Detection of corrosion damage in aircraft wing skin structures is an ongoing NDT challenge. Ultrasonic methods are known and well-accepted techniques, which are relatively simple to carry out in terms of setup, probes and instrumentation and operator training. However, with conventional inspection from the top surface using a transducer at normal incidence (0o to the normal to the surface) producing a visual picture in the form of a C-scan, it is very tie consuming to point-by-point inspects large aircraft wing skin areas. In addition it is too difficult to detect disbonds in thin multilayered and fatigue cracks in the shadow region at fastener holes in airframe structures where water and humidity then are infiltrated to create corrosion and exfoliation around and under the rivets. Ultrasonic guided waves demonstrate potential as promising, global and fast inspection method. It can be used to compliment and in some cases, be an alternative to conventional ultrasonic C-scan inspection method.
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Ultrasonic Techniques for hidden corrosion detection
Corrosion is one of the serious problem affecting airforce and other aviation industries. It affects the aircraft on its wings, surface, between joints and fasteners. The presences of corrosion underneath the paints of surface and between joints are not easy to be detected. The unnoticed presence of corrosion may cause the aircraft to crash leading to human and money loses. To detect the corrosion present on the metal surface, various methods and tests are used. These tests conducted should be such that it does not destroy or disassemble the plane to parts or damage its surface. Hence for the further use of the plane, Non-destructive tests (NDT) are carried out.
Non-destructive testing as the name suggests is testing procedure without any damage to the part being tested. The various non-destructive testing methods used are:
1) Visual inspection
2) X-ray inspection
3) Die (liquid) penetration inspection
4) Magnetic particle inspection
5) Eddy current inspection
6) Ultrasonic inspection
Ultrasonic inspection is conventionally used for corrosion detection in aircraft wings. But the conventional inspection method carries with it certain defects like:
(i) It scans perpendicular to the surface and hence rate of scanning (from point to point) is less and hence highly time consuming.
(ii) Conventional method is not capable of detecting disbonds between layers and cracks at fastener holes.
These defects are over come by a newly developed inspection method using guided ultrasonic waves.
Guided waves demonstrate an attractive solution where conventional ultrasonic inspection techniques are less sensitive to defects such as corrosion/disbonds in thin multilayered wing skin structures and hidden exfoliation under wing skin fasteners. Moreover, with their multimode character, selection of guided wave modes can be optimized for detection of particular types of defects. Mode optimization can be done by selecting modes with maximum group velocities (minimum dispersion), or analysis of their wave mode structures (particle displacements, stresses and power distributions). Guided Lamb modes have been used for long range/large area corrosion detection and the evaluation of adhesively bonded structures.
Ultrasonic guided waves are promising but require procedure development to ensure high sensitivity and reliable transducer coupling and to provide a mechanism to transport the probe(s) over the area to be scanned.
This paper describes some practical inspection setups and procedures based on guided wave modes for corrosion damage detection in single and multilayered wing skin structures and exfoliation detection immediately adjacent to fasteners in aircraft wing skin. It describes the results of their application to detection of corrosion in simulated and real components of aircraft wing skin. Using a tone burst system, the wave modes are selected, excited and tested in pulse echo and pitch catch setups. Launch angles were obtained from the calculated dispersion curves.
Theoretical group velocities were compared to tested group velocities to confirm the excited modes at frequency thickness product and launch angle. The simulated corrosion in single and multilayered wing skin structures and exfoliation located under several rivets was successfully detected. Some guided Lamb modes proved to be more sensitive to corrosion type defects and produced better results