Micromechanical model for prediction of the fatigue limit for unidirectional fibre composites

A theoretical model is developed for the prediction of a fatigue limit of unidirectional fibre composites subjected to cyclically varying tensile loads in the fibre direction. The fatigue limit is defined from the condition that failure of one fibre does not lead to progressive failure of the neighbouring fibres. The model describes the fibre/matrix interface in terms of a fracture energy and a frictional sliding shear stress. Fatigue damage in the form of cyclic debond crack growth and a decrease in the frictional sliding shear stress along the fibre matrix interface is considered. Describing the fibre strength variation in terms of a Weibull distribution, an equation for the fatigue limit is developed. The model is applied for the prediction of the fatigue limit for glass fibre composites. Effects on the fatigue limit of R-ratio, fibre volume fraction, residual stress, interfacial fracture energy as well as the interfacial frictional sliding stress and its decrease over long time forward-reverse slip are investigated. A good agreement is found between model predictions of a fatigue limit and experimental data from the literature (run-out at 10(6)-10(10) cycles).