As cities around the world expand their cycle lane networks and more cyclists take to the streets, the risks of bicycle accidents and potential collisions also increase, underscoring the need for good bicycle safety. in dense urban areas.
According to a World Health Organization report published in 2020, more than 60% of bicycle-related deaths and long-term disabilities are the result of accidents with head trauma.
Researchers at Nanyang Technological University in Singapore (NTU Singapore), in collaboration with the French leader in specialty materials, Arkema, have developed a stronger and safer bicycle helmet using a combination of materials. The new prototype helmet has higher energy absorption, reducing the amount of energy transferred to a cyclist’s head in the event of an accident and reducing the risk of serious injury.
Led by Associate Professor Leong Kah Fai of the School of Mechanical and Aerospace Engineering, the team, made up of researcher Dr Bhudolia Somen Kumar, research associate Goram Gohel and master’s student Elisetty Shanmuga, created the composite helmet with an outer shell composed mainly of a new type of thermoplastic acrylic resin, reinforced with carbon fiber.
The new thermoplastic resin, called Elium®, was developed by Arkema, one of NTU’s industrial partners. The NTU team worked with Arkema engineers to develop a molding process for Elium® to make stronger bicycle helmets.
Our partnership with Arkema is motivated by the desire to develop a new type of helmet that is stronger and safer for cyclists. Helmets have been proven time and time again to play a vital role in reducing the severity of injuries and the number of fatalities. Our prototype helmet has been subjected to a barrage of international benchmark tests and has demonstrated its ability to offer better protection to cyclists compared to conventional helmets.. “
Leong Kah Fai, Associate Professor, School of Mechanical and Aerospace Engineering, Nanyang Technological University
The research team’s findings were published in the peer-reviewed journal Composites Part B: Engineering in May.
Stronger, stiffer outer shell absorbs more energy
Bicycle helmets are made up of two parts. The first is an outer shell, typically made from a mass-produced plastic like polycarbonate. Underneath is a layer of expanded polystyrene foam – the same material used in product packaging and takeout boxes.
The outer shell is designed to crack on impact to dissipate energy across the entire surface of the helmet. The foam layer compresses and then absorbs most of the impact energy so that less energy is transferred to the head.
The team’s composite helmet replaces the conventional polycarbonate outer shell with one in carbon fiber reinforced Elium®.
This reinforcement makes the outer shell stronger, more rigid and less brittle than a polycarbonate shell. It also increases the helmet contact time, which is the total impact time during which the helmet is subjected to an impact load.
These properties allow the outer shell to absorb more impact energy over a longer period of time, while dissipating it evenly throughout the helmet. This results in less overall force reaching the head, thus reducing the chance of serious injury.
“When the helmet hits a surface at high speed, we noticed that there is deformation with the propagation failure of the composite shell, which means that the outer shell takes more load and absorbs more energy,” Dr Somen said. “This is what you really want – the more the impact is absorbed by the shell, the less it reaches the foam, and therefore there is less overall impact on the head. We have found that in existing polycarbonate helmets , about 75 percent of the energy is absorbed by the foam This is not ideal because the foam is in direct contact with the human head.
In contrast, the composite shell of the team’s helmet absorbed over 50% of the impact energy, leaving the foam to absorb much less energy at around 35%.
Security forged on NTU’s anvils
The researchers tested their helmets by lowering them at high speed over three different types of anvils – flat, hemispherical (rounded), and rim (pyramid-shaped) – to simulate different road conditions.
These are the same tests used for certification to the US Consumer Product Safety Commission (CPSC 1203), an internationally recognized safety standard for helmets. The team’s prototype helmet meets all CPSC 1203 guidelines.
The researchers paid special attention to peak acceleration forces, which measure how much force a helmet takes based on how fast it is moving at the point of impact. A helmet must have a maximum acceleration of less than 300 G (g-force) to be considered suitable for use under CPSC 1203, lower g-force values being safer.
On two flat anvil tests, the researchers’ helmets performed on par with a control polycarbonate helmet, producing results of 194.7 G and 197.2 G versus 195.4 G and 198.2 G of control. .
However, tests on the hemispherical and curbstone anvils showed substantial improvements in the team’s composite helmet over the polycarbonate one. In two hemispherical anvil tests, the composite helmet recorded 100.9 G and 103.1 G, while the control helmet had a much higher peak acceleration of 173 G and 178.7 G.
On a single edge anvil test, the researchers’ helmets registered 111.7G, a notable improvement over the benchmark helmet which produced a result of 128.7G.
The researchers referred to the most widely used measure of injury, called the Head Injury Criterion (HIC), to calculate the likelihood of serious injury and death when using the helmet. HIC values are derived from a combination of maximum acceleration values and the duration of acceleration.
The team’s analysis of the flat anvil and HIC test results showed that the composite helmet could potentially reduce critical and fatal injury rates from 28.7% and 6% to 16.7% and 3 % respectively, compared to a polycarbonate helmet.
Even though the maximum acceleration was roughly equal between the two types of helmets, the stronger outer shell of the composite helmet resulted in a longer acceleration time on impact. This allows the outer shell to absorb more energy, resulting in a lower HIC, which means less risk of critical injury and death.
More efficient manufacturing could lead to cheaper, more durable helmets
The prototype helmet is also easier to produce than a conventional helmet. Using Elium® instead of other conventional thermoplastics simplifies the manufacturing process for composite helmets.
Elium® is liquid at room temperature, which allows it to be molded at room temperature unlike other thermoplastic-based composite shells which require treatment at a higher temperature.
NTU researchers are working with Arkema to commercialize the helmet manufacturing process, which would allow interested manufacturers to produce them. Assoc Prof Leong says the helmets produced by their method would offer the same protection as current top-tier helmets, but potentially at the price of mid-range helmets ($ 100 to $ 150).
Researchers are currently working on the development of composite helmets in Elium® and polypropylene fabric, another type of thermoplastic. This is to overcome the only current compromise of composite helmets, which is that they weigh about 20% more than polycarbonate helmets.
Helmets made from Elium® fabric and polypropylene will potentially make them as light as those made from polycarbonate but will offer better protection.
Gohel, G., et al. (2021) Development and characterization of the impact of a thermoplastic acrylic composite bicycle helmet shell with improved safety and performance. Composites Part B: Engineering. doi.org/10.1016/j.compositesb.2021.109008.