When you apply nonlinear mesh adaptivity to problems that do not converge because of mesh distortion, it is necessary to use the mesh-quality-based criterion. Use care when specifying the SHAPE criterion so that the hex-to-tet remeshing occurs at a reasonable time, necessary for allowing the hex elements to model the large deformations without causing mapping difficulties for the subsequently generated tetrahedral meshes. The defined components for the criterion should include all elements that undergo large deformations and must adhere to the rules as shown in Figure 4.1: Multipart Multibody Schematic.

The SKEWNESS criterion specified for the subsequent tetrahedral meshes must adhere to the general recommendations for skewness criteria for tetrahedral elements.

When using nonlinear mesh adaptivity to improve solution accuracy, the energy-based criteria is often the best selection for general simulations. If you are unsure which parts of the model are critical regions for mesh refinement, the component should be defined on all the bodies of the model defined by hex-dom elements. A default value of 1.0 for the energy criterion should improve most problems. However, choosing all hex-dom components on the entire model would convert all of them to tetrahedral mesh components, eliminating the advantage of NLAD-ETCHG to more accurately model hex-dom mesh deformation. The recommended step is to define components carefully in the modeling phase itself, so that only a subset of them can be selected NLAD-ETCHG.

If you know which bodies are critical and require refinement, model them as SOLID186/SOLID187 and create components out of them to define NLAD-ETCHG energy-based criteria. Alternatively, you can also use position-based criteria.

When applying nonlinear mesh adaptivity to problems which would otherwise be unsolvable, but where the deformed mesh is not so distorted (for example, local deformations in buckling/bifurcation or rubber seal problems), combinations of criteria are often required. After the NLAD-ETCHG conversion to tetrahedral elements, use the position-based criterion for refining elements moving into specific particular regions (for example, small cavities). Without a fine enough mesh and sufficient degrees of freedom, the simulation may not accurately predict the behavior of material moving into such regions. To refine elements at specific intervals, use the energy-based criterion, specifying a very low value or 0 as the minimum energy for force the refinement.