## 3 Results
### Dynamic Mechanical Test Results  
#### 3.2.1 Comparison between SS 304 and NiTi WMD  
Figure 9 compares the dynamic mechanical behavior of NiTi WMDs to that of SS 304 samples. Both types of WMDs were constructed with the same relative density of 0.31 and then subjected to sinusoidal excitation with a frequency of 1 Hz and displacement amplitudes of 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, and 0.8 mm. As the displacement amplitude of the isolator increases, the symmetric hysteresis loops of SS 304 WMDs spin counterclockwise, whereas the hysteresis loops of NiTi WMDs show an opposite tendency.  
To better understand this phenomenon, the dynamic stiffness and loss factors of the two types of WMDs at various amplitudes listed in Table 5 were calculated using Equations (2) and (6) and then plotted as functions of the displacement amplitudes as shown in Figure 10.  
Figure 8: Effect of heat treatment temperature on the pseudo-static mechanical performance of NiTi WMDs. (**a**) hysteresis curves; (**b**) equivalent stiffness and loss factor.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Amplitude** **(mm)**} & \multicolumn{2}{c}{**Dynamic Stiffness \(\mathbf{K_{\mathrm{D}}}\) (kN/mm)**} & \multicolumn{2}{c}{**Loss Factor \(\mathbf{\eta}\)**} \\ \cline{2-5}  & **304** & **NiTi** & **304** & **NiTi** \\ \hline
0.3 & 1.44 \(\pm\) 0.02 & 0.59 \(\pm\) 0.03 & 0.35 \(\pm\) 0.01 & 0.48 \(\pm\) 0.01 \\
0.4 & 1.49 \(\pm\) 0.05 & 0  
On the one hand, as the amplitude increases, the SS 304 WMD's dynamic stiffness increases, whereas the NiTi WMD's dynamic stiffness gradually decreases, as shown in Figure 10a. The maximum stiffness of SS 304 WMD is higher than that of NiTi WMD. This can be explained by the differences in the hardening behaviors between the SS 304 wire and the NiTi wire. The stress of SE NiTi wire tensioned at room temperature is supposed to increase linearly at first and then suddenly decrease its slope due to the low elastic modulus of the stress-induced martensite according to the literature [14]. On the contrary, the SS 304 wire only experiences straining and dislocation hardening during the tension. As a result, the SS 304 WMD appears to be strengthened, whereas the NiTi WMD exhibits a softened behavior, as the dampers are subjected to larger displacement amplitudes.  
On the other hand, although the loss factor of both types of WMDs decreases as the displacement amplitude increases, the NiTi WMD exhibits more excellent damping capacity and a slower decline in loss factor compared to the SS 304 sample, as shown in Figure 10b. Given that the SS 304 and SE NiTi WMDs have the same relative density, the frictional dissipation produced by the dry friction joints of the two types of WMDs is comparable. Therefore, the extra damping capacity of the SE NiTi WMD can be attributed to phase transformation damping. How the phase transformation within the NiTi WMD produces the extra energy dissipation will be thoroughly discussed in Section 4.1.  
Figure 10: Variation of WMD’s dynamic stiffness and loss factor with amplitude. (**a**) Dynamic stiffness; (**b**) loss factor.  
Figure 9: Comparison of dynamic curves of WMDs fabricated with different materials: (**a**) SS 304; (**b**) SE NiTi.