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

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
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