An ultrasonic-transducer fixture containing multiple pairs of fixed transducers, plus associated electronic transducer-multiplexing circuitry have been developed to accelerate the acquisition of data on the dispersion of ultrasonic leaky Lamb waves (LLWs) in plate specimens of laminated composite (matrix/fiber) materials. The fixture and multiplexing circuitry can readily be incorporated into a previously constructed ultrasonic-LLW system that contains a single pair of movable ultrasonic transducers.

LLWs are guided waves that propagate along the surfaces of the specimens. In a typical conventional ultrasonic-LLW system, a plate specimen is immersed in water (which serves as an acoustic-coupling medium), with a transmitting and a receiving ultrasonic transducer positioned above the specimen in a pitch/catch arrangement at a specified angle of incidence, as shown in Figure 1. For each of several angles of incidence, the output of the receiving transducer is digitized and processed to extract reflection-spectrum and dispersion data. These data are then inverted, by use of special-purpose software, to obtain the coefficients of elasticity of the materials and to evaluate flaws in the specimens.

Figure 1. In a Conventional Ultrasonic-LLW System, the transmitting and receiving transducers must be moved to different positions to obtain different angles of incidence.

In the conventional system, the speed of acquisition of data is limited by the need to reposition the transducers to obtain different angles of incidence. In a system equipped with the present fixture and multiplexing circuitry, data can be acquired much more rapidly because except as explained in the next paragraph, the transducers are not repositioned during a test; instead, multiple transmitting/receiving pairs of transducers are pre-positioned at the desired angles of incidence (see Figure 2) and are switched into and out of operation in the required sequence during a test by use of the multiplexer circuits under control by a computer. The software for controlling the multiplexer circuits is part of a modified version of the software for controlling the rest of the ultrasonic-LLW system.

Figure 2. Transmitting/Receiving Pairs of Ultrasonic Transducers in this fixture are positioned and oriented, relative to an immersed specimen, for angles of incidence of 15°, 30°, 45°, and 60°.

In some cases, it could be necessary to acquire data with the plane of incidence at different polar angles (0°, 45°, and 90°) with respect to fibers projected onto the surfaces of the specimens in order to characterize the specimen materials completely. Even in such a case, it is not necessary to expend much time repositioning transducer pairs; all one need do is rotate the transducer fixture to each polar angle and to perform the measurement sequence there. Experiments have shown that dispersion data for four angles of incidence at one given polar-angle setting could be acquired in 7.4 seconds.

This work was done by Yoseph Bar-Cohen of Caltech and Susan Kersey, Cedric Daksla, and Anatoly Blanovsky of UCLA for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the category Physical Sciences

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Technology Reporting Office
JPL
Mail Stop 122-116
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-20400


This Brief includes a Technical Support Package (TSP).
Apparutus for faster Ultrasonic-Lamb-Wave Testing

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This article first appeared in the January, 1999 issue of NASA Tech Briefs Magazine.

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