2. Crash pulse fundamentals

The crash pulse is the acceleration–time history experienced by a vehicle structure during an impact. It is the single most important intermediate variable in frontal crash safety: it is the output of the barrier test configuration and the input to every restraint system, dummy, and occupant. Understanding how barrier type, impact speed, and structural design shape the pulse — and how that pulse propagates into occupant loading — is the foundation for interpreting test data and tuning restraint performance.

Barrier Types and Their Pulse Signatures

Frontal crash tests use two fundamentally different barriers, and the choice of barrier determines the character of the pulse before any vehicle design is considered.

A rigid wall (typically a fixed, immovable concrete or steel block) presents an essentially infinite stiffness. All deformation energy must be absorbed by the vehicle. The resulting pulse is short, high-amplitude, and dominated by the vehicle's own crush characteristics. Peak deceleration commonly falls in the 40–60 g range for a full frontal rigid-wall impact at 50 km/h, with the pulse duration on the order of 80–120 ms. Because the wall does not yield, the vehicle's front structure is the only compliance in the system.

A deformable barrier (for example, the honeycomb/aluminum structure used in offset deformable barrier tests) introduces a controlled crushable interface. The barrier absorbs a portion of the kinetic energy, which lowers the peak acceleration and lengthens the pulse. In an offset deformable barrier test, the barrier face deforms progressively, and the vehicle experiences a lower-magnitude, longer-duration pulse — often 25–40 g peak with a duration of 120–160 ms. The barrier's crush strength is engineered to approximate the stiffness of a typical opposing vehicle front end, making the test more representative of real-world car-to-car collisions.

The practical consequence is that the same vehicle can produce very different pulses against different barriers. A structure tuned for a rigid-wall pulse may be too stiff or too soft when evaluated against a deformable barrier, and vice versa. This is why modern frontal safety development uses both configurations.

Crash Pulse: Barrier Configuration → Vehicle Response → Occupant Loading Rigid Wall Impact Fixed wall Vehicle crush zone Peak: 40–60 g Duration: 80–120 ms Short, high-amplitude pulse Deformable Barrier Impact Crushable barrier Vehicle crush zone Peak: 25–40 g Duration: 120–160 ms Longer, lower-amplitude pulse Pulse Shape Comparison time (ms) a (g) Rigid wall Deformable Occupant Loading Pathway Barrier force input Vehicle structure crush / deceleration Occupant compartment pulse transmission Restraints belt / airbag / pretensioner Occupant injury metrics Δv = ∫ a(t) dt — the velocity change is the area under the acceleration–time curve Same Δv, different pulse shape → different occupant loading and injury outcome

Pulse Shape Metrics

Engineers characterize a crash pulse using several quantitative metrics, each of which influences occupant outcome differently.

  • Peak acceleration (g) — the maximum deceleration magnitude. High peaks drive high restraint forces and can cause belt-loading injuries, but a short high peak may be survivable if the occupant is well coupled.
  • Pulse duration (ms) — the time from impact to separation or zero velocity. Longer pulses give restraints more time to act progressively, reducing peak forces on the occupant.
  • Delta-v (Δv) — the total velocity change, equal to the integral of acceleration over time. For a full frontal rigid-wall impact at 50 km/h, Δv is approximately 50 km/h (13.9 m/s) if the vehicle comes to rest. Δv is the primary energy metric and correlates strongly with injury severity.
  • Time to peak (ms) — how quickly the pulse reaches its maximum. Early peaks are more aggressive because restraints have less time to deploy and engage.
  • Pulse shape (front-loaded vs. rear-loaded) — a front-loaded pulse rises quickly and decays slowly; a rear-loaded pulse rises slowly and peaks late. Front-loaded pulses are generally more demanding on restraints because the occupant is loaded early.

A useful way to think about Δv is the area under the acceleration–time curve. Two vehicles can experience the same Δv but very different pulses: one may have a sharp 60 g spike lasting 60 ms, while another has a broad 30 g plateau lasting 140 ms. The occupant in the first vehicle experiences a much higher rate of loading, which is why pulse shape — not just Δv — determines injury risk.

Delta-v and Occupant Loading Pathways

The occupant does not experience the barrier force directly. The loading pathway is a chain: barrier → vehicle structure → occupant compartment → restraints → occupant. Each link transforms the pulse.

Consider a 50 km/h full frontal rigid-wall impact. The vehicle's front structure crushes, generating a deceleration pulse that peaks around 50 g at roughly 40 ms. The occupant compartment decelerates with the vehicle, but the occupant — restrained by the belt and airbag — lags behind. The relative motion between occupant and compartment is the occupant ride-down. The restraint system's job is to manage this relative motion so that the occupant decelerates over the longest possible time and distance, minimizing peak forces on the chest, head, and pelvis.

If the pulse is too aggressive (high peak, short duration), the restraint must absorb the occupant's kinetic energy in a very short window. This can lead to high belt forces, chest deflection, and head acceleration. If the pulse is too soft (low peak, long duration), the occupant may not be coupled early enough, leading to excessive forward excursion and potential contact with interior surfaces.

Key relationship: Δv sets the energy the restraint must manage. Pulse shape sets how quickly that energy must be managed. Both are determined by barrier type and vehicle structure, and both must be considered together when tuning restraints.

Practical Example: Interpreting a Pulse Trace

Suppose a frontal impact test produces the following simplified acceleration–time data for the vehicle's B-pillar (a common pulse measurement location):

Time (ms)   Acceleration (g)
0           0
10          15
20          38
30          52
40          55
50          48
60          35
70          22
80          10
90          3
100         0

From this trace, we can extract several metrics. The peak acceleration is 55 g at 40 ms. The pulse duration is approximately 100 ms. The Δv is the area under the curve. Using trapezoidal integration:

Δv ≈ Σ (a_i + a_{i+1})/2 × Δt

Segment 0–10 ms:   (0+15)/2 × 0.010 = 0.075 m/s
Segment 10–20 ms:  (15+38)/2 × 0.010 = 0.265 m/s
Segment 20–30 ms:  (38+52)/2 × 0.010 = 0.450 m/s
Segment 30–40 ms:  (52+55)/2 × 0.010 = 0.535 m/s
Segment 40–50 ms:  (55+48)/2 × 0.010 = 0.515 m/s
Segment 50–60 ms:  (48+35)/2 × 0.010 = 0.415 m/s
Segment 60–70 ms:  (35+22)/2 × 0.010 = 0.285 m/s
Segment 70–80 ms:  (22+10)/2 × 0.010 = 0.160 m/s
Segment 80–90 ms:  (10+3)/2 × 0.010 = 0.065 m/s
Segment 90–100 ms: (3+0)/2 × 0.010 = 0.015 m/s

Total Δv ≈ 2.78 m/s ≈ 10 km/h

This Δv of about 10 km/h is low for a 50 km/h barrier test, which suggests the pulse data may be from a partial-overlap or lower-severity configuration, or that the measurement location is not capturing the full vehicle Δv. In a full frontal rigid-wall test at 50 km/h, the vehicle Δv should be close to 50 km/h (13.9 m/s). This discrepancy is a useful diagnostic: it tells the engineer to check the measurement location, the test configuration, or the data processing.

Practical tip: Always verify that the integrated Δv from your pulse trace matches the expected test Δv within a few percent. A mismatch usually indicates a sensor mounting issue, a filtering problem, or a data acquisition error.

Occupant Loading Pathways in Detail

The pathway from pulse to occupant injury involves several distinct mechanisms:

  • Belt loading: The lap and shoulder belts transfer deceleration forces to the pelvis and chest. High pulse peaks require high belt tension, which can cause rib fractures and abdominal injuries.
  • Airbag interaction: The airbag deploys and provides a compliant surface that distributes forces over the head and chest. The timing of airbag deployment relative to the pulse is critical — too early and the bag may be deflated before the occupant arrives; too late and the occupant may contact the steering wheel.
  • Knee bolster and lower extremity loading: The lower legs and knees interact with the knee bolster and footwell, which can cause femur and tibia forces.
  • Head kinematics: The head's motion is governed by the neck and the restraint system. High pulse peaks can cause excessive head acceleration and neck tension.

Each of these pathways is tuned to the specific pulse shape. A restraint system optimized for a rigid-wall pulse may not perform optimally in a deformable barrier test, and vice versa. This is why modern development programs use both barrier types and validate restraint performance across a range of pulse shapes.

Summary

The crash pulse is the bridge between barrier configuration and occupant injury. Rigid walls produce short, high-amplitude pulses; deformable barriers produce longer, lower-amplitude pulses. Δv quantifies the total velocity change and sets the energy the restraint must manage, while pulse shape determines how quickly that energy must be absorbed. The occupant loading pathway — barrier to structure to compartment to restraints to occupant — transforms the pulse at each stage, and understanding this chain is essential for interpreting test results and designing effective restraint systems.

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