Effect of alternating R-ratios loading on fatigue life of a woven fabric carbon/epoxy laminate has been studied. Variable R-ratio fatigue tests are carried out in which stress ratio of fatigue loading alternates between two different values, R1 and R2. Fatigue life under alternating R-ratios loading is examined for different pairs of stress ratio (R1, R2) at different constant levels of maximum or minimum fatigue stress. Experimental results demonstrate that for all the pairs of stress ratio (R1, R2) tested, the fatigue lives Nf(R1, R2) are approximately distributed in the range from min{Nf(R1), Nf(R2)} to two times the value. It is also found that the Miner sum of cycle ratios for constant R-ratio loadings at R1 and R2 included by a given alternating R-ratios loading approximately takes a value in the range from 0.5 and 2. The latter observation suggests that the Miner rule is applicable to prediction of fatigue life for alternating R-ratios loading. Based on these observations, an engineering fatigue live prediction methodology for composites is proposed which is based on the anisomorphic constant fatigue diagram and the Miner rule. Finally, it is shown that the fatigue life prediction methodology developed is applicable in alternating R-ratios loading of the woven fabric carbon/epoxy laminate with accuracy by a factor of two.
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