Fatigue of Graphite/Epoxy [0/90/45/−45]s Laminates Under Dual Stress Levels

Abstract

A model previously developed by the authors to predict the effect of loading sequence on the statistical distribution of the fatigue life and residual strength of composite materials has been generalized and applied to graphite/epoxy [0/90/45/−45]s laminates. The load sequence effects have been shown to be caused in general by the difference in residual strength when failure occurs (boundary effect) and the effect of previously applied loads (memory effect). The present model permits isolation of the memory effect from the boundary effect and estimations of the magnitude of the memory effect under dual levels of fatigue loadings. The model parameters were evaluated from baseline data, including static strength, fatigue life, and residual strength tests. Predictions of the material response were then made and verified with additional fatigue life and residual strength testing. It was shown that the material memory effect is insignificant for this material and correlations between predictions of the number of early failures agreed quite well with the verification tests. Predictions of the fatigue life and residual strength degradation under dual stress levels were also in good agreement with the test results.