Contrast enhancement of nonlinear photothermal radiometry in composite materials

Great potential has been detected in nonlinear photothermal radiometry. Experimental (Rajakarunanayake & Wickramasinghe 1986) and theoretical (Gusev et al. 1992, Gusev et al. 1993, Mandelis et al. 1994) studies have demonstrated the higher defect contrast, when detecting the second harmonic of the temperature signal with respect to the modulation frequency of the external heat source. When temperature variations inside the material are large, thermal properties can not be considered constant. This fact is simply one of the factors that render the heat diffusion nonlinear. Based on an experimental study (Kalogiannakis et al. 2002), which determined the aforementioned dependencies, a finite element model, simulating the temperature distribution of the one-dimensional transient problem, was developed using modulated thermal excitation. When a defect is introduced, spectral analysis shows that the second harmonic of the surface temperature signal is affected much more than the fundamental frequency component. When treating highly nonlinear materials like composites, the phenomenon is enhanced, demonstrating the potential of the photothermal method. Carbon fiber reinforced epoxy is widely employed in advanced constructions and its thermal properties exhibit quite strong dependence on temperature. Therefore, it was selected to verify the validity of the assertion.