Haynes Competition Car Composites: A Practical Handbook 1844257010, 9781844257010

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Composites have been around since ancient civilisations began making bricks from clay and straw. Glass fibre, carbon and aramid fibres are relatively recent innovations, yet today there are few competition cars that don’t have at least some components made out of one or more of these materials. However, while it

is well known that glass fibre technology can be used in the home workshop, what

may not be so widely realised is that carbon and aramid fibres also lend themselves to DIY methods. In this fully updated second edition, Simon McBeath examines the materials and methods that can be used, and explains basic ‘wet’ and ‘dry’ laminating techniques, pattern making and mould construction,

and the design and manufacture of components. He then goes on to consider material and technology upgrades, and how more advanced materials can be exploited to achieve improved properties and reduced weight. The use of thermoplastic materials,

resin infusion methods and especially ‘pre-pregs’ in the home workshop are also discussed, as are the composite techniques

used by top racecar manufacturers. This practical handbook — in series

with Competition Car Aerodynamics, . Competition Car Data Logging, Competition Car Suspension and Competition Car Electrics — is packed with clearly presented, authoritative information supported with photographs and diagrams. Simon McBeath is an experienced motorsport competitor and author. During three decades of involvement in UK hillclimbs and sprints he has prepared, raced and made composite components for a number of single seaters and also for other competitors in other competition categories. He lives in Dorset. £25.00 / $44.95

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GOLEEGE

LIBRARY

Yeovil College

1

Y0071083

COMPETITION CAR

COMPOSITES

Other books by this author: COMPETITION COMPETITION

CAR AERODYNAMICS CAR DATA LOGGING

COMPETITION

CAR

COMPOSITES A PRACTICAL Second

HANDBOOK

Edition

Simon McBeath Foreword by Brian O’Rourke

Haynes Publishing

© Simon McBeath 2000

All rights reserved. No part of this book may be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage or ‘retrieval system, without permission in writing from the copyright holder.

First published February 2000 Reprinted 2002, 2004, 2006 and 2008 Second edition published December 2009 A catalogue record for this book is available from the British Library Published by Haynes Publishing, Sparkford, Yeovil, Somerset BA22 7JJ, UK

Tel: 01963 442030 Fax: 01963 440001 Int. tel: +44 1963 442030 Fax: +44 1963 440001 E-mail: [email protected] Website: www.haynes.co.uk

ISBN 978 1 84425 701 0 Library of Congress catalog card no. 2009928036 Haynes North America, Inc. 861 Lawrence Drive, Newbury Park, California 91320, USA

Designed and typeset by James Robertson Printed and bound in the UK Jurisdictions which have strict emission control laws may consider any modification to a vehicle to

be an infringement of those laws. You are advised to check with the appropriate body or authority whether your proposed modification complies fully with the law. The publishers accept no liability in this regard. While every effort is taken to ensure the accuracy of the information given in this book, no liability can be accepted by the author or publishers for any loss, damage or injury caused by misuse of, errors in, or omissions from, the information given.

Contents Foreword by Brian O'Rourke Author's preface Acknowledgements

Chapter 1

Materials Fibres, fabrics, resins, laminates, cores

Chapter 2

Chapter 3

ee)

Equipment and basic techniques Tools, workspace, basic wet lay-up moulding trials Pattern making Materials, design and manufacture

Chapter 4

Making moulds

Chapter 5

Requirements, split lines, lay-up, reinforcement Component manufacture

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Product design, laminating, stiffeners, woven

Chapter 6

Chapter 7

glass fabrics Material upgrades Carbon and aramid fabrics, epoxy resin, more core materials Technology upgrades

95

108

Pressure moulding, elevated temperature, vacuum consolidation

128

Chapter 8

Pre-pregs and relatives

145

Chapter 9

Properties, mould design, using pre-pregs honeycomb, thermoplastic technologies, resin infusion The professionals Design, facilities, autoclaves, materials, testing

Iz

6

Chapter 10 Appendix 1 Appendix 2

COMPETITION

CAR COMPOSITES

Some case studies Examples that utilise techniques described in this book

182

Useful contacts

201

Further reading

202

Glossary of terms and abbreviations Index

203

206

Foreword by Brian O’Rourke, Chief Composites Engineer, Williams Grand Prix Engineering Ltd MY

FIRST

THOUGHTS,

when I learnt that Simon

McBeath

was

putting together a

book on the subject of composite materials and their application to practical racing car production, were that, most immediately, he was extremely brave to tackle such a difficult

subject;

and,

more

importantly,

that

if he

succeeded,

there

would

most

definitely be a ready market for it. I first became interested in structural composites through my job as a stress engineer in the aerospace industry in the mid-1970s, and very soon found that, whilst the fundamentals of the manufacturing process were easy to pick up, the number of books available to help me understand their behaviour amounted to one. Whilst the introductory five pages provided an appetite-whetting explanation of the basics, page six made me wish that I had studied Greek rather than engineering, and 50 per cent of the remaining pages were liberally coated with matrix algebra. Though my education had equipped me to cope with this, it was sometimes an effort to keep uppermost in my mind what exactly a design engineer was wanting to achieve by using these materials; they were, after all, only sophisticated rags and glue. Since then I have collected numerous books on the subject of composites and have found — with the exception of the chapters dealing with laminate analysis, which look suspiciously like those in the original book (the laws of physics do not change, I suppose) — that there has been a steady improvement in the presentation of information, so that a professional engineer with a broad mind can now find his or her way into the subject with reasonable confidence. Definitions, however, are still not usually put across in a conversational manner — for example, Q: What is a composite? A: ‘A macroscopic combination of two or more materials having a recognisable interface between them...’ Whilst those working with composites on a Formula 1 car should be expected to manage at this level, quite how someone approaching the subject from a ‘hands-on’ direction, and without an engineering background, would fare is not obvious to me. Somehow,

Simon McBeath has managed to make this step, and to the point where

-his knowledge of the subject is extensive. More remarkable still is that he has also been able to write this book, which sets out the whole subject in a thoroughly readable form so that others will understand it too. This, in my opinion, makes Competition Car Composites unique. It will prove invaluable not only to those actively involved in motorsport, who wish to be more adventurous (and he gives you fair warning when you are heading into potentially risky areas) in their own car construction, but also to those who have discovered composite materials and want to understand the subject for its own sake.

Author’s preface THE WORD ‘COMPOSITE’ seems to mean different things to different people, but a useful definition could be: ‘made up of two or more parts or elements, and having better mechanical properties than the individual constituents’. Composites are not new — ancient civilisations made straw and clay bricks, wattle and daub walls, and linen ‘and plaster mummy cases. The common factor is the combination of materials —a matrix and a reinforcement— to produce a composite with properties better than those of its components. There is a general tendency within motorsport to use the term ‘composite’ to. refer only to those components utilising carbon or aramid fibres. However, for the purposes of this book it is going to be interpreted as indicating all such strengthened materials as are used in motorsport, from good old glass fibre reinforced plastic (GFRP) through to the increasingly pervasive carbon fibre reinforced plastic (CFRP), as well as all the so-called ‘sandwich structures’ in which each can be employed. There are two reasons for adopting this wider definition. Firstly, GFRP is a composite material, being composed of two primary elements; and secondly, in order to be able to take advantage of the more ‘advanced’ materials using some of the methods described, it is useful to start off with GFRP techniques before progressing to more sophisticated materials and associated techniques. The primary purpose of this book is to provide a practical guide on composite materials and techniques that can be exploited by the ‘do it yourself competitor, preparer or constructor in a home workshop, without involving them in the purchase of expensive plant or equipment. Covering everything from basic ‘wet lay-up’ GFRP through to elevated temperature cure ‘pre-preg’ carbon fibre, it will facilitate the manufacture of such components as body panels, spoilers, aerofoils, ducting, dashboards and even, with suitable care, racecar chassis...

Notes on the second edition Although many of the materials and ones described in the first edition of this book nine years ago have remained essentially unchanged, two additional technologies have become more widely available. These are: resin infusion, itself available in various forms that could be used in a home workshop environment; and thermoplastic ‘resin’ systems that, though requiring a slightly modified approach to ‘tooling’, do offer potential time and cost savings. So, along with updates throughout, a new section has been added to this edition to give a practical overview of these ‘new’ techniques.

Acknowledgements SPECIAL THANKS MUST go to Vic Claydon for his creative input, as well as for checking my efforts and keeping me on my toes during the writing of this book; and to Brian O’Rourke for taking the time out of a hectic schedule to write the Foreword. My thanks also go to the following: Martin Armstrong, Gurit UK; Helen Belcher, Instron; Cellbond Composites Ltd; John Damon, GKN Westland Helicopters; Colin Evans, Amber Composites Ltd; Hexel Composites; David Hudson, Mitsubishi Ralliart Europe; Penske Cars Ltd; Del Quigley and Nathan O’Dell, DJ Racecars; Keith Read, MIRA; Phil Sharp, DPS Composites; Eric Taylor, Carr Reinforcements; Martin TaylorWilde, Multi-Sport Composites; Chris Tucker and Christine Riddle, Scott Bader Co Ltd; and Joan Villadelprat, Benetton Formula Ltd. For this second edition extra thanks go to Tim Roden, Marineware; Andy Scott, Scott Composites; Technical Resin Bonders; PRF

Composites; Mick Kouros, Fortec Motorsport; Barry Gates; Mick Hyde at Radical Sportscars; and Triple Eight Race Engineering. I am grateful, too, to the many fellow enthusiasts, competitors and constructors to whom I’ve chatted over the years regarding composites on competition cars; and especially the ones who bought components from me — there were times when I may not have been able to eat without you. And, as always, to Tracey, for ceaseless encouragement, support and constructive criticism.

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Chapter 1

Materials Introduction MATERIALS IN FIBROUS form are known to have some of the best mechanical properties of all available structural materials; yet their ability to cope on their own with bending and compression loads is very low. However, by embedding them in a matrix, such as a cured resin, it is possible to take full advantage of the outstanding strength and stiffness that become available. This is the world of fibre reinforced plastics, or FRPs. FRPs have been widely adopted in the motorsport industry for various reasons, but perhaps first and foremost because they can be moulded into a virtually limitless range of shapes relatively easily. Complex bodywork curvature that previously required the well-honed skills of a master metalworker can be achieved more readily, at less cost, and — in the early days at least — with less specialised equipment. That’s not to denigrate the expertise of the moulders, trimmers and, especially, the highly skilled pattern makers responsible for creating the shapes which are to be moulded from, since — as we shall see later — this is the most important part of the process. But once a mould has been made from the master pattern, it is then easier to produce numerous identical replicas than it was to manufacture the original. And whilst the moulding techniques do not lend themselves to mass production, being fairly labour intensive, the so called ‘tooling costs’ (that is, the cost of making the patterns and the moulds in the first place) are quite low, and make small volume production commercially viable. (Compare this to injection moulding, where tooling costs are very high and unit costs very low, rendering it more suitable to the mass production of certain plastic components in some applications, such as those found in and on a production car.) Fibre reinforced plastic moulding methods therefore lend themselves ideally to motorsport, where the quantities needed of individual components are generally small, and where designs, and hence tooling requirements, change frequently. The basis for a fibre reinforced plastic (FRP) composite material is, as the term suggests, a mass of reinforcing fibres combined with a ‘plastic’ resin matrix, which - binds and holds them. The properties of the resulting material depend on the properties of the fibre, the properties of the resin, and the manner in which the two are combined. The physical processes involved in FRP moulding are not particularly complex (even if the chemical ones are), but there is a very real and exciting magic about the application of a liquid resin mixture to a textile fabric which, once the resin matrix has ‘cured’, produces a strong, stiff and durable component.

part is what makes the whole what some people might have making composite components 4

This in

subject so fascinating. There is still, too, despite you believe, an element of the black arts about and structures, and the phrase ‘empirical design

COMPETITION

12

CAR COMPOSITES

methods’ still gets employed, even by Formula 1 car designers when discussing subjects like chassis construction. In order to put a clearer perspective on the more practical subsequent chapters, this first chapter will look at the background of the materials involved. There’s quite a bit to digest here, but it’s arranged into sections in such a way that the aspects of most interest to a reader should be easy to find. So let’s start off by looking at the various fibre reinforcements that can be used and the fabrics made from them, and then go on to examine the resins that hold them together. Using this approach will help to highlight the different mechanical properties of the materials that can be exploited, and enable us to begin to categorise composites according to their suitability in various applications. We'll begin where fibre reinforcement on competition cars began — with glass fibre. Reinforcing fibres Glass fibre Most people reading this book will be familiar with glass fibre reinforced plastic, otherwise known as GFRP, or just glass fibre (or fibreglass). This material has been around for years, since just after the end of the Second World War, in fact, when

the

builders of navy vessels started to take advantage of its properties. It wasn’t long before ‘it found its way into the automotive industry, and then the motorsport industry discovered it to be eminently suitable: for making bodywork, being pretty simple to use and relatively inexpensive. Famous American constructor and team owner Jim Hall even built the chassis of one of his Chaparral series of sports racing cars out of GFRP during the 1960s, though he reverted to aluminium thereafter. Glass fibre’s main use in motorsport is still in the construction of bodywork, its longevity in this application demonstrating just how useful it is, many components even from _ professional manufacturers still being made from GFRP. Its advantages, along with its cheapness, result from its useful strength to weight ratio and ready availability. Even if you’ve never made anything out of GFRP yourself, you will almost certainly have come across something made from it, and possibly even carried out some repairs to it! In its everyday bulk form, glass would not immediately come to mind as a useful ‘Chopped strand mat’ (or ‘CSM’) glass fibre fabric. ry

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—

MATERIALS

13

material for making competition car components. It breaks all too easily, and does so because it suffers from random surface flaws which, when a piece of glass is stressed, propagate through the material very easily, so that the glass cracks and breaks. But when the mineral ingredients to make glass are melted down at around 1600°C, then drawn through tiny holes whilst cooling, to transform them into small diameter

filaments (5 to 25 microns, or about 0.0002 to 0.001 inches), though there may still be

the same number of surface flaws in a given mass of the material the proportion of the fibres thus affected is small, which means that most of the fibres remain very strong.So in a bundle of fibres, the actual strength exhibited is much closer to the theoretical ‘flawless’ material’s strength. Consequently, bundles of glass fibres possess good stiffness and appreciable tensile strength — that is, resistance to breaking when subjected to tension along their own length. These properties allow the fibres to be produced in long filaments and wound onto reels. From here the next step is either to produce glass fibre yarns, which are bundles of close, twisted filaments; or glass fibre rovings, which are loose bundles of filaments. Yarns and rovings are classified according to their weight. In metric terms, yarns have what is called a ‘tex’ value, which is the weight in grams of 1,000 metres of yarn. In imperial terms they have a ‘denier’ value instead, consisting of the weight in pounds of 10,000 yards. Glass yarns are generally in the range 5 to 400 tex, while rovings are in the range 300 to 4,800 tex. The next stage in the process is for the yarns or rovings to be processed into one or other form of fabric, as we shall see later on in this chapter. Glass fibre comes basically in four fibre types: E, R, S and T. E-glass, made from alumina borosilicate, is by far the most commonly used, and, not coincidentally, the cheapest. But it also has the lowest mechanical properties. Nevertheless, it does produce GFRP laminates with good stiffness and strength, and is perfectly satisfactory in a lot of applications. S-glass was originally produced for aerospace and military applications. Chemically it has a higher silica content and no boron, and is a lot more expensive than E-glass. The commercial grade, actually called S2 but often referred to simply as S-glass, is made with fibre diameters 50 per cent smaller than E-glass, which furnishes them with more intrinsic strength (as a result of a lower proportion of flaws), and also increases the surface area available for bonding to the resin matrix, giving a stronger laminate. S-glass is also more water resistant, though this tends to be of more interest to boat builders than competition car constructors. R-glass is chemically and mechanically the same as S-glass, but is made in Europe rather than the USA, whilst T-glass is a Japanese-made equivalent. Aramid fibre Aramid fibres — or aromatic polyamide fibres to be more precise — are related to nylon, but by virtue of their slightly different chemical composition, they possess different, and quite extraordinary physical and mechanical properties. Better known by their trade

names

of Twaron

and,

most

widely

available,

Kevlar,

aramids

first became

available in around 1972, being used initially to replace the steel wires in commercial tyres. Versions of aramid are used in so-called ballistic clothing and body armour _ because of the material’s very high impact and penetration resistance. Produced by drawing filaments direct from a chemical brew, its full chemical name _ is polyparaphenylene terephthalamide. In the world of FRPs, the advantageous properties of aramids are their high tensile strength, reasonably high stiffness, relatively low density, and a toughness and resistance to abrasion that have to be seen to be believed. In fact these very properties contribute to one of aramid’s few drawbacks — its unwillingness to be cut by anything other than special (expensive) shears. It can be cut — albeit not very neatly — with ; certain types of snips, but patience is needed.

14

COMPETITION

CAR COMPOSITES

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The other principal disadvantage of aramid fibres is that they have low compressive strength, that is, strength in compression along the length of the fibre. This is thought to be caused by the make up of the fibres themselves from smaller, almost microscopic diameter fibrils, which tend to buckle under compressive loads. This has an unfortunate consequence in a laminate, creating a propensity for what is known as interlaminar shear, or separation of the layers within a laminate when under compressive load. For this reason aramids tend not to be used where appreciable bending loads can occur. They are at their most effective in bodywork — where their low density and strength can create stronger and lighter panels than glass fibre — and in structural applications that make use of the material’s high tensile strength and impact resistance. They are frequently employed as a ‘fail-safe’ back-up to stiffer, more

brittle fibres such as carbon fibre, which we will look at next. Aramids

are

typically made up into rovings in the range 20 to 800 tex. Carbon fibre Carbon fibre was originally produced and used commercially in the late 19th century by Swan and Edison, to provide the filament material in the first electric light bulbs, but it wasn’t until 1963 that structurally useful carbon fibres were produced at the Royal Aircraft Establishment at Farnborough in the United Kingdom. At that time it was an extremely expensive material, and you needed an aerospace programme research budget to even contemplate its use. However, it has been in regular use in motorsport since the early 1970s, and happily the cost has come down markedly. Carbon fibre is now within the budget of most motorsport competitors and constructors, especially if used in DIY projects such as those highlighted within this book. Raw carbon fibres are produced by the controlled thermal treatment of organic (ie carbon containing) precursor fibres. The most widespread precursor is polyacrilonitrile, or PAN, though cellulose and fibres produced from coal tar pitch are also used. The heat treatment involves carbonisation of the precursor fibres to burn away noncarbonaceous matter, followed by the so-called ‘graphitisation’ of the fibres at temperatures in the range 2,600 to 3,000°C. This process, and the deliberate prevention of shrinkage, serves to arrange the carbon atoms into an ordered, graphitic crystalline

MATERIALS

WS

structure along the length of the filaments. It is this aligned structure that gives the fibres their useful mechanical properties, namely high strength and stiffness. In particular, carbon fibres have the highest specific stiffness (that is, stiffness relative to density) of-any currently commercially available fibre, in addition to high tensile and compressive strength, and good resistance to corrosion and fatigue. There is: a ‘downside though — the impact strength of carbon fibres is lower than glass or aramid fibres, and the high modulus variants of carbon fibre are especially brittle in nature. By varying the temperature of graphitisation the manufacturers of carbon fibre are able to modify the end-product’s properties. So, fibres heated to around 2,600°C end up as ‘standard modulus’ fibres, also known as high strength or HS carbon, whilst those heated to 3,000°C are known as ‘high modulus’ or even ‘ultra high modulus’ fibres. An intermediate grade is known, unsurprisingly, as ‘intermediate modulus’, The tensile modulus values (see definitions in the next section) for the different carbon fibre types are shown in the table below. Table 1-1 The categories of carbon fibre

Type of carbon fibre

Standard modulus, HS

Tensile modulus, GPa

160 to 270

Tensile modulus, psi

A 21018. 02% 16"

Intermediate modulus (IM)

270 to 325

3.92 tol47iX 10"

High modulus (HM)

325 to 440

471101638 x10"

Ultra high modulus (UHM)

over 440

over 6.38 x 10!

We'll break off now, if you'll pardon the pun, and consider some of the structural terms and definitions that are likely to crop up now and again.

Structural terms and definitions You will often see terms like ‘tensile modulus’ and ‘specific strength’ cropping up in any discussion on structural materials, and although this book is not an engineering text by any stretch of the imagination, it will pay us to get a few definitions straight at this point. It won’t take long, so stick with it, and then we'll all know what’s being referred to. I promise I'll try to keep the gobbledegook to a minimum. Firstly, the term ‘strength’ is not what it may seem. For example, the ‘ultimate tensile strength’ — the value of load at which a material fails — that you may see quoted for a material is, in fact, the ultimate tensile stress value, often denoted by the Greek letter o

(sigma). Stress is defined as force or load divided by the area of a material over which that force or load is applied, and the units are thus in pounds per square inch or Newtons per square metre (also known as Pascals, Pa, where.GPa = a gigapascal = 10°Pa or a billion Pa). This is sensible if you think about it, because in order to determine the ultimate tensile strength of a material, you need to test a real, three-dimensional

piece of it, which therefore has a cross sectional area. And again it will be evident that a piece of the material twice as thick in one direction as another piece will require twice the load to break it, though the ultimate tensile stress value will be the same. Tensile strength, or stress, describes a material’s behaviour under tension. Thus, the

tensile strength of a carbon fibre is its strength when being pulled along the axis of the fibre. Compressive strength is the exact opposite — that is, the behaviour when the fibre is being compressed along the axis of the fibre. Strain, usually denoted by the Greek letter € (epsilon), is defined as the percentage elongation a material undergoes when subjected to load, and describes how that material deforms under a particular type of load. All materials deform when loaded, to some extent or other. Dividing the stress by the strain values for a particular material produces a figure

16

COMPETITION

CAR COMPOSITES

known as the ‘Young’s Modulus’. This value gives an idea of the material’s ‘elastic behaviour’, or how much it deforms under a given load over a given area. It is also seen as a measure of a material’s stiffness in bending, which after all is just a combination of tension and compression. By expressing the modulus as a function of tensile stress the tensile modulus can be determined, which was the parameter quoted in the table above for the different varieties of carbon fibres. It is now apparent that the high modulus fibres can be thought of as high stiffness fibres. One other concept that it’s worth covering here is incorporating the density values of each material to demonstrate their specific performance for a given weight. This allows a better comparison between different materials in some instances. For example, the specific tensile strength of a fibre is its tensile strength divided by its density, whilst the specific tensile modulus is the tensile modulus divided by the density. Finally, a quality which is very important is ‘toughness’. Not to be confused with strength, toughness is the measure of a material’s resistance to fracture by crack propagation. We mentioned this quality in passing when discussing the propensity of glass (as opposed to glass fibres) to fracture easily — it offers very low resistance to the propagation of cracks when put under stress. The table below illustrates ‘toughness’ values for some well-known materials, which helps put this attribute into perspective. ‘Notice that the units of toughness are in amounts of energy (kilojoules, or kJ) per square metre. This reflects the fact that toughness is assessed as the amount of work (energy) that has to be applied to propagate an area of crack. O10) (