Cumulative damage in creep

The effect of two step stress variation and intermittent loading on the creep behaviour of commercially pure aluminium and stainless steel has been investigated. In the two step stress variation, the first stress σ1 was applied for a given time t1 and the stress level was switched over to σ2. The resultant creep rate {approaches the limit}s2+ and the failure time t2 have been observed. Under the intermittent loading programme, the stress cycle was applied in the order σ1-zero-σ1 and the average creep rate on each reloading has been observed. The experimental data appear to give a cumulative damage rule in the form t1 tr1+ t2 tr2 e ̇s2+ e ̇s2=1 where tr is the creep rupture time corresponding to a given stress and {approaches the limit}s2 is the creep rate under the second stress in normal creep. © 1979.