Superelastic and cyclic response of NiTi SMA at various strain rates and temperatures

To characterize the thermomechanical response, especially the superelastic behavior of NiTi shape-memory alloys (SMAs) at various temperatures and strain rates, we have performed a series of both quasi-static and dynamic uniaxial compression tests on cylindrical samples, using an Instron servohydraulic testing machine and UCSD’s enhanced Hopkinson technique. Strain rates from 10−3/s to about 4200/s are achieved, at initial temperatures in the range of 77–400 K. The influence of the annealing temperature on the fatigue response is also examined. A few noteworthy conclusions are as follows: (1) the transformation stress and the dissipated energy of NiTi SMAs depend on the annealing temperature; (2) in cyclic loading, the dissipated energy over a cycle tends to a minimum stable value, and cyclic loading leads to a stable superelastic behavior of the alloy; (3) repeated dynamic tests of the alloy produce smaller changes in the shape of the superelastic loop and in the dissipated energy than do the quasi-static cyclic tests; and (4) the superelastic behavior of this material has stronger sensitivity to temperature than to strain rate; at very high loading rates, NiTi SMAs show properties similar to ordinary steels, as has been established by the first author and coworkers.