The degree of diminution of fracture ductility varies with prestrain level. This behavior is investigated in the present study by using carbon steel and 70/30 brass. Transition of the fracture ductility appears at about a pretorsional strain of unity in the case of carbon steel. On the other hand, in the case of 70/30 brass, fracture ductility decreases smoothly and this transition behavior is not observed. However, the main cause of the loss of fracture ductility of the two materials is that the surface layer of the material becomes brittle due to the shear strain, and tensile fracture starts from surface cracks initiated by tensile deformation. The difference in diminution behavior of ductility between the two materials is thought to be due to differences in crystal structure and thus in the brittleness of the surface layer. The ductility of this surface layer is greatly reduced after the transition in the case of carbon steel.
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