An international research team from the Universities of Gothenburg and Isfahan has developed a new concept for bicycle helmets based on special geometric structures. The aim of the work was to increase the protective effect compared to conventional helmets by improving shock absorption. The design is based on so-called auxetic metastructures – a class of materials whose mechanical behavior under load differs from conventional foams.
Auxetic materials have the property of expanding when stretched across the direction of tension and contracting when compressed. This reaction increases the energy absorption in the event of an impact.
“When exposed to energy from an impact, the liner material contracts, and this improves the absorption of impact energy, which means lower risks of injuries to a cyclist’s head in an accident,” says Mohsen Mirkhalaf, Associate Professor in the mechanics and physics of materials at the University of Gothenburg. “We used a specific design optimisation method to identify the best possible geometric configuration to minimise crash forces. The geometry of the material structure is a key factor.” says Mohsen Mirkhalaf.
The researchers combined numerical simulations with design optimization to identify an effective geometry. The inner protective layer of the helmet was printed from a hyperelastic polymer that can deform significantly and then return to its original shape. In standardized impact tests, the 3D-printed material showed improved damping properties at different impact angles compared to conventional helmet structures.
Another advantage results from the flexibility of additive manufacturing. Thanks to 3D printing, the inner structure can be individually adapted to the shape of the wearer’s head. This is particularly useful for competitive athletes or people with atypical head shapes.
“The knowledge of auxetic metastructures that expand laterally when stretched has been around for almost 40 years. However, development of different metastructures has exploded due to the advancements in 3D printing technology,” says Mohsen Mirkhalaf. “With further development, this technology could lead to a new generation of bicycle helmets that are safer, more comfortable and more customisable. Although 3D printing technology currently is more expensive than mass-produced foam liners, costs are expected to fall as the technology becomes more widely used. In the future, it may even be possible to have personalised helmets printed on demand, ensuring that each rider receives the best possible protection.”
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