An empirical description of fatigue crack propagation (FCP) under complex loadings, typical of those experienced by gas turbine disks, is presented. This approach uses a three coefficient sigmoidal curve (SINH) to model the macroscopic effects on FCP of service overloads, dwells, and load sequencing, as well as basic influences of frequency, stress ratio, and temperature. The coefficients are then related to these growth-controlling parameters through simple, linear, empirical functions. Comparisons are made of this approach with more conventional methods, and the results of laboratory verification tests are presented.

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