Pulsed laser irradiation of the surface of a medium can produce temporally and spatially distributed thermal stresses and deformations that can be exploited for enhanced detection of small fatigue cracks. Two such enhancement methods were investigated in this research project. Fast, using long-pulse laser irradiation in combination with conventional Rayleigh wave inspection, direct generation of ultrasonic Rayleigh waves can be avoided and the relatively slow laser induced crack closure can be detected as a parametric modulation effect in a manner similar to the acousto-elastic effect often used in nonlinear ultrasonic studies. This technique is particularly well suited to distinguish small fatigue cracks from nearby scattering artifacts, such as machine marks, mechanical wear, corrosion pits, etc., that could otherwise overshadow the flaw. Second, short-pulse irradiation in the thermo-elastic region, that is routinely used to generate ultrasonic Rayleigh waves, can be substantially enhanced when the irradiated area contains near-surface discontinuities. Using an expanded laser beam, direct generation of ultrasonic surface waves from intact areas can be minimized and a significant increase in amplitude occurs when a discontinuity is present in the irradiated area. This technique is better suited to distinguish small fatigue cracks from distributed material noise caused by the surrounding inhomogeneous microstructure, such as coarse grains, grain colonies, precipitations, and anomalous phases.; Numerous new experimental techniques combining laser irradiation with ultrasonic detection methods have been developed based on these two inspection principles. During the development of these techniques, in order to better understand and optimize them, several simplified models were built, numerical simulations were performed, and related optical, thermal, mechanical, and acoustical phenomena were also investigated. They include: (i) temporal and spatial distributions of temperature and thermal stress in the specimens due to repetitive long-pulse laser irradiation, (ii) photo-thermo-elastic crack closure behavior in 3-D, (iii) thermally induced refraction effects on ultrasonic Rayleigh wave propagation, and (iv) relations of the enhanced laser generated ultrasonic Rayleigh waves (amplitude, spectrum, directivity) to discontinuity parameters. This dissertation contributes to three aspects of laser-ultrasonic nondestructive evaluation (NDE), namely new enhanced inspection techniques, experimental database, and original and improved analytical models.
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