transducers are manufactured for a variety of applications and
can be custom fabricated when necessary. Careful attention must
be paid to selecting the proper transducer for the application.
A previous section on Acoustic
Wavelength and Defect Detection gave a brief overview of factors
that affect defect detectability. From this material, we know
that it is important to choose transducers that have the desired
frequency, bandwidth, and focusing to optimize inspection capability.
Most often the transducer is chosen either to enhance the sensitivity
or resolution of the system.
Transducers are classified into groups according to the application.
transducers are used for direct contact inspections, and
are generally hand manipulated. They have elements protected
in a rugged casing to withstand sliding contact with a variety
of materials. These transducers have an ergonomic design so
that they are easy to grip and move along a surface. They
often have replaceable wear plates to lengthen their useful
life. Coupling materials of water, grease, oils, or commercial
materials are used to remove the air gap between the transducer
and the component being inspected.
transducers do not contact the component. These transducers
are designed to operate in a liquid environment and all connections
are watertight. Immersion transducers usually have an impedance
matching layer that helps to get more sound energy into the
water and, in turn, into the component being inspected. Immersion
transducers can be purchased with a planer, cylindrically
focused or spherically focused lens. A focused transducer can
improve the sensitivity and axial resolution by concentrating the
sound energy to a smaller area. Immersion transducers are typically
used inside a water tank or as part of a squirter or bubbler
system in scanning applications.
More on Contact Transducers.
Contact transducers are available in a
variety of configurations to improve their usefulness for a variety
of applications. The flat contact transducer shown above is used
in normal beam inspections of relatively flat surfaces, and where
near surface resolution is not critical. If the surface is curved,
a shoe that matches the curvature of the part may need to be added
to the face of the transducer. If near surface resolution is important
or if an angle beam inspection is needed, one of the special contact
transducers described below might be used.
element transducers contain two independently operated elements
in a single housing. One of the elements transmits and the other
receives the ultrasonic signal. Active elements can be chosen for their sending and
receiving capabilities to provide a transducer with a cleaner signal,
and transducers for special applications, such as the inspection of
course grained material. Dual element transducers are especially
well suited for making measurements in applications where reflectors
are very near the transducer since this design eliminates the
ring down effect that single-element transducers experience (when
single-element transducers are operating in pulse echo mode, the
element cannot start receiving reflected signals until the element
has stopped ringing from its transmit function). Dual element transducers
are very useful when making thickness measurements of thin materials
and when inspecting for near surface defects. The two elements
are angled towards each other to create a crossed-beam sound path
in the test material.
line transducers provide versatility with a variety of replaceable
options. Removable delay line, surface conforming membrane, and
protective wear cap options can make a single transducer effective
for a wide range of applications. As the name implies, the primary
function of a delay line transducer is to introduce a time delay
between the generation of the sound wave and the arrival of any
reflected waves. This allows the transducer to complete its "sending"
function before it starts its "listening" function so
that near surface resolution is improved. They are designed for
use in applications such as high precision thickness gauging of
thin materials and delamination checks in composite materials.
They are also useful in high-temperature measurement applications
since the delay line provides some insulation to the piezoelectric
element from the heat.
beam transducers and wedges are typically used to introduce
a refracted shear wave into the test material. Transducers can
be purchased in a variety of fixed angles or in adjustable versions
where the user determines the angles of incidence and refraction.
In the fixed angle versions, the angle of refraction that is marked
on the transducer is only accurate for a particular material,
which is usually steel. The angled sound path allows the sound
beam to be reflected from the backwall to improve detectability
of flaws in and around welded areas. They are also used to generate
surface waves for use in detecting defects on the surface of a
Normal incidence shear wave transducers are unique because
they allow the introduction of shear waves directly into a test piece
without the use of an angle beam wedge. Careful design has enabled
manufacturing of transducers with minimal longitudinal wave contamination.
The ratio of the longitudinal to shear wave components is generally
Paint brush transducers are used to scan wide areas. These
long and narrow transducers are made up of an array of small crystals
that are carefully matched to minimize variations in performance
and maintain uniform sensitivity over the entire area of the transducer.
Paint brush transducers make it possible to scan a larger area
more rapidly for discontinuities. Smaller and more sensitive transducers
are often then required to further define the details of a discontinuity.