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.

“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.”

The paper, “Enhancing Safety and Crashworthiness of Vehicles Using Composite Metal Foam,” is published open access in the Journal of Composites Science. First author of the paper is Aman Kaushik, a postdoctoral researcher at NC State.

Rabiei is the inventor of composite metal foams. She has assigned her related IP to a small business of which she is a shareholder. Afsaneh Rabiei has patent #9208912, #8110143, #8105696, and #7641984 issued to Advanced Materials Manufacturing, LLC.