A procedure for detecting hidden surface defects in a plate from real thermal data by means of active thermography

Let $\Omega_\epsilon$ be a metallic plate whose top inaccessible surface has been damaged by some chemical or mechanical agent. We heat the opposite side and collect a sequence of temperature maps $u^\epsilon$. Here, we construct a formal explicit approximation of the damage $\epsilon\theta$ by solving a nonlinear inverse problem for the heat equation in three steps: (i) smoothing of temperature maps, (ii) domain derivative of the temperature, (iii) thin plate approximation of the model and perturbation theory. Our inversion formula is tested with realistic synthetic data and used in a real laboratory experiment.