Metal foam rails cut predicted head-injury risk in simulations

At 55 mph, the front rail between a car’s bumper and chassis has a narrow job: deform enough to absorb impact, but not pass a severe load into the passenger compartment. In North Carolina State University’s computer simulations, a composite metal foam rail reduced predicted crash loads and head-injury risk while keeping the same length and weight as the conventional part.

The design uses an aluminum tube around a steel core made from composite metal foam, or CMF. The material embeds hollow metal spheres in a metallic matrix. Conventional rails in the comparison used either a rectangular cross-section—a long, hollow steel rectangle—or a double-octagonal cross-section made from two hollow aluminum octagons stacked together.

At the same 55 mph impact speed, CMF reduced maximum deceleration by about 38%, overall crash severity by about 45% and the Head Injury Criterion, or HIC, by about 45% versus double-octagonal rails. Versus rectangular rails, the reductions were about 84%, 94% and 83%, respectively. HIC is a measure of the likelihood of serious head injury, not a count of actual injuries.

The simulations also indicated that a vehicle with CMF front rails could travel about 34% faster than one with double-octagonal rails before reaching the critical crash-severity limit, and about 40% faster before reaching the head-injury limit. Against rectangular rails, the corresponding figures were about 32% and 48%. Those are model outputs: the study used detailed computational modeling based on experimental CMF data and publicly available data for conventional rail materials and designs.

In the model, the practical gain is lower predicted crash severity without added mass; Rabiei says shorter front rails could still improve safety and fuel efficiency. For electric vehicles, the researchers also see a possible role in structures protecting high-voltage battery packs, since CMF has demonstrated resistance to heat and fire. Rabiei says the researchers are open to testing the design with automakers, suppliers and battery manufacturers; until those tests happen, the percentages describe predicted performance rather than protection demonstrated in a production car.