Conventional ultrasonic flaw detection techniques are based on the generation of an ultrasonic wave packet that travels through a structure and interacts with existing flaws within the structure. Either reflected or transmitted signals may be detected in pulse-echo or pitch-catch modes of operation. As might be expected, small flaws give rise to weak reflections or small changes in the amplitude of transmitted signals. These small variations are often too weak to be detected with existing laser detectors. For such cases we have proposed an alternate approach for ultrasonic detection of small surface-breaking cracks using laser-based techniques. This approach does not measure the interaction of a well-established surface ultrasonci wave with a flaw, but rather monitors the changes in the generated ultrasonic signal as the laser source passes over a defect (see Figure 1). Changes in amplitude and frequency of the generated ultrasound are observed which result from the changed constraints under which the ultrasound is generated when the laser beam passes over a defect.
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