Reconstruction Of Depth Profile Of Thermophysical Properties Of Continuously Inhomogeneous Media Using Photothermal Technique

Photothermal radiometry techniques (PTR) have been proven to be a powerful tool for characterization and nondestructive evaluation (NDE) of thermophysical properties of various materials. The surface structure of a case hardened sample is an inhomogeneous layer with continuously varying thermophysical parameters from the surface to the unhardened core part of the sample. To retrieve the depth profile of the thermophysical parameters of the inhomogeneous hardened layer, several"inverse"algorithms have been developed and demonstrated. In those studies, a 1-D measurement scheme must be employed in which the incident beam was assumed to be large enough when compared with the thermal diffusion length in the frequency range of interest, so as to simplify the mathematical algorithm. In this thesis a new method of retrieving thermophysical depth profile of continuously inhomogeneous materials is presented both theoretically and experimentally using photothermal radiometry technique. A 3D theoretical model which is suitable for characterizing materials with continuous varying thermophysical parameters of arbitrary depth profile is developed. A numerical fitting algorithm to retrieve the thermophysical profile was demonstrated with three case hardened steel samples, which are found to be in good agreement with the hardness profile obtained with conventional indent method.