Houdini 18.0 introduced a new, improved solve method for FEM, the global nonlinear (GNL) method. This method is intended as a replacement of the global single linearization (GSL) method that was used by FEM before Houdini 18.0. The GNL solve method should be used for all new projects. The GSL solve method is only retained for backwards compatibility with older scene files. You can switch between the two solve methods using the Solve Method parameter on the FEM Solver.
In addition to the Solve Method parameter, there is also a new Material Model parameter on both the FEM Solid Object and the FEM Hybrid Object. If you have a file made using Houdini 17.5 or before and you want to get as close as possible to the old behavior, you can set the Solve Method on the FEM Solver to GSL and set the Material Model to Corotated Linear.
Advantages of GNL over GSL:
More stable, higher-quality results at large substeps. It is often possible to get decent results using only 1 or 2 Substeps on the FEM Solver.
The ability to simulate highly nonlinear material models, such as the Stable Neo-Hookean Variant that’s exposed on all FEM Objects.
The ability to simulate improved skin, using the Shell settings of the FEM Hybrid Object
There are not many new parameters for the GNL method. The method was designed to be as compatible as possible with the existing FEM objects and their parameters. The idea is that using the GNL method, you get much closer to the ideal result that corresponds to the parameter settings. The only new parameter that is recognized only by GNL and not by GSL is the new Repulsion parameter. This parameter allows you to control and limit the strength of the GNL method’s soft collision response approach.
The vast majority of FEM features that worked with the GSL method, also work with the GNL method, and work in a similar way. However, there are some features that are supported by the older, GSL solve method that are not (yet) available in the GNL solve method. If you want to make use of one of this features, you can switch to the GSL solve method. The current list of limitations of the GNL solve method is:
Stress-based fracturing is not supported: The Enable Fracturing option on the FEM Solver is disabled when you choose GNL mode.
Collisions are supported only for tet-to-tet and tet-to-volume (no tet-to-polygon collisions)
The options Create Energy Attribute and Create Force Attribute on the FEM objects are not supported.
The FEM Hybrid Object’s Bend Stiffness parameter doesn’t work in GNL mode. Instead, it is recommended to use a layer of tetrahedra below your skin polygons to create bend resistance.
Some newer FEM features are available only in GNL mode. These include the FEM Slide Constraint.