Non-destructive Evaluation of Composites Materials by Pulsed-phase Thermography: Depth Inversion

This work is focused in the application of Pulsed-Phase Thermography (PPT) to non-destructive evaluation of composite materials. In this approach, the composite sample is heated by a thermal pulse briefly and recording the surface temperature decay with an infrared camera. Data processing and depth inversion is carried out by extracting and analyzing the response of the sample on the frequency domain, based on the frequency spectrum available in the thermal stimulation pulse. The extraction of the various frequencies is performed with a discrete one-dimensional Fourier Transform on each pixel of the thermogram sequence. The depth inversion technique relies on the thermal diffusion length equation in a similar manner as in Lock-In Thermography. Thus, the inversion problem reduces to the estimation of the blind frequency, i.e. the limiting frequency at which a defect at a particular depth presents enough phase contrast to be detected on the frequency spectra. As it will be showed, one of the interesting aspects of PPT is the phase image, which is relatively independent of local optical (e.g. non-uniform heating) and infrared surface features (e.g. variability in surface emissivity). This represents important advances when compared with other techniques, such as Pulsed Thermography.