Automotive-related accidents are the third most common cause of traumatic brain injuries. While seat belts and airbags restrain and cushion occupants against impact, energy-absorbing structures throughout the car also help to mitigate injuries and improve occupant safety during crashes.

The front bumper of an automobile consists of a bumper beam and crush cans, which absorb energy during collisions. During high-velocity frontal collisions, the front bumper transfers energy to front rails which are the primary absorbers of this impact.

Conventional front rails are often made from thin-walled steel or aluminium tubes that absorb energy by undergoing plastic deformation and buckling upon impact. However, as the impact becomes more severe, the tubes can undergo increasingly large and localised deformations, producing large changes in deceleration experienced by the vehicle. Exposure to dangerous levels of deceleration upon impact can increase the chances of severe or fatal injury.

One way to improve this energy absorption could be to introduce a lightweight material that can deform while providing more consistent resistance to compression. Composite metal foams (CMFs) are materials that contain hollow metal spheres suspended within a metal matrix. This creates a structure that combines the properties of a metal with the energy-absorbing behaviour of a foam.

A new study published in the Open Access Journal of Composites Science investigated whether steel CMFs could improve the performance of vehicle front rails during frontal impacts. Using computational simulations, the researchers compared front rails containing a steel CMF-core with conventional designs.

The CMF-core front rails produced a more consistent deceleration response during deformation and increased the predicted vehicle velocity required to exceed critical accident severity and head injury criterion (HIC) values.

How does a metal foam absorb crash energy

Steel CMFs are made from stainless-steel hollow spheres packed within a stainless-steel matrix. The spaces inside the spheres contain air, which becomes an important part of the material’s response when it becomes compressed.

Unlike a conventional solid metal, a CMF can undergo substantial deformation while maintaining a relatively broad plateau in its stress–strain response before densification. This means the material can continue to absorb energy over a longer range of deformation rather than rapidly becoming much harder to compress.

Prof. Afsaneh Rabiei, corresponding author of the study, explains how this can improve crash safety:

Our work here tells us that the unique structure of the steel CMF allows it to better absorb energy during high velocity impacts. This is important, because it slows the rate at which a vehicle decelerates during a crash, and slowing deceleration improves driver and passenger safety.

The researchers designed a CMF-core front rail design which incorporated the stainless-steel spheres within a steel matrix into an aluminium double-tube front rail. With the CMF forming the core between the tubes.

How the air within the CMF responded during an impact was also considered in the simulations. As the material becomes compressed, the air becomes pressurised which increases the resistance of the core to further deformation.

Comparing different front rail designs

This study compared the CMF-core front rail design against two conventional front rails:

• Rectangular front rail design: Based on the front rail of a commercially available truck.
• Double-octagon front rail design: Based on a published patent and used in a commercially available sedan.

All designs were compared using equivalent masses and lengths.

Simulations were run which considered several accident velocities. The resulting deformation, vehicle deceleration, crush force, accident severity, and HIC values were measured.

As the conventional rails deformed, the tubes buckled repeatedly, causing changes in the force and vehicle deceleration throughout the impact. At higher impact velocities, the deformation became more severe, and the structures moved towards densification.

The CMF-core rail behaved differently. As the front rail compressed, the CMF core underwent axial plastic deformation while supporting the surrounding tube. The interaction between the core, tube, and pressurised air helped to prevent localised buckling.

As a result, the CMF-core design produced a more plateau-like force deceleration response during the impact.

How collision speed changes the impact

Traumatic brain injuries are primarily caused due to the compression or stretching of brain tissue during an impact. In automotive crashes, the sudden deceleration of the head can therefore place the brain under forces that may cause serious injury.

Two measures used to assess this risk are accident severity and the Head Injury Criterion (HIC). Both are strongly influenced by the rate of deceleration during a crash, with higher deceleration generally producing more severe loading of the head. Values greater than 1000 for either measure indicate a high probability of fatal brain trauma to the occupants.

The conventional front rails showed clear differences in their ability to manage these loads. The rectangular front rail was unable to safely absorb the impact at 17.40 m s⁻¹, putting passenger safety at risk. The double-octagon rail performed better, not reaching dangerous thresholds until a speed of 24.59 m s⁻¹.

The CMF-core front rail showed improvements over both conventional designs. At 24.59 m s⁻¹, its estimated accident severity and HIC values remained within the critical limits. Compared with the double-octagon design, these values were 44.79% and 44.66% lower, respectively.

The researchers estimated that the CMF-core rail would need to reach 26.97 m s⁻¹ to exceed the critical accident severity value and 34.37 m s⁻¹ to exceed the critical HIC value. These velocities are 31.93% and 39.50% higher, respectively, than those reported for the double-octagon rail.

Prof. Afsaneh Rabiei explains these findings:

CMF outperformed the conventional rails across the board. The higher the speed, the better the CMF performed compared to the conventional front rails.

Could composite metal foams be used to improve vehicle safety?

Importantly, the improved crash performance did not require a larger or heavier structure. The CMF-core front rails were the same length and weight as the conventional designs, meaning that this improvement would not come at the expense of vehicle fuel economy. Combined with the previously demonstrated resistance of CMFs to heat and fire, this could also make them useful in electric vehicles, where crash structures could help protect high-voltage battery packs from damage during a collision.

These findings indicate that a steel CMF-core front rail can reduce the crash severity and lower the HIC during frontal impacts. This is thought to be due to the progressive deformation of the material which allows more effective impact energy dissipation which reduces the forces transmitted to the passenger compartment.

Despite the promising results, the study only presents computational simulations which would need to be verified experimentally. This would evaluate the performance of CMF-core front rails during high-velocity front impacts in accordance with National Highway Traffic Safety Administration protocols as well as testing further side-impact collisions. These findings would provide a more comprehensive understanding of steel CMF front rail performance and truly test their effectiveness in improving passenger safety.

Furthermore, the CMF-core front rails tested in this design do have the potential for further optimisation for crashworthiness applications. The authors note that such optimisation would be the subject of future work which could be done in collaboration with automobile manufacturers.

More research on automotive engineering, crash safety, and advanced materials can be found across MDPI’s Open Access journals, Materials, Vehicles, and Journal of Composites Science.

Alternatively, you can access the full MDPI journal list here.