The Effect of Inclusions on the Torsional Fatigue of Anisotropic Rolled Steel : Examinations Based on Successive Observation of Specimen Surface

Abstract

Torsional fatigue tests were carried out, after electropolishing the specimens cut out of a rolled steel in the rolling, the thickness and the 45° directions. The fatigue processes were observed successively by the metallurgical microscope. The main results obtained are as follows : 1) The torsional fatigue limit of each directional specimen is always controlled by the limiting conditions for the propagation of cracks occurring from inclusions. 2) The torsional fatigue cracks occurring from inclusions are initiated within the first 5 % of the lifetime of a specimen. 3) In both bending and torsional fatigues, the effect of inclusions on the fatigue strength is small in the bar-shaped inclusions, but large in the circular-plate shaped inclusions. 4) When the stress of the torsional fatigue limit is repeated in each directional specimen, each length of the non-propagating cracks initiated from inclusions reaches the saturated value before approximately 2×10⁶ cycle repetitions.