Electrical approach to defect depth estimation by stepped infrared thermography

An electrical modelling and analysis is presented for thermal nondestructive evaluation of materials by stepped infrared (IR) thermography. A one-dimensional (1D) electrical analysis based on the Laplace transform technique of network-analysis and time delay in an RC ladder network is given for defect depth estimation. Defect depth is evaluated based on the time instants at which surface temperature evolution over the defect and nondefect regions of the material deviates from its constant initial slope, corresponding to the response of a semi-infinite material under similar conditions of step heating. The method is applicable even for three-dimensional geometries of materials having unknown thermal properties and variable surface emissivity. Experimental and simulated results validate the proposed method and give good estimation of defect depth, even under noisy conditions for a thermally anisotropic material and a nonflat bottom type of defect.