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Smart composite detects and repairs airplane cracks

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The New Scientist,Vol196,Issue 2624,6 October 2007,Page 32 Paul MarksThe composite material could make the next generation of aeroplanes safer by sensing and repairing cracks before they become critical A smart composite material that senses cracks as they develop and then repairs the damage could make the next generation of aeroplanes safer. Composites are widely used in bicycles, fishing rods, racing cars and aircraft, and the first all-composite aircraft - the Boeing 787 and Airbus A350 - are on the way. A polymer resin typically makes up most of the bulk of a composite, while fibres of a stronger material, such as carbon or glass, are embedded in it to add strength. By varying the type and amounts of resin and fibre, composites are tuned to produce different combinations of lightness and strength. These materials can weaken through delamination, in which the fibres begin to part from the resin. This can be caused by the material experiencing a hard impact, absorbing liquids, or simply getting old. Repeated loading of a composite - caused by the pressurisation and


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.Scientists have developed a smart composite material which could sense cracks as they develop in airplanes and then repair the damage.

The carbon fibre composite was developed by engineer Nikhil Koratkar at Rensselaer Polytechnic Institute in Troy, New York. In tests it showed an ability to self-repair within seconds of cracks appearing. The material works by heating and then melting a special powder when a crack appears. The warm substance fills the gap and hardens as it cools down, retaining at least half the strength of the original material. Koratkar's work could be used in the next generation of carbon fuselage aircraft, science magazine New Scientist reported Sunday.


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Electromagnetic Wormhole' Possible with Invisibility TechnologyOct. 12 issue of Physical Review Letters

Allan Greenleaf, mathematics at the University of Rochester,

Greenleaf's coauthors:

Matti Lassas, mathematics at the Helsinki University of Technology

Yaroslav Kurylev, mathematics at the University College, London

Gunther Uhlmann, Mathematics at the University of Washington.


One of the views through the "wormhole." Different lengths result in different bending of light. Credit: University of Rochester

The team of mathematicians that first created the mathematics behind the "invisibility cloak" announced by physicists last October has now shown that the same technology could be used to generate an "electromagnetic wormhole."

In the study, Allan Greenleaf and his coauthors lay out a variation on the theme of cloaking. Their results open the possibility of building a sort of invisible tunnel between two points in space.

"Imagine wrapping Harry Potter's invisibility cloak around a tube," says Greenleaf. "If the material is designed according to our specifications, you could pass an object into one end, watch it disappear as it traveled the length of the tunnel, and then see it reappear out the other end."

Current technology can create objects invisible only to microwave radiation, but the mathematical theory allows for the wormhole effect for electromagnetic waves of all frequencies. With this in mind, Greenleaf and his coauthors propose several possible applications. Endoscopic surgeries where the surgeon is guided by MRI imaging are problematical because the intense magnetic fields generated by the MRI scanner affect the surgeon's tools, and the tools can distort the MRI images. Greenleaf says, however, that passing the tools through an EM wormhole could effectively hide them from the fields, allowing only their tips to be "visible" at work.

To create cloaking technology, Greenleaf and his collaborators use theoretical mathematics to design a device to guide the electromagnetic waves in a useful way. Researchers could then use these blueprints to create layers of specially engineered, light-bending, composite materials called metamaterials.

Last year, David R. Smith, professor of electrical and computer engineering at Duke's Pratt School, and his coauthors engineered an invisibility device as a disk, which allowed microwaves to pass around it. Greenleaf and his coauthors have now employed more elaborate geometry to specify exactly what properties are demanded of a wormhole's metamaterial in order to create the "invisible tunnel" effect. They also calculated what additional optical effects would occur if the inside of the wormhole was coated with a variety of hypothetical metamaterials.

Assuming that your vision was limited to the few frequencies at which the wormhole operates, looking in one end, you'd see a distorted view out the other end, according the simulations by Greenleaf and his coauthors. Depending on the length of the tube and how often the light bounced around inside, you might see just a fisheye view out the other end, or you might see an Escher-like jumble.

Greenleaf and his coauthors speculated on one use of the electromagnetic wormhole that sounds like something out of science fiction. If the metamaterials making up the tube were able to bend all wavelengths of visible light, they could be used to make a 3D television display. Imagine thousands of thin wormholes sticking up out of a box like a tuft of long grass in a vase. The wormholes themselves would be invisible, but their ends could transmit light carried up from below. It would be as if thousands of pixels were simply floating in the air.

But that idea, Greenleaf concedes, is a very long way off. Even though the mathematics now says that it's possible, it's up to engineers to apply these results to create a working prototype.

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