Natural holes are best for strength

Tom Shelley explains how lessons from nature can help us make much improved composite constructions complete with integral sensors

Natural holes are best for strength
Tom Shelley explains how lessons from nature can help us make much
improved composite constructions complete with integral sensors
Nature teaches us that we must not make holes in composites, but if we must, they are best moulded in rather than drilled. Now it seems such holes 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 present in insects. Earlier developments, based on studies of wood, have already show practical 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 Eureka’s July 2001 issue 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.
Jeronimidis’ first major developments were inspired by wood, which is full of longitudinal passages and 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 maufacturer of cardboard boxes, and was then taken up by Royal Mail, which was looking for lower weight, bomb-proof containers, into which sorters could toss suspect packages. One of the attractions of the idea, according to Jeronimidis 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. This means 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 strikes on aircraft skins, rotating machinery parts, knife thrusts against personal body armour, or solid structures impacted by 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 been aired in Eureka). The only example that we are aware of is the crash-energy-absorbing composite 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 Jeronimidis’ material, where air passages are only a few mm across and the repeat distance between corrugations is about 5mm.
Interest in lightweight, blast absorbing structures and better body armour has now been rekindled by recent events and Professor Jeronimidis has suddenly found himself much in demand attracting our attention by way of a seminar he was recently invited to give by ERA Technology.
As well as his established results with 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”, he says. “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, that is found in such crucial spots as the wing root in the dragonfly. 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 calls for 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 run 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 the 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 rather 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 that has yet to be discovered. (More information can be found at Reading University Biomimetics).

University of Reading - Department of Engineering Enter XXX

Tom Shelley

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