Vertical cracks characterization using lock-in thermography: II finite cracks

The aim of this work is to characterize vertical cracks of finite size and arbitrary shape using optically excited lock-in thermography. In the first place, we have solved the direct problem, which consists of calculating the surface temperature distribution when the shape, size and width of the vertical crack are known. To do this we have developed a new method based on discontinuous finite elements, which allows one to deal even with very narrow cracks, for which classical finite element methods fail. The surface temperature of steel samples containing semi-infinite cracks and illuminated with a laser beam focused close to the crack has been measured using a lock-in thermography setup. A least square fit of the amplitude and phase of the surface temperature is used to retrieve the width and depth of the semi-infinite crack. A very good agreement between the nominal and retrieved values of both parameters is found, confirming the validity of the model.