Fatigue design allowables for GRP laminates based on stiffness degradation measurements

The fatigue behaviour of a multidirectional, [0(±45)2/0]T glass/polyester laminate under various loading conditions is studied in this paper. Series of coupons cut on- and off-axis, at seven different orientations, were uniaxially loaded at constant amplitude over a wide range of R ratios. Test data were subjected to statistical analysis and S/N curves at specific reliability levels were calculated. Stiffness changes were continuously monitored during fatigue testing, and design curves corresponding to a certain level of stiffness reduction, and not to failure, were determined. It is shown, for the material studied in this work, that stiffness-based and reliability S./N curves are correlated and therefore determination of fatigue design allowables bearing information on both stiffness loss and strength can be formulated.