A football helmet design that listens to physics

ANN ARBOR—A shock-absorbing football helmet system being developed at the University of Michigan could mitigate some dangerous physics that today’s head protection ignores.

The engineering researchers making the system, called Mitigatium, were recently funded by a group that includes the National Football League. Their first prototype could lead to a lightweight, affordable helmet that effectively dissipates energy hit after hit in the field. Current helmets can’t do that, and that’s one of the reasons they’re not very effective at preventing brain damage.

“Today’s football helmets are designed to prevent skull fractures by reducing the peak force of an impact,” said Ellen Arruda, professor of mechanical engineering and biomedical engineering at UM. “And they are doing a good job. But they don’t actually dissipate energy. They leave that to the brain.

Sports like football present great challenges for designers of protective helmets. To dissipate energy, a helmet usually has to deform, like the bicycle version cracks in a crash. And disposable helmets are not practical for footballers.

When a bike helmet breaks, it absorbs what’s called “impulse,” a side effect of an initial force. Impulse, which gives objects momentum, is what transmits kinetic energy through a system. It takes into account not only the force, but also the duration during which this force was applied. To calculate impulse, you multiply the average force by the amount of time it was exerted on the subject.

For head protection to be most effective against the speed and weight of players on a football pitch, these researchers say it must block impulses.

They are not the first to say it. They found medical studies dating back 70 years that blame impetus for the damage caused by rapid, football-style impacts. Yet today, helmet manufacturers and health researchers tend to rely on other factors. For example, new helmet designs are approved based solely on the maximum force they can withstand.

“Everyone is focused on the force of an impact and just the force,” Arruda said. “But they found that when they measure peak force at the surface of the skull, they can’t correlate that with brain damage. The reason is that strength is only part of the story.

Scientists and doctors don’t fully understand how a blow to the head translates to brain damage, but UM researchers say impulse is a big factor. Arruda and his colleagues have demonstrated this.

They looked closely at the mechanical characteristics of impacts and explosions and how helmets and other armor could be designed to better protect sensitive structures. To do this, they constructed two-dimensional dummy cross-sections of materials that replaced the brain and skull in various helmet shells. Then they use a tabletop crash simulator to test the different samples. They compared the amount of energy transmitted to the brain-like layer in their own helmet system and the status quo. They used a high-speed camera to help them observe how the brain model distorted in both systems.

“Some of the insights we took from this analysis were slightly different from how the helmet community thought about design, although we found examples in early medical literature that matched our understanding,” said Michael Thouless. , Janine Johnson Weins Professor of Engineering in Mechanical Engineering and Materials Science and Engineering.

In their experiments, the current headphone design did little to block the impulse. The researchers could tell how distorted the speckled pattern of their brain layer was. The Mitigatium prototype, however, reduced the impulse to only 20% of what passed through the brain model in the conventional headset. Mitigatium reduced the maximum pressure to 30%. He lowered both by an order of magnitude, Arruda said.

The overlay on the left represents the padding of current football helmets, while the overlay on the right shows the new material (the thin, shiny, black layer). Photo: Evan Dougherty, Michigan Engineering Communications & MarketingHere’s how it works: It’s made of three materials that are more than the sum of their parts. The first layer is similar to the hard polycarbonate that is the shell of current helmets. The second is a soft plastic. Together, these substances reflect most of the initial shock wave of a collision, i.e. most of the initial force. They also do something else unique and important: they convert the frequency of this incoming pressure wave into a frequency that the next layer can, in essence, pick up and dissipate by vibrating. This third “visco-elastic” layer has the consistency of dried tar.

“We came up with a completely new concept of how to create effective impact attenuation structures that could dissipate energy without being damaged,” Thouless said. “And we used basic concepts of mechanics to develop a fundamental understanding of how to protect delicate structures like the brain.”

Late last year, the UM team was one of five winners of Head Health Challenge III, a competition to support the development of materials that better absorb or dissipate impact. Besides the NFL, the sponsors are Under Armour, GE and the National Institute of Standards and Technology. The UM researchers received $250,000 to take their technology to the full prototype stage. Doctoral student Tanaz Rahimzadeh is also contributing to this project.

The researchers also point out that their system is extremely flexible, in that different materials could be used to tune different incoming pressure waves. They envision their approach having applications for military and other protective equipment, as well as playground surfaces.

An article on some of these discoveries, entitled “Design of armor for protection against blast and impact”, is published in the Journal of the Mechanics and Physics of Solids. Rahimzadeh is the first author.

The University of Michigan is working to identify commercialization partners to help bring the technology to market.

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