📚 Course ContentsChapter 2 Occupant restraint system fundamentals

2. Occupant restraint system fundamentals

An airbag is not a single component but a complete restraint subsystem whose behavior is defined by its gas source, cushion geometry, venting strategy, and the timing logic that decides when it fires. For engineers working on contact force testing and CAE correlation, the practical question is always the same: what force does the cushion actually deliver to the occupant, and how does that force change across the deployment timeline? This chapter establishes the vocabulary and physical framework needed before any test fixture is built or any solver deck is written.

Airbag Types and Their Force Signatures

Airbags are classified by the body region they protect and, more importantly for force work, by how they manage internal pressure. The three families that dominate passenger-vehicle restraint systems are frontal driver/passenger airbags, side airbags (thorax, pelvis, and curtain), and knee airbags. Each produces a distinctly different contact force profile because of differences in volume, inflation rate, and vent design.

A frontal driver airbag is a high-volume, low-pressure cushion, typically 60–80 L for a driver unit and 100–150 L for a passenger unit. It relies on a pyrotechnic inflator that releases gas in roughly 20–40 ms, followed by controlled venting through fabric pores or discrete vents. The resulting force on the chest is spread over a large area, so peak contact pressure is relatively low but the impulse duration is long. A side thorax airbag, by contrast, may be only 8–15 L and must inflate in under 10 ms because the occupant-to-door gap is small. Its force signature is sharper and more localized, which makes sensor placement and mesh density far more critical in both testing and simulation.

Curtain airbags introduce a different challenge: they are long, thin, and often tethered, so the contact force varies significantly along the length of the cushion. A knee airbag is stiff and short-stroke, designed to load the femur and tibia rather than the torso. When you correlate CAE to test, you must match not only the peak force but the rise time and the shape of the force–time curve, because these reflect the inflator mass flow rate and the vent area.

Deployment Stages and the Force Timeline

Deployment is conventionally divided into four stages, and each stage maps to a distinct segment of the contact force curve.

Initiation and inflator firing (0–5 ms): The ECU closes the circuit, the squib ignites, and the gas generant begins to burn. No occupant contact occurs yet, but the internal pressure starts to rise. In CAE, this is modeled with a pressure–time or mass-flow–time boundary condition derived from tank tests.
Cushion expansion and membrane contact (5–20 ms): The cushion unfolds and the fabric membrane first touches the occupant. Contact force is still low, but the contact area grows rapidly. This is where out-of-position (OOP) scenarios are most dangerous, because a deploying cushion can strike an occupant who is too close to the module.
Peak inflation and occupant loading (20–50 ms): Internal pressure reaches its maximum, and the occupant decelerates against the cushion. This is the dominant phase for injury metrics such as chest deflection and femur load. The contact force here is a function of cushion pressure, contact area, and the occupant's mass and velocity.
Venting and rebound (50–120 ms): Gas escapes through vents and fabric porosity, pressure falls, and the occupant may rebound. Force decays smoothly if venting is well tuned; a secondary force spike often indicates a vent that is too small or a tether that restricts cushion collapse.

The timing boundaries above are typical for a 50th-percentile male in a 56 km/h frontal impact, but they shift with inflator output, ambient temperature, and occupant size. Always state the reference condition when quoting stage durations.

Contact Force Metrics and Their Physical Meaning

Contact force is rarely a single number. In both physical testing and CAE, engineers extract several metrics from the force–time history, and each one answers a different question.

Metric	Definition	Typical Use
Peak contact force (F_peak)	Maximum force recorded during the event	Injury risk screening; cushion stiffness validation
Time to peak (t_peak)	Elapsed time from impact to F_peak	Inflator timing and vent tuning
Impulse (∫F dt)	Area under the force–time curve	Momentum transfer; correlation of overall energy
Loading rate (dF/dt)	Slope of the rising portion	OOP injury assessment; fabric permeability effects
Contact pressure	Force divided by contact area	Localized injury prediction (rib fracture, soft tissue)

For CAE correlation, F_peak and t_peak are the primary anchors, but impulse and loading rate often reveal modeling errors that peak force alone hides. A model that matches F_peak but predicts a 15 ms earlier t_peak usually has an inflator mass-flow curve that is too aggressive or a cushion that is too stiff in the early expansion phase.

Practical Correlation Workflow

Consider a driver-side frontal airbag tested at 56 km/h with a 50th-percentile male Hybrid III dummy. The test measures chest contact force via a load cell array on the chest plate, and the CAE model uses a uniform pressure (UP) or corpuscular particle method (CPM) airbag. A reproducible correlation sequence looks like this:

Extract the test force–time curve and identify F_peak, t_peak, and impulse. Example values from a representative test: F_peak = 4.2 kN at t_peak = 38 ms, impulse = 0.18 kN·s.
Run the baseline CAE model with the nominal inflator mass-flow curve and vent area. Compare the three metrics. A common baseline result is F_peak = 5.1 kN at t_peak = 30 ms, indicating the model is too stiff and too fast.
Adjust the inflator mass-flow curve by scaling the rising portion to delay t_peak, and increase vent area by 10–15% to reduce F_peak. Re-run and compare.
Check the loading rate in the 10–25 ms window. If the model's dF/dt is more than 20% higher than test, the early cushion expansion is too aggressive; consider reducing initial gas temperature or adding a fabric permeability parameter.
Validate against a second test condition (e.g., 40 km/h or a 5th-percentile female) without re-tuning. If the model holds within ±10% on F_peak and ±3 ms on t_peak, the correlation is considered robust for that airbag family.

Common pitfall: Tuning only F_peak by scaling the entire force curve can produce a model that matches the peak but misrepresents the impulse. Always check the area under the curve and the loading rate before accepting a correlation.

Reference Conditions and Sign Conventions

All force values in this course follow the SAE J211 sign convention: positive force acts in the direction of occupant travel (forward for a frontal impact), and the coordinate origin is at the vehicle centerline at the occupant's H-point. When you switch to a side impact or a different dummy, state the new reference explicitly. Contact force is reported as a positive magnitude unless a directional component is being analyzed.

Quick check before any correlation run: Confirm that the test force channel is filtered per SAE J211 CFC 600, the CAE output is sampled at the same rate, and both curves are aligned to the same T₀ (impact trigger). A 2 ms misalignment can shift t_peak enough to invalidate an otherwise good model.

With the airbag types, deployment stages, and force metrics established, you are ready to define the specific test matrix and sensor layout for contact force measurement. The next step is to translate these metrics into a concrete data acquisition plan.