Effects of Non-Negative Mean Stress on the Off-Axis Fatigue Behavior of Unidirectional Carbon/Epoxy Composites at Room Temperature

Influences of non-negative mean stress on the off-axis fatigue behavior of unidirectional composites are elucidated on the basis of a non-fragmentary set of data on the T800H/2500 carbon/epoxy laminate at room temperature. Constant amplitude fatigue tests under different stress ratios R= -1.0, 0.1, 0.5 are performed on plain coupon specimens with six kinds of fiber orientations. For all fiber orientations, the relative fatigue strength becomes lower with decreasing stress ratio. The off-axis fatigue data normalized with respect to the static tensile strength substantially fall on a single S–N relationship for each stress ratio. It is confirmed that the S–N relationships on logarithmic scales are almost linear over the range of fatigue life up to 10 6 cycles, regardless of the fiber orientations and stress ratios. A phenomenological fatigue damage mechanics model proposed by the authors is further developed to consider the effect of mean stress on the off-axis fatigue behavior. It is demonstrated that the modified fatigue model can describe adequately the stress ratio dependence as well as the fiber orientation dependence of the off-axis fatigue behavior under non-negative mean stresses.