Thermal Wave Imaging For Characterizing Structures In Aging Aircraft
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Fig. 1(a) Imaging head, showing IR focal plane array Fig. l(b) View inside the shroud, camera (left), and the aluminum shroud upon which it showing the two flashlamps and the IR is mounted, camera lens. In practice, our aircraft version of a thermal wave imaging system uses pulsed heating (approximately 5 ms in duration) generated by two linear photographic flashlamps which are mounted in an aluminum shroud to direct the light uniformly onto the aircraft (see Fig. 1). An infrared (IR) focal plane array camera views the surface through an opening in the rear of the shroud, and monitors the surface temperature distribution following the pulse. The thermal wave image acquisition process is carried out under the control of a WindowsTM program in a portable 47 Mat. Res. Soc. Symp. Proc. Vol. 503 ©1998 Materials Research Society
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computer. Our system has the electronics and computer mounted on a two-wheeled cart, and the imaging head is remotely located, hand-held, and connected to the cart by a 50-ft umbilical cable. The thermal wave system acquires images in a series of time gates after the flash. With appropriate selection of the gate time(s) for monitoring the cooling, one can readily distinguish both thicker and thinner areas in the resulting thermal wave images. This enables one to see internal structures, such as bonded doublers, internal corrosion, impact damage, delaminations, etc. DISBONDS, DELAMINATION, AND IMPACT DAMAGE
Fig. 2 Photograph of the thermal wave imaging system in use to inspect a 747 aircraft in a maintenance hangar. The inspection was aimed at evaluating the technique for inspecting subsurface disbonded doubler structures. Thermal wave imaging has become a useful nondestructive inspection technique for imaging disbonds in aircraft structures. When a doubler is well bonded, the thermal contact through the adhesive bond to the second layer causes it to show up as a region of darker contrast, as compared to the surrounding single skin. This contrast is absent when the doubler is disbonded. Figure 2 shows a photograph of our thermal wave imager in use to image a region near the cockpit of a 747 aircraft in an airline maintenance facility. As is evident from Fig. 2, this hand-held system is easily manipulated by a single inspector. Thermal wave images of two internal doublers in the 747 aircraft imaged during this particular inspection are shown in Fig. 3. The top image in Fig. 3 shows a well-bonded doubler, whereas the bottom image reveals a disbonded doubler. These, and
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other images taken during the inspection, were confirmed by the ai
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