Chapter 16-Fatigue and Fatigue Crack Growth

Abstract

High-temperature components in a steam power plant are subject during service to fatigue straining due to thermal cycling or a combination of thermal and mechanical deformation in which the strain cycle includes a hold period. The first wall in a D-T tokamak fusion system will also undergo thermomechanical fatigue (TMF) as a result of the mechanical and electromagnetic loadings and the cyclic strains induced by the temperature changes during the plasma burn and off-burn periods [1,2]. Two approaches may be adopted for estimating the lifetimes of the component materials under combined thermal and mechanical cycling. The first involves the formulation of failure relationships directly from TMF tests; however, the acquisition of TMF data on materials by testing with simultaneously varying temperature and strain is experimentally difficult, time consuming, and expensive, and the procedures have not yet been standardized. Consequently, most of the relevant materials data has been generated by the second approach of isothermal continuous cycling fatigue and creep-fatigue (hold time) testing. However, the service lives extend over many years, and it is not practical to reproduce the conditions in laboratory tests. It has therefore been necessary to develop models of the behavior to enable the long-term service lives of components exposed to TMF to be predicted from the results of the shorter-term laboratory tests.