Conventional ultrasonic testing for the detection and characterizatin of defects in metals has traditionally relied on systems using contact piezoelectric transducers for both generation and detection of the ultrasonic field. Standard contact ultrasonic inspection systems, including pulse-echo, through transmission and tandem, work well for room temperature applications when the defects are not too small, no less than about 5 mm, and the surface of the material to be interrogated is readily accessible. If the application requires high temperatures, or surfaces on which a couplant cannot be used, such as a moving surface, the use of contact transducers is no longer practical. For these applications, a non-contacting system is generally desired. As a research tool, laser generation and detection, and generation and detection by other electromagnetic methods have been among the more successful non-contact ultrasonic inspection systems.
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