3. Pulse Shaping Using Rubber Bumper and Honeycomb Absorber

Pulse Shaping with Rubber Bumpers and Honeycomb Absorbers

In a sled test, the acceleration pulse delivered to the test article is not produced by the sled drive alone. It is the result of a deliberate mechanical filter placed between the moving sled and the reaction surface: a pulse-shaping stack. Two materials dominate this stack in practice — elastomeric rubber bumpers, which provide a progressive, velocity-dependent restoring force, and crushable honeycomb absorbers, which provide a nearly constant force over a controlled stroke. Combining them lets you tune peak level, rise time, and pulse duration somewhat independently, which is exactly what is needed to match a target crash pulse.

The core idea is energy management. The sled arrives at the shaping stack with a known kinetic energy, and the stack must absorb that energy over a prescribed displacement while producing the desired deceleration history. Rubber stores and returns energy elastically; honeycomb dissipates it through plastic buckling. A stack that is mostly rubber gives a rounded, sinusoidal-like pulse with a pronounced rise and fall. A stack that is mostly honeycomb gives a flatter, more rectangular pulse with a sharp onset. Real target pulses usually sit between these extremes, so the design task is to find the right mix and geometry.

How the Two Elements Shape the Pulse

A rubber bumper behaves approximately like a nonlinear spring with damping. Its force rises steeply with compression, so the deceleration climbs quickly at the start of contact and then rolls off as the sled slows. The effective stiffness depends on durometer (Shore A), cross-section, and the number of bumpers in parallel. Increasing durometer or reducing bumper height raises peak acceleration and shortens duration. Rubber also has a rebound component, so a purely rubber stack can produce a secondary oscillation after the main pulse — often visible as a negative acceleration tail that must be filtered or accepted within tolerance.

Honeycomb, by contrast, is a controlled crush element. Once the crush initiator triggers, the cell walls buckle at a nearly constant crush stress, so the reaction force stays flat until the honeycomb is fully densified. That flat plateau is what produces the characteristic "square" portion of a crash pulse. The plateau level scales with crush strength (a function of cell size, wall thickness, and material), and the plateau length scales with the crush stroke. Honeycomb is largely one-shot: after crushing it does not recover, so it is a single-use element in each test.

Design Workflow for a Target Pulse

Suppose the target is a 30 g plateau lasting about 60 ms with a 10 ms rise, corresponding to a velocity change of roughly 17.6 m/s (Δv = ∫a dt). The following sequence is a practical way to converge on a stack geometry.

1. Fix the sled mass and impact velocity. These are given by the test specification and determine the total kinetic energy the stack must absorb: E = ½mv². For a 500 kg sled at 17.6 m/s, E ≈ 77.4 kJ.
2. Estimate the required stroke. For a roughly constant deceleration, stroke s ≈ v²/(2a). With a = 30 g ≈ 294 m/s², s ≈ 0.53 m. This is the total crush distance the stack must provide, split between rubber compression and honeycomb crush.
3. Size the honeycomb for the plateau. Required plateau force F = ma ≈ 500 × 294 ≈ 147 kN. Select honeycomb grade whose crush strength times loaded area gives this force, then set honeycomb thickness to cover most of the stroke (e.g., 0.40 m), leaving the remainder for rubber.
4. Size the rubber for the rise and peak trim. Choose bumper count and durometer so the initial stiffness produces the 10 ms rise without overshooting the 30 g plateau. Fewer, softer bumpers lengthen the rise; stiffer or more bumpers shorten it and raise the peak.
5. Iterate on the sled. Run the stack, compare the measured pulse to the target using peak, rise time, duration, and Δv, then adjust honeycomb thickness (duration) or bumper durometer (peak/rise) and repeat.

The figure below shows how the two elements combine in the stack and how each contributes to the resulting pulse shape.

Material and Geometry Effects to Evaluate

Once a baseline stack is built, the evaluation phase is about isolating which parameter moves which pulse metric. The table below summarizes the expected direction of change for the most common adjustments. These are qualitative trends consistent with the mechanics described above; confirm them on your own sled with measured data rather than assuming exact magnitudes.

Two effects deserve special attention. First, rubber rebound can create a negative acceleration tail after the main pulse; if the target pulse requires a clean return to zero, add a small honeycomb element or a damping layer to absorb the stored elastic energy. Second, honeycomb densification — the point where crushed cells pack solid — produces a sharp spike at the end of the stroke. Keep the honeycomb thickness below the available stroke so densification occurs after the pulse has already decayed, or the spike will corrupt the tail of the pulse.

Verification Criteria

After each run, reduce the measured acceleration to the same metrics used to define the target: peak acceleration, rise time (10% to 90% of peak), plateau duration, and total Δv from integration. Accept the stack when peak and Δv fall within the test specification tolerance (commonly ±5% to ±10% depending on the program) and the pulse shape matches the target within the agreed corridor. If peak is high but Δv is correct, the stack is too stiff and too short — shift stroke from rubber to honeycomb. If Δv is low, the stack is not absorbing enough energy — increase honeycomb thickness or crush strength. If the rise is too slow, reduce rubber compliance or add bumpers. Iterating on these three levers, one at a time, converges quickly and keeps the effect of each change interpretable.