Investigation of heat source reconstruction of thickness-through fatigue crack using lock-in vibrothermography

Lock-in vibrothermography (LVT) is an active thermography nondestructive testing technique, which utilizes thermal contrast induced by lock-in modulated vibration to detect surface or subsurface defects. During the LVT testing, only the surface thermal distribution of the specimen can be directly captured. However, in most cases special cares need to be taken about the subsurface thermal distribution and the related heat source reconstruction. In this paper, a direct and inverse problem for vertical heat source reconstruction by using LVT is investigated. Specifically, a 3D transient analytical model is proposed to investigate heat source distribution of a metallic plate with a fatigue crack. The Tikhonov regularization is used to reconstruct the heat source from the analytical/experimental data. Further, H-index is introduced to quantitatively evaluate the reconstructed results. The shapes of reconstructed heat source from both analytical and experimental data are similar to a 'half-penny'. Additionally, the integral operator has a significant influence on the experimentally reconstructed result.