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‘Strain Paint’ Reveals Material Stress

Friday, June 22, 2012

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Invisible stresses that threaten the integrity of bridges, aircraft and other structures could be unmasked by a new coating developed at Rice University.

The fluorescent nanotube coating, dubbed “Strain Paint,” holds promise for non-invasive detection of even low-level material stress “well before the effects become visible to the naked eye, and without touching the structure,” the university reports.

 Slide from Bruce Weisman's presentation on strain paint to NASA in 2010

 Bruce Weisman/Rice University

Professor Bruce Weisman introduced the idea of Strain Paint with this slide from a presentation to NASA in 2010.

The researchers’ study, published online this month by the American Chemical Society journal Nano Letters, describes a composite coating that could be read by a handheld infrared spectrometer.

Clear Coat Sensing

The clear polymeric varnish can measure strain at any location and along any direction, without the need to physically connect a strain gauge to a read-out device, the university said.

The technology employs nanotube fluorescence, which shows large, predictable wavelength shifts when the tubes are deformed by tension or compression.

“The paint—and therefore each nanotube, about 50,000 times thinner than a human hair—would suffer the same strain as the surface it’s painted on and give a clear picture of what’s happening underneath,” the university reports.

The coating is the result of a collaboration among several researchers, including Rice chemistry professor Bruce Weisman; Satish Nagarajaiah, a Rice professor of civil and environmental engineering and of mechanical engineering and materials science; and Paul Withey, an associate professor of physics at the University of Houston-Clear Lake.

Laser Interface

Such a coating could, for example, drastically improve on the current practice of applying conventional strain gauges at specific locations along a wing and subjecting it to force vibration testing to see how it behaves, Nagarajaiah said. Those tests can be done only on the ground and measure only the part of the wing where the gauges are wired.

 Dr. Satish Nagarajaiah
“It’s time for new technology,” says Dr. Satish Nagarajaiah, a Rice professor of civil and environmental engineering and of mechanical engineering and materials science.

“But with our non-contact technique, they could aim the laser at any point on the wing and get a strain map along any direction,” Nagarajaiah said.

In a variety of applications—bridges, buildings and other structures—the coating could provide ongoing monitoring potential, rather than isolated inspections that may occur months or years apart, he said.

Multifunctional Potential

Strain Paint can also be designed with multifunctional properties for specific applications—for example, as a protective film that impedes corrosion or strengthens the substrate, Nagarajaiah said.

Commercialization of the coating awaits additional research.

“We’ll need to optimize details of its composition and preparation, and find the best way to apply it to the surfaces that will be monitored,” said Weisman. “These fabrication/engineering issues should be addressed to ensure proper performance, even before we start working on portable read-out instruments.”

But neither these nor other issues—including construction of a handheld optical strain reader—seems insurmountable, Weisman said.

“There are already quite compact infrared spectrometers that could be battery-operated,” he said. “Miniature lasers and optics are also readily available. So it wouldn’t require the invention of new technologies, just combining components that already exist.

“I’m confident that if there were a market, the readout equipment could be miniaturized and packaged,” Weisman said. “It’s not science fiction.”

Watch a video demonstration of Strain Paint.


Tagged categories: Coatings technology; Protective coatings; Research

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