Evaluation Methods for Non-contact Bend and Free Recovery Tests of Thin NiTi Wires and Their Effects on Measured Transformation Temperatures

Abstract Non-contact evaluation of transformation temperatures via bend and free recovery tests requires precise optical evaluation of shape changes of NiTi components. A variety of experimental setups is documented in the literature, but the influence of the evaluation method on the transformation temperatures is rarely assessed in detail. In the present work, the reverse transformation of bent wires is evaluated comparing the tracking of the lowest wire point and the tracking of the curvature. For calculating curvatures, different approaches of fitting the wire outline were applied. Fourth degree polynomials and ellipse segment fits were found to cause high noise toward the end of the reverse transformation, second degree polynomials and circle segment fits led to increased sensitivity in that region. Accordingly, the evaluation of curvature allowed to resolve a two-stage reverse transformation, which was otherwise obscured. The reasons for this effect are discussed comparing curvatures as determined by the different evaluation methods.