SME constitutes both stress induced transformation (twinned Martensite to de-twinned Martensite) and temperature induced transformation (de-twinned Martensite to Austenite). SME is a useful property of SMA for applications in the field of linear actuators as the displacement range of an actuator can be easily controlled by controlling the temperature of the SMA element.

To understand this complicated behavior and to optimally design smart actuating elements, various constitutive models have been proposed. These models may be classified into two types:

• The first group uses a micromechanical approach to follow closely the crystallographic phenomena of the SMAs by using the thermodynamics laws for describing the transformation. These models consider the martensitic variant as a transformation inclusion and use micromechanics to calculate the interaction energy due to the phase transformation in the material. Stresses and strains are obtained as averages over a volume in which many inclusions are considered representing possible variants. Most of the micromechanics-based constitutive models include implicit variables such as free energy and predict the material behaviors of the SMAs qualitatively. These models cannot be easily used for engineering application as variables such as free energy are not readily quantifiable.
  
  
• The alternate macroscopic (phenomenological) model is built on phenomenological thermodynamics and/or curve fitting of experimental data. Transition regions in the common phase diagram of SMA are experimentally determined by plotting the stress-temperature diagram. The first macroscopic constitutive model was proposed by Tanaka. This model is for one dimensional SMA and is obtained from general three dimensional theory based on the energy balance equation and the Clausius-Duhem inequality.