1), 2) 3), 4) 1) 3), 4) (JIS SUS 316L) 2 2 1 (JIS SGV410) 0.15C 0.26Si 1.16Mn, 0.017P 0.002S (mass%) 40mm 270MPa 470MPa 145 3), 4) 10.7 mm 40mm SM 16mm 2.65mm 10.7mm 3 3 4.5mm 1.07mm 0.22mm NA NB NC #2000 (9μm) (1.0μm, 0.3μm, 0.1μm)

Torsional fatigue tests were conducted for circumferentially notched bars of austenitic stainless steel (JIS SUS316L) and carbon steel (JIS SGV410) under completely reversed cyclic torsion without static tension (Case A) and with static tension (Case B). The propagation behavior of cracks formed at the notch root was examined from a viewpoint of fracture mechanics. The length of crack was evaluated by the electrical potential method under the assumption of concentrical cracks propagating inward on the minimum cross section of specimens. For Case A of both steels, the crack propagation rate decreased with crack length, and turned to increase after taking the minimum rate point. Sharper notches gave rise to a slower rate for SUS 316L, while to a faster rate for SGV410. At the same stress amplitude, the crack propagation rate in SUS316L is lower than in SGV410. For Case B, the crack propagation rate was monotonically increased with crack length. Similar tendency was observed when the crack propagation rate was correlated to the stress intensity range. The J integral range was estimated from the hysteresis loop between torque and twist angle. For Case B, the crack propagation rate was expressed as a power function of the J integral range. For Case A, the crack propagation was lower than that for Case B, because of retardation due to the sliding contact between crack faces. The amount of retardation was larger for sharper notches at lower stress amplitudes in SUS316L, while blunter notches gave larger retardation in SGV410. The amount of retardation in Case A was closely related to the roughness or the acuteness of the factory-roof fracture surface.