High temperature thermal creep under variable stress and temperature loading conditions

The present paper is concerned with thermal creep of cavitating materials subjected to time variable stresses and/or temperatures. Creep cavitation damage, because reducing the load bearing capability will influence the creep behavior of structures loaded by tension. The calculations are based on a phenomenological cavitation damage model, from which the life fraction rule (LFR) is deduced. For complex stress/temperature loading conditions by means of the LFR analytical expressions for the corresponding lifetimes are derived. Only stationary stress rupture data together with the loading conditions enter the computations. In the present procedure the damage function A(t), as a variable structure parameter, enters a constitutive strain rate/stress equation through an effective (true) stress. The latter, for given loading conditions, is derived from the solution of the general tensile test equation. Solving the strain rate/stress equation in terms of strain or time, the constitutive creep equation for ideally plastic cavitating materials exposed to non-stationary loading conditions are derived