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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.

Glass Fiber Reinforced Plastics (GFRP) have been applied to products such as building components and various structural materials in industrial plants due to their excellent lightness and corrosion resistance, as well as weather resistance since the 1980's. Use of GFRP for large-scaled components and/or components with repetitive load has increasingly widespread in recent years, and both tensile and compressive properties have thus become important. Reinforcing laminate for GFRP has been mainly woven fabric such as glass woven roving due to its easy handling. However, its fiber winding and gap occurrence have been problems to be solved. In the recent years, new fiber fabrics for reinforcement have come on the market owing to rapid development of weaving machine, and one of the new fabrics is the stitched fabric. This glass fiber stitched fabric has been increasingly developed in Europe and all over the world for over ten years, and in Japan, has extended its application for ship for about ten years. This study investigates and examines GFRP reinforced with unidirectional glass fiber stitched fabric, in respects of the effect of fiber laminated constitution, fiber content and fiber diameter on tensile and compressive properties. Additionally, this study also examines relevance between weaving patter of fiber and strength property by comparing 0 degree/ 90 degree stitched fabric with GFRP with general woven roving.