Pulse-echo ultrasonic measurements can determine the location
of a discontinuity in a part or structure by accurately measuring
the time required for a short ultrasonic pulse generated by a
transducer to travel through a thickness of material, reflect
from the back or the surface of a discontinuity, and be returned
to the transducer. In most applications, this time interval is
a few microseconds or less. The two-way transit time measured
is divided by two to account for the down-and-back travel path
and multiplied by the velocity of sound in the test material.
The result is expressed in the well-known relationship
d = vt/2 or v = 2d/t
where d is the distance from the surface to the discontinuity
in the test piece, v is the velocity of sound waves in
the material, and t is the measured round-trip transit
The diagram below allows you to move a transducer over the surface
of a stainless steel test block and see return echoes as they
would appear on an oscilloscope. The transducer employed is a
5 MHz broadband transducer 0.25 inches in diameter. The signals
were generated with computer software similar to that found in
the Thompson-Gray Measurement Model and UTSIM developed at the
Center for Nondestructive Evaluation at Iowa State University.
Precision ultrasonic thickness gages usually operate at frequencies
between 500 kHz and 100 MHz, by means of piezoelectric transducers
that generate bursts of sound waves when excited by electrical
pulses. A wide variety of transducers with various acoustic characteristics
have been developed to meet the needs of industrial applications.
Typically, lower frequencies are used to optimize penetration
when measuring thick, highly attenuating or highly scattering
materials, while higher frequencies will be recommended to optimize
resolution in thinner, non-attenuating, non-scattering materials.
In thickness gauging, ultrasonic techniques permit quick and
reliable measurement of thickness without requiring access to
both sides of a part. Accuracy's as high as ±1 micron or
±0.0001 inch can be achieved in some applications. It is
possible to measure most engineering materials ultrasonically,
including metals, plastic, ceramics, composites, epoxies, and
glass as well as liquid levels and the thickness of certain biological
specimens. On-line or in-process measurement of extruded plastics
or rolled metal often is possible, as is measurements of single
layers or coatings in multilayer materials. Modern handheld gages
are simple to use and very reliable.