High cycle fatigue under reversed bending of sandwich panels with GFRP skins and polyurethane foam core

The effect of fully reversed loading (R = −1) on high cycle fatigue performance of sandwich panels composed of polyurethane foam core and Glass Fiber Reinforced Polymer (GFRP) skins is studied and compared to that of similar panels tested under fully unloaded (R = 0) conditions. Fatigue life curves are established and compared based on maximum loads of 30–70% of the ultimate monotonic strength (Pult). It was shown that panels consistently fail in shear of the foam core. The fatigue life reduces significantly at R = −1, to about 10% of that at R = 0. In order to achieve at least 2 million cycles – the commonly acceptable fatigue life in structural engineering – the maximum service loads should be limited to 30% and 45% of Pult, respectively, for the cases of R = −1 and R = 0. It is estimated that the threshold loading levels at infinite fatigue life are 23% and 37% of Pult, for the cases of R = −1 and R = 0, respectively. By the end of fatigue life, up to 25% reduction in stiffness occurs. The transition between high and low cycle fatigue occurs between 200 and 5000 cycles. A 3-D Haigh diagram is also established for design purposes.