Effects of diameter and preparation of round shaped tensile specimen on mechanical properties

Abstract

The effect of specimen size and specimen preparation with round cross-sectional area on tensile properties in twinning-induced plasticity (TWIP) steel has been investigated. Round type tensile specimens were prepared with two ways: the one is lathe processed specimen and the other is mechanically and chemically polished specimen after lathe processing. And then, tensile tests with various specimen diameters from 2 mm to 13 mm at the fixed ratio of gage length to diameter of five were conducted. The polished specimens had similar yield strength (YS), tensile strength (TS), total elongation (TE), and reduction in area (RA) values regardless of specimen diameter. Whereas, lathe processed specimens had a variation in tensile properties with specimen size: YS increased and TE decreased with decreasing specimen size. The deformation twins was occurred in the surface layer of lathe processed specimens of undeformed and at the engineering strain of 10%, which is not related to the tensile stress during tensile test but to the torsional stress or compressive stress during specimen preparation by lathe process and torsional stress during tensile test. The tensile properties with specimen diameter were well explained using the concept of fraction of initially generated deformation twins during specimen preparation. Finally, it is recommended that bigger specimens should be used as much as possible to reduce the surface effect of specimens and experimental errors. For instance, the gage diameter of 5 mm was enough to ignore the effect of specimen preparation and size. Inversely, the surface residual stress in specimen needed to be removed before tensile test when small tensile specimen machined by lathe process was used.