Tips and Tricks for Characterizing Shape Memory Wire Part 5: Full-Field Strain Measurement by Digital Image Correlation

This is the fifth paper in a series on the experimental characterization of shape memory alloy (SMA) wires. In this installment we focus on the use of digital image correlation (DIC) to measure the strain field on the surface of the wire. After a brief overview of the principles and mathematics behind DIC, two different thermo-mechanical tension tests using DIC are presented to demonstrate the technique. The first experiment consists of Joule heating a shape memory (SM) wire to induce the shape memory effect, using 2-D DIC to measure the strain field. The second experiment measures the response of a superelastic (SE) wire to mechanical cycling at room temperature, using 3-D DIC to measure the strain field and an infrared camera to measure the temperature field. In addition to describing the experimental results, attention is paid to specimen preparation and the two experimental setups. Many of the challenges and precautions associated with using DIC are discussed, along with practical recommendations for specimen speckle patterns, digital photography, and data post processing.