# Recovery Behavior of Fe-Based Shape Memory Alloys under Different Restraints
## 5 Conclusions  
In this study, a series of experiment were conducted to understand three types of Fe-SMAs and the conclusions are listed below:  
1. While the ultimate strain in the FSMA-A specimen was 0.2256, which is about 75% less than the strain from FSMA-B and FSMA-C specimen, the yield strength estimated used 0.2% of the offset method in FSMA-A specimen. It was 599 MPa which is nearly 34% higher than the strength from FSMA-B and FSMA-C specimen. The elastic modulus calculated from FSMA-A, FSMA-B and FSMA-C specimen varied between 123 and 126 GPa and the effect of chemical composition was insignificant to the mechanical property.
2. Under the free restraint, the recovery strains are 0.00956, 0.01445, and 0.01977 for FSMA-A, FSMA-B, FSMA-C specimens after activation. Although the chemical composition in the FSMA-B and FSMA-C specimens are the same, the recovery strain differed by more than 37%. Accordingly, the heat treatment is proposed to have a high recovery strain in the Fe-based SMAs.
3. Under the rigid restraint, the effects of pre-strain in the FSMA-A, FSMA-B and FSMA-C specimens was insignificant compared to the final recovery stress when the Fe-SMA was activated. The recovery stress increased as the heating temperature was raised. Also, the FSMA-A has the higher the recovery stress than that of FSMA-B and FSMA-C. Also, FSMA-A has the less the short-term relaxation was formed than FSMA-B and FSMA-C.
4. In cases where the recovery strain developed when the Fe-SMA was activated and constrained in the same condition, the recovery stress in the FSMA-A specimen was greater than the one from the FSMA-C specimen. Thus, the FSMA-A type is suitable to be applied to a component, such as pre-stressing tendons, which requires a high recovery stress over a FSMA-C type.
5. In the case that the recovery strain developed when the Fe-SMA was activated and constrained in the same restraint condition, the recovery strain in the FSMA-C specimen was greater than the one from the FSMA-A specimen. Therefore, the FSMA-C type is applicable for use in pipe joining that encourages the component to have a high recovery strain.
6. The recovery stress in the FSMA-A and FSMA-C specimen constrained during cooling was 50% lower than those from the rigid restraint. Hence, constraining the deformation before the heating temperature is favorable in order to develop a high recovery stress.
7. When FSMA-C is used for the pipe coupler, the internal diameter of coupler should not be larger than 102% of the external diameter of connected pipe. Additionally, in order to develop an effective confining stress more than 50% or more of its recovery stress, the internal diameter of FSMA-C coupler should not be larger than 101.6% of the external diameter of connected pipe.  
Figure 15: Pipe joining process by Fe-SMA coupler.  
**Author Contributions:** [PERSON] and [PERSON] conceptualized the study; [PERSON] obtained field data; [PERSON] and [PERSON] implemented data processing under the supervision of [PERSON]; the original draft of the manuscript was written by [PERSON] and [PERSON] with editorial contributions from [PERSON] and [PERSON]. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure and Transport, Republic of Korea grant number 20 CTAP-C151899-01.  
**Acknowledgments:** This research was supported by a grant (20 CTAP-C151899-02) from Technology Advancement Research Program (TARP) funded by Ministry of Land, Infrastructure and Transport of Korean government  
**Conflicts of Interest:** The authors declare no conflict of interest.