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