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Continuous Laser
Generation of Ultrasound
Generating and detecting ultrasound is
a standard method of nondestructive
evaluation of materials. The amplitude
or speed of the ultrasound is used to
determine the strength of the
material, or to sense defects.
Pulsed lasers are used to generate
ultrasound remotely in situations
where contact transducers cannot be
used or is not beneficial. In these
laser-based systems, the scanning rate
is limited by the repetition rates of
the pulsed lasers, ranging between 10
and 400 Hz for lasers with sufficient
pulse widths and energies. A hundred
shots per second may at first seem
like a good pace, but if one needs to
scan a square meter with millimeter
resolution, the scan would take
between three hours and two days.
In place of the pulsed laser, we
believe that a high-powered continuous
wave laser (think about a laser
pointer with two million times the
power) can be used in replace of the
pulsed laser to create the ultrasound
in the material. As shown in the
figure, a scanning mirror sweeps the
beam across the surface, generating an
ultrasonic wavefront in the material.
Our
calculations reveal that with
sufficient power, detectable
ultrasound waves can be produced at a
scanning rate that is more than 20,000
times faster. All components of the
system are commercially available to
produce such a system.
Fig1: A typical CLGU setup used
for sensing generating and
detecting ultrasound in
materials. Changes in the
wavefront as it passes through
the material indicate material
defects. The deflection
shown is greatly
exaggerated. Here a GCLAD
system is used for detection,
but a scanning laser
interferomteric system may also
be used.
The
primary technology challenge is
developing a fast rotating mirror that
can operate at the desired angular
velocity and handle the high incident
power.
The primary
advantage is the time savings.
As of 2006, the Lockheed-Martin
LaserUT system had a rate of 400
inspection points per second, limited
by the pulse rates of the lasers. With
this scan rate and 2 mm steps, the
system can scan at a rate of 5.8
m2/hour. If steps are reduced to 1 mm,
it would take 42 minutes to scan a
squared meter.
In contrast, CLGU
could scan a single meter-long line in
less than a millisecond, and a squared
meter with millimeter resolution in
less than a second. If realized, CLGU
can be used for the rapid inspection
of airplane fuselages, full inspection
of composite panels and during
manufacturer, and remote inspection of
hot metals during production. As a
research tool, CLGU allows the study
of acoustic wavefronts in materials,
and can provide ultrasound ’videos’ of
materials under stress and impact.
Defect Resolution
Increased
ultrasound frequency enables the
detection of smaller flaws. The
frequency distribution created by
pulsed laser generation is inversely
proportional to the pulse width and
cannot easily be modified. For CLGU,
the center frequency distribution is
determined by the scanning velocity
and the beam radius. Thus,
ultrasound frequency can be varied by
changing either the scanning rate or
the beam spot size.
CLGU
Research
We
are currently exploring different
approaches to funding the R&D of
CLGU. As you can imagine, this
requires the use of a facility with the
appropriate lasers. If you have
any ideas on how to get this started,
feel free to share them with us.
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