Investigation on the suitability of laser-excited thermography to detect cavities in metallic components

Active thermography has become a widely used technique in nondestructive material testing and numerous excitation sources are applied. One field of application is the testing of joints. Especially the detection of defects in welds is a challenging tasks as two-dimensional defects like cracks can occur as well as volumetric inner defects as cavities. Recently, the suitability of lasers as a contactless excitation source has been investigated and it was shown, that cracks can be detected. As the specimens are locally heated, a heat flow is initiated. Those heat flows can be used to detect crack-like defects that are perpendicular to the surface. However, the suitability of the laser-excited thermography to detect inner defects like voids and the influence of geometric as well as excitation parameters on the test result has not been fully investigated. Therefore, this study focus on the detection of inner defects in metallic components. In a first step, thermographic measurements with varying excitation parameters were performed. The obtained results were then used as a basis for the numerical simulation of the testing process. A parametric FE-model has been implemented and the influence of excitation as well geometric parameters on the testing results were determined. Different approaches to evaluate the resulting surface temperatures were considered as well. The simulation results were then compared to the experimental data that was obtained by testing idealized specimens with cavities. Based on these results, the most relevant parameters are determined and limiting factors resulting from the experimental setup are discussed. (author)