Application of infrared thermography to adhesion science

Abstract Infrared thermography (IRT) is a non-contact, non-intrusive technique which converts the invisible thermal energy, radiated from the surface of an object in the infrared band of the electromagnetic spectrum, into a video signal, each energy level being generally represented by a color or a gray level. IRT has been considered an exciting scientific breakthrough since its introduction in the early 1960s. Indeed, the new generation of fully-computerized infrared imaging systems can provide both qualitative and quantitative measurements which are useful in many industrial and research fields. Nevertheless, infrared thermography is still not completely exploited. The reason for this lies, in part, in the lack of knowledge, since at first sight IRT seems to be too expensive and difficult to use and, in part, in the industrial inertia to change the routine procedures. The aim of this review article is to provide the reader with a background to infrared theory and with an overview of the most relevant applications of IRT to the adhesion field. The use of IRT as a non-destructive evaluation technique with the two different approaches of pulse thermography (PT) and lock-in thermography (LT) is discussed. Many applications are described which involve several different materials (metals, plastics, plaster, composites, hybrid composites and sandwiches) and different types of bonds (coatings, sandwiches and joints). The results show that both PT and LT are able to detect material modifications caused by surface treatments, presence of inhomogeneities in bulk materials, as well as disbonding, delamination, and cracks and slag inclusions in bonded structures. The LT is also capable of evaluating materials characteristics (e.g., variations in density, porosity, hardness, etc., which induce variations in the phase angle), the dimension of the heat affected zone in welded joints, coating thickness, bondline thickness, the effects of adhesive thickness, the effects induced in bonded structures by substrate surface treatments, and the effects of crosslinking in polymers. The LT technique is particularly advantageous in the evaluation of frescoes, mosaics and antique artworks. The reported applications provide also information which is useful for decision making about the use of IRT alone, or combined with other techniques. Keywords: INFRARED THERMOGRAPHYTEMPERATURE MAPSNON-DESTRUCTIVE EVALUATIONPULSE THERMOGRAPHYLOCK-IN THERMOGRAPHYMATERIAL TREATMENTSBONDED STRUCTURES