Ultrasonic Testing (UT) uses high frequency sound energy to conduct
examinations and make measurements. Ultrasonic inspection can
be used for flaw detection/evaluation, dimensional measurements,
material characterization, and more. To illustrate the general
inspection principle, a typical pulse/echo inspection configuration
as illustrated below will be used.
A typical UT inspection system consists of several functional
units, such as the pulser/receiver, transducer, and display devices.
A pulser/receiver is an electronic device that can produce high
voltage electrical pulses. Driven by the pulser, the transducer
generates high frequency ultrasonic energy. The sound energy is
introduced and propagates through the materials in the form of
waves. When there is a discontinuity (such as a crack) in the
wave path, part of the energy will be reflected back from the
flaw surface. The reflected wave signal is transformed into an electrical
signal by the transducer and is displayed on a screen. In the
applet below, the reflected signal strength is displayed versus
the time from signal generation to when a echo was received. Signal
travel time can be directly related to the distance that the signal
traveled. From the signal, information about the reflector location,
size, orientation and other features can sometimes be gained.
Ultrasonic Inspection is a very useful and versatile
NDT method. Some of the advantages of ultrasonic inspection that
are often cited include:
It is sensitive to both surface and subsurface discontinuities.
The depth of penetration for flaw detection or measurement
is superior to other NDT methods.
Only single-sided access is needed when the pulse-echo technique
It is highly accurate in determining reflector position and
estimating size and shape.
Detailed images can be produced with automated systems.
It has other uses, such as thickness measurement, in addition
to flaw detection.
As with all NDT methods, ultrasonic inspection also has its limitations,
Surface must be accessible to transmit ultrasound.
Skill and training is more extensive than with some other
It normally requires a coupling medium to promote the transfer
of sound energy into the test specimen.
Materials that are rough, irregular in shape, very small,
exceptionally thin or not homogeneous are difficult to inspect.
Cast iron and other coarse grained materials are difficult
to inspect due to low sound transmission and high signal noise.
Linear defects oriented parallel to the sound beam may go
Reference standards are required for both equipment calibration
and the characterization of flaws.
The above introduction provides a simplified introduction to
the NDT method of ultrasonic testing. However, to effectively
perform an inspection using ultrasonics, much more about the method
needs to be known. The following pages present information on
the science involved in ultrasonic inspection, the equipment that
is commonly used, some of the measurement techniques used, as
well as other information.