FATIGUE SENSITIVITY OF RANDOM GLASS-FIBER-REINFORCED PLASTICS

Static and fatigue tests were carried out in four-point bending on thr ee random Glass Fibre Reinforced Plastics (GFRP), different in resin t ype (thermosetting and thermoplastic), resin content, and fibre form. The experimental results were evaluated in the light of a recent two-p arameter model, allowing for a statistical analysis of the fatigue dat a. It is shown that, despite the different nature of the materials con sidered, the theoretical model accurately predicts the fatigue respons e, 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 s ensitivity to stress amplitude and number of cycles, respectively. Fro m the values of the two parameters, evaluated for the GFRPs tested, a common trend is observed: the higher the material sensitivity to stres s amplitude, the lower its sensitivity to number of cycles. This impli es that comparing different materials on the basis of their fatigue re sponse at relatively low cycles can be misleading, because a better be haviour at high stress levels can result in shorter lifetimes at low s tress levels.