Phased array beam focusing and near field

  • Phased array beam focusing and near field

    Posted by george on 27/11/2021 at 4:59 pm

    I have learnt that “Phased-array probes are composed of several piezoelectric crystals that can transmit/receive independently at different times. To focus the ultrasonic beam, time delays are applied to the elements to create constructive interference of the wave fronts, allowing the energy to be focused at any depth in the test specimen undergoing inspection”

    However, Olympus phased array tutorial states “The near field length in a given material also defines the maximum depth at which a sound beam can be focused. A beam cannot be focused beyond the end of the near field”.

    And also I have learnt that “Because of acoustic variations within a near field, it can be extremely difficult to accurately evaluate flaws in materials when they are positioned within this area”.

    Can someone please explain what is beam focusing, is the beam focusing restricted to near field and if focusing is restricted to near field how can flaws in far field be evaluated.

    charlie replied 7 months ago 2 Members · 2 Replies
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  • Phased Array Ultrasonics (PAUT)

    charlie

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    28/11/2021 at 2:11 pm
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    Niranjan, there have been a few discussions on the NDT.net forum regarding phased-array UT and focussing. Since we use short pulse excitation of elements the “acoustic variations” in the near field are perhaps not as drastic as you may have been led to believe. You need only look at the AVG diagrams for monoelement probes to see that there is usually just a slight dip prior to the maximum at the near field distance. Go to the A-Z Encyclopaedia and see the graph illustrated under the definition for DAC As for the principles of phasing, perhaps the videos in the photoelastic series can help understanding.
    NDT.net has a series I called Photoelastic Visualisation _ Phased Array Sound Fields. There are about 12-13 articles in which I attempt to illustrate how the beams are formed and interact. The phased-array portion starts at Part 14 and with each article there is a short video that you can link to via the link at the top of the first page in each case; e.g. for Part 14 it is Focussing is not something you should be doing with all applications of phased-array UT! It has limited uses where spatial resolution is important. For most “detection” applications you would use an unfocussed beam.
    Since “focussing” requires small delays of the inner elements relative to the outer elements, it is clear that once you have no delays applied to the elements you can no longer focus a beam. No relative delays applied to the elements is the same as a flat unfocussed probe which will have a natural focus at its near field distance. Since the near field distance is fixed by the aperture and wavelength you cannot “focus” beyond it.

  • Phased Array Ultrasonics (PAUT)

    charlie

    Member
    28/11/2021 at 2:16 pm
    2 Points
    Up
    0
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    ::

    Niranjan, there have been a few discussions on the NDT forum regarding phased-array UT and focussing. Since we use short pulse excitation of elements the “acoustic variations” in the near field are perhaps not as drastic as you may have been led to believe. You need only look at the AVG diagrams for monoelement probes to see that there is usually just a slight dip prior to the maximum at the near field distance. Go to the A-Z Encyclopaedia and see the graph illustrated under the definition for DAC As for the principles of phasing, perhaps the videos in the photoelastic series can help understanding.
    NDT has a series I called Photoelastic Visualisation _ Phased Array Sound Fields. There are about 12-13 articles in which I attempt to illustrate how the beams are formed and interact. The phased-array portion starts at Part 14 ) and with each article there is a short video that you can link to via the link at the top of the first page in each case; e.g. for Part 14 it is Focussing is not something you should be doing with all applications of phased-array UT! It has limited uses where spatial resolution is important. For most “detection” applications you would use an unfocussed beam.
    Since “focussing” requires small delays of the inner elements relative to the outer elements, it is clear that once you have no delays applied to the elements you can no longer focus a beam. No relative delays applied to the elements is the same as a flat unfocussed probe which will have a natural focus at its near field distance. Since the near field distance is fixed by the aperture and wavelength you cannot “focus” beyond it.

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