Cycle-based analysis of damage and failure in advanced composites under fatigue: 2. Stochastic mesomechanics modeling

Overall damage and failure in advanced composites under cyclic loading are described based on the damage development within loading cycles. The development of scattered damage in a composite over the cyclic load range and during loading and unloading was studied by acoustic emission in Part 1 of this work. In this paper, a novel general model of damage evolution and failure of laminates under cyclic loading is developed based on a stochastic mesomechanics approach. The model combines a lamination theory and a theory of stochastic processes. The randomness of the elastic and strength properties of the composite plies, the laminate microstructural parameters, and the loading are taken into account. A theory of excursions of stochastic processes beyond bounds and a mesovolume concept are utilized to evaluate the ply-level damage functions. The capablities of the approach are illustrated on an example of damage and failure analysis of a laminate under low-cycle tensile loading. The new cycle-based model is found capable of predicting the experimentally observed stages in the overall fatigue damage process, i.e. the initial damage, the gradual damage development, and the final failure. The relative durations of these stages are found to be dependent on the fatigue loading parameters. The predicted damage development over the range of fatigue loading also correlates with the experimental observations. These predictions are intrinsic to the developed stochastic-process-based approach and do not require explicit experimental information on the fatigue damage behavior or stiffness degradation. These results shed light on the fundamental mechanisms of fatigue damage development in advanced composites. The new model treats fatigue of composites as a legitimate load- and time-dependent damage accumulation process within loading cycles. It is expected to be useful for fundamental studies of the effects of variable amplitude, frequency, and cycle shape on the fatigue behavior of advanced composite materials.