Damage in carbon fibre reinforced plastics (CFRP) due to impact loading is an extremely complex phenomenon that comprises of multiple failure mechanisms like intra-laminar matrix cracks, interlaminar delamination, fibre pull-out and fibre fracture. In stitched composites, impact damage behavior is further complicated by the presence of through-thickness stitching [1, 2], which not only favorably increases mode I/II interlaminar strength [3, 4], but also inevitably creates geometrical defects like weak resin-rich pockets around stitch threads and misalignment of in-plane fibres. Computational modeling has been used to simulate progressive damage effectively . However, the complexity of impact damage progression in stitched composites would need to be first understood and appreciated by physical experimental observations. In this study, quasi-static indentation (QSI) test is performed for the first time on stitched composites. QSI offers a good validation and comparison with low-velocity impact (LVI) test , and provides good understanding on damage progression in composite structures under impact loading. Damage initiation, propagation and ultimate failure are investigated due to the effect of stitching, particularly the influence of stitch density. Nondestructive evaluation (NDE) techniques namely ultrasonic c-scan analysis, x-ray radiography and xray micro computed tomography are employed to elucidate various damage mechanisms in stitched composites.
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