EUREKA DECEMBER 1997 COVER STORY
Tom Shelley reports on a low cost and non-destructive way of absorbing impacts
A university researcher has developed a radical approach to absorbing impact, with a material that uses air or gas as the elastomeric element and liquid to distribute the force of the blow.
Much of the impact energy is converted to heat through flow in the liquid, but elastic mechanisms ensure that the struck material can recover its former shape.
The approach is up to seven times better than commercially available materials and offers a completely new approach to impact absorption in motor vehicles, in industry and for individuals. Sports protection equipment is one potential application.
The idea is the brainchild of Bill Courtney, a former further education physics lecturer now pursuing an MSc at Manchester University. As shown to Eureka at the Innovation 2000 event at Chatham Maritime, its basic principle may be demonstrated by stuffing a squeezable plastic bottle full of straws, and then half filling it with water.
On striking the side of the of the bottle, air is compressed and forced to flow through the straws in order to accommodate the shape change. The water distributes force to all straws. Energy is absorbed by compression of the air in the straws, viscous forces as air is forced out of the straws, and from viscous water flow both inside and outside the straws.
Going on from macroscopic straws and plastic bottles, Courtney has tested a wide variety of combinations of materials. Eureka was shown: 50/50 expanded polystyrene beads in grease; 50/50 1mm chopped tubes and grease, and chopped straws and grease; bubble packing and Vaseline; and 50 per cent expanded polystyrene beads, 25 per cent microspheres and 25 per cent grease. Each has its own advantages under different conditions, but all offer dramatic advantages over closed cell elastic foams, open and closed cell rigid foams, and elastomeric and visco elastic solid polymers (see box).
Manchester Material, as Courtney has chosen to call his invention, consists in its simplest embodiment of lots of small air or gas bubbles. Each is enclosed by a flexible wall. The bubbles then rest within a viscous fluid encased in a tough, flexible membrane.
On being struck, a pressure wave within the viscous fluid matrix transmits force to all the little containers full of air. These are compressed from all sides. In a conventional closed cell foam, the air bubbles tend to flatten out as well as decreasing their internal volume. Because the liquid fluid phase is viscous, energy is absorbed when the bubble containments rearrange themselves within the matrix. Viscous flow also occurs during the recovery phase after impact, preventing rebound.
A series of tests have been undertaken with a drop weight test rig. First tests were undertaken with a 0.87kg hammer, dropped from a height of a 0.9m, now followed by further tests with a 3m high test rig.
It is evident from the first series of tests that best results are obtained by using a mixture of light duty bubble packing and grease. The bubble containments need not be at all strong because of the way pressure is distributed all round them. Other material combinations investigated included: light duty bubble packing and concentrated wallpaper paste; heavy duty bubble packing and grease; and 2 x 10mm hollow plastic tubes and grease. Hollow tubes and grease tended to do less well than conventional closed cell foam, but all the other combinations did better, especially at impact speeds greater than 1m/s.
The most likely immediate use of the new material is for protective knee, elbow and shin guards and gloves for sportsmen. In motor vehicles, possible uses include: pedestrian friendly bumpers, impact and vibration absorbing seating, and padding for dashboards, door pillars and steering wheels. Enclosed in rubber or fabric bellows, the material offers damped suspension system benefits at a fraction of the cost of conventional springs and dampers.
Moulded round the handles of power tools, the material has the potential to reduce the repetitive strain injury known as 'white finger'. The material could maintain protective properties in motorcycle and horse riding helmets, while allowing them to be re-usable. In boats, large and small, the material can be used for fenders, and in large structures, as foundation pads to protect against earthquake movements and traffic vibration.
Courtney also has a scheme to incorporate his idea into aircraft and vehicle fuel tanks, with the fuel in a flexible bladder acting as an impact spreading mechanism around flexible gas filled containments. This should make fuel tanks more impact resistant. He is also interested in the idea of making shock absorbing pads filled with electro rheological and magneto rheological matrix fluids, whose viscosity could be controlled externally. For now, the simplest approach, with thin walled bubble wrap in grease, looks the best. All the developments are covered by patent application.
* The basic idea is to have a viscous liquid matrix in a flexible bladder in which are immersed a large number of small air or gas bubbles, prevented from coalescing by flexible containments
* The material is up to seven times better at absorbing and spreading impacts than commercially available materials
* Early experiments show that very good effects can be achieved by using combinations of very low cost materials
Established techniques for absorbing impacts include various types of foams and solid polymers.
Closed cell elastomeric foams combine: low stiffness, lightly damped solid materials and cavities full or air (or gas). Unfortunately, impact tests show that above two to three miles per hour, the air cells beneath the blow tend to expand sideways and flatten. The result is that the material bottoms out and the transmitted force impacts on a restricted area of the surface being protected.
Elastomeric and visco elastic solid polymers are soft, rubbery solids which maintain their shock absorbing properties at higher impact speeds than foams. Because they are 100 per cent solid, they are far stiffer than foams and a large fraction of the peak impact force is still transmitted through the material. Commercially available materials include, 'Sorbothane', 'Astrosorb' and 'Norsorex'.
Rigid foams absorb impact by compressing on impact. They are the favoured approach for the lining of motorcycle helmets but their big disadvantage is that they can only be used once.
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