Ultrasonic flaw detection is a method based on the analyzing of ultrasonic vibration propagation with a frequency of 0.5 — 25 MHz in test objects using an ultrasonic transducer and a flaw detector. One of the most commonly used NDT methods.
Sound waves keep trajectory of motion in a homogeneous material. A reflection of acoustic waves occurs from the interface of media with different specific acoustic resistances. The more the acoustic resistances differ, the more the sound waves are reflected from the interface of the media. Since inclusions in metal usually contain gas (a mixture of gases) resulting from welding process, casting, etc. and do not have time to go out when the metal solidifies, gas mixture has five orders of magnitude less specific acoustic resistance than metal itself, then reflection will be almost complete.
Resolution of acoustic analysis that is the ability to detect small flaws separately from each other, is determined by a length of a sound wave, which in turn depends on the acoustic vibrations input frequency. The higher the frequency is, the shorter the wavelength is. The effect occurs due to the fact when the size of the obstacle is less than a quarter of the wavelength, the reflection of the vibrations practically does not occur, but their diffraction dominates. Therefore, as a rule, the frequency of ultrasound tends to increase. On the other hand, as the frequency of vibrations increases, their attenuation increases rapidly, which reduces possible test area. The practical compromise is the frequency range from 0.5 to 10 MHz.