When mixing Hexa and Tetra elements in a solid mesh in Ansys LS-DYNA, ESORT = 1 is commonly recommended because it transforms tetrahedral elements to ELFORM=10, which is a one-point constant stress formulation. This approach is often used for mesh transitions, particularly when the mesh consists predominantly of hexahedral elements and only a small region requires tetrahedrons for geometric compatibility.
Tetrahedral elements with ELFORM=10 are known to be stiffer than hexahedral elements, especially in bending and incompressible regimes, due to their single integration point and susceptibility to volumetric locking. However, ESORT = 1 is preferred for mesh transitions because it ensures compatibility and avoids issues that may arise from using higher-order formulations in regions where only a few tetrahedrons are present.
ELFORM=13 (TET13) is more accurate and less prone to volumetric locking, but it is generally recommended for cases where the mesh is predominantly tetrahedral or when high accuracy is needed throughout the domain. Using ESORT = 11 (TET13) in a mixed mesh may introduce compatibility issues or require additional computational resources for only a small benefit in accuracy, especially if the tetrahedral region is limited to mesh transitions. [Element formulation 10 is preferred in mesh transitions because it is robust and avoids the need to worry about the formulation for tetrahedrons in these specific regions.
In sum:
ELFORM=10 is often used for mesh transitions where only a small region requires tetrahedrons for geometric compatibility with hexahedral elements. It is robust and avoids compatibility issues in these cases.
ELFORM=13 is recommended for predominantly tetrahedral meshes or when high accuracy is required in tetrahedral regions, especially for incompressible materials or large plastic deformations.
In mixed Hexa-Tetra meshes, ELFORM=10 is preferred for mesh transitions, while ELFORM=13 is used when tetrahedral regions dominate or require improved accuracy.
More summary:
ELFORM 10: Use primarily for limited tet regions, especially as mesh transitions where ESORT is used. Avoid for large tet-dominated, incompressible, or strongly plastic regions because of volumetric locking and stiffness in bending.
ELFORM 13: Default choice for predominantly tetrahedral meshes or regions requiring better behavior in nearly incompressible or highly plastic deformation, accepting a small performance cost and the need for a finer mesh.
Mixed hex–tet meshes: Use hexes as the dominant element type; allow tets (often ELFORM 10 via ESORT) only where necessary for geometric transitions. Where tet regions become large or physics are incompressible/plastic, switch those regions to ELFORM 13, but avoid ELFORM 13 at nodes shared by materials with very different material prop./behaviour, using ELFORM 10 there instead.
Hope this helps – but perhaps others can provide some more feedback
Erko
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