Depth estimation of surface cracks on metallic components by means of lock-in thermography

In this work, a new method to characterize vertical cracks by lock-in thermography is presented. The heat transfer process induced by a modulated thermal excitation located in the vicinity of the crack is simulated using a finite element method computer package. The propagation of heat flow along the solid surface is disturbed when crossing an inhomogeneity. The disturbance of the thermal-wave allows a quantitative analysis of the crack. The main idea consists of exploiting the second derivative of the amplitude image in order to highlight the useful signal. In addition, an image analysis procedure based on the use of Laplacian calculations is proposed. To support this approach, experimental tests were performed and compared with mathematical simulations. The results demonstrate the potential of active lock-in thermography as a contactless tool for crack-depth estimation.