Mechanics of Fatigue Damage and Degradation in Random Short-Fiber Composites, Part II—Analysis of Anisotropic Property Degradation

Abstract

Based on the microcrack density and cumulative distribution functions obtained in (Wang et al., 1986), cyclic fatigue degradation and associated damage-induced anisotropy of elastic properties of random short-fiber composites are studied. Constitutive equations of the fatigue-damaged composite are derived on the basis of the well-known self-consistent mechanics scheme in conjunction with a three-dimensional elliptic crack theory and the probabilistic functions of microcrack density and cumulative distribution. The anisotropic stiffness degradation is determined as a function of microcrack evolution and accumulation in the damaged composite. Theoretical predictions and experimental data of effective modulus decay during fatigue are in excellent agreement. A damage parameter is introduced to depict quantitatively the degree of homogeneous fatigue damage. The tensorial nature of anisotropic stiffness degradation and fatigue damage is examined in detail. A power-law relationship is established between the rate of damage development and the fatigue loading cycle. The rate of fatigue damage growth is found to decrease exponentially with the loading cycle — a phenomenon unique to the random short-fiber composite. The fundamental mechanics of composite fatigue damage and associated property degradation is elucidated in this paper.