Single Compression
Single element or straight beam transducers
are used to measure thickness and to detect
flaws on plates, bars, forgings, castings
and extrusions. During testing they are
applied directly to the flat surfaces of
the test material or object. Transducers
with smaller diameters can be applied to
test slightly curved materials.
Single element contact transducers work
by emitting
compression (longitudinal) waves into the
test material. Due to the fact that this
type of transducer comes into direct contact
with test materials when being used, the
wear plates are constructed with highly
durable material.
Twin Compression
Dual element contact (pitch-catch) transducers
measure thickness and detect flaws and corrosion
in thin materials, especially where near
surface resolution is required. They focus
very close to the front surface, making
them ideal for pitting and corrosion tests,
braze inspection and lamination evaluation.
This focusing effect of the dual
transducer makes it ideal for pipes and
other curved surfaces.
Dual element transducers utilise separate
transmitting and receiving elements, mounted
on delay lines that are usually cut at an
angle. This configuration improves near
surface resolution by eliminating recovery
problems. In addition, the “crossed beam”
design provides a pseudo-focus that makes
duals more sensitive to echoes from
irregular reflectors such as corrosion and
pitting.
One consequence of the dual element design
is a sharply defined distance-amplitude
curve. In general a decrease in the roof
angle to an increase in the transducer element
size will result in longer pseudo-focal
distance and an increase in useful range.
Immersion Transducers
Immersion transducers are usually used for
mechanised or automatic systems and, in
principle, operate the same way as normal
contact compression transducers.
Most applications take place in immersion
tanks filled with water, where the test
object is placed on a turntable or roller
system so that the object is moved at a
constant speed past the probe. This technique
offers the best coupling conditions to provide
reproducible results. Compared to contact
transducers where all the parameters are
defined “as in steel”, immersion transducer
parameters are defined in water. Since the
speed of sound in steel is approx. 5920
m per sec and in water is 1480m per sec.
This gives a ratio of 4 to 1, which means
it takes the same time to travel through
10 mm of water as it does to travel through
40 mm of steel.
Immersion transducers are available as flat
faced or focussed. There are two types of
focussing spherical and line. Spherical,
spot or point focussing gives a reduced
but concentrated beam width, which provides
the best possible flaw detection capability
, but takes longer to scan because of the
reduced beam width. Line focussed probes
give larger beam width in one axis with
a concentrated reduced beam in the other
axis. The working range of the focussed
probes is much less than the flat-faced
probes and in fact the focal length occurs
within the near field length.
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Angle Beam Transducers
Angle beam transducers allow the sound beam
to be introduced into the test material
at an angle. Plastic wedges of controlled
geometry are attached to the transducer
active element in order to establish the
desired angle. Sonatest wedges are precision
engineered to produce a refracted shear
wave within the test object at specific
angles, as indicated on the wedge or transducer
housing. The refracted beam angle should
be selected to ensure that the sound beam
angle will be, as much as possible, perpendicular
to the plane of expected flaws.
In some cases, the geometry of the test
object will dictate the selection of beam
angle. With regard to frequency however,
the same general rule applies—which is to
select the lowest frequency which provides
adequate flaw sensitivity. Both material
noise and attenuation are minimised at lower
frequencies.
Delay Line Transducers
Delay line transducers transmit and receive
sound waves with one element, coupled to
the surface as with compression transducers.
The crystal is held off from the test piece
surface by a delay block. This permits inspection
very close to the test piece surface.
Wheel Transducers
Wheel transducers operate in a similar fashion
to delay line models. They are typically
used in applications where a large area
must be scanned and/or where the test piece
material is sensitive to conventional ultrasonic
couplant's.
Custom Transducers
Custom transducers are often required for
specialist applications. These often contain
a number of elements for specific locations
and angles. An example of this is the probe
used to test railway tracks that incorporates
both forward and backward facing twin element
arrangements either side of a conventional
twin crystal arrangement.
Complex transducers such as this are designed
in-house for a variety of specific applications.
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