ANALYSIS OF THE MICROSTRUCTURE AND DEFORMATION OF WOVEN COMPOSITES USING MICROFOCUS X-RAY DIFFRACTION
Robert J Young 1, Stephen J Eichhorn 1, James A Bennett 1, Prasad Potluri 1 and Richard J Davies 2
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School of Materials, University of Manchester, M1 7HS, UK European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, 38043 Grenoble Cedex, France
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KEYWORDS: Textile composites, microstructure, X-ray diffraction, mechanical properties INTRODUCTION Unidirectional (UD) prepreg based laminates, consolidated in an autoclave, have been the principal material choice for the composites industry especially for aerospace application. There are concerns, however, about the damage tolerance and compression after impact (CAI) performance of UD composites, in addition to the high manufacturing and material costs. In recent years, the composites industry has been pursuing a cost reduction strategy through various affordable composites initiatives, and at the same time trying to improve damage tolerance. Textile preforms, in the form of interlaced or stitched tow assemblies, are prime candidates for replacing UD prepregs in order to reduce costs and improve damage tolerance. They can be used in both prepreg and dry forms. In the dry form, textiles are easy to handle, have unlimited shelf life and easy to drape over complex surfaces. However, interlaced textiles in the form of woven or braided structures have complex tow geometry leading to complex strain fields around interlacements. Additional complexity arises from the fact that tows deform readily during draping/forming and consolidation processes leading to further changes in local geometry and strain fields. RESULTS AND DISCUSSION Fig. 1 shows the experimental set-up on beamline ID13 of the ESRF [1] using a 5 µm diameter x-ray beam. A grid of 41×41 diffraction patterns were obtained over the region around a hole in a 4-harness satin weave aramid/epoxy composite. Each individual diffraction pattern contains a range of information about the embedded PPTA fibres as explained below.
Fig. 1: Experimental set-up for the microfocus x-ray diffraction analysis of an aramid composite (4-harness satin weave) showing a typical diffraction pattern of the crossing aramid fibres.
By plotting different analysis parameters according to their real-space collection position on the sample, their spatial variation can be correlated with the sample's local geometry and stress fields. For example, provided there is a sufficiently high angular difference, the superposition of different yarn orientations within the detector plane can be resolved separately. This allows information to be obtained from warp and weft yarns independently when studying woven composite geometries.
Fig. 2. Maps of average out-of-plane fibre tilt angle (in degrees) for longitudinal (left) and transverse (right) yarns within an undeformed satin weave composite with a central hole.
This capability is demonstrated in Fig. 2 for the average angle of yarn tilt out of the detector plane. This has been calculated from the ratio of meridional reflection intensities between opposing diffraction pattern hemispheres [1]. The example in Fig. 2 is shown for the woven composite specimen prior to deformation (i.e. unloaded). In this case, the average fibre tilt is calculated for both transverse (horizontal) and longitudinal (vertical) yarns within each diffraction pattern. This information is invaluable in modelling the mechanical properties of the composite. The shift in the position of the meridional reflections has also been used to map the local fibre strain in the woven composites. It is found that there is a periodic distribution of fibre strain in the structure and this gives a unique insight into deformation micromechanics [2]. CONCLUSIONS The use of synchrotron microfocus X-ray diffraction offers a unique insight into the analysis of the microstructure and deformation of woven composites. In particular it is possible to analyse both fibre tilt and orientation within the composite microstructure. It is found that local fibre strains are a function of both the off-axis angle and tow thickness. This information is being combined with finite element analysis to model accurately the dependence of fibre deformation upon fibre weave geometry in these textile-based structures. REFERENCES 1. Davies R.J., Riekel C, Bennett J.A., Eichhorn S.J., and Young R.J., “Probing the internal geometry of a woven composite during deformation using an x-ray microdiffraction imaging technique”, Applied Physics Letters, Vol. 91, 044102, 2007 2. Young R.J., Riekel C, Bennett J.A., Eichhorn S.J., Potluri P. and Davies R.J., “Analysis of fibre deformation in woven composites”, Journal of Materials Science, 2008, in press.