Thermal Wave Imaging of Surface Cracks in the Linear and Nonlinear Mode

The influence of nonlinearities in photothermal radiometry due to the static temperature distribution and Stefan-Boltzmann law are analyzed with respect to crack detection by thermal wave imaging. Nonlinearities in the harmonic signal as well as the generation of second harmonic signals are studied theoretically and experimentally. The experiments are performed on indenter-induced cracks in zirconia/titanium carbide ceramic and in Ni-Zn ferrite blocks forming thin gaps. The contrast of the cracks in the termal wave images is generally improved by the nonlinearities