3. Building a full-vehicle frontal impact model

A full-vehicle frontal impact model is not simply a large mesh. It is an assembly of subsystems — body-in-white (BIW), closures, powertrain, suspension, tires, occupant restraints, and barriers — each meshed with a strategy appropriate to its deformation mode and connected through a controlled hierarchy. This chapter walks through the practical pipeline: CAD cleanup, mid-surface extraction, shell and solid meshing, mesh quality targets, and the assembly hierarchy that keeps the model traceable and restartable.

3.1 CAD Cleanup Before Any Meshing

Meshing dirty CAD is the single largest source of wasted effort in crash modeling. Cleanup should be completed in the CAD environment (CATIA, NX, or SpaceClaim) before geometry enters the preprocessor. The goal is a watertight, defeatured, mid-surface-ready model.

• Remove non-structural detail: delete bolt threads, weld beads, small fillets below 3 mm, embossed logos, and trim clips. These create sliver surfaces that force tiny elements.
• Heal topology: stitch gaps, repair missing faces, and remove duplicate surfaces. A common target is zero free edges on each closed panel before mid-surfacing.
• Defeature for mid-surface: collapse small steps and chamfers so the offset operation produces a single continuous surface rather than fragmented patches.
• Assign thickness: record nominal sheet thickness per panel (e.g., 0.7 mm, 1.2 mm, 1.8 mm) in a property table; this becomes the shell section thickness later.

3.2 Mid-Surface Extraction and Shell Meshing

Thin-walled stamped panels — fenders, doors, floor, rails, and shock towers — are represented with shell elements on the mid-surface. Extraction offsets the outer and inner surfaces to a neutral plane. After extraction, verify that the mid-surface is continuous across beads and that the offset distance matches the recorded thickness.

Shell meshing then proceeds panel by panel. A practical sequence for a front rail is:

1. Set a target element size of 5–8 mm on the rail, refining to 3–4 mm near the crush initiators and spot-weld flanges.
2. Use quad-dominant meshing with mapped regions on regular flanges and free meshing on complex beads.
3. Place at least two element rows across every flange width so spot welds have a stable attachment.
4. Align element edges with the rail axis so axial folding is not artificially stiffened by skewed elements.

3.3 Solid Meshing for Thick and Cast Components

Not everything is a shell. Engine mounts, control arms, knuckles, and some suspension links are castings or forgings with significant thickness. These use solid elements — typically hexahedral where geometry allows, tetrahedral elsewhere. A common approach is to mesh the component with 4–6 mm hex elements, then tie the solid mesh to adjacent shell structure through a tied contact or a constrained nodal rigid body.

Powertrain blocks are often represented as a rigid body with a lumped mass and inertia tensor rather than a detailed solid mesh, because their deformation is negligible in a frontal crash. The decision rule is simple: if the component is expected to deform and absorb energy, mesh it; if it only transfers load and inertia, rigidize it.

3.4 Mesh Quality Targets

LS-DYNA is explicit and conditionally stable. Poor elements force a small time step, which multiplies run time. The following targets are widely used for full-vehicle frontal models and should be checked before assembly.

The minimum element size directly sets the stable time step. For steel with a sound speed near 5000 m/s, a 3 mm element gives a time step on the order of 0.4 µs. If a single sliver element is 0.5 mm, the whole model runs at that smaller step. This is why quality checking is not cosmetic — it is a runtime decision.

3.5 Assembly Hierarchy and Connections

Once panels are meshed, they are assembled into a hierarchy that mirrors the physical vehicle. A typical structure is:

FULL_VEHICLE
├── BIW
│   ├── FRONT_END (rails, bumper beam, crash box)
│   ├── UNDERBODY (floor, tunnel, rockers)
│   └── REAR_END
├── CLOSURES (hood, doors, fenders, trunk)
├── POWERTRAIN (rigid body + mounts)
├── SUSPENSION_FRONT / REAR
├── WHEELS_TIRES
├── OCCUPANT_RESTRAINTS
└── BARRIER

Connections are defined at the assembly level. Spot welds are modeled with *CONSTRAINED_SPOTWELD or solid hex weld elements, depending on whether weld failure is being studied. Structural adhesive uses *CONSTRAINED_INTERPOLATION or cohesive elements. Bolted joints use *CONSTRAINED_NODAL_RIGID_BODY with a defined node set. Each connection type has a distinct failure behavior, so mixing them carelessly will produce non-physical load paths.

3.6 A Worked Example: Front Rail Sub-Assembly

Consider a front rail with a 1.6 mm high-strength steel outer and a 1.2 mm inner. After mid-surfacing, the outer panel is meshed at 6 mm with mapped quads on the flange and free quads on the bead region. The inner panel uses 5 mm elements to capture the crush initiator geometry. Spot welds are placed every 25–30 mm along the flange, each represented by a *CONSTRAINED_SPOTWELD with a defined failure force. The rail is then tied to the shock tower through a *CONSTRAINED_INTERPOLATION patch, and the rear end is connected to the floor through a nodal rigid body at the firewall junction.

After assembly, run a quality check and a short 5 ms initialization run. Observe the initial penetration report and the time step. If the time step drops below 0.3 µs, locate the smallest elements and coarsen them. If initial penetrations exceed 0.5 mm in the contact regions, adjust the contact thickness or the mesh alignment before proceeding to the full 150 ms frontal run.

3.7 Verification Before the Full Run

Before committing to a 150 ms frontal simulation, run a 5–10 ms initialization. Check three things: the initial penetration report (should be near zero), the mass summary (compare against the physical vehicle mass within 2–3%), and the time step (should be stable and above the target). If the mass is off, a rigid body density or a missing component is the usual cause. If the time step is unstable, a sliver element or a bad solid element is the culprit. Fixing these at the 5 ms mark costs minutes; discovering them at 120 ms costs hours.

The assembly hierarchy is not just organizational — it determines how connections, contacts, and boundary conditions are applied. A clean hierarchy with explicit connection definitions is what makes the model debuggable, restartable, and trustworthy for the correlation work that follows.