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Beam Sweeping and Encoded Phased Array Units

by J. Mark Davis* and Michael Moles^†

 

Introduction

This article describes the differences between manual and encoded phased array ultrasonic testing units. While merely adding an encoder may not seem technically demanding, the software requirements and improved capabilities are significant. Many nondestructive testing (NDT) applications cannot be performed by manual (or "beam sweeping") phased array units due to code or technical limitations.

Ultrasonic phased arrays are one of the most popular NDT technologies at this time, and with good reason. Compared with conventional ultrasonics, phased arrays offer significant advantages in:

• speed: for welds, corrosion and other components, linear (or line) scanning can increase the scanning speed significantly (hence, reducing costs)
• imaging: S-scans, E-scans and other 2D and 3D imaging can give much better and more interpretable discontinuity assessments
• flexibility: phased arrays can perform a wide variety of scans, for a wide variety of discontinuities on many different types of components (R/D Tech, 2004)
• data storage: full or even partial data storage and display allows better discontinuity interpretation, and can be useful for archival purposes
• reproducibility: though not demonstrated yet by third party trials, using the same setup and procedure with phased arrays gives more reproducible results than manual ultrasonics.
  

Ultrasonic phased arrays are one of the most popular
NDT technologies at this time, and with good reason.

However, there is more than one type of phased array instrument available on the market. Besides the large, nonportable units which have been available for a decade, portable units are available from several manufacturers. The portable units can be divided into two categories: manual and encoded, with the differences and capabilities being significant. This article describes the various features and capabilities of these units, and discusses some of the different types of applications that can be performed with them.

The technology and theory behind phased arrays are well referenced in the literature, so this paper will not describe them. For a broad assessment of the physics, hardware, software and sample applications, see R/D Tech (2004).

Beam Sweeping Phased Array Units

Beam sweeping units - that is, manual units - operate like upscale monocrystal units that can also sweep the beam. These units use a manually held array in much the same manner as a manual probe (that is, the array is rastered toward and away from the weld or component, and the operator watches the screen for discontinuities). The main difference is that the operator also sees an S-scan or E-scan, which can assist in discontinuity analysis.

In general, data are not collected, though screen displays can be saved and reported. With some manual units, it is possible to collect scans using time based data collection; however, these data are inherently unreliable compared with encoded data since the measured discontinuity length will depend on the operator's scanning speed and the lengths are not calibrated. Time based scanning, or beam sweeping, is not acceptable for automated ultrasonic testing codes such as ASME Code Case 2235-9 (ASME, 2005).

Why buy a beam sweeping unit? There are several possible reasons. If the unit can display multiple A-scans at one time (for example, 45, 60 and 70° for AWS D1:1 tests [American Welding Society, 2006]), then potentially the operator can scan faster. (Of course, the unit must be calibrated to the meet the ASME or other code.) As mentioned above, under some circumstances, 2D images can improve discontinuity assessment and limited data storage is possible. For some applications (such as hydrogen induced cracking), a beam sweeping unit may be adequate and the cost is lower.

Figure 1 - Commercial beam sweeping (or manual) phased array unit display.

Beam sweeping units cannot realistically store full data, undertake multiple functions simultaneously or perform linear scanning. These are covered below. Beam sweeping units are available from various manufacturers; Figure 1 shows a typical manual phased array unit.

Encoded Phased Array Units

Encoded phased array units have the capability to use an encoder for collecting and storing scan and probe position data. While this may not sound like a big difference - after all, adding an encoder to an electronic unit is theoretically not a big deal - the real story is in the software. Encoded phased array units can collect full A-scan waveform data, store and manipulate displays to give "top/side/end" views and process these data. In other words, encoded phased array units are a portable, low cost automated ultrasonic testing system, whereas beam sweeping phased array units are glorified monocrystal units. Scanned data can be replayed for offline data analysis.

Encoded phased array units offer significant time and cost saving advantages for components like welds. Figure 2a shows the traditional raster testing process, with the probe being moved back and forth toward the weld. Figure 2b, on the other hand, shows weld tests using "linear scanning," that is, testing using a single pass parallel to the weld while the array performs the rastering. Beam sweeping phased array units perform raster scanning (Figure 2a), while encoded phased array units can perform linear scanning (Figure 2b) as well.

Linear scanning offers significant advantages in speed, maybe up to a factor of five or ten times over monocrystal testing. Besides collecting and displaying all of the discontinuities, some encoded phased array units can perform multiple scans simultaneously - that is, they can fulfill the code requirements in a single pass, with arrays on either side of the weld (Figure 3). The more advanced units can also perform additional scans like time of flight diffraction.

Naturally, with encoder capability, data storage, display reconstruction, more channels and some signal processing, the encoded phased array units are more expensive than beam sweeping phased array units. They also require more training. However, for the correct applications, encoded data collection is a much better, more cost-effective solution, and in many cases is the only phased array solution (see "Sample Applications," below). Figure 3 shows an example of an encoded phased array unit.

Phased Array Code Compatibility

Although phased arrays are new, there is a big push from the marketplace to get phased arrays accepted by the various codes. Fortunately, most of the major US codes, such as ASME (2003) and AWS (2006), inherently accept phased arrays as a technology, but require demonstration that the proposed technique and procedure fulfill the code (Ginzel et al., 2006). This can be performed through various routes, for example, ASME Code Case 2541 (2006), or ASME Code Case 2235-9 (2005). It is important to check if encoded data are required, as with ASME Code Case 2235-9.

Figure 2 - Phased array scanning: (a) conventional raster scanning; (b) linear scanning.

Figure 3 - Multiple group screen display for encoded scanning.

At this time, ASME is arguably the leader in codifying phased arrays. ASME Code Case 2557 is already in print (2006), and other code cases are in progress. One notable feature for phased arrays is the upcoming calibration requirements. ASME has interpreted an S-scan or an E-scan as multiple waveforms, each of which must be calibrated to code. The new ASTM standard for setting up phased arrays (2006) has taken the same approach: full angle corrected gain and time corrected gain are required.

Sample Applications

Beam sweeping phased array units are good for applications which require manual manipulation and imaging, typically some inservice applications. Here, crack sizing or discontinuity characterization is required, as with stress corrosion cracking or hydrogen induced cracking. Obviously, good data analysis capability and some image storage are beneficial, but most beam sweeping units can fulfill these minimal requirements.

Other applications, like bolt testing, can be performed by beam sweeping units, but these are best performed using mechanized and encoded scanning. Welds can be tested using beam sweeping, provided the unit can be correctly calibrated, but will not be as fast as encoded scanning, nor will the data be stored. Aerospace applications will depend on the specifics.

Encoded phased array units will be beneficial for any testing that requires reproducible, stored data. This includes code-type weld tests, manufacturing, corrosion mapping, high speed tests, remote applications and precision tests (R/D Tech, 2004), or essentially any application that is currently filled by automated ultrasonic testing.

Beam Sweeping or Encoded Phased Array Units?

The choice of unit depends on the application. For some limited applications, a beam sweeping unit may be adequate, but the purchaser should make sure that the unit can fulfill the requirements of any codes. For more advanced applications, and greater flexibility, an encoded phased array unit is the obvious choice.

One viable solution is to buy an upgradeable unit, so that the operator can move up from beam sweeping to encoded scanning when the application demands or as the operator's skills develop.

References

ASME, ASME Boiler and Pressure Vessel Code, New York, American Society of Mechanical Engineers, 2003.

ASME, Boiler and Pressure Vessel Code, Case 2235-9: Use of Ultrasonic Examination in Lieu of Radiography, New York, American Society of Mechanical Engineers, 2005.

ASME, Boiler and Pressure Vessel Code, Case 2541: Use of Manual Phased Array Examination Section V, New York, American Society of Mechanical Engineers, 2006.

ASME, Boiler and Pressure Vessel Code, Case 2557: Use of Manual Phased Array S-scan Ultrasonic Examination per Article 4, Section V, New York, American Society of Mechanical Engineers, 2006.

ASTM, E 2491-06: Standard Guide for Evaluating Performance Characteristics of Phased-array Ultrasonic Examination Instruments and Systems, ASTM International, 2006.

AWS, AWS D1.1, Structural Welding Code, Miami, Florida, American Welding Society, 2006.

Ginzel, R.G., E.A. Ginzel, J. Mark Davis, S. Labbé and M.D.C. Moles, "Qualification of Portable Phased Arrays to ASME Section V," Proceedings of ASME Pressure Vessel and Piping Conference, Vancouver, Canada.

R/D Tech, Introduction to Phased Array Ultrasonic Technology Applications: R/D Tech Guideline, Toronto, Ontario, Canada, R/D Tech, 2004.

* Davis NDE, Inc., 4060 Bent River Lane, Birmingham, AL 35216; (205) 733-0404; e-mail mkdavis@mindspring.com.

† Olympus NDT Canada, 73 Superior Ave., Toronto, ON M8V 2M7, Canada; (416) 831 4428; fax (905) 248-3546; e-mail michael.moles@olympusndt.com.

Copyright © 2007 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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