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  • Mechanics of Materials 42 (2010) 10041019

    Contents lists available at ScienceDirect

    Mechanics of Materials

    journal homepage: www.elsevier .com/locate /mechmat

    High strain rate characterisation of unidirectional carbon-epoxyIM7-8552 in transverse compression and in-plane shear using digitalimage correlation

    H. Koerber a, J. Xavier b, P.P. Camanho a,a DEMec, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugalb CITAB/UTAD, Engenharias I, Apartado 1013, 5001-801 Vila Real, Portugal

    a r t i c l e i n f o

    Article history:Received 26 February 2010Received in revised form 20 August 2010

    Keywords:Strain rate effectsComposite materialsYieldFractureMulti-axial loadingDigital image correlation

    0167-6636/$ - see front matter 2010 Elsevier Ltddoi:10.1016/j.mechmat.2010.09.003

    Corresponding author. Tel.: +351 225081716; faE-mail address: (P.P. Caman

    a b s t r a c t

    This paper presents an experimental investigation of strain rate effects on polymer-basedcomposite materials. Quasi-static and dynamic experiments at strain rates up to 350 s1

    were performed with end-loaded, rectangular off-axis compression and transverse com-pression specimens. The dynamic tests were performed on a split-Hopkinson pressurebar, where pulse shaping ensured early dynamic equilibrium and near constant strain ratesfor all specimen types. The in-plane strain field of the specimen was obtained via digitalimage correlation. With the high speed camera used for the dynamic tests, the failure pro-cess of the specimen was monitored and the fracture angle was measured. The strain rateeffect on modulus, yield, ultimate strength, strain to failure and on the in-plane shear prop-erties was studied. The experimental failure envelope for combined transverse compres-sion and in-plane shear loading was compared with the Puck failure criterion for matrixcompression and excellent correlation between experimental and predicted failure enve-lopes was observed for both strain rate regimes. The quasi-static and dynamic yield enve-lopes for combined loading are also presented.

    2010 Elsevier Ltd. All rights reserved.

    1. Introduction

    The increasing applications of polymer-based compos-ite materials in structures designed for, or exposed to,crash and impact scenarios, and the related need to simu-late the mechanical response of such structures under highstrain rate deformation, requires a reliable identification ofthe strain rate effect on the material behaviour. The studyof this strain rate effects has drawn significant attention bythe international research community and with advancesin experimental techniques it will continue to do so inthe future.

    Vogler and Kyriakides (1999) studied the nonlinearbehaviour of unidirectional AS4/PEEK in shear and trans-

    . All rights reserved.

    x: +351 225081445.ho).

    verse compression. A custom biaxial loading fixture wasdesigned to perform biaxial tests under three loading his-tory regimes: transverse compression in the presence ofconstant shear stresses, the shear response under the pres-ence of constant transverse compression and proportionalloading of transverse compression and shear. In addition,quasi-static and medium rate tests were performed forpure transverse compression, pure in-plane shear and neatPEEK resin. For the later experiments it was observed thatthe increase of the transverse and in-plane shear strengthwas similar to the increase of the neat resin strength.

    Hsiao et al. (1999) performed high strain rate testsfor the carbon-epoxy material system IM6G/3501-6 on adrop tower and split-Hopkinson pressure bar (SHPB) andreported results for dynamic transverse compressionmodulus, strength and ultimate strain as well as in-planeshear modulus and strength. It was stated that is was notpossible to monitor the failure process and the use of the

  • H. Koerber et al. / Mechanics of Materials 42 (2010) 10041019 1005

    Recovery SHPB, invented by Nemat-Nasser et al. (1991),was suggested.

    Hosur et al. (2001) performed high strain rate com-pression tests on a Recovery SHPB for carbon-epoxy inthe longitudinal and transverse material directions andfurther studied the dynamic compressive response ofcross-ply laminates. Compared to the trends found byHsiao et al. (1999), Hosur et al. (2001) reported a differentstrain rate behaviour for the unidirectional compressiontests.

    Tsai and Sun (2005) performed SHPB experiments with15, 30 and 45 off-axis compression specimens to studythe dynamic in-plane shear strength and failure strain ofunidirectional glass-epoxy S2/8552. The pure in-planeshear strength at r22 = 0 was extrapolated from the off-axistest data in the combined transverse compression/in-planeshear stress state.

    In a recent study, Kawai and Saito (2009) performedquasi-static and medium rate tension and compressiontests on a servo-hydraulic testing machine at elevated tem-perature using tabbed off-axis carbon-epoxy specimens. Adetailed discussion of the failure modes and fracture sur-face angles was given and modifications to well-knowncomposite failure criteria, such as Tsai-Wu, Tsai-Hill andHashin-Rotem, were proposed to consider the influenceof transverse compression on the in-plane shear strengthof unidirectional laminae.

    Lee et al. (2010) performed on optical study of loading-rate effects on the fracture behaviour of carbon-epoxyT800/3900-2 and reported an increase of the material frac-ture toughness for dynamic loading. The fracture tough-ness decreased with increasing degrees of anisotropy forboth quasi-static and dynamic loading.

    The objective of this work is to study the mechanicalproperties of the unidirectional carbon-epoxy materialsystem IM7-8552 which are most effected by the visco-plastic response of the polymer matrix, namely the com-pressive and in-plane shear properties. A detailed knowl-edge of the strain rate effects on the properties in themain material directions and on the material behaviourunder combined loading is required to validate and fur-ther develop existing composite constitutive models andfailure criteria. High strain rate experiments were per-formed on the SHPB from which the complete dynamicelasticplastic specimen stressstrain response can be ob-tained. To the authors knowledge, no published data ex-ists for the yield and failure envelopes of the present, orsimilar, material systems, at the strain rates consideredin this study. Digital image correlation (DIC) techniqueswere used for the quasi-static and high strain rate exper-iments to obtain the in-plane strain field over the entirespecimen surface. By using a high speed camera for thehigh strain rate experiment, fundamental assumptions ofthe SHPB technique were verified. Specimen failure wasmonitored and the angle of the fracture plane was identi-fied. The relevance of DIC for SHPB experiments was dem-onstrated in a recent study by Gilat et al. (2009). Acomprehensive review of the digital image correlationtechnique is given by Sutton et al. (1999) and Pan et al.(2009).

    2. Material, experimental procedures and datareduction

    2.1. Material and test specimens

    The unidirectional carbon-epoxy prepreg system Hex-Ply IM7-8552 was selected for the present study. Thisintermediate-modulus fibre and thoughened epoxy resinsystem is commonly used in the aerospace industry for pri-mary structure components. In accordance with the pre-preg curing cycle, 32-ply unidirectional plates weremanufactured on a SATIM hotpress. From these panels,off-axis compression specimens with fibre orientationangles h = 15, 30, 45, 60, 75 and transverse compres-sion specimens (h = 90) were cut on a water-cooleddiamond saw. All specimens have the same nominaldimensions of 20 10 4 mm3 and are in accordancewith the end-loading compression test standard ASTMD695 (2002). An in-plane aspect ratio of 2 was chosen toobtain a larger homogeneous strain field not affected byedge effects at the specimen end-surfaces. Prior to the testswith the DIC measurement system, a number of quasi-sta-tic transverse compression tests with back-to-back linearfoil strain gauges were performed to evaluate the influenceof bending within the relatively long specimen. The percentbending value was found to be within the limits of 10% asdefined by the test standard ASTM D3410 (2003). Hencethe DIC strain measurement from only one side was con-sidered valid for all successive tests. End-loaded compres-sion specimens were selected since complex test fixturesare not required and therefore the specimen is well suitedfor SHPB experiments. After cutting, the surface parallel-ism tolerances of all opposing surfaces were found to bewithin 0.02 mm and the loading surfaces were of goodquality. A further treatment of the loading surfaces wastherefore not performed.

    The specimens were prepared for DIC measurement byapplying a random black-on-white speckle pattern to thespecimen surface using aerosol spray painting. A finespeckle pattern was applied to the quasi-static specimenswhereas a slightly coarser pattern was used for the dy-namic specimens, considering the different image resolu-tions of the respective cameras.

    2.2. Quasi-static experimental setup

    Quasi-static tests were carried out on a standardINSTRON 4208 load frame at a constant displacement rateof 0.5 mm.min1. This corresponds to a quasi-static strainrate of _eqs 4 104 s1, considering the nominal specimenlength of 20 mm. The quasi-static test setup is shown inFig. 1 with a self-aligning loading setup similar to that usedby Tsai and Sun (2005). Polished tungsten-carbide (TC) in-sert