This integration of both EM and optics generates real-time images with high sensitivity, since the optical rotation angle is proportional to the local small electromagnetic field. There is no doubt that multi-wave imaging and hybrid imaging are emerging as solutions to those SHM/NDE challenges.3.?Multi-Wave Phenomena and ImagingFor SHM and NDE applications, various techniques based on the multi-wave phenomena have been proposed, studied and developed, being successful as detection methods, although with limited success in quantitative imaging, such as electromagnetic-thermal methods, electromagnetic-ultrasonic methods, ultrasonic infrared thermal wave methods, photo-thermal methods and photo-acoustic methods, to name a few.

This section provides a comprehensive review on the multi-wave methods for SHM/NDE and discusses their advantages and limitations, as well as the hurdles and the potential for future development.3.1. Electromagnetic-Thermal MethodsElectromagnetic-thermal (EM-T) nondestructive inspection has been proposed as an alternative to the classical eddy current (EC) testing for just more than a decade [7]. This technique, also known as eddy current thermography [8�C11], tone burst eddy current thermography (TBET) [12�C14], thermo-inductive inspection and induction thermography [15�C17], combines electromagnetic illumination of the work-piece, heating up of the material by induction and imaging by transient infrared thermography to provide a fast and efficient method for defect detection and material characterization over a relatively large area.

Thermographic images picked up by an infrared (IR) camera can be evaluated to figure out the major defects, and the data can be further processed to provide quantitative information about defects. Pulsed eddy current (PEC)-stimulated thermography by combining PEC and thermal cameras has also been investigated recently as one of the electromagnetic-thermal methods [18�C20]. Batimastat The method injects a short pulse of current (typically less than 1 s) with high intensity into the samples under test and then obtains images from an infrared camera. Since the broadband eddy current can penetrate deep into the conductive materials, EM-T techniques can detect both surface and subsurface anomalies, even the hidden defects in complex components.

In 2006, Oswald-Tranta and Wally modeled the eddy current distribution inside the material and investigated the temperature distribution around a crack with different penetration depths using finite element modeling (FEM) and experiments with metallic materials [15]. For a surface crack with a depth of 1 mm, the calculated temperature distributions around the surface crack are depicted in Figure la,b for different penetration depths of 1 mm and 0.1 mm, respectively.