Paper presented at the 8th ECNDT, Barcelona, June 2002

Abstract

A small (20 kWth), safe, pool-type nuclear research reactor called the SLOWPOKE-2 is located at the Royal Military College of Canada (RMC). The reactor was originally installed for teaching, training, research and semi-routine analysis, specifically, neutron activation analysis. It was envisioned that the neutrons from the SLOWPOKE-2 could also be used for neutron radiography, and so a research program was initiated to develop this technology. Over a period of approximately 15 years, and through a series of successive modifications, a neutron radiography system (NRS) was developed. Once completed, several applications of the technology have been demonstrated, including the non-destructive examination of the composite flight control surfaces from the Canadian Air Force’s primary jet fighter, the CF 188 Hornet aircraft.

An initial trial was set up to investigate the flight control surfaces of 3 aircraft, to determine the parameters for a final licensed system, and to compare the results to other non-destructive methods. Over 500 neutron radiographs were made for these first 3 aircraft, and moisture and corrosion were discovered in the honeycomb structure and hydration was found in the composite and adhesive layers. In comparison with other NDT methods, neutron radiography was the only method that could detect the small areas of corrosion and moisture entrapment. However, before examining an additional 7 aircraft, the recommended modifications to the NRS were undertaken. These modifications were necessary to accommodate the larger flight control surfaces safely by incorporating flexible conformable shielding. As well, to expedite inspections so that all flight control surfaces from one aircraft could be completed in less than two weeks, there was a need to decrease the exposure time by both faster film/conversion screen combinations and by incorporating the capability of near real-time, digital radioscopy. Finally, as there are no inspection specific image quality indicators, there was also a need to develop a gauge to evaluate the moisture trapped in the honeycomb cells of flight control surfaces.

Introduction

Neutron radiography is considered a complementary nondestructive testing technique to conventional radiography. In X- and gamma radiography, attenuation increases uniformly with mass number and density, whereas, with neutrons, attenuation is.random with a tendency for certain light elements such as hydrogen to absorb and scatter neutrons rather well. Thus, neutron radiography is especially well suited to detecting corrosion and moisture entrapment, especially in aircraft structures.

Before installation of the SLOWPOKE-2 reactor at RMC in 1985, a thermal column of heavy water was installed to provide a pathway for thermal neutrons to travel from the core region radially though the reactor container. At this point, the bottom end of a beam tube containing a shaped piece of graphite and an aperture was placed in order to extract a beam of neutrons upwards (Figure 1). Many additions and modifications to the shielding, lining and aperture have taken place since the original installation in order to produce a neutron beam adequate for neutron radiography and radioscopy [1].

Fig 1:The Neutron Beam Tube and The Reactor Container At RMC

Neutron Radiography Of CF188 Components

As part of an Aircraft Sampling Inspection (ASI) program, carried out to determine the overall condition of the aircraft and the effectiveness of the current preventative maintenance program, it was decided to inspect a CF188 aircraft with the highest flight hours using both x- and neutron radiography at the McClellan Air Force Base. The neutron radiography revealed 93 anomalies, including moisture, cell corrosion, damaged honeycomb core, foreign object material, voids, and repaired areas [2].

After returning to Canada, the component with the greatest structural significance, namely the right hand rudder from the vertical stabilizer, was removed from the aircraft and put through a rigorous program of numerous NDT inspections, including x-radiography.(film and real-time), eddy current, ultrasonics (through transmission and pitch-catch), infrared thermography, and neutron radiography. Of all the techniques investigated, only through transmission ultrasonics and neutron radiography were able to identify large areas of hydration, and only neutron radiography could identify the small areas of moisture entrapment and hydration. Finally, the rudder was disassembled to destructively determine the levels of moisture and entrapment. All areas of moisture entrapment and hydration found during this disassembly were the same areas that had been detected using neutron radiography.

Fig 2: Neutron Radiography Of The Hinge Attachment Area, Left LEF, CF188729

Fig 3: Neutron Radiograph Of The Right LEF, CF188912

As a result of these investigations, it was decided to inspect all the flight control surfaces from 3 CF188 aircraft with neutron radiography and through transmission C-scan ultrasonics. To carry out these inspections, over 500 radiographs were completed, with 57 of these indicating anomalies including 37 (or 65 %) due to moisture and corrosion. By way of illustration, Figure 2 is a copy of the neutron radiograph of the hinge attachment area of the leading edge of the outboard flap removed from aircraft CF188729, clearly showing areas of moisture entrapment and the absence of cell walls due to corrosion. Figure 3 is a copy of the neutron radiograph of the left hand aileron from aircraft CF188912, also clearly indicating areas of moisture entrapment and cell corrosion. As these radiographs had been completed with only temporary shielding and parts handling, further modifications to the Facility were required prior to completing further inspections.

Facility Development

The original neutron radiography Facility at RMC had been designed to inspect relatively small components. Therefore, in order to inspect the large aircraft flight controls, including the horizontal stabilizer (which was not inspected during the first 3-aircraft trial due to its size), significant modifications to the Facility were required (Figures 4 and 5).

Fig 4: Modified RMC Neutron Radiography Facility

Fig 5: Component Positioning System

The design incorporated flexible, conformable shielding (Figure 4) thereby eliminating the need to change shielding configurations with each part. As well, a computer-controlled 2- dimensional parts manipulation system (Figure 5) was installed that facilitates parts maneuvering during the inspections as well as guaranteeing reproducible results for each flight control.

The new design allows for either film or near real-time radioscopy inspections, which will also speed up the inspection process. Neutron radioscopy does not utilize film but rather a Vidicon video camera with screen intensifiers or a Charged Coupled Device (CCD) camera with a scintillation screen to record the image. The image is stored in a digital format on a computer for viewing and digital enhancement purposes. The major disadvantage of neutron radioscopy is poor image resolution while the advantages include good image contrast, reduced exposure time, very good image linearity and the ability to manipulate image data. A typical configuration for neutron radioscopy is shown in Figure 6.

Fig 6: Neutron Radioscopy Configuration.

With these modifications completed, the next phase of the investigations will involve the inspection of the flight controls from additional CF188 aircraft using both radiography and radioscopy.

Technique Development

In addition to the Facility Developments, the researchers have been investigating numerous technique developments. Technique development has proceeded along 2 fronts, namely to expedite the inspection times using both radiography and radioscopy, and the development of a honeycomb moisture gauge.

For radiography, an investigation was undertaken to find a faster combination than the industry standard Gadolinium (Gd) screen and Kodak SR or 400 AA film. Gadolinium oxysulphide scintillation screens and various light-sensitive medical films were tried. As no industrial screens or films were available, medical screens such as Lanex Regular, Medium and Fine X-ray and Min R mammography screens, and films such as Min R-2000, Min R-M and T-Mat were used. In the absence of an industrial image quality indicator for these trials, cadmium and lead plates with drilled holes of varying diameters were used for each radiograph. As a result of these investigations, the combination of the Lanex Medium screen and the Min R-2000 film was found to be the best combination, with a resultant image quality just slightly less than that of the Gd-AA combination. There was also a dramatic decrease in inspection times, with the new combination requiring an exposure time of only 6 minutes compared with 35 minute exposures required for the Gd-AA combination. However, for primarily economic considerations, CX film, with an exposure time of approximately 20 minutes, was chosen as the preferred film for these investigations.

The most recent developments have been with radioscopy. Using a CCD camera was first applied to neutron radioscopy in 1990 [3]. Advancements in CCD camera and cooling technology have made the use of CCD cameras both affordable and practical. There are two phases in the installation of a CCD camera system. The first phase is the assembly of the hardware, which includes the CCD camera, mirror, lens, scintillation screen, and a light-tight enclosure. The second phase is the testing of the CCD camera system to determine its capabilities and performance. Testing of the system includes quantifying the system’s resolution, optimising the exposure time, quantifying the neutron-scintillation screen interaction and quantifying the results from images taken with actual aircraft flight control surfaces.

The assembly of the hardware was relatively straightforward, with most of the components readily available. The determination of the radioscopy's performance has been conducted over the past year, with the conclusion that even with the relatively low flux values available at the image plane (2.0 x 10 ⁴ n/cm ² s), the SLOWPOKE-2 Facility is capable of producing acceptable radioscopy images. Numerous image enhancement techniques have been investigated, as illustrated in Figure 7, with the greatest image improvement produced by using a 2 x 2 Erosion filter..

(a) no enhancements (b) basic enhancements (c) advanced enhancements Fig 7: -Sequence Of Images Taken At RMC

In neutron radiography, the only industrial image quality indicator is for use with Gd-SR combinations, and not for specific applications. Therefore, the next goal in technique development was to develop a gauge for quantifying the moisture found in the honeycomb cells of the flight controls of varying thicknesses, and for all film-foil combinations and radioscopy. As a first step, a section of honeycomb structure from a deskinned CF188 rudder was sliced just under one of the composite layers and filled with various amounts of water in 0.05 ml increments, and radiographed. The minimum amount of water that could be accurately measured was 0.10 ± 0.005 ml, and the maximum amount that filled the cell was 0.30 ml.

With linearity between the amount of water and the resultant image density confirmed, high density polyethylene was placed in a test gauge and radiographed with the water in the honeycomb cells to determine if the same linearity existed for the polyethylene. Numerous radiographs using various film-foil combinations were performed, including Gd-AA and Lanex Medium-Min R-2000. These investigations again determined that there was linearity between the volume of water and the thickness of polyethylene in the test gauge. As a last test, varying separations between the test gauge and the deskinned rudder were attempted in order to simulate varying thicknesses of components. In all tests performed, there was no effect resulting from either separations or film-foil combinations. Thus, the use of this indicator will allow for comparison of the density readings of the known thicknesses of polyethylene in the gauge to the densities for located moisture in the flight control surface under investigation, thereby quantifying the moisture content in the honeycomb cells.

Conclusions

In summary, neutron radiography has been used successfully to indicate areas of moisture ingress, honeycomb cell corrosion, and adhesive/composite hydration in CF188 flight control surfaces. In order to inspect additional CF188 components, major Facility modifications were required to accommodate larger flight control surfaces and to expedite the inspections. Flexible, conformable shielding along with a computer-controlled 2-.dimensional parts maneuvering system have been installed. In order to speed up the inspections, faster film-foil combinations have been investigated, and a near real-time radioscopy system has been installed. Finally, a gauge that can quantitatively determine the moisture content in the honeycomb cells has been developed. With the efficiency and effectiveness of the neutron radiography Facility completed, additional inspections of CF188 flight controls will now be undertaken.

Acknowledgements

The authors wish to extend their deepest gratitude to H. Wieland for his expert advice and skills in making these major modifications to the NRS. The availability of O. Francescone as an experienced radiographer has been made possible by the Director of Quality Assurance (DQA), National Defence Headquarters (NDHQ). The assistance of B. LeGros, Research Assistant, was also invaluable. Financial support for this work was received from the Air Vehicle Research Detachment (AVRD) and the CF 188 Aircraft Engineering Office, Director General Aerospace Equipment Program Management, NDHQ.

References

1. W.J. Lewis, L.G.I. Bennett, "Improving The Beam Quality Of The Neutron Radiography Facility Using The SLOWPOKE-2 At The Royal Military College Of Canada", Nuclear Instruments and Methods A377, 41-44, 1996.
2. W.J. Lewis, L.G.I. Bennett, "Moisture And Corrosion Inspection Of Aircraft Composite Flight Controls With Neutron Radiography", 1998 ASNT Spring Conference, Anaheim, California, 23-27 Mar 98.
3. H.Kobayasi, Neutron Radiography, 3, Kluwer, pg. 421, 1990.

© NDT.net - info@ndt.net

|Top|