3. Occupant kinematics and injury criteria

Sled tests exist to answer one question: if this restraint system were installed in a real vehicle, would the occupant survive the crash with acceptable injury risk? The sled itself only reproduces a deceleration pulse; the actual verdict comes from the biomechanical metrics computed from the Anthropomorphic Test Device (ATD) instrumentation. This section covers the core injury criteria — HIC, chest deflection, femur load, and their supporting metrics — and explains how each is derived, what physical mechanism it represents, and how to judge whether a measured value passes or fails.

From Kinematics to Injury: What the ATD Actually Measures

During a frontal sled test, the occupant's motion follows a predictable sequence: the torso loads the belt, the pelvis migrates forward, the head rotates and translates toward the steering wheel or airbag, and the knees drive into the knee bolster. Each of these motions is captured by accelerometers, load cells, and displacement sensors embedded in the ATD. The raw signals are filtered and then reduced into scalar injury criteria that can be compared against regulatory or consumer-rating thresholds.

The most important distinction is between kinematic quantities (acceleration, displacement, velocity) and injury criteria (dimensionless or physically-scaled indices derived from those quantities). A head acceleration trace is kinematic data; HIC is the injury criterion computed from it. Confusing the two leads to misreading test reports.

Head Injury Criterion (HIC)

HIC is defined as:

HIC = max over [t1, t2] of { (t2 - t1) * [ (1/(t2-t1)) * ∫ a(t) dt ]^2.5 }

where a(t) is the resultant translational acceleration at the head center of gravity, expressed in g, and the integration window t2 - t1 is limited to 36 ms for HIC36 or 15 ms for HIC15. The 2.5 exponent heavily penalizes high-magnitude, short-duration pulses, which is why a sharp airbag deployment spike can dominate the HIC value even if the overall pulse looks benign.

In practice, you compute HIC from the filtered head accelerometer signal (typically SAE J211 Channel Frequency Class 1000 for the head). The standard regulatory threshold for a 50th-percentile male ATD in a frontal rigid barrier test is HIC15 ≤ 700. Consumer rating programs often use HIC15 ≤ 500 for a good score. A common failure mode is a HIC value that passes the 36 ms window but fails the 15 ms window — this indicates a short, sharp contact with a stiff structure, such as a steering wheel rim or an unfired airbag cover.

Chest Deflection and the Chest Injury Criterion

Chest deflection is measured by a potentiometer or string potentiometer inside the ATD chest, recording the sternum-to-spine compression in millimeters. For the Hybrid III 50th male, the peak chest deflection limit is typically 63 mm in regulatory tests, while consumer protocols may require ≤ 42 mm for a good rating. The chest injury criterion (often denoted C_max or simply "chest deflection") is not a single formula but a threshold applied to the peak value of the filtered deflection signal.

The physical mechanism is direct: excessive sternal compression fractures ribs and injures internal organs. Belt routing matters enormously here. A belt that rides too high on the abdomen produces a different deflection profile than one properly seated on the sternum. In sled tests, you will often see chest deflection increase when the shoulder belt anchor is too low, because the belt loads the lower rib cage and allows more sternal travel.

For the Q-series child ATDs, chest deflection is also measured, but the threshold scales with the ATD size. Always verify which ATD and which threshold apply to your test matrix.

Femur Load

Femur load is measured by load cells in each femur, capturing axial compression along the bone axis. The standard threshold for the Hybrid III 50th male is 10 kN per femur, often expressed as a combined or individual limit. The injury mechanism is axial compression leading to femoral shaft fracture or hip joint injury.

Femur load is highly sensitive to knee bolster geometry and knee target location. If the knee misses the bolster and loads the instrument panel directly, or if the bolster is too stiff, femur load spikes. In sled testing, a common diagnostic is to compare left and right femur loads; asymmetry greater than 1 kN often indicates a knee target misalignment or a asymmetric belt routing that rotates the pelvis.

Other Biomechanical Metrics

Beyond the three primary criteria, sled test evaluation typically includes:

• Neck injury criteria (Nij, N_km) — combine axial force and bending moment at the occipital condyle. Nij is used in frontal tests; N_km is more common in rear-impact evaluations.
• Combined Thoracic Index (CTI) — combines chest deflection and chest acceleration into a single value, useful when both are elevated but neither alone exceeds its threshold.
• Tibia Index (TI) — combines axial force and bending moment at the tibia, relevant when knee bolster contact is severe.
• Pelvic acceleration — a secondary metric for pelvic fracture risk, especially in side-impact or oblique sled configurations.

Each metric has its own filter class and threshold. Mixing filter classes — for example, applying a CFC 600 filter to a signal that should be CFC 1000 — will shift the computed criterion and can turn a passing test into a failing one. Always confirm the filter class from the test protocol before post-processing.

Practical Evaluation Workflow

When you receive a sled test data set, the evaluation sequence should be:

1. Verify channel metadata. Confirm each channel's filter class, polarity, and units. A head accelerometer recorded in m/s² must be converted to g before HIC computation.
2. Align time zero. Define T0 as the point where the sled pulse first exceeds a defined threshold (commonly 0.5 g or 1 g). All criteria windows are referenced to this T0.
3. Compute each criterion independently. Do not reuse a filtered signal across criteria with different filter requirements. HIC uses CFC 1000; chest deflection uses CFC 180; femur load uses CFC 600.
4. Compare against the correct threshold set. Regulatory (FMVSS 208), Euro NCAP, and internal targets differ. A test can pass FMVSS but fail a consumer rating.
5. Document the limiting criterion. The "worst" metric drives the design change. If HIC is at 95% of threshold and chest deflection is at 60%, the head path is the priority.

Interpreting Results: A Worked Example

Consider a frontal sled test at 56 km/h with a Hybrid III 50th male. The post-processed results show:

The limiting criterion is the right femur load at 96% of threshold. The asymmetry between left (8.2 kN) and right (9.6 kN) suggests a knee target or bolster geometry issue on the right side. Before adjusting the restraint, inspect the knee bolster contact pattern and verify that the right knee is not loading a stiffer section of the instrument panel. This is a classic case where the numeric result points directly to a physical root cause.

Key Takeaways

• HIC, chest deflection, and femur load are the three primary frontal sled test criteria; each has a distinct physical injury mechanism and filter requirement.
• Always verify filter class, units, and time-zero alignment before computing any criterion.
• Thresholds vary by ATD size, test protocol, and rating program — never assume a single universal limit.
• Asymmetry between left and right channels is a powerful diagnostic for restraint geometry problems.
• The limiting criterion, not the average, drives design decisions.