# Multi-Step Prestressing with Hybrid SMA Wires
## 3 Multi-Step Shape Recovery Test  
### Test Vasriabe and Specimens  
Before conducting prestressing experiments using the hybrid SMA wires, the behaviors of the materials constituting the hybrid SMA wires should be identified. To this end, a multi-stepwise shape recovery test observes the deformation behavior of the SMA wire according to the SME caused by the heat applied in the chamber. To evaluate ranges of austenite temperature of SMA wires and to confirm the behavior of multi-stepwise shape recovery, three specimens for each of four types of wires were fabricated; two types of SMA wires and two types of hybrid SMA wires. The comparative test for each type of SMA is an experiment to confirm the austenite temperature of individual SMA. Hybrid SMA wires are tested to compare the behaviors of different SMA combinations. Two types of NiTi SMA wires were used. The material properties of the wires used in this study are shown in Table 1 and Figure 3. The stress-strain relationship of SMA wires is different from that of conventional structural steel, in that the yield behavior appears twice in case of the SMA wires. It may appear differently depending on the manufacturing method and chemical composition of the SMA wire. The SMA wire used in this study has a primary yield point at approximately 150 MPa and a secondary yield point at approximately 800 MPa. The estimated tensile strength of this wire was approximately 1000 MPa.  
Figure 2: Stress distribution due to the prestressing method with hybrid SMA wires.  
The two major test variables in this study were single wire and hybrid wire. Single-wire specimens were used to check the austenite temperature required to activate the SME of individual SMA wires. Although the manufacturer had provided these values, to double check and prevent any error, we confirmed the austenite temperature experimentally by using a heating chamber. In addition, we conducted experiments to compare SMA wire behavior in the single-wire state with that in a hybrid wire state. As shown in Figure 4a,b, a hybrid wire is a mechanical combination of three wires. Hybrid-wire experiments were performed to verify whether the behaviors of the individual constituent wires were combined. Here, combined behavior implies that the SME effect is expressed in a stepped manner as to the heating temperature reaches the austenite temperatures of the individual constituent. Three specimens were prepared for each variable to ensure the validity of the experimental results. Table 2 summarizes the types of specimens.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Type**} & \multicolumn{2}{c}{**Austenite**} & \multicolumn{6}{c}{**Chemical Composition (\%)**} \\  & **Temperature** & **Ti** & **Ni** & **Co** & **Cu** & **Fe** & **Nb** & **C** & **Others** \\ \hline NiTi50 & 50 \({}^{\circ}\)C \(\pm\) 5 \({}^{\circ}\)C & \multirow{2}{*}{Bal.} & 55.89 & 0.050 & 0.010 & 0.050 & 0.025 & 0.046 & Each \textless{} 0.10 \\ NiTi90 & 90 \({}^{\circ}\)C \(\pm\) 5 \({}^{\circ}\)C & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Material properties of the SMA wire, as provided by the manufacturer.  
Figure 4: Details of specimens: (**a**) unbent hybrid SMA wire and bent hybrid SMA wire; (**b**) section of specimen at wire clip.  
Figure 3: Stress-strain curves of SMA wires.