Shear strain evolution during tension-induced Lüders-type deformation of polycrystalline NiTi plates

This paper reports a new aspect of the Lüders-type deformation of NiTi. This is on the occurrence of lateral shear strains in a tension-induced Lüders deformation process. The phenomenon was studied by means of digital image correlation analysis. It was found that the lateral shear strains occurred in opposite directions within the Lüders band, apparently as an effort to self-accommodate the lateral displacement caused by the shear strains. The Lüders band propagation also changed from a single band mode to a branched mode when the lateral displacement, thus the in-plane bending moment, became too large. The branches, whilst having the same axial normal strain in the loading direction, were formed with opposite shear strains in double alternation between left and right of the sample and between the branches and the gaps between them, thus, to achieve the optimum self-accommodation. In addition, both the axial normal strain field and the lateral shear strain field were nonuniform after the Lüders-type deformation. These findings provide more insight and direct evidence for the explanation of the characteristics of Lüders deformation behaviour of NiTi.