Tensile Fatigue Damage and Degradation of Random Short-Fiber Smc Composite
Fatigue damage and degradation in a random short-fiber SMC-R50 composite are investigated experimentally and theoretically. In the study of homogeneous damage, experiments were conducted to examine fundamental mechanisms and characteristics of fatigue damage and its evolution. The statistical nature of microcracks was evaluated by the introduction of damage distribution functions. Results showed that the cumulative distribution and density of microcrack length followed the form of a three-parameter Weibull function, whereas those of orientation followed a power form of the cos (THETA) function. Constitutive equations for the damaged composite are derived based on the self-consistent mechanics scheme in conjunction with a three-dimensional elastic crack theory and probabilistic functions. Agreement between theoretical predictions and experimental data is excellent. The theory and analysis are able to evaluate the tensorial nature of fatigue damage and degradation of all material elastic constants. A damage tensor is introduced to describe quantitatively the degree of homogeneous fatigue damage. Stiffness degradation is related to the rate of change of microcrack evolution and accumulation, and a power-law relationship is found between the rate of damage development and fatigue loading cycles. The exponent in the damage-growth law appears to be the same for all components of the damage tensor. In the nonhomogeneous damage study, residual tensile strength and fracture toughness were determined experimentally. Both of these quantities were found to decrease gradually with fatigue cycles. The results also revealed that the notch still dominated the fracture behavior in the damaged composite and that fracture mechanics concepts and analysis were applicable. The fatigue crack propagation rate for notched composites followed a power-law expression of Paris' type. The value of the exponent was found to be much higher for SMC-R50 composite than that reported for common metals due to cumulative damage of the heterogeneous composite occurring simultaneously in the notch-tip region and in the remote area. A distributed damage model and a ligament model to study the mechanics of fatigue crack growth were examined. The distributed damage model which contains several salient features unique to the SMC-R50 composite is too complicated for analysis while the ligament model leads to results which fit the experimental data well but the mechanics associated with the model is questionable.
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations
University of Illinois at Urbana-Champaign