The critical damage state controlling the tension-tension fatigue life of unidirectional fibre composites

A fracture mechanics model is proposed for the prediction of the critical damage state corresponding to tension-tension fatigue failure of unidirectional fibre composites. The critical damage state is characterized by the development of longitudinal splitting cracks, enabling isolated damage zones with broken fibres to link up. The critical damage state can be expressed in terms of the size of damage zones with broken fibres. The critical damage zone size depends on the maximum applied cyclic stress level, the geometry of the splitting pattern and on the Mode II fracture energy of the composite. The model was used for predicting the critical damage size of a glass fibre composites subjected to cyclic loading. In a study reported in the literature, the damage zone size was determined experimentally by X-ray tomography just prior to fatigue failure. A good agreement was found between the predicted and reported damage zone sizes. Moreover, the reported failure involved longitudinal splitting in accordance with the model.