Mapping the depth and thickness of delaminations using lock-in thermography: Towards a quantitative tomography

A method to map simultaneously the depth and thickness of delaminations is proposed. It is based on lock-in infrared thermography, which consists in illuminating the sample surface uniformly with an intensity modulated light source and recording the surface temperature evolution with an infrared camera. A 1D model of the heat propagation has been applied since we deal with delaminations featuring realistic slow variations of depth and thickness. We verify the validity of the model by taking data on metallic samples containing calibrated wedge depth and thickness. We focus on narrow delaminations (thickness < 50 μm) with the aim of enabling early failure detection and characterization. The local values of depth and thickness are obtained by performing a pixel-wise fitting of the analytical expression of the surface temperature to the experimental frequency spectra recorded in an interval of sensitive frequencies. As a result, we obtain both depth and thickness maps of the delamination, which agree very well with the geometrical calibrated values.