Impact resistance and tolerance of interleaved tape laminates

Andre Duarte a,*, Israel Herszberg b, Rowan Paton c

a Sir Lawrence Wackett Centre for Aerospace Design Technology, Royal Melbourne Institute of Technology, GPO Box 2476V, Melbourne,

Vic. 3001, Australiab Department of Aerospace Engineering, Royal Melbourne Institute of Technology, GPO Box 2476V, Melbourne, Vic. 3001, Australia

c Cooperative Research Centre for Advanced Composites Structures Limited, 506 Lorimer Street, FishermenÕs Bend, Vic. 3207, Australia

Abstract

This paper presents and discusses the results of low-velocity impact and compression-after-impact (CAI) tests conducted on

interleaved and non-interleaved carbon/epoxy tape laminates. Ole®n ®lm interleaves provided a strong interface bond, resulting in a

reduction in projected damage area. These interleaves changed the stress distribution under impact and restricted delamination

formation at the ply interface. An investigation into the compression behaviour of these laminates revealed a reduction in un-

damaged strength using ole®n interleaves. This was attributed to the lack of lateral support for ®bres at the ®bre/interleaf interface,

allowing ®bre microbuckling to occur at a low load. Low modulus copolyamide web interleaves resulted in an increase in damage

area and minor changes to CAI strength. Examination of laminate cross-sections revealed that this was due to both the open

structure of the interleaf and poor resin/interleaf adhesion. High shear modulus polyethylene interleaves resulted in a signi®cant

decrease in damage area at various impact energies, with CAI strength improved compared to the non-interleaved lami-

nates. Ó 2000 Published by Elsevier Science Ltd. All rights reserved.

Keywords: Interleaving; Impact resistance; Damage tolerance; Compression after impact; Carbon/epoxy tape

1. Introduction

The use of structural composites has been limited bytheir low impact resistance and damage toleranceproperties. Much work has been conducted to addressthis problem, with various techniques developed in-cluding resin modi®cation and 3D reinforcements.However, many of these techniques involve costlymanufacturing processes or result in degradation of themechanical properties of the composite. One methodused to improve impact properties in prepreg laminatesis the technique of interleaving, which involves the in-sertion of thin, tough, polymer layers (interleaves) be-tween selected plies of the composite laminate. Fig. 1(a)and (b) show cross-sections of a non-interleaved andinterleaved laminate respectively, with the interleaf ap-pearing as dark bands at the ply interfaces in Fig. 1(b).Various researchers have reported improvements inimpact resistance and damage tolerance with interleavedcarbon/epoxy tape laminates, including Masters [1],Evans et al. [2], Sun and Rechak [3] and Gandhe andGri�n [4]. In the current work, this technique has been

applied to carbon/epoxy tape laminates incorporatingvarious thermoplastic interleaf materials, and usingrelatively thin specimens (2 mm). This paper presentsand discusses the results of low-velocity impact tests andcompression-after-impact (CAI) tests conducted on in-terleaved and non-interleaved laminates.

2. Test apparatus and materials

2.1. Materials and specimen de®nition

The interleaved and non-interleaved carbon/epoxyprepreg specimens were produced using T300/914C tapemanufactured by Ciba±Geigy. All laminates were curedin an autoclave at 177°C and 650 kPa. The non-inter-leaved specimens used a 16 ply quasi-isotropic lay-up��45; 0;ÿ45; 90�2S as recommended by the SACMA testspeci®cation [5], while the interleaved specimens had aninterleaf (I) between dissimilar orientation tape pliessuch that the lay-up was ��45; I; 0; I;ÿ45; I; 90;I;�45; I; 0; I;ÿ45; I; 90�S. The following interleaf mate-rials were chosen for impact and CAI testing, each withdi�erent material properties. These are described in de-tail in Table 1, with three di�erent interleaf types shown

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Composite Structures 47 (1999) 753±758

* Corresponding author.

0263-8223/99/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 6 3 - 8 2 2 3 ( 0 0 ) 0 0 0 4 9 - 0

in Fig. 2. The last two listed interleaf materials weretested to investigate the in¯uence of interleaf perfora-tions on impact and CAIS.· XAF2210 ± thermofusible polyole®n ®lm,· 1a8s18 ± thermofusible copolyamide web,· Ultem 1000 ± polyetherimide ®lm,· XAF2210P ± perforated XAF2210, hole size 1

4in. at

20 mm staggered spacing,· XAF2065 ± open net form of XAF2210.

2.2. Drop weight impact test

Low velocity impact tests were conducted in accor-dance with the CRC-ACS impact test speci®cation [6],using a drop weight test rig as shown in Fig. 3. Thespecimens measured 90� 115 mm2, and were machinedwith tolerances as designated by the speci®cation. Thespecimens were clamped using an edge support framewith an impact window of 80� 90 mm2. The impactorused a hemispherical steel tup of diameter 12.5 mm, andused variable weights and heights to impact the speci-

men at incident energies up to 7 J/mm. Following im-pact, the specimens were c-scanned to determine theprojected damage area, with selected specimens sec-tioned and viewed under a microscope to investigatefailure paths and mechanisms.

2.3. Compression after impact test

Compression tests were conducted on impacted andnon-impacted specimens using a modi®ed rig developed

Fig. 3. Schematic of drop weight impact test rig.

Fig. 1. Cross-sectional micrographs of non-interleaved and interleaved specimens: (a) non-interleaved; (b) 2210 interleaved.

Table 1

Interleaf details

Designation Material Manufacturer Moulded thickness

(mm)

Melting temperature

(°C)

XAF2210 Polyle®n XIRO 0.030 145

XAF2210P Polyole®n XIRO 0.030 145

XAF2065 Polyole®n XIRO 0.039 125

1a8s18 Copolyamide Protechnic 0.018 145

Ultem 1000 Polyetherimide General Electric 0.048 225

Fig. 2. Interleaf structure.

754 A. Duarte et al. / Composite Structures 47 (1999) 753±758

at the CRC-ACS, and following the CRC-ACS CAI testspeci®cation [7]. A schematic of the rig is shown in Fig. 4.The CAI tests were conducted at a displacement rate of0.5 mm/min and were concluded at the ®rst onset offailure.

3. Results

3.1. Low-velocity impact

The ole®n ®lm interleaves (XAF2210) provided astrong bond at the interleaf/epoxy interface. These in-terleaves restricted delamination propagation at the in-terface and changed the formation and distribution ofinterlaminar stresses under impact. As shown in Fig. 5,the XAF2210 interleaves produced a signi®cant reduc-tion in projected damage area at all impact energies(�55% reduction at 6 J/mm), with the threshold energyfor damage initiation increased to 2 J/mm. At this im-pact energy of 2 J/mm, the non-interleaved specimenresulted in an extrapolated damage area of around 200±250 mm2. The perforated ®lm (XAF2210P), producedsimilar results, with slightly higher damage areas com-pared to the non-perforated ®lm.

The XAF2065 interleaf produced only a slight im-provement in damage resistance over the non-inter-leaved (�15% at 6 J/mm), with the initiation energyincreased to approximately 1 J/mm. At this impact en-ergy level, 100 mm2 damage area was produced in thenon-interleaved specimen. The behaviour of theXAF2065 interleaved composite was very di�erent tothat using the XAF2210 ®lm interleaves because of thelarge amount of matrix resin at the interface dominatingthe stress±strain behaviour of the specimen underimpact.

The 1a8s18 interleaves resulted in an increase indamage area, particularly at higher impact energies. Itwas suggested that this was due to the interleaf havinga very open web structure, allowing cracks to propa-gate easily between the gaps in the thermoplastic.Another reason for the lack of delamination suppres-sion was the poor bond between the copolyamide in-terleaf and the epoxy resin, possibly providing weakpoints for the initiation of matrix cracks. A similarbehaviour was shown using 1a8s18 interleaved RTMcomposites in a paper previously published by theauthors [8].

The polyetherimide interleaf material was chosen forits high shear modulus, to overcome problems with lackof lateral support under compression loading associatedwith the low modulus ole®n interleaves (see Section 3.2).The results presented in Fig. 6 show a substantial in-crease in impact resistance, with damage initiation en-ergy increased to 2.7 J/mm. At this impact energy, thenon-interleaved specimen had a projected damage areaof approximately 250±300 mm2. The improvement wasnot as signi®cant at the higher impact energies, with 5 J/mm extrapolated as a damage energy at which no im-provement was realised. The proposed reason for thereduced e�ect of interleaving at higher impact energies isthe increased localised ®bre breakage. The impactorbegan penetrating the impact surface of the non-inter-leaved specimen at around 3±4 J/mm. It will alsobe shown later that the reduction in CAIS of the

Fig. 4. Schematic of CAI test rig.

Fig. 5. Damage area (non-interleaved and various interleaved).

A. Duarte et al. / Composite Structures 47 (1999) 753±758 755

non-interleaved laminate was minimal after 3 J/mm(saturation energy). Hence, a large proportion of theimpact energy was absorbed by breakage of ®bres in andaround the impacted site, reducing the e�ectiveness ofthe interleaf material.

The micrographs in Fig. 7 show cross-sections of anultem-interleaved specimen impacted at 3 J/mm. It isseen that crack propagation occurred preferentiallythrough the lamina, rather than at the ®bre/matrix in-terface. This indicated a signi®cant increase in the bondstrength at the interface, and an increase in interlaminarfracture toughness of the interleaved composite. The

second ®gure shows numerous transverse cracksthrough the lamina, inclined at 45° to the lamina. Thenoticeable phenomenon here is the suppression of del-amination formation at the interfaces, with relativelyfew transverse cracks forming delaminations. Withthose that do form delaminations, the propagation dis-tance is very small, indicating increased resistance todelamination at the interface.

3.2. Compression after impact

The di�erent types of interleaf resulted in very dif-ferent CAI strengths, as shown in Fig. 8. The reductionin undamaged compression strength was very severewith the XAF2210 ole®n interleaves (almost 65% re-duction), particularly due to the separation of the 0°plies. The low compression strength of these specimenswas due to ®bre microbuckling at the edge of the lamina.This was due to a lack of out-of-plane support for the®bres under compression loading, as the ole®n interleafhad a very low shear modulus. The failure of theselaminates is seen in the micrograph in Fig. 9. This wasalso described by Evans and Masters [9], indicating ®bremicrobuckling would occur as the shear modulus of theinterleaf material decreased. Further reductions in CAISwere evident only at very high energy impacts. Thediscrete perforations in the XAF2210P specimens made

Fig. 6. Damage area (non-interleaved and ultem interleaved).

Fig. 7. Intra-tow cracks and transverse cracks (ultem interleaved ± 3 J/mm).

Fig. 8. CAIS (non-interleaved and various interleaved).

756 A. Duarte et al. / Composite Structures 47 (1999) 753±758

no di�erence to the CAIS results. The XAF2065 resultedin slightly higher undamaged and low-energy impactcompression strength. This was due to the open netproviding larger resin regions at the interface to supportthe ®bres and a shorter buckling length. The 1a8s18interleaves seemed to indicate a tolerance to very low-energy impacts, with no damage reduction seen at1J/mm. However, the strength dropped suddenly at2J/mm, to a level similar to the non-interleaved.

As seen in Fig. 10, the introduction of ultem inter-leaves produced a very small reduction in compressionstrength (7%) at 1 J/mm impact. In contrast, the non-interleaved laminate su�ered a 30% reduction in com-pression strength for a similar impact. This was similarat higher impact energy, however the damage toleranceimprovement reduced as the impact energy increased.The last result recorded was a compression strengthreduction of 47% at 4.1 J/mm compared to a 60% re-duction at 4.3 J/mm for the non-interleaved composite.

This is a signi®cant result, indicating the damage toler-ance of quasi-isotropic tape laminates can be increasedto a large degree with high shear modulus thermoplasticinterleaving.

4. Conclusions

The di�erent types of interleaf material investigatedo�er di�erent property tailoring options for interleavedRTM composites. Low modulus ole®n interleaves weresuccessfully used to increase impact resistance, shown bya decrease in projected damage area. However, this wasaccompanied by a reduction in compression strength,due to a lack of lateral support for the ®bres. Therefore,this reduction in visible damage area may be of use inlightly or non-loaded non-structural components only.

For structural components, the compression strengthmay be improved using a higher shear modulus interleafmaterial. This was shown using a polyetherimide ®lm,which produced reductions in damage area, and signif-icant improvements in compression after impactstrength.

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