Lüders band evolution in geometrically graded NiTi plates

Abstract

This study investigated the process of the Lüders band formation and propagation within geometrically graded plate samples of a pseudoelastic NiTi during the stress-induced martensitic transformation by means of digital image correlation full-field strain measurement. The geometrically graded plate samples had parabolic width profiles in concave and convex forms along the length. Such sample geometries created a situation of progressively increasing applied load and a continuous variation of the sample width during the process of the stress-induced transformation. The size (along the sample length direction) of the Lüders band front transition zone was found to remain unchanged with variation of the sample width during the process, but the local strain rate across the transition zone was varied. The transition zone of the martensite Lüders band during the forward transformation was wider than that of the austenite Lüders band on the reverse transformation, and the local strain rate was lower than that of the latter.