COMPARISON OF CARBON FIBRE/EPOXY COMPOSITESREINFORCED BY SHORT ARAMID AND CARBON FIBRES

Laurence Walker and Xiao-Zhi HuDepartment of Mechanical and Materials Engineering, The University of Western Australia,

Nedlands, W.A. 6907, Australia

(Received May 21, 1999)(Accepted June 1, 1999)

Keywords:Composites; Fibres; Fracture toughness; SEM

Introduction

The development of interlaminar reinforcement methods designed to improve the delamination tough-ness of continuous fibre reinforced polymer composites has been a strong focus amongst the aerospacefraternity for many years. Solutions to the problems of delamination due to impact damage have variedfrom interleaving methods incorporating toughened interlaminar plies and resins (1,2) to through-the-thickness stitching (3) and recently short fibre interlaminar reinforcement (4,5,6).

There are a number of distinct advantages associated with the short fibre interlaminar reinforcementtechnique. Firstly, the laminating process is unchanged so that construction of composite structures inpractice will be almost identical to the original laminating method. Secondly, aside from increasedfracture toughness the original carbon fibre/epoxy composite system properties remain unchanged asthere is no tougher polymer involved. Thirdly, without the complication of through-the-thicknessstitching, the short fibre interlaminar reinforcement technique offers an economic way to increase theimpact resistance of large composite structures via the proven method of fibre bridging (7,8).

This study is designed to examine and compare the toughening effects of short aramid and carbonfibres in carbon fibre/epoxy composites. The primary objective being to identify the tougheningmechanisms associated with the two different short fibres. The detailed information on tougheningmechanisms will provide us with a general guide on the relationship between composite interlaminardesign and composite performance. Composite design and processing, delamination testing and SEMstudy of fracture surfaces are used in conjunction in the current study for a better understanding of theshort fibre interlaminar reinforcement technique.

Correspondence to: Dr. Laurence Walker, Department of Mechanical and Materials Engineering, The University of WesternAustralia, Nedlands, WA 6907, Australia.

Pergamon

Scripta Materialia, Vol. 41, No. 6, pp. 575–582, 1999Elsevier Science Ltd

Copyright © 1999 Acta Metallurgica Inc.Printed in the USA. All rights reserved.

1359-6462/99/$–see front matterPII S1359-6462(99)00193-1

575

Experimental Details

Composite Design and Processing

Five unidirectional carbon composite panels were manufactured using Newport Adhesives NCT 301Carbon Fibre cloth with final fibre/resin volume ratios between 65–70%. The processing temperaturesand times were as recommended by the manufacturer whilst the pressure was maintained at 350kpag.The slight variation in the final fibre/resin volume ratio is due to different interlaminar designs.

A starter crack was initiated between the 5th and 6th plies of the 10 ply unidirectional laminate usinga 20 mm thick Teflon film for delamination tests. The crack plane coincided with the interlaminarreinforcement region that comprised either short carbon or aramid fibres. Short fibres were eitheraramid, cut from a roll of unidirectional Kevlar 49, or carbon fibres, supplied by Zoltek with anepoxy/phenolic binding surface coating.

The interlaminar reinforcement has been successfully introduced between all plies of continuouscarbon fibres. However, for fracture mechanics tests such as Mode-I tests using DCB specimens, amid-plane reinforcement is sufficient.

Specimen Preparation and Testing

Specimens of 18mm width were cut parallel to the continuous carbon fibre orientation using a StruersAccutom II radial diamond saw. Five DCB specimens of 18mm in width and 180mm in length and withinitial cracks between 35–40 mm were produced from each composite panel manufactured so that testsfor each composite design could be repeated if necessary. The specimen side surfaces were polishedusing 1600 grit SiC abrasive paper, then coated with thinned white correction fluid to produce a visiblecrack tip for crack growth measurements. The specimen sides were marked in 1mm increments tofacilitate crack growth measurements.

DCB specimens were tested employing the standard loading/unloading technique on an Instron 4301with a 2mm crosshead speed and a Lloyds PL3 chart recorder to plot load versus cross head movement.Because of the difficulty in determining the exact crack initiation loads, the first unloading often tookplace at 2–3mm crack extension.

SEM Fracture Analysis

All fractured surfaces were examined using SEM to identify special features of the delaminationsurfaces. A special emphasis was given to distinguish the differences between the composites with shortcarbon and aramid fibres. Selected SEM micrographs and detailed discussions on the tougheningmechanisms associated with the short carbon and aramid fibres are given in the next section.

Results and Discussion

Toughness Results Measured from DCB Specimens

As usual, the critical strain energy release rate G1c is used to characterise the delamination toughnessbecause it can be easily evaluated from the DCB specimens. Both the uncorrected and well-receivedcorrected formulae (9) for G1c was used for the determination of fracture toughness. Both calculationmethods were used due to queries raised regarding the accuracy and applicability of the correctedformula with reference to short fibre reinforced specimens (10). Hu and Mai indicate that the influence

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of fibre bridging has a more significant affect on compliance than crack tip deflection and possibly moreeffect than that found with end block rotation and shortening of the beam length. Correcting for cracktip deflection, rotation and beam shortening does not tend to reflect realistic values when applied tosamples featuring large portions of fibre bridging. Charalambides and Williams (11) argue that thecorrection factor form of

G 512P2~a 1 xh!2

B2h3EF (1)

as opposed to

G 512P2~a 1 D!2

B2h3E(2)

allows the substitution ofxh to account for the effects of crack tip rotation but should not be substitutedinto the Youngs modulus instead being applicable only to the G1c equations. After considering theabove arguments and subjecting the data to both corrected and uncorrected equations it was determinedto utilise the uncorrected form of the equations to maintain consistency with previous studies (4,5) andas the study was comparative. Another consideration was the substantial affects of large variations inD values. In this instance the correction valueD varied substantially between specimens as well asbetween laminate types. The addition of eitherD or xh dramatically alters G1c values and produced verysignificant variations in interpreted data from specimens cut from the same laminate.

Figure 1. 1% 6mm carbon short fibres.

Figure 2. 0.7% 6mm Kevlar short fibres.

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Figures 1 to 5 show the uncorrected G1c values against crack length for the seven different laminatestested. Five specimens from each laminate panel have been tested and results are shown in the figures.As shown in the figures, G1c values vary from 0.25 to 0.55 kJ/m2 for crack extension between 38 and110 mm. As expected, there is a considerable amount of scatter observed in these composites mostparticularly those with higher percentages of short fibres and increased fracture toughnesses.

Using the G1c results of the un-reinforced composite panel as the reference, one can see the slighttoughening effects of short aramid fibres. The toughness behaviour has been enhanced through theaddition of the aramid fibres though scatter has also increased significantly in agreement with previouswork (5). The effect of short carbon fibres of varying lengths and densities is less pronounced. Therelatively large scatter in the G1c values and in some cases the observable reduction in fracturetoughness with the addition of 6mm short fibres indicate a marginal, yet detrimental effect is arisingfrom the addition of short carbon fibres. The 0.5, 1 and 2.5% densities of 6mm fibres do very little toalter the fracture toughness of the material. A marginal increase in scatter is evidenced over that of theunreinforced specimen, as is an increase in the crack extension for each load/unload cycle. Thesegraphical observations indicate that the addition of small quantities of 6mm carbon fibres have analmost negligible effect on the mode 1 fracture toughness of carbon fibre/epoxy samples.

SEM Observation

The delamination fracture surfaces of the short carbon and aramid fibre reinforced composites showvery obvious differences even at low magnification e.g. Figures 6 and 7. It is obvious that the

Figure 3. 0.5% 6mm carbon short fibres.

Figure 4. Unreinforced.

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delaminated surface reinforced with short aramid fibres (Fig. 6) is much rougher than that with shortcarbon fibres (Fig. 7). The difference can be attributed to delamination occurring within the interlaminarregion in the case of short carbon fibre reinforced composites and delamination being deviated into thecontinuous carbon fibre plies (intraply) in the case of short aramid fibre reinforced composites. Withsignificant improvements to the toughness of the interlaminar region, the crack has relocated to the nextweakest point within the laminate. Observation of the aramid reinforced laminates showed intraplyfailure across all laminate surfaces with limited areas where the crack deviated through the interlaminarregion. The strong crack bridging/bonding and deviation mechanisms associated with short aramid fibrereinforcement are essential for the toughness enhancement. As a result, the continuous intraply carbonfibres must also contribute to the toughness improvement through crack bridging in order to fully realisethe improvements available. This explains why even a low volume fraction of aramid fibres can stillimprove the delamination toughness.

The difference between short carbon and aramid fibre reinforcement becomes even clearer in Figures8 and 9 with higher magnifications. De-bonding of carbon fibres from the epoxy matrix appeared to beeasy and clear cut as shown in Figure 8. Some carbon fibres were also fractured. Cracking in the carbonfibres and between the carbon fibre and matrix interface is basically elastic, and it appears that littleenergy is required to propagate a crack if it has been initiated. Therefore, to improve the effectivenessof short carbon fibre reinforcement some kind of fibre coating may be required to change the debondingcondition so that fibre bridging instead of fibre fracture is possible. Additionally, modifications to the

Figure 6. Short aramid fibre reinforcement.

Figure 5. 2–3% 6mm carbon short fibres.

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continuous carbon fibres to increase intraply fibre bridging could also enable further improvements tothe fracture toughness available with both short aramid and carbon fibres.

The bonding between the short aramid fibres and epoxy matrix appeared to be strong, different tothose previous reports by Andrews et al. (12) and Kalantar and Drzal (13,14). Strong bonding in thiscase is beneficial, as inelastic fracture activities such as fibrillation of aramid fibres and crack bridgingwere generated. That is, the “ductile” behaviour of aramid fibres is fully utilised because of the strongbond. The comparison between the failure mechanisms appears to suggest that brittle fibres (shortcarbon fibres) require a relatively weak bonding so that fibre bridging and pull-out instead of fracturecan occur. Conversely, ductile fibres (short aramid fibres) require a strong bond to take the fulladvantage of high tensile strength and energy absorption mechanisms of the aramid fibres. Crackdeviation and fibrillation of the aramid fibres further enhance the toughening effects of these shortaramid fibres.

Quantitative results obtained from the analysis of data suggest that the improvements due to theaddition of short aramid fibres is less substantial than those results obtained by Sohn and Hu (5) in theirprevious work. Earlier studies utilised an epoxy wet lay up system and commercially available uniaxialcarbon fibre cloths as opposed to this study which incorporated epoxy pre impregnated uniaxial cloths.Pre impregnated cloths are produced with substantially less fibre misalignment and fibre tangling within

Figure 7. Short carbon fibre reinforcement.

Figure 8. Carbon short fibre reinforced fracture surface.

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the ply than the older style wet lay-up cloths. Consequently these laminates are more prone to intraplycracking due to less continuous carbon fibre bridging. Fracture toughness measurements taken from thearamid reinforced laminates were most often produced by failure within the intraply region as opposedto interlaminarly and as such the intraply toughness becomes representative of the material fracturetoughness. Consequently, the ability of the short fibres to improve fracture toughness is limited by theintraply fracture toughness. As a result of this fracture relocation, fibre misalignment, continuous fibrebridging and fibre tangling become important for increasing Mode 1 toughness in uniaxial laminates.The two systems may also utilise different continuous fibre surface treatments therefore resin/fibrecompatibility becomes a variable. All of these factors can dramatically alter the intraply fracturemechanisms.

Concluding Remarks

The delamination toughness and toughening mechanisms of carbon fibre composite laminates rein-forced by short aramid and carbon fibres are studied through delamination testing and SEM observation.It has been found that the strong bonding between aramid fibres and epoxy matrix is essential to thetoughness improvement and associated toughening mechanisms such as crack deviation, fibre bridgingand fibrillation. It is not desirable in the case of short carbon fibre reinforcement as a relatively weakbond may promote fibre pull-out and bridging and thus the overall delamination toughness.

Observations comparing the aramid and carbon short fibre reinforced laminate fracture surfacesindicate different fracture mechanisms within the laminate. Aramid reinforcement has forced thefracture into the intraply region whilst carbon reinforcement has not toughened the interlaminar regionsufficiently to deviate the fracture away from between the plies. An assessment of the differences inintraply toughnesses between the wet lay-up and pre preg systems should allow verification of thesystem variables and toughness requirements of the intraply region.

Acknowledgement

One of the authors (LW) would like to thank the University of Western Australia for UniversityPostgraduate Award, and the late George Tonie Samaha for a supplementary Research Scholarship.

Figure 9. Fibrillated aramid fibres.

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