Development of a diagnostic method for thermoelastic characterization of coatings using laser ultrasonics

Characterization and evaluation of candidate materials used for structural components in future liquid-metal cooled nuclear reactors is of very high importance. One requirement for an investigation technique is that it is both non-destructive and remote such that it can be used in a radiation shielding hot cell laboratory. Laser excited acoustic and thermal waves have the interesting property that they can deliver information about both the elastic and thermal properties of materials. The aim of this work is to develop and estimate the feasibility of a laser ultrasonic diagnostic technique for the thermoelastic characterization of promising materials andengineered protective measures that are foreseen to be used in future nuclear reactors. In order to accomplish this goal the setups of three generally used all-optical laser ultrasonic are adapted to incorporate the excitation and detection of surface acoustic waves and thermal decay proles on a variety of samples, of which some have an optically rough surface finish. This will allow for the estimation of elastic and thermal properties down to very small wavelengths and very high frequencies.A theoretical approach is presented that calculates, starting from the low-frequency limit of the thermoelastic equation, a thermal dispersion curve for a thermally contrasting coating-substrate system that shows similarity with acoustic dispersion curves. From this thermal dispersion curve, which is easy to access experimentally, the thermal performance of a coating-substratelayer can be deduced.A signal response function is calculated for the heterodyne diffraction setup. With this function, the absolute amplitude of the normal displacement ment component of a surface acoustic wave can be extracted. Moreover, it was shown that a factor two increase in the signal could be achieved when the technique is used in its dierential setup. With a set of experiments on a variety of samples, the feasibility of laser ultrasonics as a supplementary diagnostic technique in the evaluation and characterization of future nuclear reactor material was armed. Prior to this characterization, a theoretic study was performed to investigate the feasibility of the used experimental and signal analyzing methods, i.e. determine whether the inverse problem is solvable. Therefore, the pairwise covariances of the least-squared cost function are examined for all the parameters that enter the inverse problem. The possible existence and shape of a minimum region allows to determine the existence and width of the interval of condence on the parameters involved.In the last part of the work, it is shown that, besides the characterization of materials, laser ultrasonics can be applied in the detection, localization and characterization of heterogeneities in materials that can pose a restriction on the integrity and durability of components. An empirical experimental and theoretic relation between the parameter that represents the diffraction of an incident acoustic eld on a crack in a plate and the vertical length of the crack is presented.