Thermoelastic Transformation Behavior of Nitinol

The transformation behavior of Nitinol under uniaxial tension and four-point bending was investigated. A novel sample geometry produced from drawn tubing was used to observe the differences caused by localized phase transformation effects between the two types of loading and between samples with different process and load histories. Phase shifted moire interferometry was used to provide full-field measurement of strain during the experiments. Optical resolution and grating coherence were sufficient to simultaneously resolve the strain fields within both the parent and transformed phases of the material. Evidence of both localized and uniformly distributed phase transformation is observed for the samples tested in tension while the bending results clearly indicate an asymmetric neutral axis and a complex reverse bending response for samples containing a strong R-phase component and tested at temperatures below critical transformation temperatures.  You can download our paper that describes our approach of using moire interferometry to characterize the mechanical behavior of Nitinol and our novel four-point bending test rig and our results showing the influence of R-phase and phase transformations..  It was published by ASTM in 2005.