Cycle based modeling of composite fatigue

Fundamental relation between overall damage and failure process in laminates under cyclic loading and damage development within cycles is addressed. A general model for damage evolution and failure analysis of laminates under cyclic loading was developed based on the stochastic mesomechanics approach. The model combines lamination theory and a theory of excursions of random processes beyond the bounds. Randomness of elastic and strength properties of plies, laminate microstructure, and cyclic loading are taken into account. Capabilities of the model were illustrated by examples. The model was found capable of predicting three stages in the overall damage accumulation process, i.e. initial damage, gradual damage development, and final failure. Relative durations of these stages depended on load cycle parameters. Theoretical predictions were compared with acoustic emission observations of damage development in an unnotched graphite-epoxy laminate under low-cycle fatigue. Overall time history of acoustic emission was separated into emission during loading and unloading portions of the loading cycles. Histories of the locations of the AE sources and distributions of the acoustic events over the stress range were analyzed. It was concluded that most of the new damage was developed during the loading portions of the loading cycles. The emission during unloading was attributed to friction between crack faces. The time history of the new damage exhibited initial, steady, and final damage development stages. The new damage was developed over the entire stress range with higher damage rates at higher stress. Experimental observation corroborated with the results of theoretical analysis.