Strength Degradation and Simple Load Spectrum Tests in Rotor Blade Composites

This paper presents comprehensive residual strength data. These data were collected to aid the validation and development of a residual strength based lifetime prediction method. Such a model can be used to predict the lifetime of rotor blade materials subjected to variable amplitude fatigue. Residual strength tests were done on one multi-directional glass-fiber/polyester material, which is representative of the material used in a wind turbine blade. This material has been characterized extensively in the DOE/MSU fatigue database. Both compressive and tensile residual strength tests were performed after three different fractions of the expected lifetime. Residual strength curves were determined in tension-tension, tension-compression, and compression-compression fatigue. In addition, simple load spectra, consisting of two load blocks with different amplitude, were investigated. These results are used, indirectly, to determine the form of the strength degradation model. The results seem consistent with the strength degradation found in the residual strength data. Nomenclature Sr = Residual Strength after n cycles at Smax Sr-1 = Current residual strength S0 = Initial Strength Smax = Maximum load applied in fatigue n = Number of cycles with Smax neq = Number of cycles at Smax, which would have led to the same current strength Sr C = Strength degradation parameter I