NC State study using professor’s patented composite metal foam could improve frontal collision safety

North Carolina State University researchers say their new study finds that composite metal foam (CMF) could significantly improve frontal vehicle collision safety by increasing injury prevention limits at higher impact speeds by up to 48%.

Using computational models to run detailed simulations, researchers found that incorporating CMF into vehicle front rails would allow them to absorb more energy during high-velocity impacts, according to an NC State press release.

The research paper, titled “Enhancing Safety and Crashworthiness of Vehicles Using Composite Metal Foam,” explains that CMFs are foams of hollow spheres, made of metals or alloys such as steel, and embedded in a metallic matrix. The resulting material is lightweight and strong at absorbing compressive and impact forces, with potential applications ranging from aircraft wings and vehicle armor to body armor, the release states.

A cross-sectional view of a permanent casting mold useful in one embodiment of the invention, according to patent no. 9,208,912.

An SEM image of a cross-section of a composite metal foam of the invention showing an aluminum matrix between two hollow steel spheres, according to patent no. 9,208,912.

NC State explains that during a high-velocity frontal car crash, the bumper system transfers impact energy to the front rails, which are designed to absorb that energy. The front rails are meant to deform in a way that limits energy transfer to people inside the vehicle and to stop the vehicle from decelerating too rapidly. Researchers note that 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 compare how common front rail designs would perform against front rails with an aluminum tube and steel CMF core. They drew on extensive CMF experimental data and publicly available material and design data for the conventional front rails to conduct detailed computational modeling of how the rails perform during high-velocity frontal impacts, the release states.

“In simple terms, the CMF front rails allowed the vehicle to withstand much higher impact speeds before reaching critical safety limits,” the release states. “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,” said Afsaneh Rabiei, one of the study’s authors and an NC State mechanical and aerospace engineering professor, in the release. “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.”

According to the release, Rabiei also invented CMFs. The United States Patent and Trademark Office has assigned four patents to Rabiei under Advanced Materials Manufacturing, LLC.

“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,” Rabiei said. “This is important because it slows the rate at which a vehicle decelerates during a crash, and slowing deceleration improves driver and passenger safety.”

A chart of the stress-strain curves of composite metal foams under monotonic compression for various embodiments of the invention, according to patent no. 9,208,912.

Compared with double-octagon rails, CMF of the same weight and length reduced maximum deceleration by about 38%, overall crash severity by about 45%, and the Head Injury Criterion (HIC) — a measure of the likelihood of serious head injury — by about 45%, according to the study. 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%.

NC State researchers concluded that, 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,” the release states.

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

He added that NC State is also interested in working with automakers, suppliers, and battery manufacturers to evaluate CMF-based structures for EVs, including battery protection systems.

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