Thermal destructive evaluation using microwave sources

The development of thermographic nondestructive evaluation techniques using microwave heating methods is described in this paper. Microwave heating provides unique capabilities for selective heating of defects and for the deposition of heat at depth in the specimen. This can improve the spatial resolution, sensitivity, and speed of the thermal inspection process relative to conventional thermographic techniques with optical heating. Analysis of the expected time-development of the surface temperature is presented for a series of specimen geometries and supporting experimental results are given. Systems studied include a test specimen with water at a series of different depths, a disbonded coating with water at the interface, a composite specimen with embedded carbon fibers, and a composite specimen which had experienced impact damage. The dependence of the thermographic response on the polarization and angle of incidence of the microwaves is examined. Quantitative information about the depth of defects and about the thermal properties of the specimens is obtained from analysis of the time-dependence and spatial-dependence of the temperature distribution during microwave heating. The results presented here provide the groundwork for the development of embedded sensors for process control which can be remotely activated by microwaves and remotely interrogated by infrared radiometry methods.