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Operating Principle Of Ultrasonic Flaw Detectors-UFD

Operating Principle Of Ultrasonic Flaw Detectors-UFD

  • March 3, 2022

Operating Principle Of Ultrasonic Flaw Detectors-UFD

1)The Principle of ultrasonic testing

 The ultrasonic inspection refers to non-destructive methods. This method is widely used for welded joints testing for different materials: low-alloyed and low-carbon steel, aluminium, copper and their alloys. Mechanical oscillations propagate in the elastic “body”. They are divided into: infrasonic (frequency of vibrations up to 30 Hz), sonic (20 – 20* 10Hz), ultrasonic (2 * 104 to 109 Hz) and hypersonic (over 109 Hz). During acoustic waves propagation, the particles of the medium, that the waves are passing though, oscillate relatively of their equilibrium points. If particles oscillate along, these waves are called longitudinal, if perpendicular – transverse.

Both: longitudinal and transverse waves can occur in hard materials. There are also surface waves which propagate only on the surface of the body. For welded joints testing for ultrasonic flaw detection, generally transverse and longitudinal ultrasonic waves are used. The speed of ultrasonic waves depends on the material or the environment properties in which they  propagated.


Ultrasonic wave carries a certain amount of energy along the direction of it’s propagation, which is characterized by intensity of ultrasound (quantity of energy that is transferred along with the wave into the object body through area, which is perpendicular to the direction of propagation).


The longer ultrasonic wave propagates, the less is it’s intensity. The length of the path travelled by wave can be measured by the value of the damping coefficient. High hardness materials have their own absorption and scattering coefficients.


Piezoelectricity is the electric charge that accumulates in certain solid materials such as crystals, certain ceramics in response to applied mechanical stress. The piezoelectric effect is a reversible process.  The reverse piezoelectric effect manifest as  internal generation of a mechanical strain resulting from an applied electrical field. High-frequency electrical oscillations given by generator are converted through the piezoelectric crystals into mechanical vibrations with frequency up to 500 – 1000 MHz.


So  a high frequency generator sends impulse to piezoelectric element that issues ultrasonic oscillations, that being reflected from a flaw, or the opposite surface fall onto the other piezoelectric element that being stressed by those oscillations sends electrical impulse to the instrument’s input. After processing thus obtained signal is displayed on the device’s monitor.


Special protective housing for emitting along with collecting piezoelectric elements is called ultrasonic piezoelectric transducer (PET) or UT-probe.


UT-probes  fall into several types: Combined transducer – ultrasonic transmitter and receiver in the same housing; Separate transducer – ultrasonic transmitter and receiver are as two separate probes; Dual element transducer – ultrasonic transmitter and receiver
as two individual crystals, located in the same probe housing. Ultrasonic waves can be emitted continuously or as separate pulses – it depends on the operating mode of the generator.