Reconstruction Of Thermal Conductivity Depth Profile Of Non-flat Metal Surface Using Photothermal Radiometry

Thermal conductivity is a very important parameter for characterizing the thermophysical properties of material and thus its measurement technology has been an important research field. Photothermal radiometry technique, a novel tool for noncontact and nondestructive thermophysical characterization of a wide variety of materials, has been used to reconstruct the thermal conductivity depth profiles of the case hardened flat samples. In this thesis, the reconstruction of depth profile of thermal conductivity of case hardened non-flat samples using photothermal radiometry is investigated. Specifically, we propose and develop a normalization procedure to suppress and/or eliminate the curvature effect of the non-flat samples. Under this procedure, the normalized photothermal field of non-flat samples is found, under certain conditions, to be approximately equivalent to that of the flat samples with the same thermophysical structure. This equivalence suggests that the reconstruction of a non-flat sample can be performed by the theoretical model of a flat sample, which breaks the limitation conventionally encountered in characterization of a non-flat sample. The effects of the radii of curvature of the curvilinear solid, the thickness of the case hardness and the measurement azimuthal angle on the validity of the technique are discussed in detail. Finally, experimental reconstructions of thermal conductivity depth profiles of a set of hardened cylindrical steel rods with various diameters are performed based on theoretical model of either cylindrical or flat samples. The reconstructed results based on flat theoretical model are consistent to those based on curved cylindrical model, and are all consistent to that obtained with conventional destructive indent test results.