Cork as Shock Absorber
Cork as Shock Absorber
Cork, which is derived from the bark of the Oak Cork Tree, primarily grown in Portugal and Spain is one of the most environmentally friendly and renewable material resources available. It can easily be recycled again and again for various applications including shoe foot beds, sound insulation, flooring and cork boards. It is most famous for its use in wine stoppers which, among other things, highlights it waterproof and antimicrobial properties.
Cork is also able to absorb a lot of punishment. Its cells have a unique 14 sided polyhedron structure that can be compressed to 15% of their normal volume and then regain most or all of its size and shape slowly. This compression and very slow rebound allows cork to act as a very effective energy dampener, thus its popular use in flooring and noise proofing. Low density cork was shown by GRANTA DESIGN to perform competitively with high density EPS as a material well suited to absorb the type of impacts targeted in helmet testing.
However, cork has not been developed for many applications. The vast majority of virgin cork is cornered for the wine closure industry. This means that it is a renewable but somewhat limited resource that has not attracted engineers looking to exploit its properties for ambitious projects. So, there has been only limited research and development of cork as a performance material.
Fortunately, cork is amenable to various manipulations that can alter its density and the size of the granules that compose agglomerated products. These two variables make it possible to optimize cork agglomerates for best performance in a variety of applications. For use in helmets a low density, low granule size cork is used which has been shown through testing to effectively dampen the energy that is transferred to the human head during a typical head impact from a bicycle accident.
By creating a pattern of perforations and layering the cork used in helmet cushioning Coyle is able to enhance the energy absorbing properties inherent in the material while at the same time significantly reducing the weight of each helmet.Layering and perforations give resilient cork cells more room to move to during impacts. Unlike EPS cork does not typically crush and is not destroyed by moderate impacts. It does rebound to its full or near its full original dimensions over 24 hours post impact. Because this is so cork benefits from having voids which it can fill while being “smushed” during an impact event.