Fracture Mechanics and Fractography of Creep and Fatigue Crack Propagation at Elevated Temperature

A fracture mechanics and fractographic study was conducted on creep and fatigue crack propagation of a 304 stainless steel under constant and repeated tensile stress at a temperature of 650°C. Linear elastic fracture mechanics could be applied to the test data fairly successfully in spite of the fact that the tests were conducted under creep conditions. A comparison with data in a literature indicated that this is not true for bending-type specimens such as the compact tension specimen because of redistribution of bending stress due to creep, and therefore a specimen geometry which avoids bending stress is preferred for creep crack testing. Under repeated stress, the crack generally started as a transgranular fatigue crack and changed to an intergranular creep crack at some length of the crack. The transition point moved to a later stage of crack propagation as the period of repeated stress increased. This transition phenomenon could be explained by assuming that the two crack propagation processes, the transgranular fatigue crack and the intergranular creep crack, are possible under repeated stress condition and that the one with the higher rate actually occurs.