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This section describes the elastic materials, which are materials defined by a hyperelastic strain-enerygy function. A distinction will be made between so-called unconstrained and uncoupled materials. The former describe materials that can undergo volumetric compression. The latter are used for modeling (nearly-) incompressible materials, thus their constitutive models are constrained to produce an isochoric deformation, as long as the user has selected a sufficiently large bulk modulus to enforce that constraint.