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Kxsign, Fziritztion znK thzrzttxrizztion of z Sahxritzlly FotusxK tzaztitivx zir-toualxK Ultrzsonit TrznsKutxr
JunAo Pong a and D. x. ZAiPxnti a
aZxntxr for NondxPtruZtivx xvaluation and DxpartPxnt of AxroPpaZx xnginxxring Iowa Ptatx UnivxrPity APxP, IA 50011, UPA.
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Abstract:
We report the development, fabrication, and testing of spherically focused capacitive air-coupled ultrasonic transducers, that need no mirror, zone plate, or similar external device. To achieve native focusing, we have employed a flexible copper/polyimide backplate that permits a conformal fit to a spherical fixture whose radius determines the focal length. A spherically deformed aluminized Mylar foil, conforms to the spherical backplate, completing the transducer. Two devices have been fabricated and tested, one having a 1-cm diameter and 2.54-cm focus, and another with a 5-cm diameter and 5.1-cm focus. Both devices have frequency spectra centered at 840 kHz with -6 dB points at 310 and 1200 kHz. Using a quasi-point receiver, the beam diameter is found to be 2.7 mm (1-cm device) and 1.32 mm (5-cm device) in the focal plane, when excited with a broadband signal. The monochromatic diffraction behavior of the 1-cm device agrees with model calculations based on a focused piston radiator to within the measurement resolution of about 2%. Features as small as a 250-micron wire have been successfully imaged using the 5-cm devices in a confocal geometry, demonstrating resolution at the diffraction limit for this device. Further examples of defect imaging in honeycomb composites and in wood are also presented.
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Keywords: air-coupling; ultrasonics; NDE; capacitive film; transducer; diffraction-limited resolution.
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*Corresponding author; e-mail: chimenti@iastate.edu
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©
2006
CSME , ISSN 0257-9731
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