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

Abstract

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 106 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.