Fatigue sensitivity of random glass fibre reinforced plastics

Static and fatigue tests were carried out in four-point bending on three random Glass Fibre Reinforced Plastics (GFRP), different in resin type (thermosetting and thermoplastic), resin content, and fibre form. The experimental results were evaluated in the light of a recent two-parameter model, allowing for a statistical analysis of the fatigue data. It is shown that, despite the different nature of the materials considered, the theoretical model accurately predicts the fatigue response, permitting the calculation of the scatter in fatigue life from the scatter in static strength. A physical interpretation is given for the two parameters appearing in the model, correlated with the material sensitivity to stress amplitude and number of cycles, respectively. From the values of the two parameters, evaluated for the GFRPs tested, a common trend is observed: the higher the material sensitivity to stress amplitude, the lower its sensitivity to number of cycles. This implies that comparing different materials on the basis of their fatigue response at relatively low cycles can be misleading, because a better behaviour at high stress levels can result in shorter lifetimes at low stress levels.