Absorbing matter

A deceptively simple device could form the basis for a new generation of shock-absorbing safety systems for vehicles, sporting equipment — and even buildings.

Developed by Fayek Osman at the University of Bath, the device could be used as part of train buffers, aircraft undercarriages, car bumpers and other assemblies built to absorb sudden large impacts — and unlike the current technologies, it’s re-usable.

The device consists of a metal bar running through a curved channel. The bar is positioned so that its end faces the direction of impact. When the force arrives, it pushes the metal along the channel, forcing it to follow the curve. This pushes the other end of the bar out of the other side of the channel, so to re-use the device, it just has to be turned round.

‘It all depends on understanding how materials behave when they’re forced round corners,’ said Osman. ‘If you pump a fluid through a bent channel with a constant cross-section, it will just flow straight through. But metals are much cleverer than that — push them through a bend, and they don’t flow like liquids, but like solids. You have to overcome the binding energy between the atoms to deform the material, and to do that, you need to put in energy.’

How much energy depends on the material. ‘You can use any deformable material, from gels through to metals, and even then there is a large range of options, from soft lead through to steel and even titanium, as long as your casing is strong enough,’ said Osman. And the amount of force that can be absorbed is surprising.

‘Aluminium, 20mm thick and passing through a 90° bend, can easily absorb 10 tonnes of force.’ Osman also believes that composites and alloys could allow engineers to tailor the acceleration or deceleration of the bar as it moves through the channel.

The other variable is the geometry of the channel through which the bar passes. ‘We’ve used a right-angle turn, which means that the device has to be turned round for re-use, but there are other possibilities,’ said Osman. For example, the channel can be U-shaped, so the bar enters and exits the casing on the same side; or it can be a shallow S-shape, so it emerges on the opposite side. The latter option could have applications for building in earthquake zones. ‘In earthquakes shocks occur up as well as down, and this configuration could absorb both,’ Osman explained.

He pointed out that the device is intended to absorb one-off shocks, and is not suitable, for example, as a replacement for car shock absorbers. ‘We aren’t competing with springs,’ he said. But there is a large potential for use in devices such as shock absorbers in artillery pieces, crane and bridge joints and, particularly, as the safety mechanism to absorb the impact of hard landings in aircraft undercarriages, he added.

One important feature of the device, Osman claims, is that unlike spring-based systems it produces no rebound when absorbing shock. This could make it useful in shock absorbers at the base of liftshafts.

Less obvious but equally promising are applications in sporting equipment, said Osman. A team of students from Bath has devised ‘weightless’ physical training equipment for physiotherapy, allowing muscles to work against a resistance without having to absorb any reaction. Osman also suggests that the device could be built in to the heels of trainers for distance running to absorb the constant shocks from the road or pavement without pressing back against the easily-bruised back of the foot.