Fatigue behavior of CFRP cross-ply laminates under on-axis and off-axis cyclic loading

This paper focuses on the damage and fracture process of CFRP cross-ply laminates under on-axis and off-axis cyclic loading. On the semi-log S–N curves obtained by fatigue tests, fatigue fracture is described by a straight line and the fatigue limit is not recognized clearly. In the on-axis fatigue tests, fiber-peeling damage in 0° plies develops in addition to ply-cracking damage in 90° plies. The fiber-peeling damage was described by peeling-off of fiber bundles from the specimen surface after micro-scale damage of matrix resin and fiber-matrix interface. Most of the ply-cracking damage develops at an early stage of stress cycles, and slightly increases during fatigue, while the fiber-peeling damage in 0° plies initiates from the edges of 0° plies and evolves in the whole area of laminates with an increase in stress cycles. The fatigue fracture seems to occur when the 0° plies lose their load carrying capacity by the evolution of the fiber-peeling damage. In the off-axis fatigue tests, the fatigue fracture is caused by the initiation and evolution of ply-cracking damage and succeeding delamination. It is concluded that the fatigue strength is governed by the fiber-peeling damage in 0° plies under on-axis cyclic loading and by the ply-cracking damage and delamination under off-axis cyclic loading.