An analytical master curve for Goodman diagram data

Estimation of the remaining safe life of structural parts which are not easily inspectable continues to be a problem. Even when load histories are available, laborious interpolation of Goodman diagram data is required in order to determine the remaining fatigue life of such parts. An analytical formulation of Goodman diagram data would expedite the life check. It is shown in this paper that, for many engineering materials at room temperature, the entire range of Goodman diagram data collapses on to a single master curve when presented as the ratio of lifetime with mean stress to lifetime at R = -1 for a given stress amplitude, as a function of a non-dimensional load parameter consisting of stress amplitude, mean stress, and material strength. The master curve is conveniently expressed in terms of two easily determined Weibull constants. Stress-concentration factor influences the value of the constants, as does the strain-rate sensitivity of some materials. By use of the master curve formula in an algorithm together with the Manson-Coffin life relation and Miner cumulative damage rule, computed fatigue lives lay within a factor of 2 of results obtained in tests under aircraft spectrum loads.