Electro-thermal modelling and analysis for estimation of defect parameters by stepped infrared thermography

This paper exploits a resistance-capacitance (RC) model of transient heat conduction for estimation of defect depth, and more significantly of the defect area, from single point surface temperature evolutions over the defect and non-defect regions of material subjected to step heating. For the initial time behaviour of the surface temperature, a distributed RC model has been presented and analysed for estimation of defect depth and area. In contrast, a lumped RC model forms the basis for obtaining an analytical expression for defect area from the later part of the surface temperature evolution. The derived analytical expression estimates the defect area from the defect depth, the saturation value of the absolute thermal contrast and the slopes of the initial and final surface temperature evolution. The presented method does not require knowledge of material thermal properties or the value of the incident heat flux for estimation of defect parameters. The method has been validated by estimating the parameters of defects of various sizes and at different depths from experimental and simulated surface temperature evolutions.