This paper presents an extension of the work described in Reference 1, which dealt only with tensile fatigue loads spectra. Experimental results and modeling have now been extended to simple spectra which cover a full range of loads including compression and reversed tension-compression, as well as WISPERX and other spectra representative of wind turbines, containing a full range of R-values. Experimental fatigue data (over 1100 tests) have been generated for a fiberglass laminate which is typical of wind turbine blade construction, tested under a variety of loads spectra. Constant amplitude data typical of the DOE/MSU fatigue database are then used with various modeling schemes to predict the lifetime under spectrum loading. Several areas of current concern in blade lifetime prediction are explored. First, it is demonstrated that the Miner’s Rule cumulative damage law results in significantly non-conservative lifetime predictions for most loads spectra. Linear and nonlinear residual strength models are easily applied and are more accurate in predicting lifetime; they also provide a prediction of remaining strength as a function of the history of loading prior to failure. Prediction accuracy is also sensitive to the model (exponential or power law) used to represent and extrapolate the constant amplitude S-N fatigue data. A power law representation appears to provide more accurate predictions of lifetime under spectral loading for longer lifetime cases.

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