and Defect Detection
In ultrasonic testing, the inspector must
make a decision about the frequency of the transducer that will
be used. As we learned on the previous page, changing the frequency
when the sound velocity is fixed will result in a change in the
wavelength of the sound. The wavelength of the ultrasound
used has a significant effect on the probability of detecting a
discontinuity. A general rule of thumb is that
a discontinuity must be larger than one-half the wavelength
to stand a reasonable chance of being detected.
Sensitivity and resolution are two terms
that are often used in ultrasonic inspection to describe a technique's
ability to locate flaws. Sensitivity is the ability to locate
small discontinuities. Sensitivity generally increases with higher
frequency (shorter wavelengths). Resolution is the ability of
the system to locate discontinuities that are close together within
the material or located near the part surface. Resolution also
generally increases as the frequency increases.
The wave frequency can also affect the
capability of an inspection in adverse ways. Therefore, selecting
the optimal inspection frequency often involves maintaining a
balance between the favorable and unfavorable results of the selection.
Before selecting an inspection frequency, the material's grain structure and thickness, and the discontinuity's type, size, and probable location should be considered. As frequency increases, sound tends to scatter
from large or course grain structure and from small imperfections
within a material. Cast materials often have coarse grains and
other sound scatters that require lower frequencies to be used
for evaluations of these products. Wrought and forged products
with directional and refined grain structure can usually be inspected
with higher frequency transducers.
Since more things in a material are likely
to scatter a portion of the sound energy at higher frequencies,
the penetrating power (or the maximum depth in a material that
flaws can be located) is also reduced. Frequency also has an effect
on the shape of the ultrasonic beam. Beam spread, or the divergence
of the beam from the center axis of the transducer, and how it
is affected by frequency will be discussed later.
It should be mentioned, so as not to be
misleading, that a number of other variables will also affect
the ability of ultrasound to locate defects. These include the pulse
length, type and voltage applied to the crystal, properties of
the crystal, backing material, transducer diameter, and the receiver
circuitry of the instrument. These are discussed in more detail
in the material on signal-to-noise ratio.