BEHAVIOR OF STITCHED LAMINATES UNDER INPLANE TENSILE AND TRANSVERSE IMPACT LOADING

Experimental results for in-plane tensile and transverse impact respon ses of stitched composite laminates were presented in this paper. (O-2 /90(2))(s) E-glass/epoxy laminates of 2.8 mm nominal thickness fabrica ted by resin transfer molding were used as the specimens. The through- the-thickness reinforcement was provided by untwisted Kevlar-29 roving s of 1,000 and 3,000 denier. In the in-plane tensile test, although th e damage mechanism of the stitched laminates was much affected by the loading directions, the stiffness was not significantly affected by th e addition of the stitch threads. In the impact test using a hemispher ically tipped impactor, stitching was found to significantly reduce th e delamination crack area, and the 3,000 denier threads provided a bet ter resistance to the propagation of these cracks. It was also found t hat, although the stitch step, stitch spacing, as well as the impact l ocation relative to the stitch threads affected the size and the shape of the delamination area during impact, the penetrating density of th e stitch threads could be better used as a criterion to evaluate this delamination resistance capability, and an optimal penetrating density could always be obtained. Consequently, the impact-induced delaminati on damage for composite laminates can always be alleviated by the stit ch reinforcement.