A probabilistic model for the onset of High Cycle Fatigue (HCF) crack propagation: Application to hydroelectric turbine runner

Abstract

A fatigue reliability model for hydroelectric turbine runners is presented in this paper. In the proposed model, reliability is defined as the probability of not exceeding a threshold above which HCF contribute to crack propagation. In the context of combined LCF–HCF loading, the Kitagawa diagram is used as the limit state threshold. Two types of crack geometries are investigated: circular surface flaw and embedded flaw in a semi-infinite medium. The accuracy of FORM/SORM approximations was considered acceptable for engineering purpose in our application given the minimal numerical burden posed by such a method compared to Monte Carlo simulations. Our results show that the probability of an embedded flaw close to the surface has a major influence on reliability. Furthermore, we observe that the assumption that crack geometrical characteristics are independent leads to non-conservative results.