Predicting damage accumulation and fatigue life of UD composites under longitudinal tension

Abstract

Unidirectional composites under cyclic longitudinal tension develop damage through the accumulation and clustering of fibre-breaks, and through fibre-matrix interface debonding growth; these processes lead to a reduction of the material's load-carrying ability with increasing loading cycles, which raises a challenge to predict the fatigue response of composite structures. This paper proposes the first model in the literature to predict the kinetics of fibre-breakage and their effect on the macroscopic response of unidirectional composites under cyclic longitudinal tension. The model couples (i) a statistical hierarchical scaling law to predict fibre failure with (ii) a Paris law to predict interfacial fatigue damage propagating from broken fibres; due to its analytical formulation, the model predicts the response of composite bundles up to virtually any size and for their entire fatigue life in less than one minute. Model predictions for the accumulation and clustering of fibre-breaks show a good correlation with experiments from the literature; the model also predicts that, although the critical cluster size does not vary significantly between static, low-cycle/high-stress fatigue, and high-cycle/low-stress fatigue, the material can withstand the highest amount of softening under high-cycle/low-stress fatigue.