Analysis of Fatigue Damage Evolution and Associated Anisotropic Elastic Property Degradation in Random Short-Fiber Composite

Abstract

A theoretical analysis of cyclic fatigue damage and associated anisotropic property degradation in a random short-fiber composite is presented. The fatigue damage takes various forms of microcracking, originated from microscopic stress concentrators in the highly heterogeneous material system. A probabilistic treatment of the microcracks is in troduced to evaluate the statistical nature of the microscopic fatigue damage. Damage evo lution and accumulation are analyzed through the development of probabilistic density functions of microcrack length and orientation during the cyclic loading history. Consti tutive equations for the damaged fiber composite are then derived on the basis of a self- consistent mechanics scheme in conjunction with a three-dimensional elliptic crack theory and the microcrack density functions. Cyclic fatigue degradation and associated damage- induced anisotropy of composite material properties are determined analytically and checked against experiments. The tensorial nature of material damage and composite stiffness changes during fatigue are evaluated explicitly. A power-law relationship between the rate of damage growth and the fatigue loading cycle is obtained. The rate of fatigue damage growth is found to decrease exponentially with load cycles—a phenomenon unique to the random short-fiber composite. This study provides a comprehensive analyti cal treatment of the homogeneous fatigue damage problem for random short-fiber com posites. The fundamental mechanics and mechanisms of fatigue damage evolution and associated anisotropic property degradation in the composite are elucidated.