Effect of stress ratio on fatigue life of GFRP composites for WT blade

Fatigue life of GFRP (glass-fiber reinforced plastic) composites used in wind turbine rotor blades has been evaluated considering the glass fiber orientations. Three different laminate composites with the respective laminating orientation of 0 degrees, +/- 45 degrees, and 0 degrees +/- 45 degrees were prepared using vacuum infusion method. Tensile properties and S-N curves for these composites were experimentally determined at room temperature. From the tensile tests, it was found that tensile properties were greatly dependent upon the fiber orientation and the tensile strength of unidirectional composite was the largest and bidirectional (45 degrees) composite was the weakest among three composites. The fatigue properties were determined under constant amplitude load control at different stress ratios, R, of 0.5, 0.1 and -0.2. The properties also show the dependency of stress ratios and fiber orientation. The fatigue life diagrams of these three composite were relatively well. presented with the double logarithmic S-N curve. The linear slopes of the respective S-N curves for three composite were not greatly different. The fatigue limits for the composites were evaluated and predicted with linear Goodman and Gerber diagrams.