ABAQUS user-defined UMAT and VUMAT. UMAT: Warning: The use of this subroutine generally requires considerable expertise. You are cautioned that the implementation of any realistic constitutive model requires extensive development and testing. Initial testing on a single-element model with prescribed traction loading is strongly recommended. User subroutine UMAT: can be used to define the mechanical constitutive behavior of a material; will be called at all material calculation points of elements for which the material definition includes a user-defined material behavior; can be used with any procedure that includes mechanical behavior; can use solution-dependent state variables; must update the stresses and solution-dependent state variables to their values at the end of the increment for which it is called; must provide the material Jacobian matrix for the mechanical constitutive model; can be used in conjunction with user subroutine USDFLD to redefine any field variables before they are passed in; and is described further in User-defined mechanical material behavior.