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 (=Δ γ 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 Δ γ fpc , and a large γ pc where Δ γ 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 Δ γ fpc is closest to 0. This value can be interpreted as plastic strain of the potential maximum shear strain.