Formulation of the Fatigue-Fracture Criterion for Composite Materials

Numerous experiments have shown that composite materials under cyclic loading are deformed and disintegrate due to the accumulation of different defects. Based on these results and taking into account the concept of scattered damage and destruction, we formulate a fatigue-fracture criterion for these materials. In a general formulation, the kinetic equation for the damage parameter is considered by Haward and Bokshitsky. Their studies show that damage to a system, in accordance with the concepts of statistical physics, occurs at a rate depending on some external factors (mechanical, physical, chemical, etc.) and on the amount of accumulated damage. Real materials feature a random structure, therefore the continuity or damage parameter is a statistical indicator that can be specified using a certain kinetic equation. The relative change in the modulus of elasticity during cyclic loading is considered as a continuity (damage) parameter. The critical damage value is taken as the failure condition and, on this basis, a fatigue-fracture criterion is formulated. The coefficients of the criterion are specified and damage accumulation curves and fatigue curves are plotted. The theoretical damage and fatigue curves and the results of the corresponding experiments on the fatigue of fiberglass are in good agreement. Thus, the approach proposed allows prediction of the fatigue strength of composite materials based on the relative change in the elastic modulus during cyclic loading.