EUREKA MAY 2005 COVER FEATURE STORY

Energy swallowed on demand by compact devices

Tom Shelley reports on a superficially simple looking family of devices likely to revolutionise much of mechanical engineering

By forcing to make solid material deform as it passes round a bend in a closed channel, it is possible to construct compact, low cost and infinitely re-usable devices that absorb large or small amounts of mechanical energy.

The ideas are derived from metal forming, but working materials already studied also include gels, polymers and complex mixtures.

Applications investigated range from multi use car impact crash absorbers through rotary mechanical and building earthquake dampers to protective materials for sportsmen.

The base idea is the brainchild of Dr Fayek Osman, a lecturer in the University of Bath Department of Mechanical Engineering. He recently told us that his inspirations came from a combination of his long industrial experience of metal forming in industry, and an article on an energy absorbing "Collision protection fuse" originally published in Eureka around 20 years ago.

The fuse was a collapsible tube with side slots to ensure that it collapsed in a predictable manner and direction. This led him to consider how it might be possible to devise a device that would absorb equal or preferably greater amounts of energy but which could be re-used.

Since he was familiar with metal forming, and was aware of how metals could be made to undergo large amounts of deformation, he hit on the idea of forcing metal through a round channel with a bend in it. Provided the input cross section of the tube is the same for entry and exit, what comes out is of exactly the same geometry as what goes in, allowing the device to be used again and again.

The substance being forced around the bend does not have to be metal, and the bend can be of almost any geometry. The device can be used as is, or in combination with mechanical mechanisms to spread a reduced absorption force over a longer movement distance. Speed of operation can be from mm per minute, up to the speed of sound in the solid material. The devices produce no rebound after impact, and are maintenance free, requiring no lubrication.

The idea and many of its potential configurations and applications are protected by patent, and a wide range of devices employing different materials and configurations either have been or are being researched by Bath University students.

For the research studies, some of the early work has been with plasticine in devices made of acrylic, a technique often used for research studies of metal forming. Dr Osman considers, however, that plasticine is not a good material for practical devices because it tends to dry and change its mechanical properties if left in contact with air. Other relatively deformable materials being studied include paraffin wax and 'Polymorph'. The idea is also being studied with devices made of tool steel and with aluminium and lead as working materials. Here, establishing performance parameters become more complicated because metals work harden when deformed. Soft, low melting point metals such as lead recrystalise and revert to their original mechanical properties if left for a while. Aluminium, on the other hand only recrystalises when heated, but recrystalisation temperatures can easily be reached during the deformation process if it occurs at speed. Even if the metal does not have a chance to recrystalise, Dr Osman points out that work hardening occurs only up to a certain limit, and experimental results have been obtained showing the resistance of such devices rising until they reach a plateau force.

The first and simplest device to be made consisted of two connecting holes drilled into a block at right angles. Even using such a simple configuration, 120mm on each side, with 25mm diameter channels, it is possible to achieve an absorption force of 100kN.

Less force is required if the channel is made to bend by less than 90 degrees. The channel can also be made 'U' shaped so that the pushed out slug emerges on the same side as the pushed in slug. If the device is then rotated, it can be made to present the pushed out slug in its original position, ready for the device to be re-used. The slug of material can be pushed in by a captive punch, presenting a larger cross section to the outside world. It may then be used to push out another captive punch at the other end of the channel. If the input and/or output punch resides in a screw thread, the device may be made to absorb energy from a rotating force attached to the punches.

If the device includes a 'dog leg' section, the emerging slug or material or punch can be moved along the same axis as the input force.

A large number of potential applications have already been identified and more are constantly being added. One of the most obvious applications is to employ a series of through acting devices behind car bumpers. After an impact, they could be turned round ready for use again, instead of having to be thrown away as at present. Because of the very large energy density of the device, it is possible to place units between inclined parallel inclined ramps to absorb linear vibration. Such a configuration is being considered for absorbing earthquake shocks in buildings.

Another arrangement achieving a similar effect is to place a through acting device on rockers attached to linearly moving elements.

By engaging punches of successively larger diameters, or having them act on slugs of material of increasing stiffness, it is possible to make devices that become progressively stiffer. Such a configuration has been put forward for absorbing impact at the bottom of a lift shaft.
Other proposed applications include end stops for machine tools, aircraft undercarriage impact safety devices and energy absorbers in artillery pieces. The devices may also be expected to be found in future joints in bridges, cranes, building trusses, train buffers and automation equipment. As well as re-usable car bumper mounts, the devices have been proposed for door safety devices, and incorporation into body members, dampers and steering columns. A 'bubble' structure has been devised, but not tested, in which slugs of soft material are pushed through channels between spherical cavities for use in protective sports wear, trainers and limb and joint prosthetics. Most areas of engineering are likely to have a use for the technology. Cost and weight savings are expected to be significant.

Analytical methods have been developed to assist design, based on the Upper Bound Method traditionally used to study metal forming as well as using ANSYS finite element analysis. The complexity of the processes involved, however, require experimental verification of designs in all cases.

For more information, email Dr Fayek Osman at Bath University

Eureka says: The devices look to be able to totally change just about everything in the field of energy absorption in mechanical engineering design. The Victorians could have thought of it but didn't, proving once again that even the most basic mechanical components often still have considerable room for innovative improvement.

Pointers

* Simple to use
* Low cost and easy to manufacture
* Negligible fatigue effects
* No rebound after impact
* Very large range of possible force ratings
* Energy absorption rates can be tailored to design requirements
* Maintenance free
* Re-usable
* Single action to initiate energy dissipation
* Compact, easy to integrate into any structure

For more technical developments see www.eurekamagazine.co.uk

Back to list of stories