by Tom Nelligan
Ultrasonic thickness gaging is a widely used nondestructive test technique for measuring the thickness of a material
from one side. The first commercial ultrasonic gages, using principles derived from sonar, were introduced in the
late 1940s. Small, portable instruments optimized for a wide variety of test applications became common in the 1970s.
Later advances in microprocessor technology led to new levels of performance in today's sophisticated,
easy-to-use miniature instruments.
1. What can be measured Virtually any common engineering material can be measured ultrasonically.
Ultrasonic thickness gages can be set up for metals, plastics, composites, fiberglass, ceramics, and glass. On-line
or in-process measurement of extruded plastics and rolled metal is often possible, as is measurement of individual
layers or coatings in multilayer fabrications. Liquid levels and biological samples can also be measured. Ultrasonic
gaging is always completely nondestructive, with no cutting or sectioning required.
Materials that are generally not suited for conventional ultrasonic gaging include wood, paper, concrete, and foam
products.
2. How ultrasonic thickness gages work Sound energy can be generated over a broad frequency
spectrum. Audible sound occurs in a relatively low frequency range with an upper limit around twenty thousand cycles
per second (20 Kilohertz). The higher the frequency, the higher the pitch we perceive. Ultrasound is sound energy at
higher frequencies, beyond the limit of human hearing. Most ultrasonic testing is performed in the frequency range
between 500 KHz and 20 MHz, although some specialized instruments go down to 50 KHz or lower and as high as 225 MHz.
Whatever the frequency, sound energy consists of a pattern of organized mechanical vibrations traveling through a
medium such as air or steel according to the basic laws of wave physics.
All ultrasonic thickness gages work by very precisely measuring how long it takes for a sound pulse that has been
generated by a probe called an ultrasonic transducer to travel through a test piece. Because sound waves reflect from
boundaries between dissimilar materials, this measurement is normally made from one side in a "pulse/echo"
mode, where the gage measures the round trip transit time of a pulse that reflects off the far side or back wall of
the test piece.
The transducer contains a piezoelectric element which is excited by a short electrical impulse to generate a burst of
ultrasonic waves. The sound waves are coupled into the test material and travels through it until they encounter a
back wall or other boundary. The reflections then travel back to the transducer, which converts the sound energy back
into electrical energy. In essence, the gage listens for the echo from the opposite side. Typically this time
interval is only a few millionths of a second. The gage is programmed with the speed of sound in the test material,
from which it can then calculate thickness using the simple mathematical relationship T = (V) x (t/2)
where
T = the thickness of the part
V = the velocity of sound in the test material
t = the measured round-trip transit time
It is important to note that the velocity of sound in the test material is an essential part of this calculation.
Different materials transmit sound waves at different velocities, generally faster in hard materials and slower in
soft materials, and sound velocity can change significantly with temperature. Thus it is always necessary to
calibrate an ultrasonic thickness gage to the speed of sound in the material being measured, and accuracy can be only
as good as this calibration.
Sound waves in the megahertz range do not travel efficiently through air, so a drop of coupling liquid is used
between the transducer and the test piece in order to achieve good sound transmission. Common couplants are glycerin,
propylene glycol, water, oil, and gel. Only a small amount is needed, just enough to fill the extremely thin air gap
that would otherwise exist between the transducer and the target.
There are three common ways of measuring the time interval that represents the sound wave's travel through the
test piece. Mode 1 is the most common approach, simply measuring the time interval between the excitation pulse that
generates the sound wave and the first returning echo and subtracting a small zero offset value that compensates for
fixed instrument, cable, and transducer delays. Mode 2 involves measuring the time interval between an echo returned
from the surface of the test piece and the first backwall echo. Mode 3 involves measuring the time interval between
two successive backwall echoes. The type of transducer and specific application requirements will usually dictate the
choice of mode.
3. Transducer types
Contact transducers: As the name implies, contact transducers are used in direct contact with the
test piece. Measurements with contact transducers are often the simplest to implement and they are usually the first
choice for most common thickness gaging applications other than corrosion gaging.
Delay Line transducers: Delay line transducers incorporate a cylinder of plastic, epoxy, or fused
silica known as a delay line between the active element and the test piece. A major reason for using them is for thin
material measurements, where it is important to separate the excitation pulse recovery from backwall echoes. A delay
line can be used as a thermal insulator, protecting the heat-sensitive transducer element from direct contact with
hot test pieces, and delay lines can also be shaped or contoured to improve sound coupling into sharply curved or
confined spaces.
Immersion transducers: Immersion transducers use a column or bath of water to couple sound energy
into the test piece. They can be used for on-line or in-process measurement of moving product, for scanned
measurements, or for optimizing coupling into sharp radiuses, grooves, or channels.
Dual element transducers: Dual element transducers, or simply "duals", are used primarily
for measurement of rough, corroded surfaces. The incorporate separate transmitting and receiving elements mounted on
a delay line at a small angle to focus energy a selected distance beneath the surface of a test piece. Although
measurement with duals is sometimes not as accurate as with other types of transducers, they usually provide
significantly better performance in corrosion survey applications.
4. Things to consider In any ultrasonic gaging application, the choice of gage and transducer will
depend on the material to be measured, thickness range, geometry, temperature, accuracy requirements, and any special
conditions that may be present. Listed below are the major factors that should be considered.
Material: The type of material and the range of thickness being measured are the most important
factors in selecting a gage and transducer. Many common engineering materials including most metals, ceramics, and
glass transmit ultrasound very efficiently and can easily be measured across a wide thickness range. Most plastics
absorb ultrasonic energy more quickly and thus have a more limited maximum thickness range, but can still be measured
easily in most manufacturing situations. Rubber, fiberglass, and many composites can be much more attenuating and
often require high penetration gages with pulser/receivers optimized for low frequency operation.
Thickness: Thickness ranges will also dictate the type of gage and transducer that should be
selected. In general, thin material are measured at high frequencies and thick or attenuating materials are measured
at low frequencies. Delay line transducers are often used on very thin materials, although delay line (and immersion)
transducers will have a more restricted maximum measurable thickness due to potential interference from a multiple of
the interface echo. In some cases involving broad thickness ranges and/or multiple materials, more than one
transducer type may be required.
Geometry: As the surface curvature of a part increases, the coupling efficiency between the
transducer and the test piece is reduced, so as radius of curvature increases the size of the transducer should
generally be decreased. Measurement on very sharp radiuses, particularly concave curves, may require specially
contoured delay line transducers or non-contact immersion transducers for proper sound coupling. Delay line and
immersion transducers may also be used for measurement in grooves, cavities and similar areas with restricted
access.
Temperature: Common contact transducers can generally be used on surfaces up to approximately 125° F
or 50° C. Use of most contact transducers on hotter materials can result in permanent damage due to thermal expansion
effects. In such cases, delay line transducers with heat-resistant delay lines, immersion transducers, or high
temperature dual element transducers should always be used.
Phase Reversal: There are occasional applications where a material of low acoustic impedance
(density multiplied by sound velocity) is bonded to a material of higher acoustic impedance. Typical examples include
plastic, rubber, and glass coatings on steel or other metals, and polymer coatings on fiberglass. In these cases the
echo from the boundary between the two materials will be phase reversed or inverted with respect to the echo obtained
from an air boundary. This condition can normally be accommodated by a simple setup change in the instrument, but if
it is not taken into account, readings may be inaccurate.
Accuracy: Many factors affect measurement accuracy in a given application, including proper
instrument calibration, uniformity of material sound velocity, sound attenuation and scattering, surface roughness,
curvature, poor sound coupling, and backwall non-parallelism. All of these factors should be considered when selected
a gage and transducer. With proper calibration, measurements can usually be made to an accuracy of +/- 0.001" or
0.01 mm, and in some cases accuracy can approach 0.0001" or 0.001 mm. Accuracy in a given application can best
be determined through the use of reference standards of precisely known thickness. In general, gages using delay line
or immersion transducers for Mode 3 measurements are able to determine the thickness of a part most precisely.
5. For further information A more detailed discussion of the principles of ultrasonic gaging can be
found in the paper Theory and Application of Precision Ultrasonic Thickness Gaging on this web site.
Also see individual application notes for discussions of particular test procedures. |