Influences of extrinsic factors on the Luders-type deformation behaviour of NiTi

NiTi shape memory alloys exhibit Luders-type deformation in tension. We propose a new explanation of the mechanism of this behaviour based on a mechanical criterion of sample length change conservation among three contributions, including machine displacement, Luders band elongation and elastic strain contraction, and established a mathematical model to describe the behaviour. This model predicts the influences of several pa-rameters some intrinsic to the testing alloy and some extrinsic by the testing conditions. In this work, we investigated the effects of machine speed and sample gauge length (L0) on the elastic strain contraction (Delta epsilon el) and the stress drop (Delta sigma) of the Luders-type deformation, and the characteristic length of the Luders band at its nucleation (l*L). It was found that Delta sigma is independent of L0 and at the same time l*L increased with increasing L0. This coordinated variation of l*L with L0 demonstrates a self-regulation mechanism of the Luders-type defor-mation behaviour to assure a constant Delta sigma, which further indicates that the upper (peak) and the lower (plateau) stresses of the Luders-type deformation are intrinsic to the metallurgical conditions of the material used. This reveals a new insight to the behaviour of the Luders-type deformation of NiTi. This study also clarified a common misperception (or confusion) in mechanical testing for Luders-type deformation, between local strain rate and global strain rate conditions.

Automated summary

NiTi shape memory alloys exhibit Luders-type deformation in tension. We propose a new explanation of the mechanism of this behaviour based on a mechanical criterion of sample length change conservation among three contributions, including machine displacement, Luders band elongation and elastic strain contraction, and established a mathematical model to describe the behaviour.