Shape memory alloys (such as Nitinol) exhibit hyperelastic effects. The term hyperelasticity is used to describe materials that can withstand large deformation without permanent deformation under the loading and unloading cycle. Under the loading and unloading cycle, and even up to 10-15% strain, Nitinol materials will show a lag reaction and undergo a steep -- gentle -- steep change during loading and unloading, but without permanent deformation. Nitinol material models can be used for solid and shell elements.
The stress-strain curves of shape memory alloys show unique macroscopic behaviors that are not found in traditional materials. This behavior is attributed to fundamental macromechanics. SMA exhibits a reversible martensite phase shift, that is, a solid to solid nondiffusion transformation between austenite with more ordered phase and martensite with less ordered phase.
The flat portion of the response curve represents the area where the phase shift occurs: austenito-martensite conversion (loading) and martensito-austenite conversion (unloading).
But, for simplicity, we call the gentle changes in the reaction curve plasticity and the steep parts elasticity.
By this definition, the material first exhibits elastic behavior until it reaches a certain stress level (initial yield stress at loading). If continued loading, the material will display elastic-plastic behavior until the plastic strain reaches its limit. After that, the material exhibits elastic behavior when loading is increased.
During unloading, the material will always start unloading in an elastic manner until the stress is reduced to the initial yield stress at unloading. The material will then be elastoplastic unloaded until all accumulated plastic strains are lost (from the loading stage). After that, the material will be unloaded in an elastic manner until it returns to its original shape (without permanent deformation) and zero stress under zero load.
