Estimation of the transient creep parameter C(t) under combined mechanical and thermal stresses

This paper reports finite element analyses for an axially loaded cylinder with three different types of thermal stress. The magnitudes of the thermal and mechanical loadings are varied and creep crack tip parameters are evaluated for part-through-wall and fully penetrating defects with a range of sizes. The relaxation rate for the creep crack tip parameter is examined in detail and is found to depend on the magnitudes of the mechanical and thermal loads and on whether or not there is plasticity on initial loading. Simplified formulae are developed for describing the rate of relaxation by a modified redistribution time.