Effect of Variable R-Ratio Loading on Fatigue Life and Damage Accumulation in Plain-Weave Fabric Carbon/Epoxy Laminates

摘要

Effect of alternating R-ratios on fatigue life of a quasi-isotropic woven fabric CFRP laminate has been studied. First, constant amplitude fatigue tests are performed on coupon specimens at different stress ratios to establish a set of fatigue data that are used as a baseline for this investigation. Then, variable R-ratio fatigue tests are carried out in which the stress ratio of fatigue loading alternates between two different values, R1 and R2, during which either the maximum or minimum fatigue stress level is kept constant depending on the pair of stress ratio (R1, R2). The fatigue life under variable R-ratio loading is examined for different pairs of stress ratios at different constant levels of maximum or minimum fatigue stress. By comparing the fatigue lives under constant amplitude and variable R-ratio loading conditions, the effect of alternation in R-ratio on fatigue life of the woven CFRP laminate is identified. Experimental results demonstrate that for any pair of stress ratio, fatigue lives for alternation of stress ratio between R1 and R2 are distributed in the range from min{Nf (R1), Nf (R2)} to two times the value. It is also found that the Miner’s sum of cycle ratios for alternating R-ratios loading takes a value in the range from 0.5 and 2. The latter suggests that the linear damage accumulation rule is applicable to prediction of fatigue life for alternating Rratios loading. Finally, an engineering methodology for prediction of fatigue lives of composites that makes use of the anisomorphic constant fatigue diagram in conjunction with the linear damage accumulation rule is shown to be applicable in alternating R-ratios loading of the quasi-isotropic woven fabric CFRP laminate with an accuracy by a factor of two.