An investigation of the residual stiffness of a glass fibre reinforced composite in high cycle fatigue experiments

Abstract

A modern wind turbine is designed to operate 20 to 25 years under various loading conditions. In operation, the current degradation and damage states of the turbine’s rotor blades are unknown and structural reserves for further years of operation may exist. But, the turbine will be disassembled after reaching its design lifetime.To approach this issue, a deeper insight in the fatigue behaviour of glass fibre reinforced composites is mandatory. For this, comprehensive experimental investigations of the stiffness degradation are conducted. The specimens are tested in the high cycle fatigue regime and the loss in stiffness is monitored. Tension-tension and tension-compression experiments are performed with specimens with 90° and ±45° fibre angles at stress ratios of -1 and 0.1. For each type, experiments at different load amplitudes are conducted with the pearl string method, from which S-N curves are derived. The strains are recorded with a camera and analysed via digital image correlation. In addition, the temperature is monitored with thermography and transmitted light images are taken by a second camera from the rear view. With the latter method, the progressive damage and crack growth can be evaluated by the decrease of the material’s transparency. Finally, the impact of the occurring damage on the stiffness degradation is correlated and coherences are discussed.