4. Impact test rig and fixture design

The validity of any pole impact test — whether it follows a free-fall drop tower, a pendulum impactor, or a guided linear impactor — depends far more on the rig than on the helmet. A helmet struck by a pole that flexes, rotates, or shifts at the moment of contact is no longer being tested against the boundary condition the standard specifies. The rig and fixture therefore form part of the measurement chain: they define the impact velocity vector, the effective striking mass, the pole's boundary condition at its base, and the geometric relationship between the pole axis and the headform. This section treats those three design responsibilities — rigid pole mounting, alignment tolerance, and structural stiffness — as a single coupled problem, because in practice they trade against one another.

What the fixture must guarantee

Before choosing hardware, state the fixture's contract in measurable terms. For a typical pole impact test derived from EN 960-style headforms and pole impact methods such as EN 1078 or Snell B-95 practice, the fixture should guarantee:

• Pole base fixity: the pole behaves as a cantilever with a base rotation small enough that the contact point does not translate more than a defined fraction of the pole diameter during the impact event.
• Impact vector control: the relative velocity between pole and headform lies within a stated angular tolerance of the intended axis (commonly ±1° to ±2° for the approach direction, tighter for the pole axis itself).
• Geometric repeatability: the pole's contact point, the headform's reference point, and the impact location on the helmet shell are reproducible from specimen to specimen within a few millimetres.
• Energy accounting: the fixture's own moving mass and any stored elastic energy are either negligible or explicitly included in the impact energy budget.

These four requirements are not independent. A very stiff pole mount reduces base rotation but raises the peak contact force and shortens the pulse, which changes the helmet's deformation mode. A compliant mount softens the pulse but introduces base translation that corrupts the velocity vector. The design task is to choose a stiffness that keeps fixture-induced artefacts below the measurement uncertainty of the test.

Rigid pole mounting

The pole is normally a steel or aluminium cylinder, 40–100 mm in diameter and 1.5–2.5 m long, mounted vertically or at a specified oblique angle. The mount must resist both bending moment and shear at the base. Three mounting schemes are common:

For a bolted flange, size the bolts for the peak bending moment rather than the static weight. As a worked example, take a 60 mm steel pole, 2.0 m free length, struck 1.8 m above the base by a 5 kg headform at 6.0 m/s with a 5 ms contact pulse. The impulse is approximately m·Δv = 5 × 6.0 = 30 N·s, giving a mean force of about 6 kN over 5 ms and a peak perhaps 1.5–2× that value. The resulting base moment is roughly 12 kN·m at peak. A four-bolt circle of M16 grade 10.9 bolts at 150 mm radius carries this moment with a safety factor above 3, but only if preload is maintained; use hardened washers and verify torque before each test series. Do not rely on the bed plate's mass alone — a 200 kg plate can still rock on an uneven floor and inject low-frequency motion into the pulse.

Alignment tolerances

Alignment has three components: the pole axis relative to the impact direction, the headform reference plane relative to the pole, and the impact point on the helmet. Define a coordinate convention once and keep it: let +Z be the pole axis (upward), +X the approach direction of the headform, and the origin at the pole's contact point at the specified impact height. All angular tolerances below refer to this frame.

Practical tolerances that hold up in a working lab are:

• Pole verticality (or specified obliquity): ±0.5° measured over the free length with a digital inclinometer.
• Approach direction of the headform relative to +X: ±1.0° in the horizontal plane, ±1.0° in elevation.
• Impact height on the pole: ±5 mm.
• Lateral offset of the headform centreline from the pole axis at contact: ±3 mm.
• Headform reference plane (Frankfort plane or equivalent) relative to horizontal: ±2°.

These numbers are chosen so that the resulting error in the impact vector stays below about 2% of the nominal velocity — the point at which the tolerance begins to matter relative to typical pass/fail margins. Verify alignment with a laser or a dial indicator on a dummy pole before mounting a helmet; checking alignment with the helmet in place hides pole error behind shell curvature.

Structural stiffness and its consequences

Stiffness is the parameter most often ignored and most often responsible for irreproducible results. The pole–mount–bedplate assembly has a first bending natural frequency. If the contact pulse contains energy near that frequency, the fixture rings and the headform sees an oscillation superimposed on the intended pulse. As a rule of thumb, keep the assembly's first natural frequency at least five times the inverse of the pulse duration: for a 5 ms pulse, that means above 1 kHz.

Estimate the pole's first cantilever frequency from f ≈ 0.56 · (1/L²) · √(EI/ρA). For the 60 mm steel pole above (I ≈ 6.4 × 10⁻⁷ m⁴, A ≈ 2.8 × 10⁻³ m², ρ = 7850 kg/m³, E = 210 GPa), this gives roughly 60 Hz — far below 1 kHz. The pole itself is therefore not rigid in the sense that matters; it is a compliant cantilever whose base fixity and effective mass must be accounted for. Two responses are possible: accept the compliance and characterise it (measure pole tip deflection and pulse shape with a dummy headform), or stiffen the assembly by shortening the free length, increasing diameter, or adding a mid-span brace. A brace changes the boundary condition to a propped cantilever and roughly quadruples the first frequency, but it also introduces a second load path that must not contact the helmet.

Commissioning check before the first specimen

Run a dummy-headform impact at the lowest energy in the test matrix and record pole base strain, pole tip displacement, and headform acceleration. A clean pulse should rise and fall monotonically within the intended window; superimposed oscillation at the pole's natural frequency indicates insufficient stiffness or a loose mount. Repeat the same impact three times: peak acceleration should agree within ±5% and pulse duration within ±0.2 ms. If it does not, inspect bolt preload, socket seating, and bed plate contact before adjusting anything else. Only after the fixture passes this repeatability check should helmet specimens be mounted, because from that point onward every result inherits the fixture's behaviour.

The figure summarises the coupled design problem: the left panel fixes the coordinate convention and the tolerances that govern the impact vector, while the right panel shows why stiffness matters — a pole whose first natural frequency sits inside the pulse bandwidth turns a clean contact into a ringing one, and the headform records the fixture as much as the helmet. Treat the rig as an instrument, commission it with a dummy headform, and document its measured pulse before any specimen data is accepted.