Although liquids or rubbers are the media most often used to couple ultrasonic waves in nondestructive evaluation applications, these are not the only choices. Researchers at CNDE are investigating air as a couplant for ultrasonic tests.
The density and lower sound velocity of air seem to make air an inappropriate ultrasonic couplant, according to Dale Chimenti, a CNDE senior scientist and professor in the Department of Aerospace Engineering and Engineering Mechanics. Now, in measurements at CNDE supported by the Federal Aviation Administration Center for Aviation Systems Reliability, Chimenti and his collaborators have demonstrated air-coupled ultrasonic C-scans and materials characterization of composites at frequencies of 0.5 to 1.2 MHz
From a technical standpoint, the major obstacle in air-coupled ultrasonics is the availability of transducers that can produce and sense very high frequency pressure waves in the air. Piezoelectric transducer materials, such as quartz, are best suited to generating waves in solids since they have both large stiffness and high density. However, this combination is poorly matched to move the volume of air needed to create high-amplitude pressure waves suitable for air-coupled ultrasonic inspections.
A recent advance in this technology is the development of high frequency
foil transducers, an extension of the "tweeter" concept found in stereo
speakers. For MHz applications, however, the foils must be much thinner
and closer to a ground plane in order to function efficiently. In the air-coupled
transducers, it is the charge separation between the foil and the ground
plane that causes its "drumhead" to flex, producing comparatively large
air displacements. A further advantage is that the foils are reciprocal
devices, functioning both as generators and detectors of air-coupled ultrasound.
A bias voltage of a few hundred volts is needed for operation.
To increase the efficiency of the coupling, CNDE researchers carefully
constructed a measurement model of the test geometry to maximize sound
energy insertion into the sample, according to Chimenti. In plate-like
test samples, this is the equivalent of generating guided waves in the
plates. Evidence of guided waves, seen as complex voltage versus frequency
and angle, is routinely recorded and analyzed. This forms the major portion
of the experimental data. By using the results of detailed voltage calculations
for two-transducer measurements, the geometric aspects of the measurement
can be accounted for and the material elastic property information contained
in the data extracted with remarkably high resolution. C-scans to detect
defects have also been carried out.
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(c) Copyright 1996, Center for Nondestructive Evaluation, Iowa State University. All rights reserved.
Rolf Diederichs 01.August 1996, info@ndt.net