Behavior of Stitched Laminates under In-Plane Tensile and Transverse Impact Loading

Experimental results for in-plane tensile and transverse impact responses of stitched composite laminates were presented in this paper. (02/902). E-glass/epoxy laminates of 2.8 mm nominal thickness fabricated by resin transfer molding were used as the specimens. The through-the-thickness reinforcement was provided by untwisted Kevlar-29 ravings of 1,000 and 3,000 denier. In the in-plane tensile test, although the damage mechanism of the stitched laminates was much affected by the loading directions, the stiffness was not significantly affected by the addition of the stitch threads. In the impact test using a hemispherically tipped impactor, stitching was found to significantly reduce the delamination crack area, and the 3,000 denier threads provided a better resistance to the propagation of these cracks. It was also found that, although the stitch step, stitch spacing, as well as the impact location relative to the stitch threads affected the size and the shape of the delamination area during impact, the penetrating density of the 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 delamination damage for composite laminates can always be alleviated by the stitch reinforcement.