Imaging using Phased Arrays

Michael MolesOlympus NDT, 48 Woerd Avenue, Waltham, MA USA 02453

AbstractUltrasonic phased arrays offer great advantages in scanning, sweeping, steering and focusing beams, as well as options for advanced imaging.Until recently, the imaging options have been limited to S-scans and E-scans, or the standard “top, side, end” views. However, more recentwork has shown great potential for alternative imaging approaches, e.g. superimposing the ultrasonic image on the component to show defectlocation. Also, advanced digital filtering techniques can be used to tighten up images. Some of these techniques are commercial, while othersare still under development. Advanced imaging techniques have significant advantages for trainees, managers etc., but the value for structuralintegrity is less obvious.

Introduction

Imaging has always been an important aspect of NDT;after all, if one cannot image a defect, how can one interpretthe readings? However, it is primarily in the last decade ortwo, when personal computers and laptops became dominant,that imaging has taken great strides. These steps haveoccurred in conjunction with technical developments in NDT,to ultrasonics in general – which lends itself to compleximaging due to its digital data – and to phased arrays (PA)in particular.

Phased arrays [1,2] are well described elsewhere, andwill not be defined in detail here. PA inherently provide a 2Dor 3D image using S-scans and E-scans, and particularly withencoded scans where all the data is stored for subsequentimaging. However, it is possible to go well beyond thestandard PA images and reprocess them into complex overlaysand others. This paper describes developments in theseareas.

Fifty Years Ago

When ultrasonics first started, the only functionaldisplay was a waveform, subsequently known as an A-scan(see Fig. 1 for an example). The operator scanned the probeover a reflector, and interpreted the defect from his experience,training, knowledge etc. While good operators could produceexcellent analyses, the overall approach had severallimitations: no recorded data, no alternative interpretation,highly operator dependent.

“T op, Side, End” Views

With the arrival of Automated Ultrasonic Testing in the80’s, A-, B-, C- and D-scan displays became the way todisplay. Figure 2 shows an example of a “top, side, end”view. These same views can be mimicked with PA usingencoded scans.

Fig. 1 : Typical A-scan display on flaw detector

This standard display can be embellished by addingextra views and overlays, which are particularly useful forlocating reflectors relative to the component, e.g. welds.These types of displays are now standard in the advancedsegments of the NDT industry (see Fig. 3 for an example).

3D Imaging

Naturally, some people wanted 3D imaging to show theflaws in the best possible way. Software programs areavailable today [3], but their engineering value isquestionable. Figure 4 shows an example, with a 3D image inthe bottom right view.

Almost all analysis approaches, be they called FractureMechanics, Engineering Critical Analysis or Fitness-For-Purpose, use an ASME-style box round the defect fordefining the defect boundaries (see Fig. 5). This suits a “top,

276

Proceedings of the National Seminar & Exhibitionon Non-Destructive Evaluation

NDE 2009, December 10-12, 2009

Fig. 2 : “Top, side, end” view of weld. Top left: “top” view or C-scan; top right, “end” view; bottom left, “side” view; bottom right,waveform or A-scan.

Fig. 3 : Multi-view display showing S-scans, C-scans, weld overlays and linked cursors.

277NDE 2009, December 10-12,2009

278 Michael Moles : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation 278NDE 2009, December 10-12,2009

Fig. 4 : Images of pipe. 3D image at bottom right.

model both the array itself and the component. There areseveral approaches: simple and advanced ray tracing, beamprofiling, beam propagation models.

Figure 6 shows a simple ray tracing program results fora weld [5]. These programs are very useful for developingScan Plans for codes, and can save a considerable amountof time and effort. With these easy-to-use programs, one candetermine coverage, appropriate angles, number of beamsrequired etc. very quickly – and fulfill the code.

More complex ray tracing programs can be used foradvanced components. For example, Fig. 7 shows a PA beamin a rail head (6) using Imagine3D. This particular programalso provides an A-scan for operator evaluation.

Commercial programs are available as well to model PAbeams. As there is (theoretically) an infinite variety of arrayshapes, sizes and frequencies, this can be important tooptimize for some special applications. Figure 8 showsmodeled results comparing unfocused beams in a pipe (left)with focused beams (right) under various parameters [7]. Itis clear that there is a substantial difference between thesetwo array designs.

side, end” view display much better than a 3D image.However, the 3D image looks great – for managers, regulatorsand clients. 3D images may also have a future for complexproducts, but that is not clear today.

A related approach is to animate the results. For example,for PA inspections of bolts where the array is rotated 360o

round the head, the data can be replayed using a freewareprogram to re-animate the scans [4]. This is useful for lookingfor differences, or “fingerprinting” components.

Ray Tracing and Beam Imaging

Ray tracing can be used to plot the beam image tooptimize an array design or inspection technique for a specificapplication. Beam modeling is well suited to PA as one can

Fig. 5 : ASME CC 2235 defect analysis “box”

Fig. 6 : Sample scan plan for 50 mm weld using S-scans [5]

279NDE 2009, December 10-12, 2009

2D and 3D Overlays

More advanced overlays can be used to give betterlocation and overview of defects [1,2]. These can beproduced in both 2D and 3D, though neither approach isstrictly commercial yet. Figure 10 shows a 2D overlay of anS-scan on a steel weld. The overlaid weld profile shows thatthe stress corrosion cracking occurs in and around the rootand HAZ.

3D images and overlays can also be developed, asshown in Fig. 11 [2].

Again, the value of these displays is not proven yet,but they are in the process of being commercialized.

Advanced filtering

One aspect of ultrasonic testing that has been largelyignored (or perhaps, not become commercial) is digital signalprocessing. Despite major efforts in the 80’s to develop theBorn Approximation, little has come of DSP in NDT. However,it has potential, especially as computing costs drop andpower rises. Figure 12 shows a series of steps to “sharpen”waveforms (9). These kinds of approaches offer potential forimproved accuracy of measurements.

Fig. 7 : Ray tracing of PA beam in rail head.

Fig 8 : Comparison of unfocused beams (left) and focused (right) in thin pipes.

Actual modeling of the beam in a component can bedone in a number of ways, but all tend to be complex,expensive and time-consuming. Figure 9 shows a commercialprogram widely used for visualizing beam responses inspecific components [8]. This particular program gives asimulation of the PA response, as shown in the top rightview.

280 Michael Moles : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation

lot of “what if?” questions. Beam profiling? Yes, particularlyfor special applications and for developing applications forcomplex components. 2D and 3D imaging? Maybe. These aregood for viewing the full picture, great for impressingmanagement, regulators, customers etc., but limited forengineering assessments.

Overall, the market will ultimately decide what iscommercially viable. Engineering applications of advanced

Fig 9 : CIVA images of complex nozzle with predicted response.

Fig. 10: S-scan overlay on weld showing cracking.

Limitations of advanced imaging

So, where do we stand with the various advancedimaging approaches? Ray tracing? Yes. This is definitelyneeded for codes and simple applications. It also answers a

Fig. 11: Example of 3D data plotting and defect reconstructionfor a crack located on turbine blade

281NDE 2009, December 10-12, 2009

References

1. Olympus NDT, 2004, Introduction to Phased Array TechnologyApplications, published by Olympus NDT.

2. Olympus NDT, Advances in Phased Array Ultrasonic TechnologyApplications, published by Olympus NDT (2007).

3. UltraVision 3, www.zetec.com/pdfs/catalog_products.pdf4. Moles M and Ginzel R G, Phased Arrays for Detecting Cracking

in Bolts, 6th International Conference on NDE for StructuralIntegrity in the Nuclear Industry, Budapest, 8-10 October, 2007

5. ESBeam Tool3, see www.eclipsescientific.com.6. Imagine3D, see www.utex.com/Products/Simulation/I3d/

Imagine3D.htm7. Moles M and Zhang J, Curved arrays for improved horizontal

sizing in small pipe welds, Insight, 50(5) (2008) P 1.8. CIVA-9, see www-civa.cea.fr/scripts/home/publigen/content/

templates/show.asp?P=55&L=EN9. Wesley L J G, Sinclair A N and Moles M, Enhanced Ultrasonic

Defect Sizing Techniques using Weiner Filtering andAutoregressive Spectral Extrapolation, 12th InternationalConference on Fatigue, ICF 12 Ottawa, 13-17 July 2009.

Fig. 12: Steps in DSP of A-scans. Top, original waveform. Second, Weiner filtering. Third, Autoregressive spectral extrapolation. Bottom,magnitude of the analytic signal.

imaging are limited to date, but imaging is good and useful,and programs are not that expensive. Also this imaging isapplicable to special applications (especially nuclear), so mostlikely has a future. And they look TERRIFIC!

Conclusions

l Phased arrays have opened the door to many advancedimaging approaches: Multiple views simultaneously; Raytracing; Beam profiling; 2D and 3D overlays and images.

l Advanced imaging looks great, and helps operators oncomplex components.

l The engineering value of advanced imaging, particularly3D imaging, is yet to be established.

Acknowledgements

Several images were used from other companies,specifically Eclipse Scientific, UTEX Scientific, CEA, andZetec. In addition, some images from the Olympus NDTbooks are courtesy of OPG.