Prediction of Fatigue Crack Initiation Life of Circumferentially Notched Steel Bars under Cyclic Torsion

Torsional fatigue tests were conducted for smooth solid bars, smooth hollow cylinders, and circumferentially notched bars of carbon steel (JIS SGV410) and austenitic stainless steel (JIS SUS316L) under completely reversed cyclic torsion. The cyclic stress strain relation was first determined for two steels. The yield stress and the flow stress were higher for SUS316L than for SGV410. The torsional fatigue life of smooth solid bars was longer than that of hollow cylinders when compared at the same nominal stress amplitude. The true strain amplitude on the surface of solid bars under torsion was calculated by the elastic-plastic analysis by the finite element method (FEM). The relation between the strain amplitude on the specimen surface calculated by FEM and the fatigue life was nearly equal to the relation obtained for hollow cylinders. The crack initiation life for notched specimens was determined by the direct electrical potential method. When compared at the same nominal stress amplitude, the crack initiation life got shorter as the notches became sharper. The distribution of the amplitude of equivalent strain near the root of circumferential notches under torsion was calculated by the elastic-plastic analysis of FEM. The crack initiation life got slightly longer for sharper notches when the life was correlated to the strain amplitude at the notch root. The relation between the crack initiation life and the strain amplitude at 0.1 mm distant from the notch root was nearly unique independent of the notch shapes and was equivalent to the relation obtained for smooth specimens.