2.1.3. From 10−<sup>1</sup> s−<sup>1</sup> to 103 s−<sup>1</sup>

An adiabatic process happens under this medium strain rate. The energy generated by phase transformation accumulates in the transformed region and cannot be released in such a short time. The temperature of the transformed region increases dramatically; however, the stress level remains the same. This is because the total transformed heat production is finite.

The range of strain rate from 102 s−<sup>1</sup> to 103 s−<sup>1</sup> can be reached by the split Hopkinson bar. Experiments conducted by Chen et al. [17] showed that when the strain rate came to 81~750 s−1, there would be a residual deformation in the specimen, which recovered in a few seconds to several hours. This residual deformation can be attributed to the thermal hysteresis as a result of the adiabatic process. Another paper published by Chen and Bo [62] in 2006 paid close attention to the unload progress of split Hopkinson bar systems. They reached a stable strain rate around 430 s−<sup>1</sup> under both loading and unloading conditions, which used to have difficulties in dynamic experiment design. Nemat-Nasser and Choi [18] also found that the initial temperature affected the deformation mechanism when strain rates came to 500~700 s<sup>−</sup>1. The transformation stress increased with the initial temperature, and austenite plastic deformation happened eventually. Regarding to the compression test, Adharapurapu et al. [63] discovered that the asymmetry between compression and tension became weaker with higher temperature, and this phenomenon was more conspicuous under high strain rates, as shown in Figure 5. Recently, Shen et al. [19] focused on the thermal evolution in this medium strain rate range by the split Hopkinson bar system and

an infrared detection system. They compared martensite NiTi SMA wires with superelastic wires between the transformation temperature As and Af, and pointed out that martensite wires had higher dissipated energy than superelastic ones. This could be explained by larger plastic deformation observed in martensite wires.

**Figure 5.** Compression and tension stress-strain curves of NiTi SMA. The effect of temperature on compression-tension asymmetry at dynamic (10<sup>3</sup> s<sup>−</sup>1) and quasi-static (10−<sup>3</sup> s<sup>−</sup>1) strain rates. (**a**–**d**) are at −196 ◦C, 0 ◦C, 200 ◦C, and 400 ◦C, respectively [63]. (Reprinted from Ref. [63], Figure 5, 2006, with permission from Elsevier.)

At medium strain rates, nucleation sites are too numerous to distinguish but merge into large martensite zones. As observed by Saletti et al. [30] at a strain rate round 20 s−1, two large martensite zones emerge on the ends of the bar specimen (one at each end) where the loading is applied, and the length of transformed martensite enlarged with two fronts move in opposite direction towards the specimen center. However, the details of the localized phase nucleation and propagation are still obscure due to the experimental difficulty [30].

However, methods are lacking to test material properties around 10<sup>−</sup>1~102 s<sup>−</sup>1, where car crashes and gravity-dropped bombs are the typical examples. Conventional mechanical test methods such as tension, compression, and bending test performed with screw-driven or servohydraulic load-frames are only applicable below this strain rate range. The split-Hopkinson bar technique has permitted the evaluation of mechanical properties over durations shorter than a millisecond, which is higher than this rate range. In another word, testing at intermediate rates is inherently challenging due to the possibility of elastic wave reflections and the difficulty in establishing dynamic equilibrium in the sample and the load sensors [71].

This problem can be partly solved by improving experimental devices. Xu et al. [72] used an impact testing system for the first time to show the influence of temperature and impact velocity. However, the device cannot control the strain rate precisely. Zurbitu et al. [20,21] explored NiTi SMA wire properties on the order of 1~10<sup>2</sup> s−<sup>1</sup> using an instrumented tensile-impact technique. Different from low strain rates, medium strain

rates led to more stable critical transformation stresses for both austinite and martensite phases. Zurbitu's paper compared the critical transformation stress results with those at low strain rates and demonstrated the adiabatic process during stretching in SMA wires.
