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Shedding light on metal fatigue 9 February 2004
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A laser originally developed for fusion power research is being
used by Rolls-Royce to strengthen aerospace components.
The laser is powerful enough to send a shockwave through the
first 1-2mm of a metal surface, causing compressive residual
stress.
Compressive stress is caused by the material's atomic structure
pushing inwards against itself, and is good for combating fatigue
and inhibiting the initiation and propagation of cracks.
Rolls-Royce has been treating its Trent jet engine fan blade
components for the past 21 months, although the work has only just
been made public.
Metal Improvement, the company that has developed the
technology, hopes to eventually offer it to the nuclear, oil and
gas, medical and automotive industries.
David Francis, senior vice-president, laser operations, at Metal
Improvement, said the technology is more effective in improving the
strength of components than the existing shot peening
technique.
'Shot peening, which fires tiny little metal balls at a surface,
would only achieve compressive stress in the top quarter of a
millimetre of a material. With the laser, we can look at peening
more complex structures. We could use this on an undercarriage, and
expect to use it on advanced fighters with monolithic structures,
such as the Joint Strike Fighter.'
At peak power the laser produces a billion watts in 20
nanosecond bursts, with five bursts per second. Between the laser
beam and the component is a layer of de-ionised water and an
ablative material - one that absorbs heat by melting - protecting
the component.
The laser heats the ablative layer in nanoseconds, vaporising
its surface and turning it into a plasma, or ionised gas. The water
layer, through which the laser passes before reaching the ablative
material, is used to contain the resulting plasma.
The rapid heating causes the ablative layer to expand, sending a
kinetic energy 'shockwave' through its structure, which is
transferred to the component. This shockwave alters the first
millimetre or two of the component's surface.
The laser is housed in a 3m x 6m clean room. A smaller
transportable laser is in development for use on military bases and
in aircraft hangars.
Laser peening was first developed in 1965 but has only recently
been used commercially.
Since 1997, Metal Improvement has been developing the laser with
the help of US government agencies.
The neodymium doped glass laser used in the process was designed
for an inertial fusion power research programme run by the US
government's California-based Lawrence Livermore National
Laboratory, the US Air force's x-ray lithography research unit, and
US military research agency Darpa.
In inertial fusion, high-powered lasers are used to repeatedly
implode fuel pellet atoms inside a chamber, creating
energy-generating plasma.
Read more:
http://www.theengineer.co.uk/news/shedding-light-on-metal-fatigue/268785.article#ixzz3CNC8TWYV
Shedding light on metal fatigue
