Fatigue life of woven fabric carbon/epoxy laminates under alternating R-ratio loading along non-proportional path in the σm-σa plane

Influence of cyclic change in R-ratio on fatigue life of a carbon/epoxy composite is examined. For this purpose, two kinds of cyclic R-ratio loading tests are performed; one is under alternation of equal-life waveforms of different R-ratios, and the other is under alternation of unequal-life waveforms of different R-ratios. Experimental results show that fatigue failure under these alternating R-ratio loadings occurs in a life that is much shorter than the life predicted using the Miner rule with the cycle ratios for the two waveforms alternated, regardless of the pair of R-ratios. The alternation of equal-life waveforms of different R-ratios results in a larger accumulation of fatigue damage than any other kind of alternation of unequal-life waveforms of different R-ratios. These observations suggest that the alternation of equal-life waveforms of different R-ratios accompanies a significant interaction between the two waveforms alternated. An engineering methodology that consists of the Miner rule, the rain flow method and the anisomorphic constant fatigue life diagram approach is tested for the accuracy of prediction of fatigue life of the composite under the variable R-ratio loadings. It is demonstrated that the values of fatigue life for the alternating R-ratio loadings that are predicted using the proposed methodology are close to or even moderately shorter than the results of fatigue tests, regardless of the pair of R-ratios.