The dependent variable is the dimensionless phase field ϕ with the default variable name phi. By default, a single phase field variable is included when the interface is added to a component, but multiple phase field variables can be added. When modeling damage and fracture in solids, the phase field is a bounded quantity that represents a smooth transition from undamaged (ϕ = 0) to fully damaged material (ϕ = 1).

The governing equations of the phase field are formulated on the material frame in a total Lagrangian formulation consistent with the Solid Mechanics interface, see the sections Material and Spatial Coordinates and Lagrangian Formulation in the Structural Mechanics Theory chapter.