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Laser ultrasonic testing (LUT)
combines the sensitivity of ultrasonic
inspection with the flexibility of optical
systems in dealing with complex inspection
problems. It works well in the testing of
metals, composite materials, ceramics, and
liquids. Its remote nature allows the rapid
inspection of curved surfaces on fixed or
moving parts. It can measure parts in
hostile environments or at temperatures well
above those that can be tolerated using
existing techniques. Its accuracy and
flexibility have made it an attractive new
option in the non-destructive testing
market.
LUT is a remote implementation of
conventional ultrasonic testing (UT) that
normally uses contact transducers, squirter
transducers, or immersion systems. A
schematic layout of a laser ultrasonic
testing system is shown in the figure below.
The setup consists of a small, fiber-coupled
measurement heads that is placed near the
part to be evaluated and a remote base
station containing all support equipment. |
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Schematic layout of laser
ultrasonic inspection system. |
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LUT systems operate by first generating
ultrasonic waves in a sample using a pulsed
laser. When the laser pulse strikes the
sample, ultrasonic waves are generated
through a thermoelastic process or by
ablation. As shown in the figure below, the
full complement of waves (longitudinal,
shear, surface, and plate) can be generated
with lasers. These waves interrogate a
feature of interest in the interior or
surface of the sample and then propagate to
the surface position of the detection laser.
The resulting surface displacement is
measured with the laser ultrasonic receiver.
The measured signal is then processed to
yield and display the required information.
As with conventional UT, measurements can be
performed in the pulse-echo,
through-transmission and pitch-catch
configuration.
laser ultrasonic system in the typical pulse-echo geometry
Laser ultrasonics allows manufacturers to
perform critical measurements early in the
manufacturing process. It enables
post-process of parts for quality control.
It also allows overhaul and repair shops to
test manufactured components of equipment
such as aircraft for structural defects and
metallurgical properties while still in
service. These types of measurements are
crucial to manufacturing industries to
improve the level of quality in products and
to control the cost of parts. They permit
industry to minimize the cost of stopping
production lines to correct part defects
that can be identified during real-time
monitoring using laser ultrasonic
technology.
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Laser Ultrasonics