Plasticity-induced martensitic phase transformation in fatigue of unnotched SUS304 plates

The present study has investigated plasticity-induced martensitic phase transformation in fatigue of unnotched SUS304 plates. Martensitic phase transformation occurred in uunotched SUS304 plate specimens fatigued at room temperature in air. Volume fraction V-alpha' of alpha' martensite in the uunotched portion of fatigued specimens was measured by ferrite scope. The relations between the maximum value of V-alpha', V-alpha'max, and the number of load cycles N were represented by reverse sigmoidal curves for all the applied stress range Delta sigma levels tested in this study. For the most portion of fatigue life, the value of V-alpha'max remained almost constant. This value was increased with increase in the value of Delta sigma. The spatial distribution of V-alpha' in the specimens varied with N: i.e., continued cycling of stress made alpha' transformation localized near the central portion of specimens where the V-alpha' value reached as high as 35-40%. This value is more than doubled compared to the highest V-alpha' value found in the tensile tests of SUS304 at room temperature in air. Invisible cracks of 200 mu m in length were found in the high V-alpha' value region. These results imply that the measurement of V-alpha' in fatigued SUS304 components may detect crack initiation sites and may predict residual fatigue life.