Accurate Reconstruction of Flaws in Materials Using a Synthetic Aperture Ultrasonic Imaging System
A compact synthetic aperture ultrasonic imaging system, which rapidly reconstructs precise sectional images of flaws in steel, has been developed. Two side-drilled holes, 1 mm in diameter, at an interval of 1 mm were separately reconstructed when tested with a pulsed 5 MHz transmitted wave (wavelength:1.2 mm) and with a transducer whose aperture angle was 90°. Round profiles of larger diameter holes (4–8 mm) were also clearly reconstructed. Simulation studies have been carried out in order to predict the lateral and range resolutions of the imaging system. Most specifications of the imaging system have been determined by these simulation studies. Acquisitions of echo signals and calculations for synthetic aperture processing were carried out with a 16 bit microcomputer and the distance calculation between the transducer and reflectors were performed by a specially designed echo location calculator (5 microseconds per distance calculation). The transducer was scanned linearly along the tested blocks, and the received echoes were processed into a sectional image after each echo acquisition. It took 20 seconds to get a full image display. Another application to evaluate the crack size of the disk plates was also carried out. In this case, circular scanning was applied, and an error of ±0.5 mm in crack depth was obtained. From the viewpoint of fracture mechanics, a more accurate evaluation is required in nondestructive testing than that obtained by the conventional B-scan system. By using the developed system, the accuracy of flaw evaluation was improved.
KeywordsMain Lobe Point Reflector Acoustical Image Aperture Angle Image Memory
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