Comparison between multi-frequency and multi-speed laser lock-in thermography methods for the evaluation of crack depths in metal

Two original methods using lock-in thermography with a laser excitation are proposed for the estimation without contact of open crack depths in metal. The first uses a modulated punctual thermal source and is well suited for the study of complicated structures. In the second, a continuous moving thermal source allows to scan homogeneous structures. Each method relies on the heat diffusion modifications induced by a crack located in the thermal diffusion area of the synchronised heat source. The thermal signature of the crack is extracted to the amplitude of surface temperature images for various modulation frequencies or various scanning speeds of the thermal source. The thermal signatures are analysed according to a length representative of the thermal diffusion length to give a local evaluation of the crack depth. The obtained results demonstrate the potentiality of active lock-in thermography as a contactless measurement tool for the evaluation of crack depths up to 3 mm.