3. Intrusion metrics and targets

Reducing passenger compartment intrusion is not a matter of intuition; it is governed by a small set of measurable quantities that translate crash energy into structural performance. This section defines the four core metrics used in side-impact and frontal structural engineering — occupant survival space, intrusion velocity, dynamic crush, and the evaluation criteria applied by IIHS, NHTSA, and C-NCAP — and shows how they are measured, interpreted, and converted into design targets.

Occupant Survival Space

Occupant survival space is the residual volume between the deformed occupant compartment boundary and the seated occupant's reference posture. It is the primary structural acceptance criterion because it directly determines whether the restraint system has room to manage occupant kinematics. In practice, survival space is quantified as the residual distance between a fixed reference point on the occupant (typically the H-point or a defined body landmark) and the intruding surface, measured after the crash pulse.

For side impact, the most common engineering surrogate is the distance from the seat centerline to the inner door trim at the occupant's hip, chest, and head heights. A typical mid-size sedan target might require ≥ 250 mm residual clearance at the hip, ≥ 200 mm at the chest, and ≥ 150 mm at the head after a 50 km/h MDB (moving deformable barrier) impact. These are example targets, not universal values; they depend on vehicle class, occupant size (5th percentile female vs. 50th percentile male), and the regulatory or consumer-test protocol being addressed.

Survival space is not a single number but a profile. Engineers plot residual clearance as a function of height and longitudinal position, producing an intrusion contour map. This map reveals whether the door trim, B-pillar, or rocker is the dominant intruder and guides countermeasure placement.

Intrusion Velocity

Intrusion velocity is the rate at which the occupant compartment boundary moves inward during the crash event. It is distinct from intrusion magnitude: a door that intrudes 150 mm slowly may be less injurious than one that intrudes 80 mm at high velocity, because the occupant's body is accelerated by the moving surface before the restraint system can engage.

Intrusion velocity is typically derived from high-speed film or accelerometer data. A common method is to differentiate the displacement-time history of a target point on the inner door or B-pillar:

v_intrusion(t) = d/dt [ x_inner(t) - x_reference(t) ]

where:
  x_inner(t)     = displacement of the intruding surface
  x_reference(t) = displacement of the occupant reference (seat or tunnel)
  v_intrusion(t) = relative intrusion velocity

Peak intrusion velocity in side impact often occurs in the first 20–40 ms and can exceed 8–12 m/s in severe pole impacts. Design targets typically aim to keep peak intrusion velocity below a threshold that the side airbag and door trim can manage — often 6–8 m/s for the chest region, depending on the restraint strategy. Again, these are illustrative values; the actual target is set by correlating intrusion velocity with injury metrics such as chest deflection and rib fracture risk.

Dynamic Crush

Dynamic crush is the maximum deformation of a structural member during the crash, as opposed to static crush measured in a quasi-static test. It is the quantity that actually absorbs energy and is therefore the bridge between structural design and intrusion metrics.

For a side door assembly, dynamic crush is measured as the difference between the original and deformed positions of the door outer panel relative to the rocker. A typical side impact test might show 300–500 mm of dynamic crush at the door outer, but only 100–200 mm of intrusion into the occupant compartment — the difference being the space consumed by the door structure, trim, and airbag.

The relationship between dynamic crush and intrusion is governed by the crush efficiency of the load path. A door that crushes uniformly transfers less force to the occupant compartment; a door that bottoms out early transfers a sharp force spike. Engineers therefore design for a progressive crush profile, often verified by comparing the force-displacement curve from a component test with the target curve derived from full-vehicle simulation.

IIHS, NHTSA, and C-NCAP Evaluation Criteria

The three major evaluation regimes differ in barrier type, impact speed, and injury criteria, which in turn drive different intrusion targets.

The IIHS side test is the most demanding for intrusion because of the heavy barrier and oblique angle, which concentrates load on the B-pillar and door. NHTSA's FMVSS 214 uses a lighter barrier but a crabbed configuration that tests the door's shear resistance. C-NCAP's 2021 protocol adds a far-side occupant requirement, meaning intrusion on the non-struck side must also be controlled to prevent occupant-to-occupant contact.

For frontal impact, the corresponding metrics are toe pan intrusion, dash panel intrusion, and steering column displacement. IIHS moderate overlap and small overlap tests impose strict limits on these, with small overlap being particularly sensitive to A-pillar and hinge pillar intrusion.

From Metrics to Targets: A Worked Example

Suppose a vehicle program targets a 5-star C-NCAP side impact rating. The engineering team must translate the rating requirement into structural targets. A typical workflow is:

1. Define the occupant reference: 50th percentile male, H-point at the seat design position.
2. Set survival space targets: Based on injury risk curves, require ≥ 220 mm residual clearance at the hip and ≥ 180 mm at the chest after 60 km/h MDB impact.
3. Derive intrusion velocity limit: From restraint system capability, set peak chest-region intrusion velocity ≤ 7 m/s.
4. Allocate dynamic crush: Using CAE, determine that the door outer must crush ≥ 350 mm while the inner panel intrusion remains ≤ 150 mm.
5. Validate with component tests: Conduct a sled or door sub-system test to confirm the force-displacement curve matches the target.

If the component test shows early bottoming (force spike before 200 mm crush), the typical failure symptom is a sharp rise in intrusion velocity and a corresponding drop in survival space. Troubleshooting focuses on the load path: adding a door beam, adjusting the rocker section, or modifying the B-pillar grade and thickness to delay buckling.

Measurement and Verification

All three metrics are verified through a combination of CAE and physical testing. In CAE, intrusion is extracted from nodal displacement of the inner door panel and B-pillar. In physical tests, string potentiometers, high-speed cameras, and accelerometers are used. The correlation between CAE and test is typically required to be within 10% for intrusion magnitude and 15% for intrusion velocity at the same time step.

For C-NCAP and IIHS, the final rating is not based on intrusion alone but on injury criteria (e.g., HIC, chest deflection, pelvic force) that are influenced by intrusion. Therefore, intrusion targets are always set in conjunction with restraint system targets — they are not independent.

The diagram above shows the cross-sectional relationship between the original door outer, the deformed door outer, the door inner panel, and the occupant reference. The dynamic crush is the distance the outer panel travels; the intrusion is the distance the inner panel travels; the survival space is what remains between the inner panel and the occupant. Intrusion velocity is the time derivative of the intrusion displacement. The evaluation criteria box summarizes the three major test regimes that define the targets.

Summary of Design Targets

In practice, intrusion metrics are not independent targets but a linked chain: dynamic crush capacity determines intrusion magnitude, which determines survival space, which determines injury risk. Intrusion velocity is the rate at which that chain unfolds and is often the limiting factor for restraint system performance. The IIHS, NHTSA, and C-NCAP protocols each impose different boundary conditions, so a vehicle designed for one market may not automatically satisfy another. Engineers must therefore define a target set that covers the worst-case combination of barrier mass, impact speed, and impact angle for the intended markets, and validate it through both CAE and physical testing.