NONDESTRUCTIVE THERMAL TESTING AND CHARACTERIZATION OF POLYMERS AND CERAMICS USING OPTICAL BEAM DEFLECTION AND INFRARED DETECTION

Applications of thermal methods to defect detection and thermal diffusivity determination of polymer and ceramic samples are presented. The optical beam deflection (mirage) method was used to determine vertical crack lengths in Si3N4. The measurements and numerical simulations show that the end of a closed crack can be localized within 10 mum. Another application of the mirage method is to measure thermal diffusivities of several medium- and low thermal diffusivity samples, including different ceramics, polymers, and rare earths. Special consideration was paid to the determination of anisotropic thermal diffusivity of polymers. The effect of the experimental mirage measurement parameters was also studied to find out the most suitable parameter values gave the best possible reliability for low thermal diffusivity measurements. The analyzing methods for mirage data have been developed by means of multi-parameter fitting and partial least squares regression. The improvements in the data analysis technique and the measurement set-up allowed the low-diffusivity limit for the applicability of mirage method to the thermal diffusivity measurements to be pushed to 0.0005 cm2/s. Before this study the limit was about 0.02 cm2/s, and thus mirage technique based thermal diffusivity determinations were not suitable for polymeric materials. The thermal diffusivity of bulk polymers can be determined with an accuracy of 10% and the method is reliable for polymer foils if the foil thickness is more than 80-100 mum. A method based on line heating and infrared (IR) detection to determine the anisotropy ratio of thermal diffusivity of polymer foil is also presented. For imaging applications, numerical simulation models have been developed to calculate thermal effects caused by vertical cracks and delaminations. Using these, resolution studies of thermal imaging were made including the first quantitative theoretical estimation of the spatial resolution of the pulsed photothermal non-destructive testing (NDT) system.