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Monday, 12 September 2016

TRANSDUCER BEAM SPREAD IN ULTRASONIC TESTING

Round transducers are often referred to as piston source transducer because the sound field resembles a cylindrical mass in front of the transducer. However the energy in the beam does not remain in a cylinder, but instead spread out as it propagates through the materials. The phenomenon is usually referred to as beam spread but is sometimes also referred to as beam divergence or ultrasonic diffraction. It should be noted that there is actually a difference between beam spread and beam divergence. Beam spread is a measure of the whole angel from side to side of the main lobe of the sound beam in the far field. Beam divergence is a measure of the angle from one side of the sound beam to the central axis of the beam in the far field. Therefore, beam spread is the twice the beam divergence.
Although beam spread must be considered when performing an ultrasonic inspection, it is important to note that in the far field, or fraunhofer zone, the maximum sound pressure is always found along the acoustic axis (centerline)of the transducer. Therefore, the strongest are likely to come Frome the area directly in front of the transducer. Beam spread occurs because the vibrating particle of the material (through which the wave is travelling) do not always transfer all of their energy in the direction of wave propagation. Recall waves propagate through the transfer of energy from one particle to another in the medium. If the particles are not directly aligned in the direction of wave propagation, some of the energy will get transferred off at an angle. (picture what happens when one ball hits another ball slightly off centre). In the near field, constructive and destructive wave interference fill the sound field with fluctuation. At the start of the far field, the beam strength is always greatest at the centre of the beam and diminishes as it spreads outward. Beam angle is important consideration in transducer selection for a couple of reasons. First beam spread lower the amplitude of reflection since sound are less concentrated and, thereby weaker. Second, beam spread may result in more difficulty in interpreting signals due to reflection from the literal sides of the test object or other features outside of the inspection area. Characterization of the sound field generated by transducer is a prerequisite to understanding observed signals.