Laser lock-in thermal wave imaging for nondestructive evaluation

This paper presents a new laser lock-in thermography (LLT) technique for nondestructive evaluation. LLT utilizes a modulated continuous wave laser beam as a heat source to obtain high fidelity thermal wave images even at the presence of background heat disturbances. The thermal waves propagating along the surface and through-the-thickness directions of a structure are visualized using newly developed laser lock-in amplitude and images, enhancing the detectability of surface and subsurface defects. The LLT technique is numerically investigated and experimentally validated using thermal images obtained from a steel specimen with low emissivity.