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Phased Array Technical Guidelines

Useful Formulas, Graphs,and Examples

R/D Tech Corp.

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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 array

probes.

• 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 different

methods. 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 this booklet.

• 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 this booklet, 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 Dubé Business Development Vice-President, R/D Tech

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

1. Phased Array UltrasonicTechnology—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 depth

c) Control of the steering angle

d) Control of the beam width

e) Program of the virtual probe aperture (VPA) [see Figure 1-1]

 f) Scan with a large number of A-scans

 g) 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

F 2

F 1

-∆β2

+∆β1

VPA1VPA2

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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 anindependent pulser (see Figure 1-2).

c) The excitation time is computer-controlled and delayedaccording 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, refracted

angle, and number of active elements. f) The individual amplitudes from each active element are

summed 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 (in

nanoseconds [10−9 s], that is, a billionth part of a second!). More

details are presented in chapter 3.h) Analog signals are rectified, smoothed, averaged, and may be

compressed 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 projection between 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; defectamplitude 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 more

details).

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

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

The main advantages of phased array technology can be summarized below:

1. Faster. Phased array inspections with linear scanning are typically

an 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 be

programmed 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 reduces

wear and tear, as well as increases significantly system reliability.6. Increase the detectability of misoriented defects. Focus beam increases

the signal-to-noise ratio (SNR). The multitude of A-scans groupedin a sector with specific angular resolution contributes to detectionprobability.

Acquisition

unit

Phased array

unit

Probe

Pulses

Incident wave front

Reflected wave front

 Trigger

Acquisition

unit

Phased array

unit

Flaw

Flaw

Echo signals

Emitting

Receiving

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6 Chapter 1 R/D Tech

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 alongitudinal 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 isuseful for detection and sizing defects at a constant depth (for example, inner wallfatigue 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 geometricpreparation.

Filters SmoothingAnalog

rectification

A/D Averaging Compression

-30˚ 30˚0˚

321 321 321

a b c d

a b c d

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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 artificialdefect of variable shape and depth. Top: scanning pattern; bottom: ultrasound

display—side (B) view.

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8 Chapter 1 R/D Tech

Figure 1-8 Example of UT range selection and sweep range for a crack detectionand 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 breaking

cracks ( ), 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.

U  T   r a n g e 

0.5 t 

1.5 t 

β start β finish

hcrack  1 3 ⁄ ( ) t  piece<

0.5 1.5 – ( ) t  piece βoptimumcos ⁄ ×

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R/D T h Ph d A Ult i T h l G l F t 9

Figure 1-9 Example of UT sweep range for a crack detection by two angles atdifference >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 isometricview 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).