Ultrasonic Flaw Detection
This 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 piezoelectric 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 Young’s 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 back wall) between that material and air reflect a considerable amount of energy in the direction equal to the angle of incidence.
Dye penetrant testing:
This method is frequently used for the detection of surface breaking flaws in non-ferromagnetic materials. The subject to be examined is first of all chemically cleaned, usually by vapour phase, to remove all traces of foreign material, grease, dirt, etc. from the surface generally, and also from within the cracks. Next the penetrant (which is a very fine thin oil usually dyed bright red or ultra-violet fluorescent) is applied and allowed to remain in contact with the surface for approximately fifteen minutes. Capillary action draws the penetrant into the crack during this period. The surplus penetrant on the surface is then removed completely and thin coating of powdered chalk is applied.
After a further period (development time) the chalk draws the dye out of the crack, rather like blotting paper, to form a visual, magnified in width, indication in good contrast to the background.