The second order tensor, E_in, is defined as transformation strain to measure the strain associated with the phase transformation:

(40–1)

where ε_L is a maximum value norm of E_in in the phase transformation after fully transformed.

The stress, σ, is therefore expressed in terms of strain:

(40–2)

During the transformation, the transformation stress is defined as:

(40–3)

where is a positive and monotonically increasing function of the room temperature, T, and the material-dependent temperature, T₀, below which no twinned martensite occurs. β is a material parameter. The material parameter h is associated with the hardening of the material in the phase transformation.

γ is defined by

The evolutionary equation for E_in has the following form:

(40–4)

where the limit function F is given in terms of the transformation stress X_tr and the elastic domain radius R in the form of the Prager-type limit function:

(40–5)

where:

Thus, the governing equations for the phase transformation are expressed as:

(40–6)

In addition to the Young’s modulus and Poisson’s ratio of martensite and austenite, six other parameters are defined: M, R, h, T₀, β, and ε_L.