Microwave induced time-resolved infrared radiometry for subsurface defect detection and characterization

The use of microwaves as a heating source in time-resolved infrared radiometry provides the ability to heat surface and subsurface microwave-absorbing regions of a specimen directly. This can improve the contrast and spatial resolution of such regions and enhance their detectability when compared with conventional laser or flash lamp sources. Applications such as subsurface water detection or the detection of carbon fiber contaminates in epoxy composites are investigated experimentally and analytically. Due to recent technical developments in the speed, detector array size, and sensitivity of infrared cameras, time resolved infrared radiometry has become an important NDE tool which allows fast area inspection at high spatial resolution. While much prior work has focused on the detection of structural defects or disbonds in a variety of materials, the increasing importance of composite materials requires new approaches to inspection which allow characterization of local material properties. Defects in such materials may have little thermal contrast compared to the matrix material and may be invisible using conventional TRIR methods. However, where the embedding material is a weak microwave absorber, localized microwave absorbing regions can be detected easily. There are three different classes of absorption processes: (1) dielectric loss (e.g. water), (2) magnetic loss, and (3) Joule heating (e.g. electromagnetic radiation interaction with conducting fibers). Examples of Processes 1 and 3 are presented in this paper. All experiments use an HP 6890B Oscillator (5-10 GHz) to produce microwaves at a frequency of 9 GHz. This signal is amplified to a maximum power of 2.3 W by a Hughes 1277 X-band traveling wave tube amplifier and fed into a single flare horn antenna through rectangular wave guide. The antenna has a beam width of about 50 deg. and is placed about 15 cm from the sample. Both the angle of incidence and the polarization of the microwave field relative to the sample are controlled. A 128x128 InSb focal plane array (Santa Barbara Focalplane) operating in the 3-5 pm band is used for detection of the IR radiation. The camera has a temperature resolution of about 3 mK and a frame rate as fast as 305 Hz or 3.3 ms per frame. The frame synchronization pulse of the infrared camera triggers the microwave oscillator and the sample temperature is monitored as a function of time during the microwave pulse. This allows longer observation times with low power input and hence small temperature rises, as in time resolved infrared radiometry with optical heating [1,2]. The fist example focuses on dielectric heating of regions of subsurface water. Water-based defects include blisters in fiberglass or composite structures, or under coatings and paints as a source of corrosion. One example which is shown in Fig. la is a disbonded epoxy coating on a steel pipe. Infrared radiometry with optical heating is not very successful with these kinds of defects since the thermal effusivity of water is well matched to the steel and hence shows little thermal contrast in a surface temperature image. In the microwave regime, water has a broad strong absorption band associated with a maximum in the dielectric relaxation near 18 GHz and there is strong absorption at 9 GHz. Fig. lb shows an infrared image of the epoxy-coated steel pipe heated with a 15 s microwave pulse when the disbond is filled with water. The image outlines the disbonded region with no heating of bonded regions showing high contrast at the water defect as compared to optical (surface) heating where the whole sample would be heated. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19947132 C7-564 JOURNAL DE PHYSIQUE IV Figure la (left): Disbonded epoxy coating on a steel pipe. Figure lb (right): Infrared image of the water filled disbond after a 15 s microwave pulse. The time-dependence of the surface temperature can provide information about the subsurface region. A second sample shown in the insert in Fig. 2 is made from Plexiglass with voids filled with water placed at different depths below the surface using Teflon spacers. The time dependence of the relative surface temperature of points is plotted in Fig. 2 for each thickness . For a 3-layer system with thermal diffusivities a i , thermal conductivities q and layer thicknesses d and L and with an absorption coefficient p in the second layer, the surface temperature vs. time during and after a microwave pulse of duration to (one-dimensional heating) is: with G(h,x,t) = H o h ~ e " + h a a o l e r f c { ~ ~ ] d ~ . (2)