Laser ultrasonics in finite structures: Comprehensive modelling with supporting experiment.

摘要

Laser ultrasonic investigations in the fields of materials characterization and nondestructive evaluation have demonstrated the utility of the laser ultrasonic technique for studying the elastic properties of materials in a variety of systems. Laser ultrasonics includes those techniques which use lasers to generate and to detect ultrasonic disturbances in a material. These techniques allow remote testing of materials systems in environments where other ultrasonic testing techniques might yield unsatisfactory results. The interpretation of the ultrasonic displacements produced by laser excitation of a material is difficult owing to the involved nature of the laser ultrasonic source. To extract meaningful information about the material from laser ultrasonic displacements, the nature of the source must be understood. In this work, the theory of electromagnetic wave propagation and the theory of thermoelasticity for anisotropic materials are developed as a basis for modelling the thermoelastic laser source in anisotropic systems. The fully coupled equations of thermoelasticity for both the classical and the temperature-rate-dependent theories are derived and the methods for solving these equations are examined. The laser source at the interface of an isotropic half-space is examined for conductive materials in which the penetration of optical energy into the bulk of the material occurs. In the case of infinitely strong optical absorbtion, the laser source is expressed in terms of a surface, heating source. Under special conditions, the laser source may be represented by an equivalent, elastic boundary source which is used to predict the laser ultrasonic displacements in an infinite, isotropic plate. These displacements are calculated using a Laplace-Hankel transform solution technique in which the solution transforms are numerically inverted directly. The calculated displacements were in excellent agreement with experimentally measured displacements in plates of varying thicknesses. The effect of source size on the plate displacements was predicted theoretically and was verified experimentally for epicentral waves, including the precursor, for surface waves and for Lamb waves. The experimental investigations included the design and use of the skewed-stabilized Michelson interferometer which allowed quantitative comparison between theoretical and experimental results.