📚 Course ContentsChapter 3 Injury biomechanics for frontal and offset impacts

3. Injury biomechanics for frontal and offset impacts

Frontal crash pulse analysis only becomes diagnostically useful when the measured deceleration history is mapped onto the occupant's body and translated into injury risk. This chapter connects three layers that must always be reasoned about together: the structural load path that shapes the pulse, the restraint interaction that determines how that pulse is delivered to the occupant, and the injury criteria (HIC, chest deflection, femur load) that convert occupant kinematics into pass/fail numbers. A pulse that looks benign on a sled trace can still produce a high HIC if the restraint couples the head to the vehicle too early; conversely, a severe pulse can be survivable when the restraint manages the coupling sequence correctly.

Load paths: from barrier to occupant

In a full-frontal rigid-barrier test, the longitudinal rails, bumper beam, and cradle absorb energy and generate the primary deceleration. In an offset deformable-barrier (ODB) or small-overlap event, the load is concentrated on one side: the impacted rail and wheelhouse take the load, the barrier element crushes asymmetrically, and the vehicle yaws. The occupant compartment then experiences a rotational and lateral component in addition to longitudinal deceleration, which changes both the timing and the direction of restraint loading.

Two structural behaviors dominate the pulse shape. First, rail buckling and fold initiation create the initial peak; a rail that folds early produces a short, high-amplitude spike. Second, powertrain and cradle engagement produces the secondary, longer-duration plateau. For diagnosis, always identify which structural event corresponds to each peak in the pulse. A common error is to attribute a late peak to the barrier when it is actually the engine cradle bottoming out against the firewall.

Reference convention used throughout this course: the vehicle coordinate system is SAE-style, with +X forward, +Y to the driver's right, +Z up. Longitudinal deceleration during a frontal impact is therefore negative in X. All occupant excursions are reported relative to the seat reference point (SRP).

Restraint interaction: coupling sequence

The restraint system does not apply a single force; it applies a sequence of couplings. The seat belt pretensioner fires first (typically 10–20 ms after trigger), removing slack and pulling the occupant forward into the belt. The load limiter then controls how much force the belt transmits to the chest. The airbag inflates and is timed to be fully deployed before the occupant's head reaches the module. The sequence matters more than any single component: an airbag that deploys 15 ms late will meet the head during its highest-velocity phase and can increase, not reduce, head injury risk.

In offset impacts, the occupant also moves laterally and rotates. The belt geometry (D-ring position, buckle stalk angle) determines whether the shoulder belt stays on the shoulder or slides off, and whether the lap belt loads the iliac crest or the abdomen. A lap belt that rides above the iliac crest produces abdominal injury and allows the pelvis to submarine; a shoulder belt that slips off the shoulder concentrates load on the neck and increases head excursion.

Injury criteria: what each number actually measures

Head Injury Criterion (HIC) is computed from the resultant head acceleration a(t) over a chosen window:

HIC = max over t1,t2 of [ (1/(t2-t1)) * ∫ a(t) dt ]^2.5 * (t2-t1)
     with t2 - t1 ≤ 15 ms (HIC15) or 36 ms (HIC36)

The 2.5 exponent makes HIC extremely sensitive to short, high-amplitude spikes. A 10 ms pulse at 150 g produces a much higher HIC than a 30 ms pulse at 80 g, even though the velocity change is similar. This is why airbag timing and belt load limiting matter: they spread the head deceleration over a longer window. In practice, HIC is computed from the filtered resultant acceleration of the head CG, and the window search must be done over the entire pulse, not just the peak.

Chest deflection is measured directly by the chest potentiometer in the Hybrid III dummy (or by rib deflection sensors in the THOR). It captures rib cage compression in millimeters and is the best single predictor of thoracic injury (rib fractures, flail chest, aortic injury). The FMVSS 208 limit for the 50th-percentile male is 63 mm; for the small female and 5th-percentile occupants, the limit is lower because the chest is smaller. Deflection is driven by belt force and belt geometry: a shoulder belt that loads the sternum centrally produces more deflection than one that loads the clavicle, and a lap belt that rides high produces abdominal rather than chest loading.

Femur load is measured by load cells in the dummy's upper legs and captures axial compression transmitted through the knee-thigh-hip complex. The 10 kN limit (per leg) corresponds to a fracture risk threshold. Femur load is primarily a function of knee bolster design and the occupant's lower-extremity position: a knee that misses the bolster and loads the instrument panel directly will produce a high femur load even in a moderate pulse.

Putting it together: a diagnostic example

Consider a 40% ODB test at 64 km/h. The pulse shows an initial peak of 28 g at 25 ms (rail fold), a plateau of 18 g from 40–70 ms (cradle engagement), and a secondary rise to 22 g at 85 ms (firewall intrusion). The dummy data shows HIC15 = 620, chest deflection = 58 mm, and left femur load = 8.2 kN. All values are below limits, but the chest deflection is close to the threshold and the left femur load is elevated relative to the right (4.1 kN).

The asymmetry is the diagnostic clue. The left femur load indicates that the left knee is loading the bolster earlier or harder than the right, consistent with the offset impact's yaw. The chest deflection of 58 mm suggests the shoulder belt is loading the sternum rather than the clavicle, likely because the pretensioner pulled the occupant forward before the airbag was fully deployed. The recommended diagnostic step is to compare the belt force trace with the airbag pressure trace: if the belt force peaks before the airbag pressure reaches 50% of its maximum, the coupling sequence is early and the load limiter should be recalibrated or the pretensioner force reduced.

Do not evaluate HIC, chest deflection, and femur load in isolation. A restraint tuning change that reduces HIC (e.g., a softer airbag) often increases chest deflection (because the belt must absorb more energy), and a stiffer load limiter reduces chest deflection but raises femur load. Always check the three criteria as a coupled set, and always verify the coupling sequence against the pulse timing.

Practical checklist for pulse-to-injury diagnosis

Identify the structural event behind each pulse peak (rail fold, cradle engagement, firewall intrusion) before interpreting occupant data.
Plot belt force, airbag pressure, and head acceleration on the same time axis; check that the airbag is fully deployed before head contact.
Compute HIC15 over the full pulse, not just the peak window; report the window (t1, t2) alongside the value.
Compare left and right femur loads; asymmetry points to offset loading or knee bolster misalignment.
Check chest deflection against the occupant-specific limit, not just the 50th-percentile value.
When a criterion is close to its limit, trace the load path back to the restraint component responsible before proposing a tuning change.

The next diagnostic layer is the pulse itself: how the measured deceleration history is filtered, integrated, and compared against the structural events identified here. That analysis is only meaningful once the load path, coupling sequence, and injury criteria are understood as a single coupled system.