Scanning thermography is a noncontact, nondestructive technique that makes it possible to find defects hidden inside structural components in a variety of settings. Scanning thermography can be used to perform inspections of objects that may have large areas and a variety of shapes and that are found in a variety of settings that include, but are not limited to, production lines, industrial tanks and pipes, aircraft, power plants, and bridges. Scanning thermography is applicable to diverse structural materials, including metals, plastics, laminated polymer-matrix composites, and bonded aluminum composites, to name a few. Defects that can be detected by scanning thermography include cracks, disbonds (delaminations), corrosion, and wear.
In scanning thermography, both a source of infrared radiation and an infrared camera are mounted together near an object to be inspected and are kept at constant distance from the object while they are moved together along the surface of the object. The infrared camera images the temperature of the region behind the moving source. The output of the infrared camera is digitized and sent to an image-data processor, which computes spatial variations of temperature across the imaged portion of the surface area. Spatial variations of temperature indicate spatial variations of heat capacities. Because portions of the object that contain damage, corrosion, or delaminations are thinner, and therefore have a reduced thermal mass, relative to portions that do not contain such defects, they exhibit corresponding differences in temperature. The resulting temperature map can be examined and/or the digital output of the image-data processor can be processed further to diagnose the structural degradation.
A scanning thermographic apparatus (see figure) is highly portable. It can scan the surface of an object to be inspected at a rate about six times that of a conventional thermographic apparatus. More specifically, it can scan at a speed that can be varied up a maximum of >6 ft/s (>1.8 m/s). This is fast enough that power-plant boilers, for example, are now being inspected by scanning thermography in a fraction of the time needed for inspection by prior techniques.
Improvements in efficient utilization of the thermal source have increased the signal-to-noise ratio, significantly improving the quality of the postprocessed image data. Compilation of the image data provides a comprehensive archive — an inspection record that can be reviewed over time to provide a means of monitoring the evolution of damage within a particular structure.
This work was done by K. Elliott Cramer and William P. Winfree of Langley Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.