Computed tomography using laser-based ultrasonics

Computed tomography incorporating a complete laser-based ultrasonic (LBU) technique has been developed. A computer controlled scanning system has been employed to collect the transmitted and/or reflected Lamb waves along different directions across the area being probed, to construct tomographic images of defects. An advantage of this technique is that it can image an area of interest in an object without observing the area directly. Applications of the system to non-destructive evaluation are investigated in this dissertation.;In this study, ultrasound is generated with a pulsed laser and detected with a dual-probe heterodyne interferometer or its fiberized version. In the dual-probe interferometer, the reference beam is guided to a second detecting point on the specimen surface to measure the difference of the displacements at the two points. The interferometer is particularly useful for accurate measurements of the speed and attenuation of surface and Lamb waves.;The use of narrow-band signal generation techniques has also been studied in order to improve the sensitivity of the system. In this work, a holographic diffraction grating has been used to form an array of illuminated strips which form a source of narrow-band surface and Lamb waves. The line-array of thermoelastic sources generates narrow-band signals whose frequency can be easily controlled. It has been verified by the close agreement between the generated narrow-band Lamb waves with theoretical dispersion curves that the spacing between the illuminated lines produced by the diffraction grating generates a "forcing" wavelength for which only a few specific frequencies can propagate.;The computed tomography system with dual-probe detection and narrow-band signal generation has been successfully applied to the imaging of through-the-thickness holes, subsurface defects and cracks. Using the dual-probe heterodyne interferometer, transmitted and reflected Lamb waves are detected as they pass through the two detecting points. The attenuation and reflection coefficients of the Lamb waves have been calculated and tomographic images have been calculated by a filtered back projection method. It has been demonstrated that reflected waves are very useful for obtaining tomographic images of cracks.