Thermal Wave Techniques for Imaging and Characterization of Materials

Abstract

Thermal wave imaging is proving to be a useful technique for the nondestructive evaluation (NDE) of subsurface features of opaque solids. This imaging is achieved with various intensity-modulated heat sources, such as laser or particle beams, and with various detectors, such as microphones, ultrasonic transducers, infrared detectors, and laser probes. The authors have recently reviewed these techniques and their application to NDE [1], Common to the techniques is the fact that they each involve the interaction of a highly damped thermal wave with surface or subsurface thermal features. They also have in common the fact that the source is localized. The techniques differ in that the detectors may be local or non-local to a greater or lesser degree. For example, the focused infrared detector is a local point temperature detector; the mirage effect laser probe is a line detector; and the microphone is an area detector. The presence of the localized source gives all of these methods the potential for high spatial resolution. The symmetry of the non-locality of the detector, however, may seriously limit detection of particular kinds of flaws. For some of the detection schemes, comparisons between experiment and theory for imaging of flaws with simple geometry (planar cracks, cylindrical or spherical inclusions) are straightforward, and good agreement has been achieved in most cases.