Method for Evaluating Potential Maximum Shear Strain for a Fine Metal Wire in Torsion Testing

The torsion number of drawn fine high carbon steel wires was measured through torsion testing. The angles between the scratches on the tested wire surface and its longitudinal axis were measured. The shear strain calculated from torsion number γ_t, shear strain at fractured point γ_f, and plastic shear strain γ_pc were evaluated. The following results were obtained. First, the shear strain distribution homogenized; further, torsion number per unit length N, γ_t, and γ_pc increased when decreasing the difference between γ_f and γ_pc where γ_pc subtracted from γ_f (=Δγ_fpc) > 0. Second, the external factors caused non-uniform shear strain distribution and reduction from the potential maximum shear strain, even for the wire that was hardly affected by the internal factors. The difference of shear strain non-uniformity caused a variation in reduction from the potential maximum shear strain. The internal factors included non-uniform microstructure and existence of inclusions and voids. The external factors were caused by the testing machine and setting of the sample. The potential maximum shear strain was obtained when the effects of internal and external factors were inhibited. Finally, two evaluation methods of the potential maximum shear strain were suggested. One method identifies a sample with a small Δγ_fpc, and a large γ_pc where Δγ_fpc > 0. This sample can be regarded as having the closest strain to the potential maximum shear strain. The other method determines γ_pc when Δγ_fpc is closest to 0. This value can be interpreted as plastic strain of the potential maximum shear strain.