# Multi-Step Prestressing with Hybrid SMA Wires
## 5 Conclusions  
To improve the conventional prestressing technique, we conceptualized a multi-step prestressing technique with a hybrid SMA wire. The applicability of the proposed technique was verified through multi-step shape recovery behavior evaluation and multi-step prestressing tests. In the multi-step shape recovery tests, hybrid SMA wires were fabricated by combining two types of SMA wires: NiTi50 and NiTi90. Shape recovery behaviors that accompanied changes in temperature were evaluated using an image processing method. In the multi-step prestressing tests, the hybrid SMA wire specimens and the dummy specimen were heated simultaneously to ensure that the prestress was introduced \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Specimen** & \multicolumn{2}{c}{**Step 1**} & \multicolumn{2}{c}{**Step 2**} \\
**Max Strain** & **Residual Strain** & **Max Strain** & **Residual Strain** \\ \hline SMA-1 & 358 & \(-\)76 & 748 & \(-\)258 \\ SMA-2 & 339 & \(-\)83 & 747 & \(-\)225 \\ SMA-3 & 346 & \(-\)80 & 714 & \(-\)272 \\ Dummy & 387 & \(-\)1 & 784 & \(-\)17 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Results of multi-step prestressing tests. Figure 17: Simultaneously plotted strain-temperature curve. in multiple steps according to the SME of the hybrid SMA wire. The conclusions of this study are as follows. The shape recovery test verified the shape recovery behavior of the single SMA wires and the hybrid SMA wire. In the single-wire experiment, we evaluated the austenite temperature range in which the SMEs of the NiTi 50 wire and NiTi 90 wires were activated. The hybrid SMA wire, which was constituted of NiTi50 and NiTi90 wires joined using mechanical connectors, showed no shape recovery at 68-78 \({}^{\circ}\)C. This range represents the interval between the austenite temperatures of NiTi50 and NiTi90. Therefore, we could distinguish between the first and the second shape recovery steps based on the temperature range. The multi-step prestressing test showed that the SMEs of NiTi50 and NiTi90 appeared in each of the two stages--step 1 corresponding to 25-65 \({}^{\circ}\)C and step 2 corresponding to 25-105 \({}^{\circ}\)C--thus indicating that the prestress was introduced in each step. When compared with the dummy specimens without any embedded SMA wires, the thermal expansion of the test specimen was similar during the initial heating phase. However, unlike the dummy specimens, residual compressive strain was observed in the specimens with embedded SMA wires after the specimens were cooled. Therefore, the applicability of the proposed multi-step prestressing with hybrid SMA wire was verified. This study aimed to explore the basic concept of hybrid SMA wires. Therefore, we conducted small-scale experiments with limited variables and limited number of specimens. Future studies should explore diverse SMA wire materials and wire diameters, building upon the specimen sizes examined in this study. Furthermore, costs of SMA wires should be considered from the viewpoint of their use in practical applications. In this study, NiTi SMA, which can be supplied relatively easily, was used because it was a small-scale experiment. However, NiTi SMA is not suitable for actual application to realized structures because of its high material cost. Therefore, future studies should compare more economical SMA wires, such as Fe-based SMA wires. From these future studies, it will be possible to get a better understanding of the practical applications. Conceptualization, C.-Y.J. and J.-H.L. ; methodology, C.-Y.J. and J.-H.L. ; investigation, T.-R.W. ; resources, J.-H.L. ; data curation, T.-R.W. ; writing--original draft preparation, T.-R.W. ; writing--review and editing, C.-Y.J. ; visualization, T.-R.W. ; supervision, J.-H.L. ; project administration, [PERSON]-Y.J. and [PERSON]-[PERSON]; funding acquisition, C.-Y.J. All authors have read and agreed to the published version of the manuscript.