DSA Volume 1 Issue 4 December 2010 - Defence Science and ...

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Sound way of detecting hidden aircraft flaws Australia and its defence partners are developing a means to find defects in aircraft parts and structures by the use of a technique called sonic thermography. The work was carried out as a project mounted by The Technical Cooperation Program (TTCP) involving the United States, Canada, Great Britain, New Zealand and Australia. The issue at large here is the need to keep today’s military aircraft flying long past the end of their design lives because of their very high replacement cost. Maintenance inspections to detect parts that are defective due to effects such as corrosion, cracking and delamination have therefore become increasingly vital, and much research effort is being put into finding ways that reduce the expense and aircraft downtime required. “One particular problem for inspection work is posed by small fatigue cracks that arise in many aircraft structures and tend to remain closed under normal conditions, making them very difficult to detect,” says DSTO researcher Dr Kelly Tsoi. “Early detection is essential in the case of parts that are critical to the safety of flight.” Several non-destructive inspection techniques are currently in use, involving such means as liquid penetrants, magnetic particles, eddy currents, x-radiography and ultrasonics, but these are seen to be less than optimal for detecting certain types of flaws. Enter sonic thermography In 2002, the TTCP consortium began a study on the potential of sonic thermography, another non-destructive inspection technique, with the hope of partly filling this performance gap. Use of the technology involves the exposure of a test specimen to high frequency sound energy, which induces frictional heating at the surfaces of defects that are in close contact. This heating is then detected by an infrared camera. One advantage it offers is the speed and ease of inspections involving large areas and Above: DSTO researcher with sonic thermography apparatus. complex shapes. It is also environmentally benign, not requiring the use of x-rays, hazardous petrochemical liquids for immersing or dousing of parts being tested. Furthermore, it can usually be performed on a structure directly, without the need for disassembly or removal of parts to a laboratory or hangar. An early concern held about its use, however, was whether the mechanically irreversible and somewhat violent processes involved in crack detection would in fact contribute to further crack growth. Dr Tsoi explains, “We found ourselves facing a rather fundamental question – was the technique in fact nondestructive? “As a potential ‘show-stopping’ issue, it needed to be settled as soon as possible. We at DSTO undertook to investigate this in a laboratory study, which fortunately confirmed that there was no measurable ill effect caused by the inspection process.” Twelve steps towards a proven technology The TTCP research program involved a series of twelve steps undertaken between 2002 and 2008. The last of these was a ‘round robin’ study to determine the effectiveness of sonic thermography as an inspection tool, and to compare the performance of the different thermographic inspection systems developed by the participating TTCP countries. This involved testing carried out by all TTCP systems on the same aircraft component; the main wheel rim of a Lockheed Martin F-16 Fighting Falcon having accumulated a number of service hours. Cracking on such components is known to occur at stem structures spaced equally around the rim, which are formed as part of the rim when cast from molten alloy. For DSTO’s test procedure, a commercially available hand-held 1200-watt ultrasonic plastics welder was used as the sound 8
DEFENCE SCIENCE AUSTRALIA energy source. This was interfaced to an infrared camera with DSTO-developed software that ensured synchronous operation of the two devices. The DSTO software suite also featured a set of powerful image processing algorithms for extracting very small temperature perturbations from the infrared signals captured by the camera. Through this means, temperature changes of well under one tenth of a degree could be detected. Testing of the wheel rim was undertaken both in the state it was received, painted matt white, and then again after being coated by DSTO with a water-based high emissivity paint, being done to improve the infrared signal quality and eliminate background thermal reflections. Study outcomes The DSTO investigation found that cracks had developed at four of the five stem sites on the rim. For three of these four instances of cracking, the damage was not visible to the naked eye, being hidden beneath the white matt paint. Of particular note in terms of outcomes, the sonic thermography cracking signals reported by DSTO were the largest obtained by any of the labs involved in the ‘round robin’ study. The position of the welder horn tip during the testing process was found to have a large influence on whether or not cracks were observable. This was of particular importance in the detection of smaller cracks not visible to the naked eye. Several areas on the main wheel rim were investigated for placement of the sound energy source. One of these positions, just above the main bulk of a stem delivered a virtually non-existent infrared signal. In contrast, a position on the edge of a stem, a relatively short distance away, produced substantially improved results. Pondering on the physical processes involved here, Dr Tsoi observes, “An efficient transfer of acoustic energy into the structure is vital for generating a significant thermal signature from the defect. “While a wheel may seem to be a simple object, it is, in fact, a complex structure from a dynamics viewpoint, comprising a collection of waveguides that cause a complex flow of acoustic energy. “One of the key challenges for implementation of this technology is ensuring that the energy gets to where it is needed, and crucial to that is the location of the acoustic horn. “While intuition and experience are often good guides to optimal energy source placement, our long term objective is to develop mathematical models to accurately predict power flow.” Continuing work Further work undertaken by DSTO has revealed that sonic thermography can ably detect defects known as ‘kissing bonds’ – defective adhesive bonding between surfaces – as well as impact damage in composite bonded repairs (CBRs) and structures, cracking beneath CBRs and also loose-interference fit fasteners in metallic structures. Although sonic thermography has been shown to operate well in detecting flaws, the researchers find themselves facing fundamental difficulties when attempting to achieve the necessary repeatability of excitation in cases where defects need to be characterised rather than merely detected – as, for example, when determining the closure forces acting on cracks. The key problem in this regard is the fact that the use of sound energy to excite a test object produces chaotic responses, meaning that it is very difficult to produce precisely the same excitation in a test object in subsequent test runs. Ongoing research at DSTO is facilitating progress in this area by providing more insight into the way energy transfer is made between the energy source and the test object. The research has made possible the identification of materials that can be used as interfaces between the sound energy source and the test object, which thereby allow for efficient transfer of energy into the structure. “These outcomes offer an encouraging basis for improving the techniques being applied here,” says Dr Tsoi. The quality of the work being done by the DSTO researchers and their international collaborators was sufficiently impressive to earn them the TTCP Achievement Award, presented late last year. Above left: The F-16 Fighting Falcon main wheel rim used in TTCP sonic thermography studies. Above: Close-up of the F-16 wheel rim (left), with colour enhanced sonic thermography image of same (right) that reveals otherwise invisible cracking on lower left-hand side. 9
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