Creep under Varying Load

The creep behaviour of nickel under cyclic loading has been studied, the aim being to relate the creep strain to the metallurgical recovery that occurs during each load reduction. Square-wave loading between stresses σ1 (40 or 80 N/mm2) and σ2 (0–σ1) for times t 1 (0.01–0.1 h) and t 2 (0.1–20 h) was applied at 650 or 750° C, and a point of principle was to apply the full stress σ1 initially until the metal was fully work-hardened so that all subsequent changes could be ascribed to recovery. The strain per cycle consists of three components, which could be expressed in terms of experimental conditions: (1) ∆e1, the normal strain in time t 1 at a stress σ1; (2) ∆e2, the strain in time t 2 at stress σ2; e2/e1 = (σ2/σ1)m, where m = 10 at 650° C and 5.2 at 750° C; (3) ∆er, the extra strain on reloading from σ2 to σ1; ∆er = bt 2⅔ + c, where b and c have been determined. The factor c can be negative, probably because strain-ageing occurs. A consequence is that three functions are needed to describe accur...