Ultrasonic Testing (UT) uses occurances attendant upon generation and diffusion of mechanical vibrations supersonic frequency, i.e. over 16000 Hz.
In the ultrasonic method the mechanical vibrations are generated and introduced into the inspected unit with an ultrasonic probe. The probe is applied to the inspected surface through a thin layer of liquid, gel or oil that would act as a couplant.
The ultrasonic method (UT) allows for detection of any heterogeneities (defects) of the inspected unit, such as: cracks and air-holes. The results are achieved by reflecting a part of the beam of ultrasonic waves, which falls on those defects.
Ultrasonic Testings have been most widely applied to detection of flaws in metallic structures - above all in the steel ones.