IMPACT DAMAGE EVALUATION ON ADVANCED STITCHED COMPOSITES BY MEANS OF ACOUSTIC-EMISSION AND IMAGE-ANALYSIS

Polymeric composites that are stitched reinforced show dissimilar beha viours with respect to regular ones. In this paper the mechanical char acteristics and different behaviours of stitched reinforced polymeric composites after impact tests are investigated using an alternative ap proach. Several factors such as grid size, stitching scheme and voids content modify the composite performances and the interlaminar delamin ating 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 pr ocesses due to impact loads are considered and the post-impact behavio urs and damage tolerance are investigated. Impact tests were performed at several energy levels known to have an appreciable damage on the s pecimens. Damage evaluation analysis was performed on post-impacted sp ecimens using different techniques: as a first step, using a digitalis ed image technique, the damaged area was measured and different zone s hape was evaluated, further compression tests were done to evaluate th e material residual properties after impact. The results coming from c ompression tests do not reveal the better awaited behaviour of the sti tched composites with respect to regular ones. On the other hand, the acoustic emission technique is capable of evidencing the different asp ects 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 mat erial. This new technique joined to acoustic emission measurements all ows important results on the residual life prediction of materials mec hanical properties to be obtained.