Fatigue damage of GFRP laminates consisting of stitched unit layers

Stitching is an effective method for improving interlaminar strength. Another advantage of stitching is the ease of stacking fabrics for large-scale structures. Stitching leads to the ease of quality control. Although there are many studies on through-thickness stitching, the characteristics of composites consisting of stitched unit layers have not been sufficiently studied. In this paper, the fatigue process for composites consisting of stitched unit layers was discussed. UD (unidirectional) and DBM (double-bias mat) laminates were used as specimens. Fatigue damage was continuously observed by the replica method at two stress ratios, namely, tension–tension and tension–compression loadings. The fatigue damage of the UD laminate was initiated from stitches, 90° strands and resin-rich regions. The propagation of the damage was affected by the stress ratio, and we considered that compressive load tends to cause delamination. In the case of the DBM laminate, the first sign of damage was that delamination almost occurred and was observed near stitches and resin-rich regions. Transverse cracks were formed by delamination. The damage process for the DBM laminate is almost independent of stress ratio.