Modelling the loading rate effects on the fatigue response of composite materials under constant and variable frequency loadings

This paper discusses the effect of loading rate on the static strength and fatigue behavior of glass/polyester and graphite/epoxy composites. It is shown that the rate-sensitivity of the static strength and the fatigue data can be analytically taken into account in the framework of a two-parameter “residual strength” model, inherently fulfilling the Strength-Life Equal-Rank Assumption (SLERA). Based on the static strength's loading rate sensitivity, a method to use the fatigue life data obtained at any fixed frequencies to predict the fatigue life under different frequencies is proposed. The method's reliability is verified by predicting different data sets of CA fatigue life and residual strength data obtained on glass/polyester composites under different loading rate conditions. The responses of angle-ply carbon/epoxy laminates to arbitrary two-block cyclic loadings in ascending and descending frequency sequences revealed the method's capability to discriminate between the applied loading sequences and the potential to develop a numerical routine accounting for the cycle-by-cycle variability during realistic loading conditions.