Effect of stress ratio on the flexural fatigue behaviour of continuous strand mat reinforced plastics

The flexural fatigue behaviour of continuous glass-mat-reinforced thermoset-based composites was investigated using four-point bending. The tests were carried out adopting different stress ratios, R, i.e., the ratio of the minimum to the maximum applied stress. It was observed that the stress ratio has a strong influence on the fatigue life: given the maximum stress, passing from R=0.1 to R=0.7 can result in a two decades increase in the fatigue life. Fractographic studies revealed that debond fracture was the dominant damage process. The final failure occurred uniquely in the material volume subjected to tension and was probably initiated by the coalescence of smaller cracks into a single dominant crack. A model based on the residual strength degradation, which explicitly accounts for the stress ratio is proposed in order to express analytically the strength variation during fatigue cycle evolution.