Phenomenological Description of High-Temperature Transient Creep for Nonstationary Loading Conditions

The application of Garofalo's equation to the description of creep behavior of materials subjected to stress or temp. ramp loading is discussed. The analysis assumes that: the steady state strain rate epsilon sub s is proportional to a power function of the effective stress sigma sub e (t ) = sigma -- sigma sub i (t ); the mean internal stress sigma sub i is a function of time and applied stress sigma ; sigma sub i approaches a stationary value irrespective of loading conditions. It is shown that for stationary creep, at otherwise unchanged conditions the acceleration at t = 0, epsilon (0) will decrease with increasing sigma and T , respectively. Similarly, for sigma - and T -ramp loading, epsilon (0) increased with increasing rates sigma (0), T (0) of the respective ramps at t = 0. Creep curves are calculated by means of Garofalo's equation choosing for sigma sub i (t ) the particular form. These are qualitatively compared to experimental ones on Zircaloy-4 alloy. 10 ref.--AA