Fatigue strength of double-bias mat composites composed of stitched unit layers

Tension–tension and tension–compression fatigue tests were carried out for a DBM (double-bias mat) laminated composite. The composite is composed of stitched unit layers. The effect of stitching is considered to be complicated. Thus, the fatigue process was observed by a replica method at certain periods, and the fatigue mechanism of different stress ratios was discussed. It is found that the fatigue damage of DBM is mainly delamination induced by stitching. The fatigue damage is observed independently of stress ratio. In the meantime, stitching has a favorable effect. It is considered that stitching increases the momentum of inertia of the layer and compressive strength. Therefore, the effect of stitching is discussed considering the trade-off mechanism of the adverse and favorable effects mentioned above.