Vertical cracks characterization using lock-in thermography: I infinite cracks

Early detection of cracks is a challenging task to prevent failures in working structures. In the last decades the 'flying spot' method, based on heating the sample with a moving laser spot and detecting the surface temperature with an infrared detector, has been developed to detect cracks in a fast manner. The aim of this work is to measure the width of an infinite vertical crack using lock-in thermography. An analytical solution for the surface temperature of a sample containing such a crack when the surface is illuminated by a modulated laser beam focused at a fixed spot close to the crack is obtained. Measurements on samples containing calibrated cracks have been performed using an infrared camera. A least square fit of the amplitude and phase of the surface temperature is used to retrieve the thickness of the crack. A very good agreement between the nominal and retrieved thicknesses of fissure is found, even for widths down to 1 mu m, confirming the validity of the model.