Pages 73-74
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 ξ_<α'> of α' martensite in the uunotched potion of fatigued specimens was measured by ferrite scope. The relations between the maximum value of ξ_<α'>, ξ_<α'max>, and the number of load cycles N were represented by reverse sigmoidal curves for all the applied stress range △σ levels tested in this study. For the most portion of fatigue life, the value ofξ_<α'max> remained almost constant. This value was increased with increase in the value of △σ. The spatial distribution ofξ_<α'> in the specimens varied with N : i.e., continued cycling of stress made α' transformation localized near the central portion of specimens where the ξ_<α'>, value reached as high as 30-40%. This value is more than doubled compared to the highest ξ_<α'> value found in the tensile tests of SUS304 at room temperature. Invisible cracks of 200μm in length were found in the high ξ_<α'> value region. These results imply that the measurement of ξ_<α'> in fatigued SUS304 components may detect crack initiation sites and may predict residual fatigue life.
