If the crystal grain size of a metal is made smaller, its strength is higher. So, many methods of grain refinement have been proposed. In this study, from the viewpoint of basic plastic working, the variations of static ultimate tensile strength and fatigue strength after the application of plastic torsional deformation on face centered cubic crystal metals, that are, aluminum and copper, were investigated. Tensile test, Vickers harness test and Rotating bending fatigue test were performed. The hardness of the materials varied from surface layer to center section in cross sectional area. In the case of aluminum, the tensile strength and fatigue limit were improved after application of torsional deformation. However, in the case of copper, the fatigue limit was not improved. This is strongly related to hardness distribution around the surface layer of the specimen. Also, it was found that the crack growth mode was changed by applying the pre-strain. From these results, one of an idea for improvement of material strength will be considered.
The strength of torsional pre-strained specimen of aluminum was investigated. After torsion, the hardness of the specimen varied from surface layer to center section in cross sectional area. The tensile strength and fatigue limit were improved after application of the pre-strain, and these were related to the surface hardness. The fracture surfaces of fatigued specimens were different depending on the pre-strained level. Also, the application of the pre-strain affected on the crack growth direction and the detailed crack growth modes.
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