Fatigue of composite laminates under off-axis loading

Results from an experimental program consisting of static and fatigue tests on flat coupons, cut at different off-axis directions from a multidirectional, (MD), Glass/Polyester, (GRP), laminate are presented in this paper. The material is similar to those used by GRP wind turbine rotor blade manufacturers, i.e. hand lay-up and room temperature curing. The stacking sequence of the MD laminate under consideration is [0/(±45) 2/0] T. Specimens were cut at five different off-axis directions from that laminate and over one hundred and forty tests were conducted under static and cyclic loading. Based on the test results the effect of off-axis loading on static and fatigue behaviour of the MD laminate is studied. A simple empirical model is used to predict the observed stiffness degradation and to determine stiffness based S–N curves by means of a limited number of test data. For the materials investigated in this program it is shown that E-modulus variation depends on the off-axis loading as much as on the applied cyclic stress level. Stiffness based S-N curves corresponding to 5–20% stiffness reduction are more conservative than standard S-N allowables of 95% reliability.