16:44 Jul-07-2009 Stan NDT Inspector, Canada, Joined Jan 2009 31
PA probe focusing
I am hoping that some one will help me out here.
I was taught that with a traditional probe, if focusing is applied to it, the focal point must be less than the distance to the near zone, that is, focusing can only bring the focal point closer to the probe, you cannot focus the probe farther away from the probe than the near zone.
I was also taught that a phased array probe of a given virtual aperture has the same properties as a conventional probe of the same size. To me this implies that the probes focusing properties should be similar to those of a conventional probe, however, perhaps when using the phased array focusing this is not the case. Is it possible to focus a phased array probe farther from the probe than the end of the near zone? If so, what effect does this have on the size of the focal spot? Generally, the farther the focal distance is from the probe, the larger the focal spot size.
Will someone enlighten me here.
A PA probe dows not have the 'same properties' as a conventional 'ducer. The focus depth of the PA 'virtual probe' can be varied based on material thickness, and should be refocused for actual indications found [assuming that the defect was not readily apparant and easily 'diagnosed' during the initial scan]. Additionally, PA is at its best in the nearfield, where conventional shearwave works best past nearfield zone.
The conventional probes that are focused are typically duals or straight-beam. Using PA as a straight scan, without any sectoral sweeping, seems to me like using an elephant gun to hunt mice. It works just fine, but simpler & cheaper methods work just as well.
Where PA 'shines' is with a 32- or 64-element probe, running a sectoral sweep, focused at [or just beyond] anticipated defects and simultaniously scanning for laminations and pitting [or crack tips, scanning at 45 deg's] by using a series of parallel beams generated by a series of 8-to-16 element 'virtual' probes. A linear 64-element transducer can generate up to 49 16-element 'virtual' probes all performing a 'ladder shaped' series of linear scans [64-16, +1]. It is all in "the power of the number of elements"
A phased array transducer cannot be focused beyond its near field, and in the case of common square and rectangular probes, focusing occurs in the steering direction only. You can find some further information in the focusing section of our phased array tutorial at
There was a thread last month about a similar topic.
The discussion related to where the near field was on a rectangular probe (which most linear phased-array probes used today are duplicating).
The principles involved in focusing are still valid (i.e. only in the near field and the focal spot gets smaller as the focal point is brought closer to the probe). There have been some really nice results (especially for zonal discrimination techniques) whereby the passive aperture axis is curved and the active aperture focused in the normal fashion for a phased-array.