Phased Array Technical Guidelines

Useful Formulas, Graphs, and Examples

R/D Tech Corp.

R/D Tech Introduction 1

Introduction

One of the major contributors to the reliability of any nondestructivetesting (NDT) method is the human factor. The personnel involved inthe phased array ultrasonic inspection must be trained and certified.Through his or her skills, education, and training, the NDT practitionermust demonstrate that he or she can handle specific requirementsrelated to the procedure and equipment (phased array ultrasonicinstrument, scanners, probes, software, analysis layouts, andreporting). The practitioner must be familiar with the basic features ofphased array ultrasonic technology applied to specific components.

The customer-oriented philosophy of R/D Tech resulted in thepublication of the first book dedicated to phased array ultrasonictechnology: Introduction to Phased Array Ultrasonic TechnologyApplications: R/D Tech Guideline. This Guideline was intended for alarge audience, with extensive chapters on basic ultrasonic testing,data representation and scanning patterns, phased array probes, andapplications. The Guideline contains more general informationcompared to the booklet. The Guideline can be purchased by e-mailorder through our Web site.

The Phased Array Technical Guidelines booklet was written for the NDTpractitioner as an aide-mémoire of the basic phased array ultrasonictechnology. It is oriented toward day-to-day activities, and know-howand how-to problems (procedure compliance, calibration,characterization, new setup construction, and solved inspectionproblem). By its contents and dimensions, the booklet was designed tofit into a pocket. The booklet must withstand field conditions, hencewe produced the book on water- and tear-resistant synthetic paper,with a sturdy cover and binding.

The Phased Array Technical Guidelines booklet contains the following:

• Chapter 1, “Phased Array Ultrasonic Technology—GeneralFeatures”Describes the PAUT principles, and presents the main hardwarecomponents and type of phased array beam forming andmovement (linear, azimuthal, depth, plane, and 3-D).

• Chapter 2, “Phased Array Probes—General Features”Describes the PA probes and their main features required to beused for day-to-day inspections. The examples are based on the

2 Introduction R/D Tech

1-D planar linear array, the most commonly used type of probe formany applications.

• Chapter 3, “Focal Laws—General Examples”Illustrates the basic steps in defining focal laws for Tomoscan III™PA (TomoView™ 2.2R9) and OmniScan® PA for linear arrayprobes.

• Chapter 4, “Scanning Patterns, Views, and Layouts”Presents the major data representations (A-scans, S-scans, B-scans,C-scans, and D-scans) and the basic layouts and scanning patternsfor Tomoscan III PA (TomoView 2.2R9) and OmniScan PA.Recommended layouts for specific applications are also noted.

• Chapter 5, “Ultrasound Settings, Calibration, and PeriodicChecking”Presents basic examples for ultrasonic settings and optimization,equipment calibration, and in-the-field periodic checking.

• Chapter 6, “Useful Tables, Charts, and Formulas”Is a useful review of the main formulas, such as: Snell’s law, near-field length, wavelength, beam width, half-angle beam spread.Special emphasis is focused on defect sizing using differentmethods. Besides tables and formulas, the chapter incorporatesgraphs for a quick evaluation of specific features: refracted angle,equivalent delay, and reflector size.

• Appendix A: “Unit Conversion”Provides the metric-English conversions for units used in thisbooklet.

• Appendix B: “Support and Training”Presents the R/D Tech Web site section where you can find or postadded information related to this booklet.

• “Selected References”Lists basic materials, which support and enrich the booklet ideas.

The booklet is written as an open dialog; we include hints, importantmarks, and caution or warning signs for specific activities.

As the R/D Tech CEO and President mentioned in the preface of thisbooklet, we welcome your opinion, comments, and ideas to improveon the Phased Array Technical Guidelines booklet with the aim ofmaking a second edition.

Please use the Web site forum link, at www.rd-tech.com, for a real-time communication. Our marketing team thanks you in advance foryour input and will contact you for specific problems you may raise.

We hope this booklet will be a great help in carrying out phased arrayultrasonic inspections.

Noël DubéBusiness Development Vice-President, R/D Tech

R/D Tech Phased Array Ultrasonic Technology—General Features 3

1. Phased Array Ultrasonic Technology—General Features

The phased array ultrasonic technology is based on the followingtechnical features:

a) Multiplexing of a large number of identical crystals as a singleprobe

b) Control of the focal depthc) Control of the steering angled) Control of the beam widthe) Program of the virtual probe aperture (VPA) [see Figure 1-1]f) Scan with a large number of A-scansg) Display of the UT data in a generic view called S-scan

Figure 1-1 Multielement probe focusing at different depths and for different angles. Note that the sweep range could be positive and/or negative; different numbers of

elements may be grouped to form a virtual probe aperture (VPA).

Sweep range 2

Sweep range 1

Probe 2 Probe 1

F2

F1

-∆β2

+∆β1

VPA1VPA2

4 Chapter 1 R/D Tech

Specific features of phased array technology include the following:

a) Probe design is based on modeling.b) Each active element of a multielement probe is excited by an

independent pulser (see Figure 1-2).c) The excitation time is computer-controlled and delayed

according to Fermat principle in such a way that the cylindrical(spherical) wave front will reach in the same time (in phase) thespecific points in space.

d) The beam is cylindrically or spherically focused (see chapter 2for more details).

e) The wave front reflected by the defect reaches the reception;time of flight is delayed according to the focal point, refractedangle, and number of active elements.

f) The individual amplitudes from each active element aresummed up (amplitude and same phase).

g) The focal law calculator determines the time delay on individualelements to steer and focus the beam at different depths andangles. See Figure 1-3 for an example of delay value (innanoseconds [10−9 s], that is, a billionth part of a second!). Moredetails are presented in chapter 3.

h) Analog signals are rectified, smoothed, averaged, and may becompressed in an 8-bit or 12-bit option (see Figure 1-4).

i) Beam movement is linked with scanner axes and part geometry.Data may be viewed in a single plane or through a projectionbetween reference and measurement cursors (see chapter 5 formore details).

j) The focus pattern of S-scan may be changed (see Figure 1-5).k) Inspection data is displayed in multiple views or layout; defect

amplitude is color-coded based on specific color palette(rainbow, gray, unrectified, specific custom-built); data isplotted into 2-D specimen for each view (see chapter 4 for moredetails).

l) Data analysis is more reliable and efficient with customizeddefect table and merging A-scans (see chapter 5 for moredetails).

R/D Tech Phased Array Ultrasonic Technology—General Features 5

Figure 1-2 Principle of phased array emitting and receiving with a multielement probe.

The main advantages of phased array technology can be summarizedbelow:

1. Faster. Phased array inspections with linear scanning are typicallyan order of magnitude faster than conventional single probe rasterscanning. This saves significantly in plant downtime and operatorcosts.

2. Flexibility. A single array can inspect many different componentswith different inspection patterns, using electronic setup files.

3. Complex inspections. Phased arrays can be programmed to inspectgeometrically complex components, for example automated weldsor nozzles, with relative ease. Phased arrays can also beprogrammed to perform special scans, for example tandem,multiple angles, multiple modes, and zone discrimination.

4. Small array size. The small size of arrays makes them perfect forspecific applications, for example turbines and discs, where spaceis limited.

5. Mechanical reliability. Fewer moving parts make a more reliableinspection system. Replacing mechanics with electronics reduceswear and tear, as well as increases significantly system reliability.

6. Increase the detectability of misoriented defects. Focus beam increasesthe signal-to-noise ratio (SNR). The multitude of A-scans groupedin a sector with specific angular resolution contributes to detectionprobability.

Acquisitionunit

Phased arrayunit

Probe

PulsesIncident wave front

Reflected wave front

Trigger

Acquisitionunit

Phased arrayunit

Flaw

Flaw

Echo signals

Emitting

Receiving

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Figure 1-3 Block diagram for RF signal processing on the receiving chain, after the summation of individual amplitudes (see Figure 1-2).

Figure 1-4 Example of delay values on individual elements for steering the beam of a longitudinal wave from −30° to +30°.

Figure 1-5 Different types of focusing will generate different S-scan views: (a) projection S-scan is very useful for narrow-gap weld inspection; (b) true depth is useful for detection and sizing defects at a constant depth (for example, inner wall fatigue cracks); (c) half-path S-scan is the most commonly used S-scan; (d) focal

plane S-scan is useful for detection of lack of fusion along the weld geometric preparation.

Filters SmoothingAnalogrectification

A/D Averaging Compression

-30˚ 30˚0˚

321 321 321

a b c da b c d

R/D Tech Phased Array Ultrasonic Technology—General Features 7

Examples of pattern recognition are given in Figure 1-6 to Figure 1-8.

Figure 1-6 Multiangle inspection of a calibration block with stacked side-drilled holes. Left: inspection setup; right: ultrasound display—sectorial scan.

Figure 1-7 Linear (electronic) scan with a static probe over a test piece with artificial defect of variable shape and depth. Top: scanning pattern; bottom: ultrasound

display—side (B) view.

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Figure 1-8 Example of UT range selection and sweep range for a crack detection and sizing with skip angles. Top: principle and UT range setting; bottom: OmniScan

results for a fatigue crack of 8 mm height.

Tip:

• For a reliable detection and sizing of inner-surface breakingcracks ( ), set the ultrasonic range between

, to display the crack facets in

direct and skip detection the crack (see Figure 1-9).

• Use the zoom and software color palette functions for a bettersizing and crack orientation.

UT range

0.5 t

1.5 t

β start β finish

hcrack 1 3⁄( ) tpiece<

0.5 1.5–( ) tpiece βoptimumcos⁄×

R/D Tech Phased Array Ultrasonic Technology—General Features 9

Figure 1-9 Example of UT sweep range for a crack detection by two angles at difference >10 degrees. Left: detection with 38.5°; right: detection with 60°. Remark

the crack facets, detected also by skip, at 60°.

Data analysis and defect characteristics (height, orientation, location)is very reliably performed by plotting UT data into 2-D and 3-Dspecimen (see Figure 1-10).

Courtesy of Ontario Power Generation Inc., Canada

Figure 1-10 Example of UT data plotting (VC S-scan) of a crack into an isometric view of a turbine component.

• Set the sweep range in such a way to detect the crack by at leasttwo angles at a difference of >10 degrees when the probe ismoved backward (see Figure 1-9).