Abstract
Fatigue crack growth in SUS 304 stainless steel was investigated at 650°C under trapezoidal and sinusoidal stress waves. Special attention was placed upon the interrelation between the high-temperature fatigue crack growth behavior and fractographs. At high and intermediate crack growth rates, ΔJf (J integral range)-controlled cycle-dependent crack growth mode and J' (modified J integral)-controlled time-dependent crack growth mode were observed, which yielded transgranular and intergranular cracking, respectively. At low crack growth rates, a threshold of ΔJf was found to exist, below which the transgranular cycle-dependent crack growth mode vanished and the intergranular time-dependent crack growth mode emerged. This generated false fatigue thresholds, the levels of which were lower than the true one and arbitrary depending on the condition of stress cycling. In ΔJf (or stress intensity range ΔK) decreasing tests, fracture mode changed from transgranular to intergranular, in accordance with the transition from the cycle-dependent crack growth to the time-dependent one, which occurred when ΔJf crossed the true threshold.
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