Effects of Stress Ratio on Fatigue Behavior of CFRP Laminates and Modeling

Influences of stress ratio on constant amplitude fatigue behavior of CFRP laminates have been examined and a rational fatigue life prediction method has been developed. First, constant-amplitude fatigue tests are performed at room temperature on a quasi-isotropic CFRP laminate [45/90/-45/0] 2s for six kinds of stress ratios. The experimental results show that fatigue strength is lower for fatigue loading with larger amplitude. It is observed that the stress amplitude for a given fatigue life becomes largest under a fatigue condition in which stress ratio is equal to the ratio of compressive strength to tensile one. A procedure to construct a nonlinear constant fatigue life (CFL) diagram is developed on the basis of the static strengths in tension and compression and the reference S-N relationship for a critical stress ratio equal to the compression-tension static strength ratio. The fatigue lives predicted on the basis of the nonlinear CFL diagram agree well with the experimental results over the range of fatigue life up to 106 cycles for all the stress ratios tested. It is demonstrated that the proposed fatigue life prediction method can also be successfully applied to the fatigue behavior of different types of CFRP laminates [0/60/-60] 2s, and [0/90] 3s.