Tensile test criterion of transformation-induced elasticity and plasticity alloys for load-displacement measurement

Abstract

The tensile test criterion to obtain a reliable stress-strain curve from load-displacement data of metallic materials exhibiting twinning or phase transformation was developed especially for environments that do not allow the use of an extensometer. The study shows that neither ASTM E8/E8M standard specimen nor strip-shaped specimen geometry was appropriate for converting load-displacement data to an actual stress-strain curve; the former exhibited an undesired two-stage yield, while the latter revealed premature fracture. To solve these problems, the relationship between the specimen geometries and load displacement data was carefully analyzed for dog-bone type specimens having a minimized fillet radius; it was found that the ratio of grip width W-gr, to gauge width W-ga should be higher than the ratio of fracture stress to the first yield stress 0.1. Otherwise, a second yield always occurred when the stress reached a value close to sigma(1) W-gr/W-ga, obstructing the data analysis to reconstruct the actual stress-strain curve from load-displacement data. Finally, a methodology to verify whether the tensile test satisfied the criterion was suggested in terms of a nominal strain hardening rate.