Getting smart

...with materials that can sense changes in the environment and adjust accordingly

Published: Saturday 15 February 1997

day in the near future, airplane wings may flex themselves like fish tails, changing shape to modify, lift or drag by themselves. Bridges and telephone poles could 'feel' when they are about to break, send out a warning and then reinforce their components. Air conditioners may supress their own vibration. Handguns may fire only when held by their owners.

These are just a few of the technological marvels expected from the new science of 'smart materials': structures that can sense changes in their environment and then respond accordingly. "The first thing that's going to happen," said James S Sirkis of the University of Maryland's Smart Materials and Structures Research Center, "is the advent of systems for an early warning about structural damage in bridges and buildings."

Most of the projects underway use fibre-optic threads as strain gauges on bridges. When the structure stretches or warps, the tugging motion alters a tiny grating in the fibre-optic system, which in turn changes the wavelength of light that travels along the fibre. Computerised detector modules translate these light shifts into stress units, providing advance notice of failure. In the long term, diagnostic fibres might be coupled with ducts that would squirt epoxy or other strengthening material directly on the spot where a crack was detected.

Many scientists believe that within 25 years smart structures will drastically alter the shape of aircrafts, which will have control surfaces that can reshape themselves in air. Already a 'smart' one-sixth scale model of an f/a-18 fighter wing has passed its first round of wind tunnel tests with flying colours. "The performance enhancements, turned out to be greater than expected," said Bob Crowe of the Defense Advanced Research Projects Agency (darpa).

Adaptive systems rely on three general classes of smart materials. One is made up of substances that expand, contract or twist when exposed to electric or magnetic fields. The second type known as piezoelectric materials are films that generate a voltage when stressed or, conversely, flex when a voltage is applied are fairly popular.

The University of Maryland and darpa are using piezoelectric elements and fibre-optic sensors to design an 'active' helicopter blade that can adjust its shape continuously to respond to vibration-engendering pressure changes in the air. In the helicopter, piezoelectric patches on the blade surfaces function as both sensors and as generators of counter-force. Researchers are experimenting by augmenting the system with fibre-optic sensors and polymer composites.

Piezoelectric sensors might also be employed on the grips of handguns that will only fire when they detect the unique pressure-pattern -- signatures of the owners hand. Piezoelectric substances can respond within a thousandth of a second, but they can only be stretched to a fraction of one per cent of their dimensions. So researchers are testing them in combination with a second class of smart materials called 'shape memory alloys' (smas). Although these materials are much slower, they are far more flexible and can 'remember' their original configuration even when deformed as much as 15 per cent and return to it when heated.

A third class of smart materials comprises of liquids that change their viscosity when exposed to electric or magnetic fields. You could stir one with a spoon effortlessly in its normal state; but run a voltage across it, and it suddenly becomes thick as concrete. "Hundreds of critical systems in modern life," Sirkis said, "require 'spontaneous monitoring' to determine maintenance needs and ideally you'd like the structure to tell you that itself."

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