3. Occupant injury criteria and targets

Every restraint CAE campaign ultimately answers one question: does the occupant stay within acceptable injury limits across the full matrix of crash modes? The answer is expressed through a small set of standardized injury criteria, each tied to a specific body region, a specific measurement channel, and a specific regulatory or consumer-test threshold. This chapter explains what each criterion physically represents, how it is computed from raw channel data, and how the same physical response maps onto different pass/fail targets under FMVSS 208, Euro NCAP, and C-NCAP.

Why Criteria Differ by Body Region

Injury criteria exist because a single acceleration or force number cannot describe injury risk across the whole body. The head tolerates short-duration, high-magnitude acceleration far better than the chest tolerates sustained compression; the femur tolerates axial load but is vulnerable to bending and to knee-thigh-hip (KTH) loading through the knee bolster. Each criterion therefore encodes a different physical mechanism: rotational or translational brain injury for the head, combined neck shear/bending for the neck, rib compression for the chest, and axial force transmission for the lower extremities.

In a CAE campaign, these criteria are not read directly from the solver. They are post-processed from the dummy model's output channels — accelerometers, load cells, and displacement/rotation sensors — using the same filtering and windowing conventions the physical dummy uses in the sled or crash test. Getting the post-processing wrong is one of the most common sources of false correlation failures.

Head Injury Criterion (HIC)

HIC is defined over a time window using the resultant translational acceleration at the head center of gravity:

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

where a_res is the resultant head CG acceleration in g,
filtered per SAE J211 Channel Class 1000 (CFC 1000),
and (t2 - t1) is limited to 36 ms for HIC36 or 15 ms for HIC15.

The 2.5 exponent makes HIC strongly sensitive to peak acceleration and to how long that peak is sustained. In practice, the analyst must confirm three things before trusting a HIC value: the acceleration is the resultant of the three head CG channels, the filter class matches the requirement, and the window limit matches the target (HIC15 for most consumer protocols, HIC36 for FMVSS 208). A common error is computing HIC on a single axis or applying the wrong CFC, which can shift the result by 15–30%.

Typical targets: FMVSS 208 requires HIC36 ≤ 700 for the 50th-percentile male in the frontal rigid barrier test. Euro NCAP and C-NCAP use HIC15, with a 5-star threshold generally at HIC15 ≤ 500 and a lower performance limit around 700. Because HIC15 and HIC36 are not interchangeable, the campaign's post-processing script must select the window from the target definition, not from a global default.

Neck Injury Criterion (Nij)

Nij combines axial neck force and bending moment into a single normalized value:

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

where Fz  = axial neck force (tension positive, compression negative)
      My  = neck bending moment about the y-axis
      Fzc = critical axial force (tension or compression, per direction)
      Myc = critical bending moment (flexion or extension, per direction)

The critical values are direction-dependent, so four combinations exist: tension-extension, tension-flexion, compression-extension, and compression-flexion. The reported Nij is the maximum over the whole event, and the analyst must track which combination produced it. A value of Nij ≤ 1.0 is the standard pass threshold; Euro NCAP and C-NCAP apply the same normalized form but with their own critical values and a lower 5-star limit, often around Nij ≤ 0.6–0.8 depending on protocol version.

Because Nij is a sum of two normalized terms, it is sensitive to sign conventions. The campaign must fix a single convention — for example, tension positive and flexion positive — and apply it consistently across all load cases and all dummy variants. Mixing conventions between the driver and passenger models is a frequent source of non-reproducible results.

Chest Deflection and Viscous Criterion

Chest deflection is measured as the maximum sternum-to-spine compression, usually reported in millimeters. FMVSS 208 limits chest deflection to 63 mm for the 50th-percentile male. Euro NCAP and C-NCAP use a lower 5-star threshold, typically around 22–28 mm for the same dummy, reflecting the more demanding consumer rating philosophy.

For side impact and for some frontal protocols, the viscous criterion (VC) is also evaluated:

VC = max over time of [ (dD/dt) * (D / C) ]

where D = chest deflection (m)
      C = initial chest depth (m)
      dD/dt = rate of chest deflection (m/s)

VC captures the rate-dependent nature of soft-tissue injury: a fast, shallow compression can be more injurious than a slow, deep one. In CAE, VC requires a clean, well-filtered deflection signal; numerical noise in the differentiation step can inflate VC dramatically. The standard practice is to filter the deflection channel at CFC 180 before differentiating, and to verify that the deflection sensor in the dummy model is oriented along the correct axis.

Femur Loads and Lower-Extremity Criteria

Femur axial force is measured at each knee, with a typical FMVSS 208 limit of 10 kN per femur for the 50th-percentile male. The criterion captures axial load transmitted from the knee bolster through the femur to the hip. In CAE, the femur load is read from the knee joint load cell, and the analyst must confirm that the load path includes the correct contact definitions between the knee, bolster, and instrument panel.

Euro NCAP and C-NCAP also evaluate knee displacement and, in some protocols, the tibia index (TI), which combines bending moment and axial force at the upper and lower tibia. The tibia index is normalized similarly to Nij and is reported as a dimensionless value with a threshold typically around 1.0.

Mapping Criteria to Regulatory and Consumer Targets

The same physical response can pass one protocol and fail another because the thresholds, dummy types, and test conditions differ. The table below summarizes the principal targets for the 50th-percentile male in frontal impact, which is the anchor configuration for most campaigns.

Note that the exact Euro NCAP and C-NCAP limits depend on the protocol year and the specific test configuration (frontal offset, mobile progressive deformable barrier, side pole, etc.). The campaign's target database must be versioned alongside the protocol release, because a mid-program protocol update can invalidate previously passing results.

From Raw Channels to Campaign Decisions

The figure below shows how the four principal criteria are derived from the dummy model's output channels and then compared against the three target sets. The key takeaway is that the same channel data feeds multiple criteria, and the campaign's post-processing pipeline must apply the correct filter, window, and normalization for each target.

Correlation and Campaign Implications

In a CAE campaign, the injury criteria are the bridge between simulation and test. Correlation is judged criterion by criterion, not by overall visual similarity of the animation. A model that matches chest deflection within 10% but over-predicts HIC by 40% is not correlated for head protection, even if the overall kinematics look correct. The practical workflow is to extract each criterion from both the simulation and the physical test using identical post-processing, then compare peak values and time histories.

Typical correlation tolerances used in industry are ±15% on peak criterion values, with tighter targets (±10%) for HIC and chest deflection because they drive restraint tuning decisions. When a criterion falls outside tolerance, the analyst should first verify the post-processing (filter, window, channel orientation, sign convention) before adjusting the model. In many campaigns, a significant fraction of apparent correlation failures are post-processing errors rather than model errors.

Finally, the campaign must maintain a traceable mapping between each criterion, its source channel, its post-processing definition, and its target threshold. When a protocol updates — for example, a new Euro NCAP release lowers the Nij 5-star limit — the campaign should be able to re-evaluate all existing results against the new target without re-running the simulations, provided the raw channel data and post-processing definitions are archived. This is the operational payoff of treating injury criteria as versioned, first-class objects in the campaign rather than as ad-hoc spreadsheet calculations.