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What is phased array ?

Phased array refers to an ultrasound technology.  It’s well known in SONAR, medical, vetenary and NDT markets.  It’s been used in obstetrics since the early 1980s, but it’s being used in NDT since the mid-1990s.  One reason for such a late use is because metal velocity is typically 3 times faster than water or human tissues, therefore requiring the electronic circuits to be a lot faster.

Phased array ultrasound is based on an array transducer and a beamformer.  The array transducer is a cluster of miniature piezo-electric elements and the beamformer controls all the elements to “form” the ultrasound beams.  The output of the beamformer is a normal time-amplitude signal, totally equivalent to a standard A-scan.

Phased array is a better technology than conventional, mono-element ultrasound.  However, not all NDT problems are worth the investment in time and instrumentation to switch to phased array.  But most flaw detection applications are worth exploring into phased array as it’s a lot faster and simpler to perform inspections with.

Before going any further, you may want to look at the phased array lexicon.  The vocabulary of phased array is somehow tricky and new to a lot of people, but certainly not difficult.

How does it compare to conventional UT ?

Phased array is just another way of generating the ultrasound beams.  If you take a normal transducer, the aperture and frequency are the basic parameters defining the ultrasound beam.  It’s pretty much fixed and you can order this in a catalog.

Let’s assume you take the same aperture at the same frequency, but you dice it into many small elements.  Now let’s connect each small element to an electronic beamformer, so that we can control how and when we transmit and receive with each element.  You now have a phased array version of your original transducer, and you can do many things like steering and focusing the beam.  One of the things you can do is reproduce the exact same “catalog” beam with the same focal length, depth-of-field and width.

The guts of phased array technology is around the fact that the beamformer can generate beams after beams at the speed of very fast electronics.  The time spent to switch the programming from one beam to another is negligible compared to propagation time of ultrasound.

Since the beamformer provides all those A-scans which correspond to beams right next to each other, phased array provides a real-time image of those A-scans where amplitude is color-coded.  Still the instrument will offer you to see your A-scans in real-time.

What are the benefits ?

The benefits are numerous:

1. •Many beam angles can be generated with one probe, providing a much larger coverage of the Region Of Interest (ROI = where you want to check for flaws).
   

2. •The larger coverage allows you to either reduce the scanning on the component, either to increase the spatial resolution of your inspection, or a combination of both.
   

3. •Phased array imaging is simpler and faster to interpret for positioning and sizing defects.
   

4. •Full data (all A-scans) can be recorded in real-time, allowing full traceability of inspection.
   

5. •Reports are based on imaging, therefore a lot simpler to understand for non-NDT personnel.
   

When HARFANG introduced world’s first portable phased array instrument in June 2002, a lot of customer’s perception towards phased array changed because of the significant price drop and the user interface simplicity.

How can I make measurements ?

Conventional UT is about an echo in a gate, reported in terms of surface distance, depth, and metal path.  Phased array is just the same when you use the A-scan features.  However, most users will prefer the use of images to perform diagnostic and measurements.  The rendering of images is based on drawing each A-scan sample on screen with an aspect ratio of 1-to-1, therefore a 45-degree A-scan is a 45-degree trace on screen.  You can move cartesian cursors on top of a flaw and the cursors location will provide you with the depth and surface distance.

Is it accepted by codes ?

Phased array is inherently supported by codes, as it’s just another way of generating the ultrasound beams often required in codes.  For example, if a code requires a 45-degree shear-wave transducer then you can use phased array as long as among all beams you generate, you include one at 45 degrees with the same properties (focal length, width, depth-of-field).  Some codes may require the use of DAC or TGC.  It’s not a problem as most phased array instruments will support this too.

Another way, sometimes possible, is to exploit the performance demonstration criteria.  A good example is ASME Sec. XI, Appendix 8.

Many of the organizations like ASME, AWS, ASTM and EN are pursuing efforts to adapt their codes in order to include explicit mentions to phased array and to provide inspection and calibration techniques specific to the latter.

How can I convince my boss to get into phased array ?

The first step is to reuse this FAQ page.  Then it’s about building your business case, showing the value-added of phased array.  The most common thing is to show the time savings expressed in man-hours.  HARFANG can assist you in the whole process.  If you have mock-ups for which detection of critical flaws is mission-critical, we can perform a feasibility study the showing detection, location and size of each flaw (assuming it’s possible at all with phased array).  We also have a list of reference customers and testimonials.

Can it brew me coffee ?

Back in the 1990s, phased array was somewhat sold as the “magic” solution that does it all.  Actually, it was promoted as “black magic” by self-proclaimed gurus.  This has somehow burnt a bit the reputation of phased array as a technology, quite unfortunately.  HARFANG takes a great pride in promoting phased array with ethics.  Phased array is far from being a panacea, the solution to every NDT problem.  It’s certainly an extraordinary powerful imaging technique, but it’s still ultrasound with the same wave physics limitations.  Back in 2003, our Founder and VP-Technology made a statement at EPRI’s 3rd Conference on Phased Array.  “It is imperative in the future that phased array evangelists make a separation between the technology and the products, for the benefit of the customers.  Credibility of the sales pitch is at stake”, he said.

Focusing

With phased array, you can focus the beam where you want, within the near-field of the overall array aperture.  When focusing close to the surface, the focal point can become very small and the depth-of-field can be pretty short.

Steering

You can also steer the beam in a wide range of angles, within the capability of the array geometry and electro-acoustic performance. A typical range will be +/- 25 degrees for contact probe, and 30 to 75 degrees for shear wave.

Moving

Of course, you can operate the array a few elements at a time, therefore moving the index point.  Fewer elements mean smaller aperture, wider beam and extended depth-of-field.  This is useful for performing electronic B-scanning.

What about probes ?

It’s possible to fabricate arrays with up to 1024 elements, from 1 to 18 MHz, as small as 8mm x 8mm active aperture, coupled on metal, wedge or water, and up to 100m of cable.  Clearly, phased array probes are technically very capable.  They are indeed more expensive than mono-transducers, but you can do a lot more with them.

Beam

A phased array beam is just like any other beam.  It has its own focal length, depth of field, beam width and divergence.

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