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### Twenty-node brick element with reduced integration and incompressibility condition at the corner nodes (C3D20RI)

This element is identical to the C3D20R element, except that in addition an isochoric condition is applied at the corner nodes of the element. This is guaranteed by creating a nonlinear multiple point constraint at each corner node of the element expressing that the Jacobian determinant of the deformation is equal to unity (J=1). Due to the nature of the shape functions only those nodes which are connected by an edge to a particular node enter its incompressibility condition.

The incompressibility embodied by the C3D20RI element is weaker than those implemented by a material law, since only the corner nodes are involved.