Uniaxial Pre-strain and Free Recovery (UPFR) as a Flexible Technique for Nitinol Characterization

The measurement of phase transformation temperatures of superelastic (SE) and shape memory (SM) NiTi alloy products and components was studied in this work. The transformation temperatures of a set of twenty different 300 μm NiTi superelastic wires were measured by two well-established and standardized techniques, namely differential scanning calorimetry (DSC) and bend and free recovery (BFR) and then compared with the results from the Uniaxial Pre-Strain and Free Recovery (UPFR) test. UPFR is a tension-based test, whose aim is to overcome the limitations associated with BFR testing. Within this work, a test procedure has been set up and validated. UPFR is found to be the only method showing a very strong correlation with the mechanical properties measured using the standard uniaxial tensile test method for superelastic NiTi alloy. Further, UPFR has been validated as a robust technique for measuring the R-phase and austenitic transformation temperatures in specimens of various sizes, composition, and of different geometries. This technique overcomes the limitations of BFR and DSC which cannot be used for testing products such as 25 μm SM wire, a 50 μm SE strip, and different springs and microsprings for actuation.