fatigue life evaluation of unidirectional composites by using residual strain energy

because of inhomogeneity and anisotropy of composite materials, defining a unique fatigue criterion to be suitable for different loading conditions is much more difficult in comparison with traditional materials. for this reason many tests on evaluating the damages such as crack density, stiffness reduction and residual strength are used by various researchers to monitor the fatigue process. in this research the residual strain energy density is defined as a suitable fatigue measure and used in a micromechanical static failure model to predict the fatigue life of unidirectional composites in different fiber load angles. in contrast to stress based fatigue models the new developed model employed both stiffness and strength reduction to predict the fatigue life with more accuracy. to measure the residual strain energy density stiffness and strength failure of models given by shokrieh et al. are modified and used in this research. available experimental data on unidirectional carbon/epoxy composites with the stress ratios of r=0 and 0.1 on stress states of 50, 60, 70 and 80 percent of static strength are used to evaluate the proposed model. the results predicted by the model underestimates the fatigue life in comparison with the experimental data.