Change of Material Properties at the Tips of Fatigue Cracks Subjected to Macroscopic Tensile and Shear Stresses for Propagation : in Rotary Bending and Cyclic Torsion

Abstract

The behavior of fatigue cracks in plain specimens of eutectoid steel, in which the cracks are subjected to macroscopic tensile and shear stresses, is examined by both rotary bending and cyclic torsion tests. The size of slipped region at the crack tip and the micro-Vickers hardness in the region are examined. At the critical propagating stress, the following results are obtained. The size of slipped region is little affected by the crack length. The micro-Vickers hardness at the crack tip of tensile mode in cyclic torsion is different from that of shear mode in cyclic torsion, and the former is the same as that of tensile mode in rotary bending. That is, the hardness change is not dependent upon the load mode (rotary bending or cyclic torsion), but upon the stress mode generated at the crack tip.