3. Barrier geometry and material specifications
Frontal offset crash testing is defined as much by the barrier as by the vehicle. Three barrier families dominate regulatory and consumer-assessment practice: the Offset Deformable Barrier (ODB) used in UN R94 and Euro NCAP, the Moving Deformable Barrier (MDB) used in FMVSS 208 and the US NCAP oblique/full-width variants, and the Progressive Deformable Barrier (PDB) used in the NHTSA oblique crash test. All three are aluminium honeycomb structures, yet they differ in face dimensions, cell geometry, crush strength, and how the honeycomb is mounted to its backing. Those differences change the load path into the vehicle and therefore the injury metrics you will later interpret. This section compares the three designs at the level of geometry and material specification so that test setup, instrumentation, and result interpretation rest on a correct physical picture.
What each barrier actually is
The ODB is a fixed barrier. A rigid, flat-faced block is bolted to the test hall floor, and a deformable honeycomb face is bonded to its front. The vehicle is propelled into it. The barrier does not move; all kinetic energy is absorbed by the vehicle structure and the honeycomb face. The ODB face is a single honeycomb block with a defined crush strength, and its geometry is fixed by regulation: a nominal 1000 mm wide by 330 mm high deformable face, mounted on a rigid backing, with the face centre aligned to the vehicle centreline and the top edge at a specified height above the ground reference plane.
The MDB is a moving barrier. The honeycomb face is mounted on a rigid, trolley-like frame that is itself propelled into a stationary vehicle. Because the barrier has mass and moves, the collision is a two-body problem: momentum is shared between barrier and vehicle, and the barrier's mass and face stiffness both shape the pulse. The MDB face is wider and taller than the ODB face and is typically built from multiple honeycomb blocks with graded crush strength across the face.
The PDB is also a moving barrier, but its face is deliberately non-uniform. Instead of a single crush strength, the PDB uses honeycomb segments of different densities and cell sizes arranged so that the load increases progressively from the outboard edge toward the centre. This produces a load profile that is intended to engage the vehicle's offset structure more gradually and to better represent the stiffness distribution of an oncoming vehicle in an oblique impact.
Face dimensions and mounting
Face dimensions are the first specification to verify before a test, because a mis-sized or mis-aligned face invalidates the comparison to the reference test. The table below summarises the nominal geometry and material characteristics of the three designs. Values are representative of the regulatory definitions and are given as nominal targets; always confirm against the current revision of the applicable standard before building or accepting a barrier.
| Characteristic | ODB (fixed) | MDB (moving) | PDB (moving, progressive) |
|---|---|---|---|
| Face width (nominal) | 1000 mm | ~1500 mm | ~1500 mm |
| Face height (nominal) | 330 mm | ~500 mm | ~500 mm |
| Honeycomb construction | Single block, uniform crush strength | Multiple blocks, graded crush strength | Segmented, progressive crush strength |
| Cell size / density | Fine cell, moderate density | Coarser cell, higher density inboard | Mixed cell sizes, density graded outboard-to-centre |
| Mounting | Bonded to rigid floor-mounted backing | Bonded to rigid moving trolley frame | Bonded to rigid moving trolley frame |
| Barrier mass contribution | None (fixed) | Significant; part of collision momentum | Significant; part of collision momentum |
Two practical points follow from this table. First, the ODB face is narrow and low; it engages a limited vertical band of the vehicle front, which is why ODB results are sensitive to bumper beam and longitudinal rail height. Second, the MDB and PDB faces are wider and taller, so they engage more of the vehicle's front structure and the barrier's own mass enters the momentum balance. When you compare an ODB result to an MDB or PDB result, you are not comparing the same physical event, and the geometry table is the reason.
Honeycomb characteristics that matter
Aluminium honeycomb is specified by cell size, foil gauge, density, and crush strength. The crush strength is the plateau stress the honeycomb sustains while crushing; it is the single most important material parameter because it sets the force the barrier applies to the vehicle. A higher crush strength produces a sharper, higher-magnitude load pulse; a lower crush strength produces a longer, softer pulse. Cell size and foil gauge control how the honeycomb folds and therefore how stable the plateau is: fine cells with thin foil crush uniformly but can be sensitive to bonding defects, while coarser cells with thicker foil are more robust but produce a less uniform plateau.
For the ODB, the face is a single uniform block, so the load pulse is essentially flat-topped until the honeycomb bottoms out. For the MDB, graded blocks mean the pulse rises as the vehicle penetrates into stiffer inboard sections. For the PDB, the progression is intentional and continuous: outboard segments are soft, centre segments are stiff, and the resulting force-displacement curve is a ramp rather than a step. This is the material-level explanation for why PDB tests are used to study oblique and small-overlap loading, where a gradual build-up of force is more representative than a single plateau.
Reading the geometry as a load path
The SVG below maps the three barrier designs as load-path diagrams rather than as pictures of hardware. It shows, for each design, how the face geometry and honeycomb grading direct force into the vehicle structure, and where the barrier's own mass enters the momentum balance. Use it to keep the physical picture straight when you move on to instrumentation and pulse interpretation.
Practical consequences for setup and comparison
Because the ODB is fixed, its setup is dominated by alignment: the face centre must sit on the vehicle centreline, the face must be square to the approach direction, and the top edge height must match the standard. A small vertical misalignment changes which part of the vehicle front engages the honeycomb and can shift the pulse noticeably. Because the MDB and PDB move, their setup adds mass and propulsion checks: the trolley mass must be within tolerance, the propulsion speed must be verified at the moment of contact, and the face must be inspected for prior crush damage before every run. A previously used honeycomb face that has been partially crushed will not reproduce the specified plateau and must be replaced.
When comparing results across barrier types, keep three distinctions explicit. First, the ODB test is a fixed-barrier event, so the vehicle absorbs all the energy and the pulse reflects vehicle structure alone plus the honeycomb plateau. Second, the MDB and PDB tests are moving-barrier events, so the pulse reflects the combined momentum and the barrier's graded stiffness. Third, the PDB's progressive grading means its force-displacement curve is a ramp, not a plateau, so a direct numerical comparison of peak force between PDB and ODB is not meaningful without normalising for the different face geometry and grading.
In short, the three barriers share a material family but not a geometry or a load path. The ODB is a narrow, uniform, fixed face; the MDB is a wider, graded, moving face; the PDB is a wider, progressively graded, moving face. Treat the face dimensions, honeycomb grading, and mounting as the primary specification, verify them before every test, and carry those differences into every comparison of pulse and injury metrics.