Defect localization in plane composite: a non intrusive automated procedure based on active thermography

Abstract : This communication is focused on the implementation of a local pulsed thermography method in order to locate a defect in a plane composite. For such a local approach, the investigated plane composite is periodically heated on a small surface (several square centimeters) and thermal waves propagation is observed up to three thermal diffusion length on the same material surface (in reflexion). Both modulus and phase lag of the measured periodic signal are modified by the defect neighborhood and the seek for the most effective area leads to the defect localization. The contrast between the composite thermal behavior with or without defect is a relevant tracker of the defect proximity. Several criteria are proposed in order to quantify the contrast. Issued from cartographies differences, they are based upon usual functional norms and do not induce the same sensitivity to defect neighborhood. A simplex method is proposed for the automated procedure leading to the defect localization. Such method is based on an iterative process in order to explore the material surface (using an infrared camera) considering the investigation of new points (potentially better candidates for the defect location). New point coordinates are calculated from the previous points which are weighted considering the above mentioned criteria. Experimentations are performed according to the following steps : heat flux calibration, reference measurements, heat source shifting for automated scan.