Load Sequence Effects on Graphite/Epoxy [±35]2s Laminates

The effect of load sequence on the fatigue behavior of composite materials is contributed by (1) the difference in residual strength when fatigue fracture occurs, referred to as the boundary effect, and (2) the material memory effect with respect to previously experienced loads. A theoretical fatigue model previously developed by the authors is generalized herein to include the memory effect. The present model permits isolation of the memory effect from the boundary effect, thus the extent of the memory effect can be identified using residual strength test results under dual-level fatigue loadings. An experimental test program using graphite/epoxy [±35]2s laminates under high-low and low-high load sequences has been conducted to generate statistical residual strength data. It is shown that the material memory effect does exist for Gr/E [±35]2s laminates, as predicted by the model. The correlation between the test results and the theoretical predictions of the median residual strength and the expected percentage of fatigue failures under dual loading is very good. However, the statistical dispersion of the residual strength test results is much smaller than that of the static ultimate strength and that predicted by the model.