Establishing a simple and reliable method of measuring ductility of fine metal wire

Background: Measurement of the ductility like elongation and reduction of area of the fine metal wire is important because of the progress for the weight reduction and miniaturization of various products. This study established a simple and reliable method of measuring the ductility of a fine metal wire. Methods: Tensile and loading-unloading tests were performed with applying initial load to high-carbon steel wire (diameters of 0.06–0.296 mm) through capstan-type grippers for non-metal fiber. The wire fastened with the grippers was separated into three parts: the fastened part, the contact part, and the non-contact part. Scanning electron microscope (SEM) images were used to measure the wire radius under uniform deformation and agreed well with the radius calculated using the radius before tensile testing and uniform elongation. Results: The following conditions were clarified: non-slippage at the fastening between gripper and wire, a longitudinally uniform elongation, negligible cross-head bending, and the stroke calculation accuracy of elongated length by the initial load. Thus, uniform elongations were calculated as the ratio of the stroke at 0 N subtracted from the stroke at maximum tensile load to the additional initial chuck distance and the stroke at 0 N. The maximum error of uniform elongation was 0.21%. The reduction of area could be calculated by using the radius at uniform deformation portion, while the radius at the most constricted point was measured using SEM image of one fractured piece and uniform elongation. The measurement error of reduction of area was 1.9%. Conclusion: This measurement method can be applied to other metal wires less than 1 mm in diameter.