Effects of prior stress history on minimum creep rate and creep fracture

The purpose of the present paper is to investigate the effects of prior stress history (step loading, unloading and stress-relaxation) on the subsequent minimum creep rates and creep fracture of two materials, RR58 Aluminium alloy at 1800C and 0.24%C Steel at 4500C, which have previously been investigated for their multiaxial stress criteria and associated anisotropy. Taking as datum, the constitutive equations for creep fracture, obtained from correlation and regression analyses of 6 tests on 0.24%C steel and 7 tests on Aluminium alloy, the results of 20 variable stress, creep fracture tests on the Aluminium alloy showed that stress changes during primary creep were more damaging than changes during tertiary creep. The life summations for the former ranged from 0.2608 to 0.8479. Stress changes in the direction of loading were found to be the more damaging during tertiary creep, but during primary creep there appeared to be no significant order effect. Prior stress-relaxation, on both materials, showed damage effects similar to that obtained from changes during primary creep. Life summations for the Aluminium alloy ranged from 0.6388 to 0.7586 while for the steel they ranged from 0.6987 to 0.9262. In conclusion, the main findings are that prior stress changes result in an increased minimum creep rate but no change in the stress index, n, while for fracture changes during primary creep are much more damaging than changes during tertiary creep