A team of researchers from Nanyang Technological University in Singapore has used a combination of materials, including a new thermoplastic resin, to develop a tougher, safer bicycle helmet.The prototype helmet absorbs more impact energy, reducing the risk of a serious head injury in the event of an accident.
Bicycle helmets consist of two components: an outer shell, usually made from a mass-produced plastic such as polycarbonate, and a layer of expanded polystyrene foam. The outer shell is designed to crack on impact, thus dissipating energy across the helmet’s entire surface, while the foam layer compresses and absorbs most of the impact energy, reducing the amount that’s transferred to the rider’s head.
In the new helmet, the outer shell is made from a novel acrylic thermoplastic resin, named Elium, that was developed in collaboration with French specialty materials company Arkema.The NTU team worked with Arkema engineers to develop a moulding process for Elium, which is liquid at ambient temperature, allowing it to be moulded at room temperature rather than the high temperatures needed for standard thermoplastic-based composite shells.
Elium is reinforced with carbon fibre, which makes the outer shell tougher, stiffer and less brittle than those made using polycarbonate, while also increasing the helmet’s contact time – the total time during which the helmet experiences the impact load. This allows the outer shell to absorb more impact energy over a longer period, while also dissipating it evenly throughout the helmet, meaning that less overall force reached the head, reducing the likelihood of a 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 research associate 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 new helmet’s shell absorbed more than half of the impact energy, so the foam only had to absorb about 35 per cent.
The researchers are now exploring helmet designs that also contain polypropylene fabric, another type of thermoplastic, which would help to reduce their weight; the prototype helmet weighs about 20 per cent more than standard polycarbonate helmets.
‘Our partnership with Arkema is driven by the desire to develop a new type of helmet that is stronger and safer for cyclists,’ said Associate Professor Leong Kah Fai, from the School of Mechanical and Aerospace Engineering, who led the research. ‘Helmets have been proven time and time again to play a critical role in reducing the severity of injuries and number of fatalities. 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.’
The team’s results have been published in the journal Composites Part B: Engineering.