4. Intrusion measurement and metrics

In side impact restraint calibration, the restraint system does not operate in isolation. Its performance is bounded by how far the struck-side structure deforms and how fast that deformation reaches the occupant. Intrusion metrics quantify this boundary. They tell the calibration engineer how much survival space remains, how quickly the door trim approaches the occupant, and whether the restraint deployment timing has enough margin to intervene before contact. Without consistent intrusion measurement, airbag and pretensioner timing decisions become guesswork.

Defining the Intrusion Reference Frame

All intrusion metrics require an unambiguous reference. The standard convention in side impact testing is to measure displacement relative to the undeformed vehicle coordinate system, typically the original body-in-white (BIW) datum established before the test. Positive X is forward, positive Y is toward the struck side (or per the OEM's defined lateral axis), and positive Z is upward. Displacement of any point is reported as the change in its position from the pre-test datum to its position at a given time during the impact.

Two reference choices are common and must not be mixed within one dataset:

• Ground-fixed (laboratory) frame — the vehicle moves into a fixed barrier or moving deformable barrier (MDB); intrusion is measured in the lab frame and includes the vehicle's own rigid-body motion.
• Vehicle-fixed frame — the vehicle body is the reference; intrusion is the deformation of the structure relative to the occupant compartment. This is the frame that matters for occupant survival space.

For restraint calibration, the vehicle-fixed frame is the primary reference because the occupant is coupled to the seat and compartment, not to the laboratory floor. When data is exchanged between CAE and test, the frame convention must be stated explicitly in the correlation plan.

Door Trim Displacement

Door trim displacement is the lateral movement of the interior door panel surface at specific points that the occupant's pelvis, thorax, or abdomen may contact. It is the most direct measure of how much the striking surface intrudes into the occupant's space. Measurement points are typically placed at:

• Pelvis height (approximately 250–350 mm above the seat cushion reference)
• Lower thorax / abdomen height (approximately 400–500 mm)
• Upper thorax / shoulder height (approximately 550–650 mm)

These points correspond to the load paths of the side airbag and the likely contact zones of the occupant. Displacement is recorded as a time history, not a single peak value, because the timing of maximum displacement relative to airbag deployment is what determines whether the restraint is effective.

A typical door trim displacement trace from a moving deformable barrier test at 50 km/h shows an initial slow rise during the first 10–15 ms as the outer door skin deforms, followed by a rapid increase as the door beam and inner panel buckle. Peak displacement often occurs between 40 and 70 ms. If the side airbag is fully inflated before the trim reaches the occupant, the bag acts as a cushion; if the trim arrives first, the occupant is loaded by the door structure directly.

B-Pillar Intrusion

The B-pillar is the primary structural load path on the struck side. Its intrusion is measured at several heights along the pillar, typically at the beltline, mid-height, and sill. B-pillar intrusion is a strong indicator of structural performance and is often used as a regulatory or rating metric.

For example, in the FMVSS 214 side impact test, B-pillar intrusion at the occupant compartment is monitored, and in Euro NCAP side impact, the B-pillar intrusion rate and peak are part of the structural assessment. A common calibration target is to keep peak B-pillar intrusion below a threshold that preserves sufficient survival space — often on the order of 150–250 mm depending on vehicle class and test configuration.

B-pillar intrusion also drives the deployment timing of the side airbag. If the B-pillar intrudes rapidly, the airbag must deploy earlier to be in position before the trim reaches the occupant. The calibration engineer uses the B-pillar intrusion velocity to set the airbag trigger time.

Occupant Compartment Reduction

Occupant compartment reduction is the loss of interior volume or the reduction of the distance between the struck-side interior surface and the occupant seating position. It is a derived metric that combines door trim displacement, B-pillar intrusion, and seat movement. The most common form is the residual survival space — the distance between the deformed door trim and the occupant's H-point or torso reference line at the time of maximum intrusion.

Compartment reduction is often expressed as a percentage of the original compartment width. For a typical mid-size sedan, the original distance from the driver's H-point to the inner door trim might be 350 mm. If intrusion reduces this to 200 mm, the compartment reduction is approximately 43%. This metric directly informs whether the occupant has enough space to avoid being crushed against the center console or the far-side interior.

In CAE, compartment reduction is extracted by tracking the minimum distance between the occupant's finite element model and the intruding structure throughout the event. In testing, it is measured using string potentiometers, optical tracking, or high-speed camera photogrammetry.

Intrusion Velocity

Intrusion velocity is the time derivative of displacement. It is arguably the most important metric for restraint timing because it determines how quickly the occupant's survival space is being consumed. A high intrusion velocity means the restraint system has less time to deploy and position itself.

Intrusion velocity is calculated from the displacement time history using numerical differentiation. A central difference scheme is standard:

v(t) = (x(t + Δt) - x(t - Δt)) / (2 * Δt)

where x(t) is the displacement at time t and Δt is the sampling interval. For a typical side impact test sampled at 10 kHz, Δt = 0.1 ms. Filtering is essential; a low-pass Butterworth filter with a cutoff frequency of 300–500 Hz is commonly applied to remove noise while preserving the physical velocity content.

Peak intrusion velocity for the door trim in a 50 km/h MDB test often falls in the range of 6–10 m/s. The time at which peak velocity occurs — typically 20–40 ms — is a critical input to the airbag trigger algorithm. If the airbag must be fully inflated by the time the trim reaches the occupant, and the trim travels the last 100 mm at 8 m/s, the available time is only about 12.5 ms. This is why side airbag deployment times are often in the 10–20 ms range from trigger.

Measurement Methods and Practical Considerations

Intrusion data comes from three sources: physical testing, CAE simulation, and hybrid correlation. Each has distinct characteristics.

When measuring intrusion, several practical issues arise. Sensor mounting can affect the measurement if the sensor itself is displaced or damaged during impact. String potentiometers must be routed to avoid interference with the deploying airbag. Optical tracking requires line-of-sight, which is often blocked by the intruding door. In CAE, the choice of output frequency and filtering must match the test data to enable valid comparison.

Using Intrusion Metrics in Calibration

The calibration workflow uses intrusion metrics in a specific sequence. First, the structural intrusion profile is established from test or validated CAE. Second, the occupant's kinematic response is simulated or measured. Third, the gap between the intruding trim and the occupant is calculated over time. Fourth, the airbag trigger time and inflation profile are adjusted so that the bag is in position before the gap closes.

A useful derived metric is the time-to-contact (TTC), defined as the time remaining before the door trim reaches the occupant's surface. TTC is calculated by dividing the current gap by the current relative velocity. When TTC drops below the airbag's required inflation time, the trigger must fire. This is the fundamental timing constraint in side impact restraint calibration.

The figure above shows the relationship between door trim displacement, intrusion velocity, and the airbag trigger window. The displacement curve rises slowly at first, then accelerates as the door structure buckles. The velocity curve peaks earlier than the displacement curve — this is the critical timing signal. The airbag trigger window is positioned so that the bag is fully inflated before the displacement curve reaches the occupant. If the trigger window is missed, the occupant loads the door trim directly, and injury metrics rise sharply.

Key Takeaways for Calibration

• Always define the reference frame before reporting any intrusion value.
• Door trim displacement, B-pillar intrusion, and compartment reduction are complementary — use all three to characterize the structural threat.
• Intrusion velocity, not displacement alone, drives airbag trigger timing.
• Filtering and sampling rate must be consistent between test and CAE for valid correlation.
• Time-to-contact is the derived metric that directly links intrusion measurement to restraint deployment timing.

With these metrics established, the calibration engineer can proceed to set the airbag trigger threshold and inflation profile with a quantitative basis rather than relying on trial-and-error. The next step is to integrate these intrusion metrics into the restraint control algorithm and validate the timing against occupant injury criteria.