2. Head injury criteria and biomechanical limits
Interior trim countermeasures exist to keep the occupant's head from being injured during a crash. To design them rationally, we need a quantitative definition of "injured." That definition comes from head injury criteria — mathematical functions of the measured head kinematics that correlate with observed injury in cadaver, animal, and reconstruction studies. This section explains the three families of metrics that drive trim design: linear acceleration–based criteria (HIC and its variants), rotational injury metrics (BRIC, DAMAGE, HIP, and rotational velocity thresholds), and the combined or tissue-level criteria that are increasingly used in regulation and rating. It then shows how each metric constrains the geometry, stiffness, and packaging of a trim countermeasure.
Linear acceleration and HIC
The Head Injury Criterion is defined over the resultant translational acceleration at the head center of gravity:
HIC = max over (t1, t2) of [ (t2 - t1) * ( (1/(t2-t1)) * ∫ a(t) dt )^2.5 ]
subject to t2 - t1 ≤ 36 ms (HIC36) or 15 ms (HIC15)In practice, the acceleration signal is filtered per SAE J211 (channel frequency class 1000 for head acceleration), and the integral is evaluated over every candidate window. The 2.5 exponent makes HIC strongly sensitive to peak magnitude and only weakly sensitive to duration, which is why a short, hard strike can produce a higher HIC than a longer, softer one of the same impulse. The regulatory threshold for the driver and passenger in FMVSS 208 is HIC15 ≤ 700; Euro NCAP and many other programs use HIC15 ≤ 700 as well, while some occupant-specific requirements (for example, child or rear-seat occupants in certain protocols) apply HIC36 ≤ 1000. These are limits on the resultant, not on any single axis, so a head that is struck obliquely can reach the limit through a combination of components.
For trim design, HIC translates directly into a stiffness and packaging constraint. The head must be decelerated over a long enough distance and time that the peak resultant stays below the level that produces HIC = 700. A useful first-order estimate: if the head (mass ≈ 4.5 kg) strikes a trim surface at 24 km/h (6.7 m/s) and is brought to rest over a crush distance s, the average deceleration is a = v²/(2s). For s = 50 mm, a ≈ 450 m/s² ≈ 46 g; for s = 25 mm, a ≈ 90 g. Because HIC grows roughly with a^2.5, halving the crush distance can raise HIC by a factor of about 5.7. This is why energy-absorbing trim — crushable ribs, honeycomb, foam with a controlled plateau stress — is specified by its crush stroke and plateau force rather than by a single "softness" number.
Rotational injury metrics
Linear acceleration alone does not explain diffuse axonal injury, subdural hematoma from bridging vein rupture, or the rotational component of concussion. Rotational kinematics are therefore evaluated with dedicated metrics. The most widely used in automotive trim work are:
- BRIC (Brain Injury Criterion): based on the maximum rotational velocity about any axis, normalized by critical values that differ by axis (approximately 66.3 rad/s for the coronal and axial axes and 53.1 rad/s for the sagittal axis in the original formulation). BRIC is used in FMVSS 208 for the oblique and far-side tests and in several global protocols.
- DAMAGE: a tissue-level criterion that predicts strain in the brain from rotational velocity and duration, used in research and in some rating protocols.
- HIP (Head Injury Power) and rotational acceleration limits: used in some European and motorsport standards, with thresholds on peak rotational acceleration (commonly cited around 6,000–10,000 rad/s² depending on duration and protocol).
The key design consequence is that rotational metrics are driven by friction and geometry, not just by crush. A head that slides across a trim surface picks up rotational velocity; a head that is caught by an edge, a gap, or a protruding feature can be spun sharply. Trim countermeasures therefore include low-friction surface finishes, radiused edges, gap fillers, and "catch-free" transitions between adjacent parts. A soft foam that stops the head linearly but has a high-friction skin can still produce a high BRIC.
How the metrics constrain trim design
The table below summarizes the practical mapping from metric to countermeasure decision. Values are typical design targets for a passenger-car interior head impact at 24 km/h, not regulatory limits for every market.
| Metric | What it responds to | Trim design lever | Typical target |
|---|---|---|---|
| HIC15 | Peak resultant linear acceleration and duration | Crush stroke, plateau force, attachment stiffness | ≤ 700 |
| Peak linear acceleration | Local stiffness at strike point | Rib geometry, foam density, substrate standoff | ≤ 80–120 g (protocol-dependent) |
| BRIC | Rotational velocity about each axis | Surface friction, edge radius, gap closure | ≤ 0.8–1.0 (protocol-dependent) |
| Rotational acceleration | Sharpness of rotational loading | Compliant attachment, avoid hard local features | Protocol-specific |
In practice, the design loop is: define the strike envelope (which trim surfaces the head can reach in the relevant test), measure or simulate the head kinematics at each point, compute HIC and BRIC, and iterate on the trim. The figure below shows this loop as a signal-and-decision chain, from the impact condition through the measured kinematics to the two metric families and the resulting trim change.
Worked example: evaluating a B-pillar trim
Consider a B-pillar upper trim with a 20 mm ribbed energy absorber over a rigid substrate. The head strikes at 24 km/h with a 30° approach angle. A first simulation gives a peak resultant of 140 g and a crush stroke of 18 mm, with HIC15 = 980 and BRIC = 1.15. Both fail. The linear fix is to increase effective stroke: replace the 20 mm absorber with a 35 mm honeycomb of lower plateau stress, which raises the stroke to about 30 mm and drops the peak to roughly 85 g, bringing HIC15 to about 620. But the rotational metric may not improve, because the head still slides across a high-friction painted surface and is caught by the trim's edge radius. The rotational fix is separate: reduce the surface friction (matte, low-friction coating), increase the edge radius from 3 mm to 8 mm, and close the gap to the adjacent headliner so the head cannot catch. Re-running the model with these changes typically brings BRIC below 0.9. The lesson is that linear and rotational metrics respond to different design levers, and a trim that passes HIC can still fail BRIC.
Practical evaluation checklist
- Define the strike envelope: list every trim surface the head can reach in the applicable test, with approach angle and speed.
- Filter and process the acceleration signal per SAE J211 before computing HIC; report the window (HIC15 vs HIC36) explicitly.
- Compute BRIC about each axis and report the governing axis; do not average across axes.
- Record the effective crush stroke at the strike point, including packaging losses.
- Check edge radii, gaps, and surface friction as separate rotational countermeasures.
- Iterate until both metric families pass at every point, then verify with a physical or validated simulation test.
The metrics are not abstract: each one maps to a specific physical property of the trim — stroke and plateau force for HIC, friction and geometry for BRIC. Designing to both, at every reachable strike point, is what separates a trim that merely looks soft from one that actually protects the head.