TWI Frequently asked questions
An ultrasonic set, or flaw detector, has an oscillator 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 crystalare 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 acouplant, 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 most of the energy of the vibration will be reflected, like anecho 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 isamplified, 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)
