A Simple Fatigue Life Model for Three-dimensional Fiber-Polymer Composites

Abstract

A simple model is proposed for predicting the fatigue life of three-dimensional (3D) fiber-polymer composite materials for certain cyclic load conditions. The fatigue life can be easily calculated using the model with three empirically determined constants: (i) the ultimate strength of the 3D composite, (ii) the ultimate strength of the two-dimensional (2D) laminate, which is identical in every respect to the 3D composite (e.g., same fiber layup pattern, fiber volume content) except for the through-thickness reinforcement, and (iii) the slope parameter (m) of the fatigue-life (S-log N) curve for the 2D laminate. It is shown that the S-log N curves for stitched composites under cyclic tension-tension and compression-compression loading and for z-pinned composites under cyclic bending loading can be accurately predicted using this model. This model can also approximate the tensile fatigue life of composite lap joints reinforced by stitches or z-pins. However, the model is not reliable in predicting the fatigue life of 3D composites in all cases. It is only accurate when the fatigue damage mechanisms and failure mode of the 3D composite are not altered significantly by the through-thickness reinforcement. Consequently, a limitation of the model is that a priori knowledge of the fatigue damage process of the 3D composite is needed before it can be used with confidence to predict fatigue life. This problem is exasperated by the paucity of information available on the fatigue mechanisms of many types of 3D composites. Until this problem is resolved, the model should only be applied to those types of 3D composites in which their fatigue processes have been determined.