The Effect of Layer Orientation on the Fatigue Behavior of 3D Printed PLA Samples 3031225724, 9783031225727

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SpringerBriefs in Applied Sciences and Technology

Computational Mechanics Series Editors Holm Altenbach , Faculty of Mechanical Engineering, Otto-von-Guericke-Universität Magdeburg, Magdeburg, Sachsen-Anhalt, Germany Lucas F. M. da Silva, Department of Mechanical Engineering, Faculty of Engineering, University of Porto, Porto, Portugal Andreas Öchsner, Faculty of Mechanical Engineering, Esslingen University of Applied Sciences, Esslingen, Germany

These SpringerBriefs publish concise summaries of cutting-edge research and practical applications on any subject of computational fluid dynamics, computational solid and structural mechanics, as well as multiphysics. SpringerBriefs in Computational Mechanics are devoted to the publication of fundamentals and applications within the different classical engineering disciplines as well as in interdisciplinary fields that recently emerged between these areas.

Fawaz Aladwani · Xiaodong Sun · Omar Es-Said · Rafiq Noorani · Mahsa Ebrahim · William Melmed · Brian Avchen · Spencer Trumpp · Nicholas Lee · Debbie Aliya

The Effect of Layer Orientation on the Fatigue Behavior of 3D Printed PLA Samples

Fawaz Aladwani Mechanical Engineering Department Loyola Marymount University Los Angeles, CA, USA

Xiaodong Sun Mechanical Engineering Department Loyola Marymount University Los Angeles, CA, USA

Omar Es-Said Mechanical Engineering Department Loyola Marymount University Los Angeles, CA, USA

Rafiq Noorani Mechanical Engineering Department Loyola Marymount University Los Angeles, CA, USA

Mahsa Ebrahim Mechanical Engineering Department Loyola Marymount University Los Angeles, CA, USA

William Melmed Mechanical Engineering Department Loyola Marymount University Los Angeles, CA, USA

Brian Avchen Mechanical Engineering Department Loyola Marymount University Los Angeles, CA, USA

Spencer Trumpp Mechanical Engineering Department Loyola Marymount University Los Angeles, CA, USA

Nicholas Lee Mechanical Engineering Department Loyola Marymount University Los Angeles, CA, USA

Debbie Aliya Grand. Rapids Aliya Analytical Inc. Grand Rapids, MI, USA

ISSN 2191-530X ISSN 2191-5318 (electronic) SpringerBriefs in Applied Sciences and Technology ISSN 2191-5342 ISSN 2191-5350 (electronic) SpringerBriefs in Computational Mechanics ISBN 978-3-031-22572-7 ISBN 978-3-031-22573-4 (eBook) https://doi.org/10.1007/978-3-031-22573-4 © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 Experimental Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3 Results and Discussions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 4.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

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Chapter 1

Introduction

Abstract Fused Deposition Modeling (FDM) which is an Additive Manufacturing (AM) process, fabricates 3-D parts from a build-up of 2-D layers. Polylactic Acid (PLA) is extruded through a heated nozzle to deposit the layers. PLA is one of the most common thermoplastic materials used in additive manufacturing. The objective of this research is to carry out experimental analysis of fatigue characteristics of FDM processed PLA samples. The specific objectives are to investigate the effects of different build orientations of single fiber samples such as 0°, 90°, and concentric (ASTM D638) and of bidirectional laminates such as 0°–90° and +45°/−45° on the fatigue, tensile, flexural and impact Izod properties of PLA samples. Samples were tested in high cyclic fatigue, fully reversed with a R ratio of −1. Each orientation was tested at different level of stresses ranging between 18 and 45.5 MPA (2.6–6.6 Ksi). The results indicate that the concentric build orientation provides the best result with highest cycles of fatigue, ultimate strength, and impact energy. On the other hand, the 90° orientation provided the poorest results. A modification to the concentric single fiber samples resulted in a 8–15% elongation in tensile tests. Keywords PLA · Fatigue · Fiber orientation · Additive manufacturing · Modified ductile concentric orientation Polylactic Acid (PLA) is a thermoplastic aliphatic polyester which was derived from renewable resources such as corn starch or sugarcane. Several studies focused on developing different adjustments to allow this material to be more suitable to a broader range of products [1]. PLA has many biomedical applications. For example, it is used in sutures, bone reconstruction, and in drug delivery devices [2, 3]. The rate at which PLA degrades as well as the mechanical properties are adjustable according to some parameters such as density, crystal structure, and molecular weight. In general, it degrades in a timespan of months to years [4]. PLA has been used in automotive parts because of its advantageous mechanical properties and light weight. PLA requires adjustments due to its brittle nature for use in most applications [5]. It has been projected as a renewable plastic alternative to produce mulch films, composting bags, various biomedical applications, and service ware [6]. Due to its biodegradability,

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 F. Aladwani et al., The Effect of Layer Orientation on the Fatigue Behavior of 3D Printed PLA Samples, SpringerBriefs in Computational Mechanics, https://doi.org/10.1007/978-3-031-22573-4_1

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1 Introduction

processability, and mechanical properties, PLA is finding increasing applications like recycling, reusing and recovering [7]. PLA can simply be designed for many applications of fiber. The second biggest application field for PLA is its use as a film [8]. Many applications like foamed articles and paper coatings are considered because the raw materials for PLA are based mainly on agricultural feedstock. This results in an increased demand for PLA resins [9]. As the applications are continuing to vary, PLA is considered a good replacement for most conventional plastics. On the other hand, some of its material properties are known to be having some disadvantages. Generally, the brittleness and low heat deflection of PLA limit its use as a suitable thermoplastic [10]. The American Food and Drug Administration (FDA) favored PLA to be directly in contact with biological fluids [11, 12]. The mechanical properties of 100% dense PLA were investigated by Song et al. [13]. The PLA samples were printed in single directions. A comparison between the unidirectional 3D printed PLA and injection Molded PLA was made. It showed that 3D printing managed to increase the toughness of the material over the injection molded parts. Saini et al. [14] studied the effect of layer orientations of a stereolithography (SLA) polymer material and the difference that it caused on the mechanical properties. Five orientations were examined at 0°, 22.5°, 45°, 67.5°, and 90°. Different mechanical tests were conducted including tensile, compression, flexural, impact, fatigue, and vibration. For fatigue, the 0° samples were printed parallel to the load and the fracture edge was a smooth straight line. Similarly, the 90° samples were perpendicularly printed to the load direction and the fracture edge was in a smooth straight line. On the other hand, the 45°, 22.5° and 67.5° samples resulted in not smoothly lined cracks. Es-Said et al. [15] discussed the effects of layer orientation on mechanical properties of ABS samples in five different layer orientations by conducting tensile, three-point bending, and impact tests. The 0° (along the length) had the highest results of the ultimate strength and yield strength. The weakest results were found at the 90° and 45° orientations. The path for the crack propagation differed from one orientation to the other. Weak interlayer bonding could cause delamination along the layer interface. The 0° had the highest MOR (flexural strength). On the other hand, the 90° had the lowest value of MOR. The 0° orientation had the highest amount of absorbed energy while the 90° orientation had the lowest. All tests prove how significantly strong the 0° orientation was and how much it was better than the rest of the orientations. The fracture paths were mainly along the weak interlayer bonding or interlayer porosity [15]. Vega et al. [16] focused on the mechanical properties and toughness of a polymer combination of two polymers, ZP 130 and ZB 58, that are fused together in the Z Corporation Spectrum Z510 RP machine in order to study the effect of layer orientation. Seven orientations were tested varying in the combination of crack arrestor, crack divider, and short transverse in a tensile, three-point bending, and Izod tests. The mentioned orientations resulted in a variety of results, which were in accordance with the results of Es-Said et al. [15]. In a study conducted by Hassanifard et al. [17], some effects of part build directions or raster orientations have been viewed in terms of the strain-life fatigue parameters of 3D printed plastic materials such as Ultem 9085, Polycarbonate (PC), and Polylactic

1 Introduction

3

Acid (PLA). It was found that fill density is the most essential factor on fatigue life-span. Thermoplastics components that are produced by FDM process tend to have higher crystallinity as opposed to the injection molding produced components [18]. Priya et al. [19] characterized both the mechanical and thermal behavior of PLA and ABS considering their variety of properties as thermoplastic polymers. For both the ABS and PLA, the tensile and flexural properties were decreasing with the increase of layer thickness. On the other hand, the Izod impact energy was higher for layer heights and the infill patterns of ABS. This is believed to be due to the ductile behavior of ABS in which it can slow down the crack propagation while absorbing the impact energy. Scanning electron microscopy was conducted for both ABS and PLA specimens. For ABS, the fracture surface turned out to be smooth with a ductile fracture. For PLA, the microstructure was rough and brittle [19]. Espin et al. [20] investigated the fatigue performance of ABS specimens that were obtained by fused filament fabrications (FFF). Four building parameters such as layer height, nozzle diameter, infill density, and printing speed were considered while experimenting on cylindrical, rectilinear, and honeycomb specimens. Based on the results, the most effective factor is the infill density followed by the layer height and nozzle diameter in both the rectilinear and honeycomb specimens. Dizon et al. [21] discussed the mechanical characterization of 3D-printed polymers that were tested with different loading types like tensile, bending, compressive, and fatigue. Also discussed was the effects of adopting different additive manufacturing (AM) methods for polymers like Fused Deposition Modeling (FDM), Stereolithography (SLA), Selective layer sintering (SLS), Three-dimensional printing (3DP), Laminated object manufacturing (LOM), and PolyJet technology. Gomez-gras et al. [22] studied the fatigue performance of PLA samples manufactured through fused filament fabrication (FFF). The influence of layer height, fill density, nozzle diameter, and velocity were noted on the fatigue performance of cylindrical samples through an experimental design of L27 Taguchi that runs through two infills (linear and honeycomb) with a total of 54 samples manufactured. A rotating fatigue bending machine was used to test the samples on. It was shown that the fill density and layer height were the most influential parameters on the fatigue life respectively. The fatigue performance of PLA samples was mainly studied through building orientation and how they are manufacturing the samples with two different infill patterns being tested. A Taguchi orthogonal array was made for each one of the two infill patterns while taking four factors at three levels (layer height, fill density, nozzle diameter, and printing velocity). It was noted that the printing velocity has been left for interactions analysis and is the least effective out of all four. Afrose et al. [23] conducted a study on the fatigue behavior of PLA parts that were processed by FDM additive manufacturing process. The study examined the effect of part build orientations on the tensile fatigue properties of PLA material. Tensile samples were printed using a cube 3D printer in three different orientations (X, Y, and 45°). The samples were tested cylindrically at 80, 70, 60, and 50% nominal values of the ultimate tensile stress. As a result, the X orientation samples had the highest tensile stress when it was under static loading. On the other hand, the 45° samples had the highest fatigue life when it was under

4

1 Introduction

cyclic loading. Ziemian et al. [24] investigated the characterization of stiffness degradation caused by tensile fatigue damage of additive manufactured ABS-P400 parts. The specimen fiber orientations ranged from unidirectional specimens (0°, 45°, and 90°) to bidirectional specimens (0°/−90°, +15°/75°, +30°/60°, and +45°/−45°). As for the cyclical tensile response from the tension fatigue tests, the 90° specimens experienced the shortest fatigue life while the +45°/−45° specimens had the longest life. The 0° specimens represent the highest orientation in fatigue life for the unidirectional group while the +45°/−45° represents the same result for the bidirectional group. Moore et al. [25] studied the fatigue life and microstructure of the 3D printed elastomer material TangoBlackPlus. The process used to print this material is the Stratasys PolyJet material jetting process and is an additive manufacturing (AM) process where layers of photopolymer are printed using inkjet print heads. Directly after printing, UV lamps were used to cure the material which are located on either side of the print head. Due to the fact that no powder bed is used during this process, multiple materials can be printed in the same build. The presence of internal voids and lamination weaknesses between layers of additive manufactured components have been linked to premature fatigue failure. Guidelines to improve the fatigue life for 3D printed elastomeric components were suggested [25, 26]. Plastic parts are used in conditions of repetitive loading in aerospace, biomedical, and automotive industries. The objective of this study is to investigate the effects of layer orientation on the fully reversed bending fatigue behavior of 3D printed PLA samples. This behavior will be correlated to other mechanical properties.

References 1. Inkinen S, Hakkarainen M, Albertsson A-C, Södergård A (2011) From lactic acid to poly(lactic acid) (PLA): characterization and analysis of PLA and its precursors. Biomacromol 12(3):523– 532 2. Madhavan Nampoothiri K, Nair NR, John RP (2010) An overview of the recent developments in polylactide (PLA) research. Bioresour Technol [online] 101(22):8493–8501. https://www. sciencedirect.com/science/article/pii/S0960852410009508 3. Jamshidian M, Tehrany EA, Imran M, Jacquot M, Desobry S (2010) Poly-lactic acid: production, applications, nanocomposites, and release studies. Compr Rev Food Sci Food Saf 9(5):552–571 4. Giordano RA, Wu BM, Borland SW, Cima LG, Sachs EM, Cima MJ (1997) Mechanical properties of dense polylactic acid structures fabricated by three dimensional printing. J Biomater Sci Polym Ed 8(1):63–75. https://doi.org/10.1163/156856297X00588 5. Huda MS, Drzal LT, Mohanty AK, Misra M (2008) Effect of fiber surface-treatments on the properties of laminated biocomposites from poly(lactic acid) (PLA) and kenaf fibers. Compos Sci Technol [online] 68(2):424–432. https://www.sciencedirect.com/science/article/abs/pii/ S0266353807002643 6. Averett RD, Realff ML, Jacob K, Cakmak M, Yalcin B (2011) The mechanical behavior of poly(lactic acid) unreinforced and nanocomposite films subjected to monotonic and fatigue loading conditions. J Compos Mater 45(26):2717–2726. https://doi.org/10.1177/002199831 1410464 7. Drumright RE, Gruber PR, Henton DE (2000) Polylactic acid technology. Adv Mater 12:1841– 1846

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8. Pang X, Zhuang X, Tang Z, Chen X (2010) Polylactic acid (PLA): research, development and industrialization. Biotechnol J 5(11):1125–1136 9. Ray SS, Okamoto M (2003) Biodegradable polylactide and its nanocomposites: opening a new dimension for plastics and composites. Macromol Rapid Commun 24:815–840 10. Wang L, Gramlich WM, Gardner DJ (2017) Improving the impact strength of poly(lactic acid) (PLA) in fused layer modeling (FLM). Polymer [online] 114:242–248. https://www.sciencedi rect.com/science/article/abs/pii/S0032386117302586 11. Mattioli S, Peltzer M, Fortunati E, Armentano I, Jiménez A, Kenny JM (2013) Structure, gasbarrier properties and overall migration of poly(lactic acid) films coated with hydrogenated amorphous carbon layers. Carbon N Y 63:274–282 12. Rhim JW, Hong SI, Ha CS (2009) Tensile, water vapor barrier and antimicrobial properties of PLA/nanoclay composite films. LWT—Food Sci Technol 42:612–617 13. Song Y, Li Y, Song W, Yee K, Lee K-Y, Tagarielli VL (2017) Measurements of the mechanical response of unidirectional 3D-printed PLA. Mater Des [online] 123:154–164. https://www.sci encedirect.com/science/article/pii/S0264127517302976 14. Saini J, Dowling L, Kennedy J, Trimble D (2020) Investigations of the mechanical properties on different print orientations in SLA 3D printed resin. Proc Inst Mech Eng C J Mech Eng Sci. https://doi.org/10.1177/0954406220904106 15. Es-Said OS, Foyos J, Noorani R, Mendelson M, Marloth R (2000) Effect of layer orientation on mechanical properties of rapid prototyped samples. Mater Manuf Process 15(1):107–122 16. Vega V, Clements J, Lam T, Abad A, Fritz B, Ula N, Es-Said OS (2011) The effect of layer orientation on the mechanical properties and microstructure of a polymer. J Mater Eng Perform 20:978–988. https://doi.org/10.1007/s11665-010-9740-z 17. Hassanifard S, Hashemi SM (2020) On the strain-life fatigue parameters of additive manufactured plastic materials through fused filament fabrication process. Addit Manuf 32:100973 18. Ngo TD, Kashani A, Imbalzano G, Nguyen KTQ and Hui D (2018) Additive manufacturing (3D printing): a review of materials, methods, applications and challenges. Compos Part B: Eng 143:172–196 19. Tiersch TR, Monroe WT (2016) Three-dimensional printing with polylactic acid (PLA) thermoplastic offers new opportunities for cryobiology. Cryobiology 72(3):396–398 20. Domingo-Espin M, Travieso-Rodriguez JA, Jerez-Mesa R, Lluma-Fuentes J (2018) Fatigue performance of ABS specimens obtained by fused filament fabrication. Materials 11:2521 21. Dizon J, Espera A, Chen Q, Advincula R (2017) Mechanical characterization of 3D-printed polymers. https://www.sciencedirect.com/science/article/pii/S2214860417302749?casa_t oken=RB4JXKdkY_8AAAAA%3ArX0pcMEGYHjREQGV7N_iSspFPlIeMWP5YveNs nRF8kXDNqvWL9ms5pa5QfPzjUfcqoDGAL5hc9M 22. Gomez-Gras G, Jerez-Mesa R, Travieso-Rodriguez J, Lluma-Fuentes J (2017) Fatigue performance of fused filament fabrication PLA specimens. https://www.sciencedirect.com/science/ article/pii/S0264127517311036. Accessed 16 Oct 2020 23. Afrose MF, Masood SH, Iovenitti P et al (2016) Effects of part build orientations on fatigue behaviour of FDM-processed PLA material. Prog Addit Manuf 1:21–28. https://doi.org/10. 1007/s40964-015-0002-3 24. Ziemian C, Ziemian R, Haile K (2019) Characterization of stiffness degradation caused by fatigue damage of additive manufactured parts. https://www.sciencedirect.com/science/article/ abs/pii/S0264127516309789. Accessed 26 Oct 2020 25. Moore JP, Williams CB (2015) Fatigue properties of parts printed by PolyJet material jetting. Rapid Prototyp J 21(6):675–685. https://doi.org/10.1108/RPJ-03-2014-0031 26. Moore JP, Williams CB (2012) Fatigue characterization of 3D printed elastomer material. In: 23rd Annual international solid freeform fabrication symposium—an additive manufacturing conference, SFF 2012. University of Texas at Austin (freeform), pp 641–655

Chapter 2

Experimental Procedures

Specimen Preparations (Method of 3D Printing fatigue samples). To 3D Print these samples, the STL file (3D Model) was imported into a slicing software (PrusaSlicer 2.1.0). Default configurations for filament type, printer type, and print settings were modified to best suit the model per specifications requested. The main modified settings varied are changing support material to soluble settings, setting the support material to Extruder 5, infill pattern and orientation (per sample requested), as well as slowing down speeds and modifying support material extrusion width. The file was then sliced and exported into a GCODE format and sent to the 3D Printer (Original Prusa i3 Mk3s + MMU2s). The samples were 3D printed in five different orientations (Fig. 2.1a): 0°, 90°, and concentric (ASTM D638) for single fiber specimens and 0°–90° and +45°/−45° for bidirectional laminates. Dizon et al. [1]. Fatigue, samples were tested in three different stresses of 36.5, 27.5, 18 MPA (5.3, 4, and 2.6 ksi) which are in accordance with stresses from Pang et al. [2], Hassanifard et al. [3], and Afrose [4] while they were below the stresses from Gomez et al. [5]. The samples underwent a fatigue rotating bending test. Fatigue tests were conducted in a rotating beam machine model (RBF-200), operating at 3000 rpm at room temperature 18 °C (65 °F, ±4 °F). Depending on the availability of each group of specimens, each one was tested at levels of stress ranging between 18 and 45.5 MPA (2.6–6.6 ksi). The specimens were run to complete failure or up to 107 cycles. The formula below explains how the moment load was calculated [6]: M=S

πd3 32Kt

(2.1)

where, S = Required test stress, Pa (psi) M = Bending moment, N-m (in-lbf) © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 F. Aladwani et al., The Effect of Layer Orientation on the Fatigue Behavior of 3D Printed PLA Samples, SpringerBriefs in Computational Mechanics, https://doi.org/10.1007/978-3-031-22573-4_2

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8 Fig. 2.1 a Sample orientation. b Fatigue sample dimensions. c Tensile sample dimensions. d Three point bending sample dimensions. e Izod sample dimensions

2 Experimental Procedures

2 Experimental Procedures

9

Fig. 2.1 (continued)

D = Specimen diameter, m (in) Kt = Stress concentration factor and is equal to 1 here FDM machine processes parameters were set to the following: Fatigue = edge width (12.3 mm) (Fig. 2.1b), length between two edges (101.6 mm), and edge height (2.3 mm) (Fig. 2.1c). Tensile testing = edge width (20.3 mm), length between two edges (177.8 mm), and edge height (3.3 mm) (Fig. 2.1d). Three point bending = edge width (3.2 mm), length between two edges (127 mm), and edge height (2.4 mm) (Fig. 2.1e). Izod = edge width (12.7 mm), length between two edges (63.5 mm), and edge height (12.7 mm). Tensile testing was performed according to ASTM D638 [7] using an Instron 4505 universal testing machine. The three-point bending test followed ASTM D790 standard [8]. Izod tests followed ASTM D256 standard [9]. Images via optical microscopy were taken to examine fractured surfaces in each sample from all five orientations through using the OLYMPUS PMES microscope. A Phantom Ultra-High Speed camera was used to capture more accurate images of the fractured surfaces from each sample. The camera attached to a DistaMax infinity lens that manages to zoom closer than the camera lens itself. Samples were captured at their fractured surfaces.

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The Phantom PPC software was used to capture the images and save them in the computer. SEM images were taken on a JEOL 5800LV microscope.

References 1. Dizon J, Espera A, Chen Q, Advincula R (2017) Mechanical characterization of 3D-printed polymers. https://www.sciencedirect.com/science/article/pii/S2214860417302749?casa_t oken=RB4JXKdkY_8AAAAA%3ArX0pcMEGYHjREQGV7N_iSspFPlIeMWP5YveNsnRF 8kXDNqvWL9ms5pa5QfPzjUfcqoDGAL5hc9M 2. Pang X, Zhuang X, Tang Z, Chen X (2010) Polylactic acid (PLA): research, development and industrialization. Biotechnol J 5(11):1125–1136 3. Hassanifard S, Hashemi SM (2020) On the strain-life fatigue parameters of additive manufactured plastic materials through fused filament fabrication process. Addit Manuf 32:100973 4. Afrose MF, Masood SH, Iovenitti P et al (2016) Effects of part build orientations on fatigue behaviour of FDM-processed PLA material. Prog Addit Manuf 1:21–28. https://doi.org/10. 1007/s40964-015-0002-3 5. Gomez-Gras G, Jerez-Mesa R, Travieso-Rodriguez J, Lluma-Fuentes J (2017) Fatigue performance of fused filament fabrication PLA specimens. https://www.sciencedirect.com/science/art icle/pii/S0264127517311036. Accessed 16 Oct 2020 6. Fatigue Dynamics (1985) Instruction manual: model RBF-200, rotating beam fatigue testing machine. MI 7. ASTM Standard D638-10 (2010) Standard test method for tensile properties of plastics. ASTM International, West Conshohocken, Pennsylvania. https://doi.org/10.1520/D0638-10. www.ast m.org 8. ASTM Standard D790 (2010) Standard test method for flexural properties of unreinforced and reinforced plastics and electrical insulating materials. ASTM International, West Conshohocken, PA 9. ASTM, D256-10 (2018) Standard test methods for determining the Izod pendulum impact strength of plastics. ASTM International, West Conshohocken. https://www.astm.org/Standa rds/D256

Chapter 3

Results and Discussions

Fatigue test data. There were sixty-eight samples that were tested by the rotating beam fatigue machine, these were: +45°/−45° (21 samples), 0°–90° (13 samples), for the bidirectional laminates, 0° (9 samples), 90° (10 samples), and Concentric (ASTM D638) [1] (15 samples) for the single fiber specimens. With stresses ranging between 36.5, 27.5, 18 MPA (5.3, 4, and 2.6 ksi) and a 3000 RPM, the samples were tested within different timeframes. To allow for more accuracy with the results, each orientation set was tested in full then moved to the next one rather than shifting samples from one orientation to the other. For single fiber specimens, the concentric (ASTM D638) [1] was tested with stresses of 36.5, 27.5, 18 MPA (5.2, 4, and 2.6 ksi). The life cycles resulted ranged from 35,300 to 47,600 at 36.5 MPA (5.3 ksi), from 213,600 to 289,300 at 27.5 MPA (4 ksi), and from 1,000,300 to 1,055,900 at 18 MPA (2.6 ksi). The concentric (ASTM D638) [1] had the largest fatigue life cycles compared with the other orientations. Some samples were rotating for more than 4 or 5 h before breaking at lower stresses. Next, the 0° samples were tested with stresses of 36.5, 27.5, 18 MPA (5.2, 4, and 2.6 ksi). The life cycles resulted ranged from 23,900 to 24,900 at 36.5 MPA (5.3 ksi), from 61,700 to 94,500 at 27.5 MPA (4 ksi), and from 336,500 to 512,400 at 18 MPA (2.6 ksi). The 0° samples did take a long time to break with taking around 4 h before breaking at lower stresses. The 90° samples were tested with stresses of 36.5, 27.5, 18 MPA (5.2, 4, and 2.6 ksi). The life cycles resulted ranged from 5800 to 6400 MPA (5.3 ksi), from 32,900 to 42,600 at 27.5 MPA (4 ksi), and from 112,100 to 135,800 at 18 MPA (2.6 ksi). The 90° samples had the shortest breakage time compared to all the orientations. For the bidirectional laminates, the 0°–90° was tested with three stresses of 36.5, 27.5, 45.5 MPA (5.3, 4, and 2.6 ksi). At 45.5 MPA (2.6 ksi), some samples did not break. A threshold of 1,000,000 cycles was considered as a point of stopping had it reached that number of cycles, and it had. The life cycles resulted ranged from 23,100 to 27,000 at 36.5 MPA (5.3 ksi), from 141,800 to 172,800 at 27.5 MPA (4 ksi), and from 527,400 to 1,010,100 at 45.5 MPA (2.6 ksi). It took longer than the © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 F. Aladwani et al., The Effect of Layer Orientation on the Fatigue Behavior of 3D Printed PLA Samples, SpringerBriefs in Computational Mechanics, https://doi.org/10.1007/978-3-031-22573-4_3

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+45°/−45° samples especially with the 18 MPA (2.6 ksi) trials. The +45°/−45° showed an interesting range of life cycles from 10,600 to 11,600 at 45.5 MPa (6.6 ksi), from 9900 to 50,200 at 36.5 MPa (5.3 ksi), from 69,500 to 297,400 at 27.5 MPa (4 ksi), and from 658,400 to 713,800 at 18 MPa (2.6 ksi). It took the longest to test in terms of trial and error. The +45°/−45° was the only orientation to withstand the 45.5 MPa (6.6 ksi) stress. This is believed to be because they are denser than the other orientations and are not too ductile. The other orientation when tested at 45.5 MPa (6.6 ksi) stress caused a bend in the ruler while rotating that made the safety pin switch to stop the rotation. Figures 3.1a–c and 3.2a–c show each stress plotted from low to high in terms of life cycles average. In all the plots, concentric (ASTM D638) [1] had the highest life cycle followed by 0° and 90° is the lowest. For the bifunctional laminates, the +45°/−45° has more life cycles at 18 MPa (2.6 ksi) as compared to the 0°–90° bidirectional laminated. At higher stresses (27.5 and 36.5 MPa), the 0°–90° has longer life cycles. Based on Fig. 3.3a–f, six concentric (ASTM D638) samples generated a crack without breaking and they were all tested at 36.5 MPa (5.3 ksi). Five 0 samples cracked without fully breaking and they were mostly tested at 36.5 MPa (5.3 ksi) stress. The 0–90 samples showed a shear lip at the breaking area. Seven 90 samples showed a tiny crack without breaking into two pieces. Lastly, three +45/−45 samples cracked without breaking, and one of them melted (tested at 27.5 MPa (4 ksi)) as seen in Fig. 3.3f. The fatigue test took the longest to conduct and perform. Samples tested at lower stress level of 18 MPa (2.6 ksi) took the longest to break while samples tested at 36.5 MPa (5.3 ksi) experienced the opposite. Not all the samples broke as some of them initiated a crack that stopped the rotating beam fatigue machine. Those samples were mostly from the 0°, 90°, and the concentric (ASTM D638). While testing a sample from the +45/−45 (82,600 cycles), the sample broke but remained attached to the gauge as seen in Fig. 3.3f. An assumption was made and was reasoned to be due to the PLA heating above its glass transition temperature (between 60 and 65 °C) [2]. The rotating beam fatigue machine designed for metallic materials can be used to conduct tests on polymers like PLA with adjusting the RPM to 3000. It was highly reliable when developing the material and evaluating it. Figure 3.4 shows that out of all stresses, the concentric has the highest number of cycles, followed by the 0° and then the 90° single fibers. The 90° printing direction being perpendicular to the length of the sample caused the samples to break very fast as opposed to the other orientations because of the crack going in between the fibers in accord with the results of Es-said et al. [3]. The 0° and concentric (ATM D638) had a parallel printing direction that caused the crack to break slowly fiber by fiber (as seen in Fig. 3.5). These results are in accord with Saini et al. [4] and Zeimian et al. [5]. In Fig. 3.6, the 0°–90° and +45°/−45° are compared. The number of cycles are higher at the +45°/−45° orientation at the lowest stress 18 MPa (2.6 ksi), but the 0°–90° has higher cycles at the other two stresses (27.5 MPa (4 ksi) and 36.5 MPa (5.3 ksi)). This is not

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in accord with Zeimian et al. [5] who found that for tension fatigue performance the +45°/−45° layering pattern was consistently better than the 0°–90° in ABS-P400 material. In Fig. 3.7a, b, the S–N curve of unidirectional and bidirectional layers are shown. Table 3.1 shows the parameters fitted with a linear plot.

Fig. 3.1 a–c Bar plots of three stresses for each orientation: 0°, 90°, and concentric ASTM (D638)

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Fig. 3.1 (continued)

Espin et al. [6] indicated that higher fatigue cycles are obtained with higher infill patterns. This paper indicated that higher infill density would yield more cycles as seen mainly in the concentric (ASTM D638) samples. Gomez-Gras et al. [7] tested PLA rectilinear sample that were printed using fused filament fabrication (FFF) with the same 3000 RPM. The rectilinear samples bare similarities with the +45°/−45° from this paper. Their results showed life cycles ranging between (~800 to 5500 cycles). A rotating fatigue bending machine was used through an experimental design of L27 Taguchi. Tensile testing data. Eighteen samples were tensile tested. The ultimate strength averages were 27.4 MPa (3.97 ksi) for the 90° samples, 49 MPa (7.12 ksi) for the 0° samples, 47.8 MPa (6.93 ksi) for the +45°/−45° samples, 45.2 MPa (6.55 ksi) for the 0°–90° samples, and 52 MPa (7.56 ksi) for the modified concentric (ASTM D638) samples. Table 3.2 displays tensile data for each tested sample from all five orientations. Figure 3.8a, b displays the average ultimate strength of 0°, 90°, and modified concentric (ASTM D638) orientations and the bidirectional laminates 0°–90° and + 45°/−45°. The modified concentric (ASTM D638) orientation was the only one to yield values for the percent elongation out of all the orientations. The percent elongation values ranged from 8.7 to 15% elongation (Table 3.2). The modified concentric (ASTM D638) was the highest in strength while the 90° is the lowest. Figure 3.9 showed the tensile stress versus strain plot of two of the modified concentric (ASTM D638) samples. The plots clearly show the ductility of the concentric orientation. The non-zero elongation is attributed to modification in the concentric samples. The

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modified concentric samples will be discussed in conjunction with scanning electron microscopy. The tensile data results were consistent with the results that were proposed by other researchers [3, 5, 8, 9]. Huda et al. [10] reported the ultimate strength values to be between 0.004 and 2.3 GPa (4–2300 MPa) which is very high as compared to this paper’s ultimate strength (27.3–52.1 MPa) values. PLA is known for its brittleness

Fig. 3.2 a–c Bar plots of three stresses for each orientation: 0°–90° and +45°/−45°

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Fig. 3.2 (continued)

[11]. The tensile data showed significant percent elongation for the modified concentric (ASTM D638) ranging between 8.7 and 15% elongation while being with higher values of ultimate strength than the other orientations. This is further discussed later. Three Point Bending data. Fifteen samples were tested in total for all five orientations and the flexural stress at maximum load was recorded. The average flexural stress ranged between 36.99 MPa (5.37 ksi) for the 90° samples, 96.06 MPa (13.93 ksi) for the 0° samples, 86.87 MPa (12.6 ksi) for the +45°/−45° samples, 78.37 MPa (11.37 ksi) for the 0°–90° samples, and 84.11 MPa (12.2 ksi) for the concentric (ASTM D638) samples. Table 3.3 shows each tested sample with the resulted flexural stress at maximum load. Figure 3.10a, b shows the average flexural stress for each orientation. It was concluded that the 0° was the highest in terms of flexural stress and the 90° was the lowest. The concentric samples had a void which is discussed in the microscopy section later. This might be the reason for this behavior. All of the concentric (ASTM D638) samples bent without fracture. The +45°/−45° specimens performed very highly compared to the 0°–90° orientation. Izod data. Twenty-four samples of the five orientations were tested by the Izod device. The maximum load averages were 0.34 J for the 0° samples, 0.32 J for the 90° samples, 0.41 J for the +45°/−45° samples, 0.36 J for the 0°–90° samples, and 0.39 J for the concentric (ASTM D638) samples. Table 3.4 displays the results from all five orientations. Figure 3.11a, b show the average maximum energy absorbed

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from each orientation. The +45°/−45° energy absorbed was higher than to the 0°– 90° and the concentric (ASTM D638) was the highest while the 90° is the lowest. The Izod results are in accord with the fatigue results. The results matched Saini et al. [4] results of SLA samples that observed five orientations at 0°, 22.5°, 45°, 67.5°, and 90°. It was also in accord with the results of Es-Said et al. [3]. Optical Microscopy. Images at 50 × magnification are shown of fractured tensile bars in Figs. 3.12, 3.13, 3.14, 3.15 and 3.16. The 0° and concentric (ASTM D638) were clearly visible under the microscope and the printing direction was clear. On the other hand, the 90° and +45°/−45° were not clear under the microscope. In

Fig. 3.3 Samples after fatigue test of orientation: a concentric (ASTM D638), b 0°, c 0°–90°, d 90°, and e +45°/−45°. f A sample from +45/−45 testing

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Fig. 3.3 (continued)

Fig. 3.4 S–N curve of orientations: 0°, 90°, and concentric (ASTM D638) superimposed

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Fig. 3.5 Schematic representation of crack path of a 0° and b 90° orientations [3]

Fig. 3.6 S–N curve of 0°–90° and +45°/45° superimposed

Fig. 3.12, small voids can be seen in between the heads of the concentric (ASTM D638) samples and the printing orientation is parallel to the length of the sample. The 0° samples are shown in Fig. 3.13 where the voids are bigger than the concentric (ASTM D638) and the printing orientation is visible to be parallel to the length of the sample. Figure 3.14 displays the 90° samples images and the printing orientation was perpendicular to the length of the sample. In Fig. 3.15, the 0°–90° printing orientation can be seen as a mixture of both parallel and perpendicular to the length of the sample. Lastly, the +45°/−45° samples can be seen in Fig. 3.16 where the cross-section and diagonal printing to the length of the sample is seen. Phantom Ultra-High-speed Camera results (macro). Figure 3.17a show images of fatigue test from all orientation and the marker is at 0.6 mm in Fig. 3.17b. A black void can be seen in the middle of the concentric (ASTM D638) samples. It is believed

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Fig. 3.7 a Unidirectional S–N curve. b Bidirectional S–N curve

that this void is from the printer itself and that a much greater performance would have resulted from the concentric (ASTM D638) samples had the void been filled. The printing orientation can be seen through the circles in the 0° samples indicating the parallel to the length (into the screen) printing direction. The 90° samples viewed a perpendicular to the length of the sample direction, while the 0°–90° was seen as a mixture of the previous two. The +45°/−45° diagonal direction is clear as well.

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Table 3.1 Material linear parameters slope Material

Slope

Intercept

R 2 goodness of fit

Concentric

−0.00002

36

0.93

0

−0.00003

33

0.76

90

−0.0001

34.3

0.76

0–90

−0.00002

32.5

0.76

45/−45

−0.00002

33.2

0.74

Table 3.2 Tensile data for all orientation Specimen

Ultimate strength (ksi)

Yield strength (ksi)

Ultimate strength (MPa)

Yield strength (MPa)

Percent elongation %

90—1

4.02

3.98

27.72

27.44

–

90—2

4.11

4.01

28.27

27.64

–

90—3

3.86

3.86

26.61

26.61

–

90—4

3.91

3.78

26.89

26.06

–

90 (averages) 3.97

3.91

27.37

26.93

0—1

6.85

6.12

47.23

42.06

–

0—2

6.83

6.41

47.09

44.19

–

0—3

7.76

6.57

53.51

45.31

–

0—4

7.05

6.31

48.62

43.43

–

0 (averages)

7.12

6.35

49.11

43.74

45/−45—1

6.93

6.21

47.57

42.81

–

45/−45—2

6.93

6.09

47.57

41.99

–

45/−45—3

6.99

6.11

48.19

42.12

–

45/−45 (averages)

6.95

6.13

47.77

42.31

–

0–90 1

6.42

5.92

44.26

40.68

–

0–90 2

6.43

5.91

44.33

40.74

–

0–90 3

6.81

6.02

46.95

41.51

–

0–90 (averages)

6.55

5.95

45.18

40.97

Modified concentric 1

7.37

5.46

50.81

37.64

15

Modified concentric 2

7.35

5.46

50.67

37.64

13.72

Modified concentric 3

7.81

5.93

53.78

40.88

8.71 (continued)

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Table 3.2 (continued) Specimen

Ultimate strength (ksi)

Yield strength (ksi)

Ultimate strength (MPa)

Yield strength (MPa)

Percent elongation %

Modified concentric 4

7.75

2.52

53.43

12.37

8.81

Modified concentric (averages)

7.57

4.84

52.17

32.13

11.56

Fig. 3.8 a Ultimate strength average data for orientations: 0°, 90°, and concentric (ASTM D638). b Ultimate tensile strength average data for the 0°–90° and +45°/−45° orientations

Figure 3.17b show images of the broken tensile samples. The absence of the void is apparent in the modified concentric samples. The three-point bending phantom images can be seen in Fig. 3.18 with a marker at 1.32 mm for all orientations except for the concentric (ASTM D638) as all of the samples bent without breaking. The printing direction can be viewed as parallel and perpendicular to the length of the samples for the 0° and 90° samples respectively, while the 0°–90° samples had both directions combined. Figure 3.19 shows images of the Izod tests from all of the orientations with a marker at 1.29 mm. The printing directions are similar to the previous tests for all orientation. The concentric (ASTM D638) sample displayed a black void just like

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Fig. 3.9 Stress versus strain plot of two modified concentric (ASTM D638) sample; 1 ksi = 6.89 MPa Table 3.3 Three point bending data for all parameters Specimen

Flexure stress at maximum load (ksi) MOR bending strength

Flexure stress at maximum load (MPa) MOR bending strength

90—1

5.6

38.6

90—2

5.3

36.5

90—3

5.2

35.8

90 (average)

5.37

36.9

0—1

13.6

93.7

0—2

14.1

97.2

0—3

14.1

97.2

0 (average)

13.93

96.1

45/−45—1

12.5

86.1

45/−45—2

12.6

86.8

45/−45—3

12.7

87.5

45/−45 (average)

12.6

86.8

0–90—1

11.3

77.9 (continued)

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Table 3.3 (continued) Specimen

Flexure stress at maximum load (ksi) MOR bending strength

Flexure stress at maximum load (MPa) MOR bending strength

0–90—2

11.5

79.3

0–90—3

11.3

77.9

0–90 (average)

11.37

78.4

Concentric 1

12.2

84.1

Concentric 2

12.2

84.1

Concentric 3

12.2

84.1

Concentric (average)

12.2

84.1

Fig. 3.10 a Flexural stress averages for orientations 0°, 90°, and concentric (ASTM D638). b Flexural stress averages for 0°–90° and +45°/−45° orientations

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Table 3.4 Izod data for all parameters Condition

ft lb

Joules

Average (ksi)

Average (MPa)

0

0.28

0.38

0.255

0.3475

0.236

0.318

0.268

0.362

0.296

0.4075

0.292

0.396

90

0 – 90

45/−45

Concentric

0.24

0.33

0.24

0.33

0.26

0.35

0.24

0.33

0.24

0.33

0.26

0.35

0.22

0.29

0.22

0.29

0.26

0.35

0.26

0.35

0.28

0.38

0.26

0.35

0.28

0.38

0.3

0.41

0.32

0.43

0.28

0.38

0.3

0.41

0.28

0.38

0.32

0.43

0.3

0.41

0.3

0.41

0.28

0.38

0.26

0.35

the fatigue sample, and the width of the void is estimated to be around 3 mm. It is believed that the void might be the reason why the concentric (ATM D638) samples were second to the +45°/−45° samples in this test, and after the 0° and +45°/−45° orientation in the three-point bending test. The Phantom Ultra High-Speed Camera managed to displayed clearer fractured surfaces that the black void was seen clearly in the concentric (ASTM D638) samples. The bar plots and images from the phantom camera agree that both the fatigue and Izod tests are in accord with each other. Thus, an increase in energy absorption would cause an increase in the number of cycles to failure. The cracks had to break fiber by fiber in the 0° and in the concentric (ASTM D638) samples but can go in between the fibers for the 90° samples. The outcomes matched the anticipated thoughts with the concentric (ASTM D638) samples performing the

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Fig. 3.11 a Maximum load averages for orientation 0°, 90°, and concentric (ASTM D638). b Maximum load averages for 0°–90° and +45°/−45° orientations

highest while the 90° samples performing the lowest. For the 90°, it is believed that the weak performance is attributed to the perpendicular printing direction and fragile pattern of the samples, this is in accord with other researches [3, 9, 12, 13]. Modified Concentric. After all the mechanical tests were performed, the Phantom Ultra-High-Speed camera was used to study the fractured surfaces. It was noted that all the concentric samples had a considerable groove in the middle, fatigue samples,

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Fig. 3.12 Optical microscopy images of concentric (ASTM D638) orientation samples in fractured tension

Fig. 3.13 Optical microscopy images of 0° orientation samples fractured in tension

Fig. 3.17a and Izod samples, Fig. 3.19. This is a significant stress concentration factor which will prevent the concentric orientation from performing the best results.

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Fig. 3.14 Optical microscopy images of 90° orientation samples fractured in tension

Fig. 3.15 Optical microscopy images of 0°–90° orientation samples fractured in tension

In the tensile test, two concentric samples were tested and although their ultimate strength was higher than all orientations (7.22 ksi (49.8 MPa)) and 7.88 ksi (54.3 MPa)), however, like all the other orientations there was a brittle fracture and 0% elongation. The void or groove in the middle was due to the limitations of

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Fig. 3.16 Optical microscopy images of +45°/−45° orientation samples fractured in tension

the slicing software. The effect of the groove is shown in the broken tensile bars, Figs. 3.20 and 3.21. To solve this issue two layers of the 45° infill in the second and fourth layer of both sides totaling 4 layers of the +45 infill were inserted to the concentric, Fig. 3.22. This modification was successful. The four modified concentric samples have higher ultimate and yield strengths compared to other orientations and produced elongations of 15, 13.7, 8.7, and 8.8%, Table 3.2. The concentric and modified concentric are compared, Fig. 3.23a, b. The modified concentric fractured at 45° angle in accord with 45° fracture features shown by Es-Said, et al. [3]. When all the broken tensile bars of the different orientations are compared, Figs. 3.24 and 3.25, the 0°, 90°, 0°– 90°, and +45°/−45° break at a 90° to the applied stress. The concentric breaks at a 90°, then follows the groove to fracture along the length of the sample. The modified concentric breaks at 45° with a significant ductility between 8 and 15% elongation, Fig. 3.24, and Table 3.2. Two types of concentric samples were printed. Type 1 samples are purely concentric. Type 2 ones are concentric but inserted two layers of 45° infill in the second and fourth layer of both sides, totally 4 layers of the 45° infill were inserted. shown in Fig. 3.23a, b. This method utilized adding extra layers of the concentric and adding +45° layer which will fracture in between layers and propagate within the concentric internal materials. This method which promoted dutility is compared with methods used by other researchers where Chen et al. [14] modified PLA with adding other components, 2-methacryloyloxyethyl isocyanate (MOI). SEM. Representative fractured tensile bars exemplars of the 0°, 90°, 0°–90°, + 45°/−45°, and Modified Concentric groups were subjected to scanning microscopy,

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Fig. 3.17 a Phantom ultra-high speed camera images of fatigue samples in three stress (a) 36.5 MPa (5.3 ksi), (b) 27.5 MPa (4 ksi), (c) 18 MPa (2.6 ksi) for 1—concentric (ASTM D638), 2—0°, 3—90°, 4—0°–90°, and 5—+45°/−45°. b Phantom ultra-high-speed camera images of tensile samples

mostly at a 90° angle to the crack surfaces. It was noted that there appeared to be an inconsistency in how each layer fused with the previously deposited layer. There also appeared to be evidence of irregularity in the cross sections of the individual beads. Significant areas of the fracture surfaces revealed strands of deposited PLA with pinched (smaller than the rest) cross sections at the crack profile, Figs. 3.26 and 3.27 for the 0° orientation.

3 Results and Discussions

Fig. 3.18 Phantom ultra-high-speed camera images of three-point bending samples

Fig. 3.19 Phantom ultra-high-speed camera images of Izod samples

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Fig. 3.20 Broken tensile bars of concentric samples

Fig. 3.21 The cross-section of the broken area of the concentric samples

As noted in Fig. 3.27, as well as Fig. 3.28, the fractures show little evidence of macro solid state ductility. However, there is evidence of microductility. It is speculated that localized adiabatic heating is facilitating sometimes limited, and sometimes fairly extreme fibril formation, Fig. 3.29.

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Fig. 3.22 Concentric samples to create a modified concentric orientation

Figure 3.30 shows the optical micrograph of the 90° print direction. The strands are running parallel to the gage width. If the strands retain their circular cross section, and do not bond with each other beyond the line of initial contact, obviously there would be little load bearing cross section. Figure 3.30 shows 11 layers in the center width of the specimen. However, significant triangular areas on both specimen edges indicate that the material was fused into a more unified acting structure. Figure 3.31 is an SEM view of the left side of this broken tensile test fragment. This area includes the concentric outer framed reportedly used for all builds, and the triangular prisms are visible at the edge. However, to the right of the frame, there is an area that has the outer five layers fused to the point that the crack did not “chase the interface,” as it did in the rest of the field of view. Even here, there was some actual bonding, as the narrow bands of true fracture features (shallow tear ridges) of Fig. 3.32 depict. Figure 3.33 shows the 0–90 exemplar specimen. The features are now familiar, based on the two specimen fracture surfaces just described. Again, approximately 12 layers of PLA beads. Figure 3.34 shows the fracture features more clearly. Figure 3.35 shows the somewhat blunted tear ridge edges that are present in some areas of most of the coupons evaluated. These again appear to be a result of localized adiabatic heating due to the deformation process. The fracture surfaces of the +45°/−45° build reveals that a total of approximately 12 layers were used. No large facets are shown, indicating that multiple small cracks

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Fig. 3.23 a Pure concentric (bottom) and modified concentric with the 45° infill (top). b A close look at the difference between the two types of samples

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Fig. 3.24 Comparison of the features of broken tensile bars of different orientations

Fig. 3.25 Comparison of the fractured surfaces of broken tensile bars of different orientations

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Fig. 3.26 The 0° specimen 0B has pairs of deposited layers fused to each other. Specimen OE has the upper 4 layers all fused into a single layer

Fig. 3.27 SEM view of 0° specimen OE. Note upper 4 layers all fused together, and 90° strands running between layers 4 and 5 and 6 and 7. The strands to the right side of the image, especially in the lower rows, appear to have been pinched or stretched, perhaps during the printing process

had to initiate independently in order for the coupon to break, Fig. 3.36. This may be the reason for the relatively high tensile strength measured for this build orientation. Figure 3.37 shows an SEM view with both fine and coarser (blunted edge) tear ridges. Even though the blunted edge tear ridges are signs of ductility, it appears to be so localized that there is no “benefit” towards measurable elongation. The Modified Concentric specimens were the only ones that showed measurable ductility. Despite the literature indicating that this build orientation may produce

3 Results and Discussions Fig. 3.28 Detail of the left edge of the 0° specimen, core layers. Flat surface with very shallow tear ridges. Triangular prismatic voids are noted where the strands are bonded along their lengths

Fig. 3.29 0° specimen, with 0% measured elongation, showing micro-ductile features

Fig. 3.30 Specimen 90. Well fused, roughly triangular areas, both sides of the gage length are noted

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Fig. 3.31 Detail of Fig. 3.30. 90° SEM image. Like the 0° specimen, there are approximately 10–11 layers that make up the thickness of the coupon

Fig. 3.32 Detail of fracture features, as distinguished from as deposited surface of bead. It appears that only 25% of the cross section is truly fused, and the rest is air

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Fig. 3.33 0–90 Specimen has pairs of well bonded layers. The frame at the edges of the gage width are noted

Fig. 3.34 Detail of Fig. 3.33 (0°–90°). The 90° layers appear well fused to the 0 layers that were underneath during the printing. The next layer of 0 beads does not appear to be as well bonded. The steps are noted

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Fig. 3.35 0–90 Specimen, edge of core, where a 0 bead is not well fused to the underlying 90, but is well fused to the one above it. There is a single fracture response in the entire area shown

Fig. 3.36 +45°/−45° build. Again, pairs of layers are well bonded, while adjacent pair forms its own unit. A small area at left, within the frame, is more fully bonded

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Fig. 3.37 The core of the fracture surface (+45/−45), shows the small facets. Longitudinal voids are seen, but they have lost their triangular cross section

greater strength and toughness, the actual coupon evaluated had 16–17 layer rather than the 10–12 for all other build configurations. Seen in the +45° build, Fig. 3.22, there were pairs of layers that were better bonded to each other, than the adjacent pairs. Figures 3.38 and 3.39. The modification included building outer layers using the +45° orientation. The first set of concentric coupons had a large central axial gap. Figure 3.38 shows that this gap, although covered by the outer +45 layers, still exists. There appeared to be little bonding between the stacks of beads within the modified concentric frame. It’s possible that this coupon acted like 20 or so parallel vertical plane stress coupons, while all of the other orientations were subject to plane strain, with the previously mentioned, extremely localized adiabatic heating creating some micro-ductility. Figure 3.41 shows the highly concentrated band of fibrils in the +45 shell layers. Such fibrils area a common feature in ductile, semi-crystalline polymers. Since PLA is normally brittle at room temperatures, Occam’s Razor suggests that localized heating is allowing the material to behave like warm taffy, as the fracture faces are moved apart.

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Fig. 3.38 Modified concentric tensile bar. The break on this coupon was mostly at a slant angle. Hence lack of focus along left end of coupon

Fig. 3.39 Detail of modified concentric specimen. The layers parallel to the gage length seem independent of each other. The +45° outer shell layers appear to have more ductility than the full +45°/−45° coupon. The shape of each bead in the central layers is creating strongly triangular prismatic longitudinal voids, Fig. 3.40

3 Results and Discussions

Fig. 3.40 Modified concentric specimen has strongly triangular, prismatic longitudinal voids

Fig. 3.41 Taffy like fibrils in outer layers of modified concentric C specimen

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3 Results and Discussions

The printer does not seem to have produced a consistent bead over the course of even a single specimen build. The extreme variability in fusion from layer to layer within each specimen in every group is likely a significant contributing factor to the variation in strength from coupon to coupon. The surprisingly ductile behavior of the Modified Concentric coupons may be a result of an unanticipated increase in the number of layers, and lack of bonding between the vertical layers running parallel to the edges of the gage length. This may have moved the stress regime from plane strain to plane stress, allowing greater ductility. Whether these big, roughly surfaced “sheet voids” are overall beneficial, or might have other detrimental characteristics, is a useful follow up study.

References 1. ASTM Standard D638-10 (2010) Standard test method for tensile properties of plastics. ASTM International, West Conshohocken, Pennsylvania. https://doi.org/10.1520/D0638-10. www.ast m.org 2. Pang X, Zhuang X, Tang Z, Chen X (2010) Polylactic acid (PLA): research, development and industrialization. Biotechnol J 5(11):1125–1136 3. Es-Said OS, Foyos J, Noorani R, Mendelson M, Marloth R (2000) Effect of layer orientation on mechanical properties of rapid prototyped samples. Mater Manuf Process 15(1):107–122 4. Saini J, Dowling L, Kennedy J, Trimble D (2020) Investigations of the mechanical properties on different print orientations in SLA 3D printed resin. Proc Inst Mech Eng C J Mech Eng Sci. https://doi.org/10.1177/0954406220904106 5. Ziemian C, Ziemian R, Haile K (2019) Characterization of stiffness degradation caused by fatigue damage of additive manufactured parts. https://www.sciencedirect.com/science/article/ abs/pii/S0264127516309789. Accessed 26 Oct 2020 6. Domingo-Espin M, Travieso-Rodriguez JA, Jerez-Mesa R, Lluma-Fuentes J (2018) Fatigue performance of ABS specimens obtained by fused filament fabrication. Materials 11:2521 7. Gomez-Gras G, Jerez-Mesa R, Travieso-Rodriguez J, Lluma-Fuentes J (2017) Fatigue performance of fused filament fabrication PLA specimens. https://www.sciencedirect.com/science/ article/pii/S0264127517311036. Accessed 16 Oct 2020 8. Song Y, Li Y, Song W, Yee K, Lee K-Y, Tagarielli VL (2017) Measurements of the mechanical response of unidirectional 3D-printed PLA. Mater Des [online] 123:154–164. https://www.sci encedirect.com/science/article/pii/S0264127517302976 9. Vega V, Clements J, Lam T, Abad A, Fritz B, Ula N, Es-Said OS (2011) The effect of layer orientation on the mechanical properties and microstructure of a polymer. J Mater Eng Perform 20:978–988. https://doi.org/10.1007/s11665-010-9740-z 10. Huda MS, Drzal LT, Mohanty AK, Misra M (2008) Effect of fiber surface-treatments on the properties of laminated biocomposites from poly(lactic acid) (PLA) and kenaf fibers. Compos Sci Technol [online] 68(2):424–432. https://www.sciencedirect.com/science/article/abs/pii/ S0266353807002643 11. Ray SS, Okamoto M (2003) Biodegradable polylactide and its nanocomposites: opening a new dimension for plastics and composites. Macromol Rapid Commun 24:815–840 12. Afrose MF, Masood SH, Iovenitti P et al (2016) Effects of part build orientations on fatigue behaviour of FDM-processed PLA material. Prog Addit Manuf 1:21–28. https://doi.org/10. 1007/s40964-015-0002-3

References

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13. Moore JP, Williams CB (2012) Fatigue characterization of 3D printed elastomer material. In: 23rd Annual international solid freeform fabrication symposium—an additive manufacturing conference, SFF 2012. University of Texas at Austin (freeform), pp 641–655 14. Chen B, Shen C, Chen S, Chen A (2010) Ductile PLA modified with methacryloyloxyethyl isocyanate improves mechanical properties. Polymer 51:4667–4672

Chapter 4

Summary

The mechanical testing results were consistent with the results of other researchers [1–6]. Table 4.1 shows a summary of the orientation performance for each test for the single layer 0°, 90°, and extended 0 (concentric ASTM D638). The concentric was superior to the 0° and 90° in every test except for the three point bending. The presence of the void might account for this behavior. Table 4.2 compares the performance of 0°–90° and +45°/−45° bidirectional laminates. The fatigue data showed the 0°–90° had longer lives in 2 out of 3 stresses as compared to the + 45°/−45°. For all other tests +45°/−45° was superior. This can be explained by the initiation of small multiple cracks independently in order for the sample to break, Figs. 3.36 and 3.37.

4.1 Conclusions . Fatigue testing of polymers can be done in the rotating beam fatigue machine, which is used for metals. . The concentric (extended 0°, ASTM D638) was superior to all other single fiber orientations, 0° and 90° in terms of fatigue, tensile and Izod properties but not in bending probably due to the groove generated. . Modified Concentric was the best because it is an improved 0° orientation. It showed ductility and deformation. The ductility was between 8.7 and 15% elongation. It was clear from the tension test that there was deformation in the sample. . If the void in the middle of the concentric (ASTM D638) samples was filled, a much better performance would result, especially in bending. . The results of the Izod test was in accord with the results of the fatigue test. The higher the energy absorption, the higher the number of cycles to failure.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 F. Aladwani et al., The Effect of Layer Orientation on the Fatigue Behavior of 3D Printed PLA Samples, SpringerBriefs in Computational Mechanics, https://doi.org/10.1007/978-3-031-22573-4_4

47

48 Table 4.1 Summary of orientations: 0°, 90°, and concentric (ASTM D638) performance for each test

Table 4.2 Summary of orientations: 0°–90°, + 45°/−45° performance for each test

4 Summary Fatigue

Tensile

Three point bending

Izod

Concentric

Concentric

0

Concentric

0

0

Concentric

0

90

90

90

90

Fatigue

Tensile Three point bending Izod

0–90 (2 out of 3 tests) 45/−45 45/−45 45/−45

0–90

0–90

45/−45 0–90

. The +45°/−45° was superior to the 0°–90° orientation in tensile, bending, Izod and one of the fatigue tests (lowest stress). The 0°–90° had a greater number of cycles at higher stresses. However, at the highest stress, the +45°/−45° was the only orientation which endured 45.5 MPa (6.6 ksi).

References 1. Song Y, Li Y, Song W, Yee K, Lee K-Y, Tagarielli VL (2017) Measurements of the mechanical response of unidirectional 3D-printed PLA. Mater Des [online] 123:154–164. https://www.sci encedirect.com/science/article/pii/S0264127517302976 2. Saini J, Dowling L, Kennedy J, Trimble D (2020) Investigations of the mechanical properties on different print orientations in SLA 3D printed resin. Proc Inst Mech Eng C J Mech Eng Sci. https://doi.org/10.1177/0954406220904106 3. Es-Said OS, Foyos J, Noorani R, Mendelson M, Marloth R (2000) Effect of layer orientation on mechanical properties of rapid prototyped samples. Mater Manuf Process 15(1):107–122 4. Vega V, Clements J, Lam T, Abad A, Fritz B, Ula N, Es-Said OS (2011) The effect of layer orientation on the mechanical properties and microstructure of a polymer. J Mater Eng Perform 20:978–988. https://doi.org/10.1007/s11665-010-9740-z 5. Ziemian C, Ziemian R, Haile K (2019) Characterization of stiffness degradation caused by fatigue damage of additive manufactured parts. https://www.sciencedirect.com/science/article/ abs/pii/S0264127516309789. Accessed 26 Oct 2020 6. Rybachuk M, Mauger C, Fiedler T, Ochsner A (2017) Anisotropic mechanical properties of fused deposition modeled parts fabricated by using acrylonitrile butadiene styrene polymer. J Polym Eng 37(7): 699–706