3. Seat and head restraint modeling

In rear-impact and whiplash simulation, the seat and head restraint are not passive background geometry — they are the primary load path between the occupant and the vehicle. The quality of the seat model directly determines whether the predicted head-to-restraint gap, torso ramping, and head rotation correlate with physical sled or vehicle tests. This section covers the three modeling pillars that dominate seat model fidelity: geometry preparation, material definitions, and joint constraints.

Geometry Preparation

Seat geometry typically arrives as CAD in STEP or IGES format. The first task is to decide what must be represented explicitly and what can be idealized. For whiplash loading, the critical surfaces are the head restraint front face, the seatback cushion front surface, and the seat frame structure that transmits moment to the recliner and floor. Trim covers, stitching, and foam surface texture are irrelevant and should be removed or smoothed before meshing.

Mid-surface extraction is appropriate for stamped steel seat frames, while solid or thick-shell meshes are needed for foam. A practical meshing strategy for a driver seat is:

• Head restraint foam: solid hexahedral mesh, 8–12 mm element size, at least three elements through thickness.
• Seatback foam: solid hex mesh, 10–15 mm, with a refined band (5–8 mm) in the thoracic and lumbar contact zone.
• Seat frame and recliner mechanism: shell elements, 5–8 mm, with rigid links representing the recliner locking pawl.
• Seat pan and rails: shell elements, 8–10 mm, tied to the floor via spot-weld or rigid constraints.

Foam geometry often requires defeaturing: small radii below 3 mm, ventilation holes, and wire-frame inserts should be removed unless they affect local stiffness. After meshing, run a free-edge and duplicate-element check. A common failure is a foam mesh with inverted or negative-volume solids near the lumbar bolster, which causes early termination with "negative volume" errors during the rearward occupant push.

Material Definitions

Seat foam is the most sensitive material in the model. The recommended LS-DYNA material is *MAT_LOW_DENSITY_FOAM (MAT_57) or *MAT_FU_CHANG_FOAM (MAT_83) for strain-rate-sensitive foams. MAT_57 requires a uniaxial compression stress–strain curve, typically obtained from a quasi-static foam compression test at 10–100 mm/min. A representative polyurethane seat foam curve is shown below in tabular form for a 45 kg/m³ foam:

For the seat frame, *MAT_PIECEWISE_LINEAR_PLASTICITY (MAT_24) with a mild steel curve (E = 210 GPa, ν = 0.3, yield 250 MPa, hardening to 400 MPa at 0.15 plastic strain) is adequate. The recliner and head restraint posts should be modeled as rigid bodies (*MAT_RIGID) unless recliner slip or post bending is a specific study objective. If the head restraint is adjustable, the post-to-bushing interface can be represented with a translational joint and a friction coefficient of 0.1–0.2.

Joint Constraints

Joint constraints determine how the seat articulates and how load transfers to the occupant. The key interfaces are:

1. Foam-to-frame: Use *CONTACT_TIED_SURFACE_TO_SURFACE or shared nodes. Tied contact is preferred because it tolerates mesh mismatch between solid foam and shell frame.
2. Recliner: Model as a revolute joint (*CONSTRAINED_JOINT_REVOLUTE) with a locking stiffness. If the recliner is expected to yield, replace the joint with a plastic hinge using MAT_24 shells and a moment–rotation curve.
3. Head restraint posts: Constrain to the seatback frame with *CONSTRAINED_NODAL_RIGID_BODY or a translational joint if adjustment is modeled.
4. Seat-to-floor: Apply *CONSTRAINED_SPOT_WELD or rigid body constraints at the rail mounting points. The floor itself is typically represented by a rigid wall or a deformable floor pan.

A frequent modeling error is over-constraining the foam-to-frame interface. If the foam is fully tied to a rigid frame, the foam cannot shear and the predicted head-to-restraint gap will be unrealistically small. A better approach is to tie only the rear foam surface to the frame and leave the front surface free to deform against the occupant.

Verification and Common Failure Modes

Before running a full sled model, perform a seat-only push test: apply a 2 kN rearward force to the head restraint and a 5 kN force to the seatback at the thoracic level. Check that the recliner rotates less than 5° and that no negative-volume errors occur. Typical failure symptoms and fixes include:

• Negative volume in foam: reduce time step, refine mesh, or add *CONTROL_TIMESTEP with DT2MS mass scaling.
• Recliner rotation exceeds 15°: increase locking stiffness or add a plastic hinge.
• Head restraint separates from seatback: check post-to-frame constraint and add a tied contact.

The diagram above summarizes the load path from head restraint through posts, seatback foam, frame, recliner, and rails to the floor. Each interface has a specific constraint type and material assignment. When building the model, verify that every interface is either tied, jointed, or in contact — an unconstrained interface will allow the seat to separate under rearward load and produce non-physical head kinematics.

In summary, a reliable seat and head restraint model requires clean mid-surface and solid geometry, rate-appropriate foam curves, and explicit joint definitions at every structural interface. The verification push test and the failure-mode checklist provide a practical gate before the model is coupled to the occupant and run in a full rear-impact sled simulation.