4. Occupant injury criteria

In the previous chapter we established that conflicting crash pulses force the restraint system to trade one body region's protection against another's. To make those trade-offs measurable, every regulatory and consumer-testing regime converts the physical response of a crash dummy into a small set of scalar injury criteria, each with a defined computation window, a defined threshold, and a defined pass/fail interpretation. This chapter explains what those criteria actually compute, how they are derived from measured signals, and where the three major regimes — FMVSS, Euro NCAP, and C-NCAP — diverge in ways that matter for restraint calibration.

Head injury: HIC and its variants

The Head Injury Criterion is the oldest and most widely used occupant metric. It is computed from the resultant translational acceleration at the head centre of gravity, a(t), expressed in multiples of gravitational acceleration g:

HIC = max over (t1, t2) of [ (t2 - t1) * ( (1/(t2-t1)) * ∫ a(t) dt )^2.5 ]

subject to: t2 - t1 ≤ 15 ms   (HIC15)
           t2 - t1 ≤ 36 ms   (HIC36, legacy FMVSS 208)

The 2.5 exponent heavily penalises short, high-amplitude spikes, which is why HIC responds strongly to stiff structures and late-firing airbags. FMVSS 208 uses HIC15 with a limit of 700 for the driver and passenger in frontal rigid-barrier tests; Euro NCAP and C-NCAP both apply HIC15 = 700 as the "good" boundary in their frontal and side protocols, with higher values scored progressively. A practical consequence: because the integration window is short, a 5 ms acceleration spike of 150 g can dominate the score even when the overall pulse looks benign. When calibrating, always inspect the raw resultant acceleration trace alongside the HIC value — a HIC of 650 driven by a single narrow spike calls for a different fix (airbag venting, tether tuning) than a HIC of 650 spread over a broad plateau (structure or belt load limiting).

Two related head metrics appear in side and pedestrian testing. The 3 ms clip (resultant head acceleration exceeding 80 g for more than 3 ms) is used in FMVSS 208 and FMVSS 214 side impact. BRIC (Brain Injury Criterion) is used in some Euro NCAP far-side and oblique configurations and combines rotational velocity about the head's principal axes; it captures diffuse axonal injury that HIC misses. For this course, treat HIC as the primary frontal metric and BRIC as a supplementary rotational check where the protocol requires it.

Chest: deflection, compression, and viscous criterion

Chest injury is the dominant differentiator between restraint strategies. The primary metric is chest deflection — the peak inward displacement of the sternum relative to the spine, measured by the dummy's chest potentiometer or IR-TRACC. FMVSS 208 sets a deflection limit of 63 mm for the 50th-percentile Hybrid III male in frontal impact; Euro NCAP and C-NCAP use the same 63 mm as the upper boundary but score lower deflections as "good" (typically ≤ 22 mm for a 5-star rating in frontal offset).

Two refinements matter in practice. First, chest compression rate and the Viscous Criterion (VC) capture the rate-dependent soft-tissue injury that pure deflection misses:

VC = max over t of [ (dD(t)/dt) * (D(t) / D_max) ]

where D(t) is instantaneous chest deflection and D_max is the
half-chest depth of the dummy (e.g. 0.14 m for Hybrid III 50th).

VC is expressed in m/s and is limited to 1.0 m/s in FMVSS 208 and used as a scoring boundary in Euro NCAP. A restraint that produces 55 mm of deflection slowly is far safer than one producing 45 mm in a sharp 10 ms ramp; VC distinguishes them. Second, in side impact the metric becomes chest compression measured by the WorldSID or ES-2re dummy, with FMVSS 214 limits around 42–45 mm depending on dummy and position, and Euro NCAP side protocols applying similar boundaries. When you tune a load limiter, watch deflection and VC together: raising the limiter force reduces deflection but can increase VC if the force rises too abruptly.

Neck: Nij and its four-quadrant interpretation

The Neck Injury Criterion combines axial force and bending moment into a single normalised value:

Nij = (Fz / Fzc) + (My / Myc)

where Fz  = axial force (tension positive, compression negative)
      My  = bending moment about the y-axis (flexion/extension)
      Fzc, Myc = critical intercept values from the protocol

Nij is evaluated in four quadrants:
  NTE  (tension + extension)
  NTF  (tension + flexion)
  NCE  (compression + extension)
  NCF  (compression + flexion)

FMVSS 208 sets Nij ≤ 1.0 for the Hybrid III in frontal impact, with critical intercepts of Fzc = 6806 N (tension), 6160 N (compression), Myc = 310 N·m (extension), 135 N·m (flexion) for the 50th male. Euro NCAP and C-NCAP apply the same 1.0 limit but score Nij values below roughly 0.4 as "good." The quadrant structure is the key practical insight: a restraint that reduces tension (e.g. by softening the belt) may increase flexion moment, and vice versa. Always report all four quadrants, not just the worst one, because the calibration fix depends on which quadrant is binding. A common failure mode in conflicting-pulse scenarios is NTE (tension-extension) driven by airbag-induced head rotation combined with belt-induced neck tension — a signature that points to airbag tether or vent tuning rather than belt geometry.

Femur force and lower extremity targets

Femur force is measured along the femoral axis and is limited to 10 kN per leg in FMVSS 208 (compression), with Euro NCAP and C-NCAP applying the same 10 kN boundary and scoring below roughly 3.8 kN as "good." The metric is sensitive to knee-bolster stiffness, belt lap geometry, and — critically — the interaction between the lap belt and the femur in the presence of a deploying knee airbag. In conflicting-pulse work, femur force often becomes the binding constraint when you stiffen the knee bolster to control chest deflection: the two targets pull in opposite directions.

Additional lower-extremity targets include tibia index (TI ≤ 1.3 in FMVSS 208, combining axial force and bending moment at the tibia) and, in Euro NCAP pedestrian and some frontal protocols, knee shear and lateral knee displacement. These are secondary in most frontal calibrations but become primary in offset and oblique configurations where the leg loads asymmetrically.

How the three regimes differ in practice

The table below summarises the principal boundaries. Note that FMVSS is a pass/fail regulation, while Euro NCAP and C-NCAP are scored consumer programmes with "good/adequate/marginal/weak/poor" bands; the numbers shown are the regulatory limits and the "good" boundaries respectively.

The practical divergence is not in the numbers but in the test configurations. FMVSS 208 uses a full frontal rigid barrier at 56 km/h and an offset deformable barrier at 40 km/h; Euro NCAP adds a 50 km/h full-width and a 64 km/h offset deformable barrier plus a mobile progressive deformable barrier (MPDB) test; C-NCAP mirrors Euro NCAP closely but with its own barrier stiffness and a 50 km/h MPDB. Because the MPDB and offset tests load the chest and femur differently from the rigid barrier, a restraint calibrated to pass FMVSS 208 can still score poorly in Euro NCAP or C-NCAP — which is exactly the conflicting-pulse problem this course addresses.

The diagram above is the mental map to carry into the next chapters: each body region has a small set of criteria, each criterion is computed from a specific dummy signal, and each signal is most sensitive to a specific restraint design variable. When a conflicting pulse forces a trade-off, the question is always which row of that map is binding and which design variable can move it without pushing another row past its limit. The next chapter applies this map to the first concrete conflict: full-frontal versus offset deformable barrier pulses, where the chest and femur rows typically fight each other.