1. Build a Baseline Reclined Occupant FE Model
Why a Baseline Model Comes First
Every reclined-occupant assessment in this course is a delta against a single reference model. If the reference is inconsistent — different mesh density, different material card revisions, different contact friction — then a 12 mm change in HIC36 between two seat-back angles may be a modeling artifact rather than a real restraint effect. The baseline model therefore has one job: be stable, documented, and reproducible, so that later parametric studies change exactly one variable at a time.
This case assembles that reference: a 50th-percentile male occupant FE model seated at a 45° seat-back angle, with defined geometry, material assignments, and contact definitions. The 45° value is the mid-point of the 30–60° range used later, which makes it the natural anchor for the sweep.
Reference Geometry and the 45° Configuration
Start from a validated occupant model (e.g., a THUMS or GHBMC 50th-male variant) and a seat assembly. The critical geometric decision is how the 45° is measured. Define it explicitly and keep it consistent across all 100 cases:
- Seat-back angle θ = angle between the seat-back cushion plane and the vertical (torso reference axis), measured in the sagittal plane. θ = 45° means the torso is reclined 45° from upright.
- H-point = hip joint center of the occupant, positioned to coincide with the seat cushion reference point within ±5 mm in X and Z.
- Pelvis angle = 45° ± 2° relative to vertical, achieved by rotating the whole occupant about the H-point, not by re-posing individual joints.
Rotate the occupant as a rigid assembly about the H-point first, then apply only the minimum joint adjustments needed to seat the buttocks and back against the cushions. This preserves the model's validated joint stiffness corridors. Record the final H-point coordinates and pelvis angle in the model log — they are the acceptance values for every downstream case.
baseline_geometry.json (or equivalent include file) holding H-point, θ, pelvis angle, and cushion reference points. Later cases should read this file, never re-derive it.Material Assignments
Material cards must be frozen at the baseline. The table below lists the assignment categories that matter for reclined loading; use the exact card IDs from your validated model's documentation rather than re-tuning them here.
| Component | Material model | Key parameters to lock | Why it matters at 45° |
|---|---|---|---|
| Seat cushion foam | Low-density foam (e.g., *MAT_LOW_DENSITY_FOAM) | Density, hysteresis unloading curve | Controls pelvis sink-in and submarining tendency |
| Seat-back foam | Foam, same family | Loading/unloading curves | Sets torso penetration depth and contact onset |
| Seat frame | Elastic-plastic steel | Yield stress, hardening | Back-frame rotation under torso load |
| Occupant soft tissue | Validated occupant tissue cards | Shear modulus, failure strain | Injury metrics depend on these; do not alter |
| Belt webbing | Fabric / seatbelt material | Load-extension curve, strain rate | Restraint coupling is the study variable later |
Verify assignments by running a short (< 5 ms) gravity-settle step and confirming that foam compression and belt tension reach steady values without element errors or negative volumes.
Contact Definitions
Reclined occupants load the seat through a different contact patch than upright occupants: the buttocks slide forward and the upper back contacts lower on the seat-back. Define contacts explicitly rather than relying on a single automatic general contact:
- Occupant–seat cushion: surface-to-surface, friction μ = 0.3 (documented value; keep constant across cases).
- Occupant–seat back: surface-to-surface, μ = 0.3, with a soft constraint penalty scaled to foam stiffness to avoid penetration spikes.
- Belt–occupant: surface-to-surface with belt segment-to-torso mapping; μ = 0.15 on webbing.
- Occupant self-contact: thigh–abdomen and arm–torso pairs, to prevent interpenetration during the settle.
Set a consistent contact thickness offset (typically half the shell thickness) and confirm that initial penetrations are below 1 mm after the settle step. Initial penetration is the most common cause of spurious early forces in later runs.
Assembling and Verifying the Baseline
Build the model in this order so that each stage can be checked independently:
- Import and position the occupant and seat; rotate the occupant about the H-point to θ = 45°; confirm H-point coincidence.
- Assign materials from the frozen card set; run a single-element sanity check on foam and belt cards.
- Define contacts as listed above; run a contact-initialization check and log maximum initial penetration.
- Gravity settle for 5 ms with the occupant restrained only by contacts; confirm steady foam compression and belt tension.
- Record H-point, pelvis angle, contact penetrations, and settle-step energy balance into the model log.
Acceptance criteria for the baseline: H-point within ±5 mm of target, pelvis angle 45° ± 2°, all initial penetrations below 1 mm, no negative-volume or zero-mass elements, and a settle step whose hourglass and internal energy remain bounded (no energy growth after the first 2 ms).
What the Baseline Locks In
Once accepted, this model is the immutable reference for the remaining 99 cases. The 45° configuration, the material card IDs, the four contact definitions with their friction values, and the recorded H-point and pelvis angle are all part of the baseline contract. Later cases that change seat-back angle, initial posture, or restraint parameters must do so by editing a single documented parameter while leaving everything else untouched — that is what makes the resulting deltas attributable to the variable under study rather than to modeling drift.