Thermal Imaging Methods

Abstract Thermal imaging methods, more particularly infrared (IR) thermography, are commonly used techniques in nondestructive evaluation. They are also beginning to be used in experimental mechanics. They could be very useful for structural health monitoring purposes, although they cannot be totally embedded in the structures. Two main approaches exist for using these techniques as structural health monitoring tools. The first approach is based on the fact that all mechanical phenomena are accompanied by correlated thermal effects (thermomechanical coupling). These effects can be reversible or irreversible. The reversible thermal phenomena are a link to the strain (and stress) state of structures. Generally, irreversible phenomena are the signature of the occurrence of damage. Mapping the thermal state of a structure is a means to detect abnormalities in the mechanical behavior of the structure. The second approach comes from classical nondestructive evaluation: damages are detected, localized, and characterized by the disturbances they introduce in the local thermal properties (heat conduction, heat diffusion, heat capacity, effusivity, interface thermal resistance) of the structure. The physical principles of IR radiometry and the state of the art IR thermography are recalled. The analysis of the thermographic process shows how to perform a quantitative measurement, a requisite for the identification of structural health and the characterization of damages. The different ways to perform structural health monitoring with IR thermography are finally detailed: diagnostic elaboration based on a thermomechanical analysis (strain mapping and fatigue detection), and damage detection and characterization by stimulated thermography.