4. Occupant injury criteria for CAB assessment

Curtain airbags (CABs) are evaluated against a family of injury criteria that translate raw sensor and dummy measurements into normalized risk values. For a CAB deployment, the relevant criteria span head, neck, and ejection-mitigation domains. This section defines each metric, explains how it is computed from measured signals, and shows how the same criteria are applied consistently in both physical impact tests and CAE correlation.

Head Injury: HIC and BrIC

The Head Injury Criterion (HIC) is the classical acceleration-based metric. It is computed from the resultant translational acceleration a(t) measured at the head center of gravity, expressed in units of g:

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

Constraint: t2 - t1 ≤ 15 ms  (HIC15)
            t2 - t1 ≤ 36 ms  (HIC36)

In practice, HIC15 is the dominant criterion for CAB head impact because the interaction window is short. A typical CAB head-impact pulse peaks between 60 g and 120 g with a duration of 10–20 ms. The 15 ms integration window captures the most injurious portion. The commonly cited threshold is HIC15 = 700 for a 50th-percentile adult male, with lower values (around 500–600) applied to smaller occupants or when the head contacts a stiffer region such as the B-pillar trim.

HIC alone is insufficient for CAB assessment because it ignores rotational loading. When the head strikes the curtain and rotates, diffuse axonal injury risk rises even if translational acceleration is moderate. The Brain Injury Criterion (BrIC) addresses this by using angular velocity components:

BrIC = max( |ωx| / ωxC , |ωy| / ωyC , |ωz| / ωzC )

Critical angular velocities (rad/s):
  ωxC = 66.3   (about x, sagittal)
  ωyC = 53.1   (about y, coronal)
  ωzC = 41.5   (about z, axial)

BrIC is dimensionless; a value of 1.0 corresponds to the reference injury threshold. For CAB evaluation, the axial component ωz often dominates because the head slides along the curtain and rotates about the vertical axis. A BrIC above 0.8 in a CAB test typically triggers a design review of curtain coverage, tether geometry, or gas pressure.

Neck Injury Metrics

Neck loading in CAB events arises from two mechanisms: direct head contact that produces a bending moment, and inertial loading as the torso is restrained while the head continues moving. The primary metrics are:

• Nij (Neck Injury Criterion): a combined axial force and bending moment metric. It is computed as Nij = (Fz / Fzc) + (My / Myc), where Fz is the axial force and My is the bending moment. The critical values Fzc and Myc depend on occupant size and are specified in the dummy certification tables. Nij is evaluated as a running sum over the event; the peak value is reported.
• Nkm (Neck Bending Moment): used in some regulatory frameworks as an alternative to Nij. It considers the moment about the occipital condyle and applies a time-dependent weighting.
• Shear force Fx: reported separately because it can produce facet joint injury even when Nij is below threshold.

For a CAB test, the neck is rarely the limiting factor unless the curtain deploys late and the head loads the neck in tension. A typical Nij limit is 1.0, with values above 0.8 warranting investigation. The neck metrics are sensitive to the timing of head-curtain contact; a 5 ms delay in curtain inflation can increase Nij by 20–30% because the head travels further before being decelerated.

Ejection Mitigation Performance Measures

Ejection mitigation is the primary regulatory purpose of a curtain airbag. The performance measures differ from injury criteria because they assess containment rather than tissue tolerance. The key measures are:

These measures are evaluated from high-speed video and from pressure sensors inside the curtain. In CAE correlation, the same measures are extracted from the simulation: head CG displacement relative to the window plane, curtain mesh coverage, and cavity pressure history. The correlation target is typically within 10% for head excursion and within 15% for pressure retention.

Applying the Criteria in Test and CAE

The workflow for a CAB assessment combines physical test and simulation. In a physical sled or vehicle test, the dummy is instrumented and the data acquisition system records head acceleration, angular rate, neck forces and moments, and curtain pressure. The post-processing script computes HIC15, BrIC, Nij, and the ejection measures. In CAE, the same computations are performed on the simulated signals.

A representative example illustrates the process. Suppose a CAB test produces the following peak values (these are illustrative, not measured):

HIC15        = 620
BrIC         = 0.72
Nij          = 0.65
Head excursion = 85 mm
Curtain coverage = 93%
Pressure retention = 58%

All values are below their respective thresholds, so the design passes. If HIC15 were 780 and BrIC 0.95, the engineer would investigate curtain pressure, tether position, and the timing of head contact. A common fix is to increase the gas generator output or adjust the tether to keep the curtain closer to the window plane, which reduces head rotation and lowers BrIC.

Visualizing the Criteria Chain

The diagram below shows how the measured signals flow into each criterion and how the criteria map to the two assessment domains: injury risk and ejection mitigation. The same signal set feeds both domains, which is why a single test can evaluate all criteria.

The diagram makes the dual-domain structure explicit: the same head and neck signals feed both the injury-risk criteria and the ejection-mitigation measures. In a correlation study, each arrow represents a computation that must be replicated in the CAE post-processor. The most common correlation errors are mismatched filtering (test data is typically filtered per SAE J211, while simulation output may be unfiltered) and incorrect head CG location. Both produce systematic offsets in HIC and BrIC that are easily mistaken for model error.

Key Takeaways

• HIC15 and BrIC are complementary: HIC captures translational loading, BrIC captures rotational loading. Both must be checked for CAB head impact.
• Nij combines axial force and bending moment; it is sensitive to the timing of head-curtain contact.
• Ejection mitigation is assessed by head excursion, curtain coverage, and pressure retention, not by tissue-based injury criteria.
• In CAE correlation, apply the same filtering and computation pipeline to test and simulation data. Verify head CG location and sensor orientation before comparing metrics.
• Thresholds are occupant-dependent; always confirm the applicable limit for the dummy size and regulatory framework used in the test.