Abstrati-A technique for modeling synergistic effects in fatigue crack propagation (FCP) is presented. First, a mission (load/temperature history) is segregated into elemental damage events. A simple three parameter model is then used to describe these events. The model coefficients are seen to be interrelated linear functions of FCP rate controlling variables such as frequency, temperature, stress ratio (~~i"/U~,,), dwell, overload ratio (P ovcr,oad/P,,,) and cycles between overload. Finally, integrating event-byevent crack advance gives the expected component crack propagation life under mission cycling. Results of this procedure applied to gas turbine disk materials IN100 and Waspaloy are discussed to examine the accuracy of the model. NOTATION a,,, maximum stress akin minimum stress P maX maximum load Stress Ratio a %I01 P overload maximum load of a cycle considered as an overload Overload Ratio (OLR) Poverioad P max Cycles between overload (CBO) number of overload effected cycles Dwell Mission SINH ATM Freq Type Thick R2 SEE rad sustained load stress-time profile hyperbolic sine program testing atmosphere test frequency orientation of specimen and geometry type specimen thickness coefficient of determination standard error of the estimate crack in radial direction