Load Introduction in Carbon Fibre Composites

for Automotive Applications

Gordon KellyDepartment of Aeronautics

Royal Institute of TechnologySE–100 44 Stockholm, Sweden

Report 2002–19ISSN 0280-4646

Load Introduction in Carbon Fibre Composites for Automotive Applications i

Preface

The work presented in this licentiate thesis has been carried out at the Departmentof Aeronautics during the period September 1999 to May 2002. The work has beenconducted within the framework of two projects concerning the development offuture transportation vehicles. The financial support provided by the Commissionof the European Union through the growth project ‘Technologies for Carbon FibreRe-inforced Modular Automotive Body Structures’ (TECABS) and by the SwedishFoundation for Strategic Research through the national research program ‘IntegralVehicle Structures’ (IVS) is gratefully acknowledged.

Firstly, I would like to thank my supervisor, Dr Stefan Hallstrom for being agreat source of optimism and encouragement. His clear and structured approachto problem solving is something which I greatly admire. Thanks Stefan! I wouldalso like to thank my former supervisor Professor Tomas Astrom for welcomingme to the department and for his support during the early stages of my work.My room-mate during the last years, Dr Lance McGarva deserves many thanksfor sharing his knowledge of composite manufacturing and for his companionship.In addition to being colleagues, we have also become great friends. My past andpresent colleagues within the department are thanked for their enthusiasm and forcreating a cheerful atmosphere. I am especially grateful to the technical staff inthe structures laboratory for their support with manufacture of test equipment andinstrumentation. The staff at Novator AB are gratefully acknowledged for showinginterest in my work and for their support and fruitful co-operation.

Finally, I would like to express my gratitude to my parents and family for theirsupport and the encouragement they have given me.

Stockholm, May 2002

Gordon Kelly

Load Introduction in Carbon Fibre Composites for Automotive Applications iii

Abstract

The introduction of carbon-fibre reinforced plastics in load bearing automotive struc-tures provides a great potential to reduce vehicle weight and fuel consumption. Areduction in overall manufacturing cost can be realised through part integration anduse of cost-effective materials such as heavy tow carbon fibre. There is a currentdrive within the automotive industry to develop the technologies necessary to allowthe introduction of carbon-fibre components in mass production vehicles. One keytechnology is the development of reliable and cost-effective joining techniques.

This thesis addresses several important aspects of joining and load introductionin carbon-fibre reinforced plastics. The introduction of in-plane and out-of-planeloads in composite laminates based on non-crimp fabrics has been investigated ex-perimentally and numerically.

The bearing strength of composite laminates has been investigated consideringthe effects of the laminate stacking sequence, geometry and lateral clamping loadand bolt-hole clearance. The laminate failure mode and ultimate bearing strengthwere found to be significantly dependent upon the laminate stacking sequence, ge-ometry, lateral clamping load. The influence of initial bolt-hole clearance on thebearing strength, at 4% hole deformation and at ultimate load, was determined ex-perimentally. Significant reduction in bearing strength at 4% hole deformation wasfound for both pin-loaded and clamped laminates. It was concluded that the effectof bolt-hole clearance is significant with regard to the design bearing strength of me-chanically fastened joints. A three-dimensional non-linear finite element model wasdeveloped to investigate the effects of bolt-hole clearance on the stress field in thelaminate adjacent to the hole. The magnitude and distribution of stress at the holewas found to be significantly dependent on the level of clearance. The importanceof minimising bolt-hole clearance through machining of high tolerance, defect freeholes was highlighted.

The introduction of out-of-plane concentrated loads is commonly avoided in com-posite laminates due to their poor interlaminar and through-thickness strengths.However, these loads occur frequently in automotive structures and thus knowledgeof the behaviour of composite laminates subject to transverse concentrated loadingis necessary. Transverse load introduction in carbon fibre reinforced plastic wasinvestigated considering the effect of the specimen size, thickness and stacking se-quence. The load-displacement behaviour was found to be essentially linear withinitial failure of the laminates occurring at 20-30% of the ultimate failure load. Thegoverning failure modes were determined by fractographic analysis and found tobe matrix intralaminar and interlaminar shear failure. The finite element methodwas used to successfully predict the structural behaviour and first-ply failure of thelaminates.

Load Introduction in Carbon Fibre Composites for Automotive Applications v

Dissertation

This licentiate thesis contains an introduction to the research area and the followingappended papers:

Paper A

Kelly, G., Bearing strength of carbon fibre/epoxy laminates: Effect of bolt-holeclearance, To be submitted for publication.

Paper B

Kelly, G., Transverse load introduction in CFRP laminates, To be submitted forpublication.

Load Introduction in Carbon Fibre Composites for Automotive Applications vii

Contents

Preface i

Abstract iii

Dissertation v

1. Introduction 11.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2. Material systems for structural automotive components . . . . . . . . 31.3. Mechanically fastened joints in composite laminates . . . . . . . . . . 4

1.3.1 Geometric effects . . . . . . . . . . . . . . . . . . . . . . . . . 61.3.2 Effect of stacking sequence . . . . . . . . . . . . . . . . . . . . 71.3.3 Lateral clamping load . . . . . . . . . . . . . . . . . . . . . . 91.3.4 Bolt-hole clearance . . . . . . . . . . . . . . . . . . . . . . . . 9

1.4. Transverse load introduction in composite laminates . . . . . . . . . . 10

References 11

Paper A A1–A24

Paper B B1–B23

Load Introduction in Carbon Fibre Composites for Automotive Applications 1

1. Introduction

1.1. Background

Polymer composite materials are being used in a wide range of structural ap-plications in the aerospace, construction and automotive industries due to theirlightweight and high specific stiffness and strength. A variety of materials are be-ing used ranging from lower performance glass fibre/polyester, used in small sailboats and domestic products, to high performance carbon fibre epoxy systems usedin military aircraft and spacecraft. One sector where the use of composite mate-rials is still evolving is the automotive industry. Composite materials offer greatpotential in reducing vehicle weight, thus increasing fuel efficiency and reducingCO2 emissions. In addition to weight reduction, the number of individual partscan be significantly reduced making the high-volume composite car concept cost-effective. Current use of composite materials in high-volume automotive structuresis limited to secondary exterior structures such as body panels, wheel housings orenergy absorbing bumpers. Body panels are commonly made from sheet mouldingcompound (SMC) which is based on thermoset matrix reinforced by discontinuousglass fibre. Bumpers and front panels have been made from glass mat thermoplastic(GMT) which is commonly discontinuous glass fibre reinforced polypropylene. BothSMC and GMT composites are based on a randomly orientated, discontinuous glassmat reinforcement with a fibre volume content of not more than 30%. Both canbe compared with the fast stamping processes used to manufacture semi-finishedsheet metal parts. Their structural efficiency is typically low being only slightlyhigher than steel in case of plate bending. However, the advantages of good forma-bility, high energy absorption and resistance to corrosion makes GMT and SMCappropriate for exterior automotive components. A typical example of a compositeautomotive structure is Ford Explorer Sport Trac cargo area illustrated in Figure 1.

Figure 1: The Ford Explorer Sport Trac with SMC cargo area.

The Ford Explorer Sport Trac features an scratch-resistant, corrosion-proof SMCcargo area with integrated liner and side panels which have allowed for a reduction

2 G.Kelly

in weight of 20% in comparison to a sheet steel solution.

However, there is a greater potential to utilise the benefits of polymer compositematerials in automotive structures. Introduction of fibre reinforced plastics in thebody structure could decrease the mass by between 50-67% in comparison to currentmetallic body structures. In comparison, the potential of alternative materials suchas high strength steel and aluminium are 25-35% and 40-55% respectively [1, 2].Reducing the weight of an automotive body structure has a follow-on effect on otherkey systems. Reduction in body weight allows for smaller and lighter drive-systemswhich will reduce the loads on the suspension and chassis. This can potentiallyreduce the mass of the vehicle further.

Recent developments in the European automotive industry has seen the presen-tation of a new multi-material car concept from BMW incorporating a carbon fibrereinforced plastic (CFRP) passenger cell and aluminium space frame structure. TheBMW Z22 concept car has a body structure comprising of approximately 20 com-ponents including the CFRP side frame as shown in Figure 2. This first prototypeis part of a long term strategy at BMW to introduce carbon fibre reinforced plasticsin high volume vehicles [3].

Figure 2: BMW’s Z22 CFRP side panel manufactured by RTM

Another ongoing project involves the development of a carbon-fibre reinforcedBody-In-White (BIW) structure. The project is aimed at developing technolo-gies and methods which enable a CFRP vehicle concept to be manufactured at50 units/day utilising technological advances in heavy tow carbon fibre and resintransfer moulding [4]. Potential reductions in number of parts and weight are pre-dicted to be 30% and 50% respectively.

The introduction of load bearing composite parts in automotive structures placesnew demands on materials, manufacture and assembly processes. In order to realisehigh-volume production of composite automotive body structures, new material andmanufacturing concepts will have to be utilised. The following section presents onepossible solution for production of automotive body structures in fibre-reinforcedplastics.

Load Introduction in Carbon Fibre Composites for Automotive Applications 3

1.2. Material systems for structural automotive components

The introduction of high performance reinforced plastic components in automotivestructures will be significantly dependent upon the raw material costs and the man-ufacturing process. While Formula 1 cars and luxury sports cars can motivate theuse of materials and technologies developed within the aerospace industry, new sys-tems will have to be developed if polymer composite materials are to compete withmetallic materials in the production of high-volume vehicles. A potential solutionto manufacture composite automotive structures involves the use of liquid mouldingtechniques together with non-crimp fabric (NCF) reinforcement. Non-crimp fabricreinforcement is manufactured from fibre tows which are placed side by side to forma layer (as classical UD). The layers are subsequently stacked on top of each otherand stitched through their total thickness. The resulting mat commonly consists of 2to 4 layers and can be easily handled and draped. The manufacturing of multi-axialreinforcement from non-crimp fabric material is illustrated in Figure 3.

Figure 3: Manufacturing of multi-axial non-crimp fabrics.

Non-crimp fabrics also provide additional advantages such as unlimited shelf-life,high deposition rates and relatively low cost. However, there is a general concernthat the mechanical properties of laminates made from non-crimp fabrics are lowerin comparison to laminates made from pre-preg material. A number of studies havebeen conducted [5–11] and comparisons made to pre-preg laminates. However,further research is required in order to quantify the difference in material propertiesbased on relevant base data.

A liquid moulding technique which can be used to mass produce large, complex,composite parts is resin transfer moulding (RTM). The process involves the infusionof a reinforcement mat with low viscosity resin which is injected under pressure.The dry (unimpregnated) reinforcement is commonly pre-shaped and oriented intoa skeleton of the actual part known as a preform, which is inserted into a matcheddie mould. The mould is then closed, allowing the resin to be injected. Heat isapplied to the mould to activate polymerization mechanisms that cure the resin and

4 G.Kelly

solidify the part. Once the part develops sufficient handling or green strength, it ismoved or demoulded. Several benefits offered by the RTM process are :

• good laminate surface quality

• ability to manufacture large, complex shapes

• integration of ribs, cores and inserts

• use of a wide range of resin systems and reinforcements

• controllable fibre volume fraction

A major challenge in the design of structural automotive parts made from fibrereinforced plastics is the joining and load introduction aspects. These can arisethrough joining composite parts to metallic spaceframes, or in attachment of sec-ondary structures such as seat mountings and suspension systems. Introducing loadsinto composite laminates has been found to be more complicated than in metallicmaterials due material anisotropy, poor interlaminar strength and the absence oflocal stress relief such as plasticity. While a number of load attachments and jointscan be integrated into the components which are manufactured by RTM, the need tojoin or attach secondary structures is unavoidable. Thus, knowledge of the structuralbehaviour of mechanically fastened joints and attachments in composite laminatesmade from non-crimp fabric materials will be necessary. A review of published sci-entific literature concerning mechanically fastened joints in composite laminates ispresented in the following section. The majority of the literature concerns the be-haviour of laminates made from pre-preg material as this has been the material ofchoice for the aerospace industry.

1.3. Mechanically fastened joints in composite laminates

Mechanically fastened joints, where two or more components are joined using bolts,rivets or screws have been the subject of intense research during the last 30 years.The theoretical and experimental research has mainly been focused on high per-formance joints tailored to the aerospace industry. However, with the increaseduse of composite materials in non-aerospace applications has come the need for thedevelopment of cost-effective joining technologies.

Significant contributions to the development of mechanically fastened joints havebeen published in symposium proceedings [12], comprehensive texts [13] and liter-ature reviews [14–16]. The following sections describe the main factors influencingthe strength of mechanically fastened joints in composite laminates.

While mechanically fastened joints may be less favourable in terms of stressconcentrations, they have the advantage that they do not require surface preparationand that they can be disassembled. They do however require hole drilling which canhave a significant effect on the subsequent strength of the joint. The hole drillingtechnique has been shown to influence the manufacturing induced damage and thusthe machining of high quality, defect free holes is vital for obtaining maximum jointstrength [17]. Additional complexity arises due to the brittle nature of composite

Load Introduction in Carbon Fibre Composites for Automotive Applications 5

materials which do not exhibit local yielding which assists redistribution of loads inmetallic joints.

The design of joints in composite laminates is significantly influenced by thepossible joint failure modes. Six possible failure modes are illustrated in Figure 4.Bearing failure develops through compression damage at the loaded section of thehole. The compressive loading often leads to high interlaminar normal and shearstresses which promote delamination at the hole edge. Bearing failure is often char-acterised by the elongation of the hole. Net-section failure is often catastrophicwhere damage initiating at the hole edge propagates across the section perpendicu-lar to the load direction. Shear-out failure is often associated with poorly designedlaminates and involves an entire section of material behind the loaded area beingremoved through shearing. Shear-out failures are induced through high shear stressconcentrations at the hole edge at an angle of 90◦ to the loading direction. Cleavagefailure is associated with laminates having too few transverse plies or inadequateedge distance. Failure is initiated in a shear-out mode followed by failure of thenet-section at one side of the laminate. Fastener pull-out is generally a problemwhen countersunk rivets or screws are used. In cases where thicker laminates areadopted, shear failure may occur in the fastener.

Bearing failure Net-section failure

Cleavage failureShear-out failure

Fastener pull-through failure Fastener failure

Figure 4: Failure modes for bolted joints in composite laminates.

Factors which influence joint behaviour have been highlighted by a number of au-thors [15,16,18] as :

1. Material parameters - fibre type and form, fibre orientation, resin type, stack-ing sequence and fibre volume fraction.

6 G.Kelly

2. Design parameters - joint type, laminate thickness, geometry (width, holediameter, edge distance etc.), load direction and loading type.

3. Fastener parameters - fastener type, fastener size, fastener material, fastenerpattern, clamping force, washer size, fastener/hole fit.

Several of the factors highlighted in the literature as the most influential with regardsto joint design are discussed in the following sections.

1.3.1. Geometric effects

Geometrical parameters such as the bolt hole diameter d, laminate thickness t, thewidth w and edge distance e have been shown to have a significant effect on thestrength of bolted joints [19–23]. The commonly adopted terminology for joint geo-metrical parameters is illustrated in Figure 5. The bearing strength of a composite

d

e

w

Figure 5: Geometry related terminology for bolted joints in composite laminates.

joint has been shown to increase up to a value e/d ≥ 4 or 5. For higher e/d values,no significant increase in joint strength has been observed. The e/d values where thejoint strength can be increased have however, been shown to be strongly dependenton the laminate stacking sequence. Laminates which contain a high percentage offibres in the 0◦ direction tend to fail by shear-out failure but increasing e/d causesa transition to a bearing failure mode. In general, increasing the isotropy of thelaminate allows for lower e/d values to be used. The Composite Materials Hand-book (MIL-HDBK-17) [24] recommends a minimum e/d value of 4.5 for [0◦/± 45◦]laminates.

The width of the joint has also been shown to influence the joint strength. Thew/d ratio is often used to determine the pitch distance in multi-fastener jointsbased on single bolt data. For w/d > 6, Rankumar and Tossavainen [25] showedthat the bearing strength remains relatively constant. For w/d < 4, the failuremode is primarily net-section failure for [0◦/ ± 45◦] laminates. Hart-Smith [21, 26]determined experimentally that the joint strength varies with d/w as shown inFigure 6. Matthews [27] illustrated the effect of the e/d and w/d ratio on jointbearing strength as illustrated in Figures 7-8. The geometrical ratios w/d and e/dare clearly dependent on the stacking sequence and degree of anisotropy of thelaminate. Collings [19] determined the required ratios for a number of differentstacking sequences of for CFRP laminates as listed in Table 1. Similar results for

Load Introduction in Carbon Fibre Composites for Automotive Applications 7

Figure 6: Joint strength vs ratio of bolt diameter to strip width [21].

the [0◦/90◦] and [0◦/ ± 45◦] laminates have been shown by other authors [15, 28].These values are however dependent on the relative proportions of plies in eachdirection and may therefore differ depending on the tested laminate.

1.3.2. Effect of stacking sequence

While highly orthotropic stacking sequences may be optimal with respect to globalloading and stiffness based design, stress concentrations as high as 7 or 8 have beendetermined for orthotropic laminates containing holes. Laminate stacking sequenceallows the designer to tailor laminates for applied loads and this possibility hasbeen used extensively in the design of composite joints. Stacking sequence has beenshown to influence the strength of joints significantly with respect to both strengthand failure mode. Collings [19] in his earlier work on joints in CFRP discovered thatwhile stacking sequences had a lesser effect on shear-out and net-section strengths,bearing strengths were sensitive to stacking sequence. Subsequent research hasshown that all failure modes are dependent on stacking sequence. Arnold [18] andHamada [29] expressed the importance of 0◦ plies on the bearing strength withHamada proposing that the strength was dependent on the degree of interlaminarfracture versus fibre buckling in the 0◦ plies. The pseudo-clamping effect obtainedthrough employing plies at 90◦ to the load direction at the outer surfaces, has beenshown both experimentally [18, 30, 31] and numerically [30, 32] for both glass fibrereinforced plastic (GFRP) and CFRP laminates. Applying 90◦ plies at the outersurface acts in reducing interlaminar normal stresses and impedes delamination. Incontrast to much of the published literature, Hamada [29] found that the highestjoint strengths were achieved through adopting outer 0◦ plies in his study of the

Table 1: Required w/d and e/d ratios for bolted joints in CFRP [19].

Stacking Sequence w/d e/d[0◦/90◦] >4 >5[±45◦] ≥8 >4

[0◦/ ± 45◦] ≥4 ≥4[0◦/60◦] ≥4 ≥4

8 G.Kelly

Figure 7: Variation of bearingstrength with w/d. Thesloping region correspondsto net-section failure, theplateau region to bearingfailure [27].

Figure 8: Effect of e/d on the bear-ing strength. The slopingregion corresponds to shearout failure, the plateau re-gion at high e/d to bearingfailure [27].

effects of stacking sequence on the strength of quasi-isotropic laminates.The role of ±45 plies in diffusing the load path in bolted joints has been reported

by a number of authors [18, 21, 29]. This has led to the current belief that efficientjoint design is only achieved when quasi-isotropic stacking sequences are used. Hart-Smith suggested the stacking sequence design guideline illustrated in Figure 9.

Figure 9: Selection of ply stacking sequence for fibrous composites [22].

The diagram has resulted from the author’s extensive experimental research onjoints in carbon fibre-epoxy laminates. Hart-Smith [21] also reported the effect ofreducing stress concentrations through use of glass fibre strips around bolt holes inCFRP. Through locally substituting 0◦ CFRP fibres by softer GFRP strips (aboutfour hole diameters wide), experimental evidence has shown the improved strength

Load Introduction in Carbon Fibre Composites for Automotive Applications 9

of bolted joints in these hybrid laminates.It has also been reported [33] that plies of similar orientation should be dis-

persed throughout the laminate and not ‘blocked’ in order to achieve higher bearingstrength. Thus the most efficient laminates, considering all possible failure modes,have a dispersed [90◦/ ± 45◦/0◦]s quasi-isotropic stacking sequence. Cross-ply lam-inates have been studied in tension by Marshall [32] and in bending by Chen andLee [30] with both authors in consensus that [90◦n/0◦n]s laminates exhibit higherultimate joint strength compared to [0◦n/90◦n]s laminates.

1.3.3. Lateral clamping load

The effect of applying lateral (through-thickness) load around the bolt hole has beenshown to be beneficial with regard to joint bearing strength [15,19–22,28,32,34,35].Collings [19] assumed the stress induced by clamping to be constant over the washerarea and given by

σzz =T

0.2 d π4

(D2 − d2)(1)

where T is the applied torque, D is the outer diameter of the washer, d is the holediameter and ‘0.2’ is a torque coefficient determined by Stewart [36]. Strength im-provements for 3 mm thick CFRP laminates of up to 170% were found for a constantlateral pressure of 22 MPa [19]. It was noted that little further improvement wasachieved through increasing the lateral pressure. In contrast, Smith et al. [28] foundthat for 1 mm CFRP laminates covering a range of stacking sequences, pressuresgreater than 35 MPa were required to achieve maximum bearing strength. Akay [37]supported the latter findings in a study on various woven and unidirectional CFRPlaminates. Slightly different behaviour was discovered for aramid fibre reinforcedplastic (AFRP) laminates with Akay and Kong Ah Mun [35] finding that bear-ing strengths could be increased by approximately 100 % by mere finger-tightening(1-2 MPa) in comparison to the pin-loaded condition. Transition from bearing totensile failure was also noted upon applying finger-tight lateral pressure. Similartransitions have also been noted for CFRP [37] but at significantly higher lateralpressures (>25 MPa) depending on the stacking sequence.

1.3.4. Bolt-hole clearance

The clearance between the bolt and the hole significantly influences the behaviourof mechanically fastened joints. Several authors have investigated the effect of bolt-hole clearance on the behaviour of mechanically fastened joints in composite lami-nates [38–41]. In general, increased clearance was found to reduce the contact areabetween the bolt and the hole thus increasing the stress levels. However, it is diffi-cult to compare the results from different authors as the problem is non-linear beingdependent upon the applied load, friction and the bolt-hole contact area.

Bolt-hole clearance can significantly affect the load distribution in multi-rowjoints. A neat fit between the bolt and the hole will ensure that the load is transferredimmediately. In contrast, there will be a delay in load transfer in clearance fitjoints until contact has been established. The effect of clearance is potentially moreimportant in the case of single-lap joints where the clearance allows the bolt to rotate

10 G.Kelly

in the hole. Bolt rotation has a detrimental effect on joint performance as a resultof the reduction in the bolt-hole contact area. No published scientific literature wasfound concerning the effect of bolt-hole clearance in multi-row or single-lap joints.

With mass production of modular composite automotive structures, the man-ufacturing and assembly tolerances will become important parameters which willhave to be considered in the design process. While larger tolerances may be ad-vantageous for manufacture and assembly operations, knowledge of the subsequenteffect on the mechanical properties of fastened structures is necessary. The effectof bolt-hole clearance on the bearing strength of carbon fibre epoxy laminates wasinvestigated in Paper A.

1.4. Transverse load introduction in composite laminates

Concentrated load introductions and attachments in CFRP structures require thatloads are introduced into the laminates in the transverse or thickness direction. Thisis arguably the most difficult load case given the poor interlaminar and through-thickness strength of the material. Transverse loads are commonly avoided in com-posite laminates and this is reflected in the volume of published literature.

Important contributions have been made by Waters and Williams [42] and byBanbury et al. [43, 44]. Waters and Williams [45] conducted an early experimentalinvestigation of bolt push-through failure in composite laminates. The authors in-vestigated the response and failure modes of a wide range of composite laminates.Clamped circular specimens of diameter 25.4 mm and 50.8 mm were loaded inthe transverse direction through a countersunk fastener. The characteristic failuremodes were found to be intralaminar fibre failure in the vicinity of the bolt holeand intralaminar and interlaminar matrix failures in the laminate interior. It wasalso concluded that the strength of transversely loaded composite laminates couldbe improved through the use of tough resins, high-strain fibres and low modulusadhesive interleaving.

Banbury and Kelly [43] conducted a two part study investigating the pull-throughstrength of carbon fibre reinforced epoxy laminates made from woven fabric andunidirectional tape pre-preg material. The authors provided a detailed investiga-tion of the observed failure modes and discussed the mechanisms responsible forpull-through failure. The internal damage pattern in the laminates was likened tothat observed in composite laminates subject to low velocity impact. Failure wasattributed to a critical level of matrix strain which was independent of the laminateand fastener geometry or material type for a given resin system. The authors alsoconducted a two-dimensional axi-symmetric finite element analysis to simulate thefastener pull-through tests [44]. A progressive damage model based on the maximumstrain criterion was used to predict failure in the laminates although no consider-ation was given to interlaminar failure modes. The results from the model wereconsistent with the experimental observations and the predicted failure loads wereshown to be in good agreement with the experimentally determined values.

There is currently no widely accepted test method for evaluating the perfor-mance of transverse load introductions. Two different methods are suggested in theComposite Materials Handbook (MIL-HDBK-17) [24], however, it is acknowledgedthat methods may not be representative of actual joints. The first method is an

Load Introduction in Carbon Fibre Composites for Automotive Applications 11

adaptation of MIL-STD-1312 Test 8 with this method being suggested for screeningpurposes only. The second proposed method is similar to that adopted in Paper Bof this thesis. This method is suggested to be suitable for design purposes.

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