Effect of Stress Ratio on Mode I Propagation of Interlaminar Fatigue Cracks in CFRP.

rowth behavior of mode I interlaminar cracks in a unidirectional carbon/epoxy laminate, Toray T800H/#3631, was studied under cyclic loading at different stress ratios. Under a constant value of the applied stress intensity range, ΔKIap, the crack propagation rate decreased with the crack extension due to crack-tip shielding by fiber bridging. The crack propagation rate was expressed by a power function of the maximum value of the crack-tip stress intensity factor: da/dN=C0(KItip, max/KIc)n, where C0 and fracture toughness KIc are the material constants, while the exponent n strongly depends on the stress ratio. The reduction rate in KItip due to fiber bridging was determined only by the crack extension Δa, but not by the applied load or the stress ratio. The upper bound of the crack propagation rate can be estimated from the scatter of fracture toughness and the fiber-bridging effect of materials. Change in load amplitude has no influence on either the da/dN-KItip, max relation nor the reduction rate in KItip.