 Table of Contents ECNDT '98 Session: Aerospace | Thermal Wave Imaging of Aircraft for Evaluation of Disbonding and CorrosionR.L. Thomas, Xiaoyan Han, and L.D. FavroInstitute for Manufacturing Research Wayne State University, Detroit, MI 48202, USA Corresponding Author Contact: Xiaoyan Han Email: han@thermal.physics.wayne.edu , URL: http://thermal.physics.wayne.edu/~han/han.html |
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Thermal wave imaging can be used to determine aircraft skin corrosion thinning.[1] In addition to imaging the pattern of subsurface corrosion, the technique can rapidly (a few seconds) make quantitative measurements of less than 1% material loss for various regions in the image. It uses pulse heating of the aircraft from photographic flashlamps mounted in an aluminum box, which is open at one end, and placed against the fuselage of the airplane (see Fig. 1). This box traps and funnels the light uniformly onto the fuselage, and an infrared (IR) focal plane array camera, aimed and focused at the surface through an opening in the rear of the hand-held shroud, monitors the rapid cooling of the surface of the fuselage.
Fig 1a: Exterior
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Fig 1b: Interior
Photographs of the exterior and interior of the thermal wave imaging head showing the IR focal plane array camera, aluminum box, and two linear flash lamps. The opening of the shroud is approximately 30 cm on a side.
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The system operates by sending a pulse of heat from the surface into the material, where it undergoes thermal wave reflection at either the rear surface or at any interior surface at which the thermal impedance changes, e.g., at disbonds, delaminations, etc. The effect of these thermal wave reflections is to modify the local cooling rate of the surface. The cooling rate, in turn, is monitored through its effect on the IR radiation from the surface, which is detected by the camera, and processed as a sequence of images by the control computer. The contrast in the processed images reveals the presence of defects in the interior or variations in the thickness of the material.
Figure 2 shows an example of the thermal wave imaging system in use in a airline maintenance facility. As can be seen from this photograph, the imaging head is easily manipulated around the aircraft by a single inspector. Such a system has been successfully used to image the entire belly skin of a DC-9 aircraft parked on an airfield, far from the hangar, with no protection from the elements. In that operation, overlapping images were taken in rapid succession, and the entire skin was imaged in one working day. The computer was controlled remotely by the single inspector who positioned the imaging head on the belly skin. To accomplish this, the inspector used push-button controls mounted on the imaging head, together with automatic sequential file saving and incremental naming in the computer. An example of an image from this inspection is shown in Fig. 3.
Fig 2: Photograph of the thermal wave imaging head in use in an airline maintenance facility to inspect the belly skin of a DC-10 aircraft.
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Fig 3: Example image of the belly skin of a DC-9, showing rows of fasteners (black), blistered paint (small white spots), and mild corrosion in the lower left region of the bottom row of fasteners, which is along the center line of the belly.
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Fig 4: Example montage thermal wave image of corrosion and disbonded doublers on a B737 aircraft (a testbed aircraft located at the FAA's Airworthiness Assurance NDI Validation Center in Albuquerque, NM). The montage covers a region between a bulkhead (left) and a cargo door (right), and is approximately 1.3 m in total width.
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Fig 5a: Corrosion analysis window , containing a thermal wave image of an aluminum test panel, the rear surface of which has five intentionally corroded regions. Also shown in the image are smaller squares which indicate the region being analyzed and two reference regions.
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In Fig. 4, we show an example montage thermal wave image of an area of a B737 aircraft which has extensive corrosion and numerous disbonded tear straps. This aircraft is out of service and is used as a testbed aircraft at the FAA's Airworthiness Assurance NDI Validation Center in Albuquerque, NM. Also shown in this image is an external patch covering up previous corrosion damage. Using the software developed in our laboratory, we are able to make quantitative corrosion thinning measurements at mouse-selectable locations in any such thermal wave image, immediately following its acquisition. To illustrate this capability, in Fig. 5, we show a corrosion analysis window from our software, which includes a thermal wave image of an aluminum test panel, the rear surface of which has five, more or less square, intentionally corroded regions. Also shown in the image are three smaller squares which indicate the region being analyzed and two reference regions, one of which is above and one of which is below the analysis. The corrosion percentage is calculated relative to the thickness of the metal in the reference regions. The reason for using two reference regions is to eliminate possible errors which could be introduced as the result of the top to bottom scanning of the focal plane array in the camera. In the top right of the window directly to the left of the image window in Fig. 5 (the "Corrosion Evaluation" window), the percentage loss is shown as 0.9% for the region under evaluation. We have evaluated each of the intentionally corroded regions on several such test panels, of two different thicknesses, and have plotted the results against direct measurements taken with a micrometer. The resulting comparison, shown on the right in Fig 5, indicates excellent agreement between the two measurements from less than one percent to nearly thirty percent material loss.
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| Fig 5b: The resulting comparison with micrometer measurements for this and similar panels, indicates excellent agreement from less than one percent to nearly thirty percent material loss. |
In a thermal wave image, metal doublers, which are bonded to the inside surface of the fuselage, cause the outside surface just above them to cool more rapidly. The result is that doublers, when properly bonded, show up as darker contrast. This situation differs from images in which where corrosion appears, because corrosion shows up as brighter contrast. An example thermal wave image showing both bonded and disbonded doublers on a B747 aircraft is shown in Fig. 6. This image was made on Section 41 of the aircraft. Inspection is currently required of the horizontal doublers in this area of the B747, and the disbond shown in Fig. 6 was confirmed by means of conventional bond testing by the manufacturer and airline inspectors. The thermal wave imaging technique is currently undergoing a validation process involving its possible use as an alternative means of compliance with this inspection requirement. To date, three aircraft have been inspected, and a fourth inspection has been scheduled as part of this validation process.
Fig 6: Thermal wave image of disbonded (left) and bonded (right and center) doublers on a B747 aircraft. Bonded doublers show up with darker contrast, whereas for a disbonded doubler, only the fasteners are seen. The characteristic "banana" shape of the doubler on the right is indicative of a good bond. The small gap between the horizontal and vertical doublers is not a disbond, but is a part of the design of the doubler structure. On the left, there is no distinct "banana" shape in the darker contrast, although there are some minor regions around the fasteners where the bond is still intact.
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In Figs. 7 and 8, examples of thermal wave imaging of composite-to-metal disbonds are shown. These images were taken for boron fiber reinforced composite patches on aluminum structures. Disbonds are clearly imaged as bright contrast features. Figure 8 is notable, because the disbond runs from the edge of the patch, through the ply drop-off area, into the thickest region of the patch, which in this case is 36 plies deep. The feature in the image corresponding to this disbond is fainter, and more blurred in this thick region, but is still easily distinguished from the surrounding, well-bonded region.
 Fig 7: Sequence of thermal wave images of a boron fiber reinforced composite patch on an aluminum structure. The image on the left (at 777 msec) shows only details of the near surface structure of the composite. The second and third images show thermal wave reflections (brighter contrast) near the center and bottom of the patch, indicative of disbonds between the patch and the metal surface. A third, smaller, feature along the line of the two disbonds shows more clearly in the third and fourth images, along with a linear feature extending to the upper right. The dark vertical band in the fourth image is the result of structure in the metal. |
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| Fig 8: Thermal wave image of an intentionally placed disbond (see arrow) which runs from the edge of the boron fiber composite reinforcement patch, through the ply drop-off area, into the thickest region of the patch, which in this case is 36 plies deep. The feature in the image corresponding to this disbond is fainter, and more blurred in this region, but is still easily distinguished from the surrounding, well-bonded region. | |
This material is based upon work performed by the FAA-Center for Aviation Systems Reliability, operated at Iowa State University and supported by the Federal Aviation Administration Technical Center in Atlantic City, New Jersey, under grant number 95-G-025, and by AFOSR under Grant No. F49620-96-1-0166.
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