Mean vs Life-Limiting Fatigue Response of a Ni-Base Superalloy, Part 2: Life Prediction Methodology (Preprint)

摘要

In Part 1, we showed that the mean fatigue behavior of IN100 separates from the life-limiting response as stress level is decreased. This separation of lifetimes was suggested to be related to the development of heterogeneity levels of local deformation that produce sequential occurrence of 'short-lifetime' and the 'mean-lifetime' dominating mechanisms. In the current paper, we show that the distribution in the life-limiting mechanism is controlled by small-crack growth from the relevant microstructural scale in the surface region, which explains the significantly less sensitive response of the lower-tail to the stress level, relative to the mean-lifetime behavior. We, therefore, describe the lifetime distribution in terms of superposition of the crack-growth-lifetime probability density and a mean-dominating density. Strategies for obtaining these probability density functions are discussed. A probabilistic life-prediction method is derived from this description and shown to provide significantly better representation of the lower-tail fatigue lifetime behavior, as compared to the traditional approach, and more reliable predictions of the probabilistic lifetime limit as a function of stress level.