2. Occupant injury metrics for small-overlap

Small-overlap frontal impacts load the occupant through a path that is fundamentally different from a full-frontal or moderate-overlap event. Because only a narrow outboard strip of the front structure engages the barrier, the longitudinal rail on the struck side is often bypassed or buckled outward, and the resulting deceleration pulse is short, steep, and accompanied by significant lateral and yaw motion of the occupant compartment. The occupant therefore moves forward and inward, loading the restraint system obliquely. This chapter explains how the four primary injury metrics used in small-overlap assessment — HIC, BrIC, chest deflection, and femur loads — are defined, how they respond to oblique loading, and how an engineer reads them together to judge restraint performance.

Why Oblique Loading Changes the Metric Picture

In a purely frontal pulse, the occupant's torso loads the belt in a near-sagittal plane, the airbag deploys into a predictable gap, and the head rotates primarily in pitch. In a small-overlap event, three things change simultaneously:

• The occupant compartment yaws and translates laterally, so the head approaches the airbag and A-pillar region at an angle.
• The shoulder belt loads the outboard shoulder and can slip toward the neck or off the shoulder if the geometry is poor.
• The knees and femurs are driven into an offset knee bolster or the instrument panel edge rather than a symmetric surface.

These changes mean that a restraint system tuned only for HIC in a full-frontal test can pass that test yet produce elevated BrIC, asymmetric chest deflection, or concentrated femur loading in a small-overlap configuration. The metrics must therefore be evaluated as a set, not individually.

Head Injury Criterion (HIC)

HIC is defined over a time window and is computed from 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 ]
      with (t2 - t1) ≤ 15 ms
a(t) = resultant acceleration at head CG, in g

In small-overlap testing, HIC is typically reported for a 15 ms window (HIC15). The characteristic failure mode is a glancing head strike: the head contacts the airbag, slides laterally, and then loads the A-pillar, roof rail, or side curtain region. This produces a sharp secondary acceleration spike that can dominate the HIC integral even when the primary airbag contact is benign. A useful diagnostic is to plot the head resultant acceleration and mark the time of first airbag contact versus the time of the peak. If the peak occurs after the head has translated laterally by more than roughly 100 mm, the restraint geometry — not the airbag pressure — is usually the limiting factor.

Brain Injury Criterion (BrIC)

BrIC addresses rotational head motion, which HIC does not capture. It is computed from peak angular velocities about three anatomical axes:

BrIC = max( |ωx|/ωxC , |ωy|/ωyC , |ωz|/ωzC )
ωxC = 66.25 rad/s, ωyC = 56.45 rad/s, ωzC = 42.87 rad/s

The critical values above are the standard normalization constants. In small-overlap, the dominant BrIC component is usually the z-axis (axial rotation) because the oblique head path induces yaw as the head slides across the airbag face. A BrIC value approaching or exceeding 1.0 indicates a high risk of rotational brain injury. The engineering lever here is airbag surface friction and the presence of a side curtain or far-side airbag that can arrest the lateral head excursion before yaw builds. Reducing airbag venting too aggressively can increase BrIC even while lowering HIC, so the two must be balanced.

Chest Deflection

Chest deflection is measured with a chest band or IR-TRACC (Infra-Red Telescoping Rod for Assessment of Chest Compression) and reported as the peak lateral compression of the rib cage, usually in millimeters. In oblique loading, the belt path determines whether deflection is concentrated or distributed. A shoulder belt that migrates toward the neck produces a narrow, high-pressure load path and elevated local deflection; a belt that stays on the shoulder distributes load across more ribs.

Two practical observations matter. First, deflection is asymmetric: the outboard side of the chest compresses more than the inboard side, and the peak value is what is compared against the injury assessment reference value (IARV). Second, chest deflection is sensitive to belt pretensioner timing. If the pretensioner fires too late, the belt has already paid out and the chest absorbs the full relative motion; if it fires too early, the occupant is pulled into the deploying airbag. The target is to have the pretensioner remove slack before the occupant's forward excursion begins, which is typically within the first 15–25 ms of the pulse.

Femur Loads

Femur load is measured along the axis of each femur and reported as the peak axial force, usually in kilonewtons. In small-overlap, the outboard femur is the critical one because the knee bolster and instrument panel edge present an offset, often angled surface. The load path runs from the knee, through the femur, into the hip and pelvis. Two failure modes are relevant: axial femur fracture and hip joint injury from the combined axial and bending load.

Because the loading is oblique, the femur is not loaded purely in compression; a bending moment is superimposed. This means the measured axial force alone can understate the injury risk. Engineers therefore also examine the knee bolster geometry and the presence of a knee airbag, which can redirect the load into a more favorable path. A common symptom of poor femur load management is a peak that occurs late in the event, after the occupant has already rotated — indicating that the knee is being loaded by the rotating pelvis rather than by direct panel contact.

Reading the Metrics Together

The four metrics are not independent. A restraint change that lowers HIC by increasing airbag pressure may raise BrIC by increasing rotational impulse. A change that lowers chest deflection by increasing pretensioner force may raise femur loads by pulling the occupant into the panel earlier. The table below summarizes the typical direction of these trade-offs in a small-overlap event.

Practical Example: Interpreting a Small-Overlap Test

Consider a hypothetical small-overlap test at 64 km/h with a 25% overlap barrier. The occupant is a 50th-percentile male Hybrid III ATD. The recorded signals show:

• HIC15 = 620 (below the 700 reference)
• BrIC = 1.12 (above the 1.0 reference)
• Peak chest deflection = 42 mm (below the 50 mm reference)
• Peak outboard femur load = 4.8 kN (below the 6.0 kN reference)

The HIC, chest, and femur values look acceptable, but BrIC is elevated. The diagnostic question is: where does the rotational velocity come from? If the head acceleration trace shows a smooth rise and the angular velocity about z peaks at roughly the same time as the lateral head excursion, the cause is likely the head sliding across the airbag face without a side curtain to arrest it. The corrective action would be to add or retune a side curtain airbag, or to increase the friction of the airbag face so the head does not slide as freely. Increasing airbag pressure alone would likely raise BrIC further, so it is not the right lever.

Summary

Small-overlap loading produces an oblique occupant trajectory that stresses the restraint system in ways that full-frontal metrics do not capture. HIC measures translational head acceleration and is sensitive to glancing A-pillar strikes. BrIC measures rotational head velocity and is typically driven by lateral head slide and yaw. Chest deflection measures rib compression and is strongly influenced by belt path and pretensioner timing. Femur load measures axial force plus bending and is driven by offset knee bolster contact. Because these metrics trade off against one another, restraint tuning for small-overlap requires balancing all four rather than optimizing any one in isolation.