reading this section you will be able to do
why it is important to know about sound refraction and Snell's
Law when performing an angle beam inspection.
what a shear wave is.
Often straight beam testing will not
find a defect. For example, if the defect is vertical and thin
enough, it will not reflect enough sound back to the transducer
to let the tester know that it exists. In cases like this, another
method of ultrasound testing must be used. The other method of
ultrasound testing is angle beam testing. Angle beam testing
uses an incidence of other than 90 degrees. In contact testing,
an angled plastic block is place between the transducer and the
object to create the desired angle. For angle beam testing in
immersion systems, a plastic block is not needed because the transducer
can simply be angled in the water.
If the angle of incidence is changed
to be anything other than 90 degrees, longitudinal waves and a
second type of sound wave are produced. These other waves are
called shear waves. Because the wave entered at an angle,
it does not all travel directly through the material. Molecules
in the test object are attracted to each other because solids
have strong molecular bonds. The molecules carrying the sound
are attracted to their surrounding molecules. Because of the angle,
those sound carrying molecules get pulled by attracting forces
in a direction perpendicular to the direction of the wave. This
produces shear waves, or waves whose molecules travel perpendicular
to the direction of the wave.
Angle beam testing and a change in
the angle of incidence also creates further complications. Remember
that when a wave hits a surface at an angle, it will be refracted,
or bent, when it enters the new medium. Thus, the shear waves
and the longitudinal waves will be refracted in the test object.
The amount of refraction depends on the speed of sound in the
two mediums between which the wave is traveling. Since the speed
of shear waves is slower than the speed of longitudinal waves,
their angles of refraction will be different. By using Snells
law, we can calculate the angle of refraction if we know the speed
of sound in our material.
- An angle
beam test cannot be performed unless the angle of refraction
is calculted using Snell's law, and the speed of sound must
be known too.
waves are produced when the angle of incidence is not 90 degrees.