New 3D finite difference method for thermal contrast enhancement in slabs pulsed thermography inspection

Abstract

The Finite Difference Thermal Contrast (FDTC) is a new technique based on the approximation to the discretization of the Fourier heat propagation model in 3D, in order to be applied on a sequence of infrared images to enhance contrast for automatic detection and characterization of flaws in composite slabs. This contrast enhancement is performed by the calculus of relative error between predicted and real temperature over the heated surface only and for each pixel, in such a way that defective regions will exhibit greater errors than sound ones. Thermal sequences from a simulated Carbon Fiber Reinforced Plastic (CFRP) slab with air-filled defects, and from a real CFRP slab sample with Teflon squared defects, are used to evaluate and compare the enhancement obtained from FDTC, Normalized Contrast (NC) and Modified Differential Absolute Contrast (m-DAC). In spite of the need of executing an additional background compensation in case of real slabs, results show that the proposed technique offers a better contrast between defects and background than the other techniques (about 33 % less residuary thermal non-uniformity with the adjusted version—FDTCa), mainly because of the more energy of the resulting thermal profiles. Also, as this technique does not estimate the temperature distribution along depth axis, but approximates temperature after a spatial step only, it can run faster than other thermal reconstruction methods like the classic 3D thermal filtering.