Damage evaluation on advanced stitched composites under impact loads by means of image analysis and acoustic emission

Polymeric composites that are stitched reinforced show dissimilar behaviours with respect to regular ones. In this paper the mechanical characteristics and different behaviours of stitched reinforced polymeric composites after impact tests are investigated using an alternative approach. Several factors such as grid size, stitching scheme and voids content modify the composite performances and the interlaminar delaminating processes. If, on the one hand, the stitching technique is used to prevent delaminating damage phenomena, then, on the other hand, the process itself introduces a sort of damage into the layers during the assembled preform, that has to be taken into account. Delaminating processes due to impact loads are considered and the post-impact behaviours and damage tolerance are investigated. Impact tests were performed at several energy levels known to have an appreciable damage on the specimens. Damage evaluation analysis was performed on post-impacted specimens using different techniques: as a first step, using a digitalised image technique, the damaged area was measured and different zone shape was evaluated, further compression tests were done to evaluate the material residual properties after impact. The results coming from compression tests do not reveal the better awaited behaviour of the stitched composites with respect to regular ones. On the other hand, the acoustic emission technique is capable of evidencing the different aspects of the stitching wire. The image acquisition technique represents a new experimental method able to fully characterise damage zones and gives more information on the interlaminar behaviour of composite material. This new technique joined to acoustic emission measurements allows important results on the residual life prediction of materials mechanical properties to be obtained.