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Ultrasonic examination in Non-Destructive Examination


Ultrasound has a frequency above 20 kilohertz (kHz). The human ear can detect frequencies below this, down to 20 hertz. Ultrasonic inspection involves sending a high frequency, mechanical vibration into material and registering and evaluating any echoes that are detected; rather like submarine detection by echo sounding. Ultrasonic examination procedures are widely used for thickness measurement, corrosion monitoring, lamination checks and flaw detection in welds, forgings, castings and pipes.

An ultrasonic set, or flaw detector, has a pulse generator circuit, which sends electrical pulses to a probe. The probe holds a piezoelectric crystal, which vibrates when it receives the electrical pulse. The vibrations from the crystal are ultrasonic, with a frequency in the range 1MHz to 15MHz. Typical frequencies used in weld examination are between 2 and 5 Mhz. The ultrasonic vibrations leave the probe and are conducted into the material to be tested by a couplant, usually grease, oil, water, paste, or gelatin.

In the material, the ultrasonic pulses travel in straight lines, until they hit an interface between two different materials (steel and air for example), or a flaw, when some of the energy of the vibration will be reflected, like an echo from a wall or mountainside. A small amount of the energy is reflected back to the probe, where it vibrates the piezoelectric crystal, generating a tiny electric current. This current returns to the flaw detector, where it is amplified, rectified, filtered and displayed on a cathode ray tube.

Further information can be found in:

R Hamshaw, Introduction to the Non-Destructive Testing of Welded Joints, 2nd edition, Abington Publishing, Cambridge, UK, 1996 (ISBN 1 85573 314 5)

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