International Journal of Applied Science and Engineering
Published by Chaoyang University of Technology

Junho Song and D. E. Chimenti*

Center for Nondestructive Evaluation and Department of Aerospace Engineering Iowa State University Ames, IA 50011, USA.


 

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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. 


Keywords: air-coupling; ultrasonics; NDE; capacitive film; transducer; diffraction-limited resolution.


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ARTICLE INFORMATION




Accepted: 2006-03-03
Available Online: 2004-04-04


Cite this article:

Song, J., Chimenti, D.-E. 2005. Design, Fabrication and characterization of a spherically focused capacitive Air-Coupled ultrasonic transducer. International Journal of Applied Science and Engineering, 4, 1–19. https://doi.org/10.6703/IJASE.2006.4(1).1