Composites Science and Technology 70 (2010) 272–283

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Composites Science and Technology

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The static and high strain rate behaviour of a commingledE-glass/polypropylene woven fabric composite

Kevin A. Brown *, Richard Brooks, Nicholas A. WarriorDivision of Materials, Mechanics and Structures, Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD, United Kingdom

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 August 2009Received in revised form 22 October 2009Accepted 25 October 2009Available online 6 November 2009

Keywords:High strain rate effectsA. Textile compositesB. Mechanical propertiesB. Stress/strain curvesD. Fractography

0266-3538/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.compscitech.2009.10.018

* Corresponding author. Tel.: +44 (0) 1159513820;E-mail address: kevin.brown@nottingham.ac.uk (K

In this paper the effect of strain rate on the tensile, shear and compression behaviour of a commingled E-glass/polypropylene woven fabric composite over a strain rate range of 10�3–102 s�1 is reported. Thequasi-static tests were conducted on an electro-mechanical universal test machine and a modified instru-mented falling weight drop tower was used for high strain rate characterisation. The tensile and com-pression modulus and strength increased with increasing strain rate. However, the shear modulus andstrength were seen to decrease with increasing strain rate. Strain rate constants for use in finite elementanalyses are derived from the data. The observed failure mechanisms deduced from a microscopic studyof the fractured specimens are presented.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

There has been a significant increase in the application of ther-moplastic composites (TPC) in the automotive industry. TPCs arebeing selected for their high stiffness and strength to weight ratio,high damage tolerance, high energy absorption capability andrecyclability. Furthermore, TPCs offer the potential for rapid man-ufacturing with low cycle times that allow for medium to high vol-ume production. The application of TPCs was initially restricted tosemi-structural components such as front-end panels, spare wheelwells, underbody closures and door panels [1]. However, morestructural applications are emerging, such as vehicle front endand crash structures [2,3]. These components are likely to sustainhigh strain rate loading during a crash event. Therefore, it is impor-tant to understand rate effects in these materials.

Today, numerical simulations are used extensively in vehicledesign and crashworthiness assessment. Accurate material modelsthat can reliably predict the elastic and damage response of TPCsunder both quasi-static and high velocity loads are required for de-sign and structural analysis. These material models need reliableexperimental materials data if they are to simulate accurately thestructural response.

However, experimental studies on the high strain rate responseof composites do present difficulties. Sierakowski [4], Barre et al.[5] and Hamouda and Hashmi [6] have reviewed a large volumeof literature on strain rate effects on the mechanical properties of

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composites. These reviews show that most of the high strain ratecharacterisation studies have focused on thermoset compositeswith epoxy and polyester matrices reinforced mainly with glassand carbon fibres. In contrast, few attempts have been made tostudy the dynamic mechanical properties of TPCs.

Todo et al. [7] conducted tensile testing of plain weave carbon/polyamide-6 (CF/PA6), carbon/modified polyamide-6 (CF/mPA6),glass/polyamide-6 (GF/PA6) and glass/modified polyamide-6 (GF/mPA6) composites in a servo-hydraulic machine at strain ratesranging from 10�2 to 40 s�1. They observed that, typically, the ten-sile strength and failure strain increased with an increase in strainrate with the exception of GF/mPA6 whose mechanical propertiesremained constant at strain rates above 10�2 s�1. Tensile testswere also performed by Vashchenko et al. [8] on glass/polyamidecomposites within the strain rate range of 10�3–105 s�1. They re-ported a linear relationship between the tensile strength and thelog of strain rate. Papadakis et al. [9,10] studied the effect of strainrate on the tensile and shear properties of a continuous glass fibre/polypropylene composite (PlytronTM) at various crosshead speeds(5, 50 and 500 mm min�1). They found that the Young’s modulusincreased with increasing loading rate while the failure straindecreased. The shear strength was reported to increase while theshear modulus decreased with increasing strain rate. Both the ten-sile strength and shear failure strain were observed to be rateinsensitive. Bai et al. [11] studied the effect of strain rate on tensileproperties of glass bead/high density polyethylene (HDPE) com-posites. Both the Young’s modulus and tensile strength increasedwith increasing strain rate. Kawata et al. [12] investigated theinfluence of strain rate on the tensile properties of graphite short

Fig. 1. Schematic of TwintexTM commingled balanced 1:1 woven fabric compositeand the material coordinate system.

K.A. Brown et al. / Composites Science and Technology 70 (2010) 272–283 273

fibre reinforced nylon 6,6 composite materials at strain rates rang-ing from 10�3 to 2000 s�1. They noted that the tensile strength in-creased with increasing strain rate. Weeks and Sun [13]characterised the strain rate dependence of a AS4/PEEK (APC-2)thermoplastic composite using a split Hopkinson pressure bar. Bal-anced angle ply laminates with fibre orientations of [90]64,[±30]16s, [±45]16s, and [±60]2s were tested at strain rates rangingfrom 100 to 1000 s�1. The material was found to exhibit significantstrain rate sensitivity. Peterson et al. [14] investigated the tensileproperties of a discontinuous fibre reinforced styrene-maleic anhy-dride (S/MA) composite at strain rates ranging from 1.67 � 10�3 to6 s�1. They reported an increase in the strength and stiffness withincreasing strain rate while the strain to failure decreased. McK-own and Cantwell [15] characterised the effect of strain rate(10�4–10 s�1) on the tensile properties of a self-reinforced polypro-pylene composite. The results indicate an increase in the initialelastic stiffness, yield strength and maximum strength withincreasing strain rate. However, the strain to failure reduced withincreasing loading rate. Recently, Bonnet [16] reported on the char-acterisation of the mechanical properties of a glass/polypropylenecommingled woven fabric unbalanced 4:1 weave composite(TwintexTM) and a short glass fibre reinforced polypropylene com-posite (PP–FVC) at strain rates ranging from 10�1 to 100 s�1. Theresults show that for TwintexTM the average tensile and shear mod-ulus decreased with increasing strain rate while the tensile andshear strength increased. The PP–FVC exhibited marginal rate sen-sitivity. There was a high level of data scatter which was partlyattributed to the test setup and inertial effects.

Although several different TPC materials are reported on in theabove literature, the general conclusion is that both tensile stiff-ness (Young’s modulus) and tensile strength both increase withstrain rate. From the very limited studies on shear properties, theindications are that, whereas shear strength shows an increasewith strain rate, shear modulus reduces.

This paper reports on the experimental characterisation of themechanical properties (tensile, compression and shear) of acommingled woven fabric glass fibre reinforced polypropylenethermoplastic composite for strain rates ranging from quasi-staticto above 100 s�1. The effect of strain rate on the mechanical prop-erties and the corresponding micro and macro scale failure modesare discussed.

The results from this experimental program can be used todevelop and validate dynamic material models required in explicitfinite element codes (e.g. LS-DYNATM and PAM-CRASHTM) for accu-rate structural analysis and design.

2. Experimental procedure

2.1. Material and manufacture

The material system used in this study is a commingled E-glass/polypropylene woven fabric composite, balanced 1:1 twill weavewith a nominal 60% fibre weight fraction. This weight fractionhas been verified in a separate study by Liu [17]. The material issupplied by OCV Reinforcements (formerly Saint Gobain Vetrotex)under the tradename TwintexTM, with product code T PP 60 745 AF.Fig. 1 illustrates the commingled roving and fabric architecture ofthe material.

A xyz Cartesian coordinate system was used to define theprincipal material coordinate system as depicted in Fig. 1. For thein-plane orthogonal directions, the x-axis is aligned with the warpfibres, while the y-axis is aligned to the weft fibres. The z-axis isaligned perpendicular to the plane of the material (the through-thickness direction). It was assumed that the in-plane materialproperties were equal because of the balanced, symmetric nature

of the TwintexTM woven fabric. Therefore, all in-plane tension andcompression tests were conducted in only the x-direction.

A non-isothermal compression moulding process was used tomanufacture all specimens in this study. The optimal processingparameters were taken from Wakeman et al. [18]. Preconsolidatedsheets of TwintexTM were stacked ply-by-ply in a 4.5 kW infraredoven and preheated at a temperature of 200 �C. The stack was thenrapidly transferred to a flat plaque heated tool (80 �C) which is in-stalled in an up stroking hydraulic press. The tool is closed and apressure of 16 bar is applied for 60 s. Test specimens were cut fromthe flat plaque laminates (260 � 120 mm) with fibre orientations[0/90] for tension and compression and [±45] for shear.

2.2. Tensile test and specimens

Quasi-static tensile tests were conducted according to ASTMstandard D3039 [19]. The tests were conducted on an InstronTM

electro-mechanical universal test machine at a crosshead speedof 5 mm min�1. The specimen strain response was determined bya 50 mm gauge length extensometer with a maximum strain capa-bility of 10%.

Dynamic tensile tests were conducted in a modified RosandTM

instrumented falling weight drop tower using a specially designedfixture (see Fig. 2). A crosshead mass of 100 kg was used in the dy-namic tensile tests. The tests were conducted at speeds of 3, 5 and7 ms�1 which corresponded to strain rates of 36, 50 and 70 s�1,respectively. The strain rate during the tests was derived fromthe gradient of the strain–time history data.

A special test fixture, developed at The University of Notting-ham [20], was used for the dynamic tensile tests (see Fig. 3). Thespecimen is clamped at each end between steel grips. The top gripis directly bolted through the load cell to the fixed carriage. A mov-ing carriage is supported by the lower grip and is guided by twosteel rods through two linear bearings on one side and two nylondowels on the other side. The drop tower striker imparts a loadon the moving carriage which loads the specimen in tensionthrough the lower grip as it travels downward.

Load was measured with a Kistler 9051A piezoelectric load cell.Strain data was recorded with Kyowa 120 Ohm strain gauges (KFG-20-120-C1), having a 20 mm gauge length. A single gauge wasbonded centrally on the specimen using cyanoacrylate adhesive.The strain gauge length was selected to be greater than the10 mm square characteristic repeating unit cell of the TwintexTM

weave. The strain gauge was connected to a quarter bridge circuit.Both load cell data and strain gauge readings were recorded at asample rate of 40 kHz.

High strain rate data is often accompanied by high frequencyoscillations caused by vibration and reflected shock waves super-

Fig. 2. Schematic of modified RosandTM instrumented falling weight drop tower.

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imposed on the load-time histories obtained from the load cell [5].Subsequently, a Butterworth filter with a cut-off frequency of600 Hz was used to treat the raw signal for all the dynamic testsin this study. This was done to increase data interpretation andreadability and improve comparability between static and highstrain rate test results.

The quasi-static tensile specimens were straight sided, 250 mmlong � 25 mm wide, with a nominal thickness of 4 mm. There wasa 150 mm gauge length between the grips. The high strain ratespecimens were also straight sided, 180 mm long � 25 mm wide,with a nominal thickness of 2.5 mm. For these there was a100 mm gauge length between grips. All tensile specimens had a

Fig. 3. (a) Picture of dynamic tensile test fixture (b) schematic diagram of dynamic tensilfixed frame, (6) top grip, (7) nylon dowels, (8) specimen, and (9) bottom grip.

[0/90] stacking sequence with eight plies for quasi-static speci-mens and five plies for dynamic specimens. The reduced thicknessof the dynamic specimens is necessary to ensure complete failureduring testing [21].

2.3. Shear test and specimens

The quasi-static in-plane shear test was based on the tensiletest but with a ±45� laminate as described by ASTM standardD3518 [22]. The same test equipment and fixtures as describedabove for the quasi-static and dynamic in-plane tensile tests wereused for the in-plane shear tests.

Strain data was recorded with bi-axial 0–90� T-rosette Kyowa120 Ohm strain gauges (KFG-10-120-D16), having a 10 mm gaugelength. A single bi-axial gauge was bonded centrally on the speci-men in a [0/90] orientation using cyanoacrylate adhesive.

The dimensions of the shear specimens are the same as the qua-si-static and dynamic tensile specimens except the shear speci-mens have a nominal thickness of 3.5 mm (eight plies) and a[±45] layup.

2.4. Compression test and specimens

A special compression fixture, shown in Fig. 4, was used forquasi-static and dynamic compression tests. The fixture is basedon a design by Montiel and Williams [23], later modified by Fernie[20]. The fixture consists of a T-shaped impactor that is guided bytwo closely toleranced vertical channels in a steel supportingblock. The specimen is clamped between two steel end-caps andplaced directly under the impactor. Montiel and Williams [23] re-corded the dynamic compression load in the specimen by placinga load cell above the T-shaped impactor. Fernie [20], however,found that placing the load cell directly below the specimenresulted in more accurate load measurements and reduced noisein the test data.

For quasi-static testing, the compression fixture was installed inan InstronTM electro-mechanical universal test machine and speci-mens were loaded at a crosshead speed of 5 mm min�1. High strainrate compression tests were conducted in the drop tower using acrosshead mass of 40 kg at speeds of 3, 5 and 7 ms�1, correspond-ing to strain rates of 56, 78 and 126 s�1, respectively.

e test fixture: (1) guide rails, (2) linear bearings, (3) moving carriage, (4) load cell, (5)

Fig. 4. (a) Picture of compression test fixture (b) schematic diagram of compression test fixture: (1) end caps, (2) supporting block, (3) T-shaped impactor, (4) specimen, and(5) load cell.

K.A. Brown et al. / Composites Science and Technology 70 (2010) 272–283 275

In both quasi-static and dynamic tests, compression strain datawas recorded with a single Kyowa 120 Ohm strain gauge (KFG-10-120-C1), having a 10 mm gauge length. The gauges were bondedcentrally on the specimen using cyanoacrylate adhesive. Previouswork by Fernie [20] has shown that no buckling effects take placewith these specimens, thus a single gauge was deemed appropriatefor measuring the compression strain.

The compression specimens were straight sided, 40 mmlong � 25 mm wide, with a nominal thickness of 4 mm. The spec-imen gauge length between grips was 30 mm and the stacking se-quence is [0/90]. Close tolerances were specified for flatness andorthogonality on the specimen ends so as to minimise out-of-planeloading and ensure uniform load introduction.

2.5. Microscopic analysis

Optical microscopy and scanning electron microscopy (SEM)were used to investigate local failure mechanisms in the fracturedcomposite specimens. For optical microscopic analysis, sample sec-tions were cut from the failure region of the fractured specimens.The samples were cast in polyester resin and polished in six stagesranging from 200 to 4000 grit abrasive wheel and 0.1 lm aluminaparticles. These prepared samples were then examined using aZeiss Axiolab optical microscope with a CCD camera attachmentgiving a resolution of 1 lm. In SEM, sample sections from thespecimen fracture zone were bonded to a sample holder and goldplated before scanning.

Table 1Strain rate constants from Eqs. (1) and (2) for the modulus and strength propertiesover the strain rate range in this study.

Mechanical property Strain rate parameters Eqs. (1) and (2)

Tensile modulus CERT ¼ 0:026

Tensile strength CSRT ¼ 0:089

Shear modulus CERT ¼ �0:187

Shear strength CSRT ¼ �0:061

Compression modulus CERT ¼ 0:251

Compression strength CSRT ¼ 0:181

3. Results and discussion

3.1. Strain rate effects on mechanical properties

The effect of strain rate on the mechanical properties of materi-als is typically modelled by strain rate dependent functions for theelastic modulus and strengths. The most popular functions includethe Cowper–Symonds power law [24] and Johnson–Cook’s loga-rithmic law [25].

Most recently Yen and Caiazzo (Y–C) developed logarithmicfunctions for strain rate effects in composites as follows [26,27]:

ERT ¼ E0 1þ CERT ln

_�e_e0

!ð1Þ

SRT ¼ S0 1þ CSRT ln

_�e_e0

!ð2Þ

where ERT are the rate adjusted moduli and E0 is quasi-static mod-ulus, SRT are the rate adjusted strengths, S0 is the quasi-staticstrength, _�e0 ¼ 1 is the reference strain rate and _�e are effective strainrates. CE

RT and CSRT are the strain rate constants for stiffness and

strength, respectively.The Y–C equations have been implemented in the MAT162

composite material model which is available in the commercialLS-DYNATM finite element software [28]. In order to support futuremodelling work and validate the Y–C equations, the strain rateconstants CE

RT and CSRT will be obtained in the present study by fit-

ting Eqs. (1) and (2) with the high strain rate test results for thetension, shear and compression tests (see Table 1).

3.2. Tensile mechanical properties

Fig. 5 shows the typical in-plane tensile stress–strain responsefor TwintexTM at different strain rates. Under quasi-static loadingthe stress–strain response is approximately linear elastic up tothe maximum stress point followed by abrupt failure at a strainof 2.18%. At a strain rate of 36 s�1, the initial stress–strain responseleading up to the peak stress is relatively similar to the quasi-staticresults. However, in contrast, the post peak stress response is veryductile with a final failure strain that is 4 times higher than at thequasi-static rate. Final failure strain is defined here as the strain atwhich the material exhibits complete cross-sectional failure. Athigher strain rates, between 50 and 70 s�1, the material stress–strain response is highly non-linear with premature yielding fol-lowed by strain hardening and then maximum peak stress andstrain softening. In addition, the final failure strain is three timeslarger than the quasi-static result. This appearance of a non-linearstress–strain behaviour at dynamic loading rates along withincreasing failure strain with increasing strain rate has also beenobserved by Bonnet [16]. These phenomena may be attributed to

Fig. 5. Typical in-plane tensile stress–strain curves for TwintexTM at quasi-static and high strain rates.

Table 2Mechanical properties for TwintexTM at quasi-static and high strain rates.

Strain rate (s�1) Tensile modulus (GPa) Standard deviation (GPa) Tensile strength (MPa) Standard deviation (MPa)

Quasi-static 14.46 0.92 269.03 10.8036 13.19 0.83 266.75 11.3250 17.51 1.22 382.40 23.2770 16.08 3.89 391.88 74.64

Shear modulus (GPa) Shear strength (MPa)Quasi-static 1.79 0.38 30.55 3.0933 0.33 0.15 14.10 35.2770 0.47 0.10 25.12 4.7491 0.22 0.07 24.17 20.21

Compression modulus (GPa) Compression strength (MPa)Quasi-static 14.55 0.76 178.14 19.7756 33.30 10.14 317.27 41.5978 36.09 5.46 316.29 36.82126 28.35 6.01 333.08 28.35

Fig. 6. The strain rate effect on the in-plane tensile modulus of TwintexTM (error bars indicate ±1 standard deviation).

276 K.A. Brown et al. / Composites Science and Technology 70 (2010) 272–283

Fig. 7. The strain rate effect on the in-plane tensile strength of TwintexTM (error bars indicate ±1 standard deviation).

K.A. Brown et al. / Composites Science and Technology 70 (2010) 272–283 277

the change in failure modes, with more progressive damage accu-mulation occurring along the gauge length at higher strain rates asdiscussed below.

The tensile modulus and strength properties at different strainrates are presented in Table 2. Figs. 6 and 7 illustrate the effectof strain rate on the tensile modulus and strength, respectively,on a logarithmic scale. The tensile modulus and strength are rela-tively insensitive to increases in strain rate up to 36 s�1. However,at strain rates greater than 36 s�1 the tensile strength increases sig-nificantly up to 46% at the highest strain rate. The tensile modulusfollows a similar trend although not so pronounced, with an in-crease of 11%. The best fit trend lines obtained from the Y to C func-tions, (Eqs. (1) and (2)) are also drawn but, due to the insensitivityto strain rate at low strain rates less than 36 s�1, overestimate thedynamic tensile modulus and strength in this region.

Fig. 8. Typical quasi-static damage modes for tensile specimens: (a) photograph of fashowing matrix cracking and interfacial debonding (�10).

The increase in tensile modulus and strength is generally ex-plained by a combination of several factors including the viscoelas-tic nature of the polymeric matrix [29], i.e. stiffening withincreased rate of loading, and following Welsh and Harding [30],the woven structure and geometry of the composite. Plain-weavecomposites show a higher strain rate sensitivity than satin-weave,unidirectional and multi-directional composites because of thehigher interaction between the matrix and fibres [31]. Further-more, the rate sensitivity of the tensile strength can also be attrib-uted to the rate dependence of the glass fibres in fibre dominatedloading mode, as suggested by Okoli [32].

Under quasi-static loading damage occurred around a localisedthrough-thickness shear fracture plane at an angle of about 45� tothe axis of loading as shown in Fig. 8a. This may be attributed tothe woven nature of the composite which tends to result in

iled specimen, (b) optical micrograph of damage zone and (c) optical micrograph

Fig. 9. Typical high strain rate damage modes for tensile specimens (70 s�1). (a) Photograph of failed specimen, (b) optical micrograph of damage zone and (c) opticalmicrograph showing matrix cracking (�20).

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through-thickness shear failure initiation across an axial fibre bun-dle [31]. The quasi-static macroscopic damage mechanisms gener-ally included matrix cracking, localised warp fibre fracture, weftfibre pull-out, and delamination along the middle plies. At highstrain rates, the damage accumulates along the entire length ofthe specimen gauge area (see Fig. 9a). As strain rate increased,significant matrix cracking, fibre fracture and weft fibre bundlepull-out was observed. In addition, the laminate delaminated alongalmost every ply.

Optical micrographs of the damage region in the fractured ten-sile specimens at quasi-static strain rates are shown in Fig. 8b–c.Matrix cracking was observed in the weft fibre bundles as depictedin Fig. 8b. These cracks propagate in an opening mode normal tothe in-plane axis due to normal stresses acting in the direction ofthe applied load. Some of these cracks propagate towards thetow interfaces resulting in interfacial debonding and interply

Fig. 10. Typical in-plane shear stress–strain curves fo

delamination as shown in Fig. 8c. Similar, but more pronounced,micro-damage mechanisms were observed in the fracture regionof the post test specimens at high strain rates (see Fig. 9b–c).

3.3. Shear mechanical properties

Typical shear stress–strain plots for quasi-static and high strainrate loading are shown in Fig. 10. Under quasi-static loading theshear stress–strain behaviour is dominated by the matrix materialand exhibits significant non-linearity over a large strain range. Athigh strain rates, inelastic deformation is less pronounced as theinitial elastic response is followed by yielding, steep strain harden-ing to peak stress and finally, strain softening. This behaviour isconsistent with findings by Bonnet [16] who concluded that theplasticity threshold reduces with increased strain rate. This maybe attributed to the time dependent nature of damage accumula-

r TwintexTM at quasi-static and high strain rates.

Fig. 11. The strain rate effect on the in-plane shear modulus of TwintexTM (error bars indicate ±1 standard deviation).

K.A. Brown et al. / Composites Science and Technology 70 (2010) 272–283 279

tion. At static loading rates the laminate has time to distribute theload and undergo steady damage accumulation. However, at highstrain rates the material has less time to respond and fails afterlimited yielding.

The in-plane shear modulus and strength properties at differentstrain rates are presented in Table 2. The effect of strain rate on thein-plane shear modulus is depicted in Fig. 11. The Y–C logarithmiclaw trend line shows a significant 80% reduction in shear modulusfrom quasi-static to dynamic rates although the line is not a goodfit to the data at intermediate rates. This reduction in modulus hasalso been observed by Bonnet [16] and is an important observation,if difficult to explain. For a matrix dominated loading mode such asshear, one would expect viscoelastic effects in the matrix to resultin an increase in shear modulus, not the observed reduction. A pos-sible explanation could be that, at dynamic loading rates, localiseddamage in the interface regions could trigger at very low shearloads and exhibit itself as a damage-induced modulus reduction.

Fig. 12. The strain rate effect on the in-plane shear strength

However, without further detailed study of the onset of damagemechanisms in this loading mode, this can only be a hypothesis.

Fig. 12 shows the effect of strain rate on the in-plane shearstrength. Shear strengths for both quasi-static and dynamic ratesof loading are taken as the peak stress point in the stress–straincurves. The Y–C trend line shows a reduction of 21% in the averageshear strength from quasi-static to the maximum dynamic rate,although, as for the modulus, it is not a good fit to the intermediaterate data point which has high scatter. Although in general, theshear strength data does show a high degree of scatter, a reducingtrend is apparent. It should be pointed out that this trend contra-dicts results presented in the literature where shear strength hasbeen reported to marginally increase with increasing strain ratein [16].

Photographs of a post test in-plane shear specimens for quasi-static and high strain rates are shown in Figs. 13 and 14, respec-tively. For both the quasi-static and high strain rate specimens,

of TwintexTM (error bars indicate ±1 standard deviation).

Fig. 13. Typical quasi-static damage modes for shear specimens: (a) photograph of failed specimen, and (b) optical micrograph of damage zone showing trans-ply matrixcracking (�10).

Fig. 14. Photograph showing typical fracture zone for high strain rate shear specimen.

280 K.A. Brown et al. / Composites Science and Technology 70 (2010) 272–283

the major macroscopic failure modes were matrix cracking, fibrepull-out, fibre bundle pull-out and delamination. Under quasi-sta-tic loading, fibre reorientation (scissoring) in the direction of theapplied load was observed which induced interlaminar stressesat the ply interfaces resulting in delamination [33]. Final failure oc-curs along a V-shaped (±45� fibre axis) in-plane failure surface, i.e.on the planes of maximum shear. In general, as the loading rate in-creases, this line of failure becomes more clearly defined indicatinga cleaner fracture compared to the more disparate fibre and fibrebundle pull-out at the lower rates. This type of failure mode hasalso been reported by Papadakis [10] and Bonnet [16].

An optical micrograph of the damage region of a quasi-static in-plane shear specimen is shown in Fig. 13b. Trans-ply matrix cracksspanning across the ply thickness were observed along withdelamination. The same microscopic failure mechanisms whereobserved at high strain rates. These trans-ply cracks are causedby in-plane normal stresses with the cracks propagating in anopening mode [33]. These in-plane normal stresses develop per-pendicular to the fibre orientation and are at a maximum valuein the surface and middle plies [33].

3.4. Compression mechanical properties

Fig. 15 shows the typical in-plane compression stress–strain re-sponse for TwintexTM at different strain rates. Under quasi-staticloading, the material response is approximately linear elastic up

to the maximum stress point followed by abrupt failure. The highstrain rate stress–strain response was characterised by an initialelastic region, then yielding followed by a steep hardening up tothe peak stress and post-failure softening. Failure occurs in bothquasi-static and dynamic tests at relatively low strains of about1–2%.

The in-plane compression modulus and strength properties atdifferent strain rates are presented in Table 2. The effect of strainrate on the compression modulus is shown in Fig. 16. The averagecompression modulus increased by a dramatic 95% between quasi-static and 126 s�1 strain rates with all the high strain rate valueslying within the dynamic experimental scatter band. Fig. 17 showsthe effect of strain rate on the compression strength. The averagecompression strength increased by a significant 87% between qua-si-static and 126 s�1 strain rates. The increase in compressionmodulus and strength may be attributed to the viscoelastic effectsin the matrix materials and the time dependent nature of damageaccumulation [29,34]. The Y–C strain rate equations fit the exper-imental results well for the dynamic compressive modulus andstrength over the entire strain rate range in this study (see Fig. 17).

Photographs of the failure regions of the compression speci-mens for quasi-static and high strain rates are shown in Figs. 18and 19, respectively. The failure mode of the specimens under bothquasi-static and high strain rates are similar; although greaterdamage occurred at high strain rates. Failure was primarily bylocalised through-thickness shear. In addition, at high strain rates,

Fig. 16. The strain rate effect on the in-plane compression modulus of TwintexTM (error bars indicate ±1 standard deviation).

Fig. 15. Typical in-plane compression stress–strain curves for TwintexTM at quasi-static and high strain rates.

Fig. 17. The strain rate effect on the in-plane compression strength of TwintexTM (error bars indicate ±1 standard deviation).

K.A. Brown et al. / Composites Science and Technology 70 (2010) 272–283 281

Fig. 18. Typical quasi-static damage modes for compression specimens: (a) photograph of failed specimen, (b) optical micrograph of damage zone, (c) optical micrographshowing fibre bundle shear fracture (�10), and (d) SEM micrograph showing the shear fracture surface of the fibres.

Fig. 19. Typical high strain rate damage modes for tensile specimens (126 s�1). (a)Photograph of failed specimen, and (b) optical micrograph of damage zone.

282 K.A. Brown et al. / Composites Science and Technology 70 (2010) 272–283

delamination (splitting) was also observed in the outer plies. Theorientation of the through-thickness shear failure bands rangefrom 36� to 55� for all specimens inspected, with the average anglebeing 48�. This shear band type of failure can be attributed to thewoven nature of the composite material as suggested by Harding[35].

Optical micrographs of the damage region in a fractured com-pression specimen at quasi-static strain rates are shown inFig. 18b–d. Fig. 18b shows fibre micro-buckling which leads to fi-bre kinking followed by failure of the fibres and matrix throughthe thickness of the specimen along a distinct shear band fractureplane. The micro-buckling in the fibres may be attributed to local

premature failure of the surrounding matrix [29]. A detailed viewof through-thickness shear failure in the fibres and matrix is shownin Fig. 18c. Fig. 18d shows a high magnification SEM fractograph ofthe fibre shear fracture surface. Similar damage mechanisms wereobserved in the fracture region of the post test compression spec-imens at high strain rates, as shown in Fig. 19b. However, extensivedelamination is observed in this case which is due in part to thecontinued displacement of the impactor after specimen failure.These compression failure modes at both quasi-static and dynamicloading rates agree with trends reported for other woven compos-ites [35,36].

4. Conclusions

The effect of strain rate on the mechanical properties and dam-age modes of a commingled E-glass/polypropylene woven fabriccomposite has been reported for a strain rate range from 10�3 to102 s�1. Dynamic tests were conducted on a falling weight droptower with specially designed fixtures. Results show that the ten-sile and compression modulus and strength increased withincreasing strain rate. However, in contrast, the shear modulusand strength decreased as strain rate increased. It was concludedthat the strain rate effects in the material appear to be influencedprimarily by the matrix viscoelasticity, fibre–matrix interfacialproperties, the composite woven reinforcement architecture andthe time dependent nature of damage accumulation. However, fur-ther detailed study is required to explain the relationship betweendamage modes and the strain rate sensitivity of the mechanicalproperties.

The data obtained from this study can be used to develop ratedependent material models required in finite element codes foraccurate structural modelling and design of components madefrom E-glass/polypropylene commingled woven thermoplasticcomposites subjected to dynamic impact loads.

Acknowledgements

The authors would like to acknowledge support from the UKEngineering and Physical Sciences Research Council (EPSRC)through the Nottingham Innovative Manufacturing Research Cen-

K.A. Brown et al. / Composites Science and Technology 70 (2010) 272–283 283

tre (NIMRC). We would also like to thank OCV Reinforcements Ltd(formerly Saint-Gobain Vetrotex) for the supply of TwintexTM

material.

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