Thermal Wave Techniques for Imaging and Characterization of Materials

Abstract

Thermal wave imaging is proving to be a useful teehnique for the nondestruetive evaluation (NDE) of subsurfaee features of opaque solids. This imaging is aehieved with various intensity-modulated heat sources, such as laser or partiele beams, and with various detectors, sueh as microphones, ultrasonic transducers, inErared detectors, and laser probes. The authors have recently reviewed these techniques and their application to t\DE (1). Common to the techniques is the fact that they eaeh involve the interaction of a highly damped the nnal wave wi th sur face or subsur face therma 1 fea ture s. They al so have in coml:lOn tlle fact that the souree 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 telnperature detector; the mirage effect laser probe is a line de tec tor; and tlle mic ro phone i s an area detec tor. The prese.nce of the localized source gives alI of these methods tlle potential for high spatial resolution. The symmetry oE 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 eracks, cylindrical or spherical inclusions) are straightforward, and good agreement has been achieved in most cases. Other schemes, such as p iezoe lec t r ic de tec t ion, are lIIore complex in na ture. For exaOl ple, the details of the conversion Erom thermal energy to acoustie energy may involve several different processes, and no quantitative three-dimensional theoretical lIIodel yet exists to assess the relative importance of these processes. In this section we give a briei review of the principles of imaging in tlle extreme near field, followed by deseriptions of three splected experimental thermal wave imaging techniques, including geometrical considerations, signal-to-noise considerations, and examples of NDE applieations. For descriptions of other thermal wave imaging teehniques the reader is referred to the literature (1).TIle resolution of any microscope depends on its ability eitller to localize tlle illumination at tlle scatterer (typical of scanning microseopes) or to localize tlle region of the seatterer to whieh the detector is sensitive (such as in conventional microscopes, whieh focus different points of the scatterer on different detectors). In microscopes which usp lenses as focusing elements to achieve this localization, t!te localization is limited by the wavelengtll of tlle 293

Nondestructive testing with thermography

“…• lock-in thermography (LT) [9][10][11], in which periodic heating at a given frequency is used in steady state to measure the amplitude and/or phase delay of the thermal response; • PT [12][13][14], in which a short (a few milliseconds) energy pulse is utilized to heat the inspected object, surface temperature is monitored under the principle that defective areas cool down (or heat up) at a different rate than non-defective areas;…”

Quantitative Thermal Wave Characterization of Coating Adhesion Defects

“…However, these frequencies are impractical for experimental work. The detection methods do not work well below 1 Hz when the usual lock-in analyzer technique is used [11], and also the measurement time increases in scanning experiments, because the balancing of the signal takes a few heating cycles at each measurement point [12]. e.. Fig.…”

An experimental and theoretical study of heat transfer effects during a laser-cutting process

“…19 shows the mass flow rate out of the nozzle at diflferent pressures of the gas upstream of the nozzle. The mass flow rate increases with increasing stagnation pressure, from a value of 1321.6 x 10"~® to 2377.5 x 10~® kg/s.…”

“…One category is the use of pulsed lasers, which provides a rapid thermal transient by local heating of a sample surface. This method has been used successfully to measure flaw location and flaw size [19,50,9]. The disadvantage of this method is that in its present form it can only be used to measure flaws very near the surface (e.g., submicron to a few mm) of a material.…”