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Neutron Radioscopy Inspection of Composite Flight Control SurfacesL.G.I. Bennett, W.S. Lewis, T.R. Chalovich, O. Fransescone
Department of Chemistry and Chemical Engineering
Royal Military College of Canada
Kingston, Ontario, Canada.
A neutron radioscopy system (NRS) has been developed on the SLOWPOKE-2 research reactor at the Royal Military College of Canada and is being used to reveal water ingress in the flight control surfaces of the CF188 Hornet aircraft. Because of the relatively low neutron flux at the image plane for this small reactor, the inspection protocol is to scan the component, some of which are quite large, using neutron radioscopy, followed by neutron radiography of the affected areas.
Keywords: neutron radioscopy, neutron radiography, SLOWPOKE-2
Non-destructive testing (NDT) and non-destructive evaluation (NDE) have become a highly developed and well-utilized activity within the airframe industry. Modern aircraft with their complex structures, multiple and unpredictable load paths, low engineering safety factors and extended lifetimes all combine to produce a requirement for continuing inspections. The materials now introduced into the manufacturing process, which can create lighter and stronger components, have made their inspection more complex and difficult.
Radiography is a NDT technique in which an image can be taken of the internal characteristics of a component by utilizing either gamma, X-ray or neutron radiation. For gamma or X-ray radiography, the source is either a radioisotopic gamma source or an X-ray tube while the recording medium is X or gamma ray sensitive or light-sensitive film. For neutron radiography, the neutron source is, preferably, a nuclear reactor while the recording medium is a converter screen used to capture the neutrons and transfer the latent image onto X or gamma ray sensitive or light-sensitive film. With neutron radioscopy, the recording medium can be any combination of a scintillation screen, a photosensitive screen, an image intensifier, an analogue video camera, or a charged coupled device (CCD) camera. A typical layout for radioscopy with a scintillation screen and CCD camera as used in this work is shown in Figure 1.
|Fig 1: Radioscopy Configuration.|
Neutrons interact with the nuclei of atoms rather than with the orbital electrons, as do X and gamma radiation and, therefore, each elemental nuclide has its own characteristic neutron cross section, showing no pattern in comparison to the increasingly uniform attenuation with mass number for X and gamma radiation. In fact, there is a slight reverse or complementary trend towards light elements for neutrons. The major advantage of neutron radioscopy is its ability to reveal light elements such as hydrogen contained in materials such as corrosion products and water.
Recently, a concern was raised over the airworthiness of the CF188 Hornet, as initial indications pointed to possible problems that may exist in some of the honeycomb structures. After a complete inspection of an intact test aircraft, the question arose as to how the components that indicated potential problems could be inspected individually off the aircraft.
After an initial comparison investigation to determine which NDT technique would be suitable, the Royal Military College (RMC), in conjunction with the NDT Section at nearby CFB Trenton, was tasked to perform neutron radiography, which was determined to be superior in detecting water ingress. A neutron radioscopy system (NRS) was commissioned at the SLOWPOKE-2 Facility at RMC in order to inspect flight control surfaces. Next, a near real-time radioscopy system was installed and characterized in order to expedite the inspection of CF188 aircraft composite flight control surfaces for water ingress.
For neutron radioscopy, the neutrons are attenuated and a sufficient amount of light is produced by a 431.8 mm (17 in.) x 431.8 mm (17 in) aluminium backed scintillation screen to detect them with a CCD camera. The scintillation screen used had a composition of  LiF:ZnS:Cu (NE 427 â). The neutrons interact with lithium due its large cross section to produce an alpha particle (4He++) and tritium (3H) daughter products. The energy from the alpha particle is deposited into zinc sulfide (ZnS), an efficient phosphor that produces visible light. The copper element acts as a wave length shifter to produce light in the yellow-green region, which has an average wavelength of 525 nm.
CCD cameras are used in neutron radioscopy due to their high resolution, high quantum efficiency, wide spectrum response, low noise and linear characteristics. The Apogee AP7 CCD camera uses a back-lit SITe502 sensor with full frame transfer in order to retain resolution and an array size of 512 x512 with a pixel size of 24 mm. The pixels have a maximum well capacity of 296000 electrons and a dark current rate of 0.8 electrons/pixel/second at -33° C. The camera had a front surface mirror with adjusting mechanism, a Vivitar AF-SC 28-70 mm F3.5 zoom lens, a red laser source (used for focusing the lens), an Apogee video capture board and was connected to a personal computer. The CCD camera, cooling unit, laser, mirror and mirror adjuster are housed in a shielded location. Image acquisition was accomplished with Image Pro Plus software and the image analysis was completed with IP Lab Spectrum software.
The Neutron Radioscopy System (NRS) (Figure 2) was installed in the 20 kW SLOWPOKE-2 research reactor, which is capable of producing a neutron flux of more than 3 x 104 n/cm2 s at the image plane. Underneath is located the Neutron Beam Tube (NBT) which is circular in cross section, tilted 8.5°from vertical and has a measured L/D of approximately 100. After installation and optimization of the NRS, the effects of the variation in neutron flux level and exposure time were studied. Although higher neutron fluxes were preferred, adequate images were obtained with the reactor operating at half power as is its norm for other applications. An exposure time of about five minutes was determined, much shorter than the almost half hour for relatively fast film.
|Fig 2: Beam Stop and Diffuser at RMC.|
The above parameters (neutron flux, exposure time) were altered when the system was not exposed to neutrons (called a dark image) and when the system was exposed to neutrons (called a unified image). A unified image is a term used to indicate when no object is placed between the neutron beam and the scintillation screen so that the neutrons uniformly illuminate the screen. If intensity values from a dark image and a unified image are combined, the resulting histogram will show the dynamic range of the system.
|Fig 3: Example of Dynamic Range Histogram.|
A small section of a CF188 rudder, measuring 14 cm x 14 cm was used as a test piece to simulate water ingress. The CF188 rudder is a composite construction that consists of two outer skins of carbon graphite/epoxy made from several plies of carbon graphite material (1- to 3-mm thick) attached to an aluminium honeycomb core by a FM-300 adhesive, as shown in Figure 4. The top skin was removed, the cells of the honeycomb were filled singly and in groups with distilled water or high density polyethylene, and the top replaced. Then, a CF188 rudder was imaged and compared to film-based radiography results.
|Fig 4: Construction of a CF188 Rudder.|
Analysis of the many exposures of the test piece with varying amounts of water or polyethylene and exposure times indicated that water amounts from 0.02 to 0.3 mL and water increments of 0.02 mL could be observed and that five minutes was a sufficient exposure time. Various digital enhancement techniques were tried, with the best being the erode filter.
A series of images of a rudder taken for an exposure time of four minutes is shown in Figure 5. The first image
Although clearer in the actual images, what are visible are the aluminium main spar, shown horizontally at the bottom with the hinge support and steel anchor nuts underneath. Above is excess adhesive, water in the honeycomb and the imaged adhesive in a honeycomb pattern.
|(a) no enhancements||(b) basic enhancements||
(c) advanced enhancements
||Fig 5: Sequence of Images of a CF188 Rudder.
Although not visible in the first image (a) in the Figure, there are some white spots indicating gamma contamination or system noise. Figure 5(b) has very little definition, as some of the individual components are not clearly distinguishable, but water is indicated, as is the hinge assembly. Water ingress and its location were previously known from neutron radiography. In Figure 5 (b) and (c), water ingress is visible and the pattern of water is identical to known images of this area. Figure 5 (c) also indicates clearer details such as the main spar, porous adhesive and the anchor nuts.
The histogram associated with the original image, Figure 5 (a) is shown in Figure 6. Thresholding was applied to the image to identify the features related to the curve and are indicated.
|Fig 6: Histogram of the Original Exposure of Figure 5 (a).|
The intensity values of the graph in Figure 6 are in the low thousands and therefore the original image is totally black as previously seen in Figure 5 (a). The intensity values have a small range, from 1536 to approximately 3500, which is only 3% of the total number of intensity values available. This small intensity range produced an image with a small dynamic range, which reduced image contrast between components as shown in Figure 5 (b). However, all of the expected features are present in the histogram. Thus, since the different items, such as water, have a characteristic location, a histogram may be considered as another means of identifying the presence of water.
A neutron radioscopy capability has been added to the neutron beam tube in the SLOWPOKE-2 Facility at RMC for the inspection of CF188 flight control surfaces. The combination of a scintillation screen and a CCD camera has provided a digital means of identifying water ingress into these structures. The use of a histogram provides a unique means of revealing the presence of the various items in the image. Radioscopy will be employed in a screening manner due to its shorter exposure time, followed by film neutron radiography where appropriate.
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