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

ABSTRACT

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.

KEYWORDS:

• Infrared thermography
• lock-in thermography
• non-destructive testing
• crack depth
• crack sizing
• laser heating

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

Christine Boué

Christine Boué received her Ph.D. degree in electrical engineering in 1991 from the University Pierre et Marie Curie in association with the Ecole Supérieure de Physique et de Chimie Industrielles, Paris France. During her Ph.D. thesis, she studied the distribution of space charges in piezoelectric polymers. She is currently an Associate Professor at Sorbonne Université (France) since 1992 where she teaches electronics, microcontroller interfacing, signal processing. Her research topics concern thermal imaging and non-destructive testing by synchronous infrared thermography.

Stéphane Holé

Stéphane Holé, born in 1968 at Pontoise (France), studied electronics and instrumentation at Université Pierre et Marie Curie-Paris6 (Paris, France). He joined Laboratoire d'Électricité Générale of École Supérieure de Physique et de Chimie Industrielles (Paris, France) to study an instrument for measuring fast development of space charges in insulators under rapid voltage variations. It was the topic of his PhD he received in 1996 and he obtained his Habilitation in 2007. Currently Professor at Sorbonne Université, he conducted his researches with Laboratoire des Instruments et Systèmes d'Ile de France from 1997 to 2007 and leads the Instrumentation Group with Laboratoire de Physique et d'Étude des Materiaux since 2007. His main research topics are space charge in insulators and semiconductors (main topic), non-destructive testing and sensors. He teaches solid state physics, electronics and sensor physics. He is coordinator of the sensors, instrumentation & mesurements master at Sorbonne Université since 2009.