## 4 Strengthening Mechanism with Activated Fe-SMA
### Recovery Stress Level of Fe-SMA  
The activation process of Fe-SMA includes two key steps: pre-stretching and heating. Therefore, the obtained recovery stress will be different under various combinations of pretension strains and activation temperatures. Table 5 lists the recovery stress of Fe-SMA reported in various activation tests. Figure 7: Reinforcement process with Fe-SMA: (**a**) concrete beam, (**b**) cracked steel plate. The prestrain is concentrated at the magnitude of 2-6%, and the activation temperatures are distributed in the range of 150-400 \({}^{\circ}\)C. The recovery stress of different Fe-SMA used for structural reinforcement obviously varies. [PERSON] et al. [98] obtained a recovery stress of 406 MPa under the prestrain of 2% and activation temperature of 260 \({}^{\circ}\)C when strengthening cracked steel plates using Fe-SMA strips. [PERSON] et al. [80] obtained a recovery stress of 160 MPa under the prestrain of 6% and activation temperature of 350 \({}^{\circ}\)C when strengthening concrete beams with Fe-SMA bars. The structural reinforcement us  
Figure 8: Effect of repairing fatigue cracks with Fe-SMA. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Alleys** & \begin{tabular}{c} **Specimens** \\ **Shape** \\ \end{tabular} & \begin{tabular}{c} **Prestrain** \\ **Size (mm)** \\ \end{tabular} & \
\begin{tabular}{c} **Activation** \\ **(\%)** \\ \end{tabular} &
\begin{tabular}{c} **Recovery Stress** \\ **Temperature (\({}^{\circ}\)C)** \\ \end{tabular} & **Refs.** \\ \hline \multirow{9}{*}{Fe-17 Mn-5 Si-10 Cr-4 Ni-1(V,C)} & Strip & 0.7 \(\times\) 3 & 4\% & 225 \({}^{\circ}\)C & 380 & [57] \\  & Strip & 0.9 \(\times\) 3 & 4\% & 160 \({}^{\circ}\)C & 330 & [57] \\  & Strip & 1.7 \(\times\) 14 & 4\% & 160 \({}^{\circ}\)C & 580 & [96] \\  & Strip & — & 4\% & 160 \({}^{\circ}\)C & 266 & [72] \\  & Strip & — & 4\% & 160 \({}^{\circ}\)C & 350 & [95] \\  & Strip & 1.5 \(\times\) 10 & 2\% & 160 \({}^{\circ}\)C & 372 & [83] \\  & Strip & 1.7 \(\times\) 25 & 2\% & 160 \({}^{\circ}\)C & 177\(\sim\)200 & [97] \\  & Bar & \(\varphi\)8 & 4\% & 160 \({}^{\circ}\)C & 277\(\sim\)305 & [71] \\  & Strip & 0.8 \(\times\) 52.5 & 2\% & 260 \({}^{\circ}\)C & 406 & [98] \\  & Strip & 1.5 \(\times\) 100 & 2\% & 160 \({}^{\circ}\)C & 292 & \\  & Strip & 1.5 \(\times\) 100 & 2\% & 180 \({}^{\circ}\)C & 330 & [99] \\ \hline \multirow{2}{*}{Fe-28 Mn-6 Si-5 Cr-0.5(Nb,C)} & Non pre-rolled & 4\% & 400 \({}^{\circ}\)C & 145 & \\  & 6\% pre-rolled & 4\% & 400 \({}^{\circ}\)C & 255 & \\  & 14\% pre-rolled & 4\% & 400 \({}^{\circ}\)C & 295 & \\ \hline Fe-28 Mn-6 Si-5 Cr-0.53 Nb-0.06C & Strip & — & 4\% & 397 \({}^{\circ}\)C & 250 & [60] \\ \hline Fe-16 Mn-5 Si-10 Cr-4 Ni-1(V,N) & — & — & 4\% & 160 \({}^{\circ}\)C & 440 & [44] \\ \hline Fe-28 Mn-6 Si-5 Cr & \multirow{2}{*}{Strip} & — & 3\% & 300 \({}^{\circ}\)C & 255 & \\  & & 3\% & 300 \({}^{\circ}\)C & 185 & \\ \hline Fe-18 Mn-8 Cr-4 Si-2 Ni-0.36 Nb-0.36N & \multirow{2}{*}{Strip} & 1.5 \(\times\) 20 & 2\% & 160 \({}^{\circ}\)C & 308 & \\  & & 4\% & 160 \({}^{\circ}\)C & 348 & \\ \hline Fe-Mn-Si alloy & Bar & \(\varphi\)14.3 & 6\% & 350 \({}^{\circ}\)C & 160–215 & [80] \\ \hline Fe-Mn-Si alloy & Strip & 1.5 \(\times\) 15.8 & \(\approx\)3\% & 155 \({}^{\circ}\)C & 268–295 & [81] \\ \hline Fe-19 Mn-4 Si-8 Cr-4 Ni-0.01C & Wire & \(\varphi\)0.7 & 4\% & 250 \({}^{\circ}\)C & 268 & \\  & & 5\% & 250 \({}^{\circ}\)C & 280 & [35] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Recovery stresses of Fe-SMA under different activation combinations. ing Fe-SMA can satisfy the precompression needs of cracked steel plates and the bearing capacity improvement of beam structures. Notably, excessive activation temperature is acceptable for steel structures, but it may damage the mechanical performance of concrete structures, which should be avoided. In addition, [PERSON] et al.