## 7 Conclusions and Prospects  
Via the shape memory effect of Fe-SMA, prestressed structural reinforcement can be realized by heating activation, thus avoiding the need for complex mechanical equipment  
Figure 21: The NSM-strengthened technology [79, 97]: (a) filling with cement mortar, (b) activation by the resistive heating.  
Figure 22: Flexural reinforcement of concrete beams with Fe-SMA [71, 99]: (a) embedding in shotcrete layer, (b) nail–anchor connections.  
and reducing the requirements for construction spaces. The available research has shown that the recovery stress level of activated Fe-SMA can satisfy the reinforcement requirements of crack control and bearing capacity improvement. As an alloy, Fe-SMA has good ductility and machinability and is thermally compatible with steel and concrete. When reinforcing steel or concrete beams, the connection between Fe-SMA members and the reinforced structures is reliable, and the bearing performance of strengthened beams under static, hysteretic, and fatigue loads is significantly improved.  
However, the research on the mechanical properties and reinforcement applications of Fe-SMA is still far from mature. This paper summarizes the following aspects that need to be further studied:  
1. The constitutive behavior of Fe-SMA is highly nonlinear, and the elastic modulus decreases with the increasing stress. For Fe-SMA, the material constitutive relationship, the energy dissipation capacity under cyclic load, and the mechanical characteristic parameters under fatigue load need further study.
2. As a metal material, the creep and relaxation under load of Fe-SMA are pronounced. The influence of creep and relaxation on the recovery stress loss under the combined action of temperature, time, and stress level needs systematic research and quantitative analysis.
3. The four characteristic temperatures have a great influence on the mechanical properties of Fe-SMA. Therefore, its mechanical properties at different ambient temperatures (low temperature, normal temperature, and high temperature) need to be studied, which can be used as the data basis for numerical simulation and the theoretical analysis of structural reinforcement.
4. It is vital to study the fire resistance of structures reinforced with Fe-SMA. Whether the fire will reactivate Fe-SMA, whether the connection between the Fe-SMA members and reinforced structures in fire is reliable, and whether Fe-SMA will play a protective role for structures in fire need to be systematically studied.
5. For the local cracking of structures, a small range of repairs can be carried out near cracks. In this case, the welding method can be considered to connect Fe-SMA and the parent steel components, and its feasibility, reliability, and durability need to be studied and analyzed.
6. Most of the developed joining modes are in the stage of method design, test verification, and model application, and lack systematic research. The theoretical calculation and optimization design of bolt anchorage, nail-anchor, adhesive bonding, and other joining methods are the focus of future research.  
Conceptualization, X.Q. and Y.W. ([PERSON]); software, [PERSON] ([PERSON]) and [PERSON] ([PERSON]); investigation, [PERSON] ([PERSON]); data curation, X.Q.; writing--original draft preparation, [PERSON] ([PERSON]) and [PERSON] ([PERSON]); writing--review and editing, X.Q. and X.J.; visualization, [PERSON] ([PERSON]); supervision, X.J.; project administration, X.Q. and X.J.; funding acquisition, X.Q. and X.J. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (52278206 and 52278207), the National Key R&D Program of China (2020 YFD1100403), Natural Science Foundation of Shanghai (21 ZR1466100), Zhejiang Province science and technology plan project (2019017) and Fundamental Research Funds for the Central Universities (02002150114).  
Not applicable.  
The authors declare no conflict of interest.