Effect of Cyclic Stress Change on Creep Crack Propagation

Abstract

In order to investigate the effect of cyclic stress change on creep crack propagation, deformation tests on smooth specimens and crack propagation tests on notched specimens of 0.16% carbon steel were carried out at 673K (400°C) under a time-dependent fatigue condition.
The crack propagation rate in time-dependent fatigue was much faster than that in static creep. It was owing to the acceleration of creep strain rate in the vicinity of crack tip because the dynamic recovery took place in the compression period. In these tests, the effect of the transition from small scale creep to large scale creep was small. The crack propagation rate had fairly good correlation with the experimentally evaluated creep J-integral range, Delta;J_c, which was equivalent to the analytically evaluated creep J-integral range based on the deformation property in time-dependent fatigue.
In detail, however, the crack propagation rate in time-dependent fatigue was rather slower than that in static creep for an equal Delta;J_c value. It was concluded to be attributable to the existence of the incubation time of crack propagation during the tensile stress hold time in time-dependent fatigue. The equivalent creep J-integral range, (Delta;J_c)_eq, which was the effective component of Delta;J_c for the creep crack propagation rate under a fatigue condition could be evaluated by the following equation;
(Delta;J_c)_eq=Delta;J_c/γ, γ=1+C₁1-R/C₂+τ_H,
where C₁ and C₂ are constants, R is the stress ratio, τ_H is the nondimensional tensile stress hold time.