6 Creep-Fatigue Analysis

In this chapter, the American Society of Mechanical Engineers (ASME) criterion for designing pressure vessels under repetitive cyclic loading conditions in the creep range is discussed. Such cycles are generally encountered in power plants as well as petrochemical plants under normal operating conditions. The assumption of repetitive cycles enables us to focus first on explaining the ASME procedure for designing components under cyclic loading in the creep range without having to deal with the complex problem of variable cyclic conditions. Variable cyclic loading due to upset, regeneration, or other emergency conditions are discussed later in this chapter.

Cyclic analysis is straightforward at temperatures below the creep range. The maximum calculated stresses are compared to fatigue curves obtained from experimental data with an appropriate factor of safety. The fatigue curves take into consideration such factors as average versus minimum stress values, effect of mean stress on fatigue life, and size effects.