hased Array Ultrasonic Testing, Sectorial scan

8/13/2019 hased Array Ultrasonic Testing, Sectorial scan

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Insight Vol 48 No 4 April 2006 1

Introduction

Ultrasonic phased arrays are a new technology which offers new

scan patterns and inspection procedures as well as emulations of

current ultrasonic procedures. Phased arrays have major advantages

over conventional ultrasonics: beams can be swept, steered and

focused. These advantages have been documented in the past(1).

However, phased array probes also offer some technical advantages

for crack sizing that have not yet been well-documented: variable

apertures for specic focusing applications, highly damped piezo-

composite arrays, frequency improvements, as well as controlled

focusing.

This paper describes the resolving capabilities of phased

arrays compared to established techniques. Specifically, the

paper investigates some resolution aspects of defect sizing using

backscatter tip diffraction techniques. Phased array probes are

particularly useful for this sizing technique since(2):

q No probe movement occurs; any slight skewing during scanning

may affect the image and results obtained;

q Coupling tends to be more constant;

q The variable angles of refraction from S-scans aid detection,

since optimum tip signals do not always correlate with xed

angle inspections; and

q S-scans allow the operator to image the corner reector and

the back-scattered tip diffracted signal at the same time. Thetwo biggest problems with back-scattered tip diffraction are

correctly identifying the tip signal from noise, and low signal-

to-noise ratio.

S-scans are the inspection procedure used in this study. An

S-scan is also termed a sectorial-scan, sector scan, swept angle

scan, or azimuthal scan; this may refer to either the beam movement

or the data display. As a data display, it is a 2D view of all A-scans

from a specific set of elements corrected for delay and refracted

angle. When used to refer to the beam movement, it refers to the

set of focal laws that sweep through a defined range of angles

using the same set of elements. Besides S-scans, phased arrays can

perform ASME raster-type electronic scans (for example E-scans at

45, 60 and/or 70), TOFD and tandem inspections; though thesescans are not the subject of this study, the same resolving capability

issues apply to them.

After an initial description of phased arrays, the paper describes

the key parameters which dictate resolution, followed by some

S-scan screen displays of typical defect sizing applications using

back-scattered tip diffracted signals.

Industrial phased arrays

Phased array probes use an array of elements, all individually

wired, pulsed and time-shifted on both pulsing and receiving.

These elements are typically pulsed in groups of ~16 and up to 32

elements at a time for weld inspections. With user-friendly systems,

a typical set-up calculates the time-delays from operator-input, oruses a pre-dened le calculated for the inspection angle, focal

distance, scan pattern etc, as illustrated in Figure 1. The time delay

values are back-calculated using time-of-ight from the focal spot,

and the scan assembled from individual Focal Laws. Time delay

circuits must be accurate to around two nanoseconds to provide the

accuracy required.

Due to the limited market, complexity, software requirements

and manufacturing problems, industrial uses of phased arrays have

been limited until the last few years, but are now becoming more

prevalent with phased array developments and training. Codes are

beginning to recognise the use and applications of phased arrays.

From a practical viewpoint, ultrasonic phased arrays are

primarily a method of generating and receiving ultrasound (though

with major imaging advantages). Consequently, many of the detailsof ultrasonic inspection remain unchanged; for example, if 5.0 MHz

is the optimum inspection frequency with conventional ultrasonics,

PHASED ARRAYS

Resolving capabilities of phased array sectorial scans

(S-scans) on diffracted tip signals

J M Davis and M Moles

This paper demonstrates the signicant improvementavailable with phased array Sectorial (S-) scans forresolving crack tip signals using back-scattered diffractionon thinner plates. Initially, phased arrays and the factorsaffecting resolution are described, such as aperture and

focusing. Another key feature is the use of piezo-compositearray probes, which have shorter pulses. A third factor isS-scan imaging, which signicantly improves crack tipsignal identication. Some experiments show the resolvingcapabilities of phased arrays.

Mark Davis is a graduate of the US Navy NDT Programme and is an ASNT

UT Level III with over 30 years of experience in Welding, Quality Assurance

and NDT. He has held Level IIIs in MT, PT, RT, ET, LT and VT. Davis has

taught ultrasonics for over 20 years, specialising in IGSCC detection, crack

sizing and weld overlay examinations. Formerly, Davis was the Inservice

Inspection Training Manager at the EPRI NDE Center in Charlotte, NC.

Currently, Davis is the Programme Administrator for the American

Petroleum Institute QUTE Shear Wave Examination, as well as the Crack

Sizing and Tank Bottom Thickness Programme. He has authored three

mathematic handbooks on UT, RT and ET, with one handbook on Advanced

Ultrasonic Crack Sizing Applications.

Davis NDE is an approved Training Organisation for Olympus NDT onadvanced phased array training for detection and sizing applications.

J Mark Davis is at Davis NDE, 4060 Bent River Lane, Birmingham, AL,

35216, USA. Tel: +1 (205) 733-0404; E-mail: [email protected]

Michael Moles is in Market Development with Olympus NDT. He has a

PhD and an MBA, and has worked in the eld of automated ultrasonics

for twenty-ve years. He was employed in the Canadian nuclear industry

for sixteen years, and has worked in petrochemical, aerospace and

manufacturing for a decade.

Michael has presented and published over one hundred papers on

ultrasonics, especially phased arrays, and edited the R/D Tech Introduction

to Phased Array Ultrasonic Technology Applications.

Michael is a member of ASME, ASNT, AWS, CINDE, and is a registered

engineer in Ontario.

Contact address: Michael Moles, Olympus NDT, 73 Superior Avenue,

Toronto, Ontario, M8V 2M7, Canada. Tel: +1 (416) 831 4428; E-mail:

[email protected]


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2 Insight Vol 48 No 4 April 2006

then phased arrays would typically start with the same frequency,

focal length, and incident angle.

While it can be time-consuming to prepare the first set-up, theinformation is recorded in a file and only takes seconds to re-load.

Also, modifying a prepared set-up is quick in comparison with

physically adjusting conventional transducers. Using electronic

pulsing and receiving provides significant opportunities for a

variety of scan patterns.

Electronic scans (E-scans)

Multiplexing along an array generates electronic scans (see

Figure 2). Typical arrays have up to 128 elements, pulsed in groups

of 8 to 16. E-scanning permits rapid coverage with a tight focal

spot. If the array is at and linear, then the scan pattern is a simple

B-scan. If the array is curved, then the scan pattern will be curved.

E-scans are straightforward to program. For example, a phased

array can be readily programmed to inspect a weld using both

45, 60 and 70 shear waves, which mimic conventional manual

inspections or automated raster scans.

Sectorial (S-) scans

S-scans use a xed set of elements, but alter the time delays

to sweep the beam through a series of angles. Generally, this

is a straightforward scan to program. One application for

S-scans involves a stationary array, sweeping across a relatively

inaccessible component like a turbine blade root(3), to map out the

features and defects. Other applications (as in this paper) involve

scanning defects to image the corner reector, crack tips, facets,

crack branches etc. Depending primarily on the array frequency,

element spacing and wedge design, the sweep angles can vary from

+ 20 up to + 80.

S-scans are unique to phased arrays, and offer excellent imaging

and data interpretation with improved resolving power. For example,

Figure 3 shows a 10% ID-connected defect with tip signal clearlydetectable on a 9.5 mm plate, and Figure 4 a 10% OD-connected

defect in a 9.5 mm sizing bar. These images are true depth; all

refracted angles, for example 45 to 60, are focused and displayed

at the same depth on the screen. It is straightforward and quick

to analyse these particular S-scan images, based on knowledge

of the component thickness, because the signals are clean and

clear. Consequently, there is a big demand to use S-scans for weld

inspections.

Combined scans

Phased arrays permit combining electronic scanning, sectorial

scanning and precision focusing to give a practical combination

of displays. Optimum angles can be selected for welds and other

Figure 1. Schematic showing generation of electronic, sectorial and dynamic depth focusing scans using phased arrays

Figure 2. Schematic illustration of electronic scanning

Figure 3. A-scan and S-scan showing 10% ID-connected crackwith diffraction tip signal (arrowed)

Figure 4. A-scan and S-scan showing 10% OD-connected crackwith diffraction tip signal (arrowed)


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components, while electronic scanning permits fast and functional

inspections. This introduces the concept of tailored inspections to

optimise detection, sizing and inspection time. The best example

of this approach is ASTM E-1961 for AUT of girth welds in

pipelines(4).

Key issues in phased array defect resolution

Array design features

Array design is a key issue, which dictates beam steering, focusingand sweeping. The key parameters are shown in Figure 5. These

factors are also described in the literature(5).

The key probe parameters are:

q Total number of elements in array (n).

qFrequency (f).

q Total aperture in steering or active direction (A).

q Height or elevation, aperture in mechanical or passive direction

(H).

q Width of an individual element (e). Typical element widths

range from 0.3 to 1 mm.

q Pitch, centre-to-centre distance between two successive

elements (p).

q Saw cut gap, or kerf, between two elements (g). Gaps are

typically ~0.05 mm.

Note:

1. The total number of elements in the array primarily dictates

coverage and is governed by the application.

2. The frequency is important for resolution, and is discussed

later.

3. The total (active) aperture dictates focusing ability, and again, is

discussed below.

4. H, the aperture in the passive direction, is ignored in this study

as we are using commercial arrays with xed H. The passive

aperture will affect focusing in the passive direction, as dened

by the equation for total (active) aperture.

5. The width of individual elements, e, is a critical parameter for

beam steering, and is discussed below.

6. p, the pitch of the array, is usually similar to e, the element

width as saw cuts are miniscule; however, sparse arrays are

possible, which introduce potential problems like grating lobes

(see sparse arrays below).

Normal phased array probe design is defined by frequency,

element width (e), number of elements (n) and pitch (p). First, this

paper will describe array manufacturing techniques, particularly

with respect to resolution.

Piezo-composite arrays

The piezo-composite manufacturing techniques for conventional

ultrasonic transducers have been transferred to arrays. Figure 6

shows a schematic of the lay-out of a matrix array. For mostapplications, a linear array is adequate, so only linear arrays were

considered in this study.

Piezo-composite arrays are made by using thin rods of ceramic

material embedded into a polymer. The signal-to-noise ratio

obtained from composite transducers is typically 10-30 dB greater

than obtained from piezo-ceramic probes. The array construction is

similar to conventional transducers, as shown in Figure 7.

Another major advantage of piezo-composites is their high

damping capability. Unlike conventional piezo-electric materials,

which ring for 2-4 cycles, piezo-composites can ring for as littleas half a cycle. In practice, 1-2 cycles are more normal, as shown

in Figure 8.

The shorter pulse from piezo-composite arrays permits better

crack tip resolution.

Aperture and focusing

The adjustable aperture with phased arrays allows focusing to be

tailored to the application. With a given array, aperture size can be

adjusted by selecting the number of elements activated. Table 1

shows the theoretical effects on focusing for a 5 MHz array with

1 mm pitch.

The focal beam width, d, shows that a larger aperture gives

a smaller spot size (but shorter depth of field), so the immediate

reaction is to use the largest aperture possible. However, fieldexperience indicates that this is not the optimum solution in most

applications. First, very large apertures give larger probe sizes,

which can present practical problems. Second, the highly-focused

beams tend to have less working range, which limits their use.

In practice, conventional transducers are sold in fixed diameters

for these practical reasons, and these sizes are emulated by standard

Figure 5. Diagram showing phased array probe manufacturingparameters

Figure 6. Schematic of piezo-composite matrix array lay-out

Figure 7. Schematic of array construction

Number of elements 10 16 32

Aperture (mm) 10 16 32

N Fresnel distance (mm) 84 216 865

Focusing depth (mm) 84 84 84

K 0.99 0.39 0.10

d (at focusing depth mm) 2.49 1.55 0.78

Table 1. Focal spot size for a selected aperture size


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arrays and apertures. Consequently, for most practical applications,

the number of pulsers is not important, provided the active aperture

is appropriate. The exceptions occur when significant beam

steering is required (see beam steering later), but such applications

are limited in the manufacturing and petrochemical industries.

With phased array focusing capabilities, ultrasonic testing can

now be performed in the near field. Historically, the near field has

been an operators problem to overcome. Table 1 shows the effect

of aperture on the near field lengths.

Examples of three different apertures are shown in Figure 9, for

10, 16 and 32 elements of the same width e.

FrequencySince phased arrays are primarily a method of generating and

receiving ultrasound, and the physics remains unchanged, the simple

approach to selecting frequency is to use the same frequency as the

equivalent conventional transducer. So, if conventional ultrasonics

uses, say, 5 MHz, plan on using the same frequency for arrays.

The same applies to the aperture: if conventional ultrasonics uses a

10 mm diameter, plan on using similar aperture with phased arrays

(for example, 10 elements of 1 mm width).

In practice, phased arrays can often use higher frequencies (and

occasionally larger apertures) to provide better signal/noise and

give a tighter, optimised focal spot. Instead of a 5 MHz conventional

transducer, a 7.5 MHz (focused) array may be optimum. Both

higher frequency and tighter focus permit better defect resolution,

sensitivity, detectability and signal-to-noise ratios. Manufacturing

problems may occur with some arrays, but these are mostly at high

frequencies (>15 MHz), and not important for ferritic steels.

Beam steering

Beam steering ability dictates the angular range of the S-scan, and

is a key issue for selecting array element width (e), and array

frequency. The relationship between the steering angle at -6 dB, st,

frequency and element size is given in equation (1).

...................................(1)

As the element width decreases, or the wavelength increases

(frequency decreases), the steering ability increases. Again, practical

considerations rule here; as the frequency decreases, resolution

decreases accordingly. As element size decreases, steering will

increase but the number of elements (and instrumentation

requirements) increases rapidly; budget considerations may rule in

this instance.

Steering range can be improved using an angled wedge; for

example, shear wave inspections typically use a wedge with a

natural refracted angle of 45, which can sweep from, say, 30

to 70(6). For different angular ranges, different wedges may be

appropriate; for example with natural refracted angles of 55 or

60. In general, for most manufacturing applications, beam steering

is designed into the array-wedge combination.

Grating lobes

All ultrasonic transducers produce side lobes, which is part of thephysics of constructive-destructive interference that generates

the beam(5). Phased arrays also generate grating lobes, which

are similar in concept to side lobes, and occur from the regularity

of the elements and spacing. In effect, these grating lobes are

standing waves. With a regular arrangement of elements (as in

most arrays), the far-eld pattern of an array probe shows a main

beam and grating lobes at regular angular spacing. These grating

lobes are predictable, both in angular direction and in amplitude.

The position is determined by the array frequency and pitch, p, as

shown in equation 2.

.....................................(2)

Figure 10 shows a simulation of grating lobes for different

numbers of elements and pitches with the same aperture size.The general rule for grating lobes is to design them out.

q For element sizes (e) , side lobes will occur; this combination

is not recommended.

q If e < /2, no side lobes will occur.

q For element sizes /2 < e < , the lobe will depend on the

specic parameters.

Sparse arrays

One solution to the requirement of high beam steering with a

limited number of elements is to make sparse arrays. These

arrays are similar in principle to standard arrays, but have many

elements effectively taken out. The main problem is sparse arrays

will produce strong grating lobes, as simulated in Figure 11. While

the main beam is little affected in the sparse array, the grating lobesare now signicant.

One solution to making sparse arrays is to use quasi-random

element locations to effectively smear the grating lobes(7). However,

this approach is not recommended for standard defect sizing since

signal-to-noise ratio suffers, as illustrated in Figure 11.

Voltage

Though different instruments have different pulser voltages, in

practice pulser voltage is not important for resolution provided

the instrument has enough power to penetrate. Under normal

conditions with a suitable array and frequency, penetration is not

an issue. Figure 12 shows a series of scans on the same crack using

different pulser voltages and gains.

The low voltage has a double advantage: it increases probe lifeand produces less heat, so no extra cooling devices are needed. The

low voltage value may be compensated by increasing gain without

Figure 8. Typical waveforms (left, rf; right, rectied), showingshort pulse effect

Figure 9. Beam simulations showing 1 mm array with threedifferent apertures (left: 10 x 1 mm, middle: 16 x 1 mm, right32 x 1 mm)


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a significant increase of the noise level. If the voltage is reduced by

50%, and the gain increased by 50% to compensate, the images are

effectively identical. The key issue with defect detection and sizing

is signal-to-noise ratio.

Experimental procedures

A standard OmniScan 16:128 phased array unit with a 5L16-A1

array (5 MHz, linear array, 16 elements) on a standard 45 wedge

was used for inspecting the defects.

A series of stainless and carbon steel plates were inspected

using S-scans. Stress corrosion cracks in the stainless steel were

grown by Davis NDE, and cracks in the carbon steel were multi-

branched fatigue cracks. Both types of defects are traditionally

hard to size and characterise. Both half-skip and skip analyses

were used, depending on the crack configuration. The inspections

were performed manually, and screen shots saved for comparisons.

Figure 13 shows a photo of typical samples.

Typical results

A 9.5 mm-thick stainless steel plate with a crack approximately

70-80% through-wall is shown in Figure 14. The crack is imaged in

direct reection (less than half skip), corner trap reector, and some

skip reection by bouncing off the plate bottom. The facets can

be individually resolved with this set-up and phased array probe.

In comparison, the A-scan at left (which correlates with the blue

line, or data cursor, at 54) is far harder to analyse. In contrast,

individual facets can be clearly seen in the S-scan image.

Figure 15 shows a 12.7 mm stainless steel plate, with an SCC

approximately 60% through wall. Imaging is direct, ie less than

half skip. Again, the phased array imaging is vastly superior to

Figure 10. Grating lobes as a function of element size and pitch

Figure 11. Simulation of two arrays: 5 MHz, focal point at50 mm, array width = 20 mm, element width (e) = 0.6 mm,wavelength = 0.3 mm in water. Left: 33 elements, no gaps.Right: sparse array with only nine elements. Note the yellowgrating lobes in the right image

Figure 12. Detection and sizing of a fatigue crack by a 3.5 MHz 1.0 mm pitch linear array probe using different voltage combinations(Courtesy of OPG)


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standard A-scan imaging (at left in Figure 15), and the shortened

pulse length, appropriate focus and overall sectorial echodynamic

display permits much better analysis. Sizing is easily performed

using the cursors and scale at right.The 9.7 mm carbon steel plate in Figure 16 contains a multi-

branched fatigue crack approximately 6 mm deep. Here, the

signal-to-noise ratio is excellent due to the lower noise level in

ferritic steels, and various facets and the main crack tip are easily

distinguished. With short pulses and good focusing, sizing is

straightforward using cursors. With the S-scan imaging, the tip

signal is easily differentiated.

For comparison, Figure 17 shows a complex crack in a

carbon steel 9.9 mm bar, with multiple facets visible. The S-scan

echodynamic display shows considerable improvement over the

A-scan image at left.

Comparing resolution with different apertures

As mentioned above, resolution is dependant on focusing

capability, short pulses, and frequency. The simplest demonstration

of resolving capability is by altering the aperture. Figures 18, 19

and 20 show a series of three scans of the same crack at the samelocation using three different apertures of 16, 8 and 4 elements.

(Note that the start points of the scans are adjusted so that the array

is always in the same effective position). The effect of altering

focusing was less notable than altering aperture.

Comparing piezo-composites and piezo-ceramics

Figure 21 shows a typical RF signal from a standard single-channel

aw detector on a 10%-deep crack in a 10 mm carbon steel bar

using a standard piezo-ceramic transducer. The tip diffracted signal

is essentially invisible, ieresolution is much diminished.

Comparing Figure 21 with Figure 18 of the same sizing bar

shows that the S-scan approach using a piezo-composite phased

array probe has much better crack tip resolution than the piezo-

ceramic transducer. The situation is complicated as the fixed 45angle is not necessarily the optimum angle for back-scattered tip

diffraction, as in this case. The combination of highly damped

array and multiple angle imaging (S-scans) allows phased arrays to

provide more accurate crack sizing results.

Discussion

This paper has illustrated the excellent defect resolution obtainable

with phased arrays using piezo-composite probes on relatively thin

plates. Coupled with the S-scan imaging, the combination permits

vastly better defect characterisation and sizing than conventional

A-scan approaches. In addition, these scans are easy to set up,

and straightforward to perform. Another major advantage is the

speed of inspection, as detection and analysis are much faster.Similar results have been obtained on complex cracks on thicker

components by other workers(8).

Figure 13. Photo of typical SCC plate and sizing bars withcracks

Figure 14. Typical S-scan of stainless steel SCC showingmultiple reections

Figure 15. S-scan display of SCC in stainless steel plate

Figure 16. C-steel plate with multiple facets and tip imaged

Figure 17. Complex crack imaged in skip


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In terms of improved resolution, the effects of aperture size

(and focusing capability) are clearly shown in Figures 18-20, and

predictable(9). These S-scans clearly demonstrate the importance of

appropriate resolution for back-scattered tip diffraction for defect

sizing. The spatial resolution of the S-scans permits the operator to

visually see all refracted angles displayed at the same time.The development of highly damped piezo-composite arrays has

certainly improved resolution. This is more difficult to demonstrate,

since equivalent piezo-ceramic arrays are not available. However,

comparing A-scans in Figures 18 and 21 shows the effect quite

clearly in general terms.

Conclusions

n Improved defect tip resolution and signal-to-noise ratios can

be obtained by using piezo-composite phased arrays, with

appropriate apertures and focusing.

n S-scan imaging and S-scan echodynamic displays make defect

characterising much simpler and more reliable, even for multi-

branched cracks.n Back-scattered tip diffraction is a viable technique for defect

sizing since S-scans permit correct tip identication, as well as

good resolution.

n With the individual facets and crack tips clearly imaged, sizing

should be much more reliable and accurate, even for small

(~1 mm defects).

References

1. G Lafontaine and F Cancre, Potential of ultrasonic phasedarrays for faster, better and cheaper inspections, NDT.net,

Vol 5, No 10, October 2000. http://www.ndt.net/article/v05n10/

lafont2/lafont2.htm

2. F Jacques, F Moreau and E Ginzel, Ultrasonic backscatter

sizing using phased array developments in tip diffraction awsizing, Insight, November 2003.

3. P Ciorau, 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,

11-15 October 2000.

4. M Moles, N Dub and E Ginzel, Pipeline girth weld inspections

using ultrasonic phased arrays, International Pipeline

Conference, IPC02-27393, Calgary, Alberta, 29 September

3 October 2002.

5. Introduction to phased array ultrasonic technology applications

R/D Tech guideline, published by R/D Tech Inc, 2004.

6. Olympus NDT, Phased-array probe ultrasonic catalogue, 2005-

2006.

7. J J Selman, J T Miller, O Dupuis, M D C Moles and P Herzog,

FASTFOCUS a novel ultrasonic phased array system for

fastener hole inspection, Aging Aircraft, San Francisco,

California, USA, 16-19 September 2002.

8. P Ciorau, Contribution to detection and sizing linear defects by

phased array ultrasonic techniques, 4th International Conference

on NDE in Relation to Structural Integrity for Nuclear and

Pressurized Components, London, UK, 6-8 December 2004.

9. J Mark Davis, Advanced ultrasonic aw sizing handbook,

Third Edition, Published by The Art Room Corporation, 2006.

Figure 18. S-scan of carbon steel plate with crack using sixteenelements. Crack tip is arrowed

Figure 19. Same S-scan as Figure 18, but with eight elements(half aperture). Note reduced resolution of crack tip

Figure 20. Same S-scan as Figures 18 and 19, but with fourelements (quarter aperture). Crack tip is barely resolved

Figure 21. A-scan of 10%-deep crack, with backscatter tipdiffracted signal undetectable

hased Array Ultrasonic Testing, Sectorial scan

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