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Laser Ultrasonic Techniques And Applications

Posted on:1996-01-07Degree:DoctorType:Dissertation
Country:ChinaCandidate:C F HeFull Text:PDF
GTID:1118360185953292Subject:Solid mechanics
Abstract/Summary:PDF Full Text Request
Laser ultrasonics is a very active forward project in the area of non-destructive testing. In this dissertation, the mechanisms of ultrasonic generation by laser pulse were discussed systematically. In consideration of thermoelastic and ablation effects existing simultaneously in some cases, a linear superposition approach was presented. Meanwhile, ultrasonic waves generated by laser pulse in aluminium in slight ablation effect were received with a PZT transducer.On the basis of discussing the principles of detecting ultrasonic signals with a confocal Fabry-Perot interferometer (CFPI), a CFPI was developed successfully. This interferometer has good characteristics, lower cost of production and is easy to operate. On the instrument construction, a straight gear was applied to the mechanism of coaxal fine turning of the interferometer to replace one of general optical instruments which is too complicated. The good experimental results were achieved.A feedback system with an analogue PI controller was used to stabilize the operation point of this interferometer, which is of great advantage for eliminating long term drift. Using the developed PZT microactuator, the photo-detecting circuit and the PI controlling circuit, the operation point was stabilized with errors less than 5%, and the accuracy needed of experiments was satisfied.A circuit with lower noise, broader bandwidth (up to video frequency) and higher gain for detecting and processing weak photo signals was developed. The experimental results show that it works well.A noncontact system detecting laser-generated ultrasound was developed. Using this system, laser-generated ultrasound in a machine-finished metallic samples, being of characteristics of weak amplitude and narrow pulse, was detected. The ratio of signal to noise is good enough for electronical signal processing.A noncontact method for measuring elastic constants was presented and prac-tised. The method is to combine longitudinal waves with Rayleigh waves. It avoids to distinguish shear waves which is always not easy to differentiate from others, and it is suitable for measuring in environments where high temperature, ionizing radiation, poor access or rapid movement make the use of contact sensors difficult if not impossible.Different kinds of simulating defects, including interface, surface and volumetric defects, were detected with this system. Meanwhile, the experimental researches for some kinds of Gr/Epoxy composite with laser ultrasonic techniques were carried out. The results showed that it is practical and effective to detect defects with this system.
Keywords/Search Tags:Non-Destructive Testing, Laser Ultrasonics, Confocal Fabry-Perot Interferometer, Rayleigh Wave, Elastic Constant, Detecting defects
PDF Full Text Request
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