Self accommodating shear strain
When the SMA is heated from the martensitic phase in the absence of stress, the reverse transformation (martensite-to-austenite) begins at the temperature ) is associated with the energy dissipated during the transformation.
The martensitic transformation is a shear-dominant diffusionless solid-state phase transformation occurring by nucleation and growth of the martensitic phase from a parent austenitic phase.During cooling of the SMA material below temperature in absence of applied stresses, the variants of the martensitic phase arrange themselves in a self-accommodating manner through twinning, resulting in no observable macroscopic shape change (see the stress-temperature diagram shown in Figure 1).By applying mechanical loading to force martensitic variants to reorient (detwin) into a single variant, large macroscopic inelastic strain is obtained.Note that subsequent cooling will result in multiple martensitic variants with no substantial shape change (self-accommodated martensite).Also, note in Figure 2 that, in going from A to B many variants will start nucleating from the parent phase, while in going from D to E there is only one variant of the parent phase that nucleates from the single remaining martensitic variant indicated by D. Schematic representation of the thermomechanical loading path demonstrating the shape memory effect in an SMA.When an SMA undergoes a martensitic phase transformation, it transforms from its high-symmetry, usually cubic, austenitic phase to a low-symmetry martensitic phase, such as the monoclinic variants of the martensitic phase in a Ni Ti SMA.
The martensitic transformation possesses well-defined characteristics that distinguish it among other solid state transformations: is an important factor in characterizing shape memory behavior.
A new definition is introduced for maximum recoverable strain, which allows the model to capture the effects of tension–compression asymmetry and transformation anisotropy.
Furthermore, the coupling effects between normal and shear deformation modes are considered by merging inelastic strain components together.
The nature of the SME can be better understood by following the process described above in a stress-temperature phase diagram schematically shown in Figure 2.
The parent austenitic phase (indicated by A in Figure 2) in the absence of applied stress will transform upon cooling to multiple martensitic variants (up to 24 variants for the cubic-to-monoclinic transformation) in a random orientation and in a twinned configuration (indicated by B).
In addition, the hysteresis size increases, and the macroscopic transformation strain decreases.