# Experimental Analysis on the Impact of Current on the Strength and Lifespan of a Ni-Ti Element
## 2 Thermomechanical Behavior of the Ni-Ti Material  
Given the properties of Ni-Ti, two aspects of phase transformations are significant. The first is that the transformation is reversible, which means that heating, above the transformation temperature, leads to the return of the crystalline structure of the austenite phase. The second key aspect is the transformation in both directions [17; 18; 19]. The cooling/heating cycle represents the thermal hysteresis of Ni-Ti transformation. The temperature difference between the two phases, austenite and martensite, is due to the composition of the two metals and the precision of processing. When a shape memory alloy is in the austenite phase, it is exposed to extremely elastic behavior. This is known as the pseudoelastic effect or pseudoelasticity. Although pseudoelasticity is an important feature of shape memory alloys, typical of applications of these materials, it requires the use of the shape memory effect, which is a result of temperature-induced transformation. The value of this transformation temperature depends on the composition of the alloy [20; 21]. Pseudoelasticity, or superelasticity as it is also called, is caused by stress-assisted martensitic transformations, the term proposed later and currently accepted being that of stress-induced martensitic transformation (MIT) at constant austenitic temperatures. Pseudoelasticity can be considered as any nonlinearity on the stress-strain curve during discharge. Depending on the percentage by which it occurs, pseudoelasticity can be maclating or transforming. Pseudoelasticity of maclating occurs in metallic materials that deform by maclating [22]. Unlike the normal elastic return from unloading, the pseudoelasticity of maclating is manifested by an additional elastic return of approximately 0.8%. The general term under which the pseudoelasticity of maclare is found is pseudomaclare. It should be noted that an alloy with pseudomaclat shape memory loses its additional recovery if it is kept in a deformed state due to the rearrangement of pseudo-maclat which is brought to s more stable position [22; 23; 24; 25]. Transformation pseudoelasticity is due to a stress-induced martensitic transformation and cannot occur at temperatures above the critical transformation point. Figure 1 defines the typical mechanical parameters of the transformation superelasticity for a schematic stress-strain curve [18; 19; 22]. Thus, the following are defined: 6!--the load plate voltage, 6!--the discharge plate voltage, \(E_{1}\)--the energy dissipated during a PSE cycle (transformation hysteresis), \(E_{2}\)--the energy released for download. Based on these mechanical parameters, two more fundamental characteristics are defined for the study of transformation superelasticity [18;19]. - degree of elongation recovery:  
\[\epsilon_{rec}=\frac{\epsilon_{t}-\epsilon_{p}}{\epsilon_{t}}.100 \tag{1}\]  
where \(\epsilon_{t}\)--the total relative elongation and \(\epsilon_{p}\)--the permanent relative elongation;  
- energy storage efficiency:  
\[\eta=\frac{E_{2}}{E_{1}+E_{2}}.100 \tag{2}\]  
It is observed that in order to be usable, superelasticity, which is one of the most useful manifestations of the shape memory effect, must have a degree of elongation recovery as close as 100% and a high yield [22]. The first condition depends on several factors (temperature, total relative elongation, speed of deformation, chemical composition, degree of hardening, heat treatment, etc.). For the second condition, it is obvious that the increase in the efficiency presupposes a higher stiffness of the initial phase, a higher charging voltage and a lower dissipated energy \(E_{1}\). The ability to store elastic energy has proven to be one of the basic characteristics of shape memory alloys. In the case of transformation superelasticity, this capacity, together with the recoverable elongation, represent the most useful properties, having values much higher than the classic elastic materials [18].