Energy Approach for Creep-Fatigue Interactions in Metals at High Temperatures

An analysis of the so-called creep-fatigue interactions in a reannealed AISI type 304 stainless steel with and without holdtime at 593 deg C (1100 deg F) is presented with a numerical example. The analysis is based on a series of papers on the thermodynamics of materials exhibiting both time-dependent and permanent-set behavior under mechanical and/or thermal loadings [15–18]. Assuming isothermal loadings, the analysis consists of an “operational” decomposition of the total mechanical work into a stored part (long-term elasticity), and two dissipated parts, namely, an instantaneous component and a delayed component due to viscoelasticity. Each of the two dissipated components is again subdivided, operationally, into an “intrinsic” part (atomic diffusion and motion of dislocations), and a “structural” part (lattice strains and propagation of microcracks). The significance of the energy approach in unifying microscopic and macroscopic testing data and in formulating multi-axial design criteria for high-temperature components such as pressure vessels, turbine rotors, steam piping, etc., is discussed.