2.1.5. Greater Than 104 s−<sup>1</sup>

When the strain rate reaches 7000 s<sup>−</sup>1, plastic deformation of austenite occurs without any martensitic transformation [27,28], which implies the yielding stress for austenite rises with the strain rate more slowly than that of the transformation stress. In the meantime, Nemat-Nasser and Choi [24] found dislocation-induced plastic slips in the austenite phase with TEM, and a similar situation was observed in NiTiCr. In other words, plastic deformation takes place before the phase interface moves, thus martensite cannot emerge at this very high strain rate. Zhang et al. [26] tested NiTi alloys in the range from 104 s−<sup>1</sup> to 10<sup>7</sup> s−<sup>1</sup> by the technique of magnetically driven quasi-isentropic compression and shock compression. They discovered that the elastic limit increased dramatically compared to that at the low strain rate. Some other shock tests with very high strain rates [81] are beyond the scope of this paper.

## *2.2. Strain Rate Effect in Different Loading Modes*

Besides the uniaxial loading mode, it is common for NiTi SMAs to serve in complex mechanical loading modes including shear and indentation. The number of shear and indentation tests is much smaller than that of uniaxial ones. In general, the critical transformation stress increases with increasing strain rate for all loading types. In cases of cyclic loading, superelasticity degeneration and temperature variations become more remarkable as the strain rate grows.

## 2.2.1. Shear

A regular transformation hardening is observed under shear loads, and the hardening is enhanced by increasing the strain rate. Simple shear experiments on NiTi SMAs were early performed by Manach and Favier [33] in quasi-static conditions. Later, a more comprehensive study on NiTi SMA behaviors under shear stress was conducted by Huang et al. [34] over a large range of strain rates from 10−<sup>4</sup> s−<sup>1</sup> to 10<sup>3</sup> s−1. The low and intermediate loading rates were realized on a modified MTS machine, while the impact loading rate was achieved by Split Hopkinson bars. Heterogeneous strain fields and increased transformation stress were found in all three strain-rate conditions. A shear band was observed in the 10◦ direction of shear loading at low and intermediate strain rates, as shown in Figure 6a, while two separated bands emerge at the impact strain rate of 290 s<sup>−</sup>1, as shown in Figure 6b. Apart from the strain localization, the thermomechanical behaviors of NiTi SMA under shear loading was similar to those under tensile loading.

**Figure 6.** (**a**) Shear strain contours with shear bands evolutions at five different strain rates from 10−<sup>4</sup> s−<sup>1</sup> to 101 s−1; (**b**) separated bands at a strain rate of 102 s−<sup>1</sup> [34]. (Adapted from Ref. [34], Figures 10 and 18, 2017, with permission from Elsevier.)
