Page 10 - Spring 2010

LISTENING TO MATERIALS:
FROM AUTO SAFTETY TO REDUCING THE NUCLEAR ARSENAL
Veerle M. Keppens
Department of Materials Science and Engineering, The University of Tennessee Knoxville, Tennessee 37996
Julian D. Maynard
Department of Physics, The Pennsylvania State University University Park, Pennsylvania 16802
Albert Migliori
National High Magnetic Field Laboratory, Los Alamos National Laboratory Los Alamos, New Mexico 87545
“This contemporary adaptation of the wheel- tapper’s technique is known as Resonant Ultrasound Spectroscopy (RUS), and its precise measurements of elastic constants have furnished valuable information about materials.”
Acoustics is an essential tool for transducer (the “tap”) and detecting
21st century society. From arts to
science to business and everyday
life, the use of acoustics, beyond the
built-in communication tool we all pos-
sess, is ubiquitous. In this article, we
focus on one aspect of acoustics—the
application of acoustic resonances to
probe the properties of materials and
objects. Such an application has a signif-
icant impact on us all as it connects to
science, engineering, quality control,
public safety and protection (including
military applications), and more. For
example, the use of acoustic resonances
to study the material properties of plu-
tonium has improved the confidence
that our military possesses in present-
day thermonuclear weapons. This has enabled the US nuclear weapons science-based stockpile stewardship program to support reduction of the nuclear arsenal from more than 36,000 to just above 5,000 today. In another more common- place example, resonances are used today to inspect the disc brake rotors of automobiles in a production environment with very high throughput. In this application, the acoustic “signature” of an automobile part coming off an assembly line can be used to determine if the part is “good.” The basic idea for this is very old. During the steam-age, one of the more unusual jobs found at railway stations was that of the carriage and wagon inspector, or wheel-tapper as they were affection- ately known—when a train paused for any period at the sta- tion, the wheel-tapper would walk the length of the train, checking the wheels for signs of stress or fracture. He would do this by striking each wheel with a special long handled hammer and listen to its “ring.” Because cracked wheels, like cracked bells, do not sound the same as their intact counter- parts, a good wheel-tapper could identify a defect by the sound his strike made. If a problem was found, the wheel- tapper would report it, the train would be delayed and the defective vehicle removed.
Modern maintenance procedures have mostly eliminat- ed the need for the wheel-tapper, but the “tap-and-ring” approach to determine a material’s properties and/or quality has become a vital tool in a material scientist’s laboratory. With the material to be measured held lightly between a drive
6 Acoustics Today, April 2010
transducer (the “ring”), the frequency of the drive is swept and a sequence of res- onance peaks can be recorded which, when processed along with the shape and mass, will yield all the elastic con- stants for the material. Or, in another application of resonances, changes in the resonance pattern or deviations in the spectrum from “known good” spec- tra are useful, nondestructive tests for quality control. This contemporary adaptation of the wheel-tapper’s tech- nique is known as Resonant Ultrasound Spectroscopy (RUS), and its precise measurements of elastic constants have furnished valuable information about materials.
Basic principles of RUS
The idea of using resonances to study the elastic response of a material goes back to the 1960’s, when Schreiber, Anderson, Soga, and Warren and Demarest devel- oped computational procedures to find the elastic moduli for
1,2
RUS, in its present form, was developed in 1988 by Migliori and Visscher,3,4 who—with the advantage of modern computing power—developed sophisticated codes and turned RUS into a powerful method for measuring elastic constants of materials with a variety of shapes and symmetry. Mechanical resonances can be calcu- lated for a sample with known dimensions, density, and elas- tic tensor, laying the basis for the reverse process—going from resonances to material properties. In a RUS experiment, the frequencies of mechanical resonances of a freely vibrating solid of known shape are measured, and an iterative proce- dure is used to adjust elastic constants until the calculated spectrum corresponds to the measured frequencies. In this way, all elastic constants are determined from a single fre- quency scan, a clear advantage of RUS. There is no need for separate measurements to measure different moduli, and multiple sample remounts and temperature sweeps are avoid- ed. Figure 1 shows the original RUS set-up, where the sample was lightly held at opposite corners between two transducers, eliminating bonding agents that can be particularly bother- some at low temperatures. Another advantage lies in the abil-
millimeter-sized mineral samples.