## 3 Results and Discussion
### Recovery Stress Test of Fe-SMA  
Figure 8 shows the typical temperature-recovery stress behavior of Fe-SMA in this paper. Figure 6: Pre-straining of Fe-SMA specimens: (**a**) 2% of pre-strain; (**b**) 4% of pre-strain; (**c**) 6% of pre-strain; (**d**) 8% of pre-strain. Figure 7: Modulus of elasticity of pre-strained Fe-SMA with activated SME. The recovery stress diagram is divided into four connected segments: (1) from points 1 to 2 is thermal expansion on the specimen; (2) from points 2 to 3 is heating the specimen; while (3) from points 3 to 4; and (4) from points 4 to 5 are cooling the specimen. The recovery stress from points 1 and 2 was decreased due to the thermal expansion effect on the Fe-SMA during temperature increase. By comparing the thermal coefficient of typical steel material of 1.0 \(\times\) 10\({}^{-5}\) C\({}^{\circ}-\)1, the Fe-SMA remaining in austenitic microstructure has the 60% larger coefficient of thermal expansion, 1.6 \(\times\) 10\({}^{-5\circ}-\)1. This is the reason for the recovery stress being decreased [10]. The recovery stress was increased back to 100 MPa at 100 \({}^{\circ}\)C due to the SME effect and was decreased again to 80 MPa at the targeted 160 \({}^{\circ}\)C due to the thermal expansion. After reaching the target temperature of 160 \({}^{\circ}\)C, the thermal contraction effect increased the recovery stress to the point of 4 during cooling of the Fe-SMA to the ambient temperature. The inclination of the recovery stress and temperature is parallel to the initial dropping inclination at the points between 1 and 2 because of similar coefficients of thermal expansion and modulus of elasticity of the Fe-SMA [10]. The points between 4 and 5 show decreased inclination because the partial phase transformation that occurred on the Fe-SMA from austenite phase to martensite phase [9]. The recovery stresses induced by the activated SME of the Fe-SMA are plotted on different recovery temperatures from 100 \({}^{\circ}\)C to 220 \({}^{\circ}\)C and different pre-strained values from 2% to 8% in Figure 9. As shown in Figure 9 and Table 4, the recovery stress increases linearly with the recovery temperature on all the pre-strained specimens. [PERSON] reported the fatigue behavior of the Fe-SMA and reported the recovery stress of 372 MPa on the 2% pre-strained Fe-SMA at 160 \({}^{\circ}\)C, referred to as \"recovery temperature\" in this paper [32]. The recovery stress reported in [32] is somewhat higher than obtained from this paper with 2% pre-strained Fe-SMA and 160 \({}^{\circ}\)C heating temperature. Figure 8: Stress-temperature diagram of a heating and cooling cycle on 4% pre-strained Fe-SMA with the recovery temperature of 160 \({}^{\circ}\)C.  
Figure 9: _Cont._  
In the case of the pre-strain level variable, the 6% or over pre-strained specimen shows somewhat reduced recovery stress when compared to 4% pre-strained one. The recovery stress of 8% pre-strained specimen was significantly lower than that of 6% pre-strained specimens. This is because the modulus of elasticity of specimen decreases with the pre-strain increases, as aforementioned in Figure 7. Additionally, with all pre-strained specimens on the 190 \({}^{\circ}\)C and 220 \({}^{\circ}\)C temperatures, the recovery stresses did not show any significant increase. This phenomenon is related to the hysteresis behavior of the Fe-SMA material at different temperatures. The SMA materials have four specific temperatures: martensite start and finish temperatures (M\({}_{\text{s}}\) and M\({}_{\text{f}}\)) and austenite start and finish temperatures (A\({}_{\text{s}}\) and A\({}_{\text{f}}\)). After the temperature reaches the A\({}_{\text{f}}\), the deformation of the specimen is a constant, which causes a constant recovery stress.