Effect of stitching on the flexure after impact behavior of thin laminated composites

The paper investigates the effect of stitching on the post-impact flexural performance of two classes of thin carbon/epoxy laminates. Unstitched and stitched laminated samples were first impacted with energies ranging between 1 J and 8 J by an instrumented drop-weight testing machine. The residual flexural properties of impacted samples were then assessed by three-point bending tests. The damage induced by impact and by post-impact bending was characterized in detail by visual inspection and penetrant-enhanced X-radiography. The results of the experimental analyses are illustrated and discussed to highlight the correlation between the residual flexural properties and the key fracture modes occurring in the laminates. The study shows that even though stitching is capable of improving the delamination resistance of the laminates, it may also drastically reduce their flexural strength. It is also seen that the residual post-impact flexural properties, which are controlled by the peculiar impact damage mechanisms of stitched and unstitched laminates, may be greatly degraded by impacts with energy above a critical threshold value.