Researchers in Singapore have worked with a French company to develop a tougher and safer helmet using a combination of materials.
They created the composite safety helmet with an outer shell made primarily of a new type of acrylic thermoplastic resin, reinforced with carbon fibre. The new thermoplastic resin, named Elium, was developed by French company Arkema. The Nanyang Technological University (NTU) team worked with Arkema engineers to develop a moulding process for Elium to manufacture stronger helmets.
The new helmet prototype, developed for cycling, has higher energy absorption, reducing the amount of energy transferred to a cyclist’s head in the event of an accident and lowering the chances of serious injury.
“Helmets have been proven time and time again to play a critical role in reducing the severity of injuries and number of fatalities,” said Associate Professor Leong Kah Fai from NTU’s from the School of Mechanical and Aerospace Engineering.
“Our prototype helmet has been subjected to a barrage of internationally benchmarked tests and has demonstrated the ability to provide greater protection for cyclists compared to conventional helmets.”
Helmets are made up of two components. The first is an outer shell, usually made from mass-produced plastic such as polycarbonate. Beneath it is a layer of expanded polystyrene foam – the same material used in product packaging and takeaway boxes.
The outer shell is designed to crack on impact in order to dissipate energy across the entire surface of the helmet. The foam layer then compresses and absorbs the bulk of impact energy so that less energy is transferred to the head.
The team’s composite helmet replaces the conventional polycarbonate outer shell with one using Elium reinforced with carbon fibre.
This reinforcement, they said, makes the outer shell tougher, stiffer, and less brittle than a polycarbonate shell. It also increases the helmet’s contact time, which is the total time of impact in which the helmet experiences impact load.
These properties allow the outer shell to absorb more impact energy over a longer period, while also dissipating it evenly throughout the helmet. This results in less overall force reaching the head, thereby reducing the chances of critical injury.
“When the helmet hits a surface at high speed, we noticed that there is a deformation along with the spread failure of the composite shell, which means the outer shell is taking more load and absorbing more energy,” said NTU’s Dr Bhudolia Somen Kumar.
“This is what you really want – the more impact absorbed by the shell, the less of it that reaches the foam, and so there is less overall impact to the head. We found that in existing polycarbonate helmets, about 75 per cent of the energy is absorbed by the foam. This is not ideal as the foam is in direct contact with the human head.”
In contrast, the team’s composite helmet shell absorbed over 50 per cent of impact energy, leaving the foam to absorb much less energy at about 35 per cent.
The researchers referred to the most widely used injury metric called the Head Injury Criterion (HIC) to calculate the probability of serious injury and fatality while using the helmet. HIC values are derived from a combination of peak acceleration values and the duration of acceleration.
The team’s analysis of the flat anvil test results and the HIC showed that the composite helmet could potentially reduce critical and fatal injury rates from 28.7 per cent and 6 per cent to 16.7 per cent and 3 per cent respectively, compared to a polycarbonate helmet.
Even though peak acceleration was roughly equal between both types of helmets, the composite helmet’s tougher outer shell led to a longer duration of acceleration during impact. This allows the outer shell to absorb more energy, therefore generating a lower HIC which means a lower chance of critical and fatal injuries.
The researchers say the prototype helmet is also easier to produce than a conventional helmet. Using Elium instead of other conventional thermoplastics simplifies the composite helmet manufacturing process.
Elium is liquid at ambient temperature, allowing it to be moulded at room temperature as opposed to other thermoplastic-based composite shells that require higher temperature processing.
The NTU researchers are working with Arkema to commercialise the helmet’s manufacturing process, which would allow manufacturers to produce them. Leong says that helmets produced through their method would offer the same protection as current top-tier helmets, but potentially at the price of mid-tier cycle helmets ($100-$150).
The researchers are currently working on developing composite helmets made from Elium and polypropylene fabric, which is another type of thermoplastic. This is to overcome the composite helmet’s one current trade-off which is that they weigh about 20 per cent more than polycarbonate helmets
Helmets made from Elium and polypropylene fabric will potentially make them just as light as polycarbonate ones but offer better protection.
The team’s research is supported by Singapore’s Agency for Science, Technology and Research (A*STAR) under the nation’s Research Innovation Enterprise 2020 Plan.