New thermal contrast definition for defect characterization by active thermography

Active thermography is a relatively young method in comparison to the classical, standardized methods of non-destructive testing. Many papers present a possibility of using this method for detection of some types of defects such as delamination, cracks or skin holes. It is important to determine the limitation of this method and possible areas of application. This paper presents an experimental setup for defect detection and characterization lying in the subsurface layers of the tested material by means of an active thermography. The work proposes two versions of a new kind of thermal contrast, called “filtered contrast – FC” and “relative filtered contrast – RFC”, which allow to detect and to characterize defects under some assumptions on the physical and thermal parameters of the tested material and defect. In opposition to the known definitions of the thermal contrast, neither a reference thermogram nor a defect-free area is necessary. Additionally, this contrast is less sensitive to the non-uniformity of the heat disposal over the material surface than classical contrasts. Theoretical considerations are illustrated by experimental results. It has to be underlined that it is a preliminary study for simple two-layer structure.

► Improvement of IR active thermography applied to defects detection is proposed.
► We examine new kind of thermal contrast as thermogram pre-processing technique.
► Knowledge of defect-free area is not necessary for defect characterization.
► Method is resistant to non-uniformity of the heat disposal over the material surface.
► Heating and cooling phases are crucial to material testing by step heating method.