THE ULTRASONIC BEAM: Dead Zone, Near Field, Far Field

THE ULTRASONIC BEAM

 Ultrasonic beam can be used as method of defect assessment, it will be necessary to produce a divergent beam in the
material concerned.

The ultrasonic beam produced by the simplest round single crystal probe is basically as shown in Figure.

 


Dead Zone 

Dead zone is a zone where it is not possible to detect defects. Due to imperfect damping of the crystals some waves will interfere with the returning waves. This problem can be overcome by using twin crystals, one transmitting, one receiving. The higher the probe frequency the shorter the Dead Zone.

 

Near Field

Near field is an area of ‘turbulence’ and varying sound intensity. Due to the effect of interference in the near field the signal height from the same size of defect may increase when it is positioned further away from the crystal. Similarly, small defects may be completely overlooked.

 

Far Field

Far field the beam diverges and the signal height from the same size of defect decreases in relation to the distance in accordance with the
inverse square law.

 

It is convenient to define the beam ‘edge’ as the point, across the beam, where the intensity of sound has fallen to one half, or sometimes one tenth of the intensity at the centre of the beam. Whenever possible we use the Far Field in ultrasonic testing,
the near field usually being accommodated within the perspex shoe of the probe.

It can be seen from the formula, that by increasing the probe diameter or increasing the frequency (shorter wavelength), the solid angle of the beam will decrease.

 



THE ULTRASONIC BEAM So that an ultrasonic beam can be used as method of defect assessment, it will be necessary to produce a divergent beam in the material concerned.  The ultrasonic beam produced by the simplest round single crystal probe is basically as shown in Figure.    Dead Zone  Dead zone is a zone where it is not possible to detect defects. Due to imperfect damping of the crystals some waves will interfere with the returning waves. This problem can be overcome by using twin crystals, one transmitting, one receiving. The higher the probe frequency the shorter the Dead Zone.    Near Field Near field is an area of 'turbulence' and varying sound intensity. Due to the effect of interference in the near field the signal height from the same size of defect may increase when it is positioned further away from the crystal. Similarly, small defects may be completely overlooked.    Far Field Far field the beam diverges and the signal height from the same size of defect decreases in relation to the distance in accordance with the inverse square law.    It is convenient to define the beam 'edge' as the point, across the beam, where the intensity of sound has fallen to one half, or sometimes one tenth of the intensity at the centre of the beam. Whenever possible we use the Far Field in ultrasonic testing, the near field usually being accommodated within the perspex shoe of the probe.  It can be seen from the formula, that by increasing the probe diameter or increasing the frequency (shorter wavelength), the solid angle of the beam will decrease.
 Ultrasonic beam



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