Estimating fatigue life of wind turbine components, where the loading and structural response are random, can be a difficult task. The life estimate is sensitive to rare large loads and can be influenced by the sequence of load applications. Sequence effects in fatigue crack initiation and crack growth are outlined. A simplified model for sequence effects in crack growth is presented along with model predictions of an American Society for Testing and Materials (ASTM) crack growth test series. The relative importance of including these effects in fatigue calculations is discussed. Analytical solutions for the time to crack initiation and crack growth provide quick estimates of fatigue life by approximating the loading as a narrow-band Gaussian process and by neglecting sequence effects. This approximation is both conservative and within the usual scatter in fatigue life estimates due to material variability. It is therefore a useful tool in estimating fatigue life, especially when turbine response data is limited.
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