A Simple Format for the Definition of Safety Factors for LCF

Abstract

Turbine components are heavy duty mechanical parts that are usually designed onto safe-life approach, where the low-cycle fatigue analysis is based onto design life curves with suitable probabilistic life margins. However, in order to design for a 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 of the case of LCF. In this paper, we firstly define a simple lognormal format for calculation of reliability for a given fatigue life under LCF conditions. Then, the partial safety factor to be applied to ε–Nf curve are then obtained for different COV’s of the applied load. The so defined design ε–Nfcurve for a rotor steel is then adopted for a complex analysis of a notched component under LCF, whose life has been estimated by an elasto-plastic analysis.