A UMAT is part of a UEL

Hadi,

Whenever you create a UEL in Abaqus, you inherently have to include a "UMAT" (constitutive update) as part of code. When you are defining the stiffness matrix and the residual vector for the element, you need the stress tensor and material tangent stiffness tensor which are exactly the items your return to Abaqus in a UMAT call (along with the state variable array).  You don't necessarily use the UMAT subroutine call, rather you embed the UMAT's coding of the const. update within the UEL.

Is it your intent to only use the XFEM element only in selective portions of the mesh (i.e., where the damage variable reaches a critical value)?  If that is the case, I really don't have the knowledge to be any help to you, as it seems that would require some sort of adaptive remeshing technique.

Regards,

Jason