Deformation and fracture of thin sheets of nitinol

Abstract

Nickel-Titanium (Nitinol) is a Shape Memory Alloy (SMA) that exhibits superelasticity (pseudoelasticity) and shape memory by a solid-solid state diffusion-less phase transformation. Phase transformation and the resulting strain localization in Nitinol has long been a topic of study, both for its inherent scientific interest and also because of the large number of practical applications of this bimetallic alloy. Although Nitinol devices are extensively used in the medical industry, there is a fundamental gap in the amount of high-quality quantitative experimental data detailing strain localization. The numerous applications of shape memory alloys provide the motivation to understand the deformation and failure mechanisms of these materials, particularly their fatigue and fracture behavior. By using an in-situ optical technique called Digital Image Correlation (DIC), quantitative measures of strain localization in Nitinol are presented for the first time in both deformation and failure modes. In addition, a finite element small-scale transformation analysis near a crack tip in Nitinol subjected to mode-I loading under plane stress conditions is performed for the first time. The experimental results and finite element analysis provide new and detailed insights concerning the structure of phase transformation and crack tip fields in Nitinol.