4. Crash pulse characterization

A crash pulse is the acceleration–time history measured at a structural reference point (typically the B-pillar base, rocker, or seat cross-member) during an impact event. For target cascading, the pulse is not treated as a single number but as a parameterized curve whose shape encodes the deceleration strategy of the vehicle. Before any target can be handed to a body, restraint, or seat team, the pulse must be reduced to a small set of physically meaningful descriptors that are unambiguous, measurable, and traceable through the development process.

Four descriptors dominate target setting practice: peak acceleration, duration, slope (rise rate and unloading rate), and energy content. Each maps to a different physical mechanism and therefore to a different owning team. Getting the parameter definitions and the sign convention right at this stage prevents months of miscommunication later.

Sign convention and reference frame

Adopt one convention and state it on every pulse chart and target sheet. The common convention in vehicle crash work is: the vehicle travels in the +X direction, the barrier or opposing structure decelerates it, and the measured acceleration is reported as a deceleration magnitude in g (positive values pointing rearward relative to the occupant). Where raw accelerometer data is used, the sensor output is typically negative during the event; the pulse is then inverted and filtered before characterization. Filtering class matters: ISO 6487 Channel Frequency Class (CFC) 60 is the usual choice for occupant-relevant pulses, while CFC 180 or CFC 1000 is used for structural and component-level work. A pulse characterized at CFC 180 will show a higher and narrower peak than the same event at CFC 60 — so the filter class is part of the parameter definition, not an afterthought.

Peak acceleration

Peak acceleration a_peak is the maximum deceleration magnitude within the event window. It is the primary driver of occupant load paths: chest deceleration, belt force, and airbag firing timing all scale with it. In target cascading, a_peak is usually expressed as a band rather than a single value, for example 42–48 g at the rocker for a 64 km/h frontal ODB (offset deformable barrier) event. The band width reflects the tolerance the downstream teams can absorb; a band narrower than the CAE correlation accuracy is not a target, it is a wish.

Two practical cautions apply. First, a single-sample spike is not a peak — require the peak to be sustained over a defined window (commonly 3 ms) so that sensor noise and high-frequency structural ringing do not set the target. Second, distinguish the first peak (often the bumper/rail engagement) from the second peak (often the engine or subframe contacting the barrier or the powertrain). They load different structures and should be specified separately.

Duration and the pulse window

Duration T is the time from impact (defined as the first crossing of a small acceleration threshold, e.g. 2 g) to the point where deceleration returns below that threshold, or to the point of maximum dynamic crush, depending on the definition adopted. For target setting, the more useful quantity is often the effective duration — the width of the equivalent rectangular pulse that produces the same velocity change. This links directly to the crash severity: the velocity change Δv is the integral of the pulse, so a longer, lower pulse and a shorter, higher pulse can deliver the same Δv with very different occupant outcomes.

Duration is owned jointly by the front-end structure (crush stroke) and the restraint team (ride-down time). A target such as "pulse duration ≥ 90 ms above 10 g" tells the body team that the crush zone must sustain load over that window, and tells the restraint team how much time is available before the occupant loads the belt and airbag.

Slope: rise rate and unloading rate

Slope is the most under-specified parameter in practice and the most common source of late-stage disputes. Two slopes matter:

• Rise rate (loading slope), typically expressed in g/ms, governs how quickly the occupant is coupled to the vehicle deceleration. A steep rise produces a sharp jerk and drives belt force peaks; a shallow rise wastes crush stroke. A typical frontal target might be 1.5–2.5 g/ms over the first 20–30 ms.
• Unloading rate (rebound slope) governs how the stored elastic energy is returned and affects rebound velocity and post-impact occupant kinematics. Excessive unloading rate can produce a secondary load spike as the occupant rebounds into the restraint.

Slope targets should be specified as a maximum rise rate and a maximum unloading rate, not as a single average, because the average hides the local gradient that actually loads the occupant.

Energy content

Energy content quantifies how much of the kinetic energy is absorbed within the pulse window and how it is distributed. Two formulations are useful for target setting:

• Kinetic energy absorbed: E = ½ m Δv², where m is the effective mass and Δv the velocity change. This is the scalar the structure must dissipate and is the natural link to crush force and stroke.
• Energy spectral density (ESD): the energy of the pulse as a function of frequency, obtained from the squared magnitude of the Fourier transform of the acceleration history. ESD reveals whether the pulse energy sits in the low-frequency band that excites the occupant–restraint system (roughly 5–20 Hz) or in the high-frequency band that excites structural modes and can drive component-level failures.

A pulse with the same peak and duration can have very different ESD depending on its shape — a rectangular pulse concentrates energy at low frequency, while a triangular pulse spreads it. Specifying an ESD envelope alongside the time-domain parameters prevents the structure team from meeting the peak target with a shape that is hostile to the occupant.

Worked example: parameterizing a frontal pulse

Consider a 64 km/h ODB event with a 1,500 kg vehicle. The measured rocker pulse (CFC 60) is idealized as a trapezoid: rise from 0 to 45 g over 25 ms, hold at 45 g for 40 ms, unload to 0 over 25 ms. The velocity change is the area under the pulse:

Δv = ∫ a dt
   = 45 g × (25/2 + 40 + 25/2) ms
   = 45 × 9.81 × 0.065 s
   ≈ 28.7 m/s  (≈ 103 km/h equivalent — check against event Δv)

If the event Δv is 16 m/s (a typical ODB value for this class), the idealized pulse over-delivers and must be rescaled: the hold level drops to about 25 g, or the hold duration shortens. This is exactly the iteration the target-setting engineer performs — the pulse must be consistent with the event Δv before any parameter is cascaded. The energy absorbed is then ½ × 1500 × 16² ≈ 192 kJ, which sets the crush force × stroke budget for the front rails.

From parameters to cascaded targets

Once the pulse is parameterized, each descriptor is assigned to an owning team with a tolerance and a verification method. The table below is a representative cascade for a frontal event; the values are illustrative and must be replaced with program-specific data.

The practical test of a well-characterized pulse is whether a downstream engineer can, from the target sheet alone, answer three questions: how hard, how long, and how fast. If any of those requires opening the raw time history, the characterization is incomplete and the cascade will drift during development.