2. Occupant protection fundamentals
An airbag cushion is not a soft pillow that simply catches an occupant. It is a controlled energy-management device whose geometry, venting, tethering, and deployment timing are all tuned against measurable injury thresholds. To design a cushion that actually protects, you must first understand how the human body is injured in a crash, how that injury is quantified, and which regulatory and consumer tests your design must satisfy. This chapter builds that foundation.
Injury Mechanisms: What Actually Happens to the Body
In a frontal impact, the vehicle decelerates rapidly while the unrestrained occupant continues forward at the original velocity. The occupant then interacts with the restraint system — seat belt, airbag, or both. Injury arises from three distinct physical mechanisms, and each maps to a different cushion design lever.
- Compression (AIS-type skeletal injury). The chest, abdomen, and head are compressed between the loading surface and the underlying skeleton. Excessive chest deflection fractures ribs and injures thoracic organs. Cushion design lever: reduce peak pressure by increasing contact area and controlling internal pressure rise rate.
- Inertial loading (acceleration-driven). The head and internal organs are accelerated by the restraint force. High head acceleration produces diffuse brain injury; rapid head rotation produces diffuse axonal injury. Cushion design lever: manage the timing and magnitude of the force applied to the head.
- Shear and bending. The neck and spine experience shear and bending moments when the head is loaded off-axis. Cushion design lever: shape the cushion face so the head is loaded near its center of gravity.
A critical concept is ride-down: the cushion should begin loading the occupant early and continue loading over a long distance, so the occupant's velocity is reduced gradually rather than abruptly. A cushion that is too stiff or deploys too late produces a sharp force spike; one that is too soft or vents too aggressively allows the occupant to "bottom out" against the module or steering column. Both extremes increase injury.
Injury Criteria: Turning Physics into Numbers
Regulators and researchers convert measured accelerations, forces, and deflections into scalar injury criteria. The most relevant to cushion design are summarized below.
| Criterion | Body Region | Physical Basis | Typical Limit (frontal) |
|---|---|---|---|
| HIC15 / HIC36 | Head | Resultant head acceleration integrated over a 15 ms or 36 ms window | HIC15 ≤ 700 (FMVSS 208) |
| Nij (Neck Injury) | Neck | Combined axial force and bending moment | Nij ≤ 1.0 |
| Chest deflection | Thorax | Rib cage compression relative to body size | ≤ 63 mm (Hybrid III 50th) |
| Viscous Criterion (VC) | Thorax | Velocity of chest deflection × deflection | VC ≤ 1.0 m/s |
| Femur force | Lower extremity | Axial load transmitted through the knee-thigh-hip complex | ≤ 10 kN per leg |
HIC is computed as:
HIC = [ (1/(t2 - t1)) * ∫ a(t) dt ]^2.5 * (t2 - t1)
where a(t) is the resultant head acceleration in g,
and t2 - t1 is the window length (15 ms for HIC15).For cushion design, HIC is the primary driver of face-of-cushion geometry and vent sizing. A cushion that contacts the head too early or too stiffly raises the early acceleration peak; one that contacts too late allows the head to strike the steering wheel or A-pillar. The designer's job is to place the cushion in the narrow window between these failure modes.
Regulatory vs. Consumer Test Requirements
Two families of tests govern airbag cushion design, and they often pull in different directions.
Regulatory tests (FMVSS 208 in the US, ECE R94/R95 in Europe, GB 11551 in China) are pass/fail mandates. They define the crash pulse, the barrier type, the occupant size (typically Hybrid III 50th percentile male), the belt configuration, and the injury thresholds. FMVSS 208, for example, requires a 56 km/h frontal rigid barrier test with unbelted and belted occupants, and includes out-of-position (OOP) tests for children and small adults. OOP tests are particularly demanding for cushion design because they constrain deployment-induced injury: a deploying cushion must not exceed specified neck and chest loads on a stationary occupant positioned close to the module.
Consumer tests (Euro NCAP, US NCAP, C-NCAP, IIHS) are rating systems. They use different crash pulses, offset deformable barriers, and additional occupant sizes (5th percentile female, 95th percentile male). Euro NCAP, for instance, applies a 64 km/h offset deformable barrier and scores both belted and unbelted occupants. A cushion optimized only for FMVSS 208 may score poorly in Euro NCAP because the offset barrier produces asymmetric loading and greater chest deflection.
How the Requirements Map to Cushion Parameters
The figure below shows the causal chain from test requirements to specific cushion design parameters. Each arrow represents a design decision that can be adjusted in CAE.
Practical Example: Translating a HIC Target into Vent Area
Consider a driver-side frontal airbag for a mid-size sedan. The CAE team must meet FMVSS 208 with an unbelted 50th percentile male at 56 km/h. The target is HIC15 ≤ 700 and chest deflection ≤ 63 mm. The baseline cushion has a 700 mm diameter, two 40 mm vents, and a 60 L inflator.
A first simulation run shows HIC15 = 820 and chest deflection = 58 mm. The head acceleration peak occurs at 45 ms and is 95 g. The chest is within limits, but the head is over. The engineer reasons: the head is decelerating too abruptly, meaning the cushion is too stiff at the moment of head contact. Two adjustments are possible: increase vent area to reduce internal pressure, or increase cushion diameter to spread the load. Increasing vent area from 40 mm to 50 mm per vent reduces peak pressure by roughly 15% in the simulation, lowering HIC15 to approximately 710. A further increase to 55 mm brings HIC15 to 680, but chest deflection rises to 62 mm — still within limit but with little margin.
The engineer then adds a tether to pull the cushion face into a flatter profile, increasing contact area without changing vent area. This reduces HIC15 to 650 and chest deflection to 57 mm. The final design uses 50 mm vents plus a single horizontal tether. This is a typical iteration: one parameter change fixes one criterion but erodes margin in another, and a second change restores the balance.
Key Takeaways for Cushion Design
- Injury is caused by compression, inertial loading, and shear/bending — each maps to a different cushion parameter.
- HIC, Nij, chest deflection, and VC are the primary criteria; HIC drives face geometry and venting, chest deflection drives cushion size and tethering.
- Regulatory tests define minimum compliance; consumer tests define competitive performance. Design to the tighter envelope.
- OOP requirements constrain how aggressively you can vent or how stiff the cushion can be during deployment.
- CAE iteration is a balancing act: every parameter change affects multiple criteria, and margin must be maintained across all occupant sizes and test configurations.