Evaluation Methods for Non-contact Bend and Free Recovery Tests of Thin NiTi Wires and Their Effects on Measured Transfo
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JMEPEG (2020) 29:5435–5441 https://doi.org/10.1007/s11665-020-05022-2
Evaluation Methods for Non-contact Bend and Free Recovery Tests of Thin NiTi Wires and Their Effects on Measured Transformation Temperatures Jonathan Apell, Markus Rettenmayr, and Andreas Undisz (Submitted February 26, 2020; in revised form June 12, 2020; published online August 20, 2020) 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. Keywords
advanced characterization, nondestructive testing
biomaterials,
1. Introduction Reliable performance of NiTi medical implants in minimally invasive therapy requires precise knowledge of phase transformation temperatures for triggering pseudoelastic deformation or the shape memory effect (Ref 1, 2). Both effects are based on the well-documented martensitic transformation of NiTi between B19’ martensite and B2 austenite (Ref 3). Depending on the material condition, an intermediate R-phase can occur (Ref 4). The phase transformation may be triggered by external stress or a temperature change. Particularly for medical applications, the functional transformation temperatures are of interest that results in an active shape change of the components. Various methods for determining transformation temperatures of NiTi including differential scanning calorimetry (DSC), bend and free recovery tests (BFR), and less conventional methods such as magnetization measurements are documented (Ref 5-7). Considering that residual mechanical stress in the material affects the transformation temperatures, the BFR test most closely resembles the application conditions in order to characterize the active response of a NiTi wire or component. Deformation temperatures determined by other methods may yield slightly different values due to the deformation gradients during bending (Ref 8). The standard procedure for BFR tests is specified in the ASTM method Jonathan Apell, Markus Rettenmayr, and Andreas Undisz, Friedrich-Schiller-Universita¨t, Otto-Schott-Institut fu¨r Materialforschung, Lehrstuhl Metallische Werkstoffe, Lo¨bdergraben 32, 07743 Jena, Germany
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