AG-20 Fractographic Aspects of Fatigue Failure in ... Reports/SM_AG-20_TP-112.pdf · Aerospace...

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GARTEUR FINAL REPORT TP 112

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G-20 Fractographic Aspectsf Fatigue Failure inomposite Materials

INETIQ/FST/SMC/TR01236

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ment is subject to the release conditionsn the reverse of this page

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he present report is distributed on a limited basis, for the information of the listed organisationsnly ; consequently filing in any central library open to others, and citation in accession lists, or as

iterature reference, are prohibited.*

ARTEUR aims at stimulating and co-ordinating co-operation between Research Establishments,ndustry and Academia in the areas of Aerodynamics, Flight Mechanics, Helicopters, Structure &aterial and Propulsion Technology.

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© Copyright of QinetiQ Ltd 2001Approval for wider use of releases must be sought from:

Intellectual Property Department, QinetiQ ltd, Cody Technology Park,Farnborough, Hampshire, GU14 0LX, UK

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Authorisation

Prepared by Mr M.J.HileyTitle Principal Scientist

SignatureDate 23/08/2001Location Room 2008, Building A7, QinetiQ ltd, Cody Technology Park,

Farnborough, Hants. GU14 0LX , UK.

Approved by Dr E.S GreenhalghTitle Principal Scientist

SignatureDate 10/09/2001

Authorised by Prof: P.T.CurtisTitle Monitoring Responsable GARTEUR SM

SignatureDate 10/09/2001

Principal AuthorsName Matthew HileyAppointment Principal ScientistLocation Room 2008, Building A7, QinetiQ Ltd, Cody Technology Park,

Farnborough, Hampshire, GUI4 0LX, UK

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Record of changes

Issue Date Detail of Changes

1 June 2001 New Document

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Executive summary

This report describes the findings of a study performed by GARTEUR AG20, aimed at examiningthe fractographic aspects of fatigue failure in polymer composites. An investigation into themechanisms of fatigue failure in unidirectional materials was performed to establish themacroscopical/microscopical features associated with fatigue fracture. The micromechanisms bywhich different fractographic features form, under different loading conditions, as well as the effect ofmaterial on their appearance, was studied. Investigations to establish whether there was arelationship between the fractographic features and crack growth rate and direction were alsoperformed.

Fractographic investigations were undertaken by exchanging fatigue specimens between memberswithin three round robin exercises. Members findings were reported at bi-annual meetings, six ofwhich were held over the three year life of the project.

The results of this study identified a number of fractographic features unique to fatigue failureincluding; striations within the fibre imprints and matrix rollers, both of which were observed withinmode II (shear) dominated failures. Striations were also observed in the matrix between the fibres insome materials, but these were only visible using electron microscopes with very high resolutions.Macroscopically the fatigue fracture surfaces were noticeably smoother that their static equivalents,which, in the mode I specimens, was attributed to a reduced level of fibre bridging. The singlephase resin systems gave rise to rollers and striations, with the fibre/matrix bond appearing to be themost important factor controlling striation formation within the fibre imprints. The main differenceobserved between materials was most apparent in the two phase system, where in mode II, rubbingout of the toughening thermoplastic particles was mainly observed.

Studies of the matrix rollers showed they were not useful for determining crack growth directions.Matrix striations could be used to indicate the local directions of crack propagation, but the striationswithin the fibre imprints were the most useful for determining global directions of fracture. Byobserving which side of the fracture surface was fibre rich or imprint rich and the appearance of thestriations (bright or dark), the crack growth direction could be ascertained. Due to the significantvariation in the inter-striation spacings observed, efforts to correlate striation spacing to crack growthrates proved inconclusive.

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List of Contributors

The following members/organisations have contributed to the compilation of this report:

Name of Member Organisation Country

Mr T. Ansart DGA/CEAT FranceMr S. Baas (Chairman) Stork FDO The NetherlandsDr H. E. Franz [4] EADS GermanyMr F. Heutling [4] EADS GermanyMr H. Huisman NLR The NetherlandsMr J-P Steyaert [8,9] NLR The NetherlandsMr M.J. Hiley (Vice-chairman) [5,6] QINETIQ EnglandDr B. Keul DaimlerChrysler

Aerospace AirbusGermany

Mr A. Lemascon [3] CETIM FranceMr P. Granstam CSM Materialteknik SwedenMr G. Spratt Bombardier Shorts N.IrelandMr M. Vancon Aérospatiale Matra FranceMr R. Muskett [7] BAE SYSTEMS England

Where numbers are shown in brackets after a member, the number indicates sections of thereport written by them.

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List of Contents

Authorisation iRecord of changes iiExecutive summary iiiList of Contributors ivList of Contents vList of Figures viiList of Tables ix1. Introduction 12. Background 23. Literature Survey 43.1 Literature Review: Fractographic Analysis of Fatigue Failure in Polymer Composite

Materials. 43.2 Examination conditions 43.3 Influence of the applied stress level 53.4 Influence of the loading mode 5

4 Literature Review: The theoretical mechanisms controlling fatiguefailure in polymer composite materials 6

4.1 Delamination growth due to fatigue 64.2. Crack growth models 74.3 Microfractographic investigations of fracture surfaces 10

5. Manufacture and testing of specimens 115.1 Background 115.2 Round Robin 1 (Mode II) 115.3 Round robin 2 (Mode I and Mixed-mode) 115.4 Mode I specimens 115.5 Mixed mode (mode I + mode II) specimens 115.6 Round robin 3 (Mode I, Mode II and Mixed-mode) 12

6. To Establish the Macroscopic/Microscopic features associated withfatigue failure. 13

6.1 Background 136.2 Macroscopic examination 136.3 Microscopic examination 136.4 Striations 136.5 Matrix Rollers 146.6 Surface Texture Effects 14

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7. Material Dependency 157.1 Introduction 157.2 Material Properties 157.3 Effect of different resin phases 167.4 Mode I fatigue fracture 167.5 Mode II fatigue fracture 167.6 Mixed -mode fatigue fracture (I+II) 177.7 Fibre Stiffness 177.8 Fibre/Matrix Interface 17

8. Relationship between Crack Growth Direction (C.G.D) and fractographicfeatures 18

9. Relationship between Crack Growth Rate (CGR) and inter-striationspacing 19

9.1 Background 199.2 Mode I Interstriation distance measurements 199.3 Mode II Interstriation distance measurements 199.4 Mixed Mode Interstriation distance measurements 20

10. Discussion 2111. Conclusions 2312. Recommendations 24References 25Tables 29Annex A - Tables A1 to A2 51Figures 57Distribution List 79Report documentation page 80

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List of Figures

Figure 1 Crack propagation rate vs. cyclic stress intensity factor ∆K.Figure 2 da/dN data generated for unidirectional DCB AS4/PEEK specimens.Figure 3 GImax as a function of cycles up to the onset of delamination.Figure 4 SEM image of a mode I static fracture surface.Figure 5 SEM image of a mode II static fracture surface.Figure 6 Singular terms for stress components of the stress field close to the crack tip under

mode I (σy) and mode II (σx).Figure 7 Model according to McEvily describing crack propagation during cyclic loading, partially

valid for metals and polymers.Figure 8 Refined version of the model according to McEvily describing crack propagation during

cyclic loading.Figure 9 Measured craze and crack opening (points) in PMMA.Figure 10 Schematic representation of the correlation of the loading phase with crack propagation

and craze growth during continuous crack growth (in PMMA).Figure 11 Schematic representation of fatigue crack propagation in a craze zone for (a)

continuous crack growth and (b) discontinuous crack growth.Figure 12 Schematic representation of secondary electrons emitted from an elevated object with

perpendicular impingement of primary electrons.Figure 13 Schematic diagram of ENF test fixture.Figure 14 Schematic diagram of DCB test fixture.Figure 15 Schematic diagram of MMF test fixture.Figure 16 Graphical representation of the loading applied to specimen P3.Figure 17 Graphical representation of the loading applied to the specimen P4.Figure 18 Striations in the fibre imprints. Left hand imprint shows ‘bright’ striations whilst right

hand imprint shows ‘dark’ striations. T800/5245 (mixed-mode) x6300.Figure 19 Striations in the fibre imprints T800/924 (mode II) x10000.Figure 20 Striations in fibre imprints IM7/8552 (mode II) x5000.Figure 21 Schematic diagram illustrating how appearance of striations is affected by viewing

direction.Figure 22 Striations on the fibre surface IM7/8552 (mode II) x4000.Figure 23 Widely spaced striations on fibre surfaces IM7/8552 (Mode II) x2000.Figure 24 Striations within the matrix oriented parallel to the fibres (mode l) T800/5245 x14000.Figure 25 Striations within the matrix T800/5245 (mode I + II).Figure 26 Matrix rollers T800/924 (mode II) x7500.Figure 27 Matrix rollers T800/924 (mode II) x5000.Figure 28 Matrix rollers between the fibres T800/924 (mode II) x1000.Figure 29 Spheroids on the surface of mode II fatigue fracture T300/914 (mode II).Figure 30 Fine detail of fractographic features visible IM7/977 (mode I) x3250Figure 31 Cusps within the resin in masked by second (modifier) phase T300/914 (mode II)

x1770Figure 32 Fractographic features resolvable despite presence of fine secondary (modifier) phase

T800/924 x 3250.Figure 33 Fractographic features resolvable despite second (modifier) phase IM7/8552 x1000.Figure 34 Fatigue striations in fibre imprint region T800/5245 (mode I) x5500.Figure 35 Fatigue striations in fibre imprint region IM7/8552 (Mode I) x5000.Figure 36 Regular features – possibly evidence of fatigue loading T300/924 (Mode II) x4000Figure 37 ‘Peened’ PES modifier spheroids T300/914 (mode II) loading.Figure 38 Rolled cusps formed during fatigue of static fracture surface T800/924 (mode II).Figure 39 Striations in the fibre imprints T800/5245 (mode I +II) x 8K.Figure 40 Striations within the resin between fibres T800/5245 (mode I + II) x9K.Figure 41 Striations in fibre imprints T800/924 (mode I + II) x1800Figure 42 Micrograph showing change in striation spacing around defect T800/8552 (mode II)

x5000.

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Figure 43 Micrograph illustrating variation in striation spacing between adjacent fibre imprintsT800/5245 (mode I + II) x3100.

Figure 44 Relation between crack growth rate and interstriation spacing.

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List of Tables

Table 1 Summary of papers reviewed.Table 2 Table of commonly used fractographic features (German, English and French).Table 3 Summary of mode II fatigue specimens issued for round robin I.Table 4 Summary of mode I fatigue specimens issued for round robin 2.Table 5 Crack length vs Cycles data for mixed mode specimen P4 distributed in round robin 2.Table 6 Summary of mixed-mode fatigue specimens issued for round robin 3.Table 7 Summary of mode I and mode II specimens issued for round robin 3.Table 8 Summary of Mixed Mode Bending (MMB) fatigue specimens issued for round robin 3.Table 9 Summary of round robin 1 results.Table 10 Summary of round robin 2 results.Table 11 Summary of round robin 3 results.Table 12 Effect of material on fractographic features.Table 13 Relation between Crack Growth Direction (CGD) and fractographic features – mode I.Table 14 Relation between Crack Growth Direction (CGD) and fractographic features – mode

II.Table 15 Relation between Crack Growth Direction (CGD) and fractographic features – mode I

+ II.Table 16 Overview of round robin results in respect to crack growth rate.Table 17 Data from round robin 1 showing relation between crack growth rate determined from

striations with that determined form crack measurement on the specimen.

Table A1 Summary of finding from mode II specimens (RR1).Table A2 Summary of findings from mode I specimens (RR2).Table A3 Summary of findings from mixed-mode specimens (RR2).Table A4 Summary of findings from mode I and mode II specimens (RR3).Table A5 Summary of findings from mixed-mode specimens (RR3)

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1. Introduction

1.1 In all stages of the life of a composite structure the systematic study of microstructure andfracture surfaces provides essential feedback into material developments, design andultimately certification. As such, fractographic analysis is increasingly being recognised asfundamental to the application of composite materials in aerospace structures.

1.2 Prior to this fractographic programme, the GARTEUR Structures and Materials ActionGroup AG14 (Fractography of Composites) [1] undertook a study aimed at establishing acommon terminology and methodology for the assessment of fracture surfaces. Whilstconducting this work it became apparent that the examination of fatigue fracture surfacespresented particular difficulties to fractographers, both in terms of their identification andtheir interpretation. To rectify this lack of understanding, the AG14 final reportrecommended that a detailed study of fatigue should be pursued within GARTEUR.

1.3 It is the general perception that current fibre reinforced composites are not susceptible tofatigue. Part of this perception has originated due to the use of conservative design strainswhich have resulted in composite components being subjected to only modest loadsinsufficient to promote fatigue failure. However, with improved understanding and thedevelopment of improved fibres and matrix materials, it is likely that design strains willincrease, which could lead to potential fatigue problems. It is essential therefore that theability to recognise and understand fatigue failures in composites structures is developedbefore they arise; either during the development of a component, or when it is in service.

1.4 The approach taken in this study involved the generation of a series of specimens, testedunder controlled conditions, which were then examined by members of the group. Threeround robin exercises, involving the exchange of mode I, mode II and mixed-mode (I+II)interlaminar fracture specimens, were conducted on a range of unidirectional materials.The fracture surfaces were examined by one or more participants and the resultsreported, discussed and evaluated at Action Group meetings. In addition, relevant datagenerated in-house was presented as appropriate by individual members to aid theinterpretation of the fracture features.

The proposed programme of work sought to address the five objectives listed below:

1) To establish the macroscopical and microscopical features which indicated that fatigueplayed a role during failure.

2) To establish the microscopical mechanisms by which the features identified (in 1) occurunder different loading modes and stress intensities.

3) To establish the material dependency of the fractographic features associated withfatigue failure (If appropriate unreinforced resins would be examined).

4) To establish the relationship between crack growth direction and the appearance of thefatigue features.

5) To establish the possibility of relating the crack growth rate to the inter-striationdistances on the fracture surface.

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2. Background

2.1 The use of carbon and glass fibre reinforced polymer composites for the manufacture ofaerospace components has seen considerable growth since their introduction in the1970’s. In applying these materials, it has widely been assumed that fatigue is not aproblem. Early fatigue tests performed on plain composite specimens showed only a smallreduction in performance even after a significant number of cycles. Residual strengthtesting after fatigue also showed minimal degradation of mechanical properties. The notionthat fatigue fracture in composites did not occur was further reinforced when thetranslaminar fracture surfaces of composites were examined. Unlike metallic samples,which often showed fractographic features such as striations on the fracture surfaces, thetransverse fractures did not show features characteristic of fatigue. The latter behaviourcan be attributed to the fact that the breaking strain of the fibres is normally less than thatof the matrix.

2.2 Although composite components are usually designed so that neither tensile forces occurorthogonally, nor shear forces parallel to the individual layers, delamination is by far thecommonest mode of failure in laminated composites. Testing of impacted laminates infatigue, has shown that there is often only a small reduction in residual strength afterfatigue loading, but that failure occurs relatively quickly after a certain number of loadcycles by a process of delamination and ultimately transverse fracture. Examination of theimpact sites using non-destructive evaluation techniques, such as C-scan, shows thatdelamination growth from the impact does occur in fatigue at loads below the staticstrength. Fatigue damage propagation in laminated composites is therefore primarily aninterlaminar process and this is where attention needs to be focussed when conducting afractographic assessment of a suspected fatigue failure.

2.3 Early fractographic assessments of fatigue failures in carbon and glass fibre reinforcedcomposites during the early 1980’s revealed the presence of striations within the matrixand fibre imprints of some materials. The former were broadly similar in appearance tofractographic features observed in bulk polymer samples subjected to fatigue. Thepresence of the striations within laminated composites served to reinforce the notion thatfatigue damage propagation could occur in composites and that the mechanisms of failurewere distinctly different to fractures occurring due to the application of a static load.

2.4 Although early fractographic studies have shown that the presence of striations mayindicate that fatigue played a role in failure, very few systematic fractographic studies havebeen made of fatigue failures. The effect of loading mode, material and stress level on thefractographic features produced by cyclic loading, still remain largely uncharacterised. Nocomprehensive study as to how fractographic features such as striations can be used todetermine local or global crack growth directions, or crack growth rates, have been made.This lack of fractographic understanding and the frequent difficulties encountered byresearchers in interpretting fatigue failures has fuelled the need for a structuredassessment of fatigue failures in laminated composites, such as the one outlined in thisreport. When considering the findings of this study, it should be remembered that all of thefractures examined have been generated in unidirectional materials. In practicedelamination in composites usually occur between plies of different orientations. Crackgrowth also seldom remains at one ply interface, but jumps through plies to one or moreplies that are preferential to delamination propagation. It was decided to restrict the initialstudy to unidirectional materials, since established test procedures for these materialswere available and the loading conditions using these tests could be carefully controlled.This enabled the basic fractographic features associated with fatigue failure to beinvestigated without the need to consider other variables introduced by using morecomplex laminates. Fatigue failures in these more complex laminates will need to beevaluated once the fundamental fractographic features associated with fatigue inunidirectional materials are understood.

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2.5 It was found by the previous action group on composite fractography (AG14), thatexchanging specimens as part of a series of round robin exercises worked well inaddressing the aims of the group. Consequently it was decided that the AG20 ActionGroup, the activities of which are described in this report, should continue to work using asimilar approach. Fracture surfaces produced by one or more participants were analysedby all members and the finding discussed at bi-annual meetings. The results of individualin-house investigations were also often presented. Since fatigue damage growth incomposite materials is predominantly controlled by delamination, the study undertakenconcentrated on the examination of mode I and mode II interlaminar failures andcombinations thereof (mixed-mode I + II). A range of carbon epoxy were investigated; withstudies being confined to the examination of unidirectional materials only.

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3. Literature Survey

3.1 Literature Review: Fractographic Analysis of Fatigue Failure in Polymer CompositeMaterials.

3.1.1 Examination of the literature showed that in most cases fractography is rarely the mainfocus of the published work and that fractographs are supplied mainly to supportexperimental and theoretical results of more complex mechanical studies. A summary ofthe available literature examined in this study [1-64] is shown in table 1. This table givesa brief overview of the fractographic finding of a selection of papers concerned with thefatigue of polymer composites. It should be noted that in some instances where thespecific researchers findings of are discussed in the general text, they have not beenalways been included in table 1. Specific references to an individuals work are, however,included towards the back of this report.

3.1.2 Over the last three decades the fatigue failure mechanisms of a large number ofpolymers have been investigated [2-5]. It is only in the last two decades, however, thatany serious attention has been paid to the microfractographic investigation of fatiguefailure in fibre reinforced composite materials. The papers in table 1 show that many ofthe studies undertaken have only involved a superficial study of the microfractographicaspects of failure, with most paper concentrating on the implications of physically deriveddata.The small quantity of literature that focuses on microfractography is oftencontradictory and does not explain in detail the observing conditions or mechanismsbehind the formation of many of the fractographic features. Some of the best accounts offatigue failure in GFRP and CFRP, have been given by Franz [6-8], , where fatiguefracture surfaces within the fractured matrix and in the fibre imprints were described.Other recent references include those by Hiley [9,10], which describes the formation ofstriations and matrix rollers in CRFP and GFRP tested under mode I, mode II and mixed-mode loading.

3.1.3 A study of the published papers dealing with fatigue in composite materials showed thatseveral parameters are influential in the formation of specific fracture features and affectthe ease by which they can be detected. The examination conditions themselves, theapplied stress level and loading mode are all important factors controlling whether aparticular fractographic features arises and whether it can be observed.

3.1.4 The presence of striations on a fracture surface of a failed component appears from theliterature to be positive evidence that fatigue played a role in failure. The reciprocal is,however, not true. When there are no striations it cannot be concluded that the part failedunder static loading only (as some materials appear to show no striations at all whenfatigued, or when the fatigue stress level is high [10] ).The type of loading applied to thecomposite also plays a major role on striation formation, but inaccurate or imprecisedescriptions of the examination conditions used in many of the studies, have madeinterpretation of the findings difficult. From the survey, it was apparent that severalfactors are important when examining fatigue fractures fractographically, the mostimportant of these are detailed below:

3.2 Examination conditions

3.2.1 In most of the papers studied, the authors did not specify all of the examinationconditions used when examining fractographic specimens. The present study and thoseby other authors [7,10], have shown that the angle of tilt of the specimens is importantwhen attempting to resolve striations. Usually an angle greater than 45° is required toobtain sufficient definition of these features. Most studies do not include this information

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which means that striations may not have been observed simply because theexamination conditions used were not favourable. The fracture surface examined (i.e thetensile or compressive surface of delamination generated in three point bending) canalso be important in characterising the fracture features observed, but again this is rarelydescribed. In some instances, scale bars or magnifications have also not been included,which makes it difficult to judge the magnitude of some fractographic features. Manyearlier fractographic papers do not therefore provide useful fractographic informationbecause of the use of inadequate examination conditions or lack of definition of theobserving conditions.

3.3 Influence of the applied stress level

3.3.1 Studies by Morris [11], showed striations under tension-compression fatigue can beformed in carbon/epoxy composite materials when the applied stresses are less than70% of the ultimate compression stress. For glass/epoxy composites tested in mode Ifatigue, Takemori [12] described how striation formation depends on the R ratio (when R= 0.01: striations are visible, when R = 0.4 or 0.5: there are no striations). In general, ahigh stress level applied during a fatigue test does not appear to contribute to thestriation development and only when the crack propagation rate is high does one cycleappear to correspond to one striation. Although crack propagation in this case may bequasi-static.

3.4 Influence of the loading mode

3.4.1 In carbon/epoxy, striations only appear to be visible in fractures dominated by mode II(shear) loading or mixed-mode (I+II) loading [7, 10]. In contrast, Laksimi [13] showed thatin glass/epoxy, striations can be found in Mode I loaded specimens. In glass/polyamide66, striations were found in Mode I by Karger Kocsis [14]. The macroscopic crack speedhas been correlated to the distance between two striations for glass/epoxy compositematerials tested in mode I and mode II fatigue (R = 0.01) by Vançon [15]. He describedhow in mode I, the lower the stress intensity, the more discontinuous the rate of crackpropagation. This can be explained by crack propagation after n cycles leading toDiscontinuous Crack Growth (DCG) bands. In mode II, the crack propagates after eachcycle, although the relationship at low stress levels appeared to break down. Hiley [9,10]has shown that at high levels of mode II (<75%), matrix rollers may form in the matrixbetween the fibres. These fractographic features, which occur under only under cyclingloading, were found in composites made from both brittle (thermosetting) resins andtough (thermoplastic) resins.

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4 Literature Review: The theoretical mechanisms controlling fatigue failure in polymercomposite materials

4.1 Delamination growth due to fatigue

4.1.1 With the increased use of composites in load-bearing components, a correspondingincrease in awareness concerning their mechanical properties is required. Althoughproperties such as tensile strength and impact strength play an important role in thematerials selection process, the more insidious accumulation of fatigue damage is oftenthe controlling factor in the final failure of the component. Therefore, the need for a betterunderstanding of the complex fatigue behaviour of composites is required to prevent thesefailures.

4.1.2 The conditions leading to the occurrence of fracture in a component are often fulfilledsolely by continuous growth of micro-cracks over a period of time as a consequence ofoperative loads. If a pre-existing crack is assumed, then this crack will lead to fracture ofthe component under static loading conditions if a certain critical stress concentration isachieved. Also, if the component is loaded in such a manner that the maximum stressconcentration at the crack tip is smaller than the critical stress concentration (at whichunstable crack growth sets in), crack elongation may occur upon alternating loading, whichdoes not immediately lead to failure of the component. This stable crack elongation maythen be ascribed to some fatigue effect of the material [17].

4.1.3 The same tests that are commonly used to measure static interlaminar fracture toughnessDouble Cantilever Beam (DCB) and End Notched Flexure (ENF) have been used tocharacterise delamination in fatigue. It has been attempted to simulate crack growth lawsby correlating the rate of delamination growth with cycles, (da/dN), with the maximumcyclic strain energy release rate (Gmax), or the cyclic range of G [9,18].

4.1.4 Composites having a brittle thermoplastic or duroplastic matrix exhibit a progressive failuremechanism and high durability. A particularly critical failure mode in laminated compositesis interlaminar fracture, which is often analysed by means of linear elastic fracturemechanics (LEFM). LEFM is based on the assumption that, macroscopically, a materialexhibits elastic behaviour until fracture and that localised plastic deformations occur only atthe crack tip in the form of a plastic zone. For fatigue fracture, the relationship shown infigure 1 is found between crack propagation rate da/dN (a crack length, N number ofcycles) and cyclic stress intensity range ∆K, where

( )∆ ∆K K K a f= − = ⋅max min σ π12 [ ]MPa m .

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4.1.5 Numerous relationships have been proposed for describing the fatigue behaviour of metalsand plastics on the basis of empirical formulas and fracture mechanics principles. Thefracture mechanics approach has been shown to be the most flexible and is the mostcommonly used. Paris [19] postulated that the stress intensity factor, itself a function ofstress and crack length, is the main controlling factor in fatigue crack propagation. Thisproposition completely agrees with the fact that the stress intensity factor also controlsstatic fracture and crack formation induced by environmental effects [17] .

4.1.6 Paris found the determining stress variable to be the stress range (σo-σu), hencedescribing da/dN values by the stress intensity range ∆K in the following form :

mKC

dNda ∆= .

C, m = f (loading conditions, material properties, environment, frequency, temperature, stressratio, etc.).

4.1.7 This so-called Paris-Erdogan equation describes regime II of the fatigue crack propagationdiagram. The pre-exponential factor C and the exponent m dependent on material, loadingconditions and environmental effects.

4.1.8 In regime III of the diagram, the crack elongation rate increases hyper-proportionally until athreshold value ∆Kth is reached, below which no crack propagation can be measured. Thisthreshold value is of great importance to the materials application. Examples of fatiguecrack propagation tests on CFRP composites as well as unreinforced plastics may befound in the papers by Pannkoke [20], Morlo [21] and Hertzberg [17].

4.1.9 Figure 2 shows da/dN data for unidirectional PEEK generated using the DCB test. Thegeneric value of the slope of the log-log plot, and the apparent threshold value, arequestionable. For instance, in the presence of fibre bridging these power law curves aremuch steeper, i.e. they have much larger slopes on a log-log plot of da/dN versus G thanwould be expected. A very small error in estimated load and hence G will result in a largeerror in the delamination growth rate. Therefore, the classical damage tolerance approachof tracking crack growth may not always be practical for composite delamination growth.

4.1.10 Another approach to characterise the onset of delamination under cyclic loads uses theDCB and ENF tests by plotting the maximum cyclic G as a function of cycles todelamination onset. Figure 3 shows a plot of GImax as a function of the number of cycles tothe onset of delamination. The curve fitted to the data reaches an asymptote, or thresholdG value, near 106 cycles.

4.2. Crack growth models

4.2.1 The durability of composites under cyclic tensile as well as shear loading is determined bymatrix-controlled failure mechanisms whereby the shear properties of the matrix, i.e. thetransference of loading between adjacent fibres, simultaneously plays a dominating role.For this reason, fatigue behaviour is a matrix-controlled property under both loadingconditions [20].

4.2.2 In a recent publication, O´Brien [22] critically examines the composite interlaminar shearfracture toughness, GIIC. He concluded that the apparent GIIC as typically measured, isinconsistent with the original definition of shear fracture. Shear failure actually consists oftension failures in the resin rich layers between plies followed be the coalescence of ligamentscreated by these failures ; and not the sliding of two planes relative to one another that isassumed in fracture mechanics theory. Figures 4 and 5 show mode I and mode II fracturesurfaces, respectively, for a brittle matrix material. The mode I fracture surface has a fairly clean

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cleavage plane, whereas the mode II fracture surface exhibits a very rough fracture planewith the characteristic hackles (cusps) observed in mode II delamination. The sketch underthe mode II image illustrates the principal tensile stress at a 45 degree angle to thedelamination plane that results from mode II loading and creates tension microcracks inthe resin-rich region between the plies. Once these cracks appear, the ligaments formedby them are forced to bend until they fracture. Then they coalesce, creating the extensionof the original delamination via mode II. This complex failure process is far removed fromthe idealised sliding of two crack planes relative to one another as postulated in thefracture mechanics theory. Hence, a mechanistic explanation of mode II fracture isneeded.

4.2.3 Several strain energy release rate solutions have been reviewed for delamination incomposite laminates and structural components where failures have been experimentallydocumented. Failures typically occur at a location where the mode I component accountsfor at least one half of the total G at failure. Hence, it is the mode I and mixed-mode (I+II)interlaminar fracture toughness data that will be most useful in predicting delaminationfailure in composite components in service. Although apparent GIIC measurements mayprove useful for completeness of generating mixed-mode criteria, the accuracy of thesemeasurements may have very little influence on the prediction of mixed-mode failures inmost structural components.

4.2.4 In order to find a suitable explanation for the macroscopic fatigue susceptibility ofpolymers, it is necessary to investigate the fracture of polymers at molecular level. Asfatigue involves the accumulation of damage over a period of time, the kinetics of thespecific deformation mechanism at the molecular level should influence the fractureprocess.

4.2.5 In the polymer structure, some chains will always exist that are more highly stressed thanothers as a result of their morphology, orientation and degree of constraint. Consequently,these highly stressed chains will probably be the first to fracture. Once a primary bond isbroken, the stress will be transferred to surrounding chains and a stress concentration willbe formed at this site. Thus, chain scission will not be truly random, but will accumulatepreferentially at the stress concentrator. Therefore, to improve the fatigue resistance of amaterial, the irreversible nucleation of molecular defects should be avoided in favour of thepreferential, high-energy-dissipative process of chain slippage, which does not lead tomolecular defects [5].

4.2.6 Macroscopically, duroplastics and thermoplastics react in a completely different way tostress concentrations. In a duroplastic matrix, stresses are reduced by the formation ofcracks and delaminations between fibre and matrix due to high brittleness and limitedadhesion between fibre and matrix. Conversely, these processes are supressed in atough, crack-resistant thermoplastic matrix. Based on these considerations, whatdetermines the fatigue resistance will be the dominating failure mechanisms. These maybe; failure in the form of longitudinal and lateral cracks, as well as delaminations or fatigueof the matrix polymer.

4.2.7 Stress fields in front of the crack tip will be considered before describing crack propagationmodels, In figure 6 the singular stress terms are shown in the form of contours close to thecrack tip [23]. These stress fields lead to the conclusion that the stress concentrationreaches much higher values in the case of mode-I than in case of mode-II loadingconditions. Hence, the volume absorbing the stresses under mode-I loading conditions is,at the same loading level, much smaller than under mode-II loading conditions.

4.2.8 The size of the process zone (plastic zone) in front of the crack tip may be estimated bymeans of the stress fields. In the literature, several crack propagation models for cyclic

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loading of polymer materials are presented. However, these are all derived from two basicmodels.

4.2.9 The model proposed by McEvily [24], see figure 7, characterising crack propagation ispartially valid for metals and polymers. The important aspects of this model are as follows.A new fracture surface is only created during the tensile period of the loading cycle. At thesame time, the crack tip is blunted, which tends to limit the extent of crack growth. Duringload relief, the material surrounding the crack contracts elastically and imposes a residualcompressive stress onto the severely plastically deformed material at the tip. As aconsequence, the crack tip is resharpened, and the residual ductility of the material justahead of the crack tip is reduced because of the severe irreversible plastic deformation ithas undergone. These last two factors serve to promote crack growth in the next loadingcycle. Consequently, the crack grows to a certain degree depending on the stress level atthe crack tip during each loading cycle.

4.2.10 Jacoby et al [25], who intensively investigated different types of markings on the fracturesurfaces of polycarbonate, gave a refined version of the McEvily model. As can be seen infigure 8, the advance of the fatigue crack front was also associated with that portion of thecycle where the load begins to rise [26].

4.2.11 For high-amplitude-loaded PE, undergoing larger deformations, McEvily et al, observedsawtooth markings in the fracture surface [24, 26]. In order to develop a modified model offatigue crack growth in glassy thermoplastics, recently available detailed information oncrazing, fibrillation and fibril behaviour should be taken into account.

4.2.12 The brittleness of thermoplastics can be traced to the formation under tensile stresses ofsmall crack-like defects called crazes. Unlike cracks, crazes are load-bearing becausetheir two surfaces are bridged by many small fibrils with diameters in the range of 5 to30nm. Cracks form due to the breakdown of the fibril structure within a craze, a processwhich is aided by the high local stresses in the fibrils. Hence, to control crack nucleationone must stop craze formation or make it more difficult relative to shear deformation.Nevertheless, under certain conditions crazing can be beneficial. Crazing is a process ofplastic deformation and hence the most important source of fracture toughness GIC inthermoplastics, which deform by crazing rather than due to shear stresses. The improvedimpact toughness produced by adding small rubber particles is an example of theexploitation of this aspect of crazing; high densities of crazes nucleate from the rubberparticles and grow before these crazes break down to form cracks.

4.2.13 In a thermoplastic material it is important to distinguish between the crack opening stretchand the maximum craze width (2v). To characterise plastic deformation and fracturebehaviour of a thermoplastic material, the maximum length of stretched fibrils and hencethe maximum craze width 2v is a more fundamental parameter than the crack openingstretch, which depends on the relaxation behaviour of the broken remnants on the fracturesurface.

4.2.14 Figure 9 shows an example of the experimentally determined shapes (points) of the crackopening and the craze zone in PMMA of high molecular weight. It is evident that the crazeis a long thin wedge. The arrangements of molecules and of stretched and broken fibrilsare indicated schematically. The measured points indicated that the crack tip is bluntedand that in this position the craze width 2v is larger than the crack opening. The location ofthe craze tip and hence the craze length may be calculated by applying the Dugdalemodel to the measured craze zone. It has been reported that in PMMA the Dugdale modelprovides a good quantitative description of the craze zone at the crack tip [26].

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4.2.15 In order to obtain direct information on the onset of crack propagation in PMMA duringfatigue, a videofilm was made using optical interference microscopy [26], the results areshown schematically in figure 10.

4.2.16 At first the crack propagates into the craze, and a little later both crack and craze grow atthe same rate. Crack propagation occurs only within a small portion of the loading cyclebefore and after the load maximum. In complete contrast to the previous model aresharpening of the fatigue crack tip does not occur and the crack propagation is notlimited by crack tip blunting. The crack starts to propagate when the crack tip has becomeblunted; when the crack tip is blunted the stress on the craze zone is highest and the fibrilsare stretched most severely and will fail.

4.2.17 Two kinds of crack propagation in the craze zone are possible under cyclic loadingconditions, see figure 11. In the first case (continuous crack growth), the length of thecraze zone remains constant during crack growth; the craze grows due to transformationof the polymer material at the craze tip into fibrillar craze. The crack grows through failureof the fibrils. During continuous crack growth, these two processes take place at the samerate. In the second case (discontinuous crack growth), the craze grows during a number ofcycles, then a crack jump occurs during one loading cycle through about 2/3 of the crazelength, resulting in additional enlarging of the craze size.

4.3 Microfractographic investigations of fracture surfaces

4.3.1 Scanning electron microscopic examination of fracture surfaces is of considerable value inthe evaluation and study of the fracture phenomenon. The fracture-morphological features,may range from the smallest of details (close to molecular level) with lengths of 1nm, tostructures up to 1000µm. The main method for investigating different kinds of fracturemorphology is the scanning electron microscope (SEM).

4.3.2 By tilting the specimen relative to the primary beam, the number of secondary electronsmay be increased even more. This is of primary importance for image contrast, since forsecondary electron imaging the topographic contrast depends on a relationship betweensecondary-electron yield and angle of incidence for the electron beam.The intensity ofsecondary electrons produced by primary electrons results in gray-level contrasting in theSEM image. Secondary electrons are repeatedly knocked out of edges of a roughspecimen, since at these sites the primary electrons impinge at a certain angle of slope,and thus a larger volume of the surface is subject to excitation. In an SEM image theseareas appear much brighter, due to the increase in secondary-electron yield. In contrast,depressions and shadowed areas, from which not all the secondary electrons emitted canreach the detector, appear much darker. The topographic contrast in a secondary-electronimage depends on these effects and is shown schematically in figure 12. This subject wasfirst published in [8]. The importance of tilt angle in resolving fractographic featuresassociated with fatigue, such as striations is discussed later on in this report.

4.3.3 If the specimen is non-conductive, then an electrically conductive material or element (e.g.Au, Au/Pt) has to be evaporated or sputtered onto the surface, thereby causing theprimary electrons to dissipated. When very low primary electron energies (<1 keV) areapplied, charging is minimised and coating is not necessary. The coating apllied to thesamples surface may obscure fine surface features associated with fatigue fracture andthe implications of this need to be considered when examining fatigue fracture surfaces.

4.3.4 In order to standardise the terminology used to describe loading conditions andmicrofractographic features, a list of terms has been compiled by the GARTEUR ActionGroup 14 (AG14) [1]. Commonly used descriptive terms are presented in table 2 and willbe used throughout this report.

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5. Manufacture and testing of specimens

5.1 Background

5.1.1 During the life of the AG20 group, three round robin exercises were undertaken. A summaryof which is outlined below. The individual findings of each members investigations aredescribed in tables A1 to A5, which are included in Annex 1 towards the back of this report.

5.2 Round Robin 1 (Mode II)

5.2.1 The first round robin involved the fractographic assessment of mode II (shear) fatiguefracture surfaces. Twenty four ply unidirectional End Notched Flexure (ENF) specimenswere manufactured at QinetiQ from Hexcel T800/924 carbon fibre prepreg. In figure 13, aschematic diagram of the ENF specimen is shown. By loading the specimen, containing amid-plane teflon insert, in three point bending the delamination was made to grow from theinsert. The specimens were initially precracked statically in shear (fracture zone A) and thenthe specimen repositioned in the fixture and cycled at a frequency of 5Hz to create asecondary region of fatigue fracture (zone B). Specimens were fatigued using an R-ratio of0.1, under constant displacement, at between 30% and 50% of the critical strain energyrelease rate GIIC to obtain a range of crack growth rates. A summary of the samplesdistributed is shown in table 3.

5.3 Round robin 2 (Mode I and Mixed-mode)

5.3.1 In round robin 2, the fractographic assessment of both unidirectional mode I and mixedmode (mode I + mode II) fatigue fracture surfaces was undertaken. Specimens weremanufactured and distributed by both QinetiQ and EADS.

5.4 Mode I specimens

5.4.1 Mode I specimens were manufactured using the Double Cantilever Beam (DCB) test aschematic diagram of which is shown in figure 14. Twenty four ply, unidirectional specimenswere fabricated from carbon/epoxy prepreg with a Teflon insert positioned at the mid-plane.Three materials were investigated; T800/924, T800/5245 and IM7/977. Tests wereconducted at a frequency of 5Hz and an R-ratio of 0.1. Specimens were all fatigued forapproximately 400 000 cycles, with fatigue delamination growth being directly from theinsert. Immediately after the fatigue zone a region of static mode I fracture was created, forcomparison with the fatigued area, by pulling the specimens apart in tension. Table 4 liststhe specimens distributed to members of the group.

5.5 Mixed mode (mode I + mode II) specimens

5.5.1 Mixed-mode tests were conducted on unidirectional T800/924 specimens loaded using theMixed Mode Flexure (MMF) test, a schematic diagram of which is shown in figure 15. Thisspecimen consisted of a unidirectional laminate containing a mid-plane delamination at oneend which was loaded in three point bending; load on one side of the specimen beingtranferred through the upper arm only. Fatigue tests were conducted using a stress ratio R =0.1. This test configuration was understood to give a mixed-mode ratio (mode I: mode II) ofapproximately (4:3). Five specimens were tested under a range of conditions, details ofwhich are described below:

5.5.2 Specimen P1

Preliminary experiment to determine the loading conditions; involved gradually increasingload. Start load = 300N, increased by 50N after every 2000 cycles. Frequency = 2Hz.Stop test at load of 900N, corresponding to 26000 cycles, crack length measured = 7.5mm

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Loading continued at 825N, Frequency of with 3HzCrack length: 10mm at 4500 cyclesCrack length: 12.5mm at 6150 cyclesCrack length: 15mm at 6500 cyclesCrack reached mid-span of the specimen after 6544 cycles

5.5.3 Specimen P2

Experiment involved gradually increasing load.Start load = 700N for 12500 cyclesLoad increased to = 750N for 12500 cyclesLoad increased to = 800N for 75000 cycles

Crack propagation was not measured optically, but a decrease in the modulus of specimenwas observed. Load increased to 825N for 16000 cycles until crack reached the mid-spanof the specimen. Specimen fatigued a further 80000 cycles before test completed.

5.5.4 Specimen P3

Experiment involved gradually increasing load.Start load = 750N for 16500 cyclesLoad increased to = 800N for 14000 cyclesLoad increased to = 825N for 50000 cycles

Testing continued until the crack reached the mid-span of the specimen. Specimenfatigued a further 20000 cycles before test completed. A graphical representation of theloading applied to specimen P3 is shown in figure 16.

5.5.5 Specimen P4

Experiment involved gradually increasing load. Crack length at beginning of experiment =3mmStart load = 600N for 1000 cyclesLoad increased to = 700N for 1000 cyclesLoad increased to = 775N for 8000 cyclesLoad reduced to = 750N until crack reached the mid-span of the specimen

Table 5 below shows crack length against the number of cycles for specimen P4. Agraphical representation of the loading applied to the specimen P4 is shown in figure 17.Table 6 lists the mixed mode specimens distributed to the group in round robin 2.

5.6 Round robin 3 (Mode I, Mode II and Mixed-mode)

5.6.1 The third round robin exercise involved the fractographic assessment of unidirectionalmode I, mode II and mixed mode (mode I + mode II) fracture surfaces. In the pure mode Iand mode II fatigue fractures, test specimens were manufactured by BAE SYSTEMS fromthree materials T800/914, T800/5245 and IM7/8552. The mixed-mode specimens weremanufactured by EADS from unidirectional T800/924 prepreg. Specimens were testedusing the Mixed Mode Flexure (MMF) test described previously in round robin 2. Tables 7and 8 shows a list of specimens distributed during this round robin exercise.

5.6.2 In each round robin exercise specimens were distributed to members for fractographicassessment. In most cases the mode of fracture was not communicated to the memberand a blind “examination” of the specimen had to be made. In all cases examinations wereconducted using optical and electron microscopy.

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6. To Establish the Macroscopic/Microscopic features associated with fatigue failure.

6.1 Background

6.1.1 Objective 1 of the AG-20 group was to “Establish the macroscopical and microscopicalfeatures which indicate that fatigue played a role during failure”. The three round robinexercises undertaken were used to identify the fractographic features associated withdelamination growth in fatigue under mode I (tension), mode II (shear) and mixed-mode (I+ II) loading. Tables 9 to 11 show a summary of fractographic findings for each specimenexamined. It can be seen from the tables that a number of key fractographic features wereidentified by the group, which appeared to be characteristic of fatigue fracture; thesefeatures are described below.

6.2 Macroscopic examination

6.2.1 Optical examination of all the fatigue fracture surfaces, whether generated under mode I,mode II or mixed mode loading, showed them to be generally smoother than similar staticfractures. The mode I static fracture surfaces appeared to be significantly rougher thantheir equivalent fatigue fractures, with the former showing many more broken fibre endscaused by fibre bridging. In mode II the surfaces tended to show evidence of frettingbetween the surfaces and the presence of rolled matrix debris. It was noted that in allcases the differences between the static and fatigued regions was subtle and that withoutboth types of fracture being adjacent to each other on the test specimens, the fatiguedregions would probably not have been distinguishable macroscopically.

6.3 Microscopic examination

6.3.1 Examination of the fatigue fracture surfaces under the SEM revealed several fractographicfeatures which appeared to be unique to fatigue loading; these features are describedbelow.

6.4 Striations

6.4.1 One of the commonest fractographic features identified on the surfaces of the fatiguespecimens were striations. These features were observed predominantly on fracturesurfaces generated under mode II dominated loading conditions, although some membersalso identified them on mode I fracture surfaces. The striations were found to occur on twoareas of the fracture surface, these included: 1) in the fibre imprints and 2) in the bulkmatrix.

6.4.2 Figures 18 to 20 show typical micrographs of striations found in the fibre imprints of threematerials. These features consisted of a series of parallel lines within the fibre imprintsoriented perpendicular to the fibre direction. The striations were found to appear as aseries of either bright or dark lines. Closer examination of the striations in the imprintsshowed them to be series of small steps in the resin, see figure 21, which appeared lightor dark depending on the direction from which they were imaged. The mechanisms bywhich the striations form was not established conclusively, but appeared to involvelocalised shearing of the matrix ,within the fibre imprints, to create the steps. Observationsby some members of the action group suggested that the appearance of the striations,light or dark, may be used to determine the direction of crack growth, an hypothesis whichis discussed in section 8 of this report. It was clear that the striation features could not beobserved unless the specimens were tilted to an angle of more than 40° and 50°.

6.4.3 In the IM7/8552 material, see figures 22 and 23, striations were also observed on thefibres themselves. These striations appeared to be made up from matrix residues left onthe fibre surfaces. This type of fractographic feature was only observed in the one material.

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The spacing of the striations observed varied considerably, ranging from 1µm to 10µm.Due to the rare occurrence of these features, no detailed studies were undertaken toestablish the mechanism by which they form.

6.4.4 Figures 24 and 25 show striations found in the bulk matrix surrounding the fibres. Thesefeatures were observed by only a few members of the group using electron microscopeswith particularly high resolutions on uncoated samples. The striations found in the matrixwere seen in both mode I and II fatigue fractures and appeared as a series of concentriclines within the resin, or as a series of parallel lines adjacent to the fibres. These striationswithin the resin were visible in the single phase resins but not within the two phase epoxy914.

6.5 Matrix Rollers

6.5.1 Matrix rollers were identified only in fatigue specimens subjected to high mode II (shear)loads. Figures 26, 27 and 28 show typical micrographs of these features which appearedin the resin between adjacent fibres. These features consisted of circular lengths of resinbetween the fibres, which were oriented perpendicular to the direction of shear.Examination of the rollers appear to suggest that they initiate due to the formation of 45°(tensile) cracks in the matrix between the fibres; in a similar manner to which cusps form.However, unlike cusps which form during a single loading event, due to the coalescence ofadjacent cracks and matrix deformation, the rollers form in a progressive manner infatigue. Cycling of the resin fillet between neighbouring cracks, appears to cause initialdisbonding of the resin fillet at the fibre surfaces. The high alternating shear stresses at thefibre/matrix interface, promotes further extension of the roller from the fibre surface into theresin until it detaches completely. In the two phase epoxy 914, matrix rollers were onlyoccasionally observed, with rubbing out of the toughening PES phase (spheroids), seefigure 29, from the base resin being a much more common feature on the fracture surface.

6.6 Surface Texture Effects

6.6.1 When both sides of the fracture surfaces from the mode II and mixed mode (I + II) wereexamined in the electron microscope it was apparent that one surface appeared to containmainly fibre imprints (lower – tensile surface) whilst the other (upper – compressivesurface) contained mainly fibres. This effect was present on both the static and fatiguedfracture surfaces. Several members suggested that there was a relationship between theappearance of the striations on each side of the fracture and the direction of crack growth;an argument discussed later in this report. A similar relationship between the angle of thecusps and the direction of crack growth was also noted; with the cusps on the fibre rich(upper – compressive) surface appearing to slope in the direction of crack propagation.On the mode I static specimens, many more fractured fibre ends (attributed to fibrebridging) could be seen than on the fatigued specimen. This difference in surface texturewould appear to present a more obvious method by which mode I dominated fatiguefractures can be distinguished from their static equivalents.

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7. Material Dependency

7.1 Introduction

7.1.1 Objective three of the GARTEUR Working Group AG 20 was “establish the materialdependency of fractographic features associated with fatigue”. Round robin 3, see table 7,was set up to address this objective. However, in order to fulfil this objective it was alsonecessary to include the findings from the other round robins. Thus a total of fiveunidirectional carbon fibre prepreg materials were examined, these included: T800/5245,T300/924, IM7/977, T800/924 and IM7/8552. Three loading modes were used: mode I,mode II, and mixed-mode (I+II). Not all the materials, however, were tested in fatigueunder all three modes.

7.1.2 Previous macroscopic and microscopic examination via round robins had identified genericfractographic features associated with different loading modes. These can be summarisedas shown below:

Mode I Fatigue Fractographic Features

Macroscopically smooth fracture surface/with less evidence of fibre bridging.Occasional localised striations in the fibre imprints.Matrix striations-at high resolution e.g. FEM.

Mode II Fatigue Fractographic Features

Macroscopically smooth fracture surface/one surface generally contains fibres, the othermainly fibre imprints.Striations in the fibre imprint regions.Matrix striations.Matrix rollers.

Mixed-mode (I+II) Fatigue Fractographic Features

Macroscopically smooth fracture surface/ one surface generally contains fibres, the othermainly fibre imprints.Striations in fibre imprint regions.Matrix striations.Matrix rollers.

7.1.3 The effect of material type (fibre and resin) and hence microstructure, on the type offeatures are now discussed. It should be noted that the ultimate resolution of ScanningElectron Microscope (SEM) equipment used to examine the fractographic features variedbetween different members of the group. This was important as the striation features(particularly those in the matrix adjacent to fibres) were close to the resolution limit of mostelectron microscopes in use during this study. The results from Field Emission Microscope(FEM) examination by EADS indicated that striation features may be more numerous thanthe results generated by individuals within the group generally indicate.

7.2 Material Properties

7.2.1 All the materials were cured/postcured(where applicable) to either manufacturersrecommendations or ‘in-house’ cure cycles. Thus the cure state of each material type wasconsidered satisfactory. The microstructure of the materials examined was generallydefect free (as determined by Ultrasonic C-scan to current aerospace acceptance criteria)and so the features observed can be considered typical. In each case the fibres were

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reasonably well distributed and fibre volume fractions (Vf) were approximately 60 to 65%.The effect of lower volume fractions on the fatigue features was not been examinedsystematically and given the similarities between specimens was not been considered inthis evaluation of material dependency.

The possible differences between fibre/resin systems were:

I. The presence/size of toughening phases and resin stiffness and toughness.

2. The fibre stiffness.

3. Fibre/matrix interface strength.

Tables 9 to 11 describe the fractographic features observed, in each material, during eachof the round robin exercises. In table 12 the features associated with each material andtest condition are summarised.

7.3 Effect of different resin phases

7.3.1 The T800/5245 and IM7/977 systems used resins with a single phase resin morphologyand hence fine microstructural features such as ‘textured microflow’ were relatively easy tofind, see figures 25 and 30. In contrast the T300/914 material, based on a two phase resinsystem (containing ~ 1µm diameter PES particles), see figure 31, had a distorting effect onthe fracture morphology making even the largest fractographic features, such as cusps,very difficult to identify. In the T800/924 and IM7/8552 materials, see figures 32 and 33,which also contained a secondary toughening phase, albeit much smaller than that in theT300/914, most fractographic features were relatively easy to discern. Due to the granularappearance of the resins, finer matrix features, such as ‘textured microflow’ weresometimes more difficult to distinguish.

7.4 Mode I fatigue fracture

7.4.1 Striations in fibre imprints, see figures 34 and 35, were occasionally observed in theT800/5245 and IM7/8552 systems. In the T800/5245 system striations were also observedin the matrix adjacent to the imprint regions, see figures 24 and 25, but these were onlyresolved using the FEM. Striations were not observed in any of the other materialssubjected to mode I fatigue loading.

7.5 Mode II fatigue fracture

7.5.1 Striations were identified in the fibre imprints in all the materials examined except for theT300/914. The 914 material, with its two phase morphology, exhibited regularly spacedfracture features oriented perpendicular to the fibres imprints and in the resin adhering tothe fibre surfaces, see figure 36. It was unclear, however, whether these regularly spacedfeatures could be described as striations. The features were thought to have originated asa consequence of cyclic crack propagation, however this was not proved conclusively.

7.5.2 Matrix rollers, see figures 26 to 28 were also identified in all systems examined, with theexception of the T300/914. The rollers all appeared to have a similar morphologyindependent of the resin system. Although the T300/914 did not show the presence ofrollers, rubbing out of the secondary PES toughing phase was observed, see figures 29and 37 with loose particles of PES being visible on the fracture surfaces.

7.5.3 In round robin I, the specimens tested in fatigue were first precracked statically beforebeing tested in fatigue. This precracking procedure allowed a zone of static fracture,subsequently subjected to fretting, to be compared with an adjacent zone of mode II

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fatigue fracture. It was noted that the fretted static fracture surfaces still showed thepresence of cusps, some of which had been rubbed out of the surface (broken cusps) dueto the repeated movement of the mating surface against it, see figure 38. The rollers foundon the fatigue fracture surface were quite different in appearance suggesting themechanisms of roller production was not the same as that resulting in cusp formation. Thedifference in the two fracture morphologies observed is important as it allows mode IIdominated static fracture surfaces, subjected to later fretting, to be distinguished fromgenuine fatigue fractures.

7.6 Mixed -mode fatigue fracture (I+II)

7.6.1 Only two systems were studied under mixed-mode loading (T800/924 and T800/5245). Incommon with the mode II specimens, examination the fracture surfaces revealed thepresence of striations in fibre imprints and in the matrix adjacent to the fibres. It neithercase were matrix rollers observed. It is thought that very high shear stresses, in excess of80%, are required to promote roller formation within the matrix.

7.7 Fibre Stiffness

7.7.1 The T300 fibre has a lower strength/modulus than IM7 which, in turn, has a slightly lowerstrength/modulus than the T800 fibre. No noticeable effects on fatigue features could bedirectly attributed to the difference in fibre modulus. In the materials examined, two fibrestypes T800 and IM7 were studied, in each case with two different resins. The effect of fibrestiffness on the fractographic features observed may have been more easily investigatedby examining fractures in one resin type only containing a number of different fibre types.Since the T300 fibre was only examined with the two phase epoxy 914 resin system, whilstthe other resins were essentially single phase, the influence the T300 fibre on fracturecould not practically be investigated. Further studies are clearly required in this area.

7.8 Fibre/Matrix Interface

7.8.1 The fibre/matrix interface of the IM7/977 was good, see figure 30, as demonstrated by thegeneral lack of fibre imprints and a good covering of resin on the fibre surfaces. TheT800/5245 system, by contrast, exhibited poor fibre/ matrix bonding, see figure 34, withlittle resin adhering to the fibre surfaces. As a consequence of the weak interface in the5245, material, fibre bridging was extensive on the mode I fracture surfaces. TheIM7/8552 and T800/924 materials showed intermediate levels of fibre/matrix bonding, seefigure 35 and 41, with some areas showing good matrix adhesion on the fibres and otherareas poorer adhesion. The T300/914, see figure 36, showed good fibre/matrix adhesionwith a large proportion of the fibres visible on the fractures surfaces being covered withresin. The condition of the fibre/matrix interface was found to influence the extent to whichfibre bridging occurred, with materials with a weak interface generally showing more fibrebridging. It was noted, however, that the extent to which fibre nesting had occurred alsohad an influence on the level of fibre bridging.

7.8.2 No noticeable effects on fatigue features could be directly attributed to the difference infibre/matrix interface strength. However, the fibre/matrix bond and presence of fibrebridging appears to be an important factor governing striation formation. The occurrence offibre bridging may explain why both light and dark striations appear on the same fracturesurface. It seems likely that a critical level of fibre/matrix bonding is necessary to promotestriation formation in the fibre imprints and materials with weak interface may not give riseto striations, due to insufficient load transfer between fibre and matrix. A very goodfibre/matrix interface may promote cohesive fracture of the resin around the fibre and mayalso prevent striation formation.

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8. Relationship between Crack Growth Direction (C.G.D) and fractographic features

8.1 Objective 4 of the AG 20 group was to “establish the relationship between crack growthdirection and appearance of the fatigue features”. Typical fatigue features that wereidentified and have been described, include striations in the matrix/fibre imprints and matrixrollers. All were investigated to see if a link with C.G.D could be established. Results fromall of the round robin exercises are summarised in table 13 to 15 for the three loadingconditions examined. The tables show the relationship observed between the fatiguefeatures and the direction of crack growth. It can seen that there potential relationshipbetween crack growth direction and appearance of striations in the fibre imprints exists.This relationship was only really proven, however, for one material and one loadingcondition; mixed mode (I+II) T800/5245. To confirm this potential relationship also appliesfor other materials, lay-up, loading conditions, additional research is necessary.

8.2 The results of round-robin 3 and previous round robin exercises showed that matrix rollers,which were formed under mode II and specific mixed-mode (I/II) loading conditions, couldbe used to show that fatigue played a role in failure. No clear relationship between crackgrowth direction and presence or appearance of the rollers was found, however, it wasnoted that in general the rollers were oriented perpendicular to the direction of shear, seefigures 26 to 28

8.3 Striations in the matrix between the fibres were only observed by a few participants.Because of the limited amount of data available no relationship between overall crackgrowth direction and appearance of striations in the matrix could be found. Since ingeneral the striations within the matrix were seen to appear as concentric bands, divergingfrom a point of origin, they may be useful for determining the local direction of crackpropagation in the resin. Striations in the fibre imprints were identified under mode II andmixed-mode (I/II) loading conditions. Very occasionally striations were also found inspecimens tested in fatigue under mode I loading. It is questionable however, whetherthese latter striations were typical mode I fatigue features, or whether they were due to amode II component being introduced locally into the fracture process by fibre-bridging.

8.4 The T800/5245 mixed-mode (I+II) specimens generated in round robin 3, which weresubject to bending loads, revealed a possible relation between the compression andtension sides of the specimens and the appearance of the fracture surfaces. The tension-side of the fracture surface was found to consist mainly of fibre-imprints, while thecompression-side contained mainly fibres. Examination of the tension-side showed that thestriations in the fibre imprints appeared dark when the specimen was tilted and viewed inthe direction of crack propagation. When the compression-side was tilted and examined inthe direction of crack propagation, the striations in the fibre imprints appeared bright. Thiswas consistent for almost all specimens. In a few instances both bright and dark striationsare found on one fracture surface, which was attributed to fibre bridging effects.

8.5 Similar observations to those in round robin 3 were seen in round robin exercises 2, withtwo T800/924 mixed-mode (I+II) specimens showing the same result. One T800/924mixed-mode (I+II) specimen, however, revealed both bright and dark striations on one halfof the fracture, which was again attributed to fibre bridging. Sometimes the results of theinvestigations were inconclusive because crucial information relating to the side of thefracture examined (tension/compression or fibre rich/imprint rich) had not been recordedby the observer. This latter point clearly shows that it is essential to examine both fracturesurfaces when examining fatigue failures.

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9. Relationship between Crack Growth Rate (CGR) and inter-striation spacing

9.1 Background

9.1.1 Objective 5 of AG20 was to establish the relation between crack growth rate (CGR) andthe inter-striation spacing. The round robins, as described in chapter 4, were alsoconducted to address this objective. The main findings of this task are summarised in table16 and 17 and are discussed below in relation to each loading mode.

9.2 Mode I Interstriation distance measurements

9.2.1 In the second round robin, a total of six fracture surfaces were examined by members. Inonly two cases were (isolated) striations observed in the fibre tracks (both with varyingspacings). However, due to their scarcity these features they were not considered typicalof fracture. In round robin 3, one member reported numerous fatigue striations in the fibreimprints and some striations in the matrix in between the fibres; a relation between crackgrowth rate and interstriation distance was not reported. With only a few reported cases ofstriations in specimens tested in this mode, many of which were contradictory, it was notpossible to relate the crack growth rate to inter-striation distance for mode I fatigue.

9.3 Mode II Interstriation distance measurements

9.3.1 From the first round robin, useful interstriation distances were measured from most of thefracture surfaces. Also from round robin number three, in one case, useful fatigue crackpropagation rate data was reported. Striations were found in the fibre tracks for both roundrobins.

9.3.2 In round robin 1 the striations in the fibre tracks were not uniformly distributed and weresometimes only observed in a single fibre track. Striations seemed to appear mostfrequently at local stress intensities (defects). Mostly the striation spacing increased atincreasing distances from stress intensities see figure 42, sometimes this effect wasreversed with a decreasing striation spacing at increasing distances from stress intensities.Different crack growth rates were also sometimes found in adjacent fibres see figure 43.This together makes it very difficult to obtain reliable crack growth data. Inter-striationdistances measured from striations in the matrix were not reported.

9.3.3 In both mode II round robins, ENF specimens were used. In round robin 1 the specimenswere fatigued in position control at an R-ratio of 0.1, using frequency of 5Hz. Since thedisplacement was fixed, the proportion of mean GIIC (which started at 50% of the staticGIIC) decreased with increasing crack length. No member however, reported a decreasinginter-striation distance over the specimen length. In round robin 3, the ENF specimenswere tested in two zones, each with an increasing proportion of mean GIIC over the cracklength (50% to 80%). This resulted in an increase in inter-striation distance over the cracklength. One member found a relationship for the two linked zones with reasonableaccuracy, see figure 44.

9.3.4 In table 17 the macroscopically determined crack growth rates and the measured inter-striation distances are presented of the specimens examined in round robin one. Assumingthat the initiating time of the precracked specimens was zero and the test was finishedwhen the crack reached the central roller, the da/dN rates were in fact a factor of 10 to 100lower than the measured crack growth rate. This suggests that there is no one cycle to onestriation relation. However it is possible that there are smaller striations in between, whichare below the detection limit of a conventional SEM. The difference in crack growth rateresults of round robin one and round robin three can be explained by the difference in theapplied proportion of mean GIIC.

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9.4 Mixed Mode Interstriation distance measurements

9.4.1 Mixed mode specimens were examined in round robin 2 and 3. Interpretation of theinterstriation distance measurements was hampered by the fact that the macroscopiccrack growth rates of the specimens were not available to the members.

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10. Discussion

10.1 A general literature review of fatigue fracture in polymers and polymer composites hasbeen conducted and shown that much of the published work does not describe in detailthe fractographic features, or processes, occurring in composites subject to cyclic loading.Where specific fractographic information has been included in published papers, importantinformation such as; which side of the fracture surface was examined, the loadingconditions, the angle of specimen tilt, the magnification or scale, have often been omitted.The absence of these details has placed a major limitation on the use of this fractographicinformation and highlights the importance of including this data when discussingfractographic results. Much of the general literature discussed in this report, relating to thefatigue fracture mechanisms, describes crack propagation theories under mode I openingloads. The relevance of these fracture mechanisms to mode II dominated fatigue fracturesis much less obvious and further studies are required in this area.

10.2 During this study a number of microscopic fractographic features unique to fatigue failureunder mode I (peel), mode II (shear) and mixed-mode (I+II) loading were identified. Thesefeatures included striations within the fibre imprints and within the matrix, as well as matrixrollers. The fracture surfaces of the fatigued materials were also observed macroscopicallyto have a smoother fracture surface, which in mode I was attributed to reduced fibrebridging, whilst in mode II dominated failures, was attributed to surface fretting. All thesefeatures provide a means by which fatigue fracture may be distinguished from there staticcounterparts. Due to the critical effect of loading mode and material on the fracturemechanisms, the absence of such features on a fracture surface does not necessarilymean that a fatigue process was not involved. It was clear from the studies of thosespecimens subject to bending loads that the two mating fracture surfaces were quitedifferent. One side of the fracture was often fibre rich, whilst the other frequently containedmainly fibre imprints. This observation emphasises how it is important to observe bothsurfaces of a fractured material when carrying out any fractographic examination.

10.3 The effect of material on the fractographic features observed was most apparent betweenthe single phase and two phase resin systems, particularly those materials incorporatinglarge thermoplastic toughening phases. The relatively large secondary phases (~1µm)present in the T300/914 obscured any signs of striation features. The formation of matrixrollers in T300/914 was also replaced by rubbing out of the PES toughening phase due tofretting. The fibre/matrix interface of different systems appears important for the formationof striations in the fibre imprints and also effects the amount of fibre bridging. Where thefibre/matrix bond is good many of the fibres are covered with a layer of resin, or a layer ofresin is stripped out of the fibre imprint and striations are not observed. An intermediatelevel of fibre matrix bond appears to promote striation formation in the fibre imprints, butwhere the fibre/matrix bond is very weak, striations are not generally observed. Other thanthe above, there was no other apparent effects of material on the formation of fatiguefractographic features. However, it should be noted that all the materials examined weregenerally similar and in a "typical" structure, the effect of fatigue propagation betweendifferent ply orientations and variation in global/local volume fractions may result in agreater material effect.

10.4 The relationship between the global direction of crack growth in the specimens andappearance of striations in the matrix was unclear. It appears, however, that striationswithin the matrix may be used to determine the local directions of crack growth within thematrix. The striations in the matrix were observed to radiate outwards from a single point ofinitiation, or progress as a series of parallel lines perpendicular to the direction of fracture.There was no identifiable relationship between crack growth direction and the presence orappearance of rollers. With regard to the striations within the fibre imprints, a potentialrelationship between crack growth direction and the appearance of the striations waspartially proven. Studies indicated that, if the striations are considered as a series of steps,

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the direction of crack propagation coincides with the downward direction of the steps. Thisrelationship was only proven conclusively for one material however, under one loadingcondition and further studies are need to confirm this hypothesis. It should be noted that, ingeneral, striations were only observed in mode II dominated fractures, with striations onmode I fractures being attributed to fibre bridging effects; introducing local shear stressesat the fibre/matrix interface. Fibre bridging is also thought to explain why both light anddark striations were observed on the same fracture surfaces, indicating opposingdirections of crack propagation. Opposing striations were also identified around defects.When using striations within the fibre imprints, to determine crack growth directions, theobserver needs to ensure that they have examined sufficient numbers of striations to berepresentative.

10.5 Studies undertaken to establish whether there was a relationship between crack growthrate and inter-striation spacing proved inconclusive. In mode I, the low number of striationsobserved made it difficult to establish a relationship due to a lack of inter-striationmeasurements. For mode II and mixed mode (I +II) fractures, the inter-striations spacingvaried considerably, even between adjacent fibre imprints. Around some defects or stressconcentrations, the crack growth rate was observed both to increase and decrease. It wasconcluded that the stresses responsible for striation formation are effected considerably bylocal stresses within the composite and that these do not necessarily reflect the globalloading conditions on the specimen. For high crack growth rates the relation with the inter-striation distance was generally better than for low crack growth rates. It seems likely thatwhen high strain energy release rates (G) are applied to a specimen, crack propagationfollows a one to one cycle/striation relation. Such a relationship may not occur at lowvalues of G and further studies are required to establish this. When the inter-striationspacing is very small, errors in the measurement of inter-striation spacing also increase.

.

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11. Conclusions

11.1 A review of the general literature has shown that much of the published work relating tofatigue fracture in composites describes only the fracture processes occurring in fatiguesuperficially. Often important information relating to the examination conditions, orspecimen test conditions, have been omitted. The review has therefore highlighted theimportance of including detailed information of this kind when describing the findings offractographic research.

11.2 In this study a number of microscopic fractographic features, unique to fatigue failure, wereobserved. These features included; matrix rollers, matrix striations and striations within thefibre imprints. The mode of loading was important in the development of these fatiguefracture features such that:

1) Matrix rollers require high shear stresses for their formation.

2) Striations in the fibre imprints also appear to require the presence of shearstresses for their formation

3) Striations in the matrix were observed in both mode I and mode II loadedspecimens. The former was attributed to the presence of local shear stresses atthe fibre/matrix interface, caused by fibre bridging.

Although the presence of such features appears to confirm that fatigue played a role infailure, their absence does not necessarily mean that cyclic loading has not played a partin failure. The fractographic features observed on the mating surfaces of specimenssubject to bending were quite different, with one side often containing many fibres, whilstthe other contained mainly imprints. Macroscopically the fatigue fractures were smootherthan their static counterparts.

11.3 Fractographic features in the material containing single phase resins were broadly similar.The PES present in the two phase 914 resin masked many of the fatigue fracture featuresor significantly modified their appearance. A relationship between crack growth directionand the appearance of the striations was only partly proven. The striations have to haveonly limited value for the determination of crack growth rates.

11.4 The work presented in this report provides increased understanding of fatigue damagedevelopment, which will be invaluable if the mode and sequence of failure in compositecomponents, in use on both civil and military vehicles, are to be identified successfully.

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12. Recommendations

12.1 The following recommendations have been made after consideration of the action groupactivities and discussion points raised during the meetings:

1) The work described in this report has illustrated the fractographic features associatedwith interlaminar fatigue for one R-ratio only (R=0.1). Further studies are required onmaterials tested at other R-ratios (for instance, mode II specimens tested using a fullyreversed fatigue cycle), in order to characterise completely the fractographic featureslikely to be encountered in a failed structure.

2) Further work is required to establish the mechanisms by which striations form inunidirectional materials and their relevance to the determination crack growth ratesand directions. Again, R-ratio will have an important effect on the fractographicfeatures produced and this needs to be considered.

3) In real structures, laminates are constructed from many layers of prepreg, withdifferent orientations. In practice delaminations seldom grow in unidirectional plies, butinitiate and grow between plies of different orientations. New studies are required tocharacterise the fractographic features produced in multidirectional laminates subjectto fatigue loading and investigate how damage propagates through the plies.

4) Significant interest in the use of composites based on woven fabrics and materialsmade from non-crimped fabrics (NCF’s) has developed over the last few years. Due tothe discontinuous nature of these materials it is likely that these materials will be moresensitive to damage growth under both static and fatigue loading. Studies in thesematerials are urgently required if ‘in-service’ fatigue failures in structures are to beidentified in the future.

12.2 The lack of any significant understanding of the fatigue fracture processes inmultidirectional laminates, and those based on woven fabrics, was viewed by the AG as inneed of immediate attention. A programme of work aimed at investigating fatigue fracturein these materials, as well as the relationship of coupon studies to structural failures, hasbeen proposed (AG27).

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32 Friedrich K, ‘Fractographic Analysis of Polymer Composites’, in Application of Fracture Mechanicsto Composite Materials, Chapter 1, Elsevier Science Publishers B.V, 1989.

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33 Gustafson C. G and Ragnar B.S, ‘Monitoring Fatigue Damage in CFRP Using Acoustic Emissionand Radiographic Techniques’, Delamination and Debonding of Materials, ASTM STP 876, EditedJohnson W.S, ASTM, Philadephia, 1985, 448-464.

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50 Paulisch C, ‘Verhalten Von Verbundwerstoffen der Neuen Generation bei Ermüdungsbelastung undAnalyse der Zugerhörigen Schadensmechanismen Bundesamt für Wehretecknik un Beschaffung’,93, P1140, 1997.

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Tables

Ref Materials Reinforcement Test Mode TestConditions

StudiedZone

ExaminationCondition

Hertzberg1987

Technicalpolymers

Unreinforced AEK High

Fatigue striations:Amorphous: fine tear linesSemi-crystalline: fine lines parallel to the crack propagation front and fatigue striations, wrinkle parallel to the crackpropagation direction.Rumpled markings: parallel to the crack front. The distance between the marks does not depend on the crackpropagation rate. These events are also visible when the crack propagation speed is high.Discontinuous Growth Bands parallel to the crack front. Important voids near the crack tip. Small microvoids in thecrazes.They are surrounded by failed fibres.Clarck1993

PMMA Unreinforced Tension 20 HzRc =0.1

3-20 kV

DGB in low, moderate and high molecular weight PMMA.No DGB in reticulated PMMA.Clarck1993

PMMA Unreinforced Tension 20 HzKcmax =Cte

3-20 kV

DGB in low molecular weight PMMA because of the complex tangle of the macromolecular chains which preventsthem from sliding.Their failure is then possible at high stress level.Franz1980/1982

Epoxy Unreinforced Tension Notchedspecimen

5Hz

The stiations starts on the notch.1 striation per cycle.Striation width (crack propagation per cycle) increase with increasing crack propagation velocity. Striation width. 2to 150µm.Franz1982

EpoxyCY221-HT972CY209-HT972FibresGraphite: T300AGlass : EC10

Unidirectional Bending Mixed-mode

23 Hz or5Hz

Static fracture: Tensile fracture rough and irregular, Compression fracture surface: smooth.Fatigue striation are only visible in the glass fibre reinforced material.Striations in the fibre imprint oriented in the stress direction. Striation in the matrix between the fibres (steppedstriations). Secondary fractures: striation shows crack propagation in the opposite direction to the primary fracturedirection.No striations when the fibres are normal to the stress direction.Franz1991

Hercules4502/AS4

UnidirectionalMulti-directional(0/+45/90/-45)

Bending(Mixed-mode)

Mixed-mode

20Hz45Hz92Hz

Striations in the fibre imprint oriented in the stress direction. Striations are only visible if the tilt angle is high.Striations are stair shaped. Crack propagation always upstairs. Bright and dark striations are visible depending ifone look in the fracture direction or in the opposite direction.The space between the striations depends on the local stresses and crack propagation rate.Regions the continuous and discontinuous increase of striation width are visible.Franz1991

Ciba913/T300

Fabric± 45 and 0/90

Bending Mixed-mode

20Hz45Hz92Hz

0/90 fabric: Striations in the fibre imprint only in the 0-play, depends on the stress intensity.Striation in the matrix.

Table 1 Summary of papers reviewed

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Franz1991

T800/924T800/5245Carbon/Epoxy

Unidirectional Bending Mixed 2300-100000

Fatigue striations in the fibre imprints.The failure surface is very rough.Franz1991

T800/924T800/5245Carbon/Epoxy

Unidirectional Bending

Generally speaking:Differences between Fatigue Striations and Discontinuous Growth Bands (DGB).On the one hand, the space between two fatigue striations corresponds to the crack propagation after one cycle.On the other hand, the DGB is formed after the crack has propagated after a great number of cycles.Hiley Curtis1991

XAS/914T300/6376AS4/PEEK

Unidirectional Bending ENFModeII

5HzR=0.1

20kVTilted upto 60degrees

Striations within the fibre imprints identified in all three materialsMatrix rollers identified in 6376 and PEEK material.Rubbing out of PES particles from matrixHiley1998

T300/V390XAS/914IM7/977T800/924AS4/PEEKE-glass/913

Unidirectional ENFDCBMMF

Mode IModeIIMixedmode

5HzR=0.1

10 – 20kVTilted upto 60degrees

Macroscopically fatigue fracture surfaces smoother than static fracturesMode I fatigue specimens do not generally show striations, although some localised regions identified (attributed tolocal shear stresses caused by fibre bridging).Mode II fatigue fractures show striations within the fibre imprints, except in V390 material (where fibre/matrix bondweaker).Glass fibre material shows striations within the fibre imprints and in adjacent resin. Unlike other materials, wherethe striations are oriented perpendicular to the axis of the fibre, the striations in the glass/epoxy were often curved.Striations also found in matrix residues on fibre surfaces.Wide variation observed in the inter-striation spacing, even on the same fracture surface. No significant correlationbetween inter-striation spacing and measured global crack growth rate.Mixed-mode fractures also exhibit striations, but diminish as mode II content decreases.Matrix rollers identified on high mode II fatigue fracture surfaces, even in brittle bismaleimide system.Morris1982

Graphite/EpoxyAS/3501-6,MMS-549Type II48/48/4 fibredominated16/80/4 matrixdominated

(+45, -45, 0, 0)3,90,(0,.0.-45,+45)3

(+45, -45, 0, +45,-45)3, +45, -45,90,-45, +45, (-45,+45,0, -45, +45)2

Tension-compression

5 HzSinglenotchsampleØ 0.64 cmR = -1, -2,-8

Every ply 3DImages(+5, -5)

No relationship between the number of striations and the number of fatigue cycles.Striations are visible when the compression stress level is less than 70%.In the matrix, the striations are parallel to the fibres. In the fibre imprints the striations are normal to the fibres.No striations near the hole.

Table 1 Summary of papers reviewed - continued

GARTEUR OPEN


Laksimi1983

Glass/epoxyEpoxy 1452/82350 % glass fibres

0/90° Tensionnotchedsamples

I 10 HzR = 0.01

5 kV*100010 kV*700025 kV*3000

Inter-striation spacing mostly irregular.Convexity of the striations locally oriented in the shear direction .No striation when R = 0.4 or R = 0.5Laksimi1983

Glass/epoxyEpoxy 1452/82350 % glass fibres

0/90° Bending onnotchedsamples

II 5 HzR = 0.01

10kV*100015 kV*500025 kV*1700

Fatigue striationsInter-striation spacing more regular than in mode IKargerKocsis1988

a) Short glass fibresMaranyl A 190

b) A 690Long glass fibres

c) Verton RF700 10700 6

d)PA 6.6Maranyl A 125

InjectedTension onnotched samples

I 5 HzR = 0.2

BeginningOf theStablePropagationZone

UnstablePropagationZone

a) Fatigue striations on the shear lips when the crack propagation rate is high (1 striation per cycle)b) Short fibres:c) Normal fracture of the fibres Good fibre/matrix adhesiond) Fibre debonding Brittle fracture of the matrixAbd Allah1996

Glass/PolyesterER 1150 F-183QL 8520 A

Unidirectional Bending 25 HzR=-1

High stresses: clean fibres.Low stresses: Fretting wear of the fibres.Awerbuch1981

Graphite/EpoxyAS/3501-5A

Unidirectional Tension7 angles/fibres0, 10, 20, 30,45, 60, 90°

18 HzR = 0.1σ==50%−75 %

3D Imageanalysis

A lot of fibre bundle failuresCracks parallel to the fibresRough failure surfacesWeak fibre-matrix interfaceMatrix failure: hackles and cleavage. A lot of microcracks in resin rich areas

Table 1 Summary of papers reviewed – continued

GARTEUR OPEN


Bhatia1975

Graphite/EpoxyThornel300/SP 286

20 plylaminates

(0, ±45, 90)

Tension Artificialdefects

Fibre pull out and debondings.Interlaminar failure near the notch.Dillon1995

Carbon AS-4PEEK APC-2

Unidirectional 20 pointsbending

σ== 70-90%

20 kVinclination<75°

Shear wear of the tensile fracture surfaces.Wear of the compression fracture surfaces.Delamination due to the shear stresses.The fatigue life and the stress level can be estimated from the degree of wear:

Limited wear: high stresses,Moderate wear: 70/75 % of the max. stress.Friedrich1986

PEEKVictrex 450 G

Short glassfibres20 and 30%weight fraction

Injected Tension onnotchedsamples

I 5HzR = 0.22

Fibre-matrix separation parallel to the crack.Smooth failure surface.When the fibres are normal to the crack propagation front, there is no fibre debonding.

Pure resin:When the crack propagation rate is small, there is no relationship between the striations and the number of cycles.When the crack propagation rate is high, I cycle = I striation.Gustafson1985

Graphite/EpoxyFiberiteT300/1304prepreg

(0/±45/90) Tension 2 HzR = 0.05σ = 181-233MPa=36%-46%σmax

Interface-45/90

No striations.Fibre imprints, Bare fibres. Hackles due to shear.Henig1989

Carbon-EpoxyGlass-EpoxyKevlar-Epoxy

0/90° ENF II Hum.Cond.+ Fatigue

x30020 kV

A few broken fibres

Table 1 Summary of papers reviewed – continued

GARTEUR OPEN


Hojo1994

Graphite/EpoxyT300/#3601T800/#3601T300/#3631T800/#3631T300/914T300/#2500

AS4/PEEK

UnidirectionalTension onnotchedsamples

I 10 HzR = 0.14

Brittle, riversBrittle, riversSlightly ductile, rivers,Slightly ductileGrain structureRough

Ductile and roughHojo1987

Carbon-EpoxyT300/914T300/P305

Unidirectional DCB I Hum.Cond.+ Fat. 10Hz0.2<R<0.5

Fatigue and static failure surfaces are similar.T300/914: Mostly matrix failure.T300/P305: Mostly fibre-matrix debonding.Hwang1989

Glass-Epoxy1002 (3M)

Unidirectional WTDCB I x100-240020 kV

Fibre-matrix debonding.Broken fibresHwang1989

Modified epoxy Unreinforced 20 Hz OpticalMicroscope

Discontinuous Growth BandsHennaf-Gardin,Lafarie-Frenot1992

Graphite/EpoxyT300/914

Graphite/PEEKAPC2

(03/90/04)Sor(07/90)S

Tension 10 HzR = 0.1

Ply 0° and90°separately

Good fibre-matrix interface.Homogeneous delamination of the Carbon-Epoxy.Non homogeneous delamination of the Carbon-PEEK which presents plastic deformation.The fatigue failure surface of Carbon-PEEK is similar to the static one.Kobayashi1989

Carbon-EpoxyPEEK-Epoxy

0/90° Tension 5 HzR variable

Examination of transverse cut.Komai1989

Carbon-Epoxy ±45° Tension 16.7 HzR = 0.06

No striationsKrey1987

Carbon, Glass,Kevlar- Epoxyor Polycarbonate

Fibres normal tothe crack

CT I 10 HzR = 0.2

A few striations are visibleLang1987

Short E glass fibresNylon or PS

Injected Tension onNotchedSamples

10 HzR=0.1

20kV

A lot of events have been observed but there was no evident relationship between the failure characteristicsAnd the loading condition


GARTEUR OPEN


Lorenzo1986

EpoxyEpon 815 /Versamid140 : ductileEpon 828/Epon Z :Brittle

E Glass fibresCarbone T.300

Unidirectional Tension R = 0.1

Higher crack density when the material is made ofGlass fibres.Cracks mostly at the fibre-matrix interface when theMaterial is made of carbon fibres.When the matrix is ductile, there are more fibre failures.

Brittle matrix under high stress level shows moreFibre-matrix debonding.Martin1989

Nylon 6Chopped glassfibres

RIM Tension onnotchedsamples

I 5 HzR = 0.1

*340

No fatigue striations, no shear lip.Poor fibre-matrix adhesionOdorico1985

T300/5208 Quasi-isotropic Tension 15 Hz

Striations in the 0° plies.Pecorini1993

PET Unreinforced Tension 5 HzR = 0.1

5 kV

Shear lips in the amorphous PETRichards-Fransen1983

Graphite/EpoxyHerculesAS-3501-6

Unidirectional 3 pointsbending

10 HzR = 0.05

Hackle formation or cleavage in the matrix.The amount of debris grows with the number of cycles.The fibres fail or are debonded.The hackles are smashed.Closer the fibres, closer are the hackles.The fatigue life depends on the defects (size, type position).Saliba1993

AZDELGlass/PP

Random Tension-compression

*500-200015 kV

Fibre pull out and debonding.Bare fibres.Matrix fibrilsMatrix debrisRadial pattern on fractured fibres.Saliba1993

AZDELGlass/PP

Random Bending *500-200015 kV

Fibre pull out on the tensile side of the sample.Fibre debonding.Bare fibres. Microbuckling on the compressive side of the sampleShih1987

E GlassSilanes Z-6020Z-6030

Unidirectional 4 pointsBendingL=21.4cmH=0.19cmSpan =3.95 cm

0.8 HzR = -0.8

35kVx500-3000

A few striations in the matrixFibre bridging. No hackles. Longitudinal and transverse cracks


GARTEUR OPEN


Wang1983

SMC 50 %weightfractionChopped glassfibres

OCF 433 AB-114PolyesterOCF E 980

Isotropic Tension onnotchedsamples

I 2 HzR = 0

Near thenotch

Far awayfrom thenotch

*3000

In rich resin areas:- Fibre pull out and debondings.- The CaCO3 filling is dissociated from the matrix- A lot of debris.The failure surface is smoother.Williams

Boron/Aluminium

(0,+45, -45, 0) Tension

15 Hz

45 Hz

Hackles on 60% of the surfaceA few fibres debonding. However the fibre/matrix interface was goodFatigue zone: 75% of the surfaceRough surface.HacklesWilliams Boron/Epoxy (0,+45, -45, 0) Tension 2.5 HzFibre debondingHeutlingFranzFriedrich1998

5245C/T800 Multidirectional[(+45/0-45/90)4]S

Mode II

Under pure Mode II loading, rollers and in addition fatigue striations appear in the fibre imprints and in the matrixon the surface of the rollers “during the initiation of roller formation”. The formation mechanism of rollers isexplained by means of high-resolution field emission SEM.HeutlingFranzFriedrich1998

5245C/T800 Multidirectional[(+45/0-45/90)4]S

Mixedmode

The ratio between the local tension and shear stress component influence influence the propagation direction ofthe secondary crack originating at the fibres. The fracture plane differs due to the ratio of the tensile and the shearstress component. The local fracture propagation of the secondary cracks can be recognised through the fatiguestriations appearing in the fracture surface of the matrix.


GARTEUR OPEN


TERMINOLOGIE TERMINOLOGY TERMINOLOGIE

Ablösung derFasernAusbreitungsfront

BiegebruchBruchausbreitung

BruchausgangBruchparabelBruchspiegel

DAFDelaminationDruckbruch

Duktiles BruchFaser/Matrix-Ablösung

FaserbettFaserbrückenFaserbündelFederstruktur

FibrillenFlußlinien

GrätenmusterGrenzschicht

HacklesHerauslösen der Fasern

Interlaminarer BruchIntralaminarer Bruch

KnickbänderKnötchen

Konzentrische RingeLanzetten

MatrixflügelMatrixteilchenMikroflußlinienMikroknickenModus I, II, IIIMixed-Modus

MuldenPseudo-Spaltbruch

RastlinienRippenRoller

SägezahnmusterStreifen

Zipfel / VersenkungScherbruch

SchwingbruchSpäne

SprödbruchStufenTäler

Translaminarer BruchZugbruch

DebondingCrack front

Flexural failureFailure propagationFracture initiation

ParabolaMirrorDAF

DelaminationCompression failure

Ductile fractureFibre-matrix debonding

Fibre imprintTied zones (fibre bridging)

Fibre bundleFeather pattern

FibrilsRiver markings

ChevronInterfaceHackles

Fibre pull-outInterlaminar failureIntralaminar failure

Kink bandsNodule

Concentric ringsLancets

Matrix wingDebris

Textured microflowsMicro-bucklingMode I, II, IIIMixed mode

ScallopsCleavage

Arrest linesRibs

RollerSerrated feet

StriationsTag / Cog

Shear failureFatigue failure

RibbonsBrittle fracture

ScarpsValleys

Translaminar failureTensile failure

DécohésionFront de propagationFlexion (Rupture par)

Propagation de ruptureAmorce de rupture

ParaboleMiroirDAF

DélaminageCompression (Rupture par)

Rupture ductileDécohésion

Empreinte de fibrePont de fibres

Paquet de fibresRamification

FibrilleRivière

ChevronInterfaceLanguette

DéchaussementInterlaminaire (Rupture)Intralaminaire (Rupture)Bande de cisaillement

NoduleAnneau concentrique

Fibrille à lancetteFlan de Matrice

DébrisFaciès à micro-rivières

Micro-flambageMode I, II, IIIMode mixteCouvette

Pseudo-ClivageLigne d’arrêt

StrieRouleau

Pied de languetteNervurre

Pointe / peigneCisaillement (Rupture par)

Fatigue (Rupture par)RubanFragileTalusVallée

Translaminaire (Rupture)Traction (Rupture par)

Table 2 – Glossary of commonly used fractographic features (German, English and French)

GARTEUR OPEN


Specimen No Number of Cycles Crack length

1 385000 22 124000 233 425000 204 420000 0-8 (Curved crack front)5 415000 236 403000 237 420000 68 855000 209 425000 2510 140000 20

Table 3 Summary of Mode II fatigue specimens issued for Round Robin I

Specimen No Material No of cycles

30 T800/5245 400 00031 T800/5245 400 00032 IM7/977 400 00035 IM7/977 400 00039 T800/924 400 00040 T800/924 400 000

Table 4 Summary of Mode I fatigue specimens issued for Round Robin 2

Crack Length Cycles

3.12mm 200003.45 260004.35 320005.00 336007.80 3800010.0 38 930

Table 5 Crack length vs Cycles data for mixed mode specimen P4 distributed in Round Robin 2

Specimen No Total Number of Cycles Final Crack length (mm)

P1 60000 45P1 60000 45P2 120000 -P3 100000 -P3 100000 -P4 200000 11P4 200000 11P6 40000 5

Table 6 Summary of Mixed-mode fatigue specimens issued for Round Robin 3

GARTEUR OPEN


Mode of Fracture Material/Specimen IdentificationT300/914

(U.D)T800/5245

(U.D)IM7/8552

(U.D)I AFD1I AFD2II AFE1I BFD1I BFD2I BFD3I CFD1I CFD2I CFD3II CFD4II CFD5

Table 7 Summary of mode I and Mode II specimens issued for Round Robin 3

Specimen Number Number of Cycles Loads (N)

P7/8 110000 170P7/10 113000 170*P7/14 63000 180**P7/15 94000 170P7/18 244000 160***

after 96000 cycles raised to 180N, ** after 90000 cycles reduced to 170N, after 170000 cycles increased to 170N

Table 8 Summary of Mixed Mode Bending fatigue specimens issued for Round Robin 3

GARTEUR OPEN


Organisation SpecimenDetails(Mode)

Cycles/mm (X103)

Fractographic features infatigued region

CEAT T800/924 (II) 5.4 Striations in fibre imprint regions.Matrix rollers between fibres.

STORK T800/924 (II) >52.5 Striations in fibre imprint regions.Matrix rollers between fibres.

EADS T800/924 (II) 70.0 No striations in fibre imprintregions. Matrix rollers betweenfibres.

CSM T800/924 (II) 17.0 Striations in fibre imprint regions.Matrix rollers between fibres.

QINETIQ T800/924 (II) N/A N/A (Not examined-supplier ofspecimens).

NLR T800/924 (II) 21.3 Striations in fibre imprint regions.Matrix rollers between fibres.

DAIMLERCHRYSLER T800/924 (II) 18.0 No striations in fibre imprintregions. No matrix rollers betweenfibres.

CETIM T800/924 (II) 17.5 Striations in fibre imprint regions.Matrix rollers between fibres.

BAE SYSTEMS T800/924 (II) 193.0 Striations? in fibre imprint regions.Matrix rollers between fibres.

BOMBARDIERSHORTS

T800/924 (II) 7.0 Striations in fibre imprint regions.Matrix rollers between fibres.

ÁEROSPATIALEMATRA

T800/924 (II) 43.0 Striations in fibre imprint regions.No matrix rollers between fibres.

Table 9 Summary of Round Robin 1 Results

GARTEUR OPEN


Organisation SpecimenDetails(Mode)

Cycles/mm(X103)

Fractographic features infatigued region

CEAT IM7/977 (I) ? No striations. No matrix rollers.Few fibre imprints-covered inmatrix.

T800/924 (I/II) P4 25.0 N/ASTORK T800/5245 (I) 70.0 Isolated striations. No matrix

rollers. Macro. smooth.EADS T800/924 (I) 70.0 No striations. No matrix rollers.

Macro. smooth.T800/924 (I/II) P1 1.4 N/A

CSM -

T800/924 (I/II) P2 ? Striations in fibre imprintregions. No matrix rollers.

QINETIQ -

T800/924 (I/II) P4 25.0 Striations in fibre imprintregions. No matrix rollers.

-


DAIMLERCHRYSLER

-

T800/924 (I/II) P2 ? N/ACETIM T800/5245 (I) ? Isolated striations. No matrix

rollers. Macroscopically, lessfibre fracture.

T800/924 (I/II) P1 1.4 Striations in fibre imprintregions. No matrix rollers.

BAE SYSTEMS -


BOMBARDIERSHORTS

T800/924 (I) ? No striations. No matrix rollers.Macro. less fibre fracture.

-

ÁEROSPATIALEMATRA

IM7/977 (I) ? N/A Examined by NLR.

NLR IM7/977 (I) ? N/A

AEROSPATIALEMATRA

T800/924 Striations in fibre imprint regions.No matrix rollers.


GARTEUR OPEN


Organisation Specimen Details(Mode)

Fractographic features in fatigued region

CEAT T300/914 (I) No striations-some regularly spaced features.No matrix rollers.

T800/5245 (I/II) Striations in fibre imprint regions. No matrixrollers.

STORK T300/914() No striations (Report not issued).EADS T300/914 (II) No striations. No matrix rollers. "Preening" of

PES spheroids.T800/5245 (I) Striations in fibre imprints and adjacent matrix.

CSM T300/914 (II) No striations. No matrix rollers.T800/5245 (I/II) Striations in fibre imprint regions. Matrix rollers?

QINETIQ T800/5245 (I) No obvious striations. Fatigue region“microscopically... smoother ....less microflow”(Report not issued)

NLR T800/5245 (I) Localised striations, fatigue region“microscopically smooth” (Report not issued)

T800/5245 (I/II) Striations in fibre imprints and adjacent matrix.No matrix rollers.

DAIMLERCHRYSLER

IM7/8552 (I) N/A (Not examined)

- -

CETIM IM7/8552 (I) Striations in fibre imprint regions.T800/5245 (I/II) Striations in fibre imprint regions. No matrix

striations/rollers.BAE SYSTEMS IM7/8552 (I) Isolated striations in fibre imprints. No matrix

striations/rollers.T800/5245 (I/II) Striations in fibre imprint regions. No matrix

striations/rollers.BOMBARDIERSHORTS

IM7/8552 (II) Striations in fibre imprint regions. Matrix rollers?

- -

ÁEROSPATIALEMATRA

IM7/8552 (II) Striations in fibre imprint regions and onfibres(!). Matrix rollers.

T800/5245 (I/II) Fibre imprint regions

Crack growth data not available


GARTEUR OPEN


Material Mode RoundRobin

Fractographic features

T300/914 I RR3 No obvious striations, some regular features.No matrix rollers.

II RR3 No obvious striations, some regular features.No matrix rollers-"preening" of PESspheroids.

I/II X -

T800/924 I RR2 No obvious striations/rollers.II RR1 Striations in fibre imprint regions – except at

very high cycle/mm. Matrix rollers present.I/II RR2 Striations in fibre imprint regions. No matrix

rollers. Macroscopically fatigue regionsmoother.

T800/5245 I RR3 Striations in fibre imprints and in adjacentmatrix. No matrix rollers.

II X -

I/II RR3 Striations in fibre imprints. No matrix rollers.

IM7/8552 I RR3 Localised striations in fibre imprints, none inadjacent matrix. No matrix rollers.

II RR3 Striations in fibre imprints/fibres and inadjacent matrix. Partially formed matrixrollers.

I/II X -

IM7/977 I RR2 No obvious striations in fibre imprints. Nomatrix rollers.

II X -

I/II X -

Table 12 Effect of Material on Fractographic Features

GARTEUR OPEN


Mode I Fatigue: T800/924 ; T800/5245 ; IM7/977 ; IM7/8552 ; T300/914

Members Mode RoundRobin

Material Fatigue Features Relationship(Yes/No)

EADS Mode I 1 T800/924 No typical fatigue features. No

BombardierShorts

Mode I 1 T800/924 No fatigue features. One fibre-rich and one imprint-rich area.

No

Fokker/STORK/FDO

Mode I 1 T800/5245 No typical fatigue features. No

CETIM Mode I 1 T800/5245 No typical fatigue features. No

EADS Mode I 3 T800/5245 Striations in fibre imprints.Striations in the matrix. Onlyone side observed.

CGD

Bright Striations in fibreimprints

No

QINETIQ Mode I 3 T800/5245 Smoother Mode IResin features in fatigue areacompared to static area. Nostriations

No

NLR Mode I 3 T800/5245 Striations in fibre imprints. No

DGA/CEAT Mode I 2 IM7/977 No typical fatigue features. No

NLR(SpecimenAerospatiale)

Mode I 2 IM7/977 No typical fatigue features.Morefibre rich area’s, less loosefibres infatigue area.

No

CETIM Mode I 3 IM7/8552 Striations in fibre imprints, bothbright and dark in one fracturehalf.

No

BAESYSTEMS

Mode I 3 IM7/8552 Isolated striations in fibreimprints.Only one side observed.

CGD

Dark striations

No

DGA/CEAT Mode I 3 T300/914 Few striations in fibre imprints. No

Table 13 Relation between Crack Growth Direction (CGD) and fractographic features –mode I

GARTEUR OPEN


Mode II Fatigue: T800/924 ; IM7/8552 ; T300/914


Material Fatigue Features Yes/NoRelationship

Daimler Benz Mode IIII

1 T800/924 Rollers. No

Shorts Mode II 1 T800/924 No fatigue features. No

Fokker/STORK/FDO

Mode II 1 T800/924 Rollers. Striations in fibreimprints, one half examined :Dark striations.

No

CETIM Mode II 1 T800/924 Rollers. Striations in fibreimprints.Side One:

Dark CGDStriations

Bright CGDStriations

Side Two:

Bright CGDStriations

Dark CGDStriations

*

BAe Mode II 1 T800/924 Rollers. Striations in fibreimprints.

No

CSM Mode II 1 T800/924 Rollers. Striations in fibreimprints.

No

DGA/CEAT Mode II 1 T800/924 Rollers. Striations in fibreimprints, one half examined :Dark striations.

No

Aerospatiale Mode II 1 T800/924 Striations in fibre imprints. No

NLR Mode II 1 T800/924 Rollers. Striations in fibreimprints.

No

*Inconclusive; Fibre-Rich/Fibre Imprint-Rich side not defined.

Table 14 Relation between Crack Growth Direction (CGD) and fractographic features – mode II

GARTEUR OPEN


: Mode II Fatigue: T800/924 ; IM7/8552 ; T300/914


Material Fatigue Features Relationship(Yes/No)

ÁerospatialeMatra

Mode II 3 IM7/8552 Striations in fibre imprints. Some matrix rollers.

Imprint-rich half:

DarkStriations CGD

Fibre-rich half:

BrightStriations CGD

Yes

BombardierShorts

Mode II 3 IM7/8552 Striations? in fibre imprints. No

EADS Mode II 3 T300/914 Globular inclusions segregatedfrom the matrix.

No

CSM Mode II 3 T300/914 No fatigue features. No

Table 14 – continued, Relation between Crack Growth Direction (CGD) and fractographicfeatures – mode II

GARTEUR OPEN


Mixed Mode (I/II) Fatigue: T800/924 ; T800/5245


Material Fatigue Features Yes/NoRelationship

DGA/CEAT Mode I/II 2 T800/924 Striations in fibre imprints.Tension:

Imprint-richCGDDark Striations

Compression:

Fibre-richBright StriationsCGD

Yes

EADS Mode I/II 2 T800/924 Striations in fibre imprints. No

QINETIQ Mode I/II 2 T800/924 Striations in fibre imprints.Tension:

Dark Striations CGD

Bright Striations CGD

Compression:


Dark Striations CGD

Yes

NLR Mode I/II 2 T800/924 Striations in fibre imprints.Fewresin rollers. Only one sideobserved.

Tension:

Dark StriationsCGD

No

Table 15 Relation between Crack Growth Direction (CGD) and fractographic features – mode I + II

GARTEUR OPEN


CETIM Mode I/II 2 T800/924 Striations in fibre imprints.Side One:

Dark StriationsCGD

Side Two:

Bright StriationsCGD

*

BAESYSTEMS

Mode I/II 2 T800/924 Striations in fibre imprints.Tension:

Imprint RichCGDDark Striations

Compression:

Fibre richDark striationsCGD

No

CSM Mode I/II 2 T800/924 Striations in fibre imprints.Side One:

Dark Striations CGD


Side Two:


Dark Striations CGD

**

Table 15 continued, Relation between Crack Growth Direction (CGD) and fractographic features – mode I + II

GARTEUR OPEN


ÁerospatialeMatra

Mode I/II 2 T800/924 Striations in fibre imprints.Tension:

BothBright Striations CGDDark Striations

Compression:

Dark Striations CGD

No

DGA/CEAT Mode I/II 3 T800/5245

Striations in fibre imprints.Tension:

Imprint-richDark Striations CGD

Compression:

Fibre-rich CGDBright Striations

Yes

QINETIQ Mode I/II 3 T800/5245

Striations in fibre imprints.Tension:Imprint-rich.Compression: Fibre-rich.One side both bright and darkstriations.

No

NLR Mode I/II 3 T800/5245

Striations in fibre imprints.Some “striations” in matrix.Tension:


Compression:

Fibre-richBright Striations CGD

Yes

BAESYSTEMS

Mode I/II 3 T800/5245

Striations in fibre imprints.Tension:


Compression:

Fibre rich CGDBright Striations

Yes


GARTEUR OPEN


CETIM ModeI/II

3 T800/5245 Striations in fibre imprints.Striations in matrix.

Tension:

Imprint-rich CGDDark Striations

Compression:

Fibre-rich CGDBright Striations

Yes

CSM ModeI/II

3 T800/5245 Striations in fibre imprints.Tension:

Imprint-rich CGDDark Striations

Bright Striations

CGDCompression:

Fibre richBright Striations CGD

Dark Striations CGD

Yes

Aerospatiale ModeI/II

3 T800/5245 Striations in fibre imprintsStriations in matrix.

Tension:

Imprint-richDark Striations

Compression:

Fibre-richBright Striations

Yes


GARTEUR OPEN


Round robin number

Specimenmanufacturer

Mode Material(s) Striations reported

Relation crack Growth

rate to interstriationdistance

1 IQ Mode II T800/924 Yes Yes

2 QINETIQ Mode I T800/924T800/5245IM7/977

NoYes (*)No

NoNoNo

2 EADS Mixed

Mode

T800/924 Yes No

3 BAESYSTEMS

Mode I T300/914T800/5245IM7/8552

Yes?YesYes

NoNoNo

3 BAESYSTEMS

Mode II T300/914T800/5245

NoYes

NoYes

3 EADS Mixed

Mode

T800/5245 Yes No

(*) Striations found considered as not being typical

Table 16 Overview of round robin results in respect to crack growth rate.

Organisation Number ofCycles

Crack Length(mm)

Crackpropagation rate

da/dN (µµµµm)

SEM StriationSpacing (µµµµm)

DGA/CEAT 124000 23 0.185 0.3Bombardier

Shorts140000 20 0.143 Not found

CSM 425000 25 0.059 -CETIM 403000 23 0.057 0.1 to 4EADS 415000 23 0.055 Not foundNLR 425000 20 0.047 0.3 to 1.3

ÁerospatialeMatra

855000 20 0.023 0.17

Fokker/STORK-FDO

420000 0-8 0.0 to 0.019 Not found

EADS 420000 6 0.014 Not foundBAE SYSTEMS 385000 2 0.0052 0.35?

QINETIQ Manufacturer ofspecimens

- - -

Table 17 Data from round Robin 1 showing relation between crack growth rate determined fromstriations with that determined form crack measurement on the specimen

GARTEUR OPEN


Annex A

RR1 - Mode IIMember No of

CyclesCrackLength (mm)

Summary of Findings

Mr Ansart 124000 23 Specimen examined at tilt angle of 0, 24 and 60 degrees.Zone A - Hackles and Scallops observed.Mode II (shear).Zone B - Matrix rollers observed. When viewed at 24 and 60 degrees,striations in fibre tracks visible with spacing of 0.3µm. Direction ofpropagation towards insert (determined using Dr Franz’s hypothesis).

Mr Baas 420000 0 - 8 Specimen showed asymmetric crack growth.Zone A - Hackles visible with modified appearance along with Scallops.Low cycle fatigue or static mode II.Zone B - Matrix rollers in fibre tracks. Striations visible in fibre tracks.Striations seen as dark lines when viewed in the direction of crackpropagation. Striations were found to be more abundant in widest area ofzone B

Dr Franz/Mr Heutling

420000 6 Zone A - Hackles visible. Static mode II fracture.Zone B - Only small area to examine. Matrix rollers visible. No striationsfound in the fibre tracks even when specimen examined at a high tiltangle. Mode II fatigue.

Mr Hiley - - Manufacturer of SpecimensMr Huisman 425000 20 Zone A - Hackles visible. Static mode II fracture.

Zone B - Matrix rollers and striations visible in fibre tracks. Striationsvisible at local stress intensities. Spacing of striations decreased asmoved away from stress intensity. Spacing typically 0.3 to 1.3µm. Norelationship between striations and crack growth direction. One side ofspecimen examined only. Mode II fatigue.

Dr Keul 415000 23 Zone A - Scallops visible. Static mode II fracture.Zone B - Some Hackles visible. No matrix rollers visible. Specimenexamined at tilt angle of 20 degrees. No striations observed.

Mr Lemascon 403000 23 Zone A - Macroscopically bright milky appearance. MicroscopicallyHackles typical of mode II fracture observed. Transverse cracks in thematrix and in fibre tracks observed. At high magnification slight striationsalso visible in fibre tracks.Zone B - Matrix rollers observed. Striations also visible in the fibre tracks.Striations not uniformly distributed and observed sometimes only in asingle fibre track. Striation spacing varied from 0.1 to 4µm. Specimenviewed from both directions at an angle of 60 degrees. Striations appearas bright lines when viewed in the direction of crack growth and as darklines when viewed in the opposite direction. When the mating fracturesurface was examined, striations appeared in reverse illumination. Couldnot use striations to identify direction of crack growth. Mode II fatigue

Mr Muskett 385000 2 Zone A - Hackles observed with modified structure “ matrix boulders”.Low cycle/high load fatigue.Zone B - Matrix rollers observed. Striations also observed but difficult tofind. Examined specimen at tilt angle of 70 degrees. Striation spacingtypically 0.35µm. High cycle/low load fatigue.

Mr Spratt 140000 20 Zone A - Hackles and scallops observed, also microcracks. Static modeII fracture.Zone B - No Hackles observed, appeared similar to peeled area (zone C).

Mrs Sorman 425000 25 Findings not presentedMr Vancon 855000 20 Zone A - Macroscopically rough. Large Hackles. Small hackles in fibre

imprints. Occasional striations visible in fibre tracks. Striation spacingtypically 0.18µm.Zone B - Macroscopically smooth. No matrix rollers observed. Striationsvisible in fibre tracks. Striation spacing typically 0.17µm.

Table A1 Summary of finding from mode II specimens (RR1)

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Mode I - DCB R = 0.1 5Hz

Member SpecimenNumber

Material Number ofCycles

Mr Lemascon 30 T800/5245 400 000Macroscopically - Static and fatigue distinguishable from each other. More fibre fracture visible instatic fracture zone.Microscopically - Less fibre bridging visible in fatigued zone. Striations found in one very localisedarea - observation not considered typical of fracture surface.

Mr Baas/Mr Eijkhout 31 T800/5245 400 000Macroscopically - Rougher surface morphology in static zone of fracture. Fewer broken fibressmoother morphology in fatigued region.Microscopically - No significant difference between fatigue and static fracture zones. Striationsfound at x35000 magnification in one area - observation not considered typical.

Mr Ansart 34 IM7/977 400 000Microscopically - No observable difference between static and fatigue fracture zones both showmicroflow and river markings.Few fibre imprints, many of fibres covered with resin.Striations were not found in the fatigue zone.

Mr Vancon 35 IM7/977 400 000Specimen not received in time - findings not presented

Mr Spratt 39 T800/924 400 000Macroscopically - Static and fatigue fracture zones distinguishable from each other. Much morefibre breakage visible in static zone of fracture.Microscopically - No striations visible in fibre imprints. Microflow and river markings found in resinadjacent to fibre imprints.

Dr Franz / MrHeutling

40 T800/924 400 000

Macroscopically - Static and fatigue zones distinguishable. Smoother surface in fatigue.Microscopically - In fatigued area matrix is smoother. Fibres are less visible as fracture confined toresin. No striations visible in fatigued zone.

Table A2 Summary of findings from Mode I specimens (RR2)

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RR2 - Mixed Mode - Bending - T800/924

Member SpecimenNumber

InitialCrackLength(mm)

NumberofCycles

Final CrackLength(mm)

DrH.Franz/MrHeutling

P1 2 60 000 45

Manufacturers of specimens.Mr A Lemascon P1 2 60 000 45

Precracked zone consistent with mode I static fracture.Mixed-mode fatigue zone - Striations found in fibre imprints, spacing appeared toincrease in direction of crack growth. Striations had dark appearance in direction of crackgrowth. Examination of mating fracture surface showed striations to be bright in directionof crack growth. Fibres also appeared to be raised on one surface only.

Continued

Mr Granstam P2 3 120 000Precracked zone shows rivers - Mode I static fractureMixed-mode fatigue zone - Hackles and scallops visible. Striations visible in fibre tracks.When striations examined on one fracture surface they appeared as white lines whenviewed in the direction of crack growth and dark lines when observed in the oppositedirection. Examination of the matching surface showed the striations to be dark lines inthe fibre direction and light in the opposite direction. The spacing of striations variedfrom 0.5µm to 3.5µm. Striations found only in the first 20mm of fracture surface.

Dr B Keul P2 3 120 000N/A

Mr H.Huisman/J.P Steyaert

P3 3 100 000

Precracked zone shows rivers and ribbons - Mode I static fractureMixed-mode fatigue zone - No significant evidence of resin rollers. Striations observed infibre imprints. Striations appear to occur at local stress intensities. Scarps visible andhackles have shallow angle indicating medium mode II component.

Mr R.Muskett P3 3 100 000Precracked zone shows rivers consistent with mode I failure.Mixed-mode fatigue zone - No matrix rollers identified. Some dark striations visible in thefibre imprints - visible in first 20mm of delaminated surface. Found striations in adjacentfibre imprints with significantly different spacings - suggests their is no clear link betweenthe striation spacing and global stress level applied to specimen.

Mr M.Hiley P4 3 200 000 11Precracked zone shows river markings microflow consistent with Mode I static fracture.Mixed mode fatigue zone - Appearance similar to static fracture, but occasional striationsvisible. Striations in fibre tracks were predominantly confined to the centre of the fatiguedzone. The striations were found to appear as white or dark bands when viewed in thedirection of crack propagation. This observation suggests that they cannot be useddirectly to determine crack growth directions. The mixed mode ratio (I/II) was estimatedas 60/40.

Mr G.Ansart P4 3 200 000 11Results not presented!?

Mr M.Vancon P6 3.2 40 000 5Striations identified in fibre imprints. When striations viewed on one side of the fracturesurface (A), striations appeared as bright lines in direction of crack growth and as darklines in the opposite direction. When the mating fracture surface (B) was observed thestriations appeared as dark lines in the direction of crack growth. The surface roughnesswas seen to decrease as the crack advanced. Striation visible in centre of delaminatedarea - spacings between 0.3µm and 0.37µm.

Table A3 Summary of findings from Mixed-Mode specimens (RR2)

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RR 3 - Mode I and Mode II Specimens

Material/Specimen IdentificationMember Mode T300/914

(U.D)T800/5245(U.D)

IM7/8552(U.D)

COMMENTS

Mr Ansart I AFD1 Microscopically - specimen examinedat 0° and 40° tilt. Fracture typical ofmode I with rivers markings visible.Two phase resin -difficult to assess.No obvious difference betweenfracture surface near insert and thatfurther away. Claim no striationsobserved - but regularly spacedfeatures are visible in the resin in someareas. Difficult to find fibre imprints.

Mr Baas I AFD2 Specimen had not been examined!Nothing to present!

Dr Franz II AFE1 Macroscopically - smooth surfacesuggesting fibre bridging had notoccurred. Microscopically - due to twophase nature of matrix no fibre imprintsvisible in fatigued region. Globularinclusions had separated formsurrounding resin. Matrix between thefibres and the globular inclusions andbetween the inclusions themselvesshow high degree of crazing. Nostriations identified.

Mr Granstam II ASE10 Macroscopically - five zones seen onthe fracture surface (including theinsert and residual fracture).Microscopically - fractures complicateddue to two phase resin. No fatiguestriations identified. Difficulty inidentifying fracture as mode II.

Mr Heutling I BFD1 Macroscopically static fracturedistinguishable from fatigue fracture.Microscopically - river markings andribbons (lancets) typical of mode Ivisible. When viewed at 45° fatiguestriations observed both in the fibretracks and in the resin between fibres.Striation spacing between individualfibre tracks varies significantly. Whenstriations observed in the direction offracture they appear bright. Striationsappear dark when viewed in theopposite direction to crack growth.

Mr Hiley I BFD2 Macroscopically two distinct zones offracture visible. Microscopically -fatigued area has smoother resinfeatures with less microflow visiblewhen compared to static zone. Riverslines visible in both static and fatiguedzones. No striations identified.

Table A4 Summary of findings from Mode I and Mode II specimens (RR3)

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Mr Huisman/Mr H. Steyaert

I BFD3 Macroscopically two distinct zonesvisible in fatigue area - two G ratios.Microscopically - fatigue area smooth,static fracture surface rougher. Rivervisible, also some shallow hacklesvisible indicating possible shearcomponent in fracture. Striationsvisible in fibre tracks when specimensviewed at 70°. Striations very hard tofind.

Dr Keul I CFD1 N/AMr Lemascon I CFD2 Macroscopically - two zones, one

bright the other dull, visible.Microscopically - many defectsidentified in this specimen includingfibre misalignment and inclusions. Anumber of features typical of mode Iidentified including river lines andribbons. Good fibre/matrix interfacelead to fatigue failure in the matrix nearthe f/m interface. Some striationsvisible in the fibre tracks in both dulland bright zones. Light and darkstriations visible of the same surface.

Mr Muskett I CFD3 Macroscopically - two zones of fracturevisible.Microscopically - fatigued zonescontain very occasional striations withsmall spacing. River lines visible infatigued region.

Mr Spratt II CFD4 Microscopically - fatigued region foundto contain rounded cusps featuresindicative of shear fatigue. Many ofthe fibre tracks had a roughappearance. Regular features, thoughtto be striations, were occasionallyfound within the fibre imprints.

Mr Vancon II CFD5 Macroscopically - clear differencebetweenstatic and fatigue zonesMicroscopically - fatigue fracturesurfaces show few fibre ruptures. Finehackles and some matrix rollersvisible. Striations also identified. Onsurface A their were predominantlyfibre imprints and the striationsappeared bright. On surface B theirwere many fibre surfaces and thestriations were predominantly dark.Thestriation spacing was found to beglobally in accordance with themacroscopic crack growth rate.

Table A4 continued, Summary of findings from Mixed-Mode specimens (RR2)

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RR 3 - Mixed-Mode Specimens

Member Specimen Number

Numberof Cycles

Loading(N)

COMMENTS

Mr Ansart P7/8 110 000 170 Microscopically - examination of tension surface at 0° showshackles. Examination of surface at 45°, in opposite direction ofcrack growth, shows bright striations in fibre tracks. Striations aredark when viewed in direction of crack growth. Striation spacingdifferent between adjacent fibre tracks. Tension facepredominantly contains fibre imprints. Examination ofcompression surface at 45° shows striations. These are darkwhen viewed in the opposite direction to crack growth and brightwhen viewed in the direction of crack growth. Fibres mainly onthis side.

Mr Baas P7/8 110 000 170 N/A

Mr Granstam P7/10 113 000 170* Macroscopically - four zones seen on fracture surface (includinginsert and residual fracture). Microscopically - fatigued regionexhibited river markings and cusps/hackles. Some bare fibresalso visible. There was little evident of matrix rollers. Fatiguestriations were observed on both fracture surfaces but their wasno consistency in their appearance (bright or dark) with respectto crack growth direction.

Mr Hiley P7/10 113 000 170* Macroscopically - two zones visible; one dark, one light (threeincluding peeled zone at end of fracture).Microscopically - light zone shows shallow cusp featuresindicating shear content to fracture. Striations visible in fibretracks. Both light and dark striations visible on same fracturesurface. One side (tensile) of the fracture has predominantly fibreimprints the other mainly fibres.

Mr Huisman/Mr H. Steyaert

P7/14 63 000 180** Microscopically - examination of fatigue fracture surfacesidentified hackles and ribbons indicative of shear. Striations alsoobserved in the fibre tracks. At beginning of fatigued areastriations easier to find. Towards the end of the fatigued zone,striations harder to find. On compression fracture surface, manyfibre imprints and striations dark. On tension fracture surface,many fibres and striations bright. Only quick examination ofspecimen conducted.

Dr Keul P7/14 63 000 180** N/A

Mr Muskett P7/15 94 000 170 Macroscopically - different crack zones distinguishable optically.Numerous areas found to contain striations. Spacing of striationsvaries systematically along the length of a single fibre imprint.Bright and dark striations visible depending on the surfaceexamined and tilt direction. Compression surface shows brightstriations when observed in crack growth direction and darkstriation when viewed in opposite direction. Tension surfaceshows dark striations when viewed in direction of crack growthand light striations when viewed in the reverse direction. Norollers visible.

Mr Spratt P7/15 94 000 170 N/A.

Mr Vancon P7/18 244 000 160*** Microscopically - bright striations visible in fibre imprints onsurface A (compression) when viewed in direction of crackpropagation. Striations also identified in matrix resin between thefibres. Striation spacings were larger than the macroscopic crackgrowth rate. Only had time only to examine surface A.

Mr Lemascon P7/18 244 000 160*** Macroscopically - the zone adjacent to the insert appears dull. Inthis zone the fracture appeared smoother.Microscopically - Only the compressive side of the fracture wasexamined. Striations visible on both bright and dull surfaces. Bothdark and light striations found in neighbouring fibre imprints.Theinter striation spacing was found to vary considerably from onefibre track to another. Ribbons also visible.

* (after 96000 cycles 180) ** (after 9000 cycles 170) *** (after 170 000 cycles 170)

Table A5 Summary of findings from mixed-mode specimens (RR3)

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Figures

Figure 1 da/dN against ∆K for unidirectional DCB AS4/PEEK specimens

Figure 2 Mode I fatigue delamination growth rates generated for unidirectional DCB(AS4/PEEK) specimens

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Figure 3 GImax as a function of cycles up to the onset of delamination (AS4/PEEK)

Figure 4 SEM image of mode I static fracture surface in brittle matrix composite

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Figure 5 SEM micrograph of mode II fracture surface in brittle matrix composite

Figure 6 Singular terms for stress components of the stress field close to the crack tip under mode1 (σy) and mode II( σx)

Cusps

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Figure 7 Model according to McEvily describing crack propagation during cyclic loading, partiallyvalid for metals and polymers

Figure 8 Refined version of the model according to McEvily describing crackpropagation during cyclic loading

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Figure 9 Measured craze and crack opening (points) in PMMA

Figure 10 Schematic representation of the correlation of loading phase with crack propagation andcraze growth during continuous crack growth (in PMMA)

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Figure 11 Schematic representation of fatigue crack propagation in a craze zoneFor (a) continuous crack growth and (b) discontinuous crack growth

Figure 12 Schematic representation of secondary electrons emitted from an elevated object withperpendicular impingement of primary electrons

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Figure 13 Schematic diagram of End Notched Flexure (ENF) Specimen

Figure 14 Schematic diagram of Double Cantilever Beam (DCB) specimen

Figure 15 Schematic diagram of Mixed-Mode Flexure (MMF) specimen

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Figure 16 Graphical representation of loading applied to specimen P3

Figure 17 Graphical representation of loading applied to specimen P4

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Figure 18 Striations in the fibre imprints. Left hand imprint shows ‘bright’ striations whilst right handimprint shows ‘dark’ striations. T800/5245 (mixed-mode) x6300

Figure 19 Striations in the fibre imprints T800/924 (mode II) x10000

LightStriations

DarkStriations

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Figure 20 Striations in fibre imprints IM7/8552 (mode II) x5000

Figure 21 Schematic diagram illustrating how appearance of striations is affected by viewingdirection

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Figure 22 Striations on the fibre surface IM7/8552 (mode II) x4000

Figure 23 Widely spaced striations on fibre surfaces IM7/8552 (Mode II) x2000

Striationson fibresurfaces

Striationson fibresurfaces

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StriationsWithin thematrix

Page 68 GARTEUR OPEN

Figure 24 – Striations within the matrix oriented parallel to the fibres (mode l) x14000 T800/5245

Figure 25 Striations within the matrix T800/5245

Striationswithin thematrix

GARTEUR TP112

(mode I + II)

Texturedmicroflowshowingdirection ofcrackpropagation

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Figure 26 Matrix rollers T800/924 (mode II) x7500

Figure 27 – Matrix rollers T800/924 (mode II) x5000

MatrixRollers

MatrixRollers

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Figure 28 – Matrix rollers between the fibres T800/924 (mode II) x1000

Figure 29 Rubbed-out secondary phase on the surface of mode II fatigue fracture T300/914(mode II)

MatrixRollers

Rubbed-outSecondaryphase

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Figure 30 Fine detail of fractographic features visible IM7/977 (mode I) x3250

Figure 31 Cusps within the resin in masked by secondary phase T300/914 (mode II) x1770

Microflowin matrix

Cusp

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Figure 32 Fractographic features resolvable despite presence of fine secondary (modifier) phaseT800/924 x 3250

Figure 33 Fractographic features resolvable despite presence of fine secondary phase IM7/8552x1000

TexturedMicroflow

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Figure 34 Fatigue striations in fibre imprint region T800/5245 (mode I). Clean imprints alsoindicative of weak f/m bond. x5500

Figure 35 Fatigue striations in fibre imprint region IM7/8552 (Mode I) x5000

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Figure 36 Regular features – possibly evidence of fatigue loading T300/924

Figure 37 Loose PES particles on fatigued surface of T300/914 (m

‘Striation – like’features on fibresurface

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(Mode II) x4000

ode II) loading

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Figure 38 Rolled cusps formed during fatigue of static fracture surface T800/924 (mode II) x1000

Figure 39 Striations in the fibre imprints T800/5245 (mode I +II) x 8K

Roundedcuspfeatures

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Figure 40 Striations within the resin between fibres T800/5245 (mode I + II) x9K

Figure 41 Striations in fibre imprints T800/924 (mode I + II) x1800

Matrixstriations

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Figure 42 Micrograph showing change in striation spacing around defect T800/8552 (mode II)x5000

Figure 43 Micrograph illustrating variation in striation spacing between adjacent fibre imprintsT800/5245 (mode I + II) x3100

Defect

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Figure 44 Relation between crack growth rate and inter-striation spacing

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Distribution List

GARTEUR Executive Committee (XC)

Dr. D. Nouailhas ONERA, France *Mr. W. Riha DLR, Germany *Prof. F.J. Abbink NLR, The Netherlands *Mr. P. Garcia Samitier INTA, Spain *Mr. B. Uggla FMV, Sweden *Mr. A. Amendola CIRA, Italy *Dr. O.K. Sodha QINETIQ, UKMr. E. Maire ONERA (GARTEUR secretary), France

GoR SM

Dr. T. Khan ONERA, France *Prof. R. Ohayon ONERA, France *Dr. M. Mahé Aérospatiale Matra Airbus, France *Dr. J. Bühlmeier Fairchild Dornier, Germany *Dr. M. Gädke DLR, Germany *Mr. H. Schnell DaimlerChrysler Aerospace Airbus, Germany *Mr. F. Holwerda NLR, The Netherlands *Dr. J.M. Pintado INTA, SpainProf. A. Blom FFA, Sweden *Dr. H. Ansell Saab AB, Sweden *Prof. P. Curtis DSTL, UKDr. R. Collins British Aerospace Airbus, UK *Mr. M. Fariola CIRA, Italy *

AG20

Mr T. Ansart DGA/CEAT, FranceMr S. Baas Stork FDO, The NetherlandsDr H.E. Franz EADS, GermanyMr J-P. Steyaert NLR, The NetherlandsMr M.J.Hiley QINETIQ, UKDr B. Keul DaimlerChrysler Aerospace Airbus, GermanyMr A Lemascon CETIM, FranceMr P.Granstam CSM Meterialteknik, SwedenMr M. Vancon Aerospatiale, FranceMr R.Muskett BAE SYSTEMS, UK Mr G.Spratt Shorts Brothers, UK

* Electronic copies only

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Report documentation page

1. Originator's report number: QINETIQ/FST/SMC/TR012369

GARTEUR Final Report TP112

2. Originator's Name and Location: M.J.Hiley, Room 2008, Building A7, QinetiQ Ltd ,Farnborough, Hants. GU14 OLX. UK

3. MOD Contract number and period covered:

4. MOD Sponsor's Name and Location:

5. Report Classification and Caveats in use: 6. Date written: Pagination: References:

GARTEUR OPEN August 2001 X + 80 64

7a. Report Title: AG-20 Fractographic Aspects of Fatigue Failure inComposite Materials

7b. Translation / Conference details (if translation give foreign title / if part of conference then giveconference particulars):

7c. Title classification: GARTEUR OPEN

8. Authors: M.J.Hiley

9. Descriptors / Key words: Fractography, polymer composites, fatigue

10a. Abstract. (An abstract should aim to give an informative and concise summary of the report in up to300 words).This report describes the findings of a study performed by GARTEUR AG20 aimed at examining the fractographic aspects of fatigue failure inpolymer composites. An investigation into the mechanisms of fatigue failure in unidirectional materials was performed to establish themacroscopical/microscopical features associated with fatigue fracture. The micromechanisms by which different fractographic features form,under different loading conditions, as well as the effect of material on their appearance, was studied. Investigations to establish whether therewas a relationship between the fractographic features and crack growth rate and crack growth direction were also performed.

Fractographic investigations were undertaken by exchanging fatigue specimens within three round robin exercises. Members findings werereported at bi-annual meeting, six of which were held over the 3 year life of the project.

The results of this study identified a number of fractographic features unique to fatigue failure including striations within the fibre imprints andmatrix rollers, both of which were observed within mode II (shear) dominated failures. Striations were also observed in the matrix between thefibres in some materials, but these were only visible using electron microscopes with very high resolutions. Macroscopically the fatigue fracturesurfaces were noticeably smoother that their static equivalents, which, in the mode I specimens, was attributed to the a reduced level of fibrebridging. The single phase resin systems gave rise to rollers and striations, with the fibre/matrix bond appearing to be the most important factorcontrolling striation formation within the fibre imprints. The main difference observed between materials was most apparent in the two phasesytem, where in mode II rubbing out of the toughening thermoplastic particles was mainly observed.

Studies of the matrix rollers showed they were not useful for determining crack growth directions. Matrix striations could be used toindicate the local directions of crack propagation, but the striations within the fibre imprints were the most useful for determiningglobal directions of fracture. By observing which side of the fracture surface was fibre rich or imprint rich and the appearance of thestriations bright or dark, the crack growth direction could be ascertained. Due to the significant variation in the inter-striation spacingsobserved, efforts to correlate striation spacing to crack growth rates proved inconclusive.

10b. Abstract classification: GARTEUR OPEN FORM MEETS DRIC 1000 ISSUE 5

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