Mike Rife
Ansys Employee

The 1st order Ogden has 1 incompressibility term. If this is not defined then the material is incompressible and mixed uP elements are used.  For incompressible hyperelastic materials the mixed uP elements enforce the uP via Lagrange multipliers.  Iterative solvers like PCG have a hard time solving when stiffness matrix has Lagrange Multipliers (or cannot solve it at all).  So the sparse solver must be used.  Or define some compressibility for the material and then don't use the mixed uP formulation (this is done automatically in WB Mechanical).  The PCG solver uses about an order of magnitude less RAM than the sparse (direct) solver does.  So in a hardware limited situation, we try to use an iterative solver if possible.