3. Deformable barrier face specification

The deformable barrier face is the component that actually converts the moving MPDB trolley into a controlled crash loading on the test vehicle. Everything upstream — trolley mass, closing speed, alignment, and braking — only sets the boundary conditions. The barrier face determines the shape of the force–displacement characteristic that the vehicle structure sees. In the Euro NCAP MPDB protocol, the barrier face is a composite assembly: an aluminium honeycomb core that provides the primary crush energy, a rigid backing plate that distributes load into the trolley frame, and a facing skin that stabilises the front surface and prevents local punch-through. Getting the geometry and the honeycomb grade right is what makes results comparable between laboratories and between test campaigns.

Overall geometry and the load-spreading envelope

The barrier face is a large rectangular block, nominally 1000 mm wide and 500 mm high, mounted on the front of the trolley with its lower edge at a defined height above the ground reference plane. The width is chosen to be wider than the front structure of a typical passenger car so that the vehicle cannot "miss" the barrier in a full-width configuration, while the height covers the bumper, grille, and lower radiator region — the load paths that dominate frontal energy absorption. The face is not a flat plate in the aerodynamic sense: the front surface is essentially planar and vertical, and the assembly is designed so that the initial contact is distributed across the full face rather than concentrated at a single point.

Behind the front skin, the honeycomb core is built up from multiple blocks rather than a single monolithic slab. This is a deliberate design decision. A single large block would be expensive to manufacture, difficult to handle, and would produce a very uniform pressure distribution that does not represent the real-world interaction between a vehicle front end and a deformable object. Instead, the core is divided into zones with different crush strengths, arranged so that the barrier presents a progressively stiffer response as crush depth increases. The zoning is what allows the barrier to generate a force–displacement curve that rises in a controlled way rather than plateauing immediately.

The barrier face is a calibrated energy absorber, not a generic cushion. Two laboratories using nominally identical barrier faces from different suppliers can produce measurably different vehicle responses if the honeycomb grade, cell size, or bonding method differs. Procurement and incoming inspection therefore matter as much as the test itself.

Honeycomb core design: cell geometry and crush strength

The core is aluminium honeycomb, typically 3003 or 5052 alloy foil, formed into a hexagonal cell structure and expanded to the required cell size. The two parameters that dominate the mechanical behaviour are the foil gauge and the cell size. Crush strength in the through-thickness direction scales approximately with the ratio of foil thickness to cell wall length, so a finer cell with thicker foil crushes at a higher stress. For the MPDB barrier, the core is specified in discrete grades — commonly expressed as a nominal crush strength in the range of a few tenths of a megapascal up to roughly one to two megapascals depending on the zone — and the grades are stacked in the through-thickness direction to build the desired force profile.

The crush behaviour of aluminium honeycomb is characterised by three regimes: an initial elastic rise to a peak, a long plateau at a nearly constant crush stress, and a final densification stage where the folded cell walls lock up and the stress rises steeply. The plateau is the useful working region, because it delivers a predictable, almost constant deceleration. The design task is to ensure that the vehicle's expected crush depth falls within the plateau of the relevant zone, not into densification. If the vehicle penetrates too far and hits densification, the barrier becomes effectively rigid and the measured deceleration spikes — a classic sign that the barrier face was under-specified for the vehicle mass and speed.

Barrier face cross-section (plan view, through-thickness horizontal) Trolley frame Backing plate Front skin High-strength zone Mid-strength zone Low-strength zone Vehicle closing speed Core depth (through-thickness) Force vs. crush depth crush depth force plateau densification (avoid)

Material properties that must be controlled

Beyond crush strength, several material properties are specified because they affect repeatability. The foil alloy and temper determine the plateau stability: 3003-H18 is common for its consistent crush behaviour and good corrosion resistance, while 5052 offers higher strength where a stiffer zone is needed. The cell size — typically in the range of 6 mm to 20 mm depending on the zone — affects both the crush stress and the sensitivity to the vehicle's local geometry. A coarse cell can be penetrated by a sharp bumper edge before the full cell wall engages, producing a localised soft spot; a fine cell distributes load more evenly but is more expensive and more sensitive to moisture ingress.

Moisture is a genuine concern. Aluminium honeycomb absorbs water through open cell faces if the facing skin is damaged, and a wet core crushes at a different stress than a dry one. The specification therefore requires the core to be sealed, the facing skin to be continuous and bonded around the full perimeter, and the barrier face to be stored in a controlled environment. Incoming inspection should include a visual check for skin delamination, a dimensional check of the face, and — where the laboratory has the capability — a coupon crush test on a sample from the same production batch.

From specification to a working barrier face

In practice, building a compliant barrier face means working from the protocol's zone table: each zone has a nominal crush strength, a cell size, and a through-thickness depth. The zones are cut to size, bonded to the backing plate with a structural adhesive, and the facing skin is applied over the front. The assembly is then weighed and dimensioned against the protocol tolerances. A useful sanity check before a test campaign is to compare the measured mass of the barrier face against the nominal value; a significant deviation usually indicates a wrong honeycomb grade or a bonding defect, both of which will shift the force–displacement curve and compromise comparability.

The key takeaway is that the barrier face is a calibrated instrument. Its geometry sets the contact envelope, its honeycomb zoning sets the force profile, and its material properties set the repeatability. Treating it as a consumable to be replaced without verification is the most common way laboratories introduce silent bias into MPDB results.

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