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ANDE Course - Phased Array Ultrasonics

October 08, 2009C.V.Krishnamurthy

Overview

• Phased Array probe

• Beam forming

• Array probe configurations– Linear

– Matrix

– Circular

– Sectorial-annular

• Probe modeling

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Array Probe

• An array is basically a large single element transducer, which has been subdivided by cutting it into small segments

• Typical element sizes are from 0.02 inches to 0.1 inches, although custom sizes are available.

p g

e

L

A

A – aperture

e – element width

g – gap

p – pitch

L – element length

Why not an array of large elements?

A large probe will give a

good flat coverage, but its

small beam angle limits

its "visibility".

Recall that the 6 dB beam

divergence is given by

A

5.0sin

Aperture 6 dB beam width

6.35 mm (0.25 inch) 4.52

12. 7 mm (0.5 inch) 2.26

25.4 mm (1.0 inch) 1.13

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Why divide the probe into small elements?

A B C

A small element has a much larger

beam divergence angle, and it is this

large angle which opens up the

useful features of arrays such as

dynamic focusing and beam

steering.

Another feature of small elements is

their energy transfer efficiency -

smaller elements take less energy to

excite and are more efficient

receivers due to the lower mass to

be energized.

Beam divergence is also a function

of frequency, lower frequencies will

give more divergence than higher.

Typical element sizes/frequencies for industrial applications

are 1mm wide for 2.5 MHz and 0.5 mm wide for 5 MHz

Overlapping beams using

Phased Array Elements

Small Flaw

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Phased Array Beamforming

Beamforming requires precise pulsing and time delays.

Receiving is the reverse of pulsing.

Array Probe Head

The array head module includes 16, 32, 64, or 128 elements (dependant on array type)

A separate pulser and preamplifier for each element, together with a multiplexer, which connects up to 8, 16, or 32 elements to create a virtual probe.

The output from the virtual probe is connected to 8, 16, or 32 coaxial wires,which connect the module to the main evaluation electronics.

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Typical Parameters

• Max No. of elements in system 256

• Max No. of elements to fire as one group 32

• Pulser Voltage 50V fixed spike

• Amplifier Bandwidth 0.25 - 20 MHz

• Max PRF 20 kHz

• Pre amplifier gain fixed 6 dB

• Digitization resolution 50 MHz

The pulser fire signals can be delayed from 0 to 2.5 s, in steps of 2.5 ns.

The returned RF echoes from each channel are amplified +/- 10 dB in steps of 0.1 dB and are digitized at 50 MHz.

The digitized echoes are delayed from 0 to 2.5 s in steps of 2.5 ns. This process is entirely digital.

Typical Array Probe Types

A Linear array is a series of transducer elements

aligned in a single housing, typically a rectangular

single element that has been segmented into smaller

individual elements.

A Curved array is similar to the linear array

with the elements curved to produce a desired

beam shape or conform to the geometry of the

part under test.

An Annular array is a series of

concentric ring elements contained

in a single housing. Imagine a single

round element being divided into

multiple individual concentric rings.

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Other Array Configurations

Electronic Linear Scan

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Electronic Focusing

Electronic Steering and Focusing

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Uniform Arrays - I

Uniform Arrays - II

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Uniform Arrays - III

Uniform Arrays - IV

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Array Beam Characteristics - I

4

2AN

A

5.0sin

FS

N

A

Fdst

Near-field to far-field distance

6 dB angular beam divergence

Focusing power (when focusing option is used)

Beam dimension at focal distance (in steering plane)

A is the dimension of the active aperture

Recap: Depth of Field

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Strong and Weak Focusing: Example

Number of active elements

10 16 32

Active Aperture (mm)

10 16 32

N (mm) 84 216 865

F (mm) 84 84 84

S 0.99 0.39 0.10

d (in mm) at F

2.49 1.55 0.78

Linear array probe pitch p = 1 mm, frequency f = 5 MHz

Calculations for water medium (v = 1480 m/s)

Array Beam Characteristics - II

• Beam width (main beam)

determined by active

aperture A

• Steering width determined

by element width e

• Angular position of lobes

determined by frequency f

and pitch p:p

lobe

est

5.0sin

A

5.0sin

p g

e

L

A

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Focusing using Phased Array - I

Focal depth: 8 mmFocal depth: 4 mm

Focusing using Phased Array - II

Focal depth: 16 mmFocal depth: 12 mm

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How many elements are needed? (Beam Formation – I)

Simulations at 5 MHz in Al

How many elements are needed? (Beam Formation – II)

Simulations at 5 MHz in Al

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Beam quality15 MHz / 6mm x 0.2mm / 0.3mm spacing / 1.5” focus

On Axis (mm) Off Axis (mm)

10 20 30 40 50 60 70 80 900

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-30 -20 -10 0 10 20 300

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Grating Lobes

10 20 30 40 50 60 70 80 900

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-30 -20 -10 0 10 20 300

0.1

0.2

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0.5

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0.9

1

Focus

Narrower

Main Lobe

Focus

16 Elements

16 Elements

8 Elements

8 Elements

Effect of Steering Angle on Directivity

N = 16, d = /2

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Directivity for small d/

Grating Lobes

1sins

c

d

1 2( ) ( ) ( )H H H

For an N-element array, inter-element spacing d,

time-delay between adjacent elements , the

steering angle is given by

Directivity is a product of the directivity of

discrete line sources H2( ), and the directivity of

a single element H1( )

For sufficiently small e/, H( ) H2( )

and is given by

Example plot for N=16, c = 5850 m/s, f = 2.3 MHz, e = /100, d = 2/(1+3) and s = 30

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Directivity for large d/

Avoiding Grating Lobes

Maximum inter-element

spacing without producing

grating lobes

Maximum steerable angle

given the inter-element

spacing d, and the

number of elements N

Note: When N is large, for a 60 sector scan corresponding to maximum steering angles 30, even an inter-element spacing of 2/3 does not produce grating lobes.

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Modeling a 1-D Phased Array

-60 -40 -20 0 20 40 60

0

20

40

60

80

100

120-60 -40 -20 0 20 40 60

0

20

40

60

80

-60 -40 -20 0 20 40 60

0

20

40

60

80

100

120-60 -40 -20 0 20 40 60

0

20

40

60

80

1

0

Axia

l D

ista

nce (

mm

)

Lateral Distance (mm)

Normal Incidence Beam Steered to 39º Focused On-axis Focused & Steered to 39º

64 rectangular elements

Centre Frequency: 7.5 MHz

Overall lateral dimension: 19.1 mm

(Near –field distance in Steel is about 115 mm)

Medium: Steel

Steering and Focusing

- Beam Quality in the Far Field of the Array

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Steering and Focusing

- Beam Quality in the Near Field of the Array

Regular and Random 2D - Arrays

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Random and Optimized 2D - Arrays

Array probes on Wedges

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Focal law for Wedge - I

Interface

X axis or Scan axis

Depth

Refraction

point

Law scan offset

Refracted Angle

The calculator searches the Snell point. It considers the center of the active aperture

(from elements 2 to 7 in this example). Then, the X, Z point of the focal point is

determined. The wedge delay is calculated and the focal law is offset accordingly.

Focal law for Wedge - II

Focal point (X,Z)

Interface

X axis or Scan axis

In wedge

In material

Sound path (time)

Time

Element number

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Examples of PAUT Applications

• Relative Arrival Time Technique (RATT)

• Absolute Arrival Time Technique (AATT)

• Linear scan - Cruciform case

• Dynamic depth focusing

• Sectorial scan

• Synthetic Aperture Focusing Technique

• Advantages of phased array

• Limitations

Tandem Scans

Phased arrays allow for dynamic scanning using the tandem technique.

Separate array groups are defined as transmit and receive "virtual

probes" and scanned to cover the test area.

This technique can be used for testing weldments in thick sections.

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Dynamic Depth Focusing

Uses the basic focusing techniques, but sequentially

focuses at various depths to cover the thickness of the part

to be tested.

Useful with linear arrays for a line scan effect, or annular

arrays to give a point focus effect

Phased array with specified

focal depth

Phased array with

dynamic depth focusing

Dynamic Depth Focusing

DDF is useful for inspecting thick components in a single pulse. The beam is refocused electronically on its return.

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Tube Inspection

Segment arrays for

large pipes

Rotating water

system segment

arrays for mid-size

pipes

Rotating water system

encircling arrays for

small size tubes.

Relative Arrival Time Technique - Principle

Ultrasound path between crack tip and corner trap signals

for a surface crack.

( )cosh CD AB

h

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RATT – Notches in Large MS Pipes

Actual

Depth (mm)

Estimated

from

Simulations

Estimated

from

Experiment

3 3.1 2.9

5 4.8 4.7

7 7.2 7.4

Comparison of the simulated and experimental

estimated notch sizes obtained by RATT on 10-mm

thick mild-steel pipe sample at 45 angle of incidence

RATT – Inspection Angle

Comparison of simulated and experimental B-scan images of 7-mm bottom surface crack

obtained for the various angles of incidence. (a) 35, (b) 45 and (c) 55 angle inspections

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Example of Signal Processing in RATT

L. Satyanarayan et al., Inverse method for detection and sizing of cracks in thin sections ..., Theor.

Appl. Fract. Mech. (2008)

AATT - Principle

2 2 1 1cos cosh UT UT

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AATT Examples

Estimated crack length: 4.9 mm (5 mm)

Angle of Inspection: 49

Estimated crack length: 6.4 mm (6.3 mm)

Top tip Inspection Angle: 74.5

Corner Inspection Angle: 42.5

Example: Monitoring Crack Growth

ABC

10 15 20 25 30 35 40

10

20

30

40

50

60

C

BA

Simulation10 15 20 25 30 35 40

10

20

30

40

50

60

70

80

90

C

B

A

Experiment

PA

SnapshotSchematic

Scanned Images

Steel specimen

Nondestructive PA results compared with that of

Destructive Dye penetrant testApplications in the Fatigue Crack Growth Studies

in Large Nuclear Components - BARC

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S-scan

Detection of four side-drilled holes (SDHs)

(a) Sectorial scanning, (b) S-scan view using 30

S-scans are stacked A-scans

Examples of S-scan

Turbine Blade Root

Turbine Welded Rotor

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Volumetric Weld Coverage - I

TOFD is sensitive to all defects

including volumetric defects

TOFD has dead zones near surfaces

PE complements TOFD

The combination covers 100% of the

weld volume

Alternative to RT (ASME CC2235,

AWS)

TOFD

PE 45 SW

PE 60 SW

Volumetric Weld Coverage - II

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TOFD 60-SW60 SW 45-SW45-SW

Pro

be

Movem

ent

Volumetric Weld Coverage - Data Visualisation

Synthetic aperture focusing

with Phased Array

Individual A- Scans

Tim

e

Small Flaw

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10 20 30 40 50 60

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0 10 20 30 40 50 60 700

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X: 33

Y: 0.3148

X: 29

Y: 0.115 X: 34

Y: 0.1011

X: 33

Y: 0.3148

X: 29

Y: 0.8049

X: 34

Y: 0.8171X: 24

Y: 0.8902

X: 27

Y: 0.9146

X: 40

Y: 0.8049

X: 31

Y: 0.3422

X: 30

Y: 0.2619

10 20 30 40 50 60

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X: 37

Y: 0.254

X: 40

Y: 0.112

X: 27

Y: 1

X: 35

Y: 0.3466

X: 32

Y: 0.6489

X: 33

Y: 0.0715

X: 44

Y: 0.9255

X: 42

Y: 0.8298

X: 36

Y: 1 X: 43

Y: 0.9681

X: 45

Y: 0.9681

X: 40

Y: 0.6809

10 20 30 40 50 60

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X: 27

Y: 0.9589

X: 34

Y: 0.5054

X: 36

Y: 0.2977

X: 35

Y: 1

X: 43

Y: 0.9178

X: 22

Y: 0.6438

X: 29

Y: 0.6986

X: 35

Y: 0.7397

X: 32

Y: 0.07272

X: 39

Y: 0.09162

X: 25

Y: 1

X: 45

Y: 0.9315

10 20 30 40 50 60

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X: 36

Y: 0.5309

X: 22

Y: 0.8764

X: 35

Y: 1

X: 49

Y: 1

X: 47

Y: 0.9505

X: 37

Y: 0.2653

X: 36

Y: 0.9146

X: 34

Y: 0.2826

X: 53

Y: 0.5802

X: 18

Y: 0.4938

X: 27

Y: 0.5802

X: 32

Y: 0.1241 X: 40

Y: 0.1107

13 mm deep SDH 19 mm deep SDH 26 mm deep SDH 45 mm deep SDH

Example of SAFT

Element by element pulse-echo from a 1.5 mm dia SDH in Al

SAFT

Image

RAW

Image

Advantages of Phased Arrays

• Inspection Speed– Real-time images

• Flexibility

• POD ( many angles and imaging)

• Access to remote areas

• Analysis Tools

• Reporting

• Good coverage (Multiple scan options)

• Real-time images

• Wedge-based applications

• Variety of probe types

• Similar to conventional procedures in TOFD

• Lends to Image processing techniques

• Simulations aid quantitative assessment

• Allows for new probe designs

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Advantages of Digital Recording

• Permanent record of all collected data

• Various signal processing options

• Enables data to be compared throughout the service life

of a component

• Re-analysis of raw data, at any time

• Variety of visual displays available

All the usual ultrasonic limitations

• Coupling

• Frequency, attenuation etc.

• Acoustic impedance mismatch requirements

• Dead zones

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ASME Codes

• Phased arrays specifically accepted as Computerized Imaging Techniques

• Code cases for manual S-scans and E-scans first submitted Feb 2006. Now approved.

• Code cases for encoded E-scans

and S-scans expected for August

2006. Now Approved

• Mandatory phased array appendix being drafted concurrently. Expect approval of appendix in a few years

• Phased arrays currently being approved through Performance Demonstration approaches, e.g. Article 14 and ASME code case 2235

• ASME CC 2235 (CIT) allows use of UT instead of RT for wall thickness > 12.7mm

• TOFD, PA and TOFD/PE techniques allowed

• Phased arrays specifically approved for ASME CC