Stress-induced martensitic phase transformation in thin sheets of Nitinol

Using the in situ optical technique known as digital image correlation (DIC), full-field quantitative strain maps of localization have been experimentally obtained for the first time in thin sheets of Nitinol. The use of DIC provides new information connecting previous observations on the micro- and macro-scales. It shows that the transformation initiates before the formation of localized bands, and the strain inside the bands does not saturate when they nucleate. The effect of rolling texture on the macroscopic stress-strain behavior was observed and it is shown that the resolved stress criterion or Clausius-Clapeyron relation does not hold for polycrystalline Nitinol. Finally, the effect of geometric defects on localization behavior was observed. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.