The UserMat subroutine is a user-programmable feature for developing a custom, constitutive model to define the stress-strain behavior of a material. User-defined materials can be used with current-technology elements that have a displacement-based element formulation.

Called at each element integration point, the incremental and total deformations are passed into the UserMat subroutine, along with stress and state variables from the previous increment. You must develop an algorithm which updates the material state and stress at the end of the increment, as well as calculates the algorithmic material tangent. In analyses with nonlinear geometry (NLGEOM,ON), the user-defined material is formulated in a co-rotational frame, and the stress and algorithmic tangent must be calculated in this co-rotated frame.