Cork May Replace Synthetic Materials for Energy Absorbing Applications in Cars

Crash-test dummies could soon be facing vehicle collision tests in cars padded with cork rather than traditional materials such as polymer foams or porous aluminium metal, according to Portuguese engineers writing in the International Journal of Materials Engineering.

Synthetic cellular materials, polymeric and metallic foams, have been extensively used in energy-absorbing systems for decades. They are commonly lightweight, stiff, and can absorb energy well. However, they suffer from some drawbacks when compared to natural materials, namely cost and a lack of sustainability.

Cork, the bark of the cork oak tree, Quercus suber, is one such cellular natural material. It can be compacted to form a micro-agglomerated material that rivals aluminium foam for its ability to absorb the energy of an impact. Now, Mariana Paulino of the University of Aveiro, and colleagues there and at the University of Coimbra have pitted cork against metal foams, polymer padding and a novel polymer foam material from Dow Automative, known as IMPAXX 300, to see which might make the optimal vehicle safety material.

The results obtained in energy-absorption tests indicate that polyurethane foam performs the worst of all the materials tested, despite its widespread use as an impact safety material in vehicles. Aluminium foam can absorb the most energy, marginally beating cork.

As an impact protection material for car bumpers, doors, headliners, knee bolsters and door pillars, cork even outperforms the novel material IMPAXX 300 in terms of the value of impact acceleration peak. Indeed, at higher energies, which would equate to a high-speed collision, cork has the best acceleration peak value.

The researchers also investigated the extent to which the different materials tested would intrude into the vechicle occupants' space in a collision. From a global point of view, aluminium foam showed the lowest displacement, followed by cork and then IMPAXX. Polyurethane foam was again the least suitable material in this test.

Aside from its well-known application as a bottle stopper material, cork is already widely used as a thermal and sound insulator and in various energy-absorbing applications including packaging and footwear. It is often used as damping pads under the keys in wind instruments such as clarinets and saxophones. However, its potential as a safety material for vehicles is only now emerging.

The researchers conclude that while aluminium foam marginally performs better than micro-agglommerated cork, cork could be a much better choice for future vehicle design as it is less costly and much easier to process than metal foam.

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