4. Geometry Preparation and Meshing for Full Vehicle Crash Models

Full vehicle crash models are dominated by thin-walled stamped structures: rails, pillars, rockers, floor pans, and closures. These parts are modeled almost entirely with shell elements, which means the geometry pipeline must deliver clean, connected midsurfaces rather than solid CAD. The quality of this preparation step determines whether the model will run stably at the target timestep, whether spot welds will land on nodes, and whether the resulting intrusion and energy absorption predictions are trustworthy. This section walks through the practical sequence: CAD cleanup, midsurface extraction, shell and solid meshing, mesh quality criteria, and transition techniques.

CAD Cleanup Before Midsurfacing

CAD from a styling or body-in-white department is built for manufacturing, not for analysis. Before extracting midsurfaces, remove or repair features that will create degenerate elements or unnecessary density. Typical cleanup actions include:

• Delete small fillets and rounds below roughly 5 mm radius on non-critical edges; keep radii that control crush initiation.
• Remove holes smaller than about 8–10 mm diameter unless they are fastener locations or intentional crush triggers.
• Repair sliver faces, gaps, and mismatched edges between adjacent panels using the CAD tool's healing functions.
• Simplify beads and stiffening ribs only if they do not carry load path or energy absorption; otherwise retain them.
• Defeature logos, embossed text, and tooling marks that have no structural role.

A useful rule of thumb: if a feature is smaller than the intended element size, it cannot be represented and should be removed or idealized. For a typical full vehicle model with a 6–8 mm target shell size, features under 5 mm are candidates for deletion.

Midsurface Extraction

Midsurface extraction converts a thin solid or closed shell body into a single surface located at the mid-thickness of the original wall. The extracted surface must preserve the original thickness as an attribute, because LS-DYNA shell elements use the *SECTION_SHELL thickness and the *PART definition to carry it. Key practices:

• Extract midsurfaces part by part, not as one global operation, so thickness assignments remain traceable.
• Verify that the extracted surface is continuous across bends and that flanges remain planar where they should be.
• Check that the midsurface offset direction is consistent; a flipped normal will invert the shell's local coordinate system and affect contact and material orientation.
• Record the original wall thickness for each panel; typical automotive steel panels range from 0.6 mm to 2.5 mm, with high-strength rails often 1.2–2.0 mm.

After extraction, run a geometry check for free edges, duplicate surfaces, and overlapping faces. Free edges that should be shared between two panels indicate a connectivity problem that will later become a missing weld or an artificial hinge.

Shell Meshing Strategy

Shell meshing is the core of the crash model. The goal is a mesh that is fine enough to capture folding and bending, coarse enough to keep the timestep acceptable, and regular enough to avoid numerical noise. A practical workflow:

1. Set a global target element size, commonly 6–8 mm for body structures and 3–5 mm for critical crash zones such as the front rail crush initiators.
2. Mesh flat panels with mapped or semi-mapped quad meshes; use trias only in transition regions and keep them below about 5% of the total.
3. Align element edges with the expected fold lines and bead directions so that bending occurs along element boundaries rather than across them.
4. Maintain at least three to four elements across any flange or narrow feature that carries load.
5. Place nodes at all spot weld and bolt locations before meshing, so welds connect existing nodes rather than creating new ones.

For solid regions such as cast nodes, suspension mounts, and battery enclosures, use hexahedral or tetrahedral meshes. Hex elements are preferred where large plastic deformation is expected; tetrahedral elements with a minimum of two elements through the thickness are acceptable for stiffer, less deformable parts.

Mesh Quality Criteria

LS-DYNA is sensitive to poor element quality, especially in explicit crash analysis where distorted elements can cause negative volume errors or timestep collapse. The following criteria are widely used as acceptance thresholds for shell meshes:

These limits are not arbitrary. A warped quad, for example, produces an inconsistent normal and can generate spurious bending stiffness. A high aspect ratio element reduces the stable timestep and can lock in shear. After meshing, run a quality check and fix the worst offenders before assembling the model.

Mesh Transitions and Connectivity

Real vehicle structures change section and thickness frequently. Abrupt mesh size changes create stress concentrations and can trigger artificial failure. Use transition techniques that preserve load path continuity:

• Gradual size transition: change element size by no more than about 20–30% between adjacent elements.
• Triangular transition bands: use a single row of trias to bridge quad regions of different density, keeping the band narrow.
• Shared nodes at panel junctions: merge coincident nodes along flanges so that adjacent panels deform together.
• Spot weld modeling: use *CONSTRAINED_SPOTWELD or *MAT_SPOTWELD at shared nodes; ensure weld spacing matches the physical pitch, typically 25–50 mm.
• Adhesive and seam connections: represent continuous bonds with tied contact or cohesive elements rather than node merging, to allow realistic debonding.

Practical Checkpoint

A geometry and mesh preparation task is complete when: all load-carrying panels have midsurfaces with correct thickness; the shell mesh meets the quality table above; welds and adhesives connect the intended nodes; solid regions are meshed with adequate through-thickness resolution; and a free-edge and duplicate-element check returns no unexpected results. At that point the model is ready for material assignment, boundary conditions, and contact definition in the next stage.