Analysis of cyclic creep and rupture. Part 2: calculation of cyclic reference stresses and ratcheting interaction diagrams

Abstract

Part 1 gives the basis for the use of cyclic reference stresses for high temperature design and assessment. The methodology relies on elastic–plastic calculations for limit loads, ratcheting and shakedown. In this paper we use a commercial non-linear finite element code for these calculations. Two fairly complex and realistic geometries with cyclic loads are analysed, namely a pipe elbow and a traveling thermal shock in a pressurized pipe. The special case of start-up shut-down cycles is also discussed. Creep and rupture predictions may be made from the results. When reference stresses can be economically calculated, their use for high temperature design has the following advantages. • Accuracy. Limit loads, shakedown and ratcheting limits are based on detailed analysis, and do not rely on rules or judgement. • Efficiency. Use of shakedown and ratcheting reference stresses to predict rupture and creep strain, respectively, allowing details of time and temperature to be dealt with as material data, not affecting the analysis. • Factors of safety. For both low and high temperature problems, factors of safety can be determined or applied, based on the real failure boundaries. • Conservatism. The rupture and strain calculations reflect the limit of rapid cycle behaviour. Cycles with relaxation will be associated with longer lives.