paut

3/17/12 April 2005 - Back to Basics

1/13www.asnt.org/publications/materialseval/basics/apr05basics/apr05basics.htm

Marketplace

Certification

Publications

Membership

Meetings andEvents

Local Sections

Links

What's New

Learning

Awards andHonors

Advertise�

�

Members Only | Contact Us

Back to Basics

Portable Phased Array Applications

by Jesse Granillo* and Michel Moles+

INTRODUCTION

Volumetric nondestructive testing (NDT) is typically performed in industry using either radiography or ultrasonics.

Radiography has the disadvantages that it can be a safety hazard and is poor at detecting the more criticalplanar discontinuities (cracks, lack of fusion and lack of penetration). Manual ultrasonics is much detecting planar discontinuities, but it is slow and the results are highly dependent on the operator. Automatedultrasonic testing typically involves large, expensive and inflexible systems, though the results are reproducible. Anew development - portable ultrasonic phased arrays - offers speed and flexibility.

Portable phased array ultrasonic equipment is highly computerized and can be operated in manual,semiautomated (encoded, with or without a scanning aid) or fully automated (operating a scanning rig) modes.This new generation of equipment offers many of the advantages of phased arrays: speed, flexibility, datastorage, imaging, reproducibility and limited footprint, with many of the advantages of manual ultrasonics:portability, ease of setup and relatively low cost.

After briefly introducing the principles of phased arrays and the types of scans, this paper describes a series ofportable phased array applications. As normal with new categories of equipment, many of the initial applicationshave been unusual in some way; more recently, general applications for weld testing have become viable.Perhaps more interesting is the observation that most of the applications are either fully manual orsemiautomated. Very few portable phased array applications are fully automated.

ULTRASONIC PHASED ARRAYS

Ultrasonic phased arrays are a novel technique for generating and receiving ultrasound. Instead of a singletransducer and beam, phased arrays use multiple ultrasonic elements and electronic time delays to createbeams by constructive and destructive interference. As such, phased arrays offer significant technicaladvantages for weld testing 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 ultrasonicallyby orienting them perpendicular to the predicted discontinuities, for example lack of fusion in automated welds.

3ortable phased arrays are commercially and technically viable

for a wide range of applications.

Electronic scanning permits very rapid coverage of the components, typically an order of magnitude faster than asingle transducer mechanical system. Beam steering (usually called sectorial or azimuthal scanning) for mapping components at appropriate angles to optimize the probability of detection of discontinuities.Sectorial scanning is also useful when only a minimal footprint is possible. Electronic focusing permits optimizingthe beam shape and size at the expected discontinuity location, as well as optimizing the probability of detection.Overall, the use of phased arrays permits optimizing discontinuity detection while minimizing testing time.

How Phased Arrays Work

Ultrasonic phased arrays are similar in principle to phased array radar, sonar and other wave physicsapplications. However, ultrasonic development is behind the other applications due to a smaller market, shorterwavelengths, mode conversions and more complex components. Several authors have reviewed ultrasonic phased arrays (Clay et al., 1999; Wustenberg et al., 1999; Lafontaine and Cancre, 2000), thoughindustrial uses have been limited until the last few years.



industrial uses have been limited until the last few years.

From a practical viewpoint, ultrasonic phased arrays are merely a technique for generating and receivingultrasound; once the ultrasound is in the material, it is independent of the generating technique (piezoelectric,electromagnetic, laser or phased arrays). Consequently, many of the details of ultrasonic testing remainunchanged; for example, if 5 MHz is the optimum testing frequency with conventional ultrasonics, then 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 areusually pulsed in groups from 4 to 16 elements. A typical user friendly computerized setup calculates delays from operator input, or uses a predefined file: test angle, focal distance, scan pattern and so forth (seeFigures in R/D Tech, 2004).

The time delay values are back calculated using time of flight from the focal spot and the scan assembled fromindividual focal laws. Time delay circuits must be accurate to around 2 ns to provide the accuracy

The setup information is electronically recorded and only takes seconds to reload. Modifying a prepared setup isquick in comparison with physically adjusting conventional transducers.

Types of ScansUsing electronic pulsing and receiving provides significant opportunities for a variety of scan patterns. The twobasic patterns are electronic and sectorial scans.

Electronic scans are performed by multiplexing along an array. Typical arrays have up to 128 elements, pulsed ingroups of 8 to 16. Electronic and linear (single axis mechanical scanning) testing permits rapid coverage tight focal spot. If the array is flat and linear, then the scan pattern is a simple B-scan. The data can be processedto provide a C-scan or combined scans (for example, top/side/end views or combined S- and A-scans).

Sectorial scans use the same set of elements, but alter the time delays to sweep the beam through a series ofangles. Again, this is a straightforward scan to program. Applications for sectorial scanning typically involve stationary array, sweeping across a relatively inaccessible component like a turbine blade root (Ciorau et al.,2000), to map out the features (and discontinuities). Depending primarily on the array frequency and spacing, the sweep angles can vary from ±20 to ±80 degrees.

Manual ultrasonic testing is performed using a single transducer, which the operator scans back and forth tocover the area to be tested. Many automated testing systems use a similar approach, with a single transducerscanned back and forth for corrosion or weld testing. This is very time consuming, since the system has dead

zones at the start and finish of the raster.

In contrast, phased arrays use a linear scanning approach. Here, the probe is mechanically scanned in a linearound or along the component (a weld in this example), while the array performs electronic or sectorial Linear scanning is frequently used in pipe mills and on pipelines.

PORTABLE PHASED ARRAY INSTRUMENTA portable phased array unit with manual, semiautomated and automated capability has been developed. Inpractice, this is a multiple technology unit, with replaceable function modules (besides phased arrays, there conventional ultrasonics, time of flight diffraction, eddy current and eddy current array modules available, withother technologies in development). The current phased array unit is a 16/128 unit (16 multiplexed 128 channels), with up to 256 focal laws (individual beam pulses). The unit can perform electronic and sectorialscans. It has ultrasonic specifications similar to an upscale single channel discontinuity detector (frequency,filtering, time corrected gain, gates, alarms, range and so forth) and can operate as such. The instrument is fullydigital and can perform encoded scans.

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 phasedarrays, to ensure uniform signal strength across the array (and wedge). The 4.6 kg (10 lb) unit also recognition" function, where the array is automatically detected and characterized when connected; thiseliminates programming the array parameters.

ArraysAs with all testing systems, the probe or transducer is critically important. This is perhaps even more the casewith arrays, though typically a single array can perform multiple tests, often with appropriate wedges. There



with arrays, though typically a single array can perform multiple tests, often with appropriate wedges. There

technical limits to arrays; individual element sizes are limited in practice to around 0.15 mm (6 x 10normally under 20 MHz. However, the real limitations of arrays are cost. The more advanced arrays, withhundreds of elements, can easily cost tens of thousands of dollars. These arrays can be matrix, circular, conicalor complex. To reduce costs, automated manufacturing of a standard series of linear arrays has been developed.

APPLICATIONSThis section lists a dozen portable phased array unit applications. This list is far from exhaustive and newapplications are arriving regularly. However, this provides a cross section of typical uses and covers a variety of industries: nuclear, petrochemical, defense, manufacturing and aerospace.

Detection and Sizing of StressCorrosion Cracking in Turbine RootsThis application involves a large number of components and high downtime costs, plus limited access in anuclear reactor. False calls must be minimized, due to outage costs, and small discontinuities (1 mm [0.04 high and as little as 3 mm [0.12 in.] long) must be detected. Discontinuity range and location varies.

The phased array solution was to model the application to optimize array design using ray tracing to optimize thetesting. The solution was to use a relatively high frequency (6 to 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, thisdoes not alter the application principles). S-scans were used, with minimal probe movement.

Small Diameter Austenitic Pipe Weld TestingThis application involved testing 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. thicknesses were generally thin. Space between pipes was minimal, necessitating a manual scan or low profilescanner. Radiography was not permitted for safety reasons. Rapid and reliable testing was required, data recording.

The portable phased array solution used two arrays generating shear waves, one on either side of the weld with asplitter cable. Linear scanning around the weld and a low profile scanner with a small encoder was used for datacollection. S-scans were used, with the data displayed as C-scans. Figure 1 shows the scanner and display.

(a)

(b)



Figure 1 - Small diameter austenitic pipe weld testing: (a) twin shear wave wedges with low profile

scanner; (b) typical A-, S- and C- scan display showing 1.5 mm (0.06 in.) calibration hole.

In Service Testing of Pipe for Stress Corrosion Cracking

This nuclear application is for detecting axial stress corrosion cracking in Canada Deuterium Uranium (CANDU)reactor feeder pipes. These pipes are ferritic steel, with very limited access between pipes. Radiation high, so testing must be quick. Crack heights are less 1 mm (0.04 in.) and wall thicknesses are typically around10 mm (0.4 in.).

The portable phased array solution is to use a small 10 MHz, 16 element array with a miniature wheel encoderattached (Figure 2). Once detected, discontinuities could be accurately sized using time of flight diffraction.

(a)

(b)

Figure 2 - Phased array detection of stress corrosion cracking in feeder pipes: (a) scanning setup; (b) crack

detection.

Butt Weld Testing

In contrast to the nuclear applications above, butt weld testing represents a huge and varied application.Typically, this testing is performed according to an established code and approved procedure and technique.

ASME code approval has been obtained using external consultants for pipes and butt welds up to 25 mm (1 in.).Typical testing criteria for practical applications include performing cost effective, rapid and reliable testing of buttwelds in plate or tubes, storing the data for reference and imaging discontinuities for optimum sizing.

The portable phased array solution uses an array on a wedge (for wear and optimum angles) to generate shearwaves as usual. S-scans or electronic scans are performed using a linear scan along the weld. The data arestored and displayed as S-scans or top/side/end views.



stored and displayed as S-scans or top/side/end views.

T-weld Testing of Bridge StructuresThese weld tests are similar to butt weld testing, but can be more challenging due to the geometry. Typically,these applications involve thicknesses of 10 to 16 mm (0.4 to 0.6 in.) and reliable detection of planardiscontinuities (cracks, lack of fusion and lack of penetration) is essential. Probe movement is limited, multipletest 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 at between 40 and 70 degrees using shear waves and the results displayed as acombination of A- and S-scans. Other scanning and display options are possible. Figure 3 shows the T-jointgeometry and testing in action.

Figure 3 - Testing T-welds using portable phased arrays with an encoded array: test geometry

Hydrogen Induced CrackingHydrogen induced cracking involves the diffusion of hydrogen into steels, where it typically forms lamellar blistersat inclusions. Standard hydrogen induced cracking is benign and easily detected by ultrasonics, but stepwisecracking can occur between blisters, which is structurally undesirable. This stress oriented hydrogen inducedcracking (or stepwise cracking) is more difficult to characterize using conventional ultrasonics. While induced cracking forms lamellar reflectors parallel to the surface, stress oriented hydrogen induced crackingforms as cracking between hydrogen induced cracking blisters, at an angle to the surface. The objective is toreliably determine if stress oriented hydrogen induced cracking exists amongst regular hydrogen inducedcracking. The testing must be rapid and comparatively inexpensive. Data storage is desirable.

The portable phased array solution is to use normal beam electronic manual scans to rapidly detect hydrogeninduced cracking. To determine if stress oriented hydrogen induced cracking is present, a second setup loaded to perform S-scans using ±30 degree S-scans. A tracking function is used to display the A-scan anglewith the highest amplitude waveform. The array is skewed back and forth to optimize the signals. beam is focused at midwall, since most hydrogen induced cracking and stress oriented hydrogen inducedcracking occurs at 1/3 to 2/3 depth. The operator looks for additional signals between hydrogen induced crackingreflections to identify stress oriented hydrogen induced cracking (Figure 4).



(a)

(b)

Figure 4 - Hydrogen induced cracking: (a) with no stepwise cracking visible (no stress hydrogen induced cracking); (b) with stress oriented hydrogen induced cracking visible.

Flange Corrosion under GasketThe requirement is to detect corrosion under a gasket seat, without removing the bolts. Testing is possible onlyfrom the pipe surfaces; scanning is needed, but the scanning area is limited. The angles are difficult forconventional ultrasonic testing (Figure 5a).

The portable phased array solution is to use a 16 element phased array probe with a 45 degree natural angleand to perform an S-scan from 30 to 85 degrees. To ensure maximum coverage with the bolts in place, a was used. Using a corrected B-scan ensured a good interpretation of the images (Figure 5b).

(a)

(b)

Figure 5 - Gasket corrosion mapping: (a) schematic showing flange gasket, area to be scanned, locations bolts and the limited access (measurements are in degrees); (b) A-scan, B-scan and corrected B-scan

displays of corrosion mapping.

Nozzle Testing



Nozzle TestingThe requirement is to detect and measure erosion/corrosion on a 175 mm (6.9 in.) nozzle inside surface. Thetesting must be performed rapidly and in service and must be cost effective.

The portable phased array solution is to use a 32 element, 10 MHz linear array and perform S-scans using L-waves from 0 to 70 degrees (Figures 6a and 6b). The nozzle is imaged as a volume corrected (true depth) scan. Erosion/corrosion is measured from the image (Figure 6c). The image can be zoomed, if required.

(a)

(b)

(c)

Figure 6 - Nozzle testing: (a) 175 mm (6.9 in.) calibration block and bevel end; (b) S-scan ofnozzle, showing bottom surface, corner and smooth end surface; (c) S-scans showing eroded

corner (the right side is a zoomed image).



Thread TestingThe requirement is to rapidly and reliably test threads on many munitions shafts to determine if they are correctlythreaded or double threaded (Figures 7a and 7b). The output display should be easy to interpret. stored.

The portable phased array solution uses a linear array with a custom wedge to fit the shaft. Focused ultrasonicbeams are used for resolution and a B-scan display to show correct or bad threading (Figure 7c). The can readily distinguish between good and double threading by interpreting the B-scan patterns (Labbé, 2004).

(a)

(b)



(c)

Figure 7 - Munitions thread testing: (a) munitions tail and mockup of probe on custom wedge; cross section through shaft showing double threading; (c) B-scan of threads showing correct

threading.

Spindle/Shaft TestingThe NDT required in this case involved testing a long spindle for cracking (Figure 8). A rapid and reliable testwas required, which should both detect and size any discontinuities. The main concern was that datainterpretation could be difficult due to multiple reflections. This type of testing is required for bridge pins, vehicleshafts and similar applications.

The portable phased array solution used a single array rotating on the top of the spindle (Figure 8a), performing anarrow angle S-scan to sweep from the centerline to the edge of spindle. The results were displayed as acorrected S-scan and known features (for example, lands) were used to determine the locations of reflectors.Calibration used machined notches.

(a)



(b)

Figure 8 - Spindle/shaft testing: (a) spindle and true depth (or volume corrected) S-scan display reflectors; (b) typical location of cracking in spindle.

Testing of Bridge Bolts

Bolts hold bridges together and undergo significant fatigue cycles. The bolts are large (around 220 mm [9 in.]long) and fatigue susceptible areas are typically hidden (Figure 9a). Normal ultrasonic testing does multitude of angles required, nor appropriate data storage and imaging. Testing must be rapid, reproducible andconvenient.

The portable phased array solution is to perform a 0 to 15 degree L-wave S scan, focused at 100 mm (4 in.). Thisis a manual scan (no encoder) with the operator manipulating the array to get full volumetric coverage. imaging makes interpretation much easier and more reproducible (Figure 9b) and tests were much faster thanwith conventional ultrasound. It would be possible to include distance amplitude correction or time corrected

(a) (b)

Figure 9 - Bolt testing: (a) typical bolt with two reference notches and array on accessible area; (b) A-scan andS-scan image from typical bolt, showing threads, reference notch and backwall.

Landing Gear TestingAircraft landing gears undergo considerable stress on landing and takeoff, and are potentially susceptible tofatigue cracking. The area to be tested has three different diameters, which makes conventional ultrasonic difficult.

The portable phased array solution is to use an S-scan to generate 40 to 65 degree shear waves inside thecomponent, with a wedge specifically contoured to the cylinder's outer diameter. This permits a single pass of the cylinder, with full data collection. Though there are several different cylinder outer diameters and multiplediameters within each, electronic setups make this testing straightforward. The imaging permits discontinuityidentification.

Laser Weld TestingThis is an aerospace test for laser weld construction. The component has a complex geometry, rapid testing isrequired and full data storage is needed.

The portable phased array solution is to use a linear array with a water box for coupling. A 10 m (32.8 ft) longlinear scan manual test is performed, using an encoder at 25 mm/s (1 in./s). The array performs a normal beam



linear scan manual test is performed, using an encoder at 25 mm/s (1 in./s). The array performs a normal beamraster test (electronic scan), giving a real time C-scan display. All the data are stored.

CompositesThere are many composite testing applications in the aerospace industry. This particular application is for a 6mm (0.24 in.) thick carbon composite. A sample simulating layup tape commonly found during the manufacturingprocess was made with known discontinuities (Figure 10). The objective was to reliably detect and sizediscontinuities and to store all data.

The portable phased array solution was to use a linear scan with electronic (normal beam) scanning. A 5 MHz, 32element probe with a 1 mm (0.04 in.) pitch was used. (In practice, a 64 element probe with 0.6 mm [0.02 would give greater resolution.) Contrary to many applications, the element grouping was set at 5. Loss ofbackwall was used for discontinuity detection. The scans were displayed as C- and A-scans and the data storedas usual.

Figure 10 - Scan results from composite specimen: loss of backwall is visible (arrowed).

DISCUSSIONThe applications listed above show that portable phased arrays can perform many different types of NDT, fromgeneric weld testing to more specialized applications. All these applications have one or more of the followingadvantages:

speed: scanning with phased arrays is an order of magnitude faster than single transducer conventionalmechanical systems, with better coverage and focusing

flexibility: setups can be changed in a few minutes and typically a lot more component dimensionalflexibility is available

testing angles: a wide variety of angles and wave modes can be used, depending on the requirements andthe array

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

small footprint: small matrix arrays can give significantly more flexibility for testing restricted areas thanconventional transducers.

As mentioned earlier, most of the listed applications are unusually specialized, largely because this is how mostnew NDT products make it into the marketplace. These special applications will continue, diversifying applications not currently thought of. Some may even use the fully automated scanning capability.

Most important, portable phased arrays now appear cost competitive for a number of applications. While it is tooearly to determine the cost of weld testing using portable phased arrays, early evidence shows that such testingis approximately five times faster than with conventional manual testing.

Besides the major labor savings, evidence also suggests that portable phased array weld testing is significantlymore reliable than manual testing; the operator's interpretation of a waveform is no longer such a key factor. Oncethe setup is prepared, the same results are repeatedly obtained. We look forward to the first weld testing trialsusing portable phased arrays.



The arrival of portable phased arrays may have one other major effect on the NDT industry: significantlyincreased productivity could offset the upcoming shortage of qualified inspectors.

CONCLUSIONSPortable phased arrays are commercially and technically viable for a wide range of applications. They have majoradvantages for high speed testing, setup flexibility, multiple test angles and wave modes and limited accesstesting. They should be cost effective for a number of standard applications (for example, welds) and standardcode compliant procedures should significantly increase their application. One should expect more portablephased array applications in the near future.

ACKNOWLEDGMENTSMany people at R/D Tech have assisted in the development of this instrument. In particular, Pierre Langloisspearheaded the development and Chris Magruder, Philippe Cyr, Simon Labbé and others worked on variousapplications. Also, several external companies have assisted with one or more of the examples here, includingEclipse Scientific Products, OPG, Materials Research Institute, Washington Group International and NorthwestAirlines.

REFERENCESCiorau, P., D. MacGillivray, T. Hazelton, L. Gilham, D. Craig and J. Poguet, "In-situ Examination of ABB 1-0Blade Roots and Rotor Steeple of Low-pressure Steam Turbine, Using Phased Array Technology," 15th Conference on NDT, Rome, Italy, October 2000.

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, p. 59.

Labbé, S., "Signal Analysis for Automated 'Go/Nogo' Inspection of Complex Geometries Using UltrasonicPhased Arrays," 16th World Conference on NDT, Montréal, Canada, August-September 2004.

Lafontaine, G. and F. Cancre, "Potential of Ultrasonic Phased Arrays for Faster, Better and CheaperInspections," NDT.net, Vol. 5, No. 10, October 2000, <www.ndt.net/article/v05n10/lafont2/lafont2.htm>.

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

Wüstenberg, H., A. Erhard and G. Shenk, "Some Characteristic Parameters of Ultrasonic Phased Array Probesand Equipments," NDT.net, Vol. 4, No. 4, 1999, www.ndt.net/article/v04n04/wuesten/wuesten.htm

* R/D Tech, 4615 E. Broadway, Suite 2, Long Beach, CA 90803; (562) 439-3102; fax (562) 439-2102; e-mail<[email protected]>.

+ R/D Tech, 73 Superior Avenue, Toronto, ON M8V 2M7, Canada; (416) 831-4428; fax (416) 255-5882; e-mail<[email protected]>.

Copyright © 2005 by the American Society for Nondestructive Testing, Inc. All rights reserved.

Copyright © 2012 by the American Society for Nondestructive Testing, Inc. ASNT is not responsible for the authenticity or accuracy of

information herein. Published opinions and statements do not necessarily reflect the opinion of ASNT. Products or services that are

advertised or mentioned do not carry the endorsement or recommendation of ASNT.

IRRSP, NDT Handbook , The NDT Technician and www.asnt.org are trademarks of the American Society for Nondestructive Testing,

Inc. ACCP, ASNT, Level III Study Guide, Materials Evaluation, Nondestructive Testing Handbook , Research in Nondestructive

Evaluation and RNDE are registered trademarks of the American Society for Nondestructive Testing, Inc. ASNT exists to create a

safer world by promoting the profession and technologies of nondestructive testing.



safer world by promoting the profession and technologies of nondestructive testing.

paut

Documents

blica ion ma

le te ing

portable phased arrays

ultrasonic phased arrays

hydrogen induced cracking

weld te ing

perform multiple tests

phased arrays