Probabilistic fatigue — theory and experimentation

Abstract

The possibility of testing to destruction a significant number of large and/or expensive engineering components in order to assess their fatigue reliability statistics is, in most cases, simply not feasible. Motivated by this and the ever increasing demands for reliability estimates, this paper reports on the present status of an ongoing research endeavour whose aim is to make meaningful judgements as to the likelihood of fatigue failure of polycrystalline metals based on a combined theoretical and experimental investigation of the stochastic nature of actual material degradation processes. It first reviews the modelling, experimental and analytic steps that have recently been completed by the author and his associates to relate probabilistic and stochastic descriptions of microstructural damage processes, including crack propagation, in polycrystalline metals under constant amplitude cyclic strain loading, with the fatigue reliability of the metal involved. The results of these theoretical and experimental investigations are then used to give an indication as to the number of cycles that are involved in initiating a fatigue crack and to distinguish this number from those involved in propagating a crack to failure in ductile materials. Using this approach in the case of O.F.H.C. copper tested in the intermediary range of cycles to failure, i.e., 20,000 – 200,000, the preliminary indications, fully discussed in the paper, are that approximately 90% of the total number of cycles to failure are involved in initiating a crack, the remainder being associated with propagation. With this delineation in mind, the results of an initial acoustic emission test program designed to detect differences in the characteristics of stress energy waves originating in the neighbourhood of an initiating as opposed to a propagating crack are also described. After a discussion of these results, the paper draws some important conclusions that already influence the direction of current and future research efforts.