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Ultrasonic phased arrays are routinely used in the medical world for imaging babies in utero. Why can't they be used for imaging welds in the same way? The quick answer is: they can, but the technology is new, and the images and requirements are different.
The idea is not new; the first industrial phased-array systems date back to the late 1950s, yet the first commercial phased-array units arrived in the 1990s.
Typically, the early industrial units were large, bulky, though had significant capability. While these large units worked in factory and non-mobile locations, they were not the ideal solution for general weld inspections.
BASICS OF PHASED-ARRAY
Arrays consist of a series of individual elements, all separately pulsed, time delayed and processed; in effect, they are a collection of individual transducers. The time delays or "phasing" is the key; this allows the unit to modify the beam by electronic control. Modern functional software allows the operator to specify the required beam focal distance and scan pattern, so no direct calculation of the time delays is required.
In practice, electronic control allows phased arrays to scan sweep, steer and focus the beam. This gives rise to a number of scans. Figure 1 shows two of the basic scans, E-scans and S-scans.
Phased arrays offer major advantages over other volumetric inspection technologies, radiography and manual ultrasonics.
Compared with radiography, phased arrays offer no radiation, licensing, chemical or waste issues and are much better at finding critical planar defects (e.g., cracks and lack of fusion). All the phased-array data are usually stored, and the scans audited. Furthermore, the inspection can be tailored to the weld profile and component.
Compared with manual ultrasonics, phased arrays offer much better imaging, much faster, are much less operator-dependant, and can use better scan plans with focusing and appropriate angles. Encoded phased array scans can be fully audited.
There are a few disadvantages of phased arrays, as always. First, the technology is new, so there is always resistance. Second, phased arrays are more expensive to buy, though the newer portable phased arrays are much closer to manual units in cost. Perhaps more important, the per-unit cost for a weld can be lower with phased arrays due to much higher scan speeds. Third, phased arrays are more difficult to use, so the major manufacturers are all running training courses. Fourth, phased arrays are not universally accepted, though new phased-array codes are becoming available from the American Society of Mechanical Engineers.
ADVANTAGES IN WELD INSPECTION
There are two approaches to weld inspection with phased arrays: manual and encoded. Manual phased arrays for welds really represent a glorified conventional scan, maybe giving some increase in speed, but with minimal data storage, and not fully auditable. Encoded phased arrays scan linearly along the weld, collecting all the data at multiple angles, imaging the whole weld—all at high speed. The problems with the encoded route are that the equipment is more expensive, and it is more complex to use.
The imaging alone from phased-arrays offers major benefits for interpreting defects. Figure 2 shows a typical S-scan of a butt weld, with defects at the root (B0) and cap (T1). A trained operator will have no problem identifying the locations at a glance. In addition, these S-scans offer the easiest method of sizing cracks. Overall, defect analysis is usually a lot easier with phased arrays.
With all the electronic power now available in small packages, phased-array units have great reporting capabilities. Automatic report generators contain all the setup data, images of the scans, and are highly repeatable.
A major advantage of phased arrays is speed. Rates for linear scanning of welds are five to 10 times faster than conventional manual scanning, depending on conditions and the number of defects.
With such major capability as electronic scanning, beam steering and focusing, phased arrays can be tailored to the code, weld profile and expected defects. In addition, other ultrasonic techniques like TOFD (time-of-flight diffraction) can be added.
SAMPLE WELD APPLICATIONS
A butt-weld inspection is fairly straightforward, as shown in Figure 3. Two arrays are set up on either side of the weld and scanned linearly along it—maybe using multiple angles—to provide full coverage to fulfill the code for the weld, heat-affected zone and some parent plate.
Fillet welds are more complex, but the imaging capability of S-scans helps a lot. With these, the operator can identify geometric reflectors compared with defects. Again, these inspections can be performed manually or encoded. The operator typically uses a calibration block for setup, which helps identify the geometry and defects.
Phased arrays have been used for a wide variety of exotic welds, including friction stir welds, electron beam welds and pipelines. Pipeline inspections use a unique approach called "zone discrimination." Here, the beams are selectively targeted and angled at various facets of the welds, and scanned at 4 inches a second. This technique works very well and very fast, but requires advanced equipment.
Construction weld inspections in particular are dominated by codes. Fortunately, the major codes (ASME Section V, API RP2X, AWS D1.1) all accept phased arrays one way or another, but require some proof that the techniques and procedures work, usually by performance demonstration. ASME now has code cases available for phased-array inspections of welds as well.
A PORTABLE FUTURE
The biggest evolution in phased arrays for weld inspections came a few years ago: portable phased arrays. These units are smaller, cheaper and lighter than the original desktop units. However, unlike laptop computers, portable phased array units tend to have less capability and be slower than the bigger desktop units. Nonetheless, the portable units can perform multiple scan patterns to fulfill codes, scan welds cost-effectively and have good detection capability. Figure 4 shows a typical portable phased array unit.
These portables point to the future of phased arrays for welds. Big desktop units will still be used for dedicated applications such as pipe mills, pipelines, special welds and some factory applications. But the bulk of the market will be for portables.
Ten years ago, there were a couple of industrial phased array manufacturers; now there are a dozen or more. The market is competitive, which is pushing technical developments. Capability is increasing, which is really good for welds as they tend to be one of the more challenging applications. Portable instruments have brought prices down by two thirds, and prices are still declining. Array prices are also dropping with volume and experience. Software, often the distinguishing feature, is also improving. Codes are starting to accept phased arrays, and training programs are well established.
The verdict: Phased-array has a bright future in the world of weld inspection.
Editor's Note: Michael Moles is business development manager at Olympus NDT, Waltham, Mass.
author: By Michael Moles