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In this non-destructive testing method, in order to detect the discontinuities in the material to be examined, it is based on the propagation of high frequency (0.1-20 MHZ) ultrasonic waves produced by the mayene probe in the test material and after hitting a discontinuity, they are reflected back to the probe and thus detected by the probe.
The waves detected by the probe (with the piezoelectric effect) are converted into electrical signals and appear on the screen of the cathode rays tube as echoes (echoes), which are the precursors of the materials internal structure. The positions and amplitudes of the echoes observed on the screen provide information about the discontinuity s location and dimensions.
It can be used to detect expected volumetric defects and crack type surface defects in metallic or non-metallic materials. Since discontinuities are perpendicular to the ultrasonic beam, they are best detected, it is difficult to apply ultrasonic method for coarse grained structures especially austenitic materials. High-frequency sound waves sent into the material are reflected when they hit an obstacle on the sound path. Depending on the impact angle, the reflected signal may or may not reach the receiving probe. The reflected signal reaching the receiving probe creates an echo indication on the screen of the ultrasonic inspection device. Based on the position of the echo, the coordinates of the reflector within the inspection piece can be calculated. In addition, the height of the echo gives an idea of the size of the reflector. It may also be possible to make a comment about the type of reflector by looking at the shape of the echo signal.
If the sound velocity and sound attenuation characteristics of the inspection piece show strong regional variations, it becomes difficult to make an accurate assessment. Inspection may sometimes be impossible in materials where sound attenuation is too great due to coarse grain structure or absorption. A sufficiently large surface should be prepared to be accessible for inspection. The surface condition directly affects the inspection parameters. Thin parts are relatively difficult to inspect. It is not possible to detect planar discontinuities located parallel to the sound beam axis. Usually reference standard blocks are needed. High-frequency sound waves are produced by a piezoelectric crystal inside a piece called a probe. The frequency range used in ultrasonic testing of metallic materials can be between 500 kHz and 10 MHz. The appropriate frequency is determined according to the microstructure characteristics of the inspection part. A suitable contact fluid (oil, grease, water, etc.) must be used so that sound waves can penetrate into the material (sensation waves cannot propagate in space) when the probe is contacted with the inspection surface. By moving the probe on the inspection surface, the positions and heights of the echoes arising from the geometry of the part are evaluated and error analysis is performed. The most commonly used wave types for ultrasonic inspection are longitudinal (pressure) and transverse (shear) waves. When working with probes with zero degree entry angle, called normal probe, the waves propagating through the material are longitudinal waves. Angle probes, on the other hand, send transverse waves into the material, usually with 45°, 60° and 70° entry angles (these values are for steel material).
