A log-normal format for failure probability under LCF: Concept, validation and definition of design curve

Turbine components are usually designed onto safe-life approach, where the low-cycle fatigue analysis is based on design life curves with suitable probabilistic life margins. However, in order to design fora given reliability, the definition of the design curve should not only include the life variability but also the scatter of applied load. Unfortunately, in the literature there are few indications which only refer to safety factors under HCF, without any specific discussion for the case of components subjected to LCF. In this paper, we firstly propose a log-normal format for calculating reliability for an assessment point ((epsilon) over cap, (N) over cap) based on a first order approximation. The validity of the approach is then proved for two different materials with a series of Monte Carlo simulations, where the material cyclic response is coupled to its epsilon - N diagram. The format is then used for estimating failure probability and for defining the design point which corresponds to a target failure probability. A safety factor is then proposed and its application to a series of steels for power generation is shown.