# Recovery Behavior of Fe-Based Shape Memory Alloys under Different Restraints
## 1 Introduction  
Shape memory alloys (SMAs) are a special type of material that can be returned to their original shape by an external stimulus, such as heating or cooling, after having gone through a large plastic deformation [1]. The behavior of SMA exhibits a shape memory effect (SME), which is the phase transformation from the martensite phase to the austenite phase depending on the temperature [2]. Around 30 alloys that show a SME have been reported so far and nickel-titanium alloy (Nitinol), is the most common SMA [3]. Since these materials have a remarkable recovery charaterisdisc in the region of strain of 0.08, Nitinol has been used in a variety of industries including aerospace, mechanical, aeronautical and medical engineering [4, 5]. However, the usage of Nitinol in any civil or architecture field has been limited due to the high cost of raw materials and limited temperature history [6, 7].  
Alternatively, many studies on Fe-based SMAs (Fe-SMA) have been carried out after a Fe-SMA was developed by [PERSON] et al. [8]. [PERSON] et al. [9] reported that Nb and C particles enhanced the SME in the Fe-SMA without a training stage. [PERSON] et al. [2] developed an alloy that has a high recovery stress of 300 to 500 MPa at 130 to 160 \({}^{\circ}\)C using a Fe-17 Mn-58i-10 Cr-4 Ni-1(V,C)-based alloy. In particular, Fe-SMAs are favorable when applied to civil and architecture engineering because theyare economically efficient compared to Nitinol [10]. [PERSON] et al. [11] made a fishplate for crane rail joints and showed the availability of Fe-SMA for joint components. [PERSON] et al. [12] examined the Fe-SMA bar by applying it to a concrete pre-stressing tendon. The Fe-SMA bar made of the Fe-17 Mn-5 Si-10 Cr-4 Ni-1(V,C) alloy had 0.04 of pre-strain, 300 MPa of recovery stress with a 200 \({}^{\circ}\)C activation temperature, and 10% of relaxation after 1000 h. [PERSON] and [PERSON] [13] studied the reinforcement of a concrete structure through the near surface mounted method (NSM) using Fe-SMA strips. In the study, the concrete structure was reinforced using Fe-SMA strips and it showed a better performance in the service stress and ultimate stress compared to the unreinforced structure [13]. [PERSON] et al. [14] proceeded with a thermal mechanical test using Fe-17 Mn-5 Si-10 Cr-4 Ni-1(V, C) alloys in order to apply the Fe-SMAs to reinforcements in civil structures. In the test, about 208 MPa to 439 MPa of recovery stresses between 100 \({}^{\circ}\)C to 220 \({}^{\circ}\)C were monitored and the recovery stresses were considered to have an effect on the pre-stressing of the concrete structure [14].  
Most of the studies regarding the Fe-SMA being applied to civil structures are focused on the recovery stress under rigid restraint, except for the work by [PERSON] and his coworkers [1]. However, the behavior of Fe-SMAs in the civil structure varies as restraints change under various service conditions. In this study, the mechanical characteristics and recovery properties of Fe-SMA under different restraints are investigated through experiments. Especially, its applicability to the tendons of prestressed concrete and pipe couplers was experimentally examined.