2.1.2. From 10−<sup>4</sup> s−<sup>1</sup> to 10−<sup>1</sup> s−<sup>1</sup>

As the loading rate grows, the heat energy generated during transformation, such as the latent heat, dissipation heat, and thermal expansion heat, cannot be released into the environment sufficiently. Excessive heat will lead to a temperature rise in the material, which in turn influences the transformation rate. The critical transformation stress typically increases with the strain rate; however, the environment temperature and heat exchange rate also play crucial roles in this thermal-mechanical coupling mechanism. Among the studies, Shaw and Kyriakides [11] compared the tensile properties between water-enclosed and air-enclosed NiTi SMA wires. Their experiments showed that the wire temperature varied remarkably between two surroundings and they found that the transformation stress in the water-enclosed wire was much lower than that of the air-enclosed one. The strain rate effect is weaker in the water-enclosed wire considering a higher efficiency of heat exchange. Bruhns [15] and Grabe's group [16] spent some effort on decoupling the thermal effect from strain-rate viscous effects at high temperatures and came to the conclusion that for a specified temperature range NiTi SMA wires can be seen as deformation-rate independent materials.

The number of martensite nucleation sites increases with increasing strain rate, and these martensite domains propagate in a parallel mode. Gadaj et al. [29] applied a thermovision camera to record the infrared radiation emitted by the specimen surface. They observed the number of martensite domains became greater as the strain rate increased, and these domains propagated parallel to the austenite strip. Similar nucleation and propagation modes were observed by Zhang et al. [31].

Tobushi et al. [14] investigated the deformation behaviors of NiTi SMA under straincontrolled and stress-controlled conditions. At a low strain rate in strain-controlled situations, they pointed out a stress overshoot at the temperature Ms and a stress undershoot at the temperature As, respectively. In contrast, the overall stress-strain curves subjected to the stress control are similar to those in strain-controlled cases with high strain rate. While at a high strain rate, stress-controlled experimental results have exhibited a smooth transition in stress around the transformation temperature Ms and As. The difference can be explained by the excessive energy needed for nucleation when the phase interface starts moving, since greater stress is needed to transform during strain-controlled situations.

In the case of compression tests, the transformation stress also increases with increasing strain rate; however, the transformation mode does not change with the strain rate from 10−<sup>4</sup> s−<sup>1</sup> to 10<sup>3</sup> s−<sup>1</sup> and the strain field is generally more uniform compared with tension tests. In contrast to the localized nucleation and propagation of martensite bands under tensile loading, NiTi SMA subjected to compressive loading always exhibits a more homogeneous transformation. Elibol and Wagner [64] employed a digital image correlation in situ technique to show that the surface strain fields in compression were always uniform during transformation without any strain localization. Meanwhile, the transformation mode was barely influenced by an increase of the strain rate in both quasi-static and dynamic conditions.

The finishing point of the martensitic transformation becomes harder to distinguish when the strain rate is higher. Dayananda and Rao [70] have tested NiTi SMA wires at strain rates from 3.3 × <sup>10</sup>−<sup>5</sup> <sup>s</sup>−<sup>1</sup> to 3.0 × <sup>10</sup>−<sup>2</sup> <sup>s</sup><sup>−</sup>1. Three "elastic" segments were identified in the stress-strain curves when the strain rate was low, which were the elastic austenite segment, superelasticity segment, and elastic Stress-Induced Martensite (SIM) segment. As the strain rate increased above 5.0 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−1, a fourth segment emerged between the superelasticity and elastic stress-induced martensite segments. This intermediate segment resulted from the overlapping of the SIM formation and elastic SIM deformation, and its length increased with increasing strain rate.
