X-ray radiography and micro-computed tomography examination of damage characteristics in stitched composites subjected to impact loading

In this study, X-ray radiography and X-ray micro-computed tomography (RCT) are employed to characterise impact damage in stitched composites, paying particular attention to the influence of stitch density and stitch thread thickness. Laminated composites, stitched with varying stitch densities and stitch thread thicknesses, are subjected to low-velocity impact loading at various energy levels. X-ray radiography is used to observe in-plane matrix cracks and overall delamination propagation: whilst mu CT is exploited to examine cross-sectional views showing detailed through-thickness matrix cracks distribution and 3D delamination damage pattern. It is revealed that stitches act as crack initiation sites, due to the presence of weak resin-rich pockets around stitch threads, thus inevitably resulting in densely stitched composites having more stitch-induced matrix cracks upon impact loading. It is also concluded that specimens with higher stitch density and thread thickness are more capable of impeding delamination growth by effectively bridging delamination cracks and arresting crack propagation. mu CT evidently shows that 3D delamination pattern in densely stitched composite is more cylindrical in shape, whereas that of a moderately stitched or unstitched composite is more conical. (C) 2011 Elsevier Ltd. All rights reserved.