| Ultrasonic
Techniques for hidden corrosion detection |
INTRODUCTION 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
TESTS 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
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