EUREKA APRIL 2002 COVER FEATURE STORY

Natural holes are best for strength

Tom Shelley explains how lessons from nature can help us make much better composite constructions with integral sensors
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Copying nature teaches us that we should not make holes in composites, but if we do, they are much best moulded in and not drilled, and can be used as the basis of novel, potential strain gauge sensors, with integral mechanical amplification.
The conclusions come from studies of natural composite constructions with integral sensors in insects. Earlier developments, based on studies of wood, have already show practicable ways of making open composites, full of air passages, three to four times as resistant to impact damage as conventional designs.
Potential applications abound in defence, aerospace, security, oil and gas and automotive constructions.
Whereas v July 2001 edition featured aspects of insect construction which might be incorporated into man made products, the Biomimetic research team lead by Professor George Jeronimidis at Reading University, has been developing and demonstrating superior, practical, nature inspired composite constructions since around 1980.
His first major developments were inspired by wood, which is full of longitudinal passages and is reinforced by fibres at an angle of 30 degrees.
His team was able to duplicate such constructions on a slightly large size scale, by draping sheets of pre-preg over corrugated formers, with the fibres at a suitable angle to the corrugations, the cured sheets could then be bonded together into constructions with three to four times the impact energy absorption capability of conventional constructions per unit weight. The development started out with support from a US maker of cardboard boxes, and was then taken up by Royal Mail, who was looking for lower weight, bomb proof containers, into which they could toss suspect packages. One of the attractions of the idea, according to the Professor is that it is possible to tailor the different layers so that while the outermost layer may be stiff and hard, subsequent layers are more elastic, so that structural collapse is propagated further into the material to absorb more energy.
Different constructions can be designed to absorb different kinds of impact, whether they be detached turbine blades in jet engines, bird strike on aircraft skins, rotating machinery parts, knife thrusts against personal body armour, or solid structures impacted against by motor cars. Despite the 1982 publication date of the original first patent, few of these ideas has so far been taken up, possibly because none of them have previously been aired in Eureka. The only example that we are aware of is the crash energy absorbing glass reinforced plastic structure in the front of the current Lotus Elise. It is made of layers of corrugated GRP bonded together, but on a much larger size scale than the Professor’s material, where air passages are only a few mm across, and the repeat distance between corrugations is about 5mm.
Interest in light weight blast absorbing structures and better body armour has now been rekindled by the events of September 11th.
Professor Jeronimidis has therefore now suddenly found himself much in demand, and came to our attention because of a seminar he was recently invited to give by ERA Technology.
As well as his established technologies on wood derived constructions, he has recently discovered ways of dealing with and making superlative use of holes in composites, inspired by studies of insects.
“Drilled holes in composites are a very bad idea”, explains the professor. “Cut fibres at any edge trigger delamination and we should, wherever possible, generate complex composite constructions by bonding and forming to shape prior to cure.” But insects, apparently, get away with holes in their constructions, by growing chitin microfibrils around them.
They also use holes, particularly in a dome shaped organ called the campaniform sensillum, found in such crucial spots as the base of the wing spar in the dragonfly.


Campaniform sensillum structures, Locust

The tops of the holes are bridged by a thin layer of membrane.

If the sides of the dome are squashed towards each other, the sides of the hole will be pushed towards each other by a greater proportional amount, because the membrane is less stiff than the material on each side of it. The narrowing of the hole is detected by a nerve cell dendrite which projects into it, producing an electric signal. This signal is thus greater than it would be otherwise, because the hole acts as a mechanical amplifier without a lever.

Transferring this to composite construction, the team has made a number of test pieces with holes in them. The optimum design is to make sheets of composite, cured around a former which is subsequently removed. In a commercial production situation, it might be possible to lay fibres using CNC equipment, so that they ran around holes or regions of material in which holes might subsequently be drilled. Test pieces with drilled holes, have, however, also been made for comparison.
In a demonstration for Eureka, deformation was detected by inserting a small plug of photoelastic elastomer, and observing colour changes between crossed polars. Such a device could be made very small in a more developed version, and interrogated by optical fibre. Alternatively, the sensing element could be made out of either: deformable polymeric optical fibre, piezoelectric material or elastomeric material loaded with carbon or metal powder. Such materials reduce their resistance when they are squeezed. In the latter two cases, readout would have to be electrical. With advances in modern microelectronics, data output could be via short range radio or induction, so there would be no vital need to embed long lengths of wire. Whatever the sensing mechanism, there would be a much smaller deleterious effect resulting from having a sensing element inserted in a small hole, than using an optical fibre laid within the rest of the fibre reinforcement.
The research programme is at present restricting itself to round holes. In insects, sensing holes are more often oval or even arrays of slits. There is doubtless some good reason for this, yet to be discovered. (More information at www.reading.ac.uk/biomimetics )

Design pointers

• Composite structures based on wood have much greater energy absorbing capacity than conventional designs
• Holes can be accommodated by forming fibres round them
• Such holes can then be used as mechanical amplifiers for novel strain sensing devices

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