3. Test article specification
In a pole impact test, the pole is not a passive backdrop — it is a calibrated test instrument. Every dimension, material property, and surface finish of the pole influences the deceleration pulse delivered to the headform, and therefore the measured Head Injury Criterion (HIC) and peak linear acceleration. If the pole specification drifts between test campaigns, results become non-comparable even when the impactor, velocity, and target point are nominally identical. This section defines the pole parameters that must be specified, controlled, and documented before any impact is executed, and explains how each parameter propagates into the measured outcome.
Geometry: Diameter, Cross-Section, and Effective Length
The pole's outside diameter is the single most influential geometric parameter. A smaller diameter concentrates contact force over a narrower area, producing a sharper, higher-amplitude deceleration pulse; a larger diameter spreads the contact patch and softens the pulse. For pedestrian head-impact work, regulatory and consumer-rating protocols typically reference poles in the 60–120 mm diameter range, with 90 mm and 114 mm being common reference sizes. The specification must state the nominal diameter, the manufacturing tolerance (for example, ±1 mm), and the measurement method — a calibrated caliper or micrometer at three axial stations (top, mid, bottom) and two orthogonal orientations per station, recorded as a diameter matrix.
Cross-section matters as much as diameter. A circular section is the default for pedestrian-protection poles, but square or rectangular posts (sign supports, gantry legs) present flat faces and sharp corners that create edge-loading effects. If the test article is non-circular, the specification must fix the impact face orientation relative to the headform trajectory, because rotating the post by 45° changes the local radius of curvature at the contact point and therefore the contact stiffness. Record the orientation as an angle referenced to a marked datum on the pole base.
Effective length is the free span between the mounting interface and the impact point, plus the overhang above it. A short, stiff cantilever behaves differently from a tall, slender one: bending compliance in the pole absorbs energy and lengthens the pulse. The specification should define the impact height above the mounting plane (for example, 1.0 m, 1.5 m, or 2.0 m depending on the headform trajectory) and the total free length, and should state whether the pole is rigidly fixed or permitted to deflect. For repeatability, most head-impact protocols require the pole to be effectively rigid over the test duration; if pole deflection exceeds a stated threshold (commonly a few millimetres at the impact point), the mounting or pole section must be revised.
Stiffness and Material Specification
Pole stiffness is governed by the elastic modulus of the material, the second moment of area of the section, and the boundary conditions at the base. For a solid circular steel pole of diameter d, the second moment of area is I = πd⁴/64, and the lateral stiffness at a cantilever tip of length L is k = 3EI/L³. As a worked example, a 114 mm solid steel pole (E ≈ 210 GPa) with a 1.5 m free length gives I ≈ 8.29 × 10⁻⁶ m⁴ and k ≈ 1.55 MN/m — stiff enough that tip deflection under a 10 kN impact load is on the order of 6 mm, which is usually acceptable for a rigid-target protocol. A hollow pole of the same outside diameter with a 6 mm wall has roughly half that stiffness, so wall thickness must be specified explicitly, not inferred.
The specification should therefore capture: material grade (for example, S355 structural steel or 6061-T6 aluminium), elastic modulus assumed for verification, wall thickness or solid/hollow designation, and the measured or calculated lateral stiffness at the impact height. Where the pole is a real roadside product rather than a laboratory surrogate, record the manufacturer, product code, and any internal reinforcement. A useful acceptance check is a static lateral load test: apply a known force (for example, 2 kN) at the impact height with a calibrated load cell and dial gauge, and confirm the measured stiffness is within ±10% of the specified value. This catches hollow-section collapse, loose base plates, and material mix-ups before dynamic testing begins.
Surface Condition and Friction
Contact friction between the headform skin and the pole surface alters the tangential force component and can change head rotation, which matters for rotational-acceleration metrics. The specification must state the surface finish (bare steel, painted, galvanized, powder-coated, or wrapped), the nominal coating thickness, and the surface roughness parameter Ra. A painted pole with a fresh, smooth coating behaves differently from a weathered, chalky one. For repeatable testing, define a surface preparation procedure: clean with isopropyl alcohol, verify no loose coating or rust scale, and replace or refinish the pole when the contact zone shows permanent deformation, cracking, or coating loss. Mark the contact zone (a band around the impact height) and inspect it before every shot.
Temperature and moisture also belong in the surface specification. Testing in a cold chamber or after rain changes both coating hardness and friction. Record ambient temperature and relative humidity at the time of impact, and if the protocol requires conditioning, state the soak duration and tolerance.
Mounting Requirements and Boundary Conditions
The base boundary condition determines how much of the impact energy is reflected back into the headform. A pole bolted to a thick steel baseplate anchored into a mass concrete block behaves as a fixed-base cantilever; a pole grouted into a sleeve with clearance behaves as a partially restrained column and will rock. The specification must define the foundation type, anchor pattern and torque, baseplate thickness, and the mass of the foundation block relative to the expected impact impulse. As a rule of thumb, the foundation mass should be at least 20–50 times the effective impact mass so that rigid-body motion of the foundation contributes negligibly to the measured pulse.
Verticality and alignment are equally important. Specify a plumb tolerance (for example, ±0.5° over the free length) and a target-point tolerance in the horizontal plane (for example, ±10 mm). The headform approach direction must be defined relative to a pole datum, and the standoff distance from the propulsion system to the contact point must be recorded so that the free-flight or guided trajectory is reproducible. Finally, document the anchor torque actually applied and re-check it before each test series; loosening anchors are one of the most common causes of drifting results.
How the Parameters Interact
The four parameter groups are not independent. Increasing diameter raises contact area and lowers peak acceleration but also raises section stiffness; reducing wall thickness lowers stiffness and lengthens the pulse but risks local denting at the contact point, which permanently changes the surface geometry for subsequent shots. A painted surface adds a thin compliant layer that slightly softens the initial contact, but coating wear after several impacts can expose bare metal and shift results. The figure below maps these dependencies so that a change in any one parameter can be traced to its likely effect on the measured pulse and on the validity of comparisons across test series.
Read the diagram left to right: the four specification groups set the contact patch and the energy-exchange path, which together shape the deceleration pulse and the resulting HIC. The dashed return path is the part most often neglected in practice — surface wear, local denting, and anchor loosening feed back into the contact behaviour and silently shift later shots. That is why the specification is not a one-time document: it is a pre-test checklist that must be re-verified before every impact, with the contact zone inspected, the anchor torque re-checked, and the plumb deviation re-measured. When a result falls outside the expected band, the first diagnostic step is to compare the current pole state against the recorded specification rather than to adjust the impactor or velocity.