Portable Phased Array Applications

Michael Moles and José Lara Cruz

ABSTRACT

This presentation describes several in-service applications for portable phased arrays. OmniScan PA is new category of instrument using ultrasonic phased arrays, as well as other technologies (conventional ultrasonics, TOFD, eddy current arrays etc.). Portable phased arrays can operate in manual, semi-automated (i.e. encoded) or fully automated modes, though most of the applications to date have been manual or semi-automated. Unlike conventional ultrasonics, portable phased arrays can provide many different displays, such as A-, B-, C-, D- and S-scans, plus combined displays which significantly help imaging. Most of the new applications so far have been specials, which take advantage of one or more of the following features: special scan patterns (e.g. S-scans), imaging (e.g. corrosion mapping or weld inspections), inspection speed, and restricted space. Portable phased arrays also offer advanced reporting capability, including pre-prepared reports and automatic pasting of images into reports for archiving. Sample applications for portable phased arrays include:

• Detection and sizing of SCC in turbine roots • Small diameter austenitic pipe weld inspections • In-service inspection of pipe for SCC • Butt weld inspections • T-weld inspections of bridge structures • HIC – Hydrogen Induced Cracking • Flange corrosion under gasket • Nozzle inspections • Thread inspections • Bridge bolt inspections • Spindle/shaft inspections • Landing gear inspections • Laser weld inspections • Composites

INTRODUCTION Volumetric inspections are typically performed in industry using either radiography or ultrasonics. Radiography has the disadvantages that it is a safety hazard, and is poor at detecting the more critical planar defects (cracks, lack of fusion, lack of penetration). Manual ultrasonics is much better than radiography for planar defects, but is slow, and the results are highly operator-dependent. Automated ultrasonics typically involved large,

expensive and inflexible systems, though the results are reproducible. A new development – portable ultrasonic phased arrays – offers speed and flexibility. Fortunately, technology has come to the rescue – in the form of portable phased array ultrasonics. This type of equipment is highly computerized, and can be operated in manual, semi-automated (encoded, with or without a scanning aid) or fully automated (i.e. operating a scanning rig). This new generation of equipment offers many of the advantages of phased arrays: speed, flexibility, data storage, imaging, reproducibility, and limited footprint, with many of the advantages of manual ultrasonics: portability, ease of set-up and relatively low cost. After briefly introducing the principles of phased arrays and types of scans, this paper describes a series of portable phased array applications. As normal with new categories of equipment, many of the initial applications have been “specials”; more recently, general applications for weld inspections have become viable. Perhaps more interesting is the observation that most of the applications are either fully manual, or semi-automated. Very few portable phased array applications use the capability of fully automated inspection. ULTRASONIC PHASED ARRAYS Ultrasonic phased arrays are a novel method of generating and receiving ultrasound. Instead of a single transducer and beam, phased arrays use multiple ultrasonic elements and electronic time delays to create beams by constructive and destructive interference. As such, phased arrays offer significant technical advantages for weld inspections over conventional ultrasonics. The phased array beams can be steered, scanned, swept and focused electronically. Beam steering permits the selected beam angles to be optimized ultrasonically by orienting them perpendicular to the predicted defects, for example Lack of Fusion in automated welds. Electronic scanning permits very rapid coverage of the components, typically an order of magnitude faster than a single transducer mechanical system. Beam steering (usually called sectorial or azimuthal scanning) can be used for mapping components at appropriate angles to optimize Probability of Detection of defects. Sectorial scanning is also useful for inspections where only a minimal footprint is possible. Electronic focusing permits optimizing the beam shape and size at the expected defect location, as well as optimizing Probability of Detection. Overall, the use of phased arrays permits optimizing defect detection while minimizing inspection time. How phased arrays work Ultrasonic phased arrays are similar in principle to phased array radar, sonar and other wave physics applications. However, ultrasonic development is behind the other applications due to a smaller market, shorter wavelengths, mode conversions and more complex components. Several authors have reviewed applications of ultrasonic phased arrays (Clay et al, 1999, Wustenberg et al., 1999, Lafontaine and Cancre, 2000), though industrial uses have been limited until the last few years.

From a practical viewpoint, ultrasonic phased arrays are merely a method of generating and receiving ultrasound; once the ultrasound is in the material, it is independent of generation method, whether generated by piezoelectric, electromagnetic, laser or phased arrays. Consequently, many of the details of ultrasonic inspection remain unchanged; for example, if 5 MHz is the optimum inspection frequency with conventional ultrasonics, then phased arrays would typically use the same frequency, aperture size, focal length, and incident angle. Phased arrays use an array of elements, all individually wired, pulsed and time-shifted. These elements are usually pulsed in groups from 4-16 elements. A typical user-friendly computerized set-up calculates the time-delays from operator-input, or uses a pre-defined file: inspection angle, focal distance, scan pattern etc (see Figure 1). The time delay values are back calculated using time-of-flight from the focal spot, and the scan assembled from individual “Focal Laws”. Time delay circuits must be accurate to around 2 nanoseconds to provide the accuracy required.

Figure 1: Schematic showing generation of linear and sectorial scans using phased arrays. The set-up information is electronically recorded, and only takes seconds to re-load. Modifying a prepared set-up is quick in comparison with physically adjusting conventional transducers. Types of scans Using electronic pulsing and receiving provides significant opportunities for a variety of scan patterns. The two basic patterns are electronic and sectorial scans. Electronic Scans

Figure 2: Schematic illustration of

electronic scanning. Electronic scans are performed by multiplexing along an array (see Figure 2). Typical arrays have up to 128 elements, pulsed in groups of 8 to 16. Electronic and linear (one-axis mechanical scanning) inspections permit rapid coverage with a tight focal spot (see below). If the array is flat and linear, then the scan pattern is a simple B-scan. The data can be processed to provide a C-scan, or combined scans (e.g. “top, side, end” views or combined S-scans and A-scans). Sectorial (Azimuthal) Scans Sectorial scans use the same set of elements, but alter the time delays to sweep the beam through a series of angles (see Figure 3). Again, this is a straightforward scan to program. Applications for sectorial scanning typically involve a stationary array, sweeping across a relatively inaccessible component like a turbine blade root (Ciorau et al, 2000), to map out the features (and defects). Depending primarily on the array frequency and element spacing, the sweep angles can vary from + 20o up to + 80o.

Figure 3: Schematic showing sectorial scanning used on turbine rotor. Linear scanning of components Manual ultrasonic inspections are performed using a single transducer, which the operator “scans” back and forth to cover the area to be covered (see Figure 4a). Many automated inspection systems use a similar approach, with a single transducer scanned back and forth for corrosion or weld inspections. This is very time consuming, since the system has dead zones at the start and finish of the raster.

Figure 4a (left). Conventional raster scanning; 4b (right). Linear scanning. In contrast, phased arrays use a “linear scanning” approach (see Figure 4b). Here the probe is mechanically scanned in a line round or along the component (a weld in this example), while the array performs electronic or sectorial scanning. Linear scanning is frequently used in pipe mills and pipe lines. PORTABLE PHASED ARRAY INSTRUMENT R/D Tech has introduced the OmniScan PA, a portable phased array unit with manual, semi-automated and automated capability. In practice, this is a multi-technology unit, with replaceable function modules (besides phased arrays, there are conventional ultrasonics, TOFD, eddy current and eddy current array modules available, with other technologies in development). The current phased array unit is a 16/128 (sixteen multiplexed pulsers with 128 channels), with up to 256 Focal Laws (individual beam pulses). The unit can perform electronic and sectorial scans. The unit has similar ultrasonic specifications to an upscale single channel flaw detector (frequency, filtering, TCG, gates, alarms, range etc.), and can operate as such (R/D Tech). The instrument is fully digital, and can perform encoded scans. Unlike conventional manual flaw detectors, the phased array unit records full waveform data at multiple angles/positions, and can display A, B, C, D, S- and combined scans. This gives much increased imaging capability. The unit also has built-in reporting capability using pasted-in scans, and internal procedure capability. There is a special calibration process for phased arrays, to ensure uniform signal strength across the array (and wedge). The unit weighs 4.6 kg with one battery. There are many electronic connections on this unit: three USB ports, video input and output, speaker, microphone, and Ethernet connection, CompactFlash® card, internal 32 MB DiskOnChip®, 2-axis encoder line, 2 TTL inputs, 4 digital outputs, RS-232 or RS485, on/off, three alarms, analog out. The instrument is shockproof and splashproof for industrial applications, and operates within a wide temperature and humidity range.

The function keys are clear and simple, following current flaw detector designs (see Figure 5). This portable phased array unit has a “probe recognition” function, where the array is automatically detected and characterized when connected; this eliminates programming the array parameters, which is a major benefit to operators.

Figure 6: The portable phased array unit showing a longitudinal wave S-scan.

Arrays As with all inspection systems, the probe or transducer is critically important. This is perhaps even more so with arrays, though typically a single array can perform multiple inspections, often with appropriate wedges. There are technical limits to arrays; individual element sizes are limited in practice to around 0.15 mm (0.006”) and are normally <20 MHz. However, the real limitations of arrays are cost. The more advanced arrays with hundreds of elements can easily cost tens of thousands of dollars. These arrays can be matrix, circular, conical, complex. To reduce costs, R/D Tech has set-up automated manufacturing of a standard series of linear arrays. Needless to say, there will always be “specials”, as normal in NDE. R/D Tech has also a standard nomenclature for arrays and wedges for convenience. With the arrival of portable phased arrays, the market is requiring lower cost, standardized, quick delivery, easy-to-use (i.e. probe recognition) arrays. APPLICATIONS This section lists a dozen portable phased array unit applications. This list is far from exhaustive, and new applications are arriving regularly. However, this should give a cross-section of typical uses, and covers a wide variety of industries: nuclear, petrochemical, defence, industrial, aerospace. Detection and Sizing of Stress Corrosion Cracking in Turbine Roots This application has a large number of components and high downtime costs, plus limited access (see Figure 7) in a nuclear reactor. False calls must be minimized due to outage

costs, and small defects (1 mm high and as little as 3 mm long) must be detected. Defect range and location varies. The phased array solution was to model the application to optimize array design, ray tracing to optimize the inspection, use relatively high frequency (6-12 MHz) and to plot the scans on a component overlay. (In practice, being a nuclear application, multiple units and multiplexed scans were used; however, this does not alter the application principles). S-scans were used, with minimal probe movement.

H 3

Figure 7: Right, schematic of turbine root; left, S-scan display showing defects.

Small Diameter Austenitic Pipe Weld Inspections This application involved inspection of stainless steel pipe welds of variable diameters for a nuclear waste application. The welds were autogenous, made by orbital welders; as such, the weld profile was near vertical. Wall thicknesses were generally thin. Space between pipes was minimal, necessitating a manual scan or low profile scanner. Radiography was not permitted for safety reasons. Rapid and reliable inspections were required, with full data recording. The portable phased array solution used two arrays generating shear waves, one on either side of the weld with a splitter cable. Linear scanning around the weld and a low profile scanner with a small MiniME® encoder was used for data collection. S-scans were used, with the data displayed as C-scans. Figure 8 shows a photo of the scanner and display.

H 3

H 4

H3 H4

Figure 8: Top; Twin SW wedges with low profile scanner for weld inspections. Bottom; Typical A-scan, S-scan and C-scan display showing 1.5 mm calibration hole.

In-service Inspection of Pipe for SCC This nuclear application is for detecting axial stress corrosion cracking in CANDU reactor feeder pipes. These pipes are ferritic steel, with very limited access between pipes. Radiation fields are high, so inspections must be quick. Crack heights are less 1 mm and wall thicknesses typically ~ 10 mm. The portable phased array solution is to use a small 10 MHz, 16 element array with miniature wheel encoder attached (see Figure 9). Once detected, defects could be sized accurately using TOFD (now available with the unit).

Figure 9: Phased array detection of SCC in feeder pipes. Left, scanning set-up; right,

crack detection.

Butt Weld Inspections In contrast to the nuclear applications above, butt weld inspections represent a huge and varied application. Typically, these inspections are performed according to an established code and approved procedure and technique. R/D Tech has been working with Eclipse Scientific Products and other companies to develop generic weld inspection techniques, and has ASME-compliant butt weld inspection procedures up to 25 mm wall (ref to come). Typical inspection criteria for practical applications include performing cost-effective, rapid and reliable inspection of butt welds in plate or tube, storing the data for reference, and imaging defects for optimum sizing. The portable phased array solution uses an array on a wedge (for wear and optimum angles) to generate shear waves as usual. S-scans or electronic scans are performed using a linear scan along the weld. The data is stored and displayed as S-scans or “top, side, end” views (see Figure 10).

Figure 10: Typical S-scan of butt weld, showing lack of fusion defects. T-Weld Inspections of Bridge Structures These weld inspections are similar to butt weld inspections, but can be more challenging due to geometry. Typically, these applications involve thicknesses of 10-16 mm, and reliable detection of planar defects (cracks, lack of fusion, lack of penetration) is essential. Probe movement is limited, multiple inspection angles are necessary, and a cost-effective solution is required. The portable phased array solution is to use an encoded hand scan with a small linear 5 MHz, 16 element array. S-scans are performed between 40o and 70o using shear waves, and the results displayed as a combination of A-scans and S-scans. Other scanning and display options are possible. Figure 11 shows the T-joint geometry and an inspection in action.

Figure 11: Inspecting T-welds using portable phased arrays with an encoded array. Top,

inspection geometry and procedure. Bottom, field inspection.

Hydrogen Induced Cracking (HIC) HIC involves the diffusion of hydrogen into steels, where it typically forms lamellar blisters at inclusions. Standard HIC is benign and easily detected by ultrasonics, but stepwise cracking can occur between blisters, which is structurally undesirable. This

SOHIC (stress-oriented hydrogen induced cracking, or stepwise cracking) is more difficult to characterize using conventional ultrasonics. While HIC forms lamellar reflectors parallel to surface, SOHIC forms as cracking between HIC blisters, at an angle to the surface. The objective is to reliably determine if SOHIC exists amongst HIC. The inspection must be rapid and comparatively low cost. Data storage is desirable. The portable phased array solution is to use normal beam electronic manual scans to rapidly detect HIC. To determine if SOHIC is present, a second set-up file is loaded to perform S-scans using + 30o S-scans. The AutoTrack function is used to display the A-scan angle with the highest amplitude waveform. The array is skewed back and forth to optimize signals. Typically the beam is focused at midwall since most HIC and SOHIC occurs at 1/3 to 2/3 depth. The operator looks for additional signals between HIC reflections to identify SOHIC (see Figures 12a and b).

Figure 12a. HIC with no stepwise cracking visible (no SOHIC)

Figure 12b. HIC with SOHIC visible

Flange Corrosion Under Gasket The requirement is to detect corrosion under a gasket seat, without removing the bolts. Inspection is possible only from the pipe surfaces; scanning is needed, but the scanning area is limited. The angles are difficult for conventional ultrasonic inspection (see Figure 13a)

Figure 13a. Schematic showing flange gasket, area to be scanned, locations of bolts and limited access

The portable phased array solution is to use a 16 element phased array probe with a 45 degree natural angle, and to perform an S-scan from 30 to 85 degrees. To ensure maximum coverage with the bolts in place, a guide was used. Using a corrected B-scan ensured a good interpretation of the images.

Figure 13b. A-scan, B-scan and corrected B-scan displays of corrosion mapping.

Nozzle Inspection The requirement is to detect and measure erosion-corrosion on a 17.5 cm (7”) nozzle inside surface. The inspection must be performed rapidly in-service, and must be cost-effective. The portable phased arrays solution is to use a 32-element, 10 MHz linear array, and perform S-scans using L-waves from 0o to 70o (see Figures 14a and 14b). The nozzle is imaged as a volume corrected (true depth) S-scan. Erosion-corrosion is measured from the image (see Figure 14c). The image can be zoomed, if required.

Figure 14a. Photo showing 175 mm calibration block and bevel end.

Figure 14b. S-scan of nozzle, showing bottom surface, corner and smooth end surface.

Bevel End Step end

Bottom Surface

Corner

SmoothEnd Surface

Figure 14c. S-scans showing eroded corner. Right, zoomed image.

Thread Inspections The requirement is to rapidly and reliably inspect threads on many munitions shafts to determine if they are correctly threaded or double-threaded (see Figures 15a and 15b). The output display should be “easy to interpret”. All data must be stored.

Figure 15a. Drawing showing munitions tail and mock-up of probe on custom wedge

Bevel End Zoomed

Figure 15b. Cross-section through shaft showing double-threading

The portable phased array solution uses a linear array with a custom wedge to fit the shaft. Focused ultrasonic beams are used for resolution, and a B-scan display to show correct or bad threading (see Figure 15c). The operator can readily distinguish between good and a double threading by interpreting the B-scan patterns (ref to Labbé).

Figure 15c. B-scan of threads showing correct threading Spindle/shaft inspections The NDE required inspecting down a long spindle for cracking (see Figure 16a). A rapid and reliable inspection was required, which both should both detect and size any defects. The main concern was that data interpretation could be difficult due to multiple reflections. This type of inspection is required for bridge pins, vehicle shafts and similar applications.

The portable phased array solution used a single array rotating on the top of the spindle (see Figure 16a), performing a narrow-angle S-scan to sweep from the centerline to the

edge of spindle. The results were displayed as a corrected S-scan, and known features (e.g. lands) were used to determine the locations of reflectors. Calibration used machined notches.

Figure 16a. Top: Spindle and true depth (or volume-corrected) S-scan display with known reflectors. Bottom, typical location of cracking in spindle.

Spindle

PA Probe 0° Law

Figure 16b. Photo showing portable phased array unit, array and inspection technique for spindles

Inspection of Bridge Bolts Bolts hold bridges together, and undergo significant fatigue cycles. The bolts are large (~22 cm long), and fatigue-susceptible areas are typically hidden (see Figure 17a). Normal ultrasonic inspections do not have the multitude of inspection angles required, nor data storage and imaging. Inspections must be rapid, reproducible and convenient.

Figure 17a: Photo showing typical bolt with two reference notches and array on accessible area.

The portable phased array solution is to perform a 0o-15o L-wave S-scan, focused at 100 mm (4”). This is a manual scan (no encoder) with the operator manipulating the array to get full volumetric coverage. The imaging makes interpretation much easier and more reproducible (see Figure 17b), and inspections were much faster than with conventional UT. It would be possible to include DAC or TCG.

Figure 17b: A-scan and S-scan image from typical bolt, showing threads, reference notch and backwall.

Landing Gear Inspections Aircraft landing gear undergo considerable stress on landing and take-off, and are potentially susceptible to fatigue cracking. The area to be inspected has three different diameters, which makes a conventional ultrasonic inspection difficult. The portable phased array solution is to use an S-scan to generate 40o to 65o shear waves inside the component, with a wedge specifically contoured to the cylinder outer diameter. This permits a single pass inspection of the cylinder, with full data collection. Though there are several different cylinder outer diameters, and multiple diameters within each, electronic set-ups make this inspection straightforward. The imaging permits defect identification (see Figure 18).

Thread area Notch Back wall

Figure 18: Portable phased array system used for landing gear inspection

Laser Weld Inspections This is an aerospace inspection for laser weld construction. The component has a complex geometry, rapid inspection is required, and full data storage is needed. The portable phased array solution is to use a linear array with a water box for coupling (see Figure 19). A 10 m long linear scan manual inspection is performed, using an encoder at 25 mm/sec. The array performs a normal beam raster inspection (electronic scan), giving a real-time C-scan display. All the data is stored.

Figure 19. Normal beam scan of aluminium laser weld with water box Composites There are many composite inspection applications in the aerospace industry. This particular application is for a 6 mm thick carbon composite. A sample simulating lay-up tape commonly found during the manufacturing process was made with known defects (see Figure 20a). The objective was to reliably detect and size defects, and to store all data. The portable phased array solution was to use a linear scan with electronic (normal beam) scanning. A 5 MHz 32 element probe with a 1 mm pitch was used. (In practice, a 64 element probe with 0.6 mm pitch would give greater resolution). Contrary to many applications, the element grouping was set at 5. Loss-of-backwall was used for defect detection. The scans were displayed as C-scan and A-scans, and the data stored as usual.

Figure 20a. Photo of composite specimen for inspection.

Figure 20b shows a combined composite scan. The loss of backwall is clearly seen.

Figure 20b. Scan results from composite specimen. Loss of backwall is visible (arrowed).

DISCUSSION The applications listed above show that portable phased arrays can perform many different types of inspections, from generic weld inspections to “specials”. All these applications have one or more of the following advantages:

• Speed: scanning with phased arrays is an order of magnitude faster than single transducer conventional mechanical systems, with better coverage and focusing;

• Flexibility: set-ups can be changed in a few minutes, and typically a lot more component dimensional flexibility is available;

• Inspection angles: a wide variety of inspection angles and wave modes can be used, depending on the requirements and the array;

• Imaging: S-scans, B-scans and C-scans offer much better data interpretation than simple A-scans;

• Small footprint: small matrix arrays can give significantly more flexibility for inspecting restricted areas than conventional transducers.

As mentioned earlier, most of the listed applications are specials, largely because this is how most new NDE products make it onto the market place. These specials will continue, and diversify into applications not currently thought of. Some may even use the full automated scanning capability. Most important, portable phased arrays now appear cost-competitive for a number of inspections. While it is too early to cost weld inspections using portable phased arrays, early evidence shows that weld inspections are approximately five times faster than with conventional manual inspections.

Besides the major labor savings, evidence also suggests that portable phased array weld inspections are significantly more reliable than manual inspections; the operator’s interpretation of a waveform is no longer such a key factor. Once the set-up is prepared, the same results are repeatedly obtained. We look forward to the first weld inspection trials using portable phased arrays. The arrival of portable phased arrays may one other major impact on the NDE industry. Significantly increased productivity could offset the upcoming shortage of qualified inspectors. CONCLUSIONS

1. Portable phased arrays are commercially and technically viable for a wide range of inspections.

2. Portable phased arrays have major advantages for: a. High speed inspections; b. Set-up flexibility; c. Multiple inspection angles and wave modes; and d. Limited access inspections.

3. Portable phased arrays should be cost-effective for a number of standard applications, e.g. welds.

4. Standard code-compliant procedures should significantly increase the application of portable phased arrays.

5. Expect more portable phased array applications in the near future!

ACKNOWLEDGEMENTS Many people in R/D Tech have assisted in the development of this instrument. In particular, Pierre Langlois, who spearheaded the development, and Chris Magruder, Philippe Cyr, Simon Labbé and others who have worked on various applications. Also, several external companies have assisted with one or more of the examples here, including Eclipse Scientific Products, OPG, Materials Research Institute, Washington Group International, and Northwest Airlines. REFERENCES

1. Clay A.C., S-C. Wooh, L. Azar and J-Y. Wang, “Experimental Study of Phased Array Beam Characteristics”, Journal of NDE, vol 18, no. 2, June 1999, page 59.

2. Wüstenberg H, A. Erhard and G. Shenk, “Some Characteristic Parameters of Ultrasonic Phased Array Probes and Equipments”, NDT.net, vol 4, no. 4, http://www.ndt.net/article/v04n04/wuesten/wsuesten.htm

3. Lafontaine G. and F. Cancre, “Potential of Ultrasonic Phased Arrays for Faster, Better and Cheaper Inspections”, NDT.net, vol 5, no. 10, October 2000 http://www.ndt.net/article/v05n10/lafont2/lafont2.htm

4. Ciorau P., D. MacGillivray, T. Hazelton, L.Gilham, D. Craig and J.Poguet, “In-situ examination of ABB l-0 blade roots and rotor steeple of low-pressure steam turbine, using phased array technology”, 15th World Conference on NDT, Rome, Italy, October 11-15, 2000.

5. See www.rd-tech.com/omniscanpa.html for details. 6. S. Labbé, "Signal Analysis For Automated ‘Go - Nogo’ Inspection Of Complex

Geometries Using Ultrasonic Phased Arrays", 16 World Conference on NDT, Montréal, Canada, August 30-September 3, 2004.

1. Michael Moles R/D Tech – Market Development 73 Superior Avenue Toronto, ON, Canada M8V 2M7 Phone and cell phone: (416) 831 4428 Fax: (416) 255 5882 E-mail: Michael.moles@rd-tech.com 2. José Lara Cruz Director General LLOG S.A. de CV Cuitlahuac No. 54 Aragón La Villa México, D.F. 07000 Tel: +52 55 57501414 Fax: +52 55 5750 1188 E-mail: llogsa@prodigy.net.mx