A study of wood inspection by infrared thermography, part II: Thermography for wood defects detection

Wood poles are among the main components of electrical distribution systems. They have to be replaced every 20-30 years due to wood decay. To reduce costs, utilities need an efficient nondestructive tool to determine the appropriate replacement time. Different techniques exist for this purpose, such as X- or gamma-ray tomography, indentation, and methods based on measurement of electrical conductivity, ultrasonic propagation, or simply bacterial culturing. Since none of these methods satisfy these utilities, it was decided to study in detail infrared thermography (NDT) in this particular context. The hypothesis is that in this particular context, wood decay corresponds to a different moisture content with respect to sound wood. In Part I of the paper the problem of wood pole NDT is analyzed using a dedicated thermal model and three different types of heating: internal through-hole, external, and by microwave. Experiments confirm modeling results: due to large defect depths, low wood thermal diffusivity, and the wood properties dependencies upon temperature, moisture, species, and fiber orientation, infrared thermography (IRT) is not appropriate for this inspection problem unless defects are close to the surface. Discussion of wood thermal properties is also included in Part I. In Part II of the paper, the wood decay inspection problem is revisited in a simpler manner: flat instead of circular geometry and shallower defects. Thermal modeling along with experimental results are presented, and the comparison is encouraging.