Contact: Sandy Embry, [email protected]
SAN FRANCISCO, Dec. 6, 1998 -- Researchers at the Department of Energy's Los Alamos National Laboratory are measuring nonlinear properties of materials to demonstrate a difference between damaged and undamaged concrete.
The research, reported today in a poster session at the fall meeting of the American Geophysical Union in San Francisco, may have broad application for the detection of damage in many materials.
PAUL JOHNSON OF THE GEOENGINEERING GROUP AT LOS ALAMOS SAID THE LABORATORY PLANNED THREE POSTER SESSIONS AT THE AGU MEETING RELATED TO RESEARCH IN THE GENERAL AREA OF NONLINEAR ELASTIC WAVE SPECTROSCOPY (NEWS).
"To our knowledge, there is no other method as sensitive in characterizing damage in materials," Johnson said. "We anticipate that in five to 10 years, application of nonlinear wave spectroscopy could have enormous economic impact."
He said the potential uses of nonlinear techniques to reveal defects in materials include inspections of aging aircraft, quality control for assembly line applications, monitoring containment walls of nuclear reactors and applications to the national infrastructure such as examining bridge pillars and other structures.
Nonlinear materials research includes a technique called nonlinear resonant ultrasound spectroscopy (NRUS), which is being used by graduate student Loren Byers, also of the Geoengineering Group, to measure the difference in the response to an acoustic wave by damaged and undamaged concrete core samples.
"This is the first work to our knowledge that shows the difference between damaged and undamaged concrete using nonlinear methods," Johnson said.
In his research using NRUS, Byers imparts energy to a concrete sample through a speaker-like device that sends a sound wave through the sample. The speaker is set up to input tones over a range from below to above the sample's resonant frequency. This is called a tonal or frequency sweep. At each tone in the range, he measures the volume and frequency of the wave at the other end of the sample. This procedure is used at progressively increasing volume levels.
In a sample that is undamaged, the volume output is directly related to the volume input, and therefore the resonant frequency remains the same as the applied volume is increased. In a sample that is damaged, the resonant frequency shifts as the applied volume increases, and the amount of the shift can be measured precisely.
Byers makes an analogy to playing a piano. "For a sample of undamaged material, it's comparable to hitting a key harder and making the sound louder," he said. "In a nonlinear sample, when you hit the key harder, the response is not only louder, it's a different note."
Moreover, he said, the frequency shift is greater in samples that are more damaged, even with microscopic cracks. So, for example, in a piece of plastic that contains no cracks, there will be no change in resonant frequency as the wave volume is increased. In a sample with a small crack, the resonant frequency will shift readily with amplitude.
Concrete is always slightly damaged in the curing process due to pressures from chemical reactions that build up inside and crack the sample. In a fresh concrete core, the resonant frequency shift is measurable, but becomes considerably larger for the damaged sample, with the same volume input.
"This change makes it very obvious what is damaged and what is fresh concrete," Byers said. "Nonlinearity is extremely sensitive to damage."
Evidence of damage is not only provided by the amount of the frequency shift, but by other manifestations of nonlinearity. For example, Byers also tests samples in a "conditioning and recovery" mode in which a loud-volume tonal sweep is followed by several soft-volume tonal sweeps. In damaged materials it takes some period of time for the sample to return to its original resonant frequency. Further, more damaged samples take longer to return to the original resonant frequency.
"Traditionally, nonlinear properties have been ignored in favor of studying the linear properties of the material to detect damage or flaws," he said. "Nonlinearity is a new frontier in damage diagnostics."
For the immediate future, Byers said, "we would like to test more core samples so we will have a large set of data in order to study variations in concrete types and damage intensities. We're working toward a correlation between the size of the frequency shift and the amount of damage in a sample, and toward making the technique available to applications indoors and outdoors."
Another technique of elastic wave spectroscopy is nonlinear wave modulation spectroscopy, which involves applying two single-frequency waves to a sample simultaneously and measuring for nonlinear modulation effects that indicate damage.
Los Alamos is collaborating on research in this area with Russian scientists from the Institute of Applied Physics in Nizhny Novgorod, who were instrumental in developing methods to study nonlinear modulation effects for the study of damage. Other Laboratory collaborations in nonlinear diagnostics are under way with the Catholic University of Belgium in Leuven and the University of Massachusetts at Amherst.
The results stem from research supported by the Office of Basic Energy Sciences of the U.S. Department of Energy.
Other poster sessions from Los Alamos at the AGU meeting involving elastic wave spectroscopy present a global view of nonlinear response in materials and present more information about the slow-time dynamic response in other materials, including rocks.
Los Alamos National Laboratory is operated by the University of California for the US Department of Energy.
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