Hyperelastic materials are more difficult to converge because they are either incompressible or nearly so. Tiny differences in nodal displacements cause large changes in the internal forces and pressure in each element causing the N-R Force Residual to bounce around, making it difficult to converge on nodal displacements that create a force equilibrium. A special element formulation has been developed to tackle this issue, called the mixed u-P element forumlation. It add an extra degree of freedom, pressure P that can be tracked to convergence separate from the nodal displacements u. This greatly improves the ability of the solver to find convergence of hyperelastic materials. The mixed u-P element formulation is turned on by inserting Keyopt,matid,6,1 using a Command Object inserted under the solid body in the Geometry branch of the Outline in Mechanical.
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