Nanocomposite material gets stronger when stressed


March 24, 2011

Graduate student Brent Carey, positioning a piece of the nanocomposite material for dynamic mechanical analysis(Photo: Jeff Fitlow/Rice University)

Graduate student Brent Carey, positioning a piece of the nanocomposite material for dynamic mechanical analysis
(Photo: Jeff Fitlow/Rice University)

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If someone does a lot of arm curls at the gym, the typical result is that the bones and muscles in their arms will get stronger. Recently, researchers at Houston's Rice University inadvertently created a nanocomposite that behaves in the same way. Although the material doesn't respond to static stress, repeated mechanical stress will cause it to become stiffer.

The discovery was made in the lab of Pulickel Ajayan, Rice professor of mechanical engineering and materials science, and of chemistry. Graduate student Brent Carey had created a composite material by infiltrating a batch of vertically aligned, multiwalled nanotubes with polydimethylsiloxane, which is an inert, rubbery polymer. He was testing the high-cycle fatigue properties of the composite, and was surprised to discover that instead of weakening when subjected to repeated loads, it actually got stronger.

Over the course of a week, the material was subjected to 3.5 million compressions. This caused its stiffness to increase by 12 percent, with indications that there was potential for further stiffening. The reason for this type of reaction is still something of a mystery.

"We were able to rule out further cross-linking in the polymer as an explanation," said Carey. "The data shows that there's very little chemical interaction, if any, between the polymer and the nanotubes, and it seems that this fluid interface is evolving during stressing."

What is known is that the use of nanomaterials greatly increases the surface area available to that fluid interface, so whatever reaction is taking place is much more pronounced than would be the case with a conventional composite.

Carey is already envisioning potential uses for materials utilizing the process. "We can envision this response being attractive for developing artificial cartilage that can respond to the forces being applied to it but remains pliable in areas that are not being stressed," he stated.

About the Author
Ben Coxworth An experienced freelance writer, videographer and television producer, Ben's interest in all forms of innovation is particularly fanatical when it comes to human-powered transportation, film-making gear, environmentally-friendly technologies and anything that's designed to go underwater. He lives in Edmonton, Alberta, where he spends a lot of time going over the handlebars of his mountain bike, hanging out in off-leash parks, and wishing the Pacific Ocean wasn't so far away. All articles by Ben Coxworth

metals do this too.

Bend a bit of steel, see how much harder it gets to bend it again.

It\'s called cold-working. People have been doing it for centuries.


but when you bend steel it weakens it and if you keep doint it it will snap

Andrew Langdon

Andrew - the material in this article is also described as getting stiffer as it is worked (the same as steel) and so will presumably begin to crack rather than bend after repeated working (the same as steel).


It doesn\'t necessarily weaken it. Depending on the structure of the steel, it gains stiffness, but loses toughness. That is, it becomes more fragile, and will break rather than bend.

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Myron J. Poltroonian

From the article, one cannot tell if the material is getting stiffer or stronger. These are completely distinct qualities. Steel is stiff and strong. Rubber is flexible and strong. Soda crackers are stiff and weak. Fruit jelly is flexible and weak. You should all read J.E. Gordon for a pleasant education. He is a superb writer, and co-discoverer of the reason that brittle glass and brittle resin can combine to make tough fiberglass. Toughness is the ability to absorb a lot of energy in the process of being damaged before breaking, and is often the most important property of a material.

Bob Stuart
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