Introduction to Ultrasonic Flaw Detection
Ultrasonic Flaw Detection Technique is used for the detection of internal and surface (particularly distant surface) defects in sound conducting materials.
The principle is in some respects similar to echo sounding. A short pulse of ultrasound is generated by means of an electric charge applied to a piezo electric crystal, which vibrates for a very short period at a frequency related to the thickness of the crystal. In flaw detection this frequency is usually in the range of one million to six million times per second (1 MHz to 6 MHz). Vibrations or sound waves at this frequency have the ability to travel a considerable distance in homogeneous elastic material, such as many metals with little attenuation. The velocity at which these waves propagate is related to the Youngs Modulus for the material and is characteristic of that material. For example the velocity in steel is 5900 metres per second, and in water 1400 metres per second.
Ultrasonic energy is considerably attenuated in air, and a beam propagated through a solid will, on reaching an interface (e.g. a defect, or intended hole, or the backwall) between that material and air reflect a considerable amount of energy in the direction equal to the angle of incidence.
For contact testing the oscillating crystal is incorporated in a hand held probe, which is applied to the surface of the material to be tested. To facilitate the transfer of energy across the small air gap between the crystal and the test piece, a layer of liquid (referred to as "couplant"), usually oil, water or grease, is applied to the surface.