Abstract
Effects of intermittent overloading on the crack propagation under creep conditions were studied with 304 stainless steels at 550 and 650°C. Intermittent loading remarkably reduced the life of specimens and the fracture took place at about the value of linear cumulative damage of 1/20∼1/40. The crack propagation rates were also remarkably accelerated to be about 20∼100 times as large as those of static creep cracks for a given value of net section stress. The fracture surface morphology under intermittent loading differed from that under a constant load of intergranular creep type, indicating extremely ductile transgranular fracture by glide plane decohesion. The remarkable reduction in life and the acceleration in propagation rate could be ascribed to the recovery of the material during the low stress period. Crack propagation rates under intermittent loading were correlated well with modified J-integral J and appeared to agree with the rates of creep cracks as well as fatigue cracks in the high stress range. Crack propagation rates at different temperatures also coincided well in the J diagram.
