In ultrasonic testing (UT), very short ultrasonic pulse-waves with center frequencies
ranging from 0.1-15 MHz and occasionally up to 50 MHz are launched into materials
to detect internal flaws or to characterize materials. A common example is ultrasonic
thickness measurement, which tests the thickness of the test object, for example, to monitor pipework corrosion.
Ultrasonic testing is often performed on steel and other metals and alloys, though it can also be used on concrete,
wood and composites, albeit with less resolution. It is a form of non-destructive testing used in many industries
including aerospace, automotive and other transportation sectors. In ultrasonic testing, an ultrasound transducer
connected to a diagnostic machine is passed over the object being inspected. The transducer is typically separated
from the test object by a couplant (such as oil) or by water, as in immersion testing. However, when ultrasonic
testing is conducted with an Electromagnetic Acoustic Transducer (EMAT) the use of couplant is not required.
There are two methods of receiving the ultrasound waveform, reflection and attenuation. In reflection (or pulse-echo) mode,
the transducer performs both the sending and the receiving of the pulsed waves as the "sound" is reflected back
to the device. Reflected ultrasound comes from an interface, such as the back wall of the object or from an imperfection
within the object. The diagnostic machine displays these results in the form of a signal with an amplitude representing
the intensity of the reflection and the distance, representing the arrival time of the reflection. In attenuation
(or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects
the amount that has reached it on another surface after traveling through the medium. Imperfections or other
conditions in the space between the transmitter and receiver reduce the amount of sound transmitted, thus revealing
their presence. Using the couplant increases the efficiency of the process by reducing the losses in the ultrasonic
wave energy due to separation between the surfaces.
