# Fatigue Characteristics of Fe-Based Shape-Memory Alloys
## 1 Introduction  
Shape-memory alloys (SMAs) are special alloys that can restore their original shape through heating and cooling--even if plastic deformation exceeding their elastic deformation capacity occurs [1]. This recovery performance is called a shape memory effect (SME), and more than 30 alloys including Ni-Ti alloy (nitiol) having this property have been reported [2]. In particular, nitiol has excellent recovery performance and is used in various fields such as the aerospace industry, machinery and medical devices [3]. However, the high raw material price of nitiol is regarded as an obstacle to the application of this alloy to the civil and architectural fields [4, 5].  
Since [PERSON] et al. [6] developed Fe-based shape-alloy alloys (Fe-SMAs) in 1982, research has been actively conducted to apply these alloys to the construction and civil engineering fields. [PERSON] et al. [7] conducted a study to use stripes made of Fe-SMA as reinforcements for the near-surface mounted method (NSM). They reported that specimens with activated stripes have an increased crack load of about 80% compared to those with inactivated stripes. [PERSON] et al. [8] conducted a study to utilize bars made of Fe-SMA as reinforcements for the NSM method. Through the results of the experiment, they confirmed that strengthening of RC structures using the Fe-SMA bars can secure high ductility, in contrast with strengthening of structures by FRP. [PERSON] et al. [9] strengthened reinforced concrete beams with Fe-SMA plates by an externally bonded method. They confirmed that specimens reinforced with Fe-SMA plates showed higher cracking loads than specimens reinforced with carbon fiber reinforced polymer (CFRP) due to prestressing force. [PERSON] et al. [10] evaluated themechanical properties and recovery characteristics of ribbed Fe-SMA bars with a diameter of 12 mm and 16 mm and proposed a stress-strain model of Fe-SMA bar.  
Civil structures have higher risk of fatigue failure due to cyclic loading than the risk of destruction due to static overload [11]. In particular, unlike normal flexural failure, fatigue failure occurs at a much lower stress than static fracture stress, and most cases exhibit sudden brittle fracture behavior [11]. In addition, if a Fe-SMA is used as the prestressing tendon of bridges or railway structures, it can be expected to receive millions of cyclic loads, and thus a review of its fatigue performance is essential.  
However, most of the existing research on Fe-SMA has been aimed at identifying the mechanical and recovery characteristics of the material, and there is a lack of research focused on evaluating fatigue characteristics. Therefore, fatigue tests for two types of alloys developed through previous studies conducted by these authors are reported in this study. One is an alloy applicable to elements requiring high recovery stress such as prestress tendons and the other is an alloy applicable to elements requiring high recovery strain such as pipe couplers. In addition, the fatigue test results were used to analyze the S-N correlation of the Fe-SMAs and to present their fatigue limits. The fatigue performance of the alloy manufactured by the process generally applied to the manufacture of Fe-SMA was evaluated in this study. Therefore, this study is expected to be referred to as meaningful data in evaluating the fatigue performance of this kind of alloy.