) G-3 type, and (D) G-4 type [34, 35].

There are two basic kinds of ceramics. The first is aluminum oxide. It is wear resistant but brittle, and used chiefly on hardened steel. The other major type is silicon nitride, which is relatively soft and tough and is used on cast irons. Between aluminum oxide and silicon nitride lies a whole host of ceramic materials called Si-AlONs that combine the two. The greater proportion of aluminum oxide, the harder the material. The more silicon nitride included, the tougher the material [34]. In the leapfrog race between work materials and tools, the laurels still go to the tools. The cutting ability of the tool is still slightly ahead of the applications (including available machine tools and their relevant characteristics) because there is a reluctance to apply the available cutting tool technology. Fig. 4.6 shows the example various design of twist drill that made for Si-AlONs material. It is a little disappointing that ceramic-reinforcement technology has not moved ahead as quickly as initially supposed. Reinforcements offer many strength advantages. They are available, but not in widespread use. It now seems that a new area that will offer many new advantages in ceramic tools is nanotechnology. The most advanced ceramics today are micro-grain materials, while the latest developments aim to move to nano-grains or particle sizes of less than a micron [35]. This technology is coming along well. The main advantage it offers is that the smaller particle size increases strength because more grain area is exposed to bonding. This strength increase translates into greater impact resistance and improved wear properties. Coatings are rarely used with ceramic inserts. On ceramics, coatings do some good but the cost is high and usually does not justify the end result because of weak adhesion between the coating materials and ceramic substrate.

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Hole-Making and Drilling Technology for Composites

4.4.1.4 Polycrystalline diamond PCD is the hardest of all the cutting tool materials. PCD is a synthetically manufactured, very hard cutting tool material made of diamond particles in a metal matrix. With Vickers hardness (HV) of 6000, PCD is significantly harder than tungsten carbide, which has a hardness of 1600–2200 HV. PCD is suited for high-speed cutting and drilling of very abrasive materials such as CFRP [36]. Garrick [37, 38] reported producing a veined drill that was capable of drilling carbon composites and its stack with titanium. However, he observed that after 200 holes had been successfully drilled, a wear land was formed on the cutting edge of the 86 series PCD veined drill, which necessitated re-sharpening of the drill. The helical PCD drill geometry gave the best overall performance when compared with other tungsten carbide drills, but was more reactive to feed rate changes when delamination was considered. Heath [38] reported that PCD being a stronger tool material could be used for machining of composites because of its ability to withstand the severe abrasion of the (CFRCs). However, PCD is found to be too fragile to withstand the high cutting forces required for metal such as titanium [38], especially when stacked with composites. Furthermore, the configuration of the core drill proved better than the traditional twist drill. Butler Smith et al. [39] reported the outstanding advantages of core drilling using a solid PCD drill. A novel designed core drill produced 26% reduction in thrust force, reduced drill surface clogging, cutting force, and drilling temperature, producing reduced delamination damage possibility during composite drilling.

4.4.2 Drill tool design There have been many different geometry drill bits made of different tool materials used in drilling of composite laminates. Based on the literature, drill bit geometry can be divided into six categories: (1) twist drill bit, (2) step drill bit, (3) Brad-point (W-point) drill bit, (4) slot drill bit, (5) straight-flute drill bit, and (6) sore drill bit. Fig. 4.7 shows a couple of typical drill bits in drilling of composite laminates. It is seen from previous study that twist drill bits made of HSS or carbides are the primary attraction in drilling of composite laminates. However, the applications of other drill bits (such as step drill bits, and straight-flute drill bits) in drilling of composite laminates are also very extensive. In addition, drill bits with special geometry design are also potentially qualified to conduct high-quality drilling of a composite material. Drill geometries are determined by a set of variables including angle characteristics (i.e., point angle and helix angles) and tool shapes. Xu et al. [40] compared the tool performances of one standard twist drill and one special drill, the “dagger drill,” in drilling of high-strength CFRP. Results showed that the dagger drill promoted better surface finish and less burr defect than the twist drill due to its smaller point angle and helix angle. The excellent performance of special drill bits globally is closely related to the mentioned geometric variables, which results in minimal hole damage and minor tool wear. Other than that, the importance of drill bit geometry in improvement of hole quality, especially in terms of delamination reduction, is well noted.

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Fig. 4.7  Example of composite tool geometry, (A) W-point; (B) twist dril 2-flute; (C) twist drill 4-flute; (D) dagger; (E) drill reamer; (F) step drill.

4.5 Influence of tool design on hole quality 4.5.1 Occurence of delamination (fiber pull-out and push-out) Delamination is an inter-ply failure phenomenon induced by drilling and is a highly undesirable problem recognized as a major damage when drilling composite laminates. Tool design has direct influence on the occurrence of delamination. Fig.  4.8 shows the schematic images of delamination damage and calculation of the delamination area. Delamination not only drastically reduces assembly tolerance and bearing strength, but also has the potential for long-term performance deterioration under fatigue loads [30]. Experimental observations show that drilling-induced delamination occurs both at the entry and at the exit of the drilled holes peripheries [9, 24, 41–45]. “Peel-up” and “push-out” are two distinguishable delamination mechanisms associated with drilling of composite. Peel-up delamination occurs around the drilled hole’s entry periphery, as shown in Fig. 4.8A. When the cutting edges of the drill bit make contact with the composite laminate, a peeling force through the slope of the drill bit flutes results in separating the plies from each other, forming a delamination zone around the drilled hole’s entry periphery. Push-out delamination occurs around the drilled hole’s exit periphery. When the drill bit approaches the hole exit side, the uncut plies beneath the drill bit become more susceptive to deformation due to decrease of its thickness. Eventually,

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Hole-Making and Drilling Technology for Composites

Fig. 4.8  (A) Mechanism of peel up and push up delamination phenomena; (B) hole area for the drilled hole; (C) delamination area identification; and (D) delamination calculation based on damaged area [9, 24].

push-out delamination appears at the drilled hole’s exit periphery if the thrust force applied to the uncut plies exceeds the inter-ply bonding strength. In practice, it has been found that the delamination associated with push-out is more severe than that associated with peel-up [4]. Hence most of previous studies paid more attention to push-out delamination. Gaitonde et al. [46] and Kilickap [47] reported the effect of angle point of a twist drill bit on delamination when drilling composite laminates. Gaitonde et  al. [46] observed that the delamination tendency increased with an increase of the point angle of the twist drill bit during both conventional drilling and high speed drilling of ­woven-ply CFRP composite laminates. However, Kilickap [47] reported that the delamination tendency decreased with an increase of the point angle of the twist drill bit during conventional drilling of UD-ply GFRP composite laminates.

4.5.2 Hole surface roughness Hole surface roughness is critical when drilling a composite laminate. In aerospace industry, the surface roughness must be less than 3 μm in order to qualify for the next process. Factors that contribute to higher hole surface roughness are internal delamination, epoxy burn, and fiber cracking. Internal delamination is normally caused by higher thrust force during the drilling process. Incorrect tool geometry will cause higher thrust force and thus contribute higher vibration during the drilling process [48, 49].

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Fig. 4.9  The type of defect that contribute to higher hole surface roughness, (A) internal delamination; (B) fiber breakage; (C) matrix cracking; and (D) combination delamination and fiber cracking [48].

Epoxy burn is caused by improper material selection during the drilling process. Since composite is naturally an abrasive material, the drilling tool material needs to be of higher wear resistance to prevent rapid or premature tool wear during drilling. If the tool material has lower wear resistance, the tool is easily worn out, generating higher friction leading to increased temperature during the drilling process [50]. The safe limit of elevated temperature for epoxy is around 300°C and temperature beyond that threshold can cause epoxy burnout. Fig. 4.9 illustrates the defects that contribute to higher surface rounghness.

4.6 Summary Drilling of composite laminates differs significantly in many aspects from drilling of conventional metals or other alloys. Proper tool selection for making holes during the drilling process is important in order to minimize tooling cost and later to improve the subsequent assembly process. Delamination of composites and rapid tool wear are the main problems encountered when drilling CFRP accounting for drilling-induced damaged as high as almost

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Hole-Making and Drilling Technology for Composites

two-thirds of the total drill product. Good knowledge of drilling parameters and composite materials and well-designed drill geometry afford a better opportunity for developing a drill that will minimize delamination on reinforced composites, reduce tool wear, and produce a high-quality drilled hole surface. Delamination is very difficult to eliminate but possible to minimize when using appropriate tool design and materials. Further research into new technology involved in the preparation of new material composition and tool design technology is needed since the behavior of the composite is continuously changing with the advancement in composite science and processing technology. Hence the drill design engineer and manufacturer will obtain a comprehensive understanding of the suitable design for the specific composite materials with the intention of improving and optimizing the efficiency of the drills and solving challenges confronting composite drilling.

Acknowledgments This work is supported by the RU-I Grant # 1001/PMEKANIK/814288 of Universiti Sains Malaysia. The authors would like to thank the Gandtrack Asia Sdn Bhd, Melaka, Malaysia for the permission to use their facilities and support for this project.

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[32] W.C. Chen, Some experimental investigations in the drilling of carbon fiber-reinforced plastic (CFRP) composite laminates, Int. J. Mach. Tools Manuf. 37 (1997) 1097–1108. [33] S.B.  Bhosale, R.S.  Pawade, P.K.  Brahmankar, Effect of process parameters on MRR, TWR and surface topography in ultrasonic machining of alumina–zirconia ceramic composite, Ceram. Int. 40 (2014) 12831–12836. [34] A. Çelik, I. Lazoglu, A. Kara, F. Kara, Investigation on the performance of SiAlON ceramic drills on aerospace grade CFRP composites, J. Mater. Process. Technol. 223 (2015) 39–47. [35] A. Çelik, I. Lazoglu, A. Kara, F. Kara, Wear on SiAlON ceramic tools in drilling of aerospace grade CFRP composites, Wear 338–339 (2015) 11–21. [36] Y.  Karpat, O.  Bahtiyar, Comparative analysis of PCD drill designs during drilling of CFRP laminates, Proc. CIRP 31 (2015) 316–321. [37] R.  Garrick, Drilling advanced aircraft structures with PCD (poly-crystalline diamond) drills, in: SAE International, 2007. [38] P.J. Heath, Developments in applications of PCD tooling, J. Mater. Process. Technol. 116 (2001) 31–38. [39] P.W.  Butler-Smith, D.A.  Axinte, M.  Daine, A.R.  Kennedy, L.T.  Harper, J.F.  Bucourt, R. Ragueneau, A study of an improved cutting mechanism of composite materials using novel design of diamond micro-core drills, Int. J. Mach. Tools Manuf. 88 (2015) 175–183. [40] J. Xu, Q. An, X. Cai, M. Chen, Drilling machinability evaluation on new developed highstrength T800S/250F CFRP laminates, Int. J. Precis. Eng. Manuf. 14 (2013) 1687–1696. [41] H. Muhammad, Q. Muhammad Asif Mahmood, S. Muhammad Qaiser, K. Sarmad Ali, B. Muhammad Mahmood Aslam, Carbon fiber-reinforced polymer composite drilling via aluminum chromium nitride-coated tools: hole quality and tool wear assessment, J. Reinf. Plast. Compos. 36 (2017) 1403–1420. [42] M.F. Ameur, M. Habak, M. Kenane, H. Aouici, M. Cheikh, Machinability analysis of dry drilling of carbon/epoxy composites: cases of exit delamination and cylindricity error, Int. J. Adv. Manuf. Technol. 88 (2017) 2557–2571. [43] T. Srinivasan, K. Palanikumar, K. Rajagopal, B. Latha, Optimization of delamination factor in drilling GFR–polypropylene composites, Mater. Manuf. Process. 32 (2017) 226–233. [44] M.A.R. Loja, M.S.F. Alves, I.M.F. Bragança, R.S.B. Rosa, I.C.J. Barbosa, J.I. Barbosa, An assessment of thermally influenced and delamination-induced regions by composites drilling, Compos. Struct. (2018), https://doi.org/10.1016/j.compstruct.2018.02.046. [45] N. Feito, J. Díaz-Álvarez, J. López-Puente, M.H. Miguelez, Experimental and numerical analysis of step drill bit performance when drilling woven CFRPs, Compos. Struct. 184 (2018) 1147–1155. [46] V.N. Gaitonde, S.R. Karnik, J.C. Rubio, A.E. Correia, A.M. Abrão, J.P. Davim, Analysis of parametric influence on delamination in high-speed drilling of carbon fiber reinforced plastic composites, J. Mater. Process. Technol. 203 (2008) 431–438. [47] E.  Kilickap, Optimization of cutting parameters on delamination based on Taguchi method during drilling of GFRP composite, Expert Syst. Appl. 37 (2010) 6116–6122. [48] J.S. Monfared Zanjani, B.S. Okan, Y.Z. Menceloglu, M. Yildiz, Nano-engineered design and manufacturing of high-performance epoxy matrix composites with carbon fiber/selectively integrated graphene as multi-scale reinforcements, RSC Adv. 6 (2016) 9495–9506. [49] D. Geng, D. Zhang, Y. Xu, F. He, D. Liu, Z. Duan, Rotary ultrasonic elliptical machining for side milling of CFRP: tool performance and surface integrity, Ultrasonics 59 (2015) 128–137. [50] H. Wang, X. Zhang, Y. Duan, Effects of drilling area temperature on drilling of carbon fiber reinforced polymer composites due to temperature-dependent properties, Int. J. Adv. Manuf. Technol. (2018), https://doi.org/10.1007/s00170-018-1810-7.

Drilling of composite laminates using a special tool point geometry

5

K. Debnath Department of Mechanical Engineering, National Institute of Technology Meghalaya, Shillong, India

5.1 Introduction Polymer composites have become a part of everyday life due to their superior multifunctional properties. The application spectrum of the polymer composites includes aerospace and automotive components, sports goods, housing and gardening equipment, consumer products, and others. The applications of polymer composites are also emerging in the field of construction of footbridges, utility poles, and repair of damaged concrete structures. These materials are largely used in train and bus interiors due to their lightweight characteristics. Also, these materials provide an alternative choice to overcome many shortcomings of conventional materials in the field of tissue engineering, orthopedics, and dental implants [1–3]. The potential payoff for polymer composites is extremely high and therefore, they have become one of the promising research and development areas for the technologists, scientists, and researchers. In particular, the research in the area of machining of composites has gained a momentum over the last few decades. The major thrust all around the world is minimizing the damage that is induced during drilling. The drilling-induced damages can be minimized by (i) optimizing cutting speed and feed, (ii) developing optimal tool point geometry, and (iii) developing theoretical models for predicting the critical forces [4–6]. The geometry of the cutting tool plays a significant role to access the extent of delamination during drilling of polymer composites. The development of dedicated tool point geometry in the context of drilling of polymer composites has been carried out extensively. The traditional twist drill bit which is frequently used for drilling of monolithic metals and their alloys is not suitable for polymer composites. This is due to the fact that drilling of polymer composites using twist drill bit causes severe damage to the drilled hole [7]. Thus, many drill point geometries have been developed and commercialized for producing good quality holes in composite laminates. Miller [8] studied the influence of 17 different drill bits on the drilled hole quality. The findings showed that the multifaceted drill bit, that is, the 8-facet drill bit can produce a better quality hole in graphite fiber-reinforced epoxy composites. Singh and Bhatnagar [9] also suggested that multifaceted drill bits such as Jo and 8-facet drill bit should be used to obtain the lower value of cutting forces during the drilling of unidirectional glass/ epoxy composites. At the same time, it is also pertinent to highlight that the multifaceted drill bits wear out at a faster rate than the standard twist drill bit at higher cutting Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00005-2 © 2019 Elsevier Ltd. All rights reserved.

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speeds [10]. Mathew et al. [11] stated that the drilling forces are significantly reduced when drilling is performed with the trepanning tool in glass/epoxy composites. The thrust force and torque are decreased by nearly 50% and 10%, respectively, when compared one on one with the traditional twist drill bit. Velayudham and Krishnamurthy [12] stated that the tripod drill bit is quite better than the web thinned tipped drill bit in drilling woven glass/phenolic composites. Drilling with the tripod drill bit resulted in the generation of minimum axial thrust force and delamination when compared with the web thinned tipped drill bit. Hocheng and Tsao [13] performed a thorough investigation to study the influence of saw, core, candlestick, and step drill bit on the extent of delamination. It was observed that the highest critical feed can be obtained using the core drill bit followed by candlestick, saw, and step drill bit. This indicates that the core drill bit can be operated at a higher feed without delamination and thus, higher production rate and good-quality holes can be obtained with the core drill bit. Marques et al. [14] experimentally investigated the influence of (i) twist, (ii) brad, (iii) step, and (iv) dagger drill bit on the forces and damages produced during drilling of carbon fiber-reinforced composites. It was comprehended that the minimum indentation of drill chisel edge is desirable for drilling of composite laminates. Tsao [15] investigated the performance of step core (i) twist, (ii) saw, and (iii) candlestick drill bit in the context of delamination produced during drilling of composites. The results indicated that the feed rate and diameter ratio have the maximum influence on the overall performance of the step core drill bit. Davim et al. [16] established the fact that the drilling with brad and spur drill bit resulted in lower axial thrust force and specific cutting pressure and thus less damage to the machined surface as compared to the stub length drill bit when material to be drilled is glass/polyester composites. Boldt et  al. [17] developed a special drill point for drilling graphite composites and called it spade drill countersink. Test results showed that 40 good quality holes per tool life cycle could be generated with no splintering and smoothly cut edges, without the use of backup material. A C-shaped drill point was designed and fabricated for drilling aramid composites as envisaged and developed by Konig [18]. It can be concluded from the ongoing discussion that the tool geometry has a substantial influence on the forces and damages induced during drilling. Thus, the design of optimal tool geometry is a fundamental field of research. Modifications in the tool geometry have resulted in better quality holes, a greater number of drilled holes, and reduction in the cutting forces. Therefore, it becomes necessary to design the optimum tool geometry for making damage-free hole in fiber-reinforced composites.

5.2 Materials and methods 5.2.1 Fabrication of laminates The hand layup method was applied to fabricate the glass/epoxy laminates. This method is mostly used for manufacturing thermosetting polymer-based composites because the technique involves simple processing steps and minimal infrastructural requirement. Moreover, the capital requirement for hand layup process is less as

Drilling of composite laminates using a special tool point geometry65

c­ ompared to other processing techniques. But the production rate is less and high volume fraction of reinforcement is difficult to achieve using this technique. Initially, a mild steel mold which consists of top and bottom flat plates was designed and fabricated. Reinforcement in the form of woven fabrics was cut and placed on the mold surface. Then, the mixture of epoxy resin and prescribed hardener (curing agent) was spread over the fiber fabric placed in the mold. Similarly, alternate layers of fiber fabric and resin were applied. The top mold plate is then placed over the layup. The complete mold setup is then placed in a hydraulic press and a compressive load (15 ton) was applied to remove the entrapped air bubbles and the excess resin. The mold was left at room temperature for curing of the epoxy resin. After curing, the mold was opened and the cured laminate was taken out.

5.2.2 Drilling test details The hole making operations were performed on a radial-type drilling machine under dry environment. Table  3.1 presents the specifications of the drilling machine. The scheme of the complete drilling setup is shown in Fig. 3.1. The drilling setup consists of a radial drilling machine, a fixture for holding the composite specimen, a piezoelectric drill dynamometer (Make: Kistler, Model: 9272A), connecting cables, multichannel charge amplifier (Make: Kistler, Model: 5070A), an analog-to-digital converter, data acquisition system, and a personal computer. The prepared composite specimen is mounted on the dedicated fixture to perform the drilling tests. The fixture is then clamped on top of the dynamometer and the dynamometer is rigidly mounted on the machine bed with the help of square-headed bolts fitted into the bed T-slots. Table 3.1  Specification of the drilling machine Make: Batliboi & Company Pvt. Ltd., India Feed rate range: 0.03–0.3 mm/rev Spindle speed range: 90–4500 RPM Elevating motor: 0.75 kW/1420 RPM Drilling main motor: 1.50 kW/1420 RPM Coolant pump motor: 0.75 kW/2800 RPM

Fig. 3.1  Scheme of the experimental setup.

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Hole-Making and Drilling Technology for Composites

The three important parameters (i) feed, (ii) spindle speed, and (iii) tool point geometry have been investigated for their influence on the drilling-induced forces, formedchip characteristics, and the accompanying damages. The drilling was performed at a feed of (i) 0.05 mm/rev, (ii) 0.12 mm/rev, and (iii) 0.19 mm/rev and spindle speed of (i) 900 RPM, (ii) 1800 RPM, and (iii) 2800 RPM using the (i) twist, and (ii) developed drill bit having a diameter of 10 mm (as shown in Fig. 3.2). It is quite clear from the figures that the design and construction of both the drill bits are substantially different.

5.2.3 Measurements The output responses considered to investigate the drilling behavior of the composite laminates are (i) axial thrust force, (ii) torque, (iii) formed-chip characteristics, and (iv) quality of the drilled hole. A drill dynamometer (piezoelectric type) coupled with charge amplifier was used to capture the cutting force signals. This type of dynamometer can measure even a small dynamic changes produced during the drilling operation. The drill dynamometer can measure three orthogonal forces and a moment. In the present work, only the axial thrust force and torque signals are recorded. The working principle of the dynamometer is based on the transmission of axial thrust force and torque through the top plate to four component load sensors. These sensors comprise three pairs of quartz plates, of which one is sensitive to compression and other two are sensitive to shear in either X or Y direction. The additional set of quartz plates is arranged to yield an electric charge proportional to the moment MZ acting about the axis of the sensor. The charge produced by quartz plates is collected by the electrodes connected to the sensor. The output voltage signals proportional to the forces acting on the dynamometer are fed to the multichannel charge amplifier. The recorded force signals are further amplified with the help of an amplifier attached to the experimental setup. The amplified signal is then transferred to

Fig. 3.2  Drill point geometries: (A) twist drill bit and (B) developed drill bit.

Drilling of composite laminates using a special tool point geometry67

the personal computer via data acquisition card. This software helps in storing individual force measurement data and its configuration in the computer. Further analyses of the signals were carried out to get the peak value of the axial thrust force and torque.

5.3 Results and discussion 5.3.1 Analysis of chip formation The machinability of the fiber-reinforced composites can be accessed by analyzing the chip formed during the drilling operation. A smooth hole wall surface is anticipated when continuous or long type of chips are formed during drilling. These types of chips are generally formed at low cutting speeds. Subsequently, at higher cutting speed, formed chips are discontinuous or short [19]. The axial feed of the drill bit also accounts for a significant change in the shape of the formed chip. The chips of varying thickness are formed when the axial feed of the drill bit is varied. The chips formed during drilling of glass/epoxy composites with the twist and developed drill bit are shown in Fig. 3.3A–D. The different types of chips formed with the twist drill bit at a different level of cutting speed (v in m/min) and feed (f in mm/rev) are shown in

Fig. 3.3  Characteristics of formed chips for (A–C) twist drill bit, and (D) developed drill bit.

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Hole-Making and Drilling Technology for Composites

Fig. 3.3A–C. It is evident from the figure that high speed coupled with low feed of the drill bit resulted in the formation of fine or powdery chips. This type of chip is formed due to the brittle nature of the glass fiber and cured epoxy resin. The matrix material (epoxy resin) fails early due to the high strain rate. Whereas, the combination of high feed and low speed of the drill bit resulted in the formation of thick continuous chips. However, the chip formed with the developed drill bit is substantially different from the chips formed with the twist drill bit. Drilling with developed drill bit resulted in the formation of a cut-out slug or cylindrical rod as shown in Fig. 3.3D. It was also observed that only one type of chip is formed during drilling of the composites using the developed drill bit at all combination of cutting speed and feed. It can also be inferred from the figure (Fig. 3.3D) that the chipping occurs at the bottom side of the cut-out slug. The thickness of the uncut slug continuously decreases as the drill bit approaches toward the bottom side of the laminate. The slug is separated from the laminates as the thickness of the uncut slug reaches its critical value. The size of the exit chipping is different at a various combination of cutting speed and feed of the drill bit.

5.3.2 Analysis of force signals The cutting force signals recorded with both the drill bits are shown in Figs. 3.4 and 3.5, respectively. It is quite evident from the figures that there is a continuous oscillation in the signals as the drill bit enters into the composite laminates. The oscillation of the force signals is expected because the drill bit encounters two different types of materials during drilling operation. These materials are the two physically and chemically distinct constituents of the composites; they are glass fiber and epoxy matrix. As the drill bit starts indenting and then cutting the composite constituents, the forces generated are repeatedly changing because the resistance offered by the fiber and the matrix is different because the yield shear strength of the fiber and the matrix is different [20]. Thus the force signal oscillates continuously during the entire drilling process. It can also be deduced from the figures that the behavior of force signals recorded during drilling with the developed drill bit are quite different from the twist drill bit because the design, construction, and cutting features of both the drill bits are substantially different. It can also be noted that the time taken to make a hole by the

Fig. 3.4  Force signals recorded at the feed of 0.05 mm/rev and spindle speed of 900 RPM during drilling with twist drill bit (A) thrust force (B) torque.

Drilling of composite laminates using a special tool point geometry69

Fig. 3.5  Force signals recorded at the feed of 0.05 mm/rev and spindle speed of 900 RPM during drilling with developed drill bit (A) thrust force (B) torque.

twist drill bit is higher than the developed drill bit because the endpoint of the twist drill bit is conical whereas the developed drill bit has two sharp cutting edges which are diametrically opposite to each other. This indicates that the developed drill bit is more suitable in industries as drilling with this drill bit leads to higher production rate.

5.3.3 Analysis of drilling-inducedforces The variation in cutting forces with feed of the drill bit at a different level of speed is presented in Figs. 3.6–3.11. The figures show that both the cutting forces increases linearly with the increase in feed from 0.05 to 0.19 mm/rev keeping the spindle speed constant. The primary and the secondary shear zone get affected with the change of the drill feed. This means the thickness of the uncut layer of chip increases with drill feed. The shearing of material becomes more difficult as the resistance offered by the uncut chip is more for higher uncut chip thickness. Thus both the cutting forces

Fig. 3.6  Axial thrust force versus tool geometry at different level of feed for spindle speed of 900 RPM.

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Hole-Making and Drilling Technology for Composites

Fig. 3.7  Axial thrust force versus tool geometry at different level of feed for spindle speed of 1800 RPM.

Fig. 3.8  Axial thrust force versus tool geometry at different level of feed for spindle speed of 2800 RPM.

boost up with an increase in the feed. This was found to be in agreement with previous studies [21–23]. It is also evident from the figures that there is no specific trend in the variation of cutting forces as the speed of the drill bit increases from 900 to 2800 RPM. However, it can be inferred from the figures that there is not much rise in the axial thrust force and torque value at higher speeds of the twist and developed drill bit. At a higher speed of the drill bit, the temperature at the machining zone increases as the friction between the workpiece and tool surface is more at the higher speed. The matrix material gets softened as the heat generation is more in the cutting zone. The softened matrix has less yield shear strength as compared to the hard or cured matrix. The resistance offered by the softened matrix is less which results in easy sharing of material and thus, the lower axial thrust force and torque is obtained at higher speed

Fig. 3.9  Torque versus tool geometry at different level of feed for spindle speed of 900 RPM.

Fig. 3.10  Torque versus tool geometry at different level of feed for spindle speed of 1800 RPM. 100

40 30 20 10

Spindle speed: 2800 RPM

0.19 mm/rev

50

0.12 mm/rev

60

0.05 mm/rev

Torque (N-cm)

70

0.12 mm/rev

0.19 mm/rev

80

0.05 mm/rev

90

0 Twist drill bit

Developed drill bit

Fig. 3.11  Torque versus tool geometry at different level of feed for spindle speed of 2800 RPM.

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Hole-Making and Drilling Technology for Composites

of the drill bit [24].The figures also revealed that there is a significant reduction in the peak values of the drilling forces while holes are generated using the developed drill bit. However, the reduction in axial thrust force is more as compared to the torque. This is due to the hollow nature of the developed drill bit. During drilling with the developed drill bit, the two cutting edges which are diametrically opposite to each other make an engagement with the workpiece. The mechanical contact between the cutting edges of developed drill bit and workpiece is less when compared one to one with the twist drill bit. Moreover, quick ejection of formed chips from the cutting zone is a challenging issue when drilling is performed with the twist drill bit. The jamming of formed chips is a common phenomenon during drilling with the twist drill bit. But in the case of developed drill bit, the chip is easily ejected in the form of cut-out slug. Also, the sharing of fiber and matrix majorly takes places at the periphery of the tooltip which render the fibers under tension during the entire drilling operation. Thus, the cutting of fibers is relatively easy. These striking features of the developed hollow drill bit make it more suitable over the twist drill bit for making holes in composites. It was also showed that the drilling of composites with hollow trepanning tool results in the generation of relatively lower axial thrust force than the twist drill bit [11, 25, 26].

5.3.4 Analysis of drilling-induced damages Two major types of damages formed during drilling of the composites are (i) peel-up and (ii) push-down delamination [27–29]. These types of delamination formed during drilling are shown in Fig. 3.12. The peel-up delamination occurs when the drill bit enters the composite laminate. A peeling force tends to develop at the starting of the drilling operation which pulls the laminate of the laminate through the flute of the drill bit. This causes separation of the upper laminate around the drill entry. This type of separation of the laminae is called peel-up delamination. The push-down delamination occurs at the bottom side of the drilled hole when the drill bit exits the hole. The separation of the bottom laminate takes place when the induced axial thrust force is more than the inter-ply bonding strength. This type of separation of the lamiane around the drill exit is known as push-down delamination [30–32].The push-down delamination is more critical as it causes rejection of most of the drilled parts [33]. Fig. 3.13 shows

Twist drill bit

Drill bit feed

Drill bit rotation Twist drill bit

Drill bit feed

Drill bit rotation

Composite laminate

Composite laminate Peel-up delamination

Push-down delamination

Fig. 3.12  Damage of the drilled hole in the form of (i) peel-up delamination and (ii) pushdown delamination.

Drilling of composite laminates using a special tool point geometry73

Fig. 3.13  Damage of the hole exit side while drilling is performed with the (A) twist drill bit and (B) developed drill bit.

the circumferential edge quality of the drilled hole exit for both the drill bits considered for the purpose of investigation. The common form of damages observed at the hole exit for the twist drill bit are (i) uncut fibers, (ii) spalling, (iii) delamination, and (iv) matrix cracking as shown in Fig.  3.13A. However, these types of damages are not seen at the hole exit while drilling is performed with the developed drill bit. A clean-cut and damage-free hole is produced with the developed drill bit (Fig. 3.13B). This indicates that the performance of the developed drill bit is much better than the twist drill bit in producing hole in composite laminates. This is primarily due to the substantial reduction in the axial thrust force and torque values. The percentage reduction in thrust force and torque values is graphically presented in Figs. 3.14 and 3.15, respectively. As shown in Fig. 3.14, at a lower value of feed (0.05 mm/rev), the axial thrust force is reduced by at least 50% at all range of spindle speed when drilling is

100

25.55

40

28.89

49.33

50

46.68

52.94

58.51

51.16

58.09

60

1800

70

57.29

80

900

Redution in thrust force (%)

90

30 20 10

Spindle speed (RPM)

Fig. 3.14  Percentage reductions in the axial thrust force.

2800

1800

900

2800

1800

900

2800

0

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Hole-Making and Drilling Technology for Composites

60 41.34 27.36

40

9.96 2800

4.83

9.50 1800

10

7.55

20

5.28

17.88

30

0.82

Redution in torque (%)

50

2800

1800

900

900

2800

1800

900

0 Spindle speed (RPM)

Fig. 3.15  Percentage reductions in the torque.

performed with the developed drill bit, taking the twist drill bit as a reference. The percentage reduction in thrust force is more than the torque. The maximum percentage reduction in torque was calculated as 41.34%, whereas the maximum reduction in the axial thrust force was 58.51%. The axial thrust force is reduced in the range of 25.55%–58.51% and torque is reduced in the range of 0.82%–41.34% at all combination of spindle speed and feed of the drill bit.

5.4 Conclusions In this chapter, the performance of an innovative drill bit has been presented and compared with the twist drill bit. The performance of the developed drill bit has been evaluated in terms of cutting forces and mechanical damages induced during drilling of glass/ epoxy composites at different levels of feed and spindle speed. It was observed during drilling with the developed drill bit that the chip is removed in the form of cut-out slug, whereas, drilling with twist drill bit resulted in the formation of short to long continuous chips. It was also noted from the recorded force signals that the time taken by the developed drill bit to make a single hole is less as compared to the twist drill bit under identical condition. Also, the peak or average value of axial thrust force and torque is substantially less when drilling is done with the developed drill bit as compared to the twist drill bit. The maximum reduction in axial thrust force and torque is calculated as 58.51% and 41.34%, respectively. The images of the drilled hole exit showed that the uncut fibers, spalling, and matrix cracking are the common form of damages induced during drilling with the twist drill bit. But such types of damages are not seen at the hole exit when drilling is performed with the developed drill bit. This indicates that the developed drill bit can produce superior quality holes in composite laminates.

Drilling of composite laminates using a special tool point geometry75

References [1] S.K. Mazumdar, Composites Manufacturing: Materials, Product, and Process Engineering, CRC Press LLC, Boca Raton, 2002. [2] R.M.  Jones, Mechanics of Composite Materials, Scripta Book Company, Washington, DC, 1975. [3] K.  Debnath, I.  Singh, A.  Dvivedi, Rotary mode ultrasonic drilling of glass fiberreinforced epoxy laminates, J. Compos. Mater. 49 (8) (2015) 949–963. [4] V. Dhawan, K. Debnath, I. Singh, S. Singh, Prediction of forces during drilling of composite laminates using artificial neural network: a new approach, FME Trans. 44 (1) (2016) 36–42. [5] V. Dhawan, K. Debnath, I. Singh, S. Singh, A novel intelligent software-based approach to predict forces and delamination during drilling of fiber-reinforced plastics, Proc. Inst. Mech. Eng. Pt. L J. Mater. Des. Appl. 230 (2) (2016) 603–614. [6] V.K. Doomra, K. Debnath, I. Singh, Drilling of metal matrix composites: experimental and finite element analysis, Proc. Inst. Mech. Eng. B J. Eng. Manuf. 229 (5) (2015) 886–890. [7] C.C. Tsao, H. Hocheng, Taguchi analysis of delamination associated with various drill bits in drilling of composite material, Int. J. Mach. Tools Manuf. 44 (10) (2004) 1085–1090. [8] J.A. Miller, Drilling graphite/epoxy at Lockheed, Am. Mach. Autom. Manuf. 131 (1987) 70–71. [9] I.  Singh, N.  Bhatnagar, Drilling of uni-directional glass fiber reinforced plastic (UDGFRP) composite laminates, Int. J. Adv. Manuf. Technol. 27 (9-10) (2006) 870–876. [10] S.C. Lin, J.M. Shen, Drilling unidirectional glass fiber-reinforced composite materials at high speed, J. Compos. Mater. 33 (9) (1999) 827–851. [11] J. Mathew, N. Ramakrishnan, N.K. Naik, Investigations into the effect of geometry of a trepanning tool on thrust and torque during drilling of GFRP composites, J. Mater. Process. Technol. 91 (1999) 1), 1–11. [12] A. Velayudham, R. Krishnamurthy, Effect of point geometry and their influence on thrust and delamination in drilling of polymeric composites, J. Mater. Process. Technol. 185 (1) (2007) 204–209. [13] H. Hocheng, C.C. Tsao, Effects of special drill bits on drilling-induced delamination of composite materials, Int. J. Mach. Tools Manuf. 46 (12) (2006) 1403–1416. [14] A.T.  Marques, L.M.  Durao, A.G.  Magalhães, J.F.  Silva, J.M.R.  Tavares, Delamination analysis of carbon fibre reinforced laminates: evaluation of a special step drill, Compos. Sci. Technol. 69 (14) (2009) 2376–2382. [15] C.C. Tsao, Experimental study of drilling composite materials with step-core drill, Mater. Des. 29 (9) (2008) 1740–1744. [16] J.P. Davim, P. Reis, C.C. Antonio, Experimental study of drilling glass fiber reinforced plastics (GFRP) manufactured by hand lay-up, Compos. Sci. Technol. 64 (2) (2004) 289–297. [17] J.A. Boldt, J.P. Chanani, Solid tool machining and drilling, in: Composites Engineered Materials Handbook, vol. 1, ASM International, Metals Park, OH, 1987, pp. 667. [18] W.  Konig, P.  Grass, A.  Heintzem, F.  Okcu, C.I.  Schmitz-Justen, New developments in drilling and contouring composites containing kevlar aramid fiber, in: Technical Symposium V, Design and Use of Kevlar Aramid Fiber in Composite Structures, 1984, pp. 95–103. [19] K. Debnath, I. Singh, A. Dvivedi, Drilling characteristics of sisal fiber-reinforced epoxy and polypropylene composites, Mater. Manuf. Process. 29 (11–12) (2014) 1401–1409.

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[20] K. Debnath, I. Singh, Low-frequency modulation-assisted drilling of carbon-epoxy composite laminates, J. Manuf. Process. 25 (2017) 262–273. [21] K.  Debnath, I.  Singh, T.S.  Srivatsan, An innovative tool for engineering good-quality holes in composite laminates, Mater. Manuf. Process. 32 (9) (2017) 952–957. [22] K. Debnath, I. Singh, A. Dvivedi, On the analysis of force during secondary processing of natural fiber-reinforced composite laminates, Polym. Compos. 38 (1) (2017) 164–174. [23] P.K. Bajpai, K. Debnath, I. Singh, Hole making in natural fiber-reinforced polylactic acid laminates: an experimental investigation, J. Thermoplast. Compos. Mater. 30 (1) (2017) 30–46. [24] K. Debnath, M. Sisodia, A. Kumar, I. Singh, Damage-free hole making in fiber-reinforced composites: an innovative tool design approach, Mater. Manuf. Process. 31 (10) (2016) 1400–1408. [25] P.K. Rakesh, I. Singh, D. Kumar, Drilling of composite laminates with solid and hollow drill point geometries, J. Compos. Mater. 46 (25) (2012) 3173–3180. [26] P.K. Bajpai, I. Singh, Drilling behaviour of sisal fibre-reinforced polypropylene composite laminates, J. Reinf. Plast. Compos. 32 (20) (2013) 1569–1576. [27] D. Liu, Y. Tang, W.L. Cong, A review of mechanical drilling for composite laminates, Compos. Struct. 94 (4) (2012) 1265–1279. [28] A.P. Singh, M. Sharma, I. Singh, A review of modeling and control during drilling of fiber reinforced plastic composites, Compos. Part B 47 (2013) 118–125. [29] J.P.  Davim, P.  Reis, Study of delamination in drilling carbon fiber reinforced plastics (CFRP) using design experiments, Compos. Struct. 59 (4) (2003) 481–487. [30] M.F. Ameur, M. Habak, M. Kenane, H. Aouici, M. Cheikh, Machinability analysis of dry drilling of carbon/epoxy composites: cases of exit delamination and cylindricity error, Int. J. Adv. Manuf. Technol. 88 (9–12) (2017) 2557–2571. [31] P. Rahme, Y. Landon, F. Lachaud, R. Piquet, P. Lagarrigue, Delamination-free drilling of thick composite materials, Compos. A: Appl. Sci. Manuf. 72 (2015) 148–159. [32] H. Ho-Cheng, C.K.H. Dharan, Delamination during drilling in composite laminates, J. Eng. Ind. 112 (3) (1990) 236–239. [33] U.A. Khashaba, Delamination in drilling GFR-thermoset composites, Compos. Struct. 63 (3) (2004) 313–327.

Application of ultrasonic-assisted machining process for making hole in composite laminates

6

K. Debnath Department of Mechanical Engineering, National Institute of Technology Meghalaya, Shillong, India

6.1 Introduction The conventional method of drilling poses several challenges while making holes in fiber-reinforced composites [1–3]. Thus the development of a new method for making superior quality hole in fiber-reinforced composites has become imperative. The ultrasonic-assisted drilling methods have been developed to meet the requirements of composite industry as these methods can be potentially applied for engineering good quality holes in fiber-reinforced composites. The mechanisms of material removal in these processes are substantially different from the conventional method of drilling. The ultrasonic-assisted drilling processes can be bifurcated into (i) intermittent contact-type ultrasonic-assisted drilling and (ii) noncontact-type ultrasonic-assisted drilling. In intermittent contact-type ultrasonic-assisted drilling processes such as (i) ultrasonic-assisted twist drilling, (ii) rotary ultrasonic machining, and (iii) rotary ultrasonic elliptical machining, the cutting edges of the tool make a mechanical contact with the workpiece intermittently. Whereas, in noncontact-type ultrasonic-assisted drilling processes such as (i) ultrasonic machining and (ii) rotary mode ultrasonic drilling, there is no direct mechanical contact between the tool surface and the workpiece. The intermittent or no-contact-type behavior of ultrasonic-assisted drilling makes it superior to the conventional drilling technique (continuous mechanical contact between the tool and the workpiece). The major benefit associated with the ultrasonic-assisted drilling processes is that these processes minimize the average value of cutting forces when compared one on one with conventional drilling. Takeyama and Kato [4] showed that the formation of burr and material distortion at the exit side of the hole can be prevented when ultrasonic vibration is applied in the longitudinal feed direction of the tool during drilling of aluminum and glass fiber-reinforced plastics using radial peripheral lip drill bit. Takeyama and Iijima [5] revealed that the average cutting forces and surface roughness of the machined part are dramatically reduced by the application of ultrasonic vibration during machining of glass fiber-reinforced plastics. In addition, burr formation and subsurface damage can be prevented by applying ultrasonic vibration. Besides drilling, the ultrasonic vibration can also be applied for cutting of composite panels. Kim and Lee [6] showed that the ultrasonic vibration-assisted cutting of carbon fiber-based composites led to the formation of a better-machined surface when cutting is performed below the critical Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00006-4 © 2019 Elsevier Ltd. All rights reserved.

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cutting speed. The machined surface quality is much better compared to the conventional cutting of carbon fiber-reinforced composites. It was also observed that the diamond tool produces a better-machined surface while cutting of carbon fiber-reinforced composites is done both conventionally and ultrasonically. Cong et al. [7] analyzed the performance of rotary ultrasonic machining and compared it with the tradition twist drilling. The rotary ultrasonic machining of carbon fiber composites resulted in lower drillinginduced thrust force, torque, delamination factor, and surface roughness as compared with tradition twist drilling. Moreover, the life of the tool in rotary ultrasonic machining was much higher as compared to the twist drilling. In rotary ultrasonic machining, the tool can produce more than 1400 holes before wearing out. But in twist drilling, the drill bit can produce only 5 holes before wearing out. However, it is also pertinent to mention that the material removal rate in twist drilling is much higher as compared to the rotary ultrasonic machining under identical cutting condition. Liu et al. [8] studied the rotary ultrasonic elliptical machining of carbon fiber-reinforced composite panels using diamond core drill bit. The rotary ultrasonic elliptical machining combines the advantages of both the core drill bit and the elliptical tool vibration toward achieving damage-free holes in composite panels. The experimental investigation showed that the rotary ultrasonic elliptical machining is better than the traditional drilling because (i) ejection of the chipis improved and thus the excess heat is rapidly removed from the machining zone, (ii) better precision of the drilled hole, (iii) reduce the tool wear, (iv) exit delamination is prevented, (v) better surface quality around the hole, and (v) reduce the average value of cutting forces. Hocheng and Hsu [9] made the first attempt for making hole in carbon fiber-reinforced thermoplastic (polyether ether ketone) and thermosetting (epoxy) composites using slurry-assisted ultrasonic machining technique. In this study, the influence of various parameters such as (i) ultrasonic energy, (ii) slurry concentration, (i) abrasive particles size, and (iii) feed rate on the (i) material removal rate, (ii) surface roughness, and (iii) hole clearance have been experimentally investigated. It was observed that the ultrasonic machining process can produce better quality hole in fiber-reinforced composites than the conventional drilling technique. The ultrasonic machining process proved to be an efficient method for producing delamination-free hole at the top and bottom side of the drilled hole. The list of the other articles published in the area of ultrasonic-assisted drilling of fiber-reinforced composites is summarized in Table 6.1. Table 6.1  Summary of publications Sl. no.

Type of drilling operation

References

1.

Rotary ultrasonic machining

2.

Rotary ultrasonic elliptical machining

3.

Ultrasonic vibration-assisted twist drilling

a. Baraheni and Amini [10] b. Amini et al. [11] c. Kumaran et al. [12] d. Cong et al. [13–16] a. Geng et al. [17, 18] b. Xu et al. [19] a. Sanda et al. [20] b. Phadnis et al. [21, 22] c. Makhdum et al. [23]

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79

From the ongoing discussion, it is clear that the majority of the research attempts made focused on investigating the performance of pulse-intermittent contact-type machining of fiber-reinforced composites using traditional twist or core drill bits. No in-depth study has yet been addressed in the context of application of slurry-assisted ultrasonic machining method for making hole in fiber-reinforced composites.

6.2 Ultrasonic machining: Process principle The main components of an ultrasonic machine are shown in Fig. 6.1. In ultrasonic machining, the tool is superimposed with high-frequency (20–40 kHz) ultrasonic vibration along the feed direction. In this process, the abrasive slurry which is a mixture of fine abrasive particles (silicon carbide, alumina, boron carbide, etc.) and liquid medium (water, glycerol, benzene, etc.) is continuously fed into the working gap, that is, the gap between the tool end face and the workpiece top surface. The vibrating tool hits the abrasive particles and then the abrasive particles further hit the work surface. A localized stress is developed due to the repetitive hammering action by the relatively large size abrasive particles which cause the removal of material from the work surface. A small amount of material is also removed due to the impact of relatively smaller size abrasive particles and implosion of bubbles (formed in the cutting zone during machining) also called cavitation. Though water is generally used as slurry medium, the chemical impurities present in the slurry medium (if any) may degrade the work material, thus resulting in the loss of material. The continuous flow of slurry flushes away the debris from the cutting zone and refills the gap with fresh slurry and the process continues. As the machining is performed in the presence of liquid media there is no thermal threat to the workpiece. But this process is only suitable for machining of hard and brittle materials as the material is removed mainly due to brittle fracture. Thus the ultrasonic machining process has been substantially modified to make it suitable for composite materials.

Fig. 6.1  Schematic illustration of ultrasonic machining process.

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6.3 Materials and methods 6.3.1 Fabrication of laminates The E(electrical)-glass fiber fabrics used in making the composite laminates have modulus of 80 GPa and density of 2.62 g/cm3. Epoxy resin (Araldite LY 556) and hardener (HY 951) having the density of 1.12 and 1 g/cm3, respectively, were used as a matrix material. One of the important characteristics of the epoxy resin is that it exhibits low shrinkage. It also has excellent adhesion property to a variety of substrate materials. Hand layup process was used to fabricate the glass fiber-reinforced epoxy composite laminates. This method is mostly used for manufacturing thermosetting polymer-based composites because it involves simple processing steps and minimal infrastructural requirement. Moreover, the capital requirement for hand layup process is less as compared to other processing techniques. A mild steel mold was designed and developed to fabricate the composite laminates. The mold consists of two plates: (i) top and (ii) bottom mold plates. A mold release gel was applied to the mold surfaces to avoid sticking of epoxy resin to the mold surface. Reinforcement in the form of woven mat was cut as per the size of the mold and placed on the mold surface. Epoxy resin was mixed with hardener in prescribed proportion and spread over the surface of fiber mat. Similarly, alternate layers of fiber fabric and resin were applied until the desired thickness of the composite laminate is achieved. The top mold plate is then placed over the layup and a compression load is applied. The mold was left at ambient temperature for curing.

6.3.2 Experimental setup The experiments were performed on a stationary ultrasonic machine (AP-500, Sonic-Mill) as shown in Fig.  6.2. The ultrasonic machining setup consists of a power supply unit, mill-module assembly, and slurry flow system. A rotary setup and a dedicated fixture were

Fig. 6.2  Ultrasonic machining setup.

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81

developed and retrofitted with the ultrasonic machine to provide rotation and hold the workpiece, respectively. The hollow cylindrical tools were also fabricated to perform the machining operation as shown in Fig. 6.3A. The micrographic image (Fig. 6.3B) shows the initial surface characteristics of the shaped tool. The stainless steel (S304) was selected as tool material because it is tough but not hard in nature as these are the most desirable characteristics of the tool material used in ultrasonic machining. The abrasive slurry used to perform the machining operation consists of abrasive particles (silicon carbide) and water. The different parameters of the ultrasonic machining are presented in Table 6.2.

6.4 Results and discussion In the present work, machining of the glass fiber-reinforced epoxy composites has been performed in the presence of abrasive slurry. A rotary setup has been developed and retrofitted with the ultrasonic machining setup to improve the machining performance.

Fig. 6.3  Tool configuration: (A) shaped tool and (B) microstructure of the tool surface. Table 6.2  Process parameters for rotary mode ultrasonic drilling Tool

Workpiece Slurry

Vibration Others

Material Geometry Outer diameter Thickness Material Thickness Abrasive type Slurry temperature Suspension medium Flow rate Frequency Amplitude Static load Machining depth Rotational speed

Stainless steel Hollow cylindrical tool 8 mm 0.625 mm Glass/epoxy laminate 4 mm SiC 28°C Water 5 L min−1 20 kHz±200 Hz 0.5 mm diameter) can be drilled by peripheral removal of material at the bore outline. In laser drilling, the prime mechanism of material removal is using high-intensity stationary laser beam with the focus on the surface where material is to be removed. At sufficient power densities of laser the material can get heated, melted and subsequently ejected in either liquid and/or vapor phase. Given the mechanism of material removal is through phase change, industrial laser processing can lead to higher magnitude of heat affected zone (HAZ) as the polymer matrices have lower vaporization temperatures as well as thermal conductivity compared to carbon fibers. It is noted that the heat affected zone causing thermal damage to unidirectional and/or crossply composites laminates is proportional to the specific laser energy (a ratio of laser power to scanning speed) [26]. It can be concluded from the above discussion that the laser processing for composite materials is intricate. Primarily the composites and its constituents have poor thermal conductivities, heat capacities, and low vaporization temperatures, thus the energy imparted during laser processing can severely damage the material.

7.2 General methods of laser drilling 7.2.1 Laser percussion drilling Two major methods/approaches are employed for laser drilling processes. Fig. 7.1 depicts the trepanning and percussion drilling processes. In order to avoid contamination to the laser optics from ejected debris, assisted gas is utilized. The assisted gas supplied coaxially also helps in material removal during the process. Basically a single pulse of laser can remove a narrow (less than 1 mm) hole of material in thin (less than 1 mm) specimens. This single pulse drilling is extended to perform percussion drilling to drill larger holes in thicker specimens (more than 1 mm) (Fig. 7.1A). In industrial laser drilling, usually high-power densities are supplied during a single pulse to irradiate enough energy which can vaporize the material in a pulse. In percussion drilling, a sequence of such short bursts/pulses (duration between 10−12 and 10−3 s) is imparted on the base material at a relatively longer time period (~10−2 s). This directed at a spot forms a larger and through hole. During each pulse, a certain amount of material at a certain depth is removed, thus a sequence of bursts can remove large chuck. For percussion drilling purposes, pulsed Nd:YAG laser is the best choice owing to higher energy per pulse [27, 28]. The physical processes involved in general laser drilling, especially percussion drilling of metal, are schematically shown in Fig. 7.2. Three stages can be identified in

Laser drilling of composite material: A review91 Laser beam

Laser beam

Single / multiple beam

Converging lens

Converging lens

Different size of holes formed

Composite sample

(B)

(A)

Fig. 7.1  General methods of laser drilling: (A) percussion [27] and (B) trepanning [20]. Laser beam

(A)

Composite sample

Laser beam

(B)

Laser beam

(C)

Fig. 7.2  Physical processes during percussion laser drilling: (A) melt pool formation, (B) splashing out of molten material, and (C) formation of recast layer [29].

the percussion drilling process. At the start, at the target surface a thin region of molten material is created by absorbing laser energy imparted through pulse (Fig. 7.2A). As the meltpool temperature increases and approaches vaporization temperature, the second phase starts with the expansion of vapors evaporating from the surface leading to the splashing stage. During splashing stage, the melt pool is pushed out radially due to the recoil pressure (Fig. 7.2B). The mechanism involved during this meltpool expulsion is the favorable molten layer, pressure gradients at the surface of the molten layer grows larger than the inward surface tension, the net force expels molten material from the hole [29]. However, during this transport some portion of melted material may stick to the walls of the hole and resolidify (Fig. 7.2C). While for the composite base materials the process becomes much more intricate due to different thermo-kinetic properties of the constituents of composite material.

7.2.2 Laser trepanning drilling For much wider holes (less than 3 mm) in thicker substrates (less than 10 mm), trepanning (Fig. 7.1B) process is employed. This involves drilling a contour on the plate by using a series of overlapping holes at the periphery. This process can be performed by either translating the workpiece or refocusing optics. This process resembles the

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contour/path cutting process and allows laser usage in either continuous wave (CW) or pulsed mode. CO2 and Nd:YAG lasers are most commonly used in trepanning. The tremendous variety offered in the laser trepanning drilling has helped industrial process to achieve repeatable, high precision larger holes at high speeds. For instance, helical trepanning involves focusing the beam at the center of the hole while revolving the substrate around the perimeter. This allows gradual deepening of hole at each cycle of rotation [30]. Focal point can also be changed sometimes during the rotation cycles in such process.

7.3 Formation of tapered hole The formation of holes during laser drilling has typically tapered cross-sectional view as depicted in Fig. 7.2. The hole diameter reduces as the depth of hole increases, thus forming a tapered wall. Such topography is formed owing to the fact that in the normal setting the laser is irradiated normal to the surface. As the depth of holes increases there is reduction in hole size primarily due to a relatively lesser contribution of heat energy due to lower reflection from the machined sides of the hole. As the depth increases the laser beam is absorbed by carbon fiber instead of reflection from metals. This results in lowering of the laser energy intensity at the hole bottom, thereby decreasing the hole size [31]. Moreover, the plasma due to assisted gas can absorb some of the laser energy and contribute toward tapering shape [32]. In CFRP the heat generated through laser is rather quickly conducted to the surrounding material owing to relatively higher thermal conductivity. This results in lowering of temperature at the zone sides than at the zone center of the machined area. Thus, the ablation of the central zone precedes the rest due to fast build-up at the central zone by incident pulses. The profile of the incident beam is also a factor that contributes to the tapered shaped holes. Gaussian distribution of the energy intensity results in higher intensity at the center resulting in the removal of more material at the center than sides. Possibility to remove tapering is proposed by Salama et al. [33] either by tilting the sample or offsetting the laser beam. For instance, for drilling cooling fluid holes in turbine blades, holes are drilled at an angle to the surface to remove taper [34].

7.4 Laser drilling methods to suppress delamination and thermal defects in composites Considering the percussion drilling process and its associated mechanism, the use of a pulsed Nd:YAG laser has demonstrated damage to matrix even at the slightest of energy impart despite extensive modulation in the pulse rate, energy intensity, and repetition rate [35]. Several defects such as discontinuities at the interfaces between layers have been found by Rodden et al. [36]. These layers contained differently orientated fibers which resultantly increased delaminative effects [36]. Another study indicated poor-quality effects in drilling microholes using 1 kHz mode-locked Ti:sapphire laser

Laser drilling of composite material: A review93

(120 fs pulses at 795 nm) on PEEK-CF. Defects such as waviness and irregular shapes were observed due to the preferential ablation of the polymer [37]. The CFRP drilling through laser trepanning showed numerous drawbacks with only success while drilling very thin laminates of up to 0.3 mm with a proper selection of drilling parameters. With laminates having thickness larger than 1 mm, ejection of ablated material from the hole in a single groove becomes difficult due to high aspect ratio between cutting depth and kerf width. This results in an extended thermal damage of the groove along the circumferential direction with irregularly shaped edges. Moreover, the rough edges trap the incident beam through multiple internal reflections preventing the progression of the cut. Localization of energy in the trap of the groove in the presence of high thermal conductivity of fibers cause extended degradation of the matrix. To circumvent the heat trapping in the groove in single-ring trepanning, Li et al. [38] demonstrated a strategy involving multiple rings thereby widening the cut kerf. This results in effective material removal and restricts heat accumulations (Fig. 7.3). Li et al. also showed that by performing multi-ring strategy the material can be removed layer by layer with a wide kerf (ring width). This not only facilitates better material ejection from the central zone where beam/material interacts, but it also reduces the shielding of incident laser beam by the plume generated. Upper surface of the substrate is set as the first focal plane, at the beginning of drill. In the multi-ring, focal plane moves downwards by an increment of 0.25 mm for each set of 100 passes. The drilling starts from the outer diameter of the hole by creating a trench to block the additional energy input generated by the internal rings from transferring along the fibers to the surrounding bulk material. The experiments demonstrated that a series of trepanning passes could be performed while reaching the hole depth of 6 mm for the cut through. Multi-ring showed a relatively lower HAZ with size of 50 μm using a nanosecond pulsed UV laser. The drilling was performed at significantly higher scanning speed (800 mm/s). It is noted that the short pulse and high scanning speed help in reducing HAZ. Moreover, this method can be employed for both trepanning [20, 33, 38] and percussion drilling [20]. For very small hole diameter (~100 μm) on a relatively thick laminate (~2.5 mm) another study by Romoli et  al. [20] used a removal mechanism based on double ring instead of percussion drilling. The advantage of internal ring ablation allows

Composite substrate

Heat conduction to the parent material is blocked by the 1st ring. Heat is retained and accumulated between successive 2nd ring nth ring

Fig. 7.3  An innovative laser drilling technique by using multiple rings to reduce heat accumulation [38].

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Hole-Making and Drilling Technology for Composites

deeper penetration of beam in the substrate. Resultantly, the conical topology of the inner surface of hole can be decreased compared to conventional percussion drilling.

7.5 Ultrashort laser pulses for composite drilling Previously lasers with pulse duration ranging in the order of 100 μs were employed in laser drilling operations to achieve target material temperature in excess of boiling point. However, for longer and small diameter holes ultrashort pulse duration laser is required due to large aspect ratio. In such ultrashort laser pulses the duration of pulse is of the order of picoseconds (10−12 s). In such short pulses the ultrashort laser offers a relatively broad spectrum with high peak intensity and the frequency of repetition can be high as well. At short wavelengths and pulse durations, laser beam-material interaction takes place at a shallow absorption depth. This helps in drafting small features and drill miniature holes of good quality, especially with pulse duration of the order of 100 ns or less. With this configuration, the peak power can be ranging above 1 kW. It is pertinent to mention that the dominant transport mechanism in laser drilling process can be purely thermal or photochemical. This depends primarily on the wavelength of the laser beam. In the UV range (shorter wavelengths), higher photon energy can be obtained leading to photochemical mechanism. While in infrared range (longer wavelengths), the dominant mechanism is thermal reaction due to low photon energy. In short-range/UV lasers, two types of laser-material interaction take place. One is photochemical ablation and other is the photothermal ablation. Critical breaking energy for bond depends on a threshold of photon energy or critical wavelength. For instance, the average binding energy of 347 kJ/mol is present between one CC bond and for CH bond the binding energy is 414 kJ/mol. To achieve such energies, required photon energies range between 3.6014 and 4.2967 eV, or in terms of wavelength such energy can be achieved between 344 and 288 nm [39]. The shortest wavelengths and pulse durations can be generated by excimer lasers but its application is generally restricted to poor beam quality and often need to be used in combination with mask projection. In several materials including unidirectional laminates, HAZ of as low as 5–30 μm can be observed at this range of wavelength and pulse durations [20]. In this range the photons carry very high energy and can overcome bond energy easily while ensuring smooth edges. However, such processing parameters are combined with low repetition frequency resulting in slower material removal rates and thus not preferred in industrial processes.

7.5.1 Nanosecond laser pulses Nanosecond laser pulses, though not considered as ultrafast laser pulses, are used for the purpose of drilling by some authors [40–42]. For drilling in GFRP and CFRP, Yalukova et al. [39] used three different types of laser wavelengths (see Table 7.1).

Laser drilling of composite material: A review95

Table 7.1  Laser specifications in drilling of GFRP and CFRP using nanosecond lasers [39]

λ (nm)

Pulse energy (mJ)

Average power (W)

Spot diameter (μm)

Irradiance (kW/cm2)

Photon energy (eV)

IR: 1064 Green: 532 UV: 266

4.8 2.8 0.5

4.8 2.8 0.5

60 40 20

170 223 159

1.165 2.331 4.662

Epoxy was used as thermoset material for the experiments. This can be influenced to various extents by the wavelengths in infrared (λ=1064 nm), visible (λ=532 nm), and ultraviolet regions (λ=266 nm). At infrared region, experiments indicate epoxy ­burning and thermal damage of glass and carbon fibers. On the other hand, by irradiating both composites in visible range the influence of thermal breakage was significantly reduced as no visible signs of burning were observed. However, some cracks appeared in epoxy matrix as it is greatly influenced by thermal and/or mechanical stress. While for UV range beam, better quality holes were drilled owing to better photochemical decomposition of the epoxy matrix and can overcome the bond energy for both bonds (CC and CH covalent bonds). Moreover, minimum burring of edges of holes was observed in UV range while maximum thermal deterioration is seen at infrared range.

7.5.2 Picosecond laser pulses For materials with high hardness, picosecond laser offers a viable machining tool ­[43–45]. For composites such as CFRP, which includes constituents of varying physical and thermal properties, laser machining is quite challenging as it results in extended HAZ. For such cases, ultrashort pulsed picosecond laser makes machining possible with improved quality. In C/SiC composites, Liu et al. [46] and Zhang et al. [47] investigated the influence of processing parameters using picosecond laser. The processing parameters include energy intensity, scanning speed, width and spacing of helical line, and machining time, while surface morphology of machined microholes were analyzed. It was noted that two parameters, that is, energy density and feed speed had the most influence on the drilled microholes in terms of its shape and depth. The morphology of the drilled hole in terms of exit side cross section is compared with the entry side. Taper in the holes were observed to increase with machining time but upon a threshold value of machining time the taper size seemed to stay unchanged. This was linked with the depleted laser energy by the debris and the sputtering from the microhole in the presence of plasma at prolonged durations of time. The influence of scanning speed was apparent on the hole depth which increased up to 700 μm with the increase in speed. However, afterwards the depth of hole did not increase owing to the reduction in laser energy in the presence of large debris/nanoparticles and shielded plasma. Moreover, debris primarily of C, Si, and O was observed on the machined surfaces. The bonding

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Hole-Making and Drilling Technology for Composites

between Si-C in the matrix was resultantly changed to SiO bond after the machining process. It is estimated that around two photons of photoenergy were necessary to break the covalent bond in C/SiC composites. The resultant crystalline structure is distorted and the formation of free atoms is observed after bond rupture. At this energy, the free atom plasma ejected from the sides of the holes. In other study, Li et al. [30] investigated the effect of picosecond laser irradiance on different SiC/SiC composites and demonstrated that good-quality circular holes can be formed through helical line modes. It was observed that 10 W was the most suitable processing power of laser owing to the considerations of material oxidation and geometrical size of holes. For larger size holes, higher quantities of debris and nanoparticles were observed in the machining zone at processing power ranging between 4 and 12 W. At low range of power, larger quantities of debris were observed to be attached to the walls of machined hole. It was primarily due to the incomplete oxidation of material for lack of laser energy. The resultant pressures at this power setting were not enough to eject the debris from the hole, resultantly particles accumulated at the walls of hole and adsorb more and more incident energy of laser thereby effecting machining process considerably. Furthermore, pits and pores of varying sizes were seen in and on the sides of the hole which reduced the oxidation resistance of composites thereby causing more damage. In another experimental study involving multiple-ring material removal in CFRP drilling employing 400 W picoseconds laser, Salama et al. [33] found that HAZ and ablation depths decrease with reduction in laser power and increase in the scanning speed. Evidently less than 25 μm HAZ were observed on the top side of 6-mm-diameter hole. The procedure was completed at a scanning speed of 2 m/s in a sample with 6 mm thickness. On the whole no apparent marks of HAZ were seen other than on the top surface. This observed HAZ at the entrance of hole was measured to be much smaller than the previously reported results employing similar laser setups [38]. In a separate study, Wang et al. [48] used picosecond laser for machining circular and square holes. It was noted that depth of holes reduced with an increase in the laser scanning speed, while hole diameter showed unappreciable change. The edges and bottom sides of the circular and square holes were flooded with small particles. Moreover, due to composite heterogeneity the bottom of square holes was found to be cone-shaped cell with waviness.

7.5.3 Femtosecond laser pulses Femtosecond laser pulses were investigated for machining in CFRP by Moreno et al. [37]. Extensive analysis revealed that the shape and dimensions of filling material (either carbon black or carbon fiber) caused geometrical deformations of drilled holes. Relatively speaking, carbon black filled polymers were observed to have better hole topology when drilled using femtosecond intense pulsed lasers. On the other hand, irregular shape, waviness, and other defects were observed in microholes drilled in CFRPs. It was concluded that fiber dimensions and preferential ablation of the polymer was the main reasons.

Laser drilling of composite material: A review97 Laser beam

Refracted beam

Converging lens Fluid Composite sample

(A)

Refraction Air Fluid

(B)

Fig. 7.4  (A) Schematic diagram of underwater laser drilling, and (B) its effect on beam spot size due to refraction at air-fluid interface [49].

7.6 Effects of underwater laser drilling Numerous studies involving materials undergoing laser machining, while submerged in water have been extensively studied such as, aluminum [49], steel [50], silicon carbide [51], and alumina [52]. However, to the best of the knowledge of authors, no such study is found in any of the publically available literature for underwater laser drilling of metallic and/or nonmetallic composites. Fig. 7.4 schematically shows the underwater laser drilling process on a material and the influence of water as media on the beam. Water being denser medium, refraction at the air-fluid interface causes reduction in spot size compared to air. In addition to beam size variation, the influence of stagnant and moving fluid also needs consideration. In a stagnant medium, the debris/particles formed in the process can partially or fully block the beam causing hole defects, while the moving fluid, in parallel or angled flow directions, can remove the debris/particles due to turbulence at the interface. But this resultantly causes irregular beam spot size.

7.7 Conclusions In this chapter, percussion, trepanning, and multiple-ring trepanning methods used in laser drilling are discussed. To mitigate the unwanted drilling-induced delamination, the use of ultrashort laser pulses for composite material drilling and their effects on hole properties are reviewed. This chapter provides a basis for future research in underwater laser drilling of composites.

Acknowledgment This work was supported by Universiti Malaysia Sarawak (UNIMAS) under Grant F02/ SpSTG/1567/2017.

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Drilling of glass fiber reinforced plastics (GFRPs): An experimental investigation and finite element study

8

S. Prakash*, P.V. Siva Teja†, J. Lilly Mercy‡, A.B. Abdullah§ *School of Mechanical Engineering, Sathyabama Institute of Science and Technology, Chennai, India, †Department of Mechanical Engineering, Dhanekula Institute of Engineering and Technology, Vijayawada, India, ‡Department of Mechanical and Production Engineering, Sathyabama University, Chennai, India, §School of Mechanical Engineering, Engineering Campus, University Sains Malaysia, Penang, Malaysia

8.1 Introduction Machining of composites is a major concern in modern-day manufacturing industries due to the non-homogeneity of their material properties [1]. Fiber reinforced plastics (FRPs) played a vital role during post-World War II in submarine parts, aircraft components, and so on. Since 1990, the thrust force and cutting force in drilling of FRP composites have been modeled and validated [2, 3]. DiPaolo et al. [4] proposed that the properties of woven FRP composites are more challenging for numerical modeling compared to their unidirectional counterparts with the same number of materials and binding matrix. In 1996 they took video images through higher-speed cameras at the onset of delamination to report the variation of thrust force and torque with change in fiber angles for unidirectional composites. Hocheng and Tsao [5] analyzed delamination in unidirectional carbon fiber composite using different types of drills (twist drill, saw drill, and candlestick drills) and concluded that the candlestick drill produced the least delamination. Rubio et al. [6] studied the effects of high-speed drilling of GFRP composites and introduced the model for adjusted delamination factor, which is considered as a dimensionless factor compensating to the total extent of delamination along the periphery. Palanikumar et  al. [7] studied in detail the effect of cutting parameters, such as spindle speed and feed rate, and their interactions, on drilling GFRP composites. Gopinath and Suresh [8] conducted static and dynamic analysis to find thrust force and torque in drilling of FRP composites. Simulating the drilling process is a method that can be used to amend drill geometry and process parameters in order to control hole quality and analyze drill wear evolution [9]. Compared with analytical methods, finite element analysis (FEA) has many advantages; by FEA, one can obtain not only

Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00008-8 © 2019 Elsevier Ltd. All rights reserved.

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variables like thrust force and chip shape, but also the specific stress and temperature distribution at an interested point [10]. Many FE studies have been conducted to predict cutting forces and delamination of composites. FEA allows for controlling all variables of the machining process and uncoupling the influencing parameters. Excellent reviews on composite machining can be found in Refs. [11–13]. There are a few works focusing on numerical modeling of composite drilling. Most literatures presentin numerical modeling of cutting FRP composites focus on orthogonal cutting due to its simplicity. Arula and Ramulu proposed FEA of orthogonal cutting of FRC using maximum stress and Tsai-Hill criteria [14]. Drilling of composite materials has been simulated as three-dimensional (3D), orthogonal cutting using FEA. Recently, numerical predictions of critical thrust force and delamination have been performed in drilling of carbon fiber reinforced plastics (CFRPs) [15–17]. Janarthanan and Nagarajan [18] conducted failure theory in GFRP and found their results to be in good agreement with the FEA results. Sadat et al. [19] used FEA analysis in 1992 to predict the load causing delamination in quasi-isotropic, graphite epoxy laminate composites. Budan and Vijayarangan [20] conducted an FEA of the drilling process to predict the effects of the drilling parameters and fiber volume fraction on surface finish, hole quality, and delamination. The failure results gave a clear idea of the damage zone resulting from the drilling operation. Durao et al. [21] developed a cohesive model in order to simulate thrust force and delamination during drilling of CFRP composites. The FE model was validated with the analytical model based on linear elastic fracture mechanics. Zitoune and Collomet [22] proposed a numerical FE method to calculate the thrust force responsible for the defect at the exit of the hole during drilling in carbon fiber composites. Their numerical results strongly agreed with the experimental values. Rahme et al. [23] developed an FE model to determine the critical thrust force for delamination using a failure mechanics approach. Bhattacharyya and Horrigan [24] developed an FE model to analyze drilling behavior using LUSAS software. In accordance with the experimental results, the FE model predicted a lower value of delamination load compared with that predicted by the model, which ignored the shearing action. Durao et al. [16] studied delamination during drilling of CFRP laminated composites using the FE method. They considered two different, simplified drill-point geometries: twist drill and C-shape drill. They observed that the FE model was not able to evaluate the effect of the operating parameters (cutting speed and feed rate) on thrust force and torque. Singh et al. [25] developed an FE model for predicting the drilling characteristics of UD-GFRP composite laminates. Vengatesan et al. [26] conducted an experiment and analysis on different orientations of GFRP composite laminate specimens at specified cutting and material parameters using ANSYS-14.5 [27]. Drilling thrust as the machining response as push-out delamination is primarily responsible for catastrophic failure of the composites. Akhil et al. used FE modeling parameters realistic with the experimental setup and loading and boundary conditions for good prediction. The ANOVA result reveals that the cutting speed is the most significant process parameter that influences delamination factor (DF) and surface roughness (Ra and Rz), respectively [28].

Drilling of glass fiber reinforced plastics

103

For our study, we used Taguchi’s methods for solving the parameter optimization problem. Taguchi’s applications not only provide the solution with the lowest number of experiments but also support a high-quality process and development. Concordantly, the method is not a statistical approach; it is a technique that can be used in all research and development actions that raises quality, reduces cost, and gives firm reliability of results [29]. We conclude that the parameters of thrust force and torque depend on drill type and feed rate. In the present work, we develop an FE model using ANSYS LS-DYNA software. The study focuses mainly on the parameters like speed, feed rate, and type of drill bit, which govern the drilling-induced damage.

8.2 Experimental procedures 8.2.1 Materials and equipment In the present study, woven GFRP composite laminate specimens with a 56% fiber volume ratio were prepared with E-glass fiber using epoxy resin by hand lay-up process. To check the process parameters and their effects on drilling GFRPs we determined thrust force and torque. The experimental study was carried out on a variable feed drive drilling machine with a Kistler 9257B piezoelectric dynamometer and a Kistler 5070A charge amplifier to capture and save outputs. We drilled the 100×50×2 mm woven glass fiber plate with different drill bits, including a solid carbide drill, TiN-coated HSS, and 5 mm HSS as shown in Fig. 8.1.

8.2.2 Experimental factors and levels We used the Taguchi design of an orthogonal experiment in this study. This method reduces dramatically the number of experiments needed. We chose an L27 orthogonal array of three levels and three factors to study the effects of factors and their interactions. Table  8.1 shows the levels and parameters we considered for our experiment.

8.3 Finite element study The theoretical approaches showed a number of facts regarding GFRP composite drilling, but a lot of experimentation still needs to be carried out. The estimation of thrust force and torque largely depends on modelling of work material and drill bit tool. There is no specific model that is used to analyze and understand the drill behavior of GFRP composites. We studied the effect of a drill with three different drill bits using a model designed with CREO 1.0 software as shown in Fig. 8.2. We used for analysis a 20-mm square plate with all the outer boundary planes constrained in all directions, including rotations. The maximum thrust force and torque were taken as the reactions obtained at the boundary nodes in X and Z directions over the steady zone.

104

(A)

(B)

Fig. 8.1  (A) Solid carbide; (B) TiN-coated HSS; (C) 5 mm HSS.

Hole-Making and Drilling Technology for Composites

(C)

Drilling of glass fiber reinforced plastics

105

Table 8.1  Experimental parameters and their levels Levels Sl. No.

Parameters

Units

1

2

3

1 2 3

Speed Feed rate Drill material

Rpm mm/rev 5 mm dia

310 0.07 Solid carbide

455 0.08 TiN coated

600 0.09 HSS

Fig. 8.2  Design of drill bit.

8.3.1 Meshing of drill bit We imported the tool geometry model from a CAD package in IGES format to HYPRMESH 11.0 for meshing. The tool was meshed with a 10-noded tetrahedral element with an average element size of 0.59 mm. A fine mesh was done only at the chisel edge of average dimension 0.032 mm, as shown in Fig. 8.3.

8.3.2 Material constitutive model The analytical model used for the workpiece was orthotropic elastic. The data used for the orthotropic model is shown in Table 8.2. The workpiece was meshed with an eight-noded hexahedral element with an average element size of 5.8 mm. A fine mesh was done only at the center of average dimension 0.074 mm as shown in Fig. 8.4.

8.3.3 Solver methodology We carried out our study using the LS-DYNA 13.0 solver. As the contact type for drill bit and laminated plate is surface to surface (STS) eroding, the loading options were taken on all axes to calculate thrust force and torque. Fig. 8.5 shows the displacement.

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Fig. 8.3  Meshed drill bit.

Drilling of glass fiber reinforced plastics

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Table 8.2  Specifications of GFRP workpiece Elastic modulus (GPa)

Poisson’s ratio

Shear modulus (GPa)

Notation

Value

Notation

Value

Notation

Value

E11 E22 E33

21 21 7

υ12 υ23 υ13

0.26 0.26 0.3

G12 G23 G13

1.52 1.52 2.65

8.4 Results and discussion In the present work, we carried out FE simulations of the drilling process and studied the effect of drill bit and increasing speed and feed rate on thrust force and torque. Figs. 8.6–8.10 show the results of thrust forces for various feed rates and speeds. A constant increase is visible in the thrust force as the feed rate increases from 0.07 mm/ rev to 0.09 mm/rev using experimental as well as analytical results. Fig. 8.6 shows the effect of speed and drill material on thrust force. Drill material 2 (HSS) shows the maximum thrust force, while drill material 3 (TiN-coated HSS) shows the minimum thrust force. Therefore drill material is the most influencing factor. Fig.  8.7 shows the effect of speed and feed rate on thrust force. A feed rate of 0.09 mm/rev shows the maximum thrust force, and a feed rate of 0.07 mm/rev shows the minimum thrust force. Therefore feed rate is the most influencing factor. Fig. 8.8 shows the effect of feed rate and drill material on thrust force. Drill material 2 (HSS) and a feed rate of 0.09 mm/rev shows the maximum thrust force. Therefore drill material and feed rate are the most influencing factors on thrust force. Fig.  8.9 shows the effect of speed and drill material on torque. Drill material 2 (HSS) produces maximum torque, while drill material 3 (TiN-coated HSS) produces minimum torque. Therefore drill material is the most influencing factor. Fig. 8.10 shows the effect of speed and feed rate on torque. A feed rate of 0.09 mm/ rev produces maximum torque, while a feed rate of 0.07 mm/rev produces minimum torque. Therefore feed rate is the most influencing factor. Fig. 8.11 shows the effect of feed rate and drill material on torque. Drill material 2 (HSS) and a feed rate of 0.09 mm/rev produces maximum torque. Therefore drill material and feed rate are the most influencing factors on torque.

8.5 Conclusions The objective of this work is to develop an FE model and to compare the experimental work in order to investigate the drill behavior of GFRPs. We draw the following conclusions based on our study: ●

●

Thrust force and torque determined by FEA showed good results when compared with the experimental results. Feed rate is the most influencing factor in thrust force and torque when drilling GFRP laminates.

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Fig. 8.4  Meshed workpiece.

ANSYS

NODAL SOLUTION

Drilling of glass fiber reinforced plastics

STEP=1 SUB =329 TIME=15.6 USUM (AVG) RSYS=0 DMX =1.95644 SMX =1.95644

Y Z

0

.217183 LS-DYNA user input

.434365

.651548

1. 08591

1. 3031

1. 52028

1. 73746

1. 95464 109

Fig. 8.5  Displacement of the specimen.

.868731

X

110

Hole-Making and Drilling Technology for Composites

Fig. 8.6  Surface plot of speed versus drill material.

Fig. 8.7  Surface plot of speed versus feed rate.

Drilling of glass fiber reinforced plastics

Fig. 8.8  Surface plot of feed rate versus drill material.

Fig. 8.9  Surface plot of speed versus drill material.

111

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Hole-Making and Drilling Technology for Composites

Fig. 8.10  Surface plots of speed versus feed rate.

Fig. 8.11  Surface plot of feed rate versus drill material.

Drilling of glass fiber reinforced plastics ●

●

●

113

The TiN-coated HSS drill bit showed good results when compared to the solid carbide and HSS drill bits. Using FE study will improve the quality of the drilled area, if online monitoring is introduced. Improvement of the drilling process for GFRPs is possible by introducing different variables and a wider range of cutting conditions.

Acknowledgments The authors wish to express their thanks to the Karunya University, Coimbatore, India for supporting the drilling operations.

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[16] L.M.P. Durão, M.F.S.F. de Moura, A.T. Marques, Numerical prediction of delamination onset in carbon/epoxy composites drilling, Eng. Fract. Mech. 75 (9) (2008) 2767–2778. [17] O. Isbilir, E. Ghassemieh, Finite element analysis of drilling of carbon fibre reinforced composites, Appl. Compos. Mater. 19 (3) (2012) 637–656. [18] M.P.  Jenarthanan, K.J.  Nagarajan, Finite element analysis of drilled holes in uni-­ directional composite laminates using failure theories, Am. J. Sci. Technol. 1 (3) (2014) 101–105. [19] A.B. Sadat, W.S. Chan, B.P. Wang, Delamination of graphite/epoxy laminate during drilling operation, J. Energy Resource Technol. 114 (1992) 139–141. [20] D.A. Budan, S. Vijayarangan, Quality assessment and delamination force evaluation in drilling of glass fiber reinforced plastic laminates—a finite element analysis and linear elastic fracture mechanics approach, Proc. Inst. Mech. Eng. B 216 (2002) 173–182. [21] L.M.P. Durao, M.F.S.F. de Moura, A.T. Marques, Numerical simulation of the drilling process on car-bon/epoxy composite laminates, Compos. Part A 37 (9) (2006) 1325–1333. [22] Z. Redouane, C. Francis, Numerical prediction of the thrust force responsible of delaminate during the drilling of the long-fiber composite structures, Compos. Part A 38 (2007) 858–866. [23] R. Rahme, Y. Landon, F. Lachaud, R. Piquet, P. Lagarrigue, Analytical model for composite laminates drilling, Int. J. Adv. Manuf. Technol. 52 (5-8) (2010) 609–917. [24] D. Bhattacharya, D.P.W. Horrigan, A study of hole drilling in Kevlar composites, Compos. Sci. Technol. 58 (2) (1998) 267–283. [25] I.  Singh, N.  Bhatnagar, P.  Viswanath, Drilling of uni-directional glass fiber reinforced plastics: experimental and finite element study, Mater. Des. 29 (2) (2008) 546–553. [26] T. Vengatesan, K. Kavitha, M. Mohanraj, G.P. Rajamani, Study and analysis of drilling in GFRP composites using ANSYS 14.5, Int. J. Innov. Res. Technol. 1 (7) (2014) 170–174. [27] N.D. Chakladar, S.K. Pal, P. Mandal, Drilling of woven glass fiber-reinforced plastic— an experimental and finite element study, Int. J. Adv. Manuf. Technol. 58 (1-4) (2012) 267–278. [28] K.T. Akhil, K. Shunmugesh, S. Aravind, M. Pramodkumar, Optimization of drilling characteristics using grey relational analysis (GRA) in glass fiber reinforced polymer (GFRP), Mater. Today: Proc. Part A 4 (2) (2017) 1812–1819. [29] M.C. Sundarraja, S. Rajamohan, Flexural strengthening effect on RC beams by bonded composite fabrics, J. Reinf. Plast. Compos. 27 (2008) 1497–1513.

Precision punching: A new method in hole-making on composite panels

9

A.B. Abdullah, H.Y. Chan, M.S.M. Zain, N. Ishak, Z. Samad School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia

9.1 Introduction Recently, composite materials are being used to replace certain metals in manufacturing of parts, such as in the aerospace and automotive industries [1, 2]. These materials are preferred because of their light weight, reliability, and strength, especially for complex and high-precision applications [3–5]. Precision composite panel assembly requires quality holes and other criteria to accommodate fasteners such as rivets, pins, bolts, and nuts. In practice, drilling is the main method used to produce holes. Drilling-induced damage, such as spalling, delamination, edge chipping, fiber pullout, crack formation, and excessive tool wear, may affect structural integrity [6–8]. Hence improved methods of producing quality holes are needed to ensure the integrity of fasteners without compromising the advantages of composite strength characteristics. In addition, drilling is considered time-consuming because drilling tools need to be changed frequently [9]. For an aircraft, thousands of holes need to be produced. Punching is another method used to produce holes, particularly on metal. To date, direct punching on composite materials has not been researched. Most of the punching processes used in the sheet metal industry can produce complex profiles and high-­precision holes. However, on composites, this technology is still new, and there are only a few published works on this topic. In addition, the amount of practical work done is relatively insufficient, thus, further experiment-based investigations are needed. Campbell [10] reported the potential of using a punching operation to produce holes. Qiao et al. [11] managed to produce holes using shear punching on metal-based composites. Comparisons of drilling and punching on composite laminate panels have been made based on wear [12]. The holes produced by punching have better quality compared to holes produced by drilling because the puncher tool undergoes less wear. The main objective of this research is to study punching as an alternative to replacement drilling for making holes on composite material. In this study, a simple punching was conducted on a laminated composite panel to examine the effects of various design and process parameters, such as die clearance, puncher profile, multistage puncher, and hole size. Hole quality in terms of hole diameter and complete shearing were among the measurements made at the produced holes. In addition, we measured load pattern and, most importantly, delamination level.

Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00009-X © 2019 Elsevier Ltd. All rights reserved.

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Hole-Making and Drilling Technology for Composites

9.2 Principles of the punching process Punching on composite is the same as punching on other materials. Principally, both drilling and punching create different cutting patterns on laminates; punching will press the material to cause shear according to the edge of die. These conditions tend to affect the integrity of the structure, which results in delamination. The pattern of layers separation compared to drilling is different as in punching the forces are applied downwards. In addition, multi-directional orientation of the fiber may cause incomplete shearing resulting in poor quality hole edges. However, punching will not produce peel-up edge, whereas drilling produces both peel-up and push-down edges. Furthermore, the process is faster and there is less wear compared to drilling. Fig. 9.1 illustrates the basic principles of punching.

9.3 Materials and methods We divide our work into five distinct steps. First, we identified the parameters. Second, we set up the experiment and prepared the materials (e.g., puncher, die material and composite panel). Third, we designed, fabricated, and assembled the die set and test rig. Fourth, we performed the experiment, and finally, we measured the results.

9.3.1 Studied parameters In this study, we used a hybrid laminated composite with 3.4 mm thickness. The material was obtained from industry and made by pre-preg using vacuum bagging and cured using an autoclave. The composite consists of 26 ply of unidirectional carbon fiber and 2 ply of glass fiber at the outer layer.

Puncher Composite panel

Die

Slug

Fig. 9.1  Principle in punching process on composite panel.

Precision punching: A new method in hole-making on composite panels117

9.3.1.1 Die clearance The die clearance was determined based on Eq. (9.1), as suggested in the Handbook of Die Design [13]. Although the equation was developed specifically for metals, the die clearance was set based on this equation because of limited resources. c =K S t

(9.1)

where c is the single die clearance, t is the strip thickness, S is the material shearing strength, and K is the clearance coefficient, whose scale is K=0.008–0.01. The shearing strength can be estimated using S=0.7 UTS. Hence 10 die sets (Fig. 9.1) with different punch diameters (3 mm, 5 mm, and 10 mm) and die clearances (25%, 30%, and 35%) were selected. We selected the punch diameters based on the hole sizes that are most commonly required in composite panel assemblies, whereas we selected the die clearances according to actual industrial applications. We tested at least three specimens for every cutting condition for repeatability. Table 9.1 outlines the experimental design.

9.3.1.2 Puncher profile In this experiment, we fabricated six different puncher profiles: single shear, double shear at 12.5 degrees, 20 degrees, and 30 degrees, conical, and inverted cup (Table 9.2) [14]. Generally, a puncher can be divided into two sections, namely, puncher head and puncher body. A minimum die clearance of 1% was implemented for the die; the puncher diameter was (Ø5.0±0.15 mm) and the die diameter was (Ø5.068±0.182 mm). The punchers were made of tool steel grade D2. For the purpose of this study, the punchers were hardened to 62 HRC.

Table 9.1  Experimental design No.

Punch diameter (mm)

Clearance (%)

Die diameter (mm)

1 2 3 4 5 6 7 8 9 10

3.0 3.0 3.0 5.0 5.0 5.0 10.0 10.0 10.0 10.0

25 30 35 25 30 35 10 25 30 35

4.70 5.04 5.38 6.70 7.04 7.38 10.68 11.70 12.04 12.38

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Hole-Making and Drilling Technology for Composites

Table 9.2  Puncher profiles No.

Puncher profile

1

30-Degree double shear

2

20-Degree double shear

3

12.5-Degree double shear

CAD

Precision punching: A new method in hole-making on composite panels119

Table 9.2  Puncher profiles—cont'd No.

Puncher profile

4

Conical

5

Inverted cup

6

Single shear

CAD

120

Hole-Making and Drilling Technology for Composites

9.3.2 Experimental setup A laboratory die rig is as shown in Fig. 9.2. The die was placed on an Instron 3367 UTM [15] with a punch travel speed of 5 mm/s. As the punch traveled downward, the initial action was clamping the composite panel. The bottom die was pressed by the top die to generate a holding pressure on the panel. The punch proceeded further to cut the composite panel successively. Three samples were collected for each parameter set in the experimental design.

9.3.3 Quality assessment We evaluated hole quality based on the captured image. First, the image of the specimen needs to be the captured and then analyzed using Alicona IFM. As a result, two different color patterns represent topography (rise or sunken) of the surface resulting from the punching process. Second, a straight line is drawn using available features within the Alicona IFM as shown in Fig. 9.3A; the profile can be seen in the Fig. 9.3B. The gained profile clearly shows the affected and non-affected area around the hole due to the punching process. The affected and non-affected zone is based on the risen profile relative to the flat section.

9.3.3.1 Top and bottom surface diameters First, we performed calibration using a ruler along the sample, where the images were captured. Second, we manually illustrated the perimeter of the surface diameter of the holes. Finally, we generated and measured the value of the top or bottom surface diameter. It is required to measure both surfaces, since it may have different size.

9.3.3.2 Shearing edge We assessed shearing edge quality based on completeness of the sheared cutting edge. First, we performed calibration using a ruler along the image samples collected. Punch(s)

Restoring spring

Composite panel

Bottom die(s)

(A) Fig. 9.2  (A) laboratory die rig and (B) die set.

(B)

Precision punching: A new method in hole-making on composite panels121

'HODPLQDWHG DUHD

3XQFKHG KROH

$ >FP@ $IIHFWHGDUHD

'HSWK]



8QDIIHFWHGDUHD

8QDIIHFWHGDUHD





 









%



  3DWKOHQJWK,











>PP@

Fig. 9.3  The resulted profile from the scanned hole.

Second, we manually illustrated the perimeter of the surface diameter of the hole. Subsequently, the value of the area illustrated was generated automatically by the operating software. This area value was regarded as hole area A (Fig. 9.4A). Third, we manually illustrated the perimeter of the clean hole area. The value of the illustrated clean hole area, A, was then generated automatically through the operating software. This area value was regarded as clean hole area, Ac (Fig. 9.4B). Finally, we obtained the incomplete shearing ratio from Eq. (9.2) [16]: ratio of incomplete shearing =

A − Ac × 100% A

(9.2)

9.3.3.3 Burr We assessed burr height utilizing the image obtained from Alicona IFM. For this study, we measured only burr from holes produced on an Al-Composite-Al stack.

122

(A)

Hole-Making and Drilling Technology for Composites

(B)

Fig. 9.4  Image of hole (A) hole area, A, and (B) clean hole area, Ac.

9.3.3.4 Delamination We created a new method as an alternative to utilizing 2D images obtained from Alicona IFM to determine delamination. First, we used two parameters to determine delamination: maximum distance of raised area (E), representing the damage area, and hole diameter (Dext and Dint) as can be seen in Fig. 9.5. Second, we computed delamination based on maximum delaminated area, E, to the average diameter of the hole, D. The closest to 1 represents ideal hole quality. Note that the image is obtained from the bottom surface of the composite panel.

9.4 Results and discussion In this study, we measured four main indicators: roundness of the holes, completeness of shearing, burr, and damage level or delamination. The former three measurements relate to quality of the holes, while the latter relates to structural integrity of the holes. In addition, we measured the load required for complete punching. Therefore two parameters were taken into consideration: die clearance and puncher profiles.

9.4.1 Effect of die clearance The effect of die clearance is observed in terms of entrance and exit diameter of the hole, incomplete shearing, and load required for the punching.

9.4.1.1 Top and bottom diameter The values of the top surface diameters with different puncher diameters minimally changed regardless of the difference in the die clearance values. For the Ø10 and

Dext Dint

[cm]

1.85

Depth - z

1.84

1.83

1.82 [mm] 0

1

2

3

4

5

6

7

Path length - I

Fig. 9.5  2D image for delamination.

8

9

10

11

12

Precision punching: A new method in hole-making on composite panels123

E

Hole-Making and Drilling Technology for Composites

Deviation of top surface diameter from punch diameter (%)

124 20 15 10 5 0 –5

Die clearance (%)

10

25

30

35

–0.419935576

–0.212344502

–0.812995191

0.685883457

Ø5mm Punch

–2.368129799

–0.790480074

–0.578825244

Ø3mm Punch

8.817087382

9.698911773

16.49581739

Ø10mm Punch

Fig. 9.6  Effect of die clearance on the deviation of top surface diameter from punch diameter.

Ø5 mm puncher diameters, the top surface diameter values are very close in size to the puncher diameters of between −0.08 and +0.07, and −0.12 mm, respectively. However, for the Ø3-mm puncher, the deviation reaches up to 0.50 mm. Fig. 9.6 shows the deviation of the top surface diameter from the puncher diameter. The trends of the graph suggest that the values of the top surface diameter are close to the Ø10and Ø5-mm puncher diameters. The deviations of the Ø10-mm puncher are between −0.813% and 0.686%. Meanwhile, for the Ø5-mm puncher, the deviations are between −2.368% and −0.579%. However, the same phenomenon is not observed on the Ø3-mm puncher because of the great deviations of between 8.817% and 16.496%. This phenomenon is inapplicable on the Ø3-mm puncher mostly because of the puncher damage after several operations. Hence the top surface diameters of all the samples (except for the Ø3-mm puncher) seem to follow the corresponding puncher diameter values. The Ø3-mm puncher is bent after several punchings. This bending occurs because the ratio of the puncher diameter d to the material thickness t does not satisfy Eq. (9.3), as recommended by Ivana [13]. d = 1.10 minimum t

(9.3)

Eq. (9.3) is used widely when dealing with materials that consist of metals or metal alloys. However, this equation also seems to work for composite panels. Smaller ratios are generally not recommended in shearing operations because of the more complicated manufacturing methods required, and not because smaller holes cannot be produced. The effect of die clearance on the bottom surface diameter is further studied and the trends suggest that the bottom surface diameter increases as the die clearance (die diameter) increases. Based on Fig.  9.7, the deviations between the bottom surface diameter and the die diameter are relatively minimal. The deviations of the ∅10 mm

Deviation of bottom surface diameter from die diameter (%)

Precision punching: A new method in hole-making on composite panels125 1.4 1.2 1 0.8 0.6 0.4 0.2 0 –0.2 –0.4 Die clearance (%) Ø10mm Punch

10

25

30

35

0.172124045

1.034240125

–0.179282084

0.611967384

Ø5mm Punch

0.568952254

0.2909639

1.263439918

Ø3mm Punch

0.491864798

0.295855196

0.391239056

Fig. 9.7  Effect of die clearance on the deviation of bottom surface diameter from die diameter.

puncher lie between −0.179%and 1.034%. For the ∅5mm puncher, the deviations are between 0.291% and 1.263%. Meanwhile, for the ∅3 mm puncher, the deviations are between 0.296% and 0.492%. Hence the bottom surface diameter seems to follow the corresponding die diameter.

9.4.1.2 Complete shearing Complete shearing is measured by considering the ratio of incomplete shearing to hole area. Fig. 9.8 summarizes the effect of die clearance on the incomplete shearing area ratio. The trends suggest that the incomplete shearing area ratio increases at larger die clearance. However, this trend is inapplicable on the Ø10-mm puncher with 30% clearance.

9.4.1.3 Punching load The compressive stress and load increase as the die clearance (die diameter) decreases. Fig. 9.9 shows the effect of die clearance on the punching load. A bigger gap between the die and puncher allows for material to shear and results in less load required.

9.4.2 Effect of puncher profile Similar to the previous section, here we examine the effect of puncher profile on various indicators on the composite panel, including size of the entry and exit diameter of the holes, incomplete shearing, and damage level or delamination.

Hole-Making and Drilling Technology for Composites Ratio of incomplete shearing area (%)

126

100 90 80 70 60 50 40 30 20 10 0

Die clearance (%) Ø10mm Punch

10

25

30

35

17.026

25.304

59.493

33.478

Ø5mm Punch

38.936

77.363

86.441

Ø3mm Punch

16.222

24.135

46.029

Mean maximum compressive load (N)

Fig. 9.8  Ratio of incomplete shearing area. 30,000 25,000 20,000 15,000 10,000 5000 0

Die clearance (%) 10mm Punch

10 28,074.67

25

30

35

21,975.94

21,057.87

19,780.37

5mm Punch

11,691.46

11,152.67

10,259.37

3mm Punch

10,415.08

9490.02

8916.15

Fig. 9.9  Effect of die clearance on maximum compressive load.

9.4.2.1 Top and bottom diameter Based on the observations, almost all the types of punchers have small deviations in diameter. For puncher #2 (20-degree double shear puncher) and puncher #4 (conical puncher), the diameters of their entries are very close to their puncher diameters, that is, 4.98 mm and 5.03 mm, respectively. In contrast, for puncher #1 (30-degree double shear), puncher #3 (12.5-degree double shear), and puncher #5 (inverted cup puncher),

Precision punching: A new method in hole-making on composite panels127

the entry hole diameters deviate more than the puncher diameters, that is, 4.89, 5.12, and 5.41 mm, respectively. The puncher with tip-initiated break prior to punching results in a balanced load distribution. In this way, punching proceeds smoothly and results in improved accuracy of holes. Punchers #1 and #3 have unsuitable degrees of chamfering for this operation. Therefore larger deviation in terms of hole accuracy can be produced. A similar case is observed for puncher #5 with an inverted cup shape (as summarized in Table 9.3). The deviations of the 30-degree double shear punch and conical punch are close to zero, that is, −0.38% and 0.59%, respectively. However, a similar pattern is not found for punchers #1, #3, and #5, where the deviation is relatively higher at −6.21%, 2.52%, and 8.13%, respectively. The negative value indicates that the diameter of the entry hole is less than the diameter of the puncher. This may happen due to larger incomplete cut of the fibers. Note that the tolerance of the puncher diameter is Ø5±0.15 mm. The trend at the exit diameters is slightly different. Table 9.3 summarizes the differences between the exit diameter and the die diameter. Puncher #4 shows a high deviation value of 15.46%, while puncher #2 depicts a lower deviation value of 6.54%. However, the exit hole diameters produced by each type of tool geometry do not tend to follow the die diameter because the value is out of the tolerance of the die diameter, that is, Ø5.068±0.182 mm. Puncher #5 broke during punching indicating that this geometry may not suitable for punching a composite material. As stated in the previous section, puncher geometry also affects hole accuracy. However, in this case, the conical puncher with a sharp edge produces a less accurate hole size compared to the double shear puncher that produced better accuracy. This is due to a larger contact area between the puncher edge and the die edge, result in better shearing capability.

9.4.2.2 Complete shearing The incomplete shearing ratio varies between these punching tools. Laminate composites consist of a multilayer fiber that is arranged at different orientations for each of the alternate layers, commonly between 0 degrees and 90 degrees. Increasing the tapered tip angle from 12.5 degrees to 30 degrees increases the area of incomplete shearing. Therefore net shearing throughout the hole is difficult to obtain. In this study, puncher #3 (12.5-degree double shear) shows a low average value of incomplete shearing of 53.98%. Table 9.4 summarizes the effect of puncher geometry on the completeness of the sheared section. Table 9.3  The deviation between entry and exit diameters of the holes for different types of profiles Types of profiles

Entry diameter (%)

Exit diameter (%)

30-Degree double shear 20-Degree double shear 12.5-Degree double shear Conical Inverted cup shape

6.21 0.38 2.52 0.60 8.14

12.74 6.54 12.42 15.46 –

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Hole-Making and Drilling Technology for Composites

Table 9.4  The result of incomplete shearing and delamination for different types of profiles Types of profiles 30-Degree double shear 20-Degreee double shear 12.5-Degree double shear Conical

Incomplete shearing (%)

Delamination

Punching load (kN)

69.40

1.53

13.51

55.78

1.60

13.75

53.98

1.53

12.55

68.72

1.46

12.12

9.4.2.3 Delamination Delamination is critical for holes because it may affect the structural integrity (i.e., strength) of the panel. Table 9.4 summarizes the effect of puncher geometry on delamination. The trend shows the same pattern seen in the incomplete shearing case. Moreover, the value of the delamination between punchers is slightly different, and different types of geometry yield different results for the composite panel. Puncher #4 (conical puncher) shows the lowest value of delamination, whereas puncher #5 (inverted cup puncher) shows the highest value of delamination. A higher value of delamination factor represents that the damaged area due to punching is higher. Therefore, in terms of delamination, we conclude that puncher #4 is the most suitable puncher for making holes on composite because it produced the least damage. In addition, the value of the delamination of puncher #4 is close to one. Again, the introduction of a sharp tip helps in reducing the affected area as the tip initiates pre-breaking prior to hole punching.

9.4.2.4 Punching load The average values of the maximum load required for complete punching show that the differences in the four types of punchers are slightly high. Moreover, the conical puncher shows the lowest load required to produce holes on the composite, approximately 12.2 kN. For the conical puncher, the existence of the sharp tip helps in initiating pre-breaking prior to the larger penetration of the main puncher. The double shear puncher with a 20-degree tapered edge depicted the highest load, approximately 13.8 kN. Interestingly, the same types of puncher with larger tapered edges resulted in lower load as summarized in Table 9.4. This study is further extended to observe the effect of punching in hole-making by modifying the punching mechanism from single to dual-stage punching. This is based on initial results where multi-stage punching may improve hole quality.

9.4.3 Modification of the punching mechanism The higher load required to precede the punching operation required enlargement of the die clearance. However, too large of a die clearance results in poor hole quality.

Precision punching: A new method in hole-making on composite panels129

Hence we propose a dual-stage punching mechanism [17]. In this trial, we selected two sizes of holes: Ø5 and Ø10 mm. For the Ø5-mm hole, three samples were produced using the ∅4–∅5 mm puncher, as shown in Fig .9.10. Punching began with the ∅4 mm puncher in the first stage followed by the ∅5 mm punch in the second stage. The material is expected to fracture at 2.5 mm or 73.5% of the material thickness. Hence the penetration of the second-stage puncher (∅5 mm puncher) stopped when the puncher reached the fracture zone. The bottom die clearance is at the minimum (i.e., 1%). Similarly for the Ø10-mm hole, three samples were produced for measurement using the ∅5.00–∅10 mm puncher as shown in Fig. 9.11. Punching began with the ∅5 mm puncher in the first stage followed by the ∅10 mm punch in the second stage. Note that the blank holder force, punch velocity and measurement method are similar to those of the previous experiment. The experimental design for the dual-stage punching mechanism is shown in Table 9.5. The experimental results are summarized in Tables 9.6 and 9.7. Results showed the hole produced by the Ø5–Ø10-mm puncher has the lowest difference value (0.084 mm) between the top and bottom diameters compared with the other holes produced by the previous Ø10 mm puncher. In addition, this hole has the lowest amount of incomplete shearing with a value of 12.25%. Through the dual-stage punching, the die clearance value can be reduced greatly without causing or increasing the load required. The hole produced by the Ø5–Ø10-mm puncher requires a lower load of 25.4 kN even at minimal die clearance (1%). A similar observation is seen in the Ø4–Ø5-mm puncher. The hole produced by the Ø4–Ø5-mm puncher has the lowest difference value (0.08 mm) between the top and bottom diameters compared with the other hole produced by the previous Ø5 mm puncher. Moreover, this hole has the lowest amount of incomplete shearing with a value of 31.52%. The hole requires the lowest load, 8.74 kN, even at the lowest die clearance value (1%). For the dual-stage Ø4–Ø5 mm puncher, the maximum load occurred when the first puncher penetrated the composite panel. The first puncher produced a higher compressive load compared with the second puncher because the first puncher sheared off a higher proportion of material than the second

15

Ø35

90

65

Ø10

10

Ø5

Fig. 9.10  Detail drawing of ∅5 mm and ∅10 mm puncher.

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Hole-Making and Drilling Technology for Composites

70

100

15

Ø35

Ø10

5

10

Ø5 Ø4

Fig. 9.11  Detail drawing of ∅4 mm and ∅5 mm dual-stage puncher. Table 9.5  Experimental design for dual-stage puncher No.

Punch diameter (mm)

Clearance (%)

Die diameter (mm)

1

5.00 (1st stage punch)–10.00 (2nd stage punch) 4.00 (1st stage punch)–5.00 (2nd stage punch)

1

10.068

1

5.068

2

Table 9.6  Observations for dual-stage Ø5–Ø10 mm punching mechanism

Ø5–Ø10 mm Punch (die clearance 1%) Ø10 mm Punch (die clearance 25%) Ø10 mm Punch (die clearance 30%) Ø10 mm Punch (die clearance 35%)

Difference between entry and exit diameter (mm)

Incomplete shearing (%)

Punching load (kN)

0.08

12.25

25.405

1.84

25.30

21.976

2.10

59.49

21.058

2.38

33.47

19.780

puncher. However, for the dual-stage Ø5–Ø10 mm puncher, the maximum load occurred when the second puncher penetrated the composite panel. Fig. 9.12 shows the load pattern of the dual-stage punching mechanism as described previously. Results showed the load required in dual-stage punching is much less compared to the load required in single punching (Tables 9.6 and 9.7).

Precision punching: A new method in hole-making on composite panels131

Table 9.7  Observations for dual-stage Ø4–Ø5 mm punching mechanism

Ø4–Ø5 mm Punch (die clearance 1%) Ø5 mm Punch (die clearance 25%) Ø5 mm Punch (die clearance 30%) Ø5 mm Punch (die clearance 35%)

Difference between entry and exit diameter (mm)

Incomplete shearing (%)

0.08

31.52

8.744

1.85

38.93

11.691

2.10

77.42

11.153

2.50

86.44

10.259

Stress (MPa)

500

1st stage punch

Punching load (kN)

2nd stage punch

400 300 300 100 0 0.0

1.0

2.0

3.0

Strain (mm/mm)

Fig. 9.12  The load changes for the dual-stage puncher.

9.5 Conclusion In this chapter, we discussed the effect of a few parameters on punched hole quality and integrity. Specifically, we studied two parameters: die clearance and puncher profiles. Results showed that die clearance does not significantly affect entry hole diameter. Exit hole diameter tends to be close in size to die diameter. The amount of incomplete shearing decreased as the die clearance decreased. For the same puncher diameter, the required load increased as the die clearance decreased. For the same clearance, the puncher with a larger diameter had higher punching load. To produce Ø10 and Ø5 mm holes with favorable surface cut quality and dimensional accuracy, the bottom die clearance value needed to be minimal because the bottom surface diameter of a hole tended to be close in size to the bottom die diameter. We also studied the effect of puncher profiles on the holes produced. Based on the observations, puncher #2 (20-degree double shear) achieved the best hole quality in terms of the accuracy of the entry diameter, which is closest to the punch diameter, and the exit diameter compared with the die diameter. Regarding completeness of shearing, results show that the double shear-type puncher yields the best shearing quality. Furthermore, the conical puncher yields the lowest delamination. Notably,

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we observed a relationship between the load required and delamination. Overall, the conical puncher is the best option among the proposed puncher profiles. We also evaluated the performance of the punching operation after making modifications to the punching mechanism, introducing dual-stage punching. The dual-stage punching mechanism proved to reduce the load required to make holes on the composite panel.

Acknowledgment The authors want to acknowledge Universiti Sains Malaysia for their sponsorship through Research University Grant (Acc. No. 1001/PMEKANIK/814270). For Mr. Fakhrul, who helps in conducting the experiments.

References [1] E.C. Botelho, R.A. Silva, L.C. Pardini, M.C. Rezende, A review on the development and properties of continuous fiber/epoxy/aluminum hybrid composites for aircraft structures, Mater. Res. 9 (3) (2006) 247–256. [2] T. Edward, Composite materials revolutionize aerospace engineering, Ingenia 36 (2008) 24–28. [3] E.D. Eneyew, M. Ramulu, Experimental study of surface quality and damage when drilling unidirectional CFRP composites, J. Mater. Res. Technol. 3 (4) (2014) 354–362. [4] O. Isbilir, E. Ghassemieh, Finite element analysis of drilling of carbon fibre reinforced composites, Appl. Compos. Mater. 19 (3-4) (2012) 637–656. [5] V.A. Phadnis, A. Roy, V.V. Silberschmidt, Finite element analysis of drilling in carbon fiber reinforced polymer composites. J. Phys. Conf. Ser. 382 (2014): 012014, https://doi. org/10.1088/1742-6596/382/1/012014. [6] I.S. Shyha, S.L. Soo, D.K. Aspinwall, S. Bradley, R. Perry, P. Harden, S. Dawson, Hole quality assessment following drilling of metallic-composite stacks, Int. J. Mach. Tools Manuf. 51 (7–8) (2011) 569–578. [7] S.  Arul, L.  Vijayaraghavan, S.K.  Malhotra, R.  Krishnamurthy, The effect of vibratory drilling on hole quality in polymeric composites, Int. J. Mach. Tools Manuf. 46 (3–4) (2006) 252–259. [8] Kim, D., Kwon, P., Lantrip, J., Beal, A., Park, K.H. Tool Wear and Hole Quality in Drilling of Composite/Titanium Stacks With Carbide and PCD Tools. SAE Technical Paper 2010-01-1868, doi: https://doi.org/10.4271/2010-01-1868, 2010. [9] M.P.D.  Luis, M.R.S.T.  Joao, G.M.  Antonio, T.M.  Antonio, P.M.B.  Antonio, Damage analysis of carbon/epoxy plates after drilling, Int. J. Mater. Prod. Technol. 32 (2/3) (2008) 226–242. [10] F.C.  Campbell, Composite Materials, ASM International, Metals Park, Ohio, USA, 2010. [11] J.W. Qiao, H.Y. Ye, H.J. Yang, W. Liang, B.S. Xu, P.K. Liaw, M.W. Chen, Dynamic shear punching of metallic glass matrix composites, Intermetallics 36 (2013) 31–35. [12] A.B. Abdullah, N.A. Ghaffar, Z. Samad, Precision hole making on laminates composite: comparison between drilling and punching, Mater. Sci. Forum 857 (2016) 291–295. [13] S. Ivana, Handbook of Die Design, second ed., 2006. 0071462716.

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[14] M.S.M.  Zain, A.B.  Abdullah, Z.  Samad, Effect of puncher profile on the precision of punched holes on composite panels, Int. J. Adv. Manuf. Technol. 89 (9–12) (2017) 3331–3336. [15] Instron Corporation, Instron Series 3300 Load Frames Including Series 3340, 3360, 3380, in Reference Manual-Equipment, I. Corporation, ed., 2004. [16] H.Y. Chan, A.B. Abdullah, Z. Samad, Precision punching of hole on composites panel, Ind. J. Eng. Mater. Sci. 22 (2015) 641–651. [17] H.Y.  Chan, A.B.  Abdullah, Z.  Samad, M.S.M.  Zain, Precision punching on laminates composite panel: effect of dual-stages puncher, Int. J. Mater. Eng. Innov. 6 (4) (2015) 288–296.

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10

M.H. Hassan, J. Abdullah School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Malaysia

10.1 Introduction Carbon fiber reinforced composites are widely used as parts of wing structure, fuselage, and other secondary parts in commercial aircrafts like the Airbus A350 and Boeing 787. This is due to their high strength-to-weight ratio as compared to their metallic counterparts [1, 2]. Composites are used in aircraft components as the skin encapsulating the metal structure. The composite skins are assembled with the metal structure by fastener, which requires drilling of holes in a single shot on stacked composites and metal (usually aluminum). Drilling is one of the most crucial processes in composite manufacturing of aircraft panels and structure because it directly affects the aircraft’s flight performance and service life. As such, it has received a lot of research interest over the last few decades [3–5]. Achieving a minimum difference in diameter between stack-up, a minimum hole surface roughness, and a minimum burr height formation that fulfills the stringent requirements for aircraft components in drilling of composites and metal stack-ups, including carbon fiber reinforced composites (aluminum, titanium, or a combination of both), is a challenging task due to the different machining characteristics between carbon fiber reinforced composites and metal. The major challenge in drilling a stack-up of composites and metal is their inherently different machining characteristics. Improper machine set-up normally yields poor-quality holes in terms of roundness, surface roughness, chipping, and so on, thus leading to rejection. Hence a basic understanding of the interaction between the drill’s cutting edge and the drilled material is important to avoid defects on the wall of the drilled hole [6, 7]. Single-shot drilling of stacked composites and metal is the preferred technique for minimizing positioning error and process time. However, single-shot drilling of the stacked carbon fiber reinforced composites/aluminum requires a cutting tool suitable for both types of materials. The special geometry design of the composite drills, such as spur drills or straight flute drills, reported in previous research are not suitable for drilling stack-up material, especially when the stack-up consists of a metal part. Most previous researchers applied the normal design of twist drills [8–15] when drilling composite and metal stack-up. In a different drilling process, the application of step drills [8, 14] had also been studied. For the type of cutting tools material, Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00010-6 © 2019 Elsevier Ltd. All rights reserved.

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most ­researchers [3–5, 16] used tungsten carbide because of its high strength and wear resistance compared with high-speed steel (HSS) or high-speed steel with cobalt (HSS-Co) tools in order to produce good hole quality especially at the composite part. In single-shot drilling process, cutting conditions between the composite and metals are different in terms of chip formation after the drilling process. The better machining parameters for metals is discontinuous chip formation, while for composites a continuous chip is required to produce better hole surface roughness [17]. However, it is difficult to produce continuous chip formation in the composite part since its construction is made of layers that create dust-type chips [12].

10.2 Current practice in drilling of stacked material (multistep process) To achieve good hole quality using manual processes, a drill jig was used. Each manual drill template consists of a machined aluminum plate with various drills and fixture bushings. The jigs are located on the workbenches and because of the location of the drilling template, drilling is not performed in a good ergonomic position. Manual drilling technology is not suited for accurately and efficiently cutting tight holes. One hole requires as many as four drilling steps to achieve its final size. Fig. 10.1 shows the steps of the manual drilling process from hole sizes 4.81 to 7.92 mm. The hole is started with a pilot hole and eventually stepped up to the final size. The hole is completed with a final reaming operation. To be cost-effective, a metallic part is normally drilled with HSS or HSS-Co. This is because the material is cheaper than tungsten carbide and polycrystalline diamond (PCD) resulting in less cost per hole. However, for composite materials, tungsten carbide is a suitable material especially for manual applications. PCD is the best material to drill composite laminate, however, it is suitable for the control feed and automatic application.

10.3 Hole quality issues in single-shot drilling process of stacked material Fig. 10.2 illustrates the common hole defects that normally occur during drilling a stack-up material. Drilled hole quality in the aircraft industry can be defined based on hole roundness or circularity, hole wall surface roughness, hole diameter difference between materials, and burr height formation. These four measurements are critical and need to be controlled according to the customer’s specification. Poor hole surface roughness may contribute to stress formation leading to damage of the rivet joints during the assembly process. Moreover, a high difference in diameter between stack materials and burr height formation would interrupt the assembly process and increase the amount of scrap from the panel due to it being out of the customer’s specifications.

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Fig. 10.1  Multistep process from pilot hole (4.81 mm) until final size (7.92 mm).

10.3.1 Hole diameter error between materials The main problem during drilling of stack-up material is the difference in tolerances between the materials being too tight. This is due to the different material properties, especially the modulus of elasticity of the materials, which contribute to different elastic deformation making it hard to control the diameter between the stack-up materials [18, 19]. If the hole in one material of the stack is undersized (hole diametertool diameter), a repair process needs to be implemented, and that process generally adds extra time and costs to the assembly process. Fig.  10.3 shows the actual situation when drilling a stack-up material in a single-shot process and the ideal situation with the same diameters after applying a reaming process. Normally, the composite plate is always undersized compared to the metal parts (titanium and aluminum) due to the fiber shrinkage of composites after the drilling process. For a metallic part, the high temperature during the drilling process transfers to the material and expands the hole diameter. Oversized holes can easily occur after drilling if the drilling is performed in dry conditions.

10.3.2 Hole surface roughness Batzer et al. [20] reported the parameters that affect hole surface finish. They found that the machining parameters of material to be drilled and drill type significantly

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Fig. 10.2  Common defect during drilling stack-up material [10].

Single-shot titanium/carbon fiber reinforced composites/aluminum stacks holes drilling139

Actual situation with different diameter

Ideal situation with same diameter

Composite

Composite

Titanium

Titanium

Aluminum

Aluminum

Fig. 10.3  Schematic diagram hole diameter error between stack-up materials.

contribute to hole surface finish. Kim and Ramulu [9, 21] proved that in order to achieve acceptable hole quality during the drilling of Gr/Bi-Ti material, the suitable cutter material is tungsten carbide used in drilling at a lower cutting speed and feed rate. Ramulu et al. [11] and Kim et al. [9] revealed that increasing the feed rate may reduce the entrance and exit burr height on drilling graphite-bismaleimide/Ti stack-up material and increase the tool life span due to shorter tool engagement and lower cutting temperature. In a different study, an increase in the feed rate increased the surface roughness especially for the composite panel during drilling stack-up materials [9]. However, when increasing the cutting speed, the surface roughness was reduced in drilling of a graphite-bismaleimide/Ti stack due to the shorter tool engagement and lower machining temperature. Fig. 10.4 illustrates the cross-section view of a drilled hole for titanium/carbon fiber reinforced composites/aluminum stack materials. It clearly indicates that at the same drilling parameters, the titanium material has higher surface roughness compared with the aluminum material due to the effect of the feed marks combined with smearing [10]. The hole surface quality for drilling stack-up material may improve when drilled with adapted step drills [14]. In drilling a composite panel, continuous chips are produced at a low feed rate and become dust-like chips with increased feed rate. However, aluminum chips are continuous when the cutting speed is high and the feed rate is less [21]. When aluminum is stacked at the bottom of carbon fiber reinforced composites, improper cutting parameters will produce a continuous and high-temperature chip passing through the carbon fiber reinforced composite hole. Zitoune et al. [12] studied the influence of cutting conditions and tool diameter on thrust force measurement when drilling stack-up carbon fiber reinforced composite and aluminum plate. In their research, a discontinuous chip was generated at a high cutting speed and lower feed rate. Their findings are supported by Rawat et al. [22] and Batzer et al. [20] who found that the chip would deteriorate the carbon fiber reinforced composite drilled hole diameter and surface roughness [12]. Fig. 10.5 shows the mechanism of aluminum chips would obstruct the composite surface roughness during the chip evacuation. The long, hot, and sharp metal chips rotated with the drill body, causing damage to the composite hole wall and affecting the surface finish during evacuation from the hole. To alleviate the continuous chip formation, proper utilization of machining

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Fig. 10.4  Cross-section of stack-up materials; (A) titanium layer cross-section, (B) feed mark on aluminum layer, (C) chip at the entrance edge of titanium, (D) entry burr between the CFRP and aluminum plate [10].

Single-shot titanium/carbon fiber reinforced composites/aluminum stacks holes drilling141

Aluminum continues chip

Damages located on the wall of the hole Damage of the first ply

0.5 mm

(A)

(B)

Fig. 10.5  Aluminum chip that contribute to damages at wall of the hole [12].

p­ arameters at high speed and high feed rate is suggested by some researchers in drilling of stack-up materials [9, 11].

10.3.3 Burr height formation at exit hole Burr height at hole exit of metal parts is also an issue to address. The burr formation induced in drilling is primarily dependent on the tool geometry and the parameters during the drilling process. If the burr height occurs at the exit hole after the drilling process, the deburring process would consume almost 30% of the cost of the full assembly process [23]. The smallest burr height formations at hole edge were reported by Rivero et al. [24] when increasing the feed rate during the drilling process. Normally, the drilling-induced metallic part burrs can be categorized into three types: type A (uniform), type B (transient), and type C (crown). Burrs are classified according to the location of the initiated crack as illustrated in Fig. 10.6. Burr type A was formed with a very small and uniform burr formation, while burr type C typically had a severe roller-back shape and crown shape due to initiation of the crack at the drill point of the cutting edge. Burr type B was produced as a result of some degree of plastic deformation; this type is determined by the burr height. Hassan et  al. [25] discovered the effect of twist drill geometry and drilling parameters on burr type formation in single-shot drilling of carbon fiber reinforced composites/aluminum materials. From their observation, a drill with a 130-degree point angle would produce a uniform burr type (type A) and a 110-degree point angle would produce transient (type B) and crown burrs (type C). In order to achieve a uniform and minimum burr height, the drill cap must produce at the exit of the hole. Hence to produce the drill cap, the initial and secondary fracture must occur at different stages.

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Type A

Type B

Crack

(A)

Type C

Crack

(B)

Crack

(C)

Fig. 10.6  Burr type classification; (A) Type A, (B) Type B, and (C) Type C, for drilling a stack material and top view of the hole of aluminum exit [25, 26].

10.4 Benefits and limitations of single-shot drilling process Single-shot drilling is based on machining the material in one drilling operation. Reducing the number of drilling steps improves drilling costs, reduces process time, and reduces the amount of cutting tools needed. The main driver for achieving this reduction of steps is to reduce temperature on the drill bit and to be able to cut the chips. To achieve hole tolerance of 25 μm, it is very important to have small chips, especially when drilling carbon fiber reinforced composite/Al stacks. Otherwise the chips will increase the hole size of the carbon fiber reinforced composites and decrease the hole surface quality. Drilling of stack-up material, such as carbon fiber reinforced composite/Al, carbon fiber reinforced composite/Ti, and Al/carbon fiber reinforced composite/Ti is a challenging task because of different machining properties between the composite and metallic parts. However, only a few studies on this topic have been carried out. Compared with drilling of carbon fiber reinforced composite or glass fiber reinforced composite laminates, drilling of stack-up material has the following limitations: 1. The hole diameter error of a drilled hole between composite and metallic parts is large when drilling stack materials [12, 13]. The difference of material properties between composite and metallic parts causes different material behavior, therefore the diameter in different layer of the drilled hole in the feed direction [10, 27]. The hole diameter error of a drilled hole can be improved by using an optimized drill bit for the stack material. In addition, the effect of

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build-up edge of aluminum or titanium at the cutting edge of the drill bit on the diameter of the hole drilled contributes to premature wear that reduces the cutting tool’s life [15]. 2. High-wear-resistant tool materials are required when drilling a composite, especially carbon fiber reinforced composite with titanium sheets [11, 21]. Extreme tool wear on HSS drill bits occurs rapidly when drilling carbon fiber reinforced composite/Ti stack-up material, even after drilling only one hold. To reduce machining costs, PCD drill bits and tungsten carbide drill bits coated with diamond, TiB2, or C7 were used to drill the stack-up material [10]. 3. The quality of drilled holes in the composite layer deteriorate owing to friction and erosion from hot, sharp, and continuous metallic chips during the evacuation process [12]. Optimizing the drilling parameters and tool geometry will help to avoid this problem [10, 28–30]. 4. It is important to select proper drilling parameters when drilling composite and metallic parts of stack material since they require a different set of drilling parameters because of their dissimilar machinability [9, 21]. To obtain a good hole quality according to customer requirements, an automated machine tool is suggested for drilling a stack material in a ­single-shot drilling process.

The current research on stack-up carbon fiber reinforced composite/Ti drilling is in search of efficient processing techniques capable of producing high quality and excellent surface integrity. To this aim, high-quality drilling of hybrid carbon fiber reinforced composites/Ti composites is a key pursuit in the modern manufacturing community. Due to the disparate natures of carbon fiber reinforced composites and Ti phases, the criteria for high-quality drilling are different with each stacked constituent. For a carbon fiber reinforced composite phase, the criteria require low-extent delamination and low fiber breaking, minimum hole shrinkage, and low surface roughness [12]. For a Ti phase, the criteria are the elimination of burrs and producing an excellent surface finish. The effective approaches to high-quality drilling of carbon fiber reinforced composite/Ti stack typically have close relation with the cutting parameters, cutting tool, and cutting environment. It is estimated that approximately 60% of the rejections of carbon fiber reinforced composites produced in the aerospace industry are caused by the use of improper cutting parameters, non-optimal cutting tools, and unfavorable cutting environments [10]. Improvement in tool geometry design and novel cutting strategies have been proposed such that tool design and cutting strategy adapt to the conditions at every stage of the drilling operation in order to take into account the properties of the material that is being drilled.

10.5 Tool strategies for drilling of stacked material Cutting tools with superior thermo-physical properties often ensure excellent toolwork interaction, provide outstanding resistance against rapid tool wear hence offering the possibility for high-quality drilling of stacked material. When drilling composite layers, the cutting tool suffers severe edge rounding wear and intense flank wear due to machining high-abrasive fibers [22, 31, 32]. While for metallic part drilling, the serious metal chip adhesion coupled with the high, localized temperature concentrated at the tool-chip interface easily results in severe adhesion wear, edge chipping, and tool fracture.

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Hence, to compromise the appropriate drill for stack material, cutting tools with high hardness, high toughness, high wear resistance, good chemical inertness, and high thermal conductivity are strongly preferred. Up to now, a wide range of tool materials, including high-speed steel (HSS), carbide tools, coated tools, and super hard materials like polycrystalline cubic boron nitride (PCBN) or PCD, have been examined in single-shot drilling applications. For tungsten carbide material tools, drills with low cobalt content around 7%–9% wt are recommended for hybrid composite drilling due to their increased tool hardness and expanded abrasion resistance. For coated tools, previous research indicates that only part of them with required properties demonstrated prominent ability to generate excellent surface finish and yield long tool life when cutting hybrid composite stacks [4, 15].

10.5.1 Coating application for tool life extension The suitability level of coated tools for stack-up material drilling greatly depends on the extent of their improvement on the tribological behavior of both tool-composite interaction and tool-metallic part interaction [33]. Fujiwara et al. [34] evaluated different coated tools made of titanium aluminum nitride (TiAlN), titanium silica nitride (TiSiN), and titanium aluminum chromium/titanium silica (TiAlCr/TiSi) coatings when drilling carbon fiber reinforced composite/Ti6Al4V stack materials. Results confirmed that TiAlCr/TiSi coating outperformed the TiAlN and TiSiN coatings due to its superior wear resistance and the ability to reduce chip adhesion in the material removal process. Among the tooling materials, carbides have been found to be an excellent substrate, irrespective of all types of coating application. In s study by Sivarao et al. [35], the application of a TiALN-coated 8 mm diameter twist drill tool caused minimal burr formation compared to a TiN-coated type drill under the same dry drilling conditions and parameters. Furthermore, Kuo et al. [36] performed a comparative experimental work on diamond-like carbon (DLC) and chemical vapor deposition (CVD) methods in order to produce high quality of drilled holes. Their findings showed that the application of CVD diamond-coating had better performance compared to DLC in terms of burr reduction and hole circularity.

10.5.2 Polycrystalline diamond application The PCD tool was primarily reported to have superior cutting performance when machining standard composites due to its high wear resistance and high thermal conductivity [37]. The veined PCD drills modified with K-land design yielded increased tool life and improved hole quality compared to the conventional geometrical PCD drills. When used in stack-up drilling, the PCD coating could also yield excellent wear resistance and effectively alleviate the serious chip adhesion encountered in metal part drilling [38]. PCD tools are commonly preferred for cutting composites because they have high wear resistance against abrasion due to their high hardness [31]. The main problem when drilling a stacked material with metal parts is build-up edge and build-up layer. When drilling metallic parts, the chips produced from drilling

Single-shot titanium/carbon fiber reinforced composites/aluminum stacks holes drilling145

process can easily weld to the cutting edge of the drill thus forming a layer that leads to premature failure of the tool [12, 15]. PCD is one of the materials that consists of high wear resistance, low friction characteristics, and low thermal conductivity that is suitable for drilling stack material with minimum drilling temperature. However, due to PCD’s inherent brittle nature, a PCD drill can easily fracture when drilling a metallic part with high-speed machining. Therefore it is recommended that a PCD drill be operated at a relatively smaller range of cutting parameters compared to tungsten carbide materials.

10.5.3 Customization drill geometry In addition, drill bits with special geometry design are also potentially qualified to conduct high-quality drilling of stacks. The drill geometries are determined by a set of variables including characteristic angle (e.g., point and helix angles), edge geometry (e.g., chisel edge, primary clearance, rake angle, etc.) and tool shape (e.g., twist shape, helical shape, etc.). The excellent performance of special drill bits globally are closely related to the mentioned geometrical variables, which results in good hole quality and longer tool life during the drilling process thus indirectly improving the assembly process and production rate. Qinglong et al. [39] and An et al. [40] compared the tool performances of one standard twist drill and one special drill (“dagger drill”) in drilling of high-strength, carbon fiber reinforced composite. They found that the dagger drill promoted better surface finish (i.e., less burr defect and smaller delamination damage) than the twist drill due to its smaller point angle and helix angle. However, the dagger drill was not preferable for metallic parts because of its poor chip evacuation capability due to its small helix angle. Wika et al. [41] conducted several drilling trials of carbon fiber reinforced composite/Ti stacks by using four different drill bits varying in flute number and helix angle. Results showed that the two-flute drill bit with higher helix angle generated the least cutting force and lowest cutting temperature as compared to other drills due to its large flute volume for chip evacuation and heat dissipation. Senthil Kumar et al. [28] examined the effects of point angle on tool performance when drilling a composite/Ti stack by using 118- and 130-degree point angle drills. It was concluded by evaluating tool wear and chip evacuation that the drills with a higher point angle (130 degrees) outperformed those with a lower point angle (118 degrees). Recently, Kuo et  al. [36] revealed that the two-stage point design for a drill bit could offer improved “self-centering” capability, thereby reducing tool deflection and guaranteeing excellent hole accuracy. Besides, it was also reported that drill bits designed with small chisel-edge width can also promote lower force generation and minimal delamination damage encountered in drilling [26, 42]. Overall, the ideal cutting tool for high-quality drilling of stack-up material should be a good match of proper tool material and optimal tool geometry. From the aspect of tool material, drill bits with high wear resistance, high hardness, and high thermal conductivity are the primary choice. With regard to tool geometry, despite the fact that various research has been done for hybrid composite drilling in the past few decades, most of the work was still

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p­ erformed by simple comparison of special and conventional tools in terms of one or multiple aspects of drilling responses [43]. The tool geometry design was mostly based on the empirical experience rather than the reasonable theoretical criteria. No explicit theoretical explanation was proposed to reveal the intrinsic mechanisms governing tool geometry optimization or improvement. The detailed theoretical standards and criteria for tool geometry design of stack-up material drilling urgently need to be established in the future.

10.6 Summary Drilling composite/metal stacks has usually involved multistep operations to permit the use of drill tools optimized for each material. The single-shot drilling process reportedly reduces tooling cost and improves the assembly process. However, hole damage induced in stack material is a bit challenging since it comprises both the polymeric and metallic defects. The delamination damage in the composite panel and the burr defect in the metallic part are regarded as the key problems that obstruct the assembly process since these two defect types lead to hole tolerance and high rejection of the machined components. To achieve high-quality results, profound expertise, rich experience, and good understanding of tool material, tool geometry, and drilling parameters are required to propose a superior tool configuration for single-shot drilling of stack-up material.

Acknowledgments This work is supported by the RU-I Grant # 1001/PMEKANIK/814288 of Universiti Sains Malaysia. The authors would like to thank the Gandtrack Asia Sdn Bhd, Melaka, Malaysia for the permission to use their facilities and support for this project.

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[7] N. Bhatnagar, D. Nayak, I. Singh, H. Chouhan, P. Mahajan, Determination of machininginduced damage characteristics of fiber reinforced plastic composite laminates, Mater. Manuf. Process. 19 (2004) 1009–1023. [8] R. Neugebauer, U. Ben-Hanan, S. Ihlenfeldt, M. Wabner, A. Stoll, Acoustic emission as a tool for identifying drill position in fiber-reinforced plastic and aluminum stacks, Int. J. Mach. Tools Manuf. 57 (2012) 20–26. [9] D.  Kim, M.  Ramulu, Drilling process optimization for graphite/bismaleimide–titanium alloy stacks, Compos. Struct. 63 (2004) 101–114. [10] I.S. Shyha, S.L. Soo, D.K. Aspinwall, S. Bradley, R. Perry, P. Harden, S. Dawson, Hole quality assessment following drilling of metallic-composite stacks, Int. J. Mach. Tools Manuf. 51 (2011) 569–578. [11] M. Ramulu, T. Branson, D. Kim, A study on the drilling of composite and titanium stacks, Compos. Struct. 54 (2001) 67–77. [12] R. Zitoune, V. Krishnaraj, F. Collombet, Study of drilling of composite material and aluminium stack, Compos. Struct. 92 (2010) 1246–1255. [13] E.  Brinksmeier, S.  Fangmann, R.  Rentsch, Drilling of composites and resulting surface integrity, CIRP Ann. Manuf. Technol. 60 (2011) 57–60. [14] E. Brinksmeier, R. Janssen, Drilling of multi-layer composite materials consisting of carbon fiber reinforced plastics (CFRP), titanium and aluminum alloys, CIRP Ann. Manuf. Technol. 51 (2002) 87–90. [15] R. Zitoune, V. Krishnaraj, B.S. Almabouacif, F. Collombet, M. Sima, A. Jolin, Influence of machining parameters and new nano-coated tool on drilling performance of CFRP/ Aluminium sandwich, Compos. Part B 43 (2012) 1480–1488. [16] J.P.  Davim, P.  Reis, C.C.  Antonio, Experimental study of drilling glass fiber reinforced plastics (GFRP) manufactured by hand lay-up, Compos. Sci. Technol. 64 (2004) 289–297. [17] S. Kalpakjian, S. Schmid, Manufacturing Processes for Engineering Materials, fifth ed., Pearsons, 2007. [18] K. Sakurai, K. Adachi, G. Kawai, T. Sawai, K. Ogawa, High feed rate drilling of aluminum alloy, Mater. Sci. Forum (2000) 625–630. Trans Tech Publications. [19] R.  Hamade, F.  Ismail, A case for aggressive drilling of aluminum, J. Mater. Process. Technol. 166 (2005) 86–97. [20] S. Batzer, D. Haan, P. Rao, W. Olson, J. Sutherland, Chip morphology and hole surface texture in the drilling of cast aluminum alloys, J. Mater. Process. Technol. 79 (1998) 72–78. [21] D. Kim, M. Ramulu, W. Pedersen, Machinability of titanium/graphite hybrid composites in drilling, Trans. NAMRI/SME 33 (2005) 445–452. [22] S. Rawat, H. Attia, Wear mechanisms and tool life management of WC–Co drills during dry high speed drilling of woven carbon fibre composites, Wear 267 (2009) 1022–1030. [23] Avila, M., Gardner, J. D, Reich-Weiser, C., Vijayaraghavan, A., Dornfeld, D. Strategies for Burr Minimization and Cleanability in Aerospace and Automotive Manufacturing. UC Berkeley: Laboratory for Manufacturing and Sustainability. Retrieved from https://escholarship.org/uc/item/9ks6b6dp, 2006 [24] A. Rivero, G. Aramendi, S. Herranz, L.L. de Lacalle, An experimental investigation of the effect of coatings and cutting parameters on the dry drilling performance of aluminium alloys, Int. J. Adv. Manuf. Technol. 28 (2006) 1–11. [25] M.H. Hassan, A. Jamaluddin, M. Abdus Samad, S. Azwan, Burr height as quality indicator in single shot drilling of stacked CFRP/aluminium composite, Key Eng. Mater. 744 (2017) 327–331. [26] J. Xu, A. Mkaddem, M. El Mansori, Recent advances in drilling hybrid FRP/Ti composite: a state-of-the-art review, Compos. Struct. 135 (2016) 316–338.

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[27] C.L. Kuo, S.L. Soo, D.K. Aspinwall, W. Thomas, S. Bradley, D. Pearson, R. M'Saoubi, W. Leahy, The effect of cutting speed and feed rate on hole surface integrity in single-shot drilling of metallic-composite stacks, Proc. CIRP 13 (2014) 405–410. [28] M. SenthilKumar, A. Prabukarthi, V. Krishnaraj, Study on tool wear and chip formation during drilling carbon fiber reinforced polymer (CFRP)/titanium alloy (Ti6Al4V) stacks, Proc. Eng. 64 (2013) 582–592. [29] B. Wang, H. Gao, B. Cao, Y. Zhuang, Z. Zhao, Mechanism of damage generation during drilling of carbon/epoxy composites and titanium alloy stacks, Proc. Inst. Mech. Eng. B J. Eng. Manuf. 228 (2014) 698–706. [30] R. Zitoune, V. Krishnaraj, F. Collombet, S. Le Roux, Experimental and numerical analysis on drilling of carbon fibre reinforced plastic and aluminium stacks, Compos. Struct. 146 (2016) 148–158. [31] K.-H. Park, A. Beal, D. Kim, P. Kwon, J. Lantrip, Tool wear in drilling of composite/titanium stacks using carbide and polycrystalline diamond tools, Wear 271 (2011) 2826–2835. [32] K.-H. Park, A. Beal, D. Kim, P. Kwon, J. Lantrip, A comparative study of carbide tools in drilling of CFRP and CFRP-Ti stacks, J. Manuf. Sci. Eng. 136 (2013) 014501–014509. [33] C.L.  Kuo, S.L.  Soo, D.K.  Aspinwall, C.  Carr, S.  Bradley, R.  M’Saoubi, W.  Leahy, Development of single step drilling technology for multilayer metallic-composite stacks using uncoated and PVD coated carbide tools, J. Manuf. Process. 31 (2018) 286–300. [34] J. Fujiwara, R. Nagaura, T. Tashiro, Drilling of CFRP/Ti6Al4V stack board, Int. J. Autom. Technol. 7 (2013) 426–432. [35] S. Sivarao, Rahim, Taifuk, Ammar, Coating characterization of TiN & TiAIN on burr formation in drilling pragmatic investigation, in: International Conference on Engineering and ICT, 27–28 November 2007, Melaka, 2007. [36] C.L. Kuo, S.L. Soo, D.K. Aspinwall, S. Bradley, W. Thomas, R. M’Saoubi, D. Pearson, W.  Leahy, Tool wear and hole quality when single-shot drilling of metallic-composite stacks with diamond-coated tools, Proc. Inst. Mech. Eng. B J. Eng. Manuf. 228 (2014) 1314–1322. [37] J.R. Ferreira, N.L. Coppini, G.W.A. Miranda, Machining optimisation in carbon fibre reinforced composite materials, J. Mater. Process. Technol. 92-93 (1999) 135–140. [38] K.H. Park, P. Kwon, D. Kim, Wear characteristic on BAM coated carbide tool in drilling of composite/titanium stack, Int. J. Precis. Eng. Manuf. 13 (2012) 1073–1076. [39] A. Qinglong, C. Xiao Jiang, X. Jin Yang, C. Ming, Experimental investigation on drilling of high strength T800S/250F CFRP with twist and dagger drill bits, Int. J. Abras. Technol. 6 (2014) 183–196. [40] Q.L.  An, X.  Jin Yang, C.  Xiao Jiang, C.  Ming, Experimental investigation on drilling force and hole quality when drilling of T800S/250F CFRP laminate, Adv. Mater. Res. 797 (2013) 155–160. [41] K.K. Wika, A.R. Sharman, D. Goulbourne, K. Ridgway, Impact of Number of Flutes and Helix Angle on Tool Performance and Hole Quality in Drilling Composite/Titanium Stacks, SAE Technical Paper, 2011. [42] J. Xu, Q. An, M. Chen, A comparative evaluation of polycrystalline diamond drills in drilling high-strength T800S/250F CFRP, Compos. Struct. 117 (2014) 71–82. [43] S.L.  Soo, A.M.  Abdelhafeez, M.  Li, R.  Hood, C.M.  Lim, The drilling of carbon fibre ­composite-aluminium stacks and its effect on hole quality and integrity, Proc. Inst. Mech. Eng. B J. Eng. Manuf. (2017), https://doi.org/10.1177/0954405417728312.

Sustainability issues in hole-making technologies: Current practices and challenges

11

A.B. Abdullah*, S.M. Sapuan†, Z. Samad* *School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia, †Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, Serdang, Malaysia

11.1 Introduction Composite materials are now becoming the preferred materials in various industries, especially aerospace and automotive. This is due to composite’s attractive thermal, mechanical, and environmental properties [1–2]. For example, substitution of metals with composite materials in airplane structural parts leads to improved efficiency and can reduce up to 25% of CO2 production [3]. One of the applications of composite material in aerospace is for structural parts that require assembly, such as wing panels, and therefore holes are necessary. To date, various technologies have been developed to produce holes on composite panels. Drilling is one of the most common methods in hole-making and variations of drilling methods have recently been proposed. For example, there is vibration-assisted drilling [4], friction drilling [5], and ultrasonic drilling [6], among others. Other methods including electrical discharge machining [7], laser machining [8], ultrasonic machining [9], and abrasive waterjet machining [10]. However, all these technologies generate waste and even the composite itself become waste at the end of its life. The produced waste takes various forms, such as slug material and chips, and requires effective and proper disposal methods. Composite is a material made from two or more constituent materials with different physical and mechanical properties that, when combined, produce a new material that may behave differently from the original materials. It consists of resin from either thermoset or thermoplastic polymer and either glass, carbon, or natural fiber reinforcement, as summarized in Fig. 11.1. Composite materials are in general energy-intensive materials and considered as possible candidates for recycling. However, they have a few inherent characteristics that make them difficult to recycle [11]. 1. Composite materials largely utilize cross-linked thermoset polymers for their matrices, which cannot be re-melted or remolded. 2. Composite materials consist of various types of material that behave differently, such as fiber, polymer, core materials, paints, and metallic inserts, therefore creating complex multiphase waste types.

Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00011-8 © 2019 Elsevier Ltd. All rights reserved.

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Composite

Resin (Matrices)

Thermoplastic (Biodegradable)

PCL

Thermoset (Non-biodegradable)

Epoxy Resin

Fibres (Reinforcement)

Synthetic

Natural

Carbon, Glass, Kevlar etc.

Jute, Hemp etc.

Fig. 11.1  General structure of the materials contain in composite. 3. In general, there is no standard composition for composite material, which results in variability among waste products. 4. Identifying different compositions is technically challenging, making collection and separation from other waste types problematic.

There are ecological constraints on recycling composite materials and a better way of waste management is needed in order to recycle them in an environmentally appropriate way [12]. In addition, the processes required for disposal of waste or end-oflife composite materials require energy. Shuaib and Mativenga [13] modeled energy demand for mechanical recycling of glass fiber reinforced polymer (GFRP). Prior to that, Ribycka et al. [14] proposed a flow mapping to identify the potential of produced scrap either to be recycled or reused. Based on that mapping, the lack of infrastructure and lack of the waste material specification barrier that exists between manufacturers and waste processors can be identified and solved. This may help in identifying the best and most efficient method to deal with waste. This chapter reviews the methods for dealing with composite material waste for the sake of the environment. The discussion covers sustainable composite material, machining performance improvement, waste management systems, and disposal methods. The chapter ends with a discussion about the challenges that may arise from implementing disposal methods.

11.2 Sustainability issues This section is divided into three sub-sections depicting the pre-action and post-action of dealing with composite material from the beginning until to the end of the material’s life.

Sustainability issues in hole-making technologies: Current practices and challenges 151

11.2.1 Sustainable composite material As indicated in the 2011 Guidelines from the Department for Environment, Food & Rural Affairs (DEFRA), the first step and the most significant action in managing waste is using non-hazardous or at least less hazardous material in manufacturing. Composite material contains resin and fiber, which are completely mixed and hence almost impossible to separate. Therefore use of non-hazardous material may be advantageous for the environment. The waste treatment policy in the European Union aims to reduce the impacts from produced waste on the environment. The policy also promotes reuse of waste as a resource. The waste framework directive is the legislative cornerstone of this policy, setting out a five-step waste hierarchy for dealing with waste [15]. One of the main alternatives in providing an added sustainable value to polymeric materials is through the use of biomass as raw materials for production. Apart from conserving the non-­ renewable resources (i.e., petrochemicals), the bio-based polymer possesses better recyclability, hence contributing to a greater environmental advantage to these materials. Besides, the replacement of synthetic fibers (i.e., glass and carbon) with natural fibers to form bio-composites is another sustainable potential for polymeric composites, which is also based on the route of adoption of renewable resources. Recently much research has been conducted to develop a bio-based polymer. Kam and Kueh [16] briefly reviewed recent efforts made to promote sustainability of composite material that the authors believe is the key to success in promoting sustainability. They proposed a next level of improvement. The next level of improvement considers zero waste in production, such as machining instead of recycling, use of green composites, and composites that have optimized self-healing functionality. Other work by Soroudi and Jakubowicz [17] and Faruk et  al. [18] focuses on the review of manufacturing and applications of bio-based polymers. Zini and Scandola [19] presented the major environmental benefits of bio-composites. In the discussion, they listed few commercialized products benefitted from bio-composites. Even though bio-­composites have good durability properties, the non-degradable material limits management of the waste produced. In other review work, Song et al. [20] presented bio-composite materials that undergo biodegradation and compost. Niaounakiset et al. [21] reviewed both biopolymer and its blends that are commercially available in the market and their general applications. Yazdiet al [22]. discussed the development, potential issues, and applications of bio-composite materials.

11.2.2 Machining performance improvement Holes can be produced mainly via machining, such as drilling [23–24], and non-­ machining methods, such as punching [25]. In their review, Ramnath et al. [26] found that there are a few parameters that may affect machining performance: cutting speed, feed rate, and depth of cut. Nassar et al. [27] listed various parameters that may influence the quality of machined features and suggested a guideline for selecting parameters, for example, in drilling, based on the fishbone diagram method. Another method is minimizing the waste during production by improving machining performance. For example, in drilling holes on the composite panel, optimal parameters need to

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be used to reduce reject. Reject usually refers to a high degree of damage around the machined holes, namely, delamination. Delaminated holes tend to fail the structure during application. To reduce this effect, Madhavan et al. [28] and Turki et al. [29] proposed optimal machining parameters. Similarly Zain et al. [30] studied the effect of die clearance and puncher profile on delamination in the punching process. Babu et  al. [31] suggested optimal milling parameters to improve surface quality. Ismail et  al. [32] presented his work about the effects of drilling parameters such as feed rate, thrust force, and cutting speed on the types of chips formed in holes drilling. Their results showed that a higher feed rate, f, and cutting speed, v, produces wider, longer, lighter, and more ribbon-like hybrid fiber reinforced polymer (HFRP) chips. The delamination phenomenon is illustrated in Fig. 11.2. Powdery or dusty, smaller, and darker CFRP chips from at converse conditions. These types of chips result in a lower surface roughness for the CFRP and lower delamination and drill wear for the HFRP. This can be seen in Fig. 11.3. Moisture from any coolant or lubricant used in the machining of composite panel may affect the integrity of the material [33]. For that reason, Josyula et al. [34] improved machinability of metal-based composites (MMCs) by introducing pressurized liquid nitrogen (LN2) to replace ordinary coolant. They found that LN2 reduces surface roughness, tool wear, and cutting temperatures. In addition, a reduction in built-up edge formation can be observed. LN2 has the added advantages of being harmless and ecofriendly. Optimization of machining parameters such as feed rate, cutting speed, and thrust force is addressed in many publications by studies that observe the diameter of drilled holes, formation of chips, delamination, and surface roughness. Based on the research, minimal surface roughness and delamination factor are associated with low feed rate, moderate-to-high cutting speed, and a small drill diameter [32, 35–36]. In optimizing the energy required for machining, torque and thrust force should be at the minimum level. Therefore Ramesh et al. [37] and Debnath et al. [38] recommend a combination of low feed, high spindle speed, and parabolic drill. Ghalme et al. [39] found that for milling GFRP, speed is the most influential parameter on surface roughness. Similar findings can be found in Meena and Kumar [40] and Panduranguda et al. [41]. In determining optimal parameters, various tools and methods were developed and used such as the Taguchi Method [42], grey-fuzzy algorithm [43], and neural network [44–45].

11.2.3 Waste management system Ribycka et al. [14] conducted research on comprehensive and systematic waste management and not only highlighted the source of scrap, but also gathered data on type and amount of scrap before suggesting recycling technology development. Based on the case studies conducted, they identified three fiber-related wastes: dry fibers, fiber material sheet off-cuts, and cured composite off-cuts. In addition, the developed tool is able to identify the material composition, which lead to identification of the material specification. Witik et al. [12] compared recycling via pyrolisis and disposal via landfilling.

Thermoplastic - Polyester - Epoxy Thermosetting - Polypropylene - Polyethylene

Plant origin - Flax - Jute - Kenaf - Hemp

Natural fibers

Machining operation

Fiber processing Fiber treatment - Cleaning - Drying - Chemical treat.

- Phenolic - Urethane

Fiber arrangement - Random Bio-based resin - Woven - Soybean oil - Directional Mineral - Asbestos

Animal - Hair - Wool - Feathers

Coupling agent - MAPP Fiber geometry - Long fiber

EBM

Griding

Thermoplastic - Needle punched - D-LFT - Extrusion - Injection molding

Production methods

Fig. 11.2  The delamination phenomenon during drilling [32].

Micro machining

Turning

- Cross sectional area

Hand lay-up

ECM

Milling

- Short fiber - Particulate & powder

Thermosetting

- Press molding - Filament winding - Hand pultrusion - Compression molding

EDM

Drilling

LBM

USM

HSM

Machining mode - Ductile mode - Brittle mode Tools - Geometry - Materials

Chips formation

Machined surface quality - Delamination degree - Surface roughness - Appearance - Feature geometry - Integrity

Shear mechanism

Machining parameters & conditions

Coolant Machining conditions

Sustainability issues in hole-making technologies: Current practices and challenges 153

Matrix

154 Hole-Making and Drilling Technology for Composites

Fig. 11.3  Example of waste produced from drilling operations at different parameters, (A) f=0.05 and v=10; (B) f=0.15 and v=30; (C) f=0.20 mm/rev and v=40 m/mm [32].

Sustainability issues in hole-making technologies: Current practices and challenges 155

Meira Castro et al. [46] developed a cost-effective waste management solution for GFRP waste materials. This end-use application system can contribute to better environmental sustainability in the fiber-reinforced polymer composites industry. In the system, the authors incorporated GFRP waste materials into polyester-based mortars as sand aggregates and filler replacement. They found that the produced composite had improved mechanical strengths. Another way of managing waste is by recycling the materials as done by Uhlmann and Meier [47]. In their research, they used milling chips and CFRP dust as filler material in thermoplastic granulate. Their results showed that the material produced improved in terms of rigidity and tensile strength. Recycling will definitely lead resource and energy saving. However, lack of markets for recycled composites, high recycling cost for developing the system, and lower quality of the recycled composite are the major problems in commercializing the material. Yang et al. [48] reviewed all these factors. Oliviux et al. [49] found that pyrolysis and solvolysis are the most preferred techniques for recycling composites. However, other techniques are also used to recycle composite material, such as mechanical recycling, and from they found that the commercial product produced from recycled composite will be commercialized soon. Fig. 11.4 summarizes the available recycling technologies and the most recent works done on each technique in recycling of composite materials. In addition, disposal systems need to be environmentally friendly and cost-­effective. Vijay et al. [1] proposed a recycling methodology where waste disposal is adopted. In general recycling can be done by either mechanical or thermal methods. Both methods have advantages and disadvantages. For example, fluidized-bed technology was developed to recover glass or carbon fibers, and the organic resins are used as an energy source while the combustion heat is recovered through a waste-heat recovery system [50]. Life cycle assessment (LCA) is also being utilized in assessing end-oflife treatments such as in Sweden by Hedlund-Astrom [51] and Witik et al. [12]. By performing the LCA, benefits of the recycling process and treatment made on the composite waste to the environment can be identified. They found that recycling is strongly linked to the impacts of the selected recovery process, the materials replaced, and types of secondary application. In agreement, Howards et al. [52] found that mechanical recycling through a milling process will benefit the environment, as the unit process energy for mechanical recycling is significantly lower than the embodied energy of virgin carbon fiber.

11.3 Conclusions and challenges in hole-making technologies Based on the preceding discussion, the challenges in ensuring waste from activities of hole-making does not have a negative impact on the environment can be seen from the following three perspectives: 1. To avoid or minimize the difficulties of recycling composite material after it is used environmentally friendly matrices and reinforcement materials should be selected at the beginning of the composite manufacturing process. However, this is challenging because use

156

Recycling Technologies

Chemical

Thermal

Mechanical

(Solvolysis) Micro-assisted pyrolysis [56]

Low Temperature pressure (LTP) [58]

Grinding [53] Fluidized bed pyrolysis [55]

Fig. 11.4  Recycling technologies and the most recent works for each technique.

High Temperature pressure (HTP) [57]

Hole-Making and Drilling Technology for Composites

Pyrolysis [54]

Sustainability issues in hole-making technologies: Current practices and challenges 157

of ­biodegradable composites is still low and requires further or more intensive promotion, especially from the government. 2. Selecting optimal machining and processing parameters will ensure less waste, or at least waste with a manageable condition or form, is produced. There are a number of works in the literature that discuss the development of optimal parameters, but total optimization requires effort and time. 3. Management of the waste or scrap should be effective not only in terms of the quality of the recycled product produced, but also the energy needed for the recycling. Some of the techniques may produce quality products but may not be economically feasible.

Acknowledgment The authors want to acknowledge Universiti Sains Malaysia for their sponsorship through Research University Grant (Acc. No. 1001/PMEKANIK/814270).

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[12] R.A. Witik, R. Teuscher, V. Michaud, C. Ludwig, J.A.E. Månson, Carbon fibre reinforced composite waste: an environmental assessment of recycling, energy recovery and landfilling, Compos. Part A 49 (2013) 89–99. [13] N.A. Shuaib, P.T. Mativenga, Energy demand in mechanical recycling of glass fibre reinforced thermoset plastic composites, J. Clean. Prod. 120 (2016) 198–206. [14] J. Rybicka, A. Tiwari, P.A. Del Campo, J. Howarth, Capturing composites manufacturing waste flows through process mapping, J. Clean. Prod. 91 (2015) 251–261. [15] Council Directive 2008/98/EC (Waste Framework Directive) [16] C.Z. Kam, A.B.H. Kueh, Towards sustainable polymeric materials: zero waste, green and self-healing, J. Teknol. 74 (4) (2015) 159–163. [17] A. Soroudi, I. Jakubowicz, Recycling of bioplastics, their blends and bio-composites: a review, Eur. Polym. J. 49 (10) (2013) 2839–2858. [18] O. Faruk, A.K. Bledzki, H.P. Fink, M. Sain, Progress report on natural fiber reinforced composites, Macromol. Mater. Eng. 299 (1) (2014) 9–26. [19] E. Zini, M. Scandola, Green composites: an overview, Polym. Compos. 32 (12) (2011) 1905–1915. [20] J.H. Song, R.J. Murphy, R. Narayan, G.B.H. Davies, Biodegradable and compostable alternatives to conventional plastics, Philos. Trans. R. Soc. B: Biol. Sci. 364 (1526) (2009) 2127–2139. [21] M. Niaounakis, Biopolymers: Reuse, Recycling, and Disposal, William Andrew, Elsevier, UK, 2013. [22] M.F.A. Yazdi, R. Zakaria, M. Mustaffar, M.Z. Abd Majid, R.M. Zin, M. Ismail, K. Yahya, Bio-composite materials potential in enhancing sustainable construction, Desalin. Water Treat. 52 (19–21) (2014) 3631–3636. [23] K. Shunmugesh, K. Panneerselvam, Machinability study of carbon fiber reinforced polymer in the longitudinal and transverse direction and optimization of process parameters using PSO–GSA, Eng. Sci. Technol. 19 (3) (2016) 1552–1563. [24] A.A.  Abdul Nasir, A.I.  Azmi, A.N.M.  Khalil, Measurement and optimisation of residual tensile strength and delamination damage of drilled flax fibre reinforced composites, Measurement 75 (2015) 298–307. [25] H.Y.  Chan, A.B.  Abdullah, Z.  Samad, M.S.M.  Zain, Precision punching on laminates composite panel: effect of dual-stages puncher, Int. J. Mater. Eng. Innov. 6 (4) (2015) 288–296. [26] R.A. Ramnath, P.R. Thyla, N.M. Kumar, S. Aravind, Optimization of machining parameters of composites using multi-attribute decision-making techniques: a review, J. Reinf. Plast. Compos. 37 (2) (2018) 77–89. [27] M.M.  Nassar, A.R.  Arunachalam, K.I.  Alzebdeh, Machinability of natural fiber reinforced composites: a review, Int. J. Adv. Manuf. Technol. 88 (9-12) (2017) 2985–3004. [28] S. Madhavan, P.B. Prabu, An experimental study of influence of drill geometry on drilling of carbon fibre reinforced plastic composites, Int. J. Eng. Res. Dev. 3 (2012) 36–44. [29] Y. Turki, M. Habak, R. Velasco, Z. Aboura, K. Khellil, P. Vantomme, Experimental investigation of drilling damage and stitching effects on the mechanical behaviour of carbon/ epoxy composites, Int. J. Mach. Tools Manuf. 87 (2014) 610–672. [30] M.S.M.  Zain, A.B.  Abdullah, Z.  Samad, Effect of puncher profile on the precision of punched holes on composite panels, Int. J. Adv. Manuf. Technol. 89 (9–12) (2017) 3331–3336. [31] G.D. Babu, K.S. Babu, B.U.M. Gowd, Effect of machining parameters on milled natural fibre-reinforced plastic composites, J. Adv. Mech. Eng. 1 (2013) 1–12.

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[32] S.O. Ismail, H.N. Dhakal, I. Popov, J. Beaugrand, Comprehensive study on machinability of sustainable and conventional fibre reinforced polymer composites, Eng. Sci. Technol. 19 (2016) 2043–2052. [33] J.  Turner, R.J.  Scaife, H.M.  El-Dessouky, Effect of machining coolant on integrity of CFRP composites, Adv. Manuf. Polym. Compos. Sci. 1 (1) (2015) 54–60. [34] S.K. Josyula, S.K.R. Narala, E.G. Charan, H.A. Kishawy, Sustainable machining of metal matrix composites using liquid nitrogen, Proc. CIRP 40 (2016) 568–573. [35] D.F. Liu, Y.J. Tang, W.L. Cong, A review of mechanical drilling for composite laminates, Compos. Struct. 94 (2012) 1265–1279. [36] G.D. Babu, K.S. Babu, B.U.M. Gowd, Optimisation of machining parameters in drilling hemp fibre reinforced composites to maximise the tensile strength using design experiments, Ind. J. Eng. Mater. Sci. 20 (2013) 385–390. [37] M.  Ramesh, K.  Palanikumar, K.H.  Reddy, Experimental investigation and analysis of machining characteristics in drilling hybrid glass-sisal-jute fiber reinforced polymer composites, in: 5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014), 2–7 December, India, 2014. [38] K. Debnath, I. Singh, A. Dvivedi, Drilling characteristics of sisal fiber-reinforced epoxy and polypropylene composites, Mater. Manuf. Process. 29 (2014) 1401–1409. [39] S. Ghalme, A. Mankar, Y.J. Bhalerao, Parameter optimization in milling of glass fiber reinforced plastic (GFRP) using DOE-Taguchi method, Springerplus 5 (2016) 1376–1384. [40] M.  Gupta, S.  Kumar, Investigation of surface roughness and MRR for turning of UDGFRP using PCA and Taguchi method, Eng. Sci. Int. J. 18 (2015) 70–81. [41] V. Pandurangadu, K. Palanikumar, H. Syed, Altaf Surface roughness analysis in machining of GFRP composites by carbide tool (K20), Eur. J. Sci. Res. 41 (1) (2010) 84–98. [42] V. Vankanti, V. Ganta, Optimization of process parameters in drilling of GFRP composite using Taguchi method, J. Mater. Res. Technol. 3 (2014) 35–41. [43] T. Rajmohan, K. Palanikumar, S. Prakash, Grey-fuzzy algorithm to optimise machining parameters in drilling of hybrid metal matrix composites, Compos. Part B 50 (2013) 297–308. [44] M. Muthuvel, G. Ranganath, Optimization of machining parameters in milling of composite materials, Int. J. Mech. Eng. Robot. Res. 1 (2) (2012) 277–285. [45] C.C. Tsao, H. Hocheng, Evaluation of thrust force and surface roughness in drilling composite material using Taguchi analysis and neural network, J. Mater. Process. Technol. 203 (2008) 342–348. [46] A.C.  Meira Castro, M.C.S.  Ribeiro, J.  Santos, J.P.  Meixedo, F.J.G.  Silva, A.  Fiúza, M.L. Dinis, M.R. Alvim, Sustainable waste recycling solution for the glass fibre reinforced polymer composite materials industry, Constr. Build. Mater. 45 (2013) 87–94. [47] E.  Uhlmann, P.  Meier, Carbon fibre recycling from milling dust for the application in short fibre reinforced thermoplastics, Proc. CIRP 66 (2017) 277–282. [48] Y. Yang, R. Boom, B. Irion, D.J. van Heerden, P. Kuiper, H. de Wit, Recycling of composite materials, Chem. Eng. Process. 51 (2012) 53–68. [49] G. Oliveux, L.O. Dandy, G.A. Leeke, Current status of recycling of fibre reinforced polymers: review of technologies, reuse and resulting properties, Prog. Mater. Sci. 72 (2015) 61–99. [50] J.M. Henshaw, W. Han, A.D. Owens, An overview of recycling issues for composite materials, J. Thermoplast. Compos. Mater. 9 (1) (1996) 4–20. [51] Hedlund-Åström, A. Model for End-of-Life Treatments of Polymer Composite Materials. Doctoral Thesis. Stockholm: Royal Institute of Technology, 2005.

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[52] J. Howarth, S.S.R. Mareddy, P.T. Mativenga, Energy intensity and environmental analysis of mechanical recycling of carbon fibre composite, J. Clean. Prod. 81 (2014) 46–50. [53] M. Roux, C. Dransfeld, N. Eguémann, L. Giger, Processing and recycling of a thermoplastic composite fibre/peek aerospace part, in: Proceedings of the 16th European conference on composite materials (ECCM 16), 22–26 June, Seville, Spain, 2014. [54] Pimenta, S. Toughness and Strength of Recycled Composites and Their Virgin Precursors. PhD Thesis, Imperial College London, London, UK, 2013. [55] S.J. Pickering, Recycling technologies for thermoset composite materials—current status, Compos. Part A 37 (2006) 1206–1215. [56] D. Åkesson, Z. Foltynowicz, J. Christéen, M. Skrifvars, Microwave pyrolysis as a method of recycling glass fibre from used blades of wind turbines, J. Reinf. Plast. Compos. 31 (2012) 1136–1142. [57] J.A.  Onwudili, E.  Yildirir, P.T.  Williams, Catalytic hydrothermal degradation of carbon reinforced plastic wastes for carbon fibre and chemical feedstock recovery, Waste Biomass Valor. 4 (2013) 87–93. [58] P.L. Xu, J. Li, J.P. Ding, Chemical recycling of carbon fibre/epoxy composites in a mixed solution of peroxide hydrogen and N,N-dimethylformamide, Compos. Sci. Technol. 82 (2013) 54–59.

Machinability studies in drilling carbon fiber reinforced composites

12

A. Krishnamoorthy*, S. Prakash*, J. Lilly Mercy*, S. Ramesh† School of Mechanical Engineering, Sathyabama Institute of Science and Technology, Chennai, India, †Department of Mechanical Engineering, KCG College of Technology, Chennai, India

*

12.1 Introduction Traditional materials can be classified into three major categories known as metals, ceramics, and polymers. Composites are structures in which two or more materials are combined to produce a new material whose properties would not be attainable by conventional means. Composites are composed of two phases, namely, matrix and reinforcement. The reinforcement in a composite material acts as the main load-­ bearing component and the matrix acts as a medium to distribute the applied load to the reinforcement. Fiber reinforced plastics (FRPs) are composed of fibers as reinforcement and polymer as the matrix. FRPs are manufactured in laminate form by stacking a number of thin layers of fibers and a polymer matrix to a desired thickness. By controlling the stacking sequence and orientation of the fiber in each layer, different ranges of physical and mechanical properties can be obtained. Composites have the edge over metals and alloys because they can be manufactured with the required properties according to the need of their applications. Composite materials possess peculiar characteristics that govern their behavior during machining. It is true that the technological development in engineering depends on advances in the field of materials. Carbon fiber composites have turned out to be the best candidate among the others for high modulus, fiber-reinforced materials with low density. Some of the most important and useful properties of carbon fiber composites are light weight, high strength at high temperature (3000°C) in non-oxidizing atmospheres, low coefficient of thermal expansion, and high thermal shock resistance. The predominant reinforcement used to achieve high stiffness and high strength in composites is carbon fiber. It is produced by the pyrolysis of the precursor fibers such as polyacrylonitrile or pitch. The filaments of the precursors are oxidized at 300°C and heated further to 1500–3500°C in a nitrogen atmosphere. Then only the carbon chain remains, and black and bright filaments are obtained. The fibers are drawn at high temperature to increase the modulus of elasticity. Their high production cost has so far excluded them from widespread commercial applications. Since the CFRP composites are manufactured at very high cost, more attention should be paid to the operations to be performed on it. The carbon fibers are very thin fibers with a diameter of 6–10 μm, Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00012-X © 2019 Elsevier Ltd. All rights reserved.

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which consist of 95% to 99.9% chemically pure carbon. These materials possess high specific modulus, high specific strength, good corrosion resistance, high service temperature, and high fatigue strength.

12.2 Effects of machining on composites The major problems faced in machining composites are the shearing and cracking of matrix material and brittle fracture across the fiber. Drilling is widely used to make holes in metals and composites as well. The major defects caused on drilling composite plates are delamination at hole entry and exit and fiber pull-out. Most of these defects are similar for both thermoset and thermoplastic matrix composites. Delamination can occur at any time in the life of a laminate for various reasons and it has various effects. It can affect the tensile strength performance depending on the region of delamination. Delamination can result in a reduction in the durability of the composite material and can cause a reduction in the bearing strength of the material and the structural integrity, resulting in performance issues. The defects that arise on drilling of a composite plate are shown in Fig 12.1. Delamination has been one of the major forms of failure in drilled materials due to the composite’s lack of strength in the direction of drilling. Delamination caused by tensile forces has been attributed mainly to the stacking sequence of laminates. It is usually caused between plies due to out-of-plane tensile stresses as well as by cyclic loads. This type of delamination is slow because the crack growth rate is very slow. Cyclic loads could be either tensile or compressive. Delamination can cause a reduction in the compressive load-carrying capability of the structure. Some of the major reasons for the occurrence of delamination are high Entry Hole size error Top

D Fiber pull out

Composite Bottom

Exit Hole size error

Exit damages Uncut ply

Delamination

Fig. 12.1  Defects caused by drilling a composite plate.ART: As response "This is the best quality that I can provide" is received, please proceed as is.

Machinability studies in drilling carbon fiber reinforced composites 163

thrust force and feed rate. Other reasons include rapid tool wear and power. Drilling is the standard process for producing holes and is the most often practiced machining method in aerospace, automotive, and fluid industries to fit the composites to other structures of the system. A typical aircraft wing might have as many as 5000 holes. Hence machining is a cost factor in the production of composites. Bolting and riveting are currently the preferred methods of fastening composite skins to the metal or other composite parts in assembly of aerospace and automotive composite structures, and the quality of drilled holes strongly affects the fatigue strength of the structure as well as the reliability of the product. Therefore there is a need to produce damage-free holes on the composite before it they are taken to the assembly section. Machining of CFRP has many undesirable effects due to the change in the process parameters. Moreover the carbon fiber reinforced composites are manufactured at very high cost; more attention should be taken on the operations to be performed on it. In this work a carbon fiber reinforced composite plate of 2 mm thickness was fabricated using hand lay-up process with [0°/90°]S orientation. A high-speed steel (HSS) drill tool of 4 mm with different point angles of 85 and 118 degrees was used for drilling in a vertical machining center (VMC). We studied the effects of the drilled holes with respect to the cutting parameters in an attempt to find the optimal combination of parameters to produce a lower delamination factor. The first half treatment of the experimental results is based on the analysis of variance (ANOVA), which is used to measure the percentage of contribution and the physical and statistical significance of the independent variables over the dependent variable. The second half is based on the regression and correlation techniques, which derive a mathematical equation to predict the effects of the independent variables. A considerable amount of research has been conducted on the effects of drilling on composite materials. The machining of carbon fiber composite materials is not the same as machining of conventional metals. Hence the cutting speed and feed rate of the machining operation should be selected carefully when machining carbon fiber composite materials. The most frequent defects caused by drilling are delamination, fiber pull-out, and interlaminar cracking. Davim and Reis [1] studied the cutting parameters for damage-free drilling in carbon fiber reinforced epoxy composite material. They studied delamination factor by drilling CFRP laminates in an autoclave using HSS and cemented carbide (k10) drills. Results showed that the k10 drill performed better than the HSS drill and the delamination factor increased with an increase in cutting speed and feed rate. Gaitonde et al. [2] investigated the effects of process parameters on delamination during high-speed drilling of CFRP composite by considering cutting speed, feed rate, and point angle as process parameters. They analyzed the effects of cutting speed, feed rate, and point angle on delamination factor using the models by generating response surface plots. The investigations reveal that the delamination tendency decreased with an increase in cutting speed. The study also suggests a combination of low values of feed rate and point angle for reducing the damage. Hocheng and Tsao [3] studied the effect of tool wear on delamination in drilling composites. They used a twist drill for drilling the material and concluded that though the critical thrust force is higher with increasing wear, the delamination becomes more liable to occur because the actual thrust force increases with wear.

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Tsao [4] used the Taguchi method to investigate the effects of drilling parameters (diameter ratio, feed rate, and spindle speed) when using three step-core drills for drilling composite material. Results showed that drilling-induced delamination of various step-core drills increased with a decrease in diameter of the drill and spindle speed and an increase in feed rate. Basavarajappa et  al. [5] studied the influence of cutting parameters on drilling characteristics of hybrid metal matrix composites. They employed the Taguchi method and ANOVE to analyze the drilling characteristics of the composites and study the effect of spindle speed and feed rate on feed force, surface finish, and burr height using solid carbide multifaceted drills of 5 mm diameter. The results revealed that the dependent variables are greatly influenced by feed rate rather than spindle speed for both composites. Ceramic-graphite reinforced composites have better machinability than those reinforced with SiCp composites. The work done so far in drilling of CFRP composites deals with changing the drill bit diameter size, drill tool materials, and drilling parameters like cutting speed and feed rate to determine which parameter most influences damage. Although much research has been carried out on the effects of cutting parameters on delamination during drilling of CFRP composites, not much work is reported on the effect of point angle. This work focuses on the effect of cutting speed, feed rate, and point angle on delamination factor.

12.3 Fabrication of composites We used composite laminates produced by a hand lay-up process that are composed of an epoxy matrix reinforced with 50% weight fraction of unidirectional carbon fiber. The laminates had a stacking sequence of [0°/90°]S and a thickness of 2 mm. We used an HSS drill with a 4-mm diameter on a VMC with a maximum spindle speed of 5000 rpm to perform the experiments. We used the Taguchi technique, which is widely used in engineering analysis. The technique consists of a plan of experiments with the objective of acquiring data in a controlled way and executing these experiments in order to obtain information about the behavior of a given process. For the elaboration of the experiments plan, we used the Taguchi method for three factors at two levels. The array chosen was the L18, which has 18 rows corresponding to the number of tests with three parameters (i.e., point angle, cutting velocity, and feed rate). Table 12.1 shows the orthogonal array used. The plan of experiment is made of eighteen tests in which the first column corresponds to the point angle (°), the second column to the cutting velocity (V), and the third column to the feed rate (f). The thrust force during drilling is measured by using the Kistler Quartz 3-Component dynamometer, type 9257B. Fig. 12.2 shows the experimental setup with the workpiece clamped on to the dynamometer in the VMC. The Kistler Quartz 3-component dynamometer is well clamped on the T-slot provided on the work table and then the CFRP composite plate with wooden back-up plate is clamped above the dynamometer. The wooden back-up plate is used in order

Machinability studies in drilling carbon fiber reinforced composites 165

Table 12.1  Orthogonal array Exp. no

Point angle (deg) (°)

Cutting velocity (m/min) (V)

Feed rate (mm/ min) (f)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

85 85 85 85 85 85 85 85 85 118 118 118 118 118 118 118 118 118

6.28 6.28 6.28 12.56 12.56 12.56 18.84 18.84 18.84 6.28 6.28 6.28 12.56 12.56 12.56 18.84 18.84 18.84

50 100 150 50 100 150 50 100 150 50 100 150 50 100 150 50 100 150

Fig. 12.2  Clamping of the workpiece with wooden back-up plate on the dynamometer.

to reduce the delamination factor at the exit of the drill tool. Delamination is defined as the separation of the laminates from each other. Fig. 12.3 shows the delamination factor due to drilling of FRP composites. The delamination factor is the ratio between maximum diameter of hole damage to hole diameter. Delamination factor ( Fd ) = Dmax / Do

(12.1)

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Hole-Making and Drilling Technology for Composites

Dmax D0

Delamination

Fig. 12.3  Delamination factor.

Where Dmax=Maximum diameter of hole and Do=Diameter of the hole. The image of the drilled hole was captured using the cam scope and analyzed with Clemex Vision image analysis software and the values of the delamination factor were obtained.

12.4 Mathematical formulation The treatment of the experimental results is based on ANOVA and the regression and correlation techniques to estimate the contribution of the variable parameters and their relations towards the response variables. Regression analysis is used for observing the line of best fit through the data in order to make estimates and predictions about the behavior of the variables. Finally the optimal combinations of the variables were analyzed to obtain high-quality drilled holes. Sir Ronald Fisher developed ANOVA. It is a statistically based, objective ­decision-making tool for detecting any difference in average performance of a group of items tested. ANOVA is a technique to estimate quantitatively the relative contribution each controlled parameter makes on the overall measured response expressed as a percentage. The decision, rather than pure judgment, takes variation into account. Regression analysis is the mathematical process of using observations to find the line of best fit through the data in order to make estimates and predictions about the behavior of the variables. This line of best fit may be linear (straight) or curvilinear to some mathematical formula. Correlation analysis is the process of finding how well (or badly) the line fits the observations, such that if all the observations lie exactly on the line of best fit, the correlation is considered to be 1 or unity. In a regression problem, levels of independent variables (X1, X2, X3, …, Xn) are set and observations are made on the dependent variable Y and the given level of X is assigned at random to each experimental unit in study. The linear population model is given as, 

Y = B0 + B1 X1 + B2 X 2 +…+ Bn X n

(12.2)

Machinability studies in drilling carbon fiber reinforced composites 167

B’s are the true coefficients to be used to weight of the observed X’s; Point angle (X1): Independent variable 1; Cutting velocity (X2): Independent variable 2; Feed rate (X3): Independent variable 3; Response factor (Y): Dependent variable. The equation describing the relationship among four variables is 

(12.3)

Y = a + b1 X1 + b2 X 2 + b3 X3

By summing the squares of the errors of estimate the following least square equations can be derived:

Σ Y = na + b1Σ X1 + b2 Σ X 2 + b3 Σ X3

(12.4)

Σ X1Y = aΣ X1 + b1Σ ( X1 ) + b2 Σ X1 X 2 + b3 Σ X1 X3

(12.5)

Σ X 2Y = aΣ X 2 + b1Σ X1 X 2 + b2 Σ ( X 2 ) + b3 Σ X 2 X3

(12.6)

Σ X3Y = aΣ X3 + b1Σ X1 X3 + b2 Σ X 2 X3 + b3 Σ ( X3 )

(12.7)

2

2

2

The values of a, b1, b2, and b3 can be found by substituting the values of n,ΣY, ΣX1, ΣX2, ΣX3, ΣX1Y, Σ(X1)2, ΣX1X2, ΣX2Y, and Σ(X2)2 in the preceding equations and solving them. Then the values of a, b1, b2, and b3 are substituted in Eq. (12.3) to get the corresponding regression equation. From the experimental values the regression equation is derived for the delamination factor (Df) and the thrust force (Tf). The percentage prediction accuracy of the model is checked by the formula given in Eq. (12.8). PPA = 100 / n Σ ( Exp value − Pre value ) / Pre value 

(12.8)

Where n=Number of experiments; Exp value=Experimental value; Pre value=Predicted value. Drilling operations were conducted on the carbon fiber composite material and the thrust forces (Tf) and delamination factor were observed for each experiment. The assignment of the results for the 18 tests is plotted in Table 12.2.

12.5 Thrust force Thrust force during drilling can be defined as “the force acting along the axis of the drill during the cutting process.” Thrust force is considered the major contributor of delamination during drilling. Critical thrust force causes delamination and thrust force below that will constrain or eliminate delamination during drilling. The thrust forces for various cutting speeds and feed rates have been observed. The thrust force for cutting velocity of 12.56 m/min and 100 mm/min is shown in Fig. 12.4.

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Table 12.2  Thrust force and delamination factor for various cutting speeds and feed rates

Exp. no

Point angle (deg) (°)

Cutting velocity (m/min)

Feed rate (mm/min)

Thrust force (N)

Delamination factor

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

85 85 85 85 85 85 85 85 85 118 118 118 118 118 118 118 118 118

6.28 6.28 6.28 12.56 12.56 12.56 18.84 18.84 18.84 6.28 6.28 6.28 12.56 12.56 12.56 18.84 18.84 18.84

50 100 150 50 100 150 50 100 150 50 100 150 50 100 150 50 100 150

40.79 46.07 67.03 17.13 29.58 45.97 14.04 25.34 36.15 55.28 75.14 94.36 33.67 50.75 77.6 28.31 45.79 52.61

1.083 1.102 1.188 1.077 1.089 1.133 1.058 1.083 1.103 1.146 1.185 1.225 1.128 1.146 1.164 1.107 1.133 1.149

Drilling

Fz [N]

220 215 210 205 200 195 190 185 180 175 170 165 0

1

2

3

4

5 Time [s]

6

7

8

9

10 Cycle No.: 1

Fig. 12.4  Thrust force versus time for cutting velocity of 12.56 m/min and feed rate of 100 mm/min.ART: As response "This is the best quality that I can provide" is received, please proceed as is.

Machinability studies in drilling carbon fiber reinforced composites 169

Fig. 12.4 clearly shows that up to 4.3 s the system is idle and after reaching 4.4 s the drill bit is in contact with the composite material and the thrust force increases with time up to 5.1 s and attains the maximum thrust force of 203.73 N. As the drill bit moves down through the laminates the thrust force decreases with time up to 5.5 s. The remaining thrust force generated from 5.5 to 7.8 s is for wood that is mounted beneath the composite material as a back-up plate. The average is taken and used for further calculation.

12.6 Analysis of variance Table 12.3 presents the ANOVA table for thrust force. The analysis shown in Table 12.3 was undertaken for a level of significance of 5%, that is, for a level of confidence of 95%. In the table, the value of frequency for degree of freedom (1, 12) for point angle is less than calculated value, and the calculated value presents a statistical significance for point angle is 4.747. Note that, in the table the value of frequency for degree of freedom (2, 12) for both feed rate and cutting velocityis less than calculated value, and the calculated value presents a statistical significance for feed rate and cutting velocity is 3.885. Table  12.3 shows the percentage of each variable on thrust force. Point angle is the greater influence on thrust force compared to feed rate and cutting velocity. The percentage contribution of error is less than the percentage contribution of cutting velocity and feed rate. The ANOVA table for delamination factor (Df) for the holes drilled by an HSS tool is shown in Table 12.4. The analysis shown in Table 12.5 was undertaken for level of significance of 5%, that is, for level of confidence of 95%. Similar pattern found here, where in table, the value of Table 12.3  ANOVA for thrust force Source of variance

Degrees of freedom

Sum of squares

Point Angle Velocity Feed Error Total

1 2 2 12 17

2845.3 2035.4 2736.0 301.6 7618.4

Variance

Percentage of contribution

2845.3 1017.7 1368 25.1

37.34 26.70 35.91 3.95

F-test 80.98 54.43 56.60

Table 12.4  ANOVA for delamination factor Source of variance

Degrees of freedom

Sum of squares

Point Angle Velocity Feed Error Total

1 2 2 12 17

0.0121161 0.0075164 0.0111814 0.0026917 0.0335056

Variance

Percentage of Contribution

0.0121161 0.0037582 0.0055907 0.0002243

36.16 22.43 33.37 8.03

F-test 54.02 16.75 24.92

Table 12.5  Regression table for the thrust force on HSS drill Y

X1

X2

X3

X12

X22

X32

X1X2

X1X3

X2X3

X1Y

X2Y

X3Y

40.79 46.07 67.03 17.13 29.58 45.97 14.04 25.34 36.15 55.28 75.14 94.36 33.67 50.75 77.6 28.31 45.79 52.61 835.61

85 85 85 85 85 85 85 85 85 118 118 118 118 118 118 118 118 118 1827

6.28 6.28 6.28 12.56 12.56 12.56 18.84 18.84 18.84 6.28 6.28 6.28 12.56 12.56 12.56 18.84 18.84 18.84 226.08

50 100 150 50 100 150 50 100 150 50 100 150 50 100 150 50 100 150 1800

7225 7225 7225 7225 7225 7225 7225 7225 7225 13924 13924 13924 13924 13924 13924 13924 13924 13924 190341

39.44 39.44 39.44 157.75 157.75 157.75 354.95 354.95 354.95 39. 44 39.44 39.44 157.75 157.75 157.75 354.95 354.95 354.95 3312.83

2500 10000 22500 2500 10000 22500 2500 10000 22500 2500 10000 22500 2500 10000 22500 2500 10000 22500 210000

533.8 533.8 533.8 1067.6 1067.6 1067.6 1601.4 1601.4 1601.4 741.04 741.04 741.04 1482.08 1482.08 1482.08 2223.12 2223.12 2223.12 22947.12

4250 8500 12750 4250 8500 12750 4250 8500 12750 5900 11800 17700 5900 11800 17700 5900 11800 17700 182700

314 628 942 628 1256 1884 942 1884 2826 314 628 942 628 1256 1884 942 1884 2826 22608

3467.15 3915.95 5697.55 1456.05 2514.30 3907.45 1193.40 2153.90 3072.75 6523.04 8866.52 11134.48 3973.06 5988.50 9156.80 3340.58 5403.22 6207.98 87972.68

256.16 289.32 420.95 215.15 371.52 577.38 264.51 477.41 681.07 347.16 471.88 592.58 422.90 637.42 974.66 533.36 862.68 991.17 9387.28

2039.50 4607.00 10054.50 856.50 2958.00 6895.50 702.00 2534.00 5422.50 2764.00 7514.00 14154.00 1683.50 5075.00 11640.00 1415.50 4579.00 7891.50 92786.00

Machinability studies in drilling carbon fiber reinforced composites 171

frequency for degree of freedom (1, 12) for point angle is less than calculated value, and the calculated value presents a statistical significance for delamination factor is 4.747. In the table, the value of frequency for degree of freedom (2, 12) for both feed rate and cutting velocity is less than calculated value, and the calculated value presents a statistical significance for both feed rate and cutting velocity is 3.885. Table 12.4 shows the percentage of each variable on the delamination factor. Point angle is the greater influence on delamination factor compared to feed rate and cutting velocity. The percentage of contribution of error is less than the percentage of contribution of cutting velocity and feed rate.

12.7 Thrust force for various cutting velocities and feed rates Fig. 12.5 shows the variation of the thrust force with varying cutting velocities for holes drilled with a 4-mm HSS twist drill of point angle 118 degrees for three different feed rates. The thrust force is plotted along the Y-axis and the cutting velocity along the X-axis. It is seen in Fig.  12.5 that the thrust force decreases with an increase in cutting velocity for a point angle of 118 degrees. When the cutting velocity is increased the heat generated in the drilling process also increases. Due to increase in heat, matrix softening occurs easily and this influences the reduction of thrust force. For lower feed rates, thrust force is minimum. Fig. 12.6 shows the variation of the thrust force with varying feed rates for holes drilled holes with a 4-mm HSS twist drill of point angle 118 degrees for three different cutting velocities. The thrust force is plotted along the Y-axis and the feed rate along the X-axis. It is seen in Fig. 12.6 that the thrust force increases with an increase in feed rate for a point angle of 118 degrees. An increase in feed rate makes the rapid movement of the

Thrust force (N)

Point angle 118 100 90 80 70 60 50 40 30 20 10 0

Fr 50 mm/min Fr 100 mm/min Fr 150 mm/min

6.28

12.56

18.84

Cutting velocity (m/min)

Fig. 12.5  Thrust force versus cutting velocity for a point angle of 118 degrees.

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Thrust force (N)

Point angle 118 100 90 80 70 60 50 40 30 20 10 0

Cv 6.28 m/min Cv 12.56 m/min Cv 18.84 m/min

50

100

150

Feed rate (mm/min)

Fig. 12.6  Thrust force versus feed rate for a point angle of 118 degrees.

Point angle 85 80 Thrust force (N)

70 60 50

Fr 50 mm/min

40

Fr 100 mm/min

30

Fr 150 mm/min

20 10 0 6.28

12.56

18.84

Cutting velocity (m/min)

Fig. 12.7  Thrust force versus cutting velocity for a point angle of 85 degrees.

drill bit inside the workpiece. Due to rapid penetration, the thrust force also increases rapidly with an increase in feed rate. Thrust force is minimum for higher cutting velocities. Fig. 12.7 shows the variation of the thrust force with varying cutting speeds for holes drilled with a 4-mm HSS twist drill of point angle 85 degrees. The thrust force is plotted along the Y-axis and the cutting velocity along the X-axis. It is seen in Fig.  12.7 that the thrust force decreases with an increase in cutting velocity for a point angle of 85 degrees. When the cutting velocity is increased the heat generated in the drilling process is also increased. Due to increase in heat, matrix softening occurs easily and that influences the reduction of thrust force. Thrust force is minimum for lower feed rates. Fig. 12.8 shows the variation of thrust force with

Machinability studies in drilling carbon fiber reinforced composites 173

Point angle 85 80 Thrust force (N)

70 60 50

Cv 6.28 m/min

40

Cv 12.56 m/min

30

Cv 18.84 m/min

20 10 0 50

100

150

Feed rate (mm/min)

Fig. 12.8  Thrust force versus feed rate for a point angle of 85 degrees.

Delamination factor

Point angle 118 1.24 1.22 1.2 1.18 1.16 1.14 1.12 1.1 1.08 1.06 1.04

Fr 50 mm/min Fr 100 mm/min Fr 150 mm/min

6.28

12.56

18.84

Cutting velocity (m/min)

Fig. 12.9  Delamination factor versus cutting velocity for a point angle of 118 degrees.

varying feed rates for holes drilled with a 4-mm HSS twist drill of point angle 85 degrees. The thrust force is plotted along the Y-axis and the feed rate along the X-axis. It is seen in Fig. 12.8 that the thrust force increases with an increase in feed rate for a point angle of 85 degrees. An increase in feed rate makes the rapid movement of the drill bit. Due to rapid penetration, the thrust force also increases rapidly for various feed rates. Thrust force is minimum for lower cutting velocities. Fig.  12.9 shows the variation of the delamination factor with varying cutting speeds for holes drilled with a 4-mm HSS twist drill of point angle 118 degrees. The ­delamination factor is plotted along the Y-axis and the cutting velocity along the X-axis.

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Delamination factor

Point angle 118 1.24 1.22 1.2 1.18 1.16 1.14 1.12 1.1 1.08 1.06 1.04

Cv 6.28 m/min Cv 12.56 m/min Cv 18.84 m/min

50

100

150

Feed rate (mm/min)

Fig. 12.10  Delamination factor versus feed fate for a point angle of 118 degrees.

It is seen in Fig. 12.9 that the delamination factor decreases with an increase in cutting velocity for a point angle of 118 degrees. When the cutting velocity is increased the heat generated in the drilling process is also increased. Due to increase in heat, matrix softening occurs easily and that influences the reduction of the delamination factor. The delamination factor is minimum for lower feed rates. Fig. 12.10 shows the variation of the delamination factor with varying feed rates for holes drilled with a 4-mm HSS twist drill of point angle 118 degrees. The delamination factor is plotted along the Y-axis and the feed rate along the X-axis. It is seen in Fig. 12.10 that the delamination factor increases with an increase in feed rate for a point angle of 118 degrees. An increase in feed rate makes the rapid movement of the drill bit. Due to rapid penetration, the delamination factor also increases rapidly for various feed rates. The delamination factor is minimum for lower cutting velocities. Fig. 12.11 shows the variation of the delamination factor with varying cutting speeds for holes drilled with a 4-mm HSS twist drill of point angle 85 degrees. The delamination factor is plotted along the Y-axis and the cutting velocity along the X-axis. It is seen in Fig. 12.11 that the delamination factor decreases with an increase in cutting velocity for a point angle of 85 degrees. When the cutting velocity is increased the heat generated in the drilling process is also increased. Due to increase in heat, matrix softening occurs easily and that influences the reduction of the delamination factor. Delamination factor is minimum for lower feed rates. Fig. 12.12 shows the variation of the delamination factor with varying feed rates for holes drilled with a 4-mm HSS twist drill of point angle 85 degrees. The delamination factor is plotted along the Y-axis and the feed rate along the X-axis. It is seen in Fig. 12.12 that the delamination factor increases with an increase in feed rate for a point angle of 85 degrees. An increase in feed rate makes the rapid movement of the drill bit. Due to rapid penetration, the delamination factor also increases rapidly for various feed rates. Delamination factor is minimum for lower cutting velocities.

Machinability studies in drilling carbon fiber reinforced composites 175

Point angle 85

Delamination factor

1.2 1.15 Fr 50 mm/min

1.1

Fr 100 mm/min

1.05

Fr 150 mm/min

1 0.95 6.28

12.56

18.84

Cutting velocity (m/min)

Fig. 12.11  Delamination factor versus cutting velocity for a point angle of 85 degrees.

Point angle 85

Delamination factor

1.2 1.15 Cv 6.28 m/min

1.1

Cv 12.56 m/min

1.05

Cv 18.84 m/min

1 0.95 50

100

150

Feed rate (mm/min)

Fig. 12.12  Delamination factor versus feed rate for a point angle of 85 degrees.

12.8 Regression analysis We analyzed the experimental values for thrust force and damage factor using the regression table. The corresponding equation that describes the thrust force and the damage factor is derived by substituting the values of the regression table and solving the normal equations. The regression table for thrust force and damage factor obtained from the carbide drill and core drill tools is shown in Tables 12.5 and 12.6.

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12.8.1 Thrust force By substituting the values of n,ΣY, ΣX1, ΣX2, ΣX1Y, Σ(X1)2, ΣX1X2, ΣX2Y and Σ(X2)2 in the Eqs. (12.4), (12.5), (12.6) and (12.7) the following equations were obtained. Table  12.5 shows the regression table on the analysis of thrust force for the holes drilled by the HSS drill tool with point angle, cutting velocity, and feed rate as the independent variables 835.61 = 18 a + 1827 b1 + 226.08 b2 + 1800 b3

(12.9)

87972.68 = 1827a + 190341b1 + 22947.12b2 + 182700b3

(12.10)

9387.28 = 226.08a + 22947.12b1 + 3312.83b2 + 22608b3

(12.11)

92786.00 = 1800 a + 182700b1 + 22608b2 + 210000b3

(12.12)

After solving these four equations the values of a, b1, and b2 were found to be as follows: a = −20.3 b1=0.644 b2 = −2.34 b3=0.308

Now these coefficient values of a, b1, b2, and b3 are substituted in the normal equation to get Eq. (12.13) describing the thrust force for the HSS drill. In Eq. (12.13) X1 is the point angle, X2 is the cutting velocity, and X3 is the feed rate. The regression equation for 

Thrust force ( N ) = Y 1 = −20.3 + 0.644 X1 − 2.34 X 2 + 0.308 X3

(12.13)

Eq. (12.13) is the regression equation for thrust force, which is used to predict the dependent variable.

12.8.2 Delamination factor Table 12.6 shows the regression table on the analysis of the delamination factor for the holes drilled by the HSS drill tool with point angle, cutting velocity, and feed rate as the independent variables. 20.299 = 18a + b11827 + b2 226.08 + b3 1800

(12.14)

2068.05 = 1827a + 190341b1 + 22947.12 + 182700b3

(12.15)

Y

X1

X2

X3

X12

X22

X32

X1X2

X1X3

X2X3

X1Y

X2Y

X3Y

1.083 1.102 1.188 1.077 1.089 1.133 1.058 1.083 1.103 1.146 1.185 1.225 1.128 1.146 1.164 1.107 1.133 1.149 20.299

85 85 85 85 85 85 85 85 85 118 118 118 118 118 118 118 118 118 1827

6.28 6.28 6.28 12.56 12.56 12.56 18.84 18.84 18.84 6.28 6.28 6.28 12.56 12.56 12.56 18.84 18.84 18.84 226.08

50 100 150 50 100 150 50 100 150 50 100 150 50 100 150 50 100 150 1800

7225 7225 7225 7225 7225 7225 7225 7225 7225 13924 13924 13924 13924 13924 13924 13924 13924 13924 190341

39.44 39.44 39.44 157.75 157.75 157.75 354.95 354.95 354.95 39.44 39.44 39.44 157.75 157.75 157.75 354.95 354.95 354.95 3312.83

2500 10000 22500 2500 10000 22500 2500 10000 22500 2500 10000 22500 2500 10000 22500 2500 10000 22500 210000

533.8 533.8 533.8 1067.6 1067.6 1067.6 1601.4 1601.4 1601.4 741.04 741.04 741.04 1482.08 1482.08 1482.08 2223.12 2223.12 2223.12 22947.12

4250 8500 12750 4250 8500 12750 4250 8500 12750 5900 11800 17700 5900 11800 17700 5900 11800 17700 182700

314 628 942 628 1256 1884 942 1884 2826 314 628 942 628 1256 1884 942 1884 2826 22608

92.06 93.67 100.98 91.55 92.57 96.31 89.93 92.06 93.76 135.23 139.83 144.55 133.10 135.23 137.35 130.63 133.69 135.58 2068.05

6.80 6.92 7.46 13.53 13.68 14.23 19.93 20.40 20.78 7.20 7.44 7.69 14.17 14.39 14.62 20.86 21.35 21.65 253.10

54.15 110.20 178.20 53.85 108.90 169.95 52.90 108.30 165.45 57.30 118.50 183.75 56.40 114.60 174.60 55.35 113.30 172.35 2048.05

Machinability studies in drilling carbon fiber reinforced composites 177

Table 12.6  Regression table on delamination factor for HSS drill tool

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253.10 = 226.08a + 22947.12b1 + 3312.83b2 + 22608b3

(12.16)

2048.05 = 1800 a + 182700b1 + 22608b2 + 210000b3

(12.17)

The equation describing the relationship among four variables is: 

Y = a + b1 X1 + b2 X 2 + b3 X3 a=0.957 b1=0.00157 b2=−0.00393 b3=+0.000605

Now these coefficient values of a, b1, b2, and b3 are substituted in the normal equation to get Eq. (12.18) describing the delamination factor for the HSS drill. In Eq. (12.18) X1 is the point angle, X2 is the cutting velocity, and X3 is the feed rate. The regression equation for delamination factor is obtained as   Delamination factor  Y 2  = 0.957 + 0.00157 X1 − 0.00393 X 2 + 0.000605 X3 (12.18)   Eq. (12.18) is the regression equation for delamination factor, which is used to predict the dependent variable.

12.9 Validation 12.9.1 Thrust force of the holes drilled using HSS drill tool Fig. 12.13 shows the validation of the predicted values with the experimental values of the thrust force for the holes drilled using the HSS drill tool. It is observed from Fig.  12.13 that the predicted values of the thrust force obtained from the regression equation trace a similar path to that of the experimental values of the thrust force for the holes drilled using HSS drill tool. The associated error rate is 6%.

12.9.2 Delamination factor of the holes drilled using HSS drill tool Fig. 12.14 shows the validation of the predicted values with the experimental values of the delamination factor for the holes drilled using the HSS drill tool. It is observed from Fig. 12.14 that the predicted values of the delamination factor obtained from the regression equation traces the same path more similar to that of the experimental values of the delamination factor for the holes drilled using the HSS drill tool. The associated error rate is 1%.

Machinability studies in drilling carbon fiber reinforced composites 179

Thrust force (N)

Experimental value

Predicted value

100 90 80 70 60 50 40 30 20 10 0 1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18

No. of. Experiments

Fig. 12.13  Validation of the predicted values with the experimental values of the thrust force for HSS drill tool.

Experimental value

Predicted value

Delamination factor

1.25 1.2 1.15 1.1 1.05 1 0.95 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18

No. of. experiments

Fig. 12.14  Validation of the predicted values with the experimental values of the delamination factor for HSS drill tool.

12.10 Conclusion In this chapter, we studied the effect of point angle, cutting velocity, and feed rate on thrust force when drilling CFRP composites. We used a 4-mm HSS twist drill with point angles of 85 degrees and 118 degrees to drill on 2 mm thick CFRP material and found that a point angle of 85 degrees, a cutting velocity of 18.84 m/min, and a feed rate of 50 mm/min produce lower thrust force and thus delamination factor. Our results

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Hole-Making and Drilling Technology for Composites

suggest that these are the optimal parameters for machining CFRP laminates. In summary, we make the following conclusions: ●

●

●

●

●

Thrust force depends on the drill point angle and the feed rate and increases with an increase in both point angle and feed rate. Thrust force and delamination factor increase with an increase in feed rate, and thrust force decreases with an increase in cutting velocity in both point angles of 85 degrees and 118 degrees. However, both thrust force and delamination factor decrease with a decrease in point angle irrespective of the feed rate and cutting velocity. It is proved statistically using ANOVA that the drill point angle and the feed rate, and their interactions, significantly influence the thrust force in drilling. The model predicted for thrust force and torque correlates with the experimental value and is confirmed by the confirmation test.

References [1] J.P.  Davim, P.  Reis, Study of delamination in drilling carbon fibre reinforced plastics (CFRP) using design experiments, Compos. Struct. 59 (2003) 481–487. [2] V.N. Gaitonde, S.R. Karnik, C. Rubio, Analysis of parametric influence on delamination in high-speed drilling of carbon fibre reinforced plastic composites, J. Mater. Process. Technol. 203 (2007) 431–438. [3] H. Hocheng, C.C. Tsao, Taguchi analysis of delamination associated with various drill bits in drilling of composite material, Int J Mach Tool Manu 44 (2007) 1085–1090. [4] C.C. Tsao, Investigation into the effects of drilling parameters on delamination by various step-core drills, J. Mater. Process. Technol. 206 (2007) 405–411. [5] S. Basavarajappa, G. Chandramohan, J.P. Davim, Some studies on drilling of hybrid metal matrix composites based on Taguchi techniques, J. Mater. Process. Technol. 196 (2008) 332–338.

Further reading [6] M.D. Luis, R.J. Campos, A.M. Abrao, A novel approach based on digital image analysis to evaluate the delamination factor after drilling composite laminates, Compos. Sci. Technol. 67 (2007) 1939–1945. [7] A. Krishnamoorthy, J. Lilly Mercy, K.S.M. Vineeth, M.K. Salugu, Delamination analysis of carbon fibre reinforced plastic composite plates by thermographic technique, Mater. Today Proc. 2 (2015) 3132–3139. [8] S.  Prakash, J.L.  Mercy, Putti Venkata Siva Teja, P.  Vijayalakshmi, ANFIS modelling of delamination during drilling of Medium Density Fibre(MDF) Board, Proc. Eng. 97 (2014) 258–266.

Burr assessment of punched holes on Al/CFRP/Al-stacked panel by profile measurement technique

13

N. Ishak, A.B. Abdullah, Z. Samad School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia

13.1 Introduction Composite materials are widely used as alternatives to certain metals in manufacturing applications, particularly in the aerospace industry [1]. Punching is one method used to produce holes, particularly on metal. However, this technology is still new for composites, and there are only a few published works on this topic. Chan et al. [2] proposed the technology and evaluated the effect of die clearance on the quality of the produced holes, while Zain et al. [3] extended the study on various puncher profiles. The precision of the punching process can be influenced by the tool geometry, punching design, and process parameters and can be assessed on the basis of a few criteria, including crack formation, damage of peripheral zone, roundness and dimensional error, surface roughness, and damage of surface layer (e.g., delamination and edge chipping) [4]. Other influencing factors include the hole diameter, cylindricity error, out of diameter, and burr formation [5]. Studies have shown that drill geometry is the major influencing factor on the quality of drilled holes compared with drill size, workpiece thickness, volume fraction, fiber orientation, speed, and feed. All the process parameters, except speed, significantly affect thrust force, while workpiece thickness, drill size, lip angle, and speed significantly affect surface roughness [6]. Fuzzing is the formation of burrs at the entry/exit of the hole due to the uncut fibers during machining, and this machining error can be corrected by further machining [4]. Jin et al. [7] investigated the disfigurement formation and control in drilling carbon fiber-reinforced composites (CFRPs) and found that burrs at the exit are longer and larger than those at the entrance. Deburring may add much cost and time for part assembly [8–10]. Ramulu et al. [11] found that in drilling of composite and titanium stacks, dissimilar mechanical and thermal properties increase the burr height at the exit. Wei et al. [12] studied the parameters that affect interlayer burr formed during dry drilling of metal materials, and they found that preloading pressing force is effective in controlling the formation of burr. Kumar and Krishnaraj [13] investigated the effect of spindle speed and feed rate on burr height in drilling of composite/metal stacks under minimal fluid lubricating conditions. They observed that exit burr decreases with an increase in feed rate. Nakao and Watanabe [14] developed an effective measurement of burr using an image-processing technique. Images of burr produced during hole drilling were captured from the side surfaces with the use of four mirrors arranged Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00013-1 © 2019 Elsevier Ltd. All rights reserved.

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around the hole with burr. Rezende et al. [15] studied the effect of drill geometry on burr height during drilling of aluminum/PE sandwich material. They found that tool geometry and feed rate are the most influential parameters. The use of a pilot hole increases the burr height at the exit. They employed a toolmaker microscope to measure the burr height. Bi and Liang [16] conducted burr height measurement during drilling of titanium (Ti) and aluminum (Al) alloy stacked materials. The optimal parameters of spindle speed, feed rate, pressure, and stacking sequence were also identified. They used a toolmaker microscope for burr measurement. Zhang et  al. [17] studied hole quality on CFRP/aluminum stacked material produced by drilling based on hole accuracy, roughness, and burr height. The burr height was measured at ×50 magnification using a 3D morphology of white light interferometry instrument (RTEC3D). They observed that drill geometry affects burr formation whereas cutting parameter insignificantly affects it. Isbilir and Ghassemieh [18] studied the drilled hole quality produced on CFRP/Ti stacks. The burr height was measured with the use of a surface profilometer (Mitutoyo SV-602; Mitutoyo, Japan). They found that burr height and width increase with an increase in drill wear. Avila et  al. [19] comprehensively reviewed the burr formation on various materials (e.g., metal, composite, and composite/metal stack) and methods (e.g., milling and drilling) from different perspectives. They found that burr is unavoidable but can be controlled. Melkote et al. [10] studied the effect of drilling parameters on interfacial burr formation. Kuo et al. [20] evaluated the hole quality of drilled holes on Ti-64/CFRP/AA7050 stacks based on diametrical accuracy, cylindricality, and burr height at different feed rates and tool coatings. Burr height was measured at the entrance and the exit of the hole at four points around the hole via a level-type dial gauge with a resolution of 0.002 mm. They found that burr is attributed to the enlargement of the corner chamfers and the greater wear of the cutting edge than in the former due to chipping/fracture. Brinksmeier and Fangmann [21] studied the effect of orbital drilling on the burr formation of composite/AA2024 stacks. The study considered different tool geometries, different coatings, different cutting parameters, tool wear, and minimum quantity lubrication. Profile measurement is a method that allows the measurement of a real surface topography with the use of optics with limited depths of field and vertical scanning [22]. The scanned image can be utilized to evaluate a few quality indicators, such as roundness [23], twist springback [24], and geometrical defect [25]. The current study aims to investigate the quality of punched holes on composite material by utilizing the profile measurement technique. The quality assessment focused on the burr formed at the hole exit. This study contributes to literature mainly by determining the effect of fiber orientation, panel thickness, and fiber types on burr amplitude.

13.2 Methodology The research methodology involved three stages: preparation of the composite panel, experimentation, and analysis of the results.

Burr assessment of punched holes

183

13.2.1 Preparation of the composite panel The composite used in the experiment is a laminar composite and was prepared using a hand lay-up technique. For stacking, AA7075 with 0.5 mm thickness was used (Fig. 13.1). A total of 18 specimens were prepared with different fiber orientations; as odd numbers (i.e., 1, 3, 5, 7, 9, 11, 13, 15, and 17), representing the orientation of 0 degrees/90 degrees, and even numbers (i.e., 2, 4, 6, 8, 10, 12, 14, 16, and 18) representing the orientation of 90 degrees/0 degrees (Table 13.1).

Fig. 13.1  Cross-sectional view of punched hole. Table 13.1  Specimens prepared for the experiment Orientation Specimen No

0/90 degrees

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

X

Fiber type

90/0 degrees

Glass

X

X X

X

Carbon

X X

X X

X X

X X X

X X

X

X X

X X

X X

X X X X X X X

Hybrid

X X X X X X

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Hole-Making and Drilling Technology for Composites

13.2.2 Experimental setup A universal testing machine was used in performing the punching operation. Four punch travel speeds (i.e., 0.5, 1.5, 3, and 5mm/s) were used. The experiment was repeated for different specimens (composite panels with different thicknesses and orientations). The diameter of the puncher was 10±0.5 mm. The setup is shown in Fig. 13.2. The puncher was purposely designed to belong to position the retractable spring and was hardened to 60 HRC.

13.2.3 Specimen analysis The cut edge quality was evaluated on the basis of the burr formation (i.e., amplitude). During the punching process, the maximum load required for successful punching was recorded to observe the load pattern. Then, the specimens were processed using the 3D surface measurement tool called Alicona IFM for image capturing and analysis of the hole quality. Fig. 13.3A shows the line constructed on the captured image to obtain the profile of the burr. The height of the burr was based on the distance between the flat surface of the panel and the peak of the burr, as shown in Fig. 13.3B.

13.3 Results and discussion Table  13.2 shows the entrance and the exit of the holes produced by the punching process. For the tensile test, we divided the observation into two orientations: unidirectional orientations of 0 degrees/90 degrees and 90 degrees/0 degrees. The results showed that the glass fiber with four plies and a unidirectional orientation of 0

Fig. 13.2  Test rig on Instron Series-3367 for punching operation.

Fig. 13.3  The captured image showing (A) the line constructed and (B) the profile measured using 3D surface measurement method on the exit of the hole. Table 13.2  The entry and exit hole of the m aespl

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d­ egrees/90 degrees presents higher Young’s modulus than does the specimen with an orientation of 90 degrees/0 degrees. A higher Young’s modulus means higher stiffness. This is due to the fiber form included in the samples. On the contrary, a lower Young’s modulus means higher flexibility of the structure.

13.3.1 Punching load In the experiment, we used an aluminum sheet with a thickness of 0.5 mm to form the stack, and we sandwiched the composite panel between the aluminum sheets. Fig. 13.4 shows the effect of the thickness of glass fiber composite panel on the required load (i.e., maximum value required for a complete punching operation). The sample used was made of glass fiber with three plies. The observation was divided into two orientations: 0 degrees/90 degrees and 90 degrees/0 degrees. At a constant speed, the maximum load increases with an increase in the thickness of the composite panel. Fig. 13.5 shows the effect of the thickness of the carbon fiber composite panel on the maximum load. The graph is divided into two orientations: unidirectional orientations of 0 degrees/90 degrees and 90 degrees/0 degrees. Carbon fiber composite presents a high elastic modulus and is rigid. When the thickness of the panel increases and the speed of the punching is constant, the stress required and the maximum load increase thereby completing the punching process. Fig. 13.6 shows the effect of the thickness of the hybrid composite panel on the maximum load. Hybrid composite is a combination of glass fiber and carbon fiber. The graph trend shows that the composite thickness increases with the maximum load. Samples 5 and 6 require the lowest load compared with the other samples due to their thickness, which is low (four plies). Notably, the thicknesses of samples 5 and 6 are 2.22 and 2.27 mm, respectively. 12,000

Maximum load, N

10,000 8000 6000 4000 2000 0 1

7

13

2

0/90° 0.5mm/s

8 90°/0

1.5mm/s

3.0mm/s

5.0mm/s

Fig. 13.4  Effect of glass fiber composite panel thickness on the maximum load.

14

Burr assessment of punched holes

187

14,000 12,000

Max. load, N

10,000 8000 6000 4000 2000 0 3

9

15

4

0/90° 0.5mm/s

10

16

90°/0 1.5mm/s

3.0mm/s

5.0mm/s

Fig. 13.5  Effect of carbon fiber composite panel thickness on the maximum load.

14,000 12,000

Max. load, N

10,000 8000 6000 4000 2000 0 5

11

17

6

0.5mm/s

12

18

90°/0

0/90° 1.5mm/s

3.0mm/s

5.0mm/s

Fig. 13.6  Effect of hybrid composite panel thickness on the maximum load.

13.3.2 Burr height The burr height was measured at the hole exit using the profile measurement technique as described in the previous section. Figs.  13.7, 13.8, and 13.9 show the results of burr height for glass fiber composite, carbon fiber composite, and hybrid composite panels, respectively. The specimens were separated into two orientations: unidirectional orientations of (a) 0 degrees/90 degrees and (b) 90 degrees/

188

Hole-Making and Drilling Technology for Composites 450 400 Burr height, mm

350 300 250 200 150 100 50 0

1

7

13

1.5mm/s

360.9 330.07

203.81 396.28

217.96 241.01

3.0mm/s

115.46

242.53

203.57

5.0mm/s

350.69

190.62

185.75

0.5mm/s

(A) 350

Burr height, mm

300 250 200 150 100 50 0

2

8

14

1.5mm/s

303.81 168.48

246.09 177.26

254.3 211.32

3.0mm/s

303.65

169.42

181.32

5.0mm/s

212.24

187.23

119.27

0.5mm/s

(B) Fig. 13.7  Burr height versus thickness of glass fiber composite panel (A) unidirectional (0/90 degrees) orientation (B) 90 degrees/0 orientations.

0 degrees. The burr height occurring at the hole exit at low punching speed is larger than that occurring at the hole exit at high punching speed. From the graph, the resulting punching hole is found to possess the highest burr height at a punching speed of 0.5 or 1.5 mm/s. The burr formation is mainly influenced by the ductility of the material and cutting speed. Therefore the punching speed and the thickness

Burr assessment of punched holes

189

600

Burr height, mm

500 400 300 200 100 0

3

9

15

1.5mm/s

238.21 561.61

246.43 335.63

247.93 390.57

3.0mm/s

419.63

146.33

360.04

5.0mm/s

411.09

138.37

280.14

0.5mm/s

(A) 600

Burr height, mm

500 400 300 200 100 0

4

10

16

124.44 514.55

248.09 260.12

346.6 261.35

3.0mm/s

95.9

234.03

187.04

5.0mm/s

367.84

132.34

191.45

0.5mm/s 1.5mm/s

(B) Fig. 13.8  Burr height versus thickness of carbon fiber composite panel (A) unidirectional (0/90 degrees) orientation (B) 90 degrees/0 orientations.

of the composite panel play a minor role in affecting the burr height. Although low burr height is always preferred, burr ­formation in the punching process is undesirable. The result of unidirectional orientations of (a) 0 degrees/90 degrees and (b) 90 degrees/0 degrees is inconsistent because the mechanical properties of the composite panels differ due to their thickness.

190

Hole-Making and Drilling Technology for Composites 300

Burr height, mm

250 200 150 100 50 0

5

11

17

1.5mm/s

162.65 207.59

176.2 246.78

99.26 153.16

3.0mm/s

95.75

168.29

88.31

5.0mm/s

74.46

184.42

39.58

0.5mm/s

(A)

Fig. 13.9  Burr height versus thickness of hybrid fiber composite panel (A) unidirectional (0/90 degrees) orientation (B) 90 degrees/0 orientations.

13.4 Conclusions and recommendations for future work This work aimed to study the effect of process parameters on the quality of punched holes in composite/metal stacked panels. The mechanical properties of the composite were obtained from tensile test. The maximum punching load is inconsistent when various punching speeds are applied but increases when the thickness of the composite

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191

with aluminum stacked panel increases. Next, the burr height of the produced hole was measured at the hole exit. The diameter measured at the entry of the hole is influenced by tool wear and delamination. The results of the diameter analysis showed that the resulting hole is not perfectly round, and its diameter is slightly different from that of the puncher. The deviation in the diameter can be minimized by producing low delamination ratio with low maximum load. The burr height at the hole exit is high at low punching speed. For future work, other parameters, such as die clearance, stacked metal, and tool wear, will be studied to obtain an improved hole for strong and accurate assembly.

Acknowledgment The authors want to acknowledge Universiti Sains Malaysia for their sponsorship through Research University Grant (Acc. No. 1001/PMEKANIK/814270). For Mr. Fakhrul, who helps in conducting the experiments.

References [1] T. Edward, Composite materials revolutionize aerospace engineering, Ingenia 36 (2008) 24–28. [2] H.Y. Chan, A.B. Abdullah, Z. Samad, Precision punching of hole on composite panels, Ind. J. Eng. Mater. Sci. 22 (2015) 641–651. [3] M.S.M.  Zain, A.B.  Abdullah, Z.  Samad, Effect of puncher profile on the precision of punched holes on composite panels, Int. J. Adv. Manuf. Technol. 89 (2017) 3331–3336. [4] W. Konig, P. Grab, Quality definition and assessment in drilling of fibre reinforced thermosets, Ann. CIRP 38 (1) (1989) 119–124. [5] I.S. Shyha, S.L. Soo, S. Bradley, R. Perry, P. Harden, S. Dawson, Hole quality assessment following drilling of metallic-composite stacks, Int. J. Mach. Tools Manuf. 51 (7-8) (2011) 569–578. [6] B.R.N. Murthy, L.L.R. Rodrigues, N.Y. Sharma, D. Anjaiah, Influence of process parameters on the quality of hole in drilling of GFRP composites—an experimental investigation using DOE, in: International Conference on Mechanical and Electrical Technology, 10-12 September 2010, Singapore, 2010, pp. 87–90. [7] Z.J. Jin, Y.J. Bao, H. Gao, Disfigurement formation and control in drilling carbon fiber reinforced composites, Int. J. Mater. Prod. Technol. 31 (1) (2008) 46–53. [8] X. Rimpault, J.F. Chatelain, J.E. Klemberg-Sapieha, M. Balazinski, Burr height monitoring while drilling CFRP/titanium/aluminium stacks, Mech. Ind. 18 (1) (2017) 114. [9] R. Zitoune, V. Krishnaraj, F. Collombet, Study of drilling of composite material and aluminium stack, Compos. Struct. 92 (2010) 1246–1255. [10] S.N.  Melkote, T.R.  Newton, C.  Hellstern, J.B.  Morehouse, S.  Turner, Interfacial Burr Formation in Drilling of Stacked Aerospace Materials, in: J.C. Aurich, D. Donfeld (Eds.), Burr—Analysis, Control and Removal, Springer-Verlag, Berlin Heidelberg, 2010, pp. 89–98. [11] M. Ramulu, T. Branson, D. Kim, A study on the drilling of composite and titanium stacks, Compos. Struct. 54 (1) (2001) 67–77.

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[12] T.  Wei, H.  Jian, L.  Wenhe, B.  Yin, Z.  Lin, Formation of interlayer gap and control of interlayer burr in dry drilling of stacked aluminum alloy plates, Chin. J. Aeronaut. 29 (1) (2016) 283–291. [13] M. Senthil Kumar, A. Prabukarthi, V. Krishnaraj, An experimental investigation on drilling of CFRP/Ti stacks using minimal flow lubricating- (MFL) technique using coated (TiAlN) and uncoated drills, in: 5th International & 26th All India Manufacturing Technology, Design and Research Conference, Assam, India, 2014. 314-1-6. [14] Y. Nakao, Y. Watanabe, Measurements and evaluations of drilling burr profile, Proc. Inst. Mech. Eng. B J. Eng. Manuf. 220 (4) (2006) 513–523. [15] B.A. Rezende, M.L. Silveira, L.M.G. Vieira, A.M. Abrão, P.E. de Faria, J.C.C. Rubio, Investigation on the effect of drill geometry and pilot holes on thrust force and burr height when drilling an aluminium/PE sandwich material, Materials 9 (2016) 774–784. [16] S. Bi, J. Liang, Experimental studies and optimization of process parameters for burrs in dry drilling of stacked metal materials, Int. J. Adv. Manuf. Technol. 53 (2011) 867–876. [17] L. Zhang, Z. Liu, W. Tian, W. andLiao, Experimental studies on the performance of different structure tools in drilling CFRP/Al alloy stacks, Int. J. Adv. Manuf. Technol. 81 (2015) 241–251. [18] O.  Isbilir, E.  Ghassemieh, Comparative study of tool drilling of CFRP/titanium stack using coated carbide drill, Mach. Sci. Technol. 17 (3) (2013) 380–409. [19] M. Avila, J. Gardner, C. Reich-Weiser, A. Vijayaraghavan, S. Tripathi, D. Dornfeld, Burr minimization strategies and cleanability in the aerospace and automotive industry, SAE Trans. J. Aerospace 114 (1) (2005) 1073–1082. [20] C.L. Kuo, S.L. Soo, D.K. Aspinwall, S. Bradley, W. Thomas, R. M'saoubi, D. Pearson, W.  Leahy, Tool wear and hole quality when single-shot drilling of metallic-composite stacks with diamond-coated tools, Proc. Inst. Mech. Eng. B J. Eng. Manuf. 228 (10) (2014) 1314–1322. [21] E. Brinksmeier, S. Fangmann, Burr and cap formation by orbital drilling of aluminum, in: J.C. Aurich, D. Donfeld (Eds.), Burr—Analysis, Control and Removal, Springer-Verlag, Berlin Heidelberg, 2010, pp. 31–45. [22] R. Danzl, F. Helmli, S. Scherer, Focus variation—a robust technology for high resolution optical 3D surface metrology, J. Mech. Eng. 57 (3) (2011) 245–256. [23] A.B. Abdullah, S.M. Sapuan, Z. Samad, H.M.T. Khaleed, N.A. Aziz, Twist springback measurement of autonomous underwater vehicle propeller blade based on profile deviation, Am. J. Appl. Sci. 10 (5) (2013) 515–524. [24] A.B. Abdullah, S.M. Sapuan, Z. Samad, Profile measurement based on focus variation method for geometrical defect evaluation—a case study of cold forged propeller blade, Adv. Mech. Eng. (2014) 874691. [25] A.B. Abdullah, S.M. Sapuan, Z. Samad, Roundness error evaluation of cold embossed hole based on profile measurement technique, Int. J. Adv. Manuf. Technol. 8 (2015) 293–300.

Electro-discharge drilling of metal matrix composites

14

A. Singh*, S. Kachhap*, R. Kumar† *National Institute of Technology Patna, Patna, India, †Indian Institute of Technology Roorkee, Roorkee, India

14.1 Introduction The present demand of industries for a material that is light in weight and possesses good mechanical properties has led to the development of composite materials. Metal matrix composites (MMCs) are becoming very applicable from the industrial point of view, as they possess superior mechanical properties like high toughness, high specific strength, high temperature resistance, high stiffness, low density, and so on compared to monolithic metals [1]. Their superior properties make them applicable in the aerospace, defense, and automotive industries. Further development in MMCs has led to the conceptualization of hybrid MMCs (HMMCs), made by the addition of two or more reinforcing materials in the metallic matrix. The sole purpose of fabricating the MMC or HMMC is to replace the heavier monolithic metals by providing the same level of mechanical properties. Many researchers have used MMCs for their experimental work and established their special characteristics. Kim et  al. [2] developed an aluminum-based hybrid MMC and found that increasing the volume of Al2O3 increases the compressive strength compared to the base metal (A356). Du and Li [3] studied the wear-resistant properties of an Al/Si/SiC/Gr HMMC and recorded that the dimensional stability increased by increasing the graphite content. Suresha and Sridhara [4] performed wear analysis on an Al/SiC/Gr HMMC and found an optimal value of reinforcement addition (around 7.5%) at which wear rate is at a minimum. Rajmohan et al. [5] conducted a drilling operation on an Al356/SiC MMC and an Al356/SiC/mica HMMC. The authors recorded higher surface roughness (SR) in the HMMC than the MMC. The machining of MMCs poses challenges because of their superior mechanical properties and the presence of hard reinforcements [6]. Conventional machining is not economical for the processing of MMCs or HMMCs as it causes high tool wear and leads to high energy consumption [7, 8]. For economical machining, nonconventional machining is the best alternative. Electro-discharge machining (EDM) plays a big role in industries to process the MMCs to their required shape. As EDM generates the replica of the tool on to the workpiece, it is applicable for the production of dies and permanent molds. In EDM, the workpiece and the tool do not come in physical contact; therefore the output responses are not affected by the strength and hardness of the workpiece material. Electro-discharge drilling (EDD) has been developed based on EDM in which the tool electrode is made to rotate in order to achieve a lower tool wear rate (TWR). The research is being carried out on EDM of Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00014-3 © 2019 Elsevier Ltd. All rights reserved.

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MMCs to make MMCs applicable for various sectors. Singh et al. [9, 10] conducted EDM operation on Al/10%SiC as cast MMC with variables as current, pulse on time, and flushing pressure, and found that the dimensional accuracy is affected at higher current and on-time ratings. Rozenek et al. [11] conducted a wire EDM operation on AlSi7Mg/SiC and AlSi7Mg/Al2O3 MMCs and recorded the lower cutting speed for MMC as compared to the base material (AlSi7Mg) due to the introduction of reinforcement. Ramulu et al. [12] found the softening effect of the EDM process up to a depth of approximately 200 μm below the recast layer on a 15%SiC/Al MMC workpiece. Singh et al. [9, 10] optimized the process parameters of EDM in Al/10%SiC composite by using the Taguchi method and found considerable improvement in the process. Kachhap et al. [13] found that TWR decreases when using a solid copper tool electrode and TWR increases when using a solid brass tool electrode during EDD of Al6063/SiC/Gr/Al2O3 HMMCs. Kuppan et al. [14] used different material tool electrodes for EDD of Inconel 718 superalloy and also found that the copper electrode resulted in low SR. The graphite electrode produced low TWR and high SR, and the copper-tungsten electrode produced moderate TWR and SR. The surface topography analysis of alloy materials, machined by EDM, was performed through scanning electron microscopy (SEM) and energy dispersive spectrograph (EDS). The minimum SR found with negative electrode polarity followed by pulse on time and discharge current [15, 16]. The literature discussed suggests that not much investigation has been made into EDD of HMMCs. In the present study, we used the EDD process and a rotary tool electrode on a hybrid MMC workpiece to drill holes. We examined the effect of various input parameters on output responses and identified an optimal set of input parameters that produce minimum TWR and SR.

14.2 Scheme of experiments The scheme of experimentation is divided into two phases. The selection of the process parameters and their respective quality characteristics in each phase were decided based on the first phase itself. The objective of the experimentation phases are as follows. Phase I: The first phase is based upon the basic requirement of experimental study of process parameters. This first phase is a process variation in which we studied the effects of copper and quenched copper tool electrodes on the HMMC workpiece. We used the one-factor-at-a-time (OFAT) approach to measure the results. The results indicate that discharge current, duty factor, flushing pressure, and tool speed are significant input parameters, while pulse off time and gap voltage are insignificant variables. The results also show a significantly improved performance of the quenched tool electrode over the normal copper electrode. Phase II: The second phase of experimentation extends the results of phase I and is an in-depth study of EDD using a quenched copper tool (which had better response compared to the normal copper tool discussed in phase I) on a hybrid MMC. We used

Electro-discharge drilling of metal matrix composites195

response surface methodology (RSM) to obtain optimal machining parameters for TWR and SR. We verified the results by conducting confirmation experiments with optimal process conditions.

14.3 Phase I: Experimental study We conducted pilot experiments at different levels of process parameters using the OFAT approach. This section presents an investigation on the relationships and parametric effects of the independently controllable process variables (input parameters), such as discharge current, duty factor, pulse off time, gap voltage, tool speed, and flushing pressure, on the material removal rate, TWR, and SR value (output responses). We used both copper and quenched copper electrodes in the experiment and attempted to predict the optimal set of process parameters for maximizing the response characteristics.

14.3.1 Experimental procedure We used a ZNC-EDD machine set-up, which we designed and developed in house, for carrying out the experiment. The tool electrodes’ configuration and geometry are shown in Fig. 14.1A and B. The effect of the input process parameters on TWR and SR was studied by changing one parameter at a time, keeping all other parameters constant. Table 14.1 shows the range of the input parameters based on the machine setting and shows the constant parameters used for the present study. Changes in tool electrode, electrode weight, and elapsed time were recorded after each experiment. The inter-electrode gap was maintained constant at 0.05 mm for each experiment. We conducted the experiments in triplicate and took the average value for further evaluation.

Fig. 14.1  Tool electrodes (A) pure copper, (B) quenched copper.

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Table 14.1  Process parameters and their range Parameters

Range of the machine

Discharge current (amp) Duty factor Pulse off time (μs) Gap voltage (V) Tool speed (rpm) Flushing pressure (kg/cm2)

3–6–12–15–21–30 0.67–0.72–0.77–0.82–0.87–0.90 4–8–15–45–90–200 30–40–50–60–70–100 0–200–500–800–1000–1200 0–0.15–0.25–0.50–0.75–1

Constant parameters Input parameters

Discharge current

Pulse on time

Pulse off time

Gap voltage

Tool speed

Flushing pressure

Constant level

12 amp

120 μs

45 μs

50 V

500 rpm

0.50 kg/ cm2

●

●

●

Tool electrode: Copper material (99.7% Cu, 0.12% Zn, 0.02% Pb, 0.02% Sn), crosssection area of electrode: 44.18 mm2 and 7.5 mm diameter. Workpiece: Metal matrix composite (Al6063 base metal [reinforcement particles 5%SiC/2.5%Gr/2.5%Al2O3]). Dielectric fluid: ELF EDM oil (viscosity at (20°C) 7.0 mm2/s).

14.3.2 Results and discussion 14.3.2.1 Effect on tool wear rate

TWR, mm3/min

Fig. 14.2 shows the influence of discharge current on TWR for the different electrodes. It has been observed that TWR increases with an increase of discharge current for both tool electrodes. The increase in TWR is due to higher heat energy at higher current values. The experimentally observed data for the duty factor for different values of TWR is given in Fig.14.3. The TWR increases with the increases in the duty factor. The quenched electrode experiences higher TWR as compared to the copper electrode. Shabgard et  al. [17] observed that by dispersing more heat from the spark-stricken

Discharge current, amp

Fig. 14.2  Variation of TWR with discharge current.

TWR, mm3/min

Electro-discharge drilling of metal matrix composites197

Duty factor

TWR, mm3/min

Fig. 14.3  Variation of TWR with duty factor.

Pulse off time, µs

Fig. 14.4  Variation of TWR with pulse off time.

position and increasing the amount of heat transferred from the plasma channel to the electrodes, the plasma channel efficiency in removing molten material from the molten crater at the end of each pulse decreases, while the dimensions of the molten crater on the electrodes increases. This effect is more pronounced for the copper electrode since its thermal conductivity is much higher than that of the workpiece. It was observed that TWR of tool electrodes initially decreases with an increase in pulse off time and increases thereafter (Fig. 14.4). The increased pulse off time means that less time is available for sparking. The lowest value of TWR is at pulse off time of 45 μs for the quenched copper tool, whereas the lowest value of TWR is at pulse off time of 90 μs for the copper tool. Fig. 14.5 depicts the effect of gap voltage on TWR. With increasing gap voltage, TWR increases, which is again due to an increase in the spark intensity between the tool and the workpiece. The increased spark intensity increases the melting and vaporization of the tool electrode resulting in increased TWR. The copper tool electrode experiences more TWR compared to the quenched tool electrode. The variation of TWR with tool speed for various electrodes is shown in Fig. 14.6. The value of TWR initially tends to reduce with an increase in tool speed and increases thereafter. The tool rotation provides additional flushing to the process by generating a centrifugal

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TWR, mm3/min

198

Gap voltage, V

TWR, mm3/min

Fig. 14.5  Variation of TWR with gap voltage.

Tool speed, rpm

Fig. 14.6  Variation of TWR with tool speed.

force into the dielectric, which clears the debris from the discharge gap. The quenched tool electrode provides lower value of TWR compared to the copper tool electrode. Chattopadhyay et al. [18] observed that there is a carbon deposition on the surface of the tool electrode, migrated from the hydrocarbon dielectric fluid, which prevents its surface from wearing. In contrast, by increasing the tool speed, the centrifugal force increase subsequently, which throws out the pyrolytic carbon particles from the surface of the electrode resulting in increased TWR [19]. TWR decreases with an increase in flushing pressure as shown in Fig. 14.7. The decrease in TWR with the increase in flushing pressure is due to the increase in cooling rate of the tool. On further increase in flushing pressure, the ionized channel is continuously washed away by the fluid pressure making the machining unstable resulting in the increased TWR.

14.3.2.2 Effect on surface roughness The effect of discharge current on SR (Ra) is shown in Fig. 14.8. The Ra value was found to increase with an increase in the discharge current. The Ra value of quenched

TWR, mm3/min

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Flushing pressure, kg/cm2

SR, µm

Fig. 14.7  Variation of TWR with flushing pressure.

Discharge current, amp

Fig. 14.8  Variation of SR with discharge current.

tool electrode was observed to be higher than the copper tool electrode. The higher values of discharge current generate large discharge energy, which results in larger craters on the surface of the workpiece. The relation between SR and duty factor is shown in Fig. 14.9. The SR value increases with an increase in the value of duty factor. The highest value of SR was found at maximum value of duty factor (0.90) for both the electrodes. The higher duty factor generates large discharge energy, which creates a shallow crater on the surface of the HMMC resulting in higher SR [20]. Soni and Chakraverti [21] observed that short pulse duration allows close overlapping of craters and therefore a good SR was obtained. The effect of pulse off time conditions on SR is shown in Fig. 14.10. The SR value decreases when the value of duty factor increases. The pulse off time is the time required for re-establishment of insulation in the discharge gap or de-ionization of the dielectric medium at the end of each discharge. Very low value of duty factor does not provide sufficient time to clear out the debris from the discharge gap causing the occurrence of arcing resulting in higher SR.

Hole-Making and Drilling Technology for Composites

SR, µm

200

Duty factor

SR, µm

Fig. 14.9  Variation of SR with duty factor.

Pulse off time, µs

Fig. 14.10  Variation of SR with pulse off time.

Fig. 14.11 shows the variation in SR at different levels of gap voltage. The lowest value of SR was obtained at a voltage value of 70 V for the copper tool electrode. The higher value of SR at lower gap voltage is due to a stronger electric field and increased spark frequency. The values of SR are in agreement with Gupta et al. [22]. It was found that increased arcing is due to insufficient pulse energy to dispel the debris effectively. The effect of tool speed on SR is shown in Fig.  14.12. The SR value decreases with increasing tool speed. The quenched tool electrode provides a higher value of SR compared to the copper tool electrode. Higher tool speed provides additional flushing to the process by quickly throwing away the molten material out of the machining zone, thereby reducing the possibility of the re-solidified material on the surface of the workpiece [23]. Fig. 14.13 shows the effect of flushing pressure on SR. The SR value decreases with an increase in flushing pressure. The effective flushing pressure flushes out the debris as well as prevents the re-solidification of the molten material on the surface of the workpiece. The reduction of debris adherence on the surface of the workpiece results in reduced SR.

SR, µm

Electro-discharge drilling of metal matrix composites201

Gap voltage, V

SR, µm

Fig. 14.11  Variation of SR with gap voltage.

Tool speed, rpm

SR, µm

Fig. 14.12  Variation of SR with tool speed.

Flushing pressure, kg/cm2

Fig. 14.13  Variation of SR with flushing pressure.

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14.4 Phase II: Response surface method By studying the experimental results in phase I, we found that the input process ­parameters have a significant effect on the output responses. It was felt that there ­exists a need to optimize the input parameters for getting the desired output responses. Therefore we conducted statistically designed experiments to find the optimal levels for the input process parameters. Since the quenched copper tool electrode was observed to have the better performance compared to the copper electrode, we decided to use the quenched copper tool electrode for further study. The target of the statistical analysis is to minimize TWR and improve SR. The input parameters that were found to have the least effect on the responses, and for which there was no clear trend available, were not considered for further statistical analysis.

14.4.1 Optimization methodology RSM accepts both mathematical and statistical techniques (began in the early 1930s), which are useful for modeling and analyzing problems in which a response of interest is influenced by several variables and the objective is to optimize the response [24]. Benyounis and Olabi [25] presented a comprehensive review on the application of the statistical and numerical methods in the area of machining. The authors have suggested that RSM performs better than other techniques. The main benefit of RSM is its ability to exhibit the factor contributions from the coefficients in the regression model. The other important field of RSM consists of the low-order, non-linear behavior with regular experimental domain. This ability is powerful in identifying the insignificant factors, insignificant interaction, or insignificant quadratic terms in the model and thereby can reduce the complexity of the problem. Table 14.2 presents a comparison of common modeling and optimization techniques based on this literature review.

14.4.2 Response surface methodology Khuri and Cornell [26] defined RSM as a set of techniques that includes the seven steps listed in this section. These steps include the setting the objective of the investigation and selecting the responses to measure, the variables to study, and the range to cover. In determining the functional relationship between the response and the explanatory input parameters, it is customary to start with a first-order relationship, after which optimization can be improved by a second-order strategy, for the seven steps of RSM are as follows: 1. 2. 3. 4. 5. 6. 7.

Identify the optimization problem Choose input variables and their levels and select response variables Choose appropriate experimental design Perform experiments to collect data Analyze data using model relationship-regression analysis Check for model adequacy Apply optimization techniques

Techniques Comparison

Artificial neural network (ANNs)

Genetic algorithm (GA)

RSM

Taguchi

Factorial designs

Computational time Experimental domain Model developing Optimization Availability in software Optimization accuracy level Application

Long Regular or irregular Yesa Through model Available High Frequently

Very long Regular or irregular No Straight Available High Rarely

Short Regular only Yes Through model Available Very high Frequently

Medium Regular or irregular No Straight Available Normal Rarely

Short Regular only Yesb Through model Available Very high Frequently

a

No factors interaction effects. No factors quadratic effects.

b

Electro-discharge drilling of metal matrix composites203

Table 14.2  Comparison of common optimization techniques [25]

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14.4.3 Parameter design for the EDD process Identification of the controllable parameters and definition of level of each factor are equally crucial to the success of any optimization problem. Pilot experiments were conducted for the EDD process using the OFAT approach. The controllable parameters taken in the present study are discharge current (6–18 amp), duty factor (067–0.87), tool speed (0–1000 rpm), and flushing pressure (0–1 kg/cm2). Beyond the selected range of the input process parameters, it was found that the EDD process becomes unstable and does not produce the desired results. In many cases, it was also observed that arcing led to the stopping of the process. The surface finish observed using a visual inspection technique also helped in identifying the suitable range for various input process parameters. The controllable parameters, their range, and their levels selected for the EDD process are shown in Table 14.3.

14.4.4 Design matrix and experimental details The selected design matrix (shown in Table 14.4) is a four-factor, five-level central composite rotatable design (CCRD) with full replications, consisting of 30 sets of coded conditions and comprising a full replication of 16 factorial design plus six center points and eight star points. CCRD is more useful than full-factorial designs since it requires fewer tests and has shown to be sufficient to describe the responses. All the machining variables at the intermediate (zero) level constitute the center points, while the combination of each of the machining variables at either its lowest value (−2) or its highest value (+2) with the other three variables at the intermediate levels constitute the star points. Thus the 30 experimental runs allows for the estimation of the linear, quadratic, and two-way interactive effects of the input process parameters on the response parameters. Statistical software (Design-Expert v7) has been used to code the variables and to establish the design matrix [27]. RSM was applied to the experimental data using the same software to obtain the regression equations and to generate the statistical and response plots. The experiments were conducted on a hybrid MMC with a quenched copper tool electrode as per the design of experiments using a random order. Table 14.4 shows the values of TWR and SR obtained in the experiments for EDD of HMMCs with the quenched tool. The data was collected with respect to the influence of the predominant process parameters on TWR and SR. The 30 experiments were conducted in duplicate and the average value of responses is presented in Table 14.4. The results of the investigation justify the practical applicability of the EDD process for HMMCs with its objective of minimizing TWR and improving SR. However, the objective of better understanding of the process from a research perspective persists. Furthermore, the results obtained would provide a better technical database for improving the potential applicability of EDD for hole making. The mathematical models for responses such as TWR and SR can be used for prediction within same design space. The selection of appropriate models and the development of regression equations for responses was done using design expert statistical software [18]. The second-order model (which is known as the quadratic model or equation) is flexible because it can take a variety of functional forms and approximates the response

Notations A B C D

Process parameters Discharge current Duty factor Tool speed Flushing pressure

Levels/Limits

Units

Actual range of OFAT

Decided range

−2

−1

0

+1

+2

amp

3–30

6–18

6

9

12

15

18

– rpm kg/cm²

0.67–0.90 0–1200 0–1

0.67–0.87 0–1000 0–1

0.67 0 0

0.72 250 0.25

0.77 500 0.50

0.82 750 0.75

0.87 1000 1.00

Electro-discharge drilling of metal matrix composites205

Table 14.3  Input parameters and their levels identified by the OFAT method

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Table 14.4  Experimental values of TWR and SR for EDD of HMMCs with the quenched copper tool Experimental information (Quenched tool electrode with HMMC) Process parameters Std. order

A (amp)

B

C (rpm)

D (kg/ cm²)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

9 15 9 15 9 15 9 15 9 15 9 15 9 15 9 15 6 18 12 12 12 12 12 12 12 12 12 12 12 12

0.72 0.72 0.82 0.82 0.72 0.72 0.82 0.82 0.72 0.72 0.82 0.82 0.72 0.72 0.82 0.82 0.77 0.77 0.67 0.87 0.77 0.77 0.77 0.77 0.77 0.77 0.77 0.77 0.77 0.77

250 250 250 250 750 750 750 750 250 250 250 250 750 750 750 750 500 500 500 500 0 1000 500 500 500 500 500 500 500 500

0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.5 0.5 0.5 0.5 0.5 0.5 0 1 0.5 0.5 0.5 0.5 0.5 0.5

Results TWR (mm3/ min)

SR (μm)

0.44 0.72 0.38 0.52 0.27 0.79 0.50 1.02 0.26 0.70 0.21 0.87 0.31 0.62 0.45 0.69 0.05 1.63 0.22 0.44 0.54 0.46 0.53 0.27 0.52 0.50 0.42 0.51 0.30 0.58

3.58 5.38 4.02 6.24 3.14 6.16 2.43 6.11 3.43 5.22 6.8 4.03 3.37 6.12 2.36 4.18 6.41 8.24 2.31 6.15 7.14 3.58 6.48 2.24 3.11 3.17 3.38 3.44 4.48 4.89

s­ urface locally. Therefore this model is usually a good estimation of the true response surface. The method of least squares can be applied to estimate the coefficients in a quadratic model and the quadratic model of “yi” for four process parameters is expressed by Eq. (14.1) [28]. 4

4

i =1

i =1

4

yi = β 0 + ∑β i xi + ∑β ii xi2 + ∑∑β ij xi x j ± ε ; i < j =2

(14.1)

Electro-discharge drilling of metal matrix composites207

Where, yi is desired response parameters; β0 is a constant; βi, βii, and βij represent the coefficients of linear, quadratic, and interaction terms, respectively; xi is variables corresponding to process parameters under the study; and ε is the experimental error. Regression analysis has been used to determine relationships between the parameter characteristics [29, 30]. The TWR and SR respresented by y1 and y2, respectively, were analyzed. The backward elimination process was used to eliminate the insignificant terms to adjust the quadratic equations. The quadratic equations for TWR and SR during EDD using the qunched copper tool electrode are obtained in terms of actual values of design factors as, y1 = −5.02 − 0.18 × A + 17.35 × B − 3.08 × 10 −003 × C − 0.45 × D + 1.84 × 10 −003 × A × B + 4.09 × 10 −006 × A × C + 0.017 × A × D + 4.08 × 10 −003 × B × C + 0.67 × B × D − 4.62 × 10 −004 × C × D + 0.01 × A2 − 12.28 × B2 + 1.90 × 10 −007 ×C 2 − 0.21 × D 2 ± ε y2 = −24.28 − 0.41 × A + 55.21 × B + 0.01 × C + 11.16 × D − 1.83 × A × B + 6.85 × 10 −004 × A × C − 0.59 × A × D − 0.03 × B × C − 6.55 × B × D − 2.07 × 10 −003 × C × D + 0.08 × A2 − 3.70 × B2 + 4.37 × 10 −006 ×C 2 + 0.37 × D 2 ± ε

(14.2)

(14.3)

14.4.5 Quenched tool with HMMC The adequacy of the model is tested using the sequential F-test and the analysis of variance technique (ANOVA) to obtain the best fit model. ANOVA tests for individual and interative process parameters for responses have been conducted. The fit summary reveals that the fitted quadratic equations are statistically significant to analyze the value of desired responses as shown in Table 14.5. The multiple regression coefficient R-squared value for TWR and SR is found to be 0.83 and 0.69, respectively. These R2 values are close to 1, which is desirable. The associated P-value of less than 0.05 (i.e., 95% confidence level) indicates that the model terms can be considered as statistically significant. The lack of fit value of the model indicates non-significance as desired.

14.4.6 Optimization of process parameters The optimization of the individual response parameters was performed for achieving the desired TWR and SR. The standard Design Expert software was used to optimize the responses during EDD of HMMCs. The target value for TWR and SR were set as minimum. The experiments were carried out at the optimal parametric settings for TWR and SR so that targeted value of the response parameters could be obtained. The optimal values of input process parameters are shown in Table 14.6. The parametric combination with the highest desirability value (0.770) is selected as the best machining conditions, according to the desirability criteria. This approach makes use of a technique for combining multiple responses into a dimensionless measure of performance called the desirability function (d), bounded by 0≤d≤1. The developed models can predict the responses adequately within the limits of machining parameters being used. The value of desirability was taken as 1 [24, 31]. Table 14.6 shows the predicted

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Table 14.5  Analysis of variance (ANOVA) for the fitted RSM models for the responses Sum of squares

Source

Degree of freedom

Mean squares

F-value

Prob>F

5.40 3.92

0.0012 0.0725

Significant Not significant

2.45 3.84

0.0484 0.0752

Significant Not significant

Results for TWR Model Lack of fit

2.12 0.37

14 10

0.15 0.037

Pure error Cor. total

0.048 2.54

5 29

9.532E-003

Results for SR Model Lack of fit

55.71 21.60

14 10

3.98 2.16

Pure error Cor. total

2.81 80.12

5 29

0.56

Table 14.6  Optimized process parameters and optimum value of TWR and SR Optimized value of input parameters

Modified value of input parameters

A

B

C

D

A

B

C

D

12.59

0.82

665.50

0.75

12

0.82

500

0.75

Actual Predicted |Error|%

TWR (mm3/ min)

SR (μm)

0.52 0.50 3.85

3.07 2.97 3.26

values of TWR and SR obatined from the mathematical relationships as given in Eqs. (14.2) and (14.3). For testing the prediction ability of the model, prediction error in each output node has been calculated using Eq. (14.4).

( Actual value − Predicted value ) ×100

(14.4) Actual value It is observed from the validation experiments that there is a minor error percentage between the predicted and the experimental values, which designates that the developed model can yield nearly accurate results. The percentage error between the actual and the predicted values for TWR and SR are ±3.72%, 3.85%, and 3.26%, respectively. Predicting error% =

14.5 Conclusions The following conclusions can be drawn from the present research: ●

●

The EDD process is sutabile for hole making (drilling) in HMMCs. We used the OFAT approach to experimentation, which gave significant results.

Electro-discharge drilling of metal matrix composites209 ●

●

●

●

●

●

●

●

We used a solid cylindrical copper tool electrode and a quenched copper tool electrode and compared the performance of each in terms of TWR and SR. The quenched copper electrode is comparatively a better electrode as it produces minimum TWR and improved SR compared to the normal copper tool electrode. We used RSM to analyze the data. We reported on optimization of the input process parameters for obtaining minimum TWR and SR values. The discharge current has a direct effect on TWR and SR. This parameter contributes positively with a statistically significant effect on TWR and SR. Duty factor of the spark duration and flushing pressure have a considerable effect on TWR and SR. There is an imminent need to improve the performance of the EDD process, and the present research initiative is a step in this direction. In the future, experimental investigation may be carried out to establish optimum parameters for generating cost-effective, high-quality holes in difficult-to-machine materials such as HMMCs.

References [1] I. Singh, S. Chaitanya, R. Kumar, Material removal processes for metal matrix composites, in: J. Paulo Davim (Ed.), Metal Matrix Composites, De Gruyter, Aveiro, Portugal, 2014, pp. 141–155. ISBN: 978-3-11-031544-8. [2] H.H. Kim, J.S.S. Babu, C.G. Kang, Fabrication of A356 aluminum alloy matrix composite with CNTs/Al2O3 hybrid reinforcements, Mater. Sci. Eng. A 573 (2013) 92–99. [3] Z.M. Du, J.P. Li, Study of the preparation of Al2O3/SiCp/Al composites and their wear-­ resisting properties, J. Mater. Process. Technol. 151 (2004) 298–301. [4] S.  Suresha, B.K.  Sridhara, Wear characteristics of hybrid aluminium matrix composites reinforced with graphite and silicon carbide particulates, Compos. Sci. Technol. 70 (2010) 1652–1659. [5] T. Rajmohan, K. Palanikumar, J. Davim, Analysis of surface integrity in drilling metal matrix and hybrid metal matrix composites, J. Mater. Sci. Technol. 28 (2012) 761–768. [6] R.  Kumar, A.  Singh, I.  Singh, Electric discharge hole grinding in hybrid metal matrix composite, Mater. Manuf. Process. 32 (2017) 127–134. [7] R. Kumar, I. Singh, Electric discharge sawing of hybrid metal matrix composites, Proc. Inst. Mech. Eng. B J. Eng. Manuf. 231 (2017) 1775–1782. [8] R. Kumar, I. Singh, D. Kumar, Electro discharge drilling of hybrid MMC, Proc. Eng. 64 (2013) 1337–1343. [9] N. Singh, P. Raghukandan, B.C.K. Pai, Optimization by grey relational analysis of EDM parameters on machining Al 10%SiCP composites, J. Mater. Process. Technol. 155 (2004) 1658–1661. [10] N.  Singh, P.  Raghukandan, K.  Rathinasabapathi, B.C.K.  Pai, Electric discharge machining of Al 10%SiCPas-cast metal matrix composites, J. Mater. Process. Technol. 155 (2004) 1653–1657. [11] M.  Rozenek, J.  Kozak, L.  Dabrowski, K.  Lubkowski, Electrical discharge machining characteristics of metal matrix composites, J. Mater. Process. Technol. 109 (2001) 367–370. [12] M. Ramulu, G. Paul, J. Patel, EDM surface effects on the fatigue strength of a 15vol% SiCp/Al metal matrix composite material, Compos. Struct. 54 (2001) 79–86.

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[13] S.  Kachhap, A.  Singh, K.  Debnath, Electric discharge drilling of hybrid metal matrix composites using different tool electrodes, J. Sci. Ind. Res. 77 (2018) 525–529. [14] P. Kuppan, S. Narayanan, R. Oyyaravelu, A.S.S. Balan, Performance evaluation of electrode materials in electric discharge deep hole drilling of Inconel 718 superalloy, Proc. Eng. 174 (2017) 53–59. [15] S. Arooj, M. Shah, S. Sadiq, S.H.I. Jaffery, S. Khushnood, Effect of current in the EDM machining of aluminum 6061 T6 and its effect on the surface morphology, Arab. J. Sci. Eng. 39 (2014) 4187–4199. [16] R.  Choudharya, V.K.  Gupta, Y.  Batra, A.  Singh, Performance and surface integrity of Nimonic75 alloy machined by electrical discharge machining, Mater. Today Proc. 2 (2015) 3481–3490. [17] M. Shabgard, M. Seyedzavvar, S.M.B. Oliaei, Influence of input parameters on the characteristics of the EDM process, J. Mech. Eng. 57 (2011) 689–696. [18] K.D. Chattopadhyay, P.S. Satsangi, S. Verma, P.C. Sharma, Analysis of rotary electrical discharge machining characteristics in reversal magnetic field for copper-EN8 steel system, J. Adv. Manuf. Technol. 38 (2008) 925–937. [19] R. Teimouri, H. Baseri, Study of tool wear and overcut in EDM process with rotary tool and magnetic field. Adv. Tribol. (2012) 1–8, https://doi.org/10.1155/2012/895918. [20] B. Mohan, A. Rajadurai, K.G. Satyanarayana, Effect of SiC and rotation of electrode on electric discharge machining of Al-SiC composite, J. Mater. Process. Technol. 124 (2002) 287–304. [21] J.S.  Soni, G.  Chakraverti, Surface characteristics of titanium with rotary EDM, Bull. Mater. Sci. 16 (1993) 213–227. [22] N. Gupta, V. Bajpai, R.K. Singh, Characterization of micro-EDM process for pyrolytic carbon, in: Proceeding of the 7th International Conference on Micro Manufacturing (ICOMM 2010), Evanston, vol. 1214, 2012, pp. 204–207. [23] S. Kumar, S.K. Choudhury, Prediction of wear and surface roughness in electro-discharge diamond grinding, J. Mater. Process. Technol. 191 (2007) 206–209. [24] D.C. Montgomery, Design and Analysis of Experiments: Response Surface Methods and Designs, John Wiley and Sons, Inc, New Jersey, 2005. [25] K.Y. Benyounis, A.G. Olabi, Optimization of different welding process using statistical and numerical approaches—a reference guide, Adv. Eng. Softw. 39 (2008) 483–496. [26] A.I. Khuri, J.A. Cornell, Response Surface, Marcel Dekker, New York, 1987. [27] K.D.  Chattopadhyay, S.  Verma, P.S.  Satsangi, P.C.  Sharma, Development of empirical model for different process parameters during rotary electrical discharge machining of copper steel (EN-8) system, J. Mater. Process. Technol. 209 (2009) 1454–1465. [28] P. Kumar, P. Goel, Product quality optimization using fuzzy set concepts: a case study, Qual. Eng. 15 (2002) 1–8. [29] K.T. Chiang, Modeling and analysis of the effects of machining parameters on the performance characteristics in the EDM process of Al2O3+TiC mixed ceramic, Int. J. Adv. Manuf. Technol. 37 (2008) 523–533. [30] Y.F. Tzeng, F.C. Chen, A simple approach for robust design of high speed electrical discharge machining technology, Int. J. Mach. Tools Manuf. 43 (2003) 217–227. [31] P. Shandilya, P.K. Jain, N.K. Jain, Modeling and analysis of surface roughness in WEDM of SiCp/6061 Al MMC through response surface methodology, Int. J. Eng. Sci. Technol. 3 (2011) 531–535.

Quality and performance assessments of drilling and punching in the hole-making process of a composite panel: A comparative study

15

S. Norisam, A.B. Abdullah School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia

15.1 Introduction Composite materials have been extensively used in various applications, particularly in the aerospace industry, due to their capability to provide the required engineering properties, such as high strength-to-weight and stiffness-to-weight ratios. For structural applications, two types of composites are available, namely, composite laminates and sandwich composite structures. A composite laminate is a combination of fibrous composite materials (fibers in a matrix) that are bonded together layer by layer to obtain the required engineering properties, such as bending stiffness, strength, and in-plane stiffness. The individual layers consist of high-modulus, high-strength fibers in a polymeric, metallic, or ceramic matrix material. The most common fibers used include graphite, glass, boron, and silicon carbide. The typical matrix materials used are epoxies, polyimides, aluminum, titanium, and alumina. The general structure of a composite laminate is shown in Fig. 15.1. Assembly is inevitable for structural composites. Holes are typically produced for assembly. Hole-making technology for the composite panels can be generally categorized into two, namely, machining and non-machining. The most commonly used machining technique in hole-making industries is drilling. Previous research has reported that drilling induces damage, such as spalling, delamination, edge chipping, fiber pullout, crack formation, and excessive tool wear. In addition, drilling is time consuming because tools should be changed frequently given the different sizes of holes and due to wear. Nevertheless, holes produced from drilling exhibit good quality and low delamination level. In the aircraft manufacturing industry, thousands of holes are required to be produced; hence, time is a crucial factor. Punching is another technique that is commonly used to produce holes, particularly on metals. However, this approach is still new for composites, and only a few published works related to this application are available. Previous research has identified die clearance as one of the most significant factors that influences punching. The cut surface quality of a hole produced on a composite panel via the punching method remains Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00015-5 © 2019 Elsevier Ltd. All rights reserved.

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Fig. 15.1  Laminated composite structure.

undetermined. In addition, practical work that applies the punching method is limited and relatively insufficient; thus, further experimental research should be conducted. Although punching and drilling are common methods used in the hole-making process, they exhibit several differences in terms of operation. Drills use a rotating bit, whereas punches use reciprocating male and female dies and do not involve any rotating mechanism. Drilling is one of the cutting processes that use a drill bit to cut and enlarge a hole with a circular cross section and profile in solid materials. The drill bit is pressed against the specimen and rotated at a rate of hundreds to thousands of revolutions per minute, depending on the thickness of the specimen. A punch does not involve any rotating mechanism, and thus, it provides several advantages, including low cost and economical mass production. Previous works have found that punching is a potential and promising alternative to replace drilling in hole-making operations. However, quality, such as complete shearing and hole neatness, has become a major issue in punching.

15.2 Hole quality 15.2.1 Hole diameter The quality of a produced hole depends on its diameter. An inaccurate diameter may affect the quality of the assembled structure. Therefore, the diameter is the main concern and the commonly measured parameter in most hole-making methods. Sakib et al. [1], Caggiano et al. [2], and other researchers used the exit and entry diameters when referring to quality assessment performed on a hole produced via the drilling technique. They observed that the sizes of the entry and exit diameters would be affected by the number of holes produced. Die clearances were based on the actual industrial applications according to the precision punching experiment carried out on the composite panels by Chan et al. [3]. The quality of the cut surface was evaluated based on the three aspects: top surface diameter, bottom surface diameter, and incomplete shearing ratio. The experiment indicated that die clearance did not considerably affect the top surface diameter compared with the bottom surface diameter.

15.2.2 Hole neatness The characteristic of composites that comprise resin and fiber results in difficulty in hole-making operations. Apart from causing wear, this condition leads to difficulty in achieving neat fiber cut, which is typically nonuniformly distributed and oriented.

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213

On the basis of Ghabezi et al. [4], the uncut fiber factor (UCFF) can be calculated using Eq. (15.1), where AHole is the diameter of the drill in mm2, Ao is the area between the hole circle and the maximum delamination zone in mm2, and A1 is the area between the hole circle and the minimum damage zone. Uncut fiber factor, UCFF =

A1 AHole

(15.1)

In accordance with Chan et al. [3], hole neatness is evaluated based on the incomplete shearing and calculated using the following equation: Ratio of incomplete shearing =

A − AC A

(15.2)

The perimeter of the surface diameter of a hole was illustrated manually to measure incomplete shearing. The value of the illustrated area was generated automatically using operating software. The value was regarded as the hole area (A), where AC is the clean hole area that can be generated automatically via operating software. The incomplete ratio is calculated using Eq. (15.2).

15.2.3 Delamination factor The delamination level indicates whether the hole-making method affects the composite panel. A high delamination level may result in low hole strength. Chan et al. [3] provided a comparison measure called the delamination factor (Fd), which enables the analysis and evaluation of the delamination extent of the composite laminates. The delamination factor is defined as the quotient of the maximum delaminated diameter (Dmax) and the hole nominal diameter (Do), as shown in the following equation: Delamination factor, Fd =

Dmax Do

(15.3)

The delamination factor is measured based on the maximum delaminated and hole diameters. Delamination extension is evaluated through nondestructive testing (NDT) based on Durão et  al. [5]. Examples of NDT include using a tool maker’s microscope [6], ultrasound techniques [7], acoustic emission [8], enhanced radiography [9], C-scan [10], computerized tomography [11], and image analysis [2]. All these methods are used to capture images that represent a hole’s surrounding area, which can then be analyzed and measured in terms of area and diameter. In accordance with Ghabezi et al. [4], the holes produced on the composite using the drilling technique was assessed based on the two quality aspects: the delamination factor and UCFF. The delamination factor and UCFF can be calculated using Eq. (15.4), where AHole is the diameter of the drill in mm2 and Ao is the area between the hole circle and the maximum delamination zone in mm2. A Delamination factor, Fd = o (15.4) AHole

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Tsao and Hocheng [10] introduced the equivalent delamination factor (Fed), which is expressed as Equivalent delamination factor, Fed =

De DUCFF

(15.5)

 4 ( Ad + Ao )  (15.6) Equivalent delamination diameter, De =   , π   where De and D represent the equivalent delamination diameter and the hole diameter, respectively; Ad is the delamination area; and Ao is the area of the hole. 0.5

15.2.4 Surface roughness Although surface roughness does not considerably affect the assembly, it reflects the cleanliness of a hole. Wern et al. [12] investigated the surface structure of composite drilled holes through an experiment. In their study, profilometry was used to study the texture of surfaces. The results indicate that the surface produced using Drill b was nearly four times rougher than that produced using Drill a when the feed rate was low. Surface roughness decreased with an increase in feed rate. A similar approach was adopted by Ashrafi et al. [13] to evaluate the fiber pullout effect on surface roughness when drilling carbon fiber-reinforced plastic. In accordance with Kumar and Singh [14], the surface roughness of holes drilled on the composite panels was measured using an SJ-210 stylus-type profilometer (Mitutoyo America, Inc.). Surface roughness was measured at the entrance and exit of the produced holes. Four measurements were obtained for each hole at various cut sections of the entrance and exit. Analysis was conducted using the average value. In this work, a cutoff length of 0.08 mm was selected for surface roughness measurement at the entrance and exit of a hole. The result indicated that surface roughness at the entrance is higher than that at the exit because less damage occurred at the exit. Tan et  al. [15] investigated the surface roughness of drilled hybrid carbon/glass composite. They used the average arithmetic surface roughness (Ra) for surface quality response. The Ra value was measured using a surface measuring tool, that is, Tokyo Seimitsu Handysurf (E-35A). The results of surface roughness frequently depend on the deviation of the nominal surface with respect to the cutting surface. The method used for the hole-making process may also affect the surface roughness of a hole. This conclusion is based on the observation of Alberdi et al. [16] on abrasive water jet.

15.3 Performance Performance is a measure of efficiency for a given task. It can be assessed based on different criteria, and the aspects depend on the set target. For machining, processing and setup times may be used as criteria for assessing performance. Uhlmann et  al. [17] compared the processing times for axial drilling and helical milling from the perspective of economical production. They produced 10, 5, and 5 bore holes with

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215

diameters of 5, 8, and 12 mm, respectively, using two types of machining techniques, namely, axial drilling and helical milling. The processing times for each technique were compared. The results showed that helical milling reduced processing time by 5.85 s although it was slower than drilling due to obsolete tool changes. Ramulu and Spaulding [18] found that an electrical discharge machining die sinker is insufficiently fast to accommodate industrial expectations because the amount of time required to produce a hole with acceptable quality is relatively high. Meanwhile, drilling typically requires more than one step. However, single-shot drilling was recently introduced to minimize positioning error and processing time [19]. The simplest solution for increasing productivity is increasing the feed rate. However, this process may cause high tool wear [20]. Therefore, the process should be optimized to obtain optimal quality hole at less time.

15.4 Methodology The methodology used in the current study consists of five stages. The specimen was prepared in Stage 1. The hole-making method that would be used was selected in Stage 2. The experiment was carried out in Stage 3. Data were collected and analyzed in Stage 4. Finally, the performance and quality of the produced holes were assessed in Stage 5.

15.4.1 Fabrication of the specimen In this study, glass fiber-reinforced composite panel was fabricated using a hand lay-up technique. A composite panel that measured 300×300 mm was cut into three smaller panels (the samples for the experiment) with dimensions of 90×270 mm, as shown in Fig.  15.2. Six samples (called batch) were collected and five holes were produced on each batch. Each sample was marked, and the details of the specimen are provided in Fig. 15.3. The thickness of the panel is between 2.20 and 2.35 mm due to manual layup.

15.4.2 Hole-making methods The performance of the drilling and punching methods is investigated in this section.

15.4.2.1 Method 1: Drilling Several holes with Ø10 mm were produced on the composite panels using the conventional drilling method. The parameters were studied by many researchers, and the experiment was carried out on the composite panels. The productivity of the conventional drilling method was measured by recording the time consumed to produce several holes with Ø10 mm. The quality of the produced holes was evaluated and measured based on the two quality aspects: delamination factor and surface roughness.

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300 mm

300 mm

Composite panel

270 mm 90 mm

270 mm 90 mm

Composite panel

Composite panel

270 mm 90 mm

Composite panel

Fig. 15.2  Preparation of specimen. 45 mm

45 mm 90 mm

270 mm

Fig. 15.3  Specimen geometries.

15.4.2.2 Method 2: Punching Several holes with Ø10 mm were produced on the composite panels using the conventional punching method. Recent studies have reported that die clearance exerts a major influence on punching. Therefore, die clearance was determined using the equation proposed by Suchy [21]. c = K S, t where c is a single die clearance; t is the strip thickness; S is the material shearing strength; and K is the clearance coefficient, whose scale is K=0.008–0.01. Shearing strength can be estimated using S=0.7 ultimate tensile strength. Hence, a die set with a 10-mm punch diameter was selected for this experiment. The quality of the produced holes was measured based on the three aspects: delamination factor, top and bottom surface diameters of the holes, and surface roughness. The productivity of the

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217

p­ unching method was measured based on the time consumed to produce several holes on the composite panels.

15.4.3 Experimental setup A laboratory die rig was placed on an Instron 3367 universal testing machine (UTM) with a punch travel speed of 5 mm/s, which is 50% of the speed adopted in the industry (10 mm/s). The composite panel was clamped and precisely positioned to prevent the punch from moving downward. Three samples were used for this punching method to produce five holes with Ø10 mm on each sample. The composite panel was clamped to the working table of a conventional milling machine. A Ø10-mm highspeed steel CO 8% 4 flute end mill tool was used for the drilling method to produce Ø10 mm holes on the composite panel. A spindle speed of 1500 rpm and a feed rate of 0.11 mm/min were set as the ideal parameters in performing this method. Three samples were used for this drilling method to produce five holes with Ø10 mm on each sample.

15.4.4 Specimen analysis The images of the produced holes were captured using a USB microscope. An ordinary ruler was placed beside the holes to ensure accurate measurements. Fig.  15.4 shows the setup of the USB microscope. The images captured by the USB microscope were analyzed using the software ImageJ. This software can be used to obtain measurements, such as areas, perimeters, and lengths, of selected surfaces of images. Moreover, this software is accurate, time saving, and user friendly. The area of the produced holes can be determined by manually sketching the holes and then using ImageJ to obtain the measurement. The maximum and nominal areas of the produced holes were measured in this stage. The maximum and nominal diameters were calculated using the formula for the area of a circle. Fig. 15.5A and B shows the measurement of the maximum diameter of the produced holes using ImageJ.

Display camera

Microscope Specimen

Fig. 15.4  USB microscope.

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Fig. 15.5 (A)  Setting up scale using ruler. (B) ImageJ software.

15.4.4.1 Surface roughness The surface roughness of the produced holes for each sample was measured using the surface and roughness tester Supercom-130A. This machine analyzed the holes produced on the sample and measured the mean roughness value Ra in microns. Fig. 15.6 shows the setup of the Supercom-130A.

15.4.5 Performance assessment The performance of the punching and drilling techniques was compared in terms of the time consumed to produce the holes on the composite panels. Processing time was recorded using a stopwatch for the drilling technique. The feed rate and spindle speed were constant. The feed rate was set to the highest value that could be achieved by the machine. For the punching technique, the time consumed to produce the holes was obtained using the UTM. For both techniques, the setup time considered every step required before the operation could proceed.

15.5 Result and discussion The image captured using the USB microscope is shown in Table 15.1. Only three out of the five holes produced on the six batches are shown in the table for both techniques.

Fig. 15.6  Supercom-130A surface and roughness tester.

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Table 15.1  Image of produced holes by both punching and drilling techniques Technique

Batch

Punching

A1

A2

A3

Drilling

B1

B2

B3

Hole 1

Hole 2

Hole 3

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The quality of the produced holes is clearly shown between the two techniques based on the images. Tables 15.2 and 15.3 provide the pattern of the delamination factor for both techniques. The trends of the result suggest that the punching method has a relatively higher value (with an average delamination ratio of 1:15) than the Table 15.2  Delamination factor pattern on hole produced by punching Batch

Hole #

Delamination factor Fd=(Dmax/D)

A1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1.073 1.100 1.048 1.087 1.080 1.118 1.105 1.204 1.210 1.232 1.234 1.206 1.202 1.200 1.154

A2

A3

Average

1.1502

Standard Deviation

0.065

Table 15.3  Delamination factor pattern on hole produced by drilling Batch

Hole #

Delamination factor Fd=(Dmax/D)

B1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1.043 1.048 1.034 1.032 1.017 1.031 1.021 1.036 1.043 1.035 1.018 1.023 1.031 1.043 1.034

B2

B3

Average

1.033

Standard Deviation

0.010

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Table 15.4  Images of hole for both techniques with highest delamination factor Technique

Highest delamination factor

Punching

1.234

Drilling

1.048

Image

drilling method (with an average delamination ratio of 1:032) or approximately 11% higher. This finding is expected because the pressure applied during punching causes considerable damage around the circular profile of the puncher. Furthermore, the high ductility of the composite material causes a large damage and is possibly noticeable if the die clearance is large. The smallest die clearance, that is, 5% of the specimen thickness, was used in this study. Compared with that in drilling, peel-up and push-down delaminations caused the separation of the top and bottom surfaces of the specimen [22], thereby leading to more efficient cutting and less damage to the surrounding area. The low standard deviation, which is close to the mean value of the delamination factor, indicates that the result is reliable. Table 15.4 shows the image of the highest delamination factor for both techniques. A clear damage area can be observed on the hole produced using the punching method compared with that produced using the drilling method. Therefore, drilling produced more net hole than punching. Another factor is the shape of the tools used to make holes. Compared with that punching, in which the tool used has a flat surface that directly comes in contact with the specimen, the drill bit used in drilling has a conical shape. This pattern may be achieved by introducing a puncher with a conical or tapered shape to lessen damage in the area and reduce the load required for punching, similar to that in Zain et al. [23]. Tables 15.5 and 15.6 provide the details of the measurement conducted on the holes produced via punching and drilling. The average value of surface roughness for drilling is 4.722 μm. Meanwhile, the average value of surface roughness for punching is 6.295 μm. This observation is expected because the shear-cut operation performed from the side (i.e., from the cutting edge to the leading edge) in drilling, whereas this operation is performed from the top (i.e., the sharp edge of the puncher and the die cause shear cutting) in punching. The fibers contained in composites are typically nonuniformly distributed. Furthermore, the orientation of the fibers is also not uniform, as shown in Fig. 15.1. Therefore, a clean cut is nearly impossible in punching. Tables 15.7–15.9 present the results of the processing and setup times of the punching and drilling techniques. Processing time is the time required to ­perform

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Table 15.5  Pattern of surface roughness measured on hole surface produce by punching Batch

Hole #

Surface roughness, Ra (μm)

A1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

5.800 6.535 6.250 6.309 6.546 6.100 6.893 6.185 6.235 6.310 6.498 6.263 5.962 6.111 6.431

Average

6.295

Standard Deviation

0.265

A2

A3

Table 15.6  Pattern of surface roughness measured on hole surface produce by drilling Batch

Hole #

Surface roughness, Ra (μm)

B1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

5.241 5.453 5.549 4.926 5.151 4.568 4.133 4.094 4.821 5.873 5.122 4.156 3.986 3.452 4.302

Average

4.722

Standard Deviation

0.688

B2

B3

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Table 15.7  Performance of the punching technique Batch

Hole #

Time taken to produce a hole

Total time taken to produce five holes

Set up time

A1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1.228 s 1.248 s 1.310 s 1.496 s 1.422 s 1.516 s 1.438 s 1.262 s 1.252 s 1.294 s 1.534 s 1.418 s 1.308 s 1.250 s 1.336 s

464 s

457.30 s

440 s

433.24 s

424 s

417.15 s

20.312 s

1328 s

1307.69 s

A2

A3

Total

Table 15.8  Performance of drilling technique

Batch

Hole #

Time taken to produce hole, To

B1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

36.60 s 36.40 s 36.50 s 36.80 s 37.00 s 35.66 s 37.00 s 38.00 s 38.50 s 38.00 s 38.30 s 39.20 s 38.50 s 37.80 s 38.50 s

548 s

364.7 s

450 s

262.84 s

460 s

267.7 s

562.76 s

1458 s

895.24 s

B2

B3

Total

Total processing time to produce five holes, T1

Set up time = (T1−To)

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Table 15.9  Comparison of processing time for different techniques in hole-making Technique

Time taken to produce 15 holes of Ø10 mm

Set up time to produce 15 holes of Ø10 mm

Punching Drilling

1.35 s 35.51 s

21.79 min 14.92 min

a hole-making operation. As mentioned earlier, 15 holes with a diameter of 10 mm were produced using each technique. In addition, time started only when the puncher or drill touched the specimen. The punching technique was conducted on the UTM; thus, the time for obtaining the hole profile for the punching technique is highly precise compared with that for the drilling technique, in which a stopwatch was used. Setup time is the time required to prepare the machine and jig prior to operation. The processing time for punching is considerably faster than that for drilling based on the result. The feed rate is the most influential parameter of processing time. Therefore, the maximum feed rate of the machine was used in this study. Nevertheless, drilling still requires a longer time than punching. In particular, drilling requires an average of 35.51 s, whereas punching requires only 1.35 s. Setup time is always restarted for each batch to observe the time consumed and to ensure consistency of measurement. On average, punching requires approximately 46% longer setup time than drilling.

15.6 Conclusions and future works The main objective of this work was to compare two hole-making techniques, namely, punching and drilling, in terms of hole quality and process performance. Two quality aspects, namely, delamination factor and surface roughness, were evaluated. From the experiment results, punching and drilling have less considerable effect difference in the delamination factor. The average value of the delamination factor for the drilling technique is 1.032, whereas that for the punching technique is 1.150. The surface roughness values for the punching and drilling techniques are 6.295 and 4.722 μm, respectively. In terms of performance, the punching technique consumed shorter processing time than the drilling technique. However, the setup time required for drilling is shorter than that for punching. In the future, the quality of holes made using different methods, including punching and drilling, may be evaluated and compared from the economic perspective.

Acknowledgment The authors want to acknowledge Universiti Sains Malaysia for their sponsorship through Research University Grant (Acc. No. 1001/PMEKANIK/814270).

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[19] J. Fernández-Pérez, J.L. Cantero, J. Díaz Álvarez, M.H. Miguélez, Composite fiber reinforced plastic one-shoot drilling: quality inspection assessment and tool wear evaluation, Procedia Manuf 13 (2017) 139–145. [20] C.L. Kuo, S.L. Soo, D.K. Aspinwall, W. Thomas, S. Bradley, D. Pearson, R. M’Saoubi, W. Leahy, The effect of cutting speed and feed rate on hole surface integrity in single-shot drilling of metallic-composite stacks, Procedia CIRP 13 (2014) 405–410. [21] I. Suchy, Handbook of Die Design, McGraw-Hill, 2006. [22] A.B. Abdullah, M.S.M. Zain, Z. Samad, Delamination assessment of punched holes on laminated composite panels based on the profile measurement technique, Int. J. Adv. Manuf. Technol. 93 (1-4) (2017) 993–1000. [23] M.S.M.  Zain, A.B.  Abdullah, Z.  Samad, Effect of puncher profile on the precision of punched holes on composite panels, Int. J. Adv. Manuf. Technol. 89 (9–12) (2017) 3331–3336.

Further reading [1] T.B. Hilditch, P.D. Hodgson, Development of the sheared edge in the trimming of steel and light metal sheet, J. Mater. Process. Technol. 169 (2) (Nov. 2005) 184–191. [2] D.  Iliescu, D.  Gehin, M.E.  Gutierrez, F.  Girot, Modeling and tool wear in drilling of CFRP, Int. J. Mach. Tools Manuf. 50 (2) (Feb. 2010) 204–213. [3] M. Khoran, P. Ghabezi, M. Frahani, M.K. Besharati, Investigation of drilling composite sandwich structures, Int. J. Adv. Manuf. Technol. 76 (2015) 1927–1936. [4] E.D. Eneyew, M. Ramulu, Experimental study of surface quality and damage when drilling unidirectional CFRP composites, J. Mater. Res. Technol. 3 (4) (2014) 354–362. [5] A.I. Azmi, R.J.T. Lin, D. Bhattacharyya, Machinability study of glass fibre-reinforced polymer composites during end milling, Int. J. Adv. Manuf. Technol. 64 (1–4) (2013) 247–261. [6] M. Kurt, Y. Kaynak, E. Bagci, Evaluation of drilled hole quality in Al 2024 alloy, Int. J. Adv. Manuf. Technol. 37 (11–12) (2008) 1051–1060. [7] Y. Tyagi, V. Chaturvedi, J. Vimal, Parametric optimization of drilling machining process using Taguchi design and ANOVA approach, Int. J. Emerg. Technol. Adv. Eng. 2 (7) (2015) 2250. Website: www.ijetae.com. [8] P.N.E. Naveen, M. Yasaswi, R.V. Prasad, Experimental investigation of drilling parameters on composite materials, IOSR J. Mech. Civ. Eng. 2278-1684, 2 (3) (2012) 30–37. [9] A.M. Abrão, J.C.C. Rubio, P.E. Faria, J.P. Davim, The effect of cutting tool geometry on thrust force and delamination when drilling glass fibre reinforced plastic composite, Mater. Des. 29 (2) (2008) 508–513. [10] T.C.  Lih, A.I.  Azmi, N.  Muhammad, Delamination and surface roughness analyses in drilling hybrid carbon/glass composite, Mater. Manuf. Process. 31 (10) (2016). [11] A.  Shirobokov, S.  Kerchnawe, D.  Trauth, P.  Mattfeld, Characterization of the sheared edge quality of blanked carbon fibre reinforced plastics, Adv. Mater. Res. 1140 (2016) 280–287.

Index Note: Page numbers followed by f indicate figures and t indicate tables. A Abrasive water jet machining (AWJM), 7–8, 8f Adhesive/adhesive-bonded joints, 33, 34 Al/CFRP/Al-stacked panel, punched hole burr assessment burr height, 187–189, 188–190f composite materials, 181–182 composite panel, preparation of, 183, 183f, 183t drill geometry, 181–182 entry and exit hole, 185 experimental setup, 184, 184f precision of, 181–182 punching load, 186, 186–187f specimen analysis, 184, 185f Al/CFRP/Ti composite, 22–23, 23f Aluminium stacks holes drilling, 142 Analysis of variance (ANOVA), 102, 163, 166, 169–171, 169–170t B Bearing strength, 31 Bearing test, 33, 35–38, 35f procedure A, 36–37, 36f procedure B, 37, 37f procedure C, 37–38, 38f procedure D, 38, 39f Bio-based polymer, 151 Bolted and adhesive joint, 34 Brad and Spur drill, 20–22, 20f, 48 Brad-point drill bit, 56, 57f Burr assessment, punched holes burr height, 187–189, 188–190f composite materials, 181–182 composite panel, preparation of, 183, 183f, 183t drill geometry, 181–182 entry and exit hole, 185 experimental setup, 184, 184f precision of, 181–182

punching load, 186, 186–187f specimen analysis, 184, 185f C Carbon fiber reinforced composite (CFRC), 48–49, 49f, 135 ANOVA, 169–171, 169–170t composite fabrication, 164–166, 165t cutting velocities and feed rates, thrust force, 171–174, 171–175f FRPs, 161 machining composites, 162–164, 162f mathematical formulation, 166–167, 168t regression analysis, 175–178, 177t thrust force, 167–169, 168t ultrasonic vibration-assisted cutting of, 77–78 validation, 178, 179f Carbon fiber reinforced polymers (CFRPs), 5–6, 19 Ceramic/zirconia, 54–55, 55f CFRC. See Carbon fiber reinforced composite (CFRC) Chemical vapor deposition (CVD) method, 144 Chip formation analysis, drilling, 67–68, 67f Composite laminates delamination, 22–23, 23f drilling of chip formation analysis, 67–68, 67f cutting tool, geometry of, 63–64 drilling-inducedforces analysis, 69–72, 69–71f drilling test, 65–66, 65–66f, 65t force signals analysis, 68–69, 68f laminates, fabrication of, 64–65 measurements, 66–67 Composite materials, 149 Al/CFRP/Al-stacked panel, burr assessment of punched holes, 181–182

228Index

Composite materials (Continued) burr assessment, punched holes, 181–182 hole punching, burr assessment of, 181–182 laser drilling delamination suppress and thermal defects, composites, 92–94, 93f femtosecond ultrashort laser pulses, 96 laser percussion drilling, 90–91, 91f laser trepanning drilling, 91–92, 91f machining and fabrication processes, 89–90 material and manufacturing processes, 89 material removal, mechanism of, 90 nanosecond ultrashort laser pulses, 94–95, 95t PEEK-CF, 89 picosecond ultrashort laser pulses, 95–96 tapered hole formation, 92 ultrashort laser pulses, 94–96 underwater laser drilling, 97, 97f sustainability, 151 Composite panels, delamination, 17–18, 18f causes of, 19–24 CFRP, 19 core drills, type of, 21, 21f damage onset and propagation, 22 drilling parameters, 19 FRP drilling, prediction thrust force and torque, 19–20 HSD, 21–22 machining parameters and tool geometry study, 20 surface quality and damage, 22–23 composite fabrication, 164–165, 166f fiber-reinforced composites, drilling, 47–48 machining composites, 162–163 measurement, 24–27, 25–27f occurence, 57–58, 58f peel-up delamination, 18 push-down delamination, 18 regression analysis, 176–178 sustainability, 151–152, 153f in unidirectional carbon fiber composite, 101 Conventional drilling, 2–3 Core-candlestick drill, 21, 21f Core-saw drill, 21, 21f

Core-twist drill, 21, 21f Countersinking/counterboring, 52 Customization drill geometry, stacked materials, 145–146 D Dagger drill, 56, 57f Delamination, 17–18, 18f causes of, 19–24 CFRP, 19 core drills, type of, 21, 21f damage onset and propagation, 22 drilling parameters, 19 FRP drilling, prediction thrust force and torque, 19–20 HSD, 21–22 machining parameters and tool geometry study, 20 surface quality and damage, 22–23 composite fabrication, 164–165, 166f fiber-reinforced composites, drilling, 47–48 hole-making process, quality and performance assessments, 213–214, 220–221t machining composites, 162–163 measurement, 24–27, 25–27f occurence, 57–58, 58f peel-up delamination, 18 precision punching, 122, 123f puncher profile, 128 push-down delamination, 18 regression analysis, 176–178 sustainability, 151–152, 153f ultrasonic-assisted machining process, 77–78, 83–84 in unidirectional carbon fiber composite, 101 Diamond-like carbon (DLC) method, 144 Die clearance, precision punching, 117, 117t complete shearing, 125, 126f punching load, 125, 126f top and bottom diameter, 122–125, 125f Drilling, 1–2, 135 CFRC, machinability studies ANOVA, 169–171, 169–170t composite fabrication, 164–166, 165t cutting velocities and feed rates, thrust force, 171–174, 171–175f

Index229

FRPs, 161 machining composites, 162–164, 162f mathematical formulation, 166–167, 168t regression analysis, 175–178, 177t thrust force, 167–169, 168t validation, 178, 179f of composite laminates chip formation analysis, 67–68, 67f cutting tool, geometry of, 63–64 drilling-inducedforces analysis, 69–72, 69–71f drilling test, 65–66, 65–66f, 65t force signals analysis, 68–69, 68f laminates, fabrication of, 64–65 measurements, 66–67 FRC (see Fiber-reinforced composites (FRC), drilling) hole-making process, quality and performance assessments, 219t, 220–221, 221t composites, 211 delamination factor, 213–214, 220–221t experimental setup, 217 hole diameter, 212 hole neatness, 212–213 methods, 215–217 performance assessment, 218, 223t processing time, 221–224, 224t setup time, 221–224 specimen analysis, 217–218, 217–218f specimen fabrication, 215, 216f surface roughness, 214, 222t Drilling-inducedforces analysis, 69–72, 69–71f Drill point geometries, 66, 66f Drill reamer, 56, 57f Drill tool material, FRC, 52–56, 53f E E/D ratio, 36–37 E(electrical)-glass fiber fabrics, 80 Electro-discharge drilling (EDD), 9–10, 9f, 193–194 experimental study experimental procedure, 195, 195f, 196t surface roughness, 198–200, 199–201f TWR, 196–198, 196–199f experiments scheme, 194–195

RSM design matrix and experimental details, 204–207, 206t optimization methodology, 202, 203t parameter design, 204, 205t process parameter optimization, 207–208, 208t quenched tool with HMMC, 207, 208t steps of, 202 Epoxy burn, 59, 59f F Femtosecond laser pulses, 96 Fiberglass reinforced composite (FRP), 19–20 Fiber metal composite (FMC) laminates, 49, 50f Fiber pull-out delamination, 57–58, 58f Fiber-reinforced composites (FRC), drilling aircraft manufacturing, laminates material types, 48–49 CFRC and GFRC composite laminates, 48–49 FMC laminates, 49, 50f composite structures, 47 delamination, 47–48 drill tool design, 56, 57f drill tool material, 52–56, 53f ceramic/zirconia, 54–55, 55f HSS, 54 PCD, 56 tungsten carbide, 54 hole-making procedures, 50–51, 51f countersinking/counterboring, 52 predrilling process, 51–52 reaming, 52 hole quality, tool design influence delamination occurence, 57–58, 58f hole surface roughness, 58–59, 59f Fiber-reinforced polymers, 31, 101, 161 Finite element analysis (FEA), 101–102 Fishbone diagram method, 151–152 Force signals analysis, drilling, 68–69, 68f Fuzzing, 181–182 G Glass fiber reinforced composite (GFRC) laminates, 48–49, 49f Glass fiber reinforced epoxy (GFRE) composites, 19–20, 81–82

230Index

Glass fiber reinforced plastic (GFRP), 150, 152, 155 cohesive model, 102 cutting parameters, 101–102 experimental factors and levels, 103, 105t FEA analysis, 102 feed rate vs. drill material, 111–112f finite element study, 103–105, 105f drill bit meshing, 105, 106f material constitutive model, 105, 107t, 108f solver methodology, 105, 109f FRP and, 101 materials and equipment, 103, 104f speed vs. drill material, 107, 110–111f speed vs. feed rate, 107, 110f, 112f thrust force, 101–103, 105, 107 torque, 101–103, 107 unidirectional carbon fiber composite, delamination in, 101 Grinding drilling, 3–4, 4–5f H High speed drilling (HSD), 3, 21–22 High-speed steel (HSS) drill tool, 54, 163, 178, 179f Hole diameter error, 137, 139f Hole integrity, 31 Hole-making process defects, delamination, 17–24, 18f CFRP, 19 composite fabrication, 164–165, 166f core drills, type of, 21, 21f damage onset and propagation, 22 drilling parameters, 19 fiber-reinforced composites, drilling, 47–48 FRP drilling, prediction thrust force and torque, 19–20 HSD, 21–22 machining composites, 162–163 machining parameters and tool geometry study, 20 measurement, 24–27, 25–27f occurence, 57–58, 58f peel-up delamination, 18 push-down delamination, 18 regression analysis, 176–178 surface quality and damage, 22–23

sustainability, 151–152, 153f in unidirectional carbon fiber composite, 101 drilling and punching, quality and performance assessments, 219t, 220–221, 221t composites, 211 delamination factor, 213–214, 220t experimental setup, 217 hole diameter, 212 hole neatness, 212–213 methods, 215–217 performance assessment, 214–215, 218, 223t processing time, 221–224, 224t setup time, 221–224 specimen analysis, 217–218, 217–218f specimen fabrication, 215, 216f surface roughness, 214, 222t laminates, 1–11 machining, 2–7 conventional drilling, 2–3 grinding drilling, 3–4, 4–5f HSD, 3 milling, 6–7, 7f orbital drilling, 5–6, 6f VATD, 4–5 nonmachining methods, punching, 11, 11f nontraditional machining, 7–10 AWJM, 7–8, 8f EDD, 9–10, 9f LBM, 10, 10f sustainability issues in environment, 151, 155 machining performance improvement, 151–152, 153–154f sustainable composite material, 151 waste management system, 152–155, 156f Hole perpendicularity error, 32–33 Hole punching, burr assessment of burr height, 187–189, 188–190f composite materials, 181–182 composite panel, preparation of, 183, 183f, 183t drill geometry, 181–182 entry and exit hole, 185 experimental setup, 184, 184f precision of, 181–182

Index231

punching load, 186, 186–187f specimen analysis, 184, 185f Hole surface roughness, 58–59, 59f, 137–141, 140–141f Hollow cylindrical tools, 80–81 Hybrid joint, 34 Hybrid MMCs (HMMCs), 193–194, 204–207, 206t I Image processing, 24, 27 Intermittent contact-type ultrasonic-assisted drilling processes, 77–78 Internal delamination, 59, 59f

M Machining, hole-making technology, 2–7 conventional drilling, 2–3 grinding drilling, 3–4, 4–5f HSD, 3 milling, 6–7, 7f orbital drilling, 5–6, 6f VATD, 4–5 Material constitutive model, 105, 107t, 108f MATLAB, 24 Matrix cracking, 59, 59f Mechanical joints, 32–33, 35f Metal matrix composites (MMCs), 9–10, 152, 193–194 Milling, 1–2, 6–7, 7f

J Joints, structural integrity assessment failure mode, 42 standard test procedure bearing test, 35–38, 35f open hole and filled hole test, 39, 40f pin-bearing strength test, 41, 41f pull-through test, 40 types of, 32–34, 32f adhesive/adhesive-bonded joints, 33–34 hybrid joint, 34 mechanical joints, 32–33, 35f L Laser beam cutting technology, 10, 10f Laser drilling, composite material delamination suppress and thermal defects, composites, 92–94, 93f laser percussion drilling, 90–91, 91f laser trepanning drilling, 91–92, 91f machining and fabrication processes, 89–90 material removal, mechanism of, 90 materials and manufacturing processes, 89 PEEK-CF, 89 tapered hole formation, 92 ultrashort laser pulses, 94–96 femtosecond, 96 nanosecond, 94–95, 95t picosecond, 95–96 underwater laser drilling, 97, 97f Laser percussion drilling, 90–91, 91f Laser trepanning drilling, 91–92, 91f

N Nanosecond laser pulses, 94–95, 95t Nontraditional machining, hole-making technology, 7–10 AWJM, 7–8, 8f EDD, 9–10, 9f LBM, 10, 10f O One-factor-at-a-time (OFAT) approach, 194 Open hole and filled hole test, 39, 40f Orbital drilling (OD), 5–6, 6f Orthogonal array, 164, 165t P Peel-up delamination, 18, 57, 72–74, 72f Picosecond laser pulses, 95–96 Pin-bearing strength test, 41, 41f Polycrystalline diamond (PCD), 56, 144–145 Polycrystalline diamond-tipped eight-facet drill, 22–23 Polyetheretherketone (PEEK), 89 Polymer composites, 63 Precision punching, 115 die clearance complete shearing, 125, 126f punching load, 125, 126f top and bottom diameter, 122–125, 125f experimental setup, 120, 120f principles of, 116, 116f puncher profile complete shearing, 127, 128t

232Index

Precision punching (Continued) delamination, 128 punching load, 128 top and bottom diameter, 126–127, 127t punching mechanism, modification of, 128–130, 129–131f, 130–131t quality assessment, 120–122, 121f burr, 121 delamination, 122, 123f shearing edge, 120–121 top and bottom surface diameters, 120 studied parameters die clearance, 117, 117t puncher profile, 117, 118–119t Predrilling process, 51–52 Profile measurement technique, 181–182, 187–189 Pull-through test, 40 Pulsed Nd:YAG laser, 92–93 Punched holes. See Hole punching Puncher profile, precision punching complete shearing, 127, 128t delamination, 128 punching load, 128 top and bottom diameter, 126–127, 127t Punching nonmachining methods, hole-making technology, 11, 11f quality and performance assessments, holemaking process, 219t, 220–221, 221t composites, 211 delamination factor, 213–214, 220t experimental setup, 217 hole diameter, 212 hole neatness, 212–213 methods, 215–217 performance assessment, 218, 223t processing time, 221–224, 224t setup time, 221–224 specimen analysis, 217–218, 217–218f specimen fabrication, 215, 216f surface roughness, 214, 222t Push-down delamination, 18, 57–58, 58f, 72–74, 72f R Reaming, 52 Regression analysis, carbon fiber reinforced composites, 175–178, 177t

Response surface methodology (RSM) design matrix and experimental details, 204–207, 206t optimization methodology, 202, 203t parameter design, 204, 205t process parameter optimization, 207–208, 208t quenched tool with HMMC, 207, 208t steps of, 202 Rotary mode ultrasonic drilling, process parameters for, 81t Rotary ultrasonic elliptical machining, 77–78 S Si-AlONs, 55, 55f Single-shot drilling, 135–136 benefits and limitations, 142–143 hole quality issues, stacked materials, 136–141, 138f burr height at hole exit, 141 hole diameter error, 137, 139f hole surface roughness, 137–141, 140–141f Stacked materials practice in drilling of, 136, 137f single-shot drilling, hole quality issues in, 136–141, 138f burr height at hole exit, 141 hole diameter error, 137, 139f hole surface roughness, 137–141, 140–141f tool strategies, drilling, 143–146 customization drill geometry, 145–146 PCD application, 144–145 tool life extension, coating application for, 144 Standard test procedure, joints bearing test, 35–38 open hole and filled hole test, 39 pin-bearing strength test, 41, 41f pull-through test, 40 Step-core-candlestick drill, 21, 21f Step-core-saw drill, 21, 21f Step-core-twist drill, 21, 21f Step drill bit, 56, 57f Structure assembly, 32–34 Stub Length drill, 20, 20f Surface roughness (SR) EDD, 198–200, 199–201f

Index233

hole-making process, quality and performance assessments, 214, 218, 222t Sustainability environment, 151, 155 machining performance improvement, 151–152, 153–154f sustainable composite material, 151 waste management system, 152–155, 156f T Taguchi method, 103, 164 Tapered hole formation, 92 Thrust force carbon fiber reinforced composites, 167–169, 168t regression analysis, 176 validation, 178, 179f GFRPs, 101–103, 105, 107 Titanium-lamella-reinforced CFRP, 32–33 Tool wear rate (TWR), 196–198, 196–199f Tungsten carbide material tools, 54, 144 Twist dril 2-flute, 56, 57f Twist dril 4-flute, 56, 57f Twist drill bit, 56, 57f, 63–64, 66f, 72–74 U Ultrashort laser pulses, 94–96 femtosecond, 96 nanosecond, 94–95, 95t picosecond, 95–96 Ultrasonic-assisted machining process, 77–78

conventional drilling process, 82–83 delamination, 77–78, 83–84 drilled composite specimens, 81–82, 83f drilled hole cylindrical surface, 83–84f drilled hole wall, surface morphology of, 83–84 drilling operation, type of, 78t experimental setup, 80–81, 80f glass fiber-reinforced epoxy composites, machining of, 81–82 laminate fabrication, 80 machined hole, surface quality of, 82–83 material removal rate, 77–78 process parameters, rotary mode ultrasonic drilling, 81t process principle, 79 rotary mode ultrasonic drilling, 85f traditional twist drilling, 85f Ultrasonic-assisted twist drilling, 77–78 Underwater laser drilling, 97, 97f Unidirectional carbon fiber composite, delamination in, 101 V Vibration-assisted twist drilling (VATD), 4–5 W Waste management system, sustainability, 152–155, 156f Wooden back-up plate, 164–165 W-point drill bit, 56, 57f