Plastic strains in metals result from deviatoric stresses that exceed the yield strength of the material. However, during phase transformations, inelastic strains may appear already at smaller stress levels. This transformation-induced plasticity (TRIP) is therefore different from classical plasticity in that it does not involve a yield criterion and that it appears at stress levels that would otherwise be insufficient to cause plastic straining even in the softest of the phases. A description of TRIP strain rate, common in the literature (see, for example, Ref. 4), is

where the strain rate is proportional to the deviatoric part of the second Piola–Kirchhoff stress tensor S through the transformation-induced-plasticity parameter , the derivative of the saturation function Φ(ξd), and the rate of formation of the destination phase . Values for the transformation-induced-plasticity parameter will depend on the type of phase transformation. It can depend on, for example, carbon content and temperature (see Ref. 5). In Ref. 6, the transformation-induced-plasticity parameter for a given steel is expressed in terms of the relative volumetric thermal strain between the source and destination phases, and the yield stress of the source phase:

Several propositions exist for the saturation functions; see Table 3-1 and Ref. 10. Through the user-defined option, you can define the derivative of the saturation function.