AUTONEWS

Study suggests metal foam would make cars safer in collisions

A new study finds that composite metal foam (CMF) could significantly improve automobile safety during frontal collisions. Using computational models to run detailed simulations, researchers found that incorporating CMF into automobile “front rails” would allow them to absorb more energy during high-velocity impacts.

CMFs are foams that consist of hollow spheres – made of metals or alloys such as steel – embedded in a metallic matrix. The resulting material is both lightweight and remarkably strong at absorbing compressive and impact forces, with potential applications ranging from aircraft wings to vehicle armor to body armor.

“CMF is lighter than conventional metals without sacrificing strength, and our work here tells us that its unique structure allows it to better absorb energy during high velocity impacts,” says Afsaneh Rabiei, corresponding author of the study and a professor of mechanical and aerospace engineering at North Carolina State University. “This is important, because it slows the rate at which a vehicle decelerates during a crash – and slowing deceleration improves driver and passenger safety.”

During a high-velocity, frontal car crash, the bumper system of an automobile transfers the impact energy to the front rails, which are designed to absorb that energy. These front rails are meant to deform in a way that limits the amount of energy that is transferred to people in the vehicle, and to stop the vehicle from decelerating too rapidly. If deceleration is too sudden, vehicle occupants can sustain severe injuries, including head injuries.

Two common front rail designs in commercially-available vehicles are “rectangular cross-section” and “double-octagonal cross-section.” Rectangular cross-section rails are long, hollow steel rectangles. Double-octagonal rails consist of two long, hollow aluminum octagons stacked on top of one another.

For this study, researchers wanted to see how those common front rail designs would perform compared to front rails consisting of an aluminum tube with a steel CMF core. The researchers drew on extensive experimental data on CMF and publicly-available material and design data for the conventional front rails in order to conduct detailed computational modeling of how the rails perform during high-velocity frontal impacts.

In simple terms, the CMF front rails allowed the vehicle to withstand much higher impact speeds before reaching critical safety limits. Compared with conventional double-octagon front rails, the vehicle could travel about 34% faster before reaching the critical crash-severity limit and about 40% faster before reaching the head-injury limit. Compared with rectangular front rails, those limits increased by about 32% and 48%, respectively.

Inside the university’s engineering lab, the material looks at first like a simple metal block sprinkled with tiny bubbles. Those bubbles are actually airtight hollow spheres fused inside a metal matrix. When struck, they collapse inward and absorb energy that would otherwise pass straight through a solid steel shell.

“This is the sample we put in front of a 300,000-pound ram car,” lead researcher Afsaneh Rabiei said as she held a cross-section of the material. “It didn’t disintegrate or punch through. The spheres squeezed down like bubble wrap and protected the plate behind it.”

Earlier federal testing showed that a ram car traveling just over 5 mph tore through a steel plate the same thickness used in tank cars. When a layer of composite metal foam was added, the railcar bounced backward and the steel remained intact.

In smaller demonstrations, the differences were just as clear. Rabiei held up two aluminum samples to show the contrast. One bent and cracked. The foam version compressed uniformly.

“This is before loading, and this is after,” Rabiei said. “The foam goes straight down and absorbs the energy instead of sending it through.”

Tank cars carrying crude oil, ethanol and chemical feedstocks can spill large quantities of hazardous material if their steel shells rupture. The study outlines how a thin layer of composite metal foam could be added between steel walls to prevent puncture in a derailment.

CMFs are foams that consist of hollow spheres – made of materials such as stainless steel, nickel, or other metals and alloys – embedded in a metallic matrix. The resulting material is both lightweight and remarkably strong at absorbing compressive forces. (Source: Jie Sun/ NC State University)

“CMF outperformed the conventional rails across the board,” Rabiei says. “And the higher the speed, the better the CMF performed compared to the conventional front rails. Comparing front rail performance during a 55 mph crash offers a good overview of the difference between designs.”

Compared with double-octagon rails, CMF of the same weight and length reduced the maximum deceleration experienced during the crash by about 38%, reduced overall crash severity by about 45%, and reduced the Head Injury Criterion (HIC) – a measure of the likelihood of serious head injury – by about 45%. Compared with rectangular rails at the same 55 mph impact speed, CMF of the same weight and length, reduced maximum deceleration by about 84%, crash severity by about 94%, and HIC by about 83%.

Combined with CMF’s previously demonstrated resistance to heat and fire, the new findings also point to potential applications in electric vehicles, including structures designed to help protect high-voltage battery packs from crash-induced damage.

And the improved safety does not require sacrificing fuel economy: the CMF front rails were the same length and weight as the conventional front rails they were being compared to.

“This also means that you could reduce the length of the front rails, still improve safety, and also make the vehicles more fuel efficient,” Rabiei says.

“We are open to working with the automobile industry to conduct testing of rails that make use of CMF in their existing front rail designs – or to develop new rail designs that make use of CMF to improve safety and fuel efficiency,” says Rabiei.

“We are also interested in working with automakers, suppliers and battery manufacturers to evaluate CMF-based structures for electric vehicles, including battery protection systems. CMF’s combination of lightweight impact-energy absorption and resistance to heat and fire creates opportunities to address multiple vehicle safety challenges with a single material system.”

Composite metal foam (CMF) could revolutionize automotive safety by absorbing impacts and drastically reducing the severity of accidents. A recent study led by researcher Afsaneh Rabiei at North Carolina State University demonstrated that replacing the interior of a car's front frame rails with this material reduces violent deceleration and protects occupants from fatal head injuries, without adding weight to the vehicle.

The material functions similarly to bubble wrap, but with ultra-high strength. It consists of hollow metal spheres (such as steel) embedded in a metallic matrix. This structure allows the material to compress to up to 80% of its original size, progressively absorbing mechanical impact energy rather than transferring it to the passenger cabin.

Additional benefits:

Fuel Efficiency: Since CMF offers the same level of protection while weighing less, vehicles do not suffer fuel economy penalties. Alternatively, the front ends of cars could be shortened to make them even lighter.

Protection for Electric Vehicles (EVs): The material is highly resistant to fire and heat, sparking interest in using it to encase and protect high-voltage battery packs against collision damage and fire risks.

The technology—which has previously been successfully tested for disintegrating armor-piercing .50-caliber projectiles—is now seeking partnerships with automakers and battery manufacturers for real-world road testing.

North Carolina State University