Non-destructive evaluation of composites materials by pulsed-phase thermography : depth inversion

This work is focused in the application of PulsedPhase Thermography (PPT) to non-destructive evaluat ion of composite materials. In this approach, the compo site sample is heated by a thermal pulse briefly an d recording the surface temperature decay with an infrared camera. D ta processing and depth inversion is carried out by extracting and analyzing the response of the sample on the fre quency domain, based on the frequency spectrum avai lable in the thermal stimulation pulse. The extraction of the va rious frequencies is performed with a discrete onedimensional Fourier Transform on each pixel of the thermogram s equence. The depth inversion technique relies on t he thermal diffusion length equation in a similar manner as in Lock-In Thermography. Thus, the inversion problem r duces to the estimation of the blind frequency, i.e. the limitin g frequency at which a defect at a particular depth resents enough phase contrast to be detected on the frequency spec tra. As it will be showed, one of the interesting a spects of PPT is the phase image, which is relatively independent of loc al ptical (e.g. non-uniform heating) and infrared surface features (e.g. variability in surface emissivity). This repr sents important advances when compared with other techniques, such as Pulsed Thermography.