Abstract
Creep fatigue tests were conducted on smooth specimens of a Type 304 stainless steel under a tensile stress hold (cp-type) condition at 1073K, and pre-creep damage was introduced in the form of small intergranular cracks distributed uniformly through the thickness. Macrocrack propagation tests in the high temperature fatigue were then carried out with the pre-damaged specimens. The results obtained are summarized as follows:(1) The pre-damage accelerated the successive macrocrack propagation in cp-type fatigue at 923K for an equal value of creep J-integral range, ΔJc, which governs the crack propagation rate of virgin material in time dependent fatigue.(2) Although the crack propagation rate of pre-damaged material in cp-type fatigue at 1073K was faster than that at 923K for ah equal ΔJc, it coincided with that of virgin one. The multiple small cracks were found on the specimens of virgin material as well as pre-damaged one near the macrocrack, which implied that the small cracks were generated not only in prior creep fatigue but also during the propagation test. Thus, the acceleration in cp-type is caused by the small cracks.(3) The crack propagation rate of pre-damaged material in cycle dependent (pp-type) fatigue was nearly 10 times faster than that of virgin one. The observation of crack morphology revealed that the accelerated fatigue crack propagation was caused by the coalescence with intergranular small cracks.
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