extended abstract development of an ultrasonic phased ... · development of an ultrasonic phased...

1

Extended Abstract

Development of an ultrasonic Phased Array system to inspect

welded joints of low thickness austenitic steel

Maria Inês Freitas1

Supervisor: Prof. Luísa Coutinho1

1Instituto Superior Técnico, Universidade de Lisboa, Portugal

May 2016

Abstract

The inspection of austenitic welds is a challenge in the field of nondestructive testing by

ultrasounds, in particular low-thickness components. Conventional ultrasonic testing of austenitic

materials is difficult due to their microstructure. This microstructure is influenced by the welding

process, causing an anisotropic acoustic behavior and creating a dispersion of sound in all directions.

This acoustic behavior leads to an uncertainty in the positioning and sizing of discontinuities that may

be present in the weld. As a result, on one hand there might appear unnecessary repairs and, on the

other hand, the acceptance of critical defects might occur.

In this project, an ultrasonic inspection procedure for automated Phased Array to inspect the

austenitic material welds sheets with a thickness of 8mm to build LNG (Liquefied Natural Gas) storage

tanks was been developed.

The CIVA simulation software was used to model the appropriate Phased Array probes, which

allowed the study of the acoustic beam pressure distribution, as well as the beam response to

discontinuities. The modeling allowed the optimization of the parameters to be used in the inspection.

Afterwards the experimental validation process models developed were validated by inspecting a test

piece with discontinuities characteristics of this type of joint. The detection and characterization, in size

and location, of these discontinuities allowed the validation of the inspection system developed.

Key-words: Nondestructive testing, Phased Array, austenitic steel, low thickness, automated

inspection system

1. Introduction

The nondestructive testing (NDT) are based

on different physical principles according to the

type of test. As such, the efficacy of the

application of a particular test involves the

understanding of their physical principles and

how they interact with the properties of the

materials to be inspected. Then it is necessary

to assess which of the NDT techniques are the

most appropriate for what is intended to be

evaluated in the inspection. Through NDT it is

possible to determine if there are discontinuities

in the components and characterize them

accordingly; they can also be used to measure

thicknesses and thus assess the level of

corrosion or to characterize material without

inducing any permanent damage, saving time

and costs. This way, the nondestructive

inspection stands out in numerous industries in

various stages of construction operations or

maintenance of structures and industrial

equipment, as well as in evaluating the quality

of the manufacturing processes of components

and products [1–3].

The construction of LNG (liquefied natural

gas) storage tanks involves weld of 9%nickel

steel plates with nickel-based filler metal,

because in structural applications for cryogenic

service the filler material should have a similar

composition, but not identical, to the base

material, in this case it was Inconel (Ni-Cr alloy)

2

which has a full austenitic microstructure. In

this construction, welding is usually carried out

by SMAW (Shielded Metal Arc Welding), GTAW

(Gas tungsten arc welding), SAW (Submerged

Arc Welding) and FCAW (Flux-Cored Arc

Welding) processes[4].

An austenitic weld exhibits an anisotropic

grain structure (grain elongation parallel to the

lines of heat dissipation and according to a

privileged crystallographic direction) and

heterogeneous (change of grain orientation in

the welded volume). These characteristics

result of how the grain growth occurs during

solidification; being that the physical

phenomena affecting grain growth are the local

direction of the temperature gradient, the

epitaxy and the competition between grains

(selective growth)[5].

Ultrasonic testing (UT) of austenitic

structures is difficult to interpret, because this

type of structure causes a high dispersion and

attenuation of sound in all directions. This

anisotropic acoustic behavior results in the

appearance of noise that can mask the true

indications of discontinuities, leading to

uncertainty in the positioning and sizing of

discontinuities that may be present in the weld.

As a result, on one hand there might appear

unnecessary repairs and, on the other hand,

the acceptance of critical defects might occur.

To circumvent these problems, longitudinal

waves generated by low frequency probes (1.5

to 3.5 MHz) are usually suggested to minimize

sound attenuation in the component, increasing

the signal to noise ratio and to increase the

penetration of waves.

The subject of this project is to inspect the

austenitic welded plates with a thickness of

8mm used in the construction of LNG storage

tanks. Thus, the component in study represents

an inspection challenge in the world of NDT not

only for being an austenitic joint but also for

being a low thickness (up to 12mm) sample. In

the inspection of austenitic materials the

detection mode must be the direct mode; the

indirect (wave mode conversion on the back

wall) cannot be used. This implies that the

probe must be as near as possible to weld so

that with in direct mode the acoustic beam can

inspect the entire volume of weld with the

proper angles, which reveals a problem when

inspecting welds without grinding the surface.

To overcome these and other limitations the

Phased Array inspection technique shows

promising preliminary results, requiring further

studies to optimize its use. Phased Array (PA)

is an advanced ultrasound technique involving

the use of a probe comprising of multiple

elements that can be individually excited and

having the ability to steer and/or focus the

sound beam[6] unlike the conventional UT in

which the probe has a single crystal and

produces divergent beams. To focus and steer

the ultrasonic beam, specific time delays are

applied to each elements to create a

constructive interference of the wavefronts,

allowing the energy to be focused at any depth

in the test specimen undergoing inspection[7].

Contrary to the mechanical translation of a

conventional probe, phased array allows to

perform electronic scanning steps, with a single

probe position. Thus, it is possible to increase

the efficiency of an inspection, reducing

inspection time and costs, also enabling

automated inspection and the data

recording[8,9].

The aim of this work is to develop a phased

array system of inspection that covers the entire

volume of a low thickness weld of austenitic

material. For this purpose, an acoustic model

was developed through modeling in CIVA

simulation software (developed by the French

Atomic Energy Commission (CEA)). In a

second phase, the validation of the model was

done on a mock-up with the same

characteristics as the LNG storage tank, with

3

artificial discontinuities. To validate the model,

the inspection procedure that has been

developed has to detect and measure the

discontinuities in accordance with the results

obtained from modeling.

2. Modeling

CIVA proved to be an essential tool to

develop and optimize the inspection process,

saving time and money. This tool when used to

prepare an inspection by UT allows the user to

establish which probe should be used for a

particular case, since it enables to simulate the

parameters to be applied. This helps in

understanding the results that are expected to

be obtained, based on the characteristics of the

discontinuities predicted for the specific

component.

The mock-up used in this study is an 8mm

thick plate of a steel alloy containing 9% nickel

and the filler metal used is Inconel®, having a

V-joint welded with FCAW. From the criteria

established for the selection of probes and

taking into account the characteristics of the

mock-up the selected probes were linear with

frequencies of 2, 3,25 and 5MHz each having

respectively 32, 20 and 32 elements. The

3,25MHz had cylindrical mechanical focus with

75mm of radius. The remaining characteristics

of the probes are in Table 1, and these were

built by Imasonic SAS.

To enable comparison of results, it has been

guaranteed that the active aperture was equal

(Table 1) for the three probes, as far as

possible.

Probes Nº active elements,

n

Length of elements, e, [mm]

Gap, g,

[mm]

Active aperture, (𝒏 × 𝒆) +𝒈(𝒏 − 𝟏),

[mm]

Passive aperture, h [mm]

2 MHz 11 1,25 0,25 16,25 22

3,25 MHz 13 1 0,2 15,4 16

5 MHz 32 0,4 0,1 15,9 10

Table 1 – Characteristics and parameters of Phased Array probes

The inspection is by direct contact, so it is

required that the contact probes are mounted

on a wedge. Thus the dimensions and

properties of wedges are essential to ensure

the efficiency of the inspection, so it is also

necessary to model them.

The acoustic beam pressure distribution of

the selected probes was studied, as well as the

response of their beam in the detection of

discontinuities of the mock-up.

2.1. Acoustic beam pressure distribution

The acoustic probe pressure depends on

the active aperture and on the frequency, on

the compounds characteristics (mainly of the

sound velocity in material) and beam’s

orientation which is related to the beam

focusing. In an inspection, the ideal condition is

to have the acoustic pressure as higher as

possible in the area which is intended to be

inspected. This means to transmit and receive

the highest energy possible. So, in the graph of

the component’s thickness vs evolution in

amplitude, it is intended to get to the maximum

achieved amplitude (Fig. 1, 2, 3 and 4). In this

way, it is ensured a greater sensitivity in the

detection of a discontinuity.

To study this factor, taking into account the

characteristics of the mock-up in the simulation

performed for the three probes, it was defined

the focusing points along the fusion zone/base

material interface, from 2.6 up to 8mm in depth.

It was necessary to take into account that it is

required generate and propagate longitudinal

waves using refracted angles have above 35°,

and those angles should not exceed 77°

because the sound ceases to have enough

acoustic energy. In these models it has never

been taken into account the attenuation.

Fig. 1, 2, 3 and 4 show the results obtained

respectively for the refracted angle of 49°, 60°,

70° and 78°. On the left side of each figure are

graphs of the acoustic pressure distribution for

the probes of (a)2MHz, (b)3,25MHz and

4

(c)5MHz; and on the right side is the graph of

the thickness of the mock-up in function of the

evolution of the amplitude in dB for the focus

point relating to the graphs on the left, obtaining

the maximum amplitude of each of the probes.

Fig. 1 - Modeling results for the refracted angle of 49°: left side graphs of the acoustic pressure

distribution for (a) 2MHz, (b) 3,25MHz and (c) 5MHz probes; right side graph of the thickness (mm) of the mock-up versus the evolution of the amplitude (dB)






distribution for (a)2MHz, (b)3,25MHz and (c)5MHz probes; right side graph of the thickness (mm) of the mock-up versus the evolution of the amplitude (dB)

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In the graphs of acoustic pressure

distribution, the higher acoustic pressure zones

are indicated by the cyan color. The ideal

situation is that the focus point is in the cyan

area. It is also intended that the beam has a

higher gradient shape and that the focal area

(black box) is as large as possible with the

focus point inside it. The graphs on the right

show the probe that has a larger amplitude

value, verifying that for all angles the probe that

has greater amplitude is the 3,25MHz, then the

2MHz and finally the 5MHz.

Thus, by analyzing acoustic pressure

distribution of the beams and comparing the

maximum amplitude of each probe for each

focusing point it is concluded that 3,25MHz

probe is the one that have the best results for

all scanning angles studied.

2.2. Characterization of discontinuities

From this study of the discontinuities

modeling in the mock-up it is possible to

examine the influence that the type, size,

location and orientation of a discontinuity has in

the beam-discontinuity interaction.

Through qualitative and quantitative analysis

of the results, it is possible to evaluate the

capacity and the sensitivity of detection and

sizing of discontinuities by the simulated

acoustic beam in the previous point. So with

this characterization of discontinuities, first of all

it is intended to realize how the parameters

previously defined to generate the beams can

detect discontinuities; secondly it is intended to

assess how probes detect them, i.e., how the

probes sensitivity affected the readings from the

detection of discontinuities, depending on their

size and location in the mock-up.

Therefore, the three discontinuities that are

located between 2,6 and 8mm deep in the

mock-up were modeled, once it is the area

covered by the PA beams. Letters were

attributed to these discontinuities to facilitate

the analysis of the modeling and validation

results. In the following figures (Fig. 5, Fig. 8,

Fig. 11) the positions of them on the mock-up,

as well as the respective results of the modeling

are presented.

Discontinuity A

Fig. 5 - Position of the discontinuity A (red) in the mock-up in mm

In Fig. 6, there are the S-scans from the

simulation resulting using the three probes in

studying the detection of discontinuity A. An S-

scan is a sectorial scanning representing a

cross-sectional view of the inspected mock-up

and is constructed by software from the A-

scans for each shot, being a set of all the shots,

thereby displaying all refracted angles using the

same focal distance and elements. These S-

scans show the relative position of the

discontinuity and depth. The horizontal axis

corresponds to the test-piece width, and the

vertical axis corresponds to the depth. In these

graphs the higher amplitude zone are indicated

by the cyan color and, as such, represent the

area of the discontinuity that reflects more

energy.

In Fig. 9 and Fig. 12, there are the S-scans

corresponding to discontinuities B and C,

respectively.

Fig. 6 – S-scan using the probe of (a)2MHz, (b)3,25MHz and (c)5MHz, in the detection of

discontinuity A

6

Table 2 presents the values of loss of

amplitude in relation to the maximal amplitude

response (in this case the 3,25MHz probe),

taken from Fig. 7 which is the A-scan (time (μs)

versus amplitude (dB)) of all probes on the

detection of discontinuity A. From these values

in dB, calculated by CIVA, it was determined by

the equation (1), values equivalent to the

screen percentage to facilitate the comparison

between the simulated results and those

obtained in the experimental validation.

𝑑𝐵 = 20 log10 (𝐴0

𝐴1) ⇔ 𝐴1[%] =

100

10(𝑑𝐵 20⁄ ) (1)

In Table 3 and Table 4, there are the same

values corresponding to discontinuities B and

C, respectively.

Fig. 7 – Simulation of A-scans for discontinuity A

Probes Loss of

amplitude [dB]

Normalized amplitude

[% screen]

2MHz 7 45

3,25MHz 0 (reference) 100

5MHz 10,9 29

Table 2 - Values of loss of amplitude in dB and in %screen taken from the A-scan of the detection

of the discontinuity A using the three probes

Discontinuity B

Fig. 8 - Position of the discontinuity B (red) in the mock-up in mm

Fig. 9 - S-scan using the probe of (a)2MHz, (b)3,25MHz and (c)5MHz, in the detection of

discontinuity B

Fig. 10 - Simulation of A-scans for discontinuity B

Probes Loss of

amplitude [dB]


[% screen]

2MHz 7 45


5MHz 10,5 30

Table 3 - Values of loss of amplitude in dB and in %screen taken from the A-scan of the detection

of the discontinuity B using the three probes

Discontinuity C

Fig. 11 - Position of the discontinuity C (red) in the mock-up in mm

Fig. 12 – S-scan using the probe of (a)2MHz, (b)3,25MHz and (c)5MHz, in the detection of

discontinuity C

Fig. 13 - Simulation of A-scans for discontinuity C

Probes Loss of

amplitude [dB]


[% screen]

2MHz 2 79


5MHz 17,8 13

Table 4 – Values of loss of amplitude in dB and in %screen taken from the A-scan of the detection

of the discontinuity C using the three probes

Observing the S-scans for all the

discontinuities, it is concluded that all probes

detect all the discontinuities, but the 2MHz

7

probe does not locate or size them correctly

contrary of the 3,25 and 5MHz probes. The

other goal is to find the beam that detects the

discontinuities with greater range. For this it

was analyzed the A-scans, and in this case, by

analyzing the values of the tables it is

concluded that the probe with a beam with

greater amplitude is the 3,25MHz, then the

2MHz, ending with the 5MHz.

Thus, it is expected that 3,25MHz probe

mechanically focused has a better performance

in detection of discontinuities, which confirms

the results of the analysis of the distribution of

acoustic beam pressure.

2.3. Optimization of 3,25MHz probe

To optimize the 3,25MHz probe it was

studied the beam behavior by analyzing the

acoustic pressure of the modelling of the

3,25MHz probes (using 13 active elements)

with and without mechanical focus. The images

of the respective acoustic beams modeled in

CIVA for each scanning angle are in Fig. 14.

Fig. 14 – Results of the acoustic pressure distribution, simulated in the CIVA for a 3,25MHz

probe with 13 elements, with and without mechanical focus for the different angles

In Table 5, are the values (in dB) of loss of

amplitude for each angle of the probes with and

without mechanical focus, taken from the A-

scans corresponding to the graphs of Fig. 14.

Loss of amplitude [dB] for the scanning angles in study

Probes 49° 60° 70° 78°

3,25MHz mechanical focus

0 0 0 0

3,25MHz no mechanical focus

7,6 6,9 6,3 6

Table 5 – Modeling results between the 3,25MHz probes with and without mechanical focus, using

13 active elements

To increase to more than thirteen the

number of active elements, it is necessary

maintain the commitment of beam entry point

on the workpiece versus distribution of the

acoustic beam, which is a critical factor, and

take into account the minimization of noise. Like

this, the best compromise established was to

use 16 active elements.

3. Experimental Validation

3.1. Experimental setup

The inspection procedure was performed

with the Phased Array Imasonic probes

modeling in CIVA, with the characteristics of

Table 1; it was modeled three wedges with

different dimensions for each one of the probes,

which were built and coupled to them. It was

used too two creeping (CR) probes RTD 2MHz

TR for austenitic to cover the surface, until

2,5mm of depth. It was used the MultiX Phased

Array device as acquisition unit, where it was

introduced the inspection setups that were

established in the modeling study. Scanning

was done with an automatic scanner with a

position encoder. The model approach has

been validated experimentally on the mock-up.

3.2. Experimental results

The general results taken from the

acquisitions (A-scans, B-scans e C-scans) of

the phased array probes and the creeping

probes are presented respectively in Table 6

and Table 7. All discontinuities of the block

were detected.

Using the criteria for an echo loss of -6 dB

on all A-scans of all probes, it was possible to

8

determine the start position of the

discontinuities, and their lengths; depths were

taken from B-scans (it is not possible in the CR

probes). It was withdraw from the X-ray done to

the mock-up the length and the start position of

all discontinuities that will be used as reference

values to compare with the values of

acquisitions; reference depths values are the

planned in the construction of the mock-up (in

the tables: theoretical).

Dimensions of discontinuities

[mm] Theoretical X-rays

Phased Array Probes

2MHz 3,25MHz (13 elem)

3,25MHz (16 elem)

5MHz

A

Start Position

250 255 253,22 252,8 254,63 255,78

Length 15 15 19,79 15,25 15,21 15,06

Depth 7,5 - 6,99 7,33 7,31 8,1

B

Start Position

160 162 163,77 165,8 164,77 167,31

Length 15 15 18,19 13,79 15,08 15,04

Depth 5 - 3,03 3,38 4,15 5,17

C

Start Position

135 137 139,42 138,92 138,45 140,27

Length 10 8 15,06 9,13 10,57 10,62

Depth 5,75 - 4,51 5,25 5,34 5,5

Table 6 - Results taken from the acquisitions of the phased array probes

Dimensions of discontinuities

[mm] Theoretical X-rays

Creeping probe

D

Start Position

70 70 71,22

Length 15 14 15,35

Depth 2,5 - -

E

Start Position

40 39 41,95

Length 15 15 14,68

Depth 0,5 - -

F

Start Position

345 350 346,14

Length 10 10 10,85

Depth 0,5 - -

Table 7 - Results taken from the acquisitions of the creeping probes

In the tables below - 8, 9 and 10- are the

angle value with the highest signal intensity

taken from the B-scans which corresponds the

maximum amplitude in the A-scans of all

probes that detected the respective

discontinuities.

Bellow, images from acquisitions using the

3,25MHz probe with 16 active elements are

presented, for being the most important results,

since they are those that correspond to the best

solution.

Discontinuity A

Probes Angle [°] Maximum

amplitude [%]

2MHz 63,3 32,82

3,25MHz (13 elem) 53,1 66,14

3,25MHz (16 elem) 57 57,43

5MHz 55 41,58

Table 8 - maximum amplitude of detection and the corresponding angle

Fig. 15 – (A) B-scan, (b) A-scan and (c) C-scan of discontinuity A using 3,25MHz probe with 16

elements

9

Discontinuity B

Probes Angle [°] Maximum

amplitude [%]

2MHz 80,4 36,17

3,25MHz (13 elem) 75,3 48,24

3,25MHz (16 elem) 73,5 57,05

5MHz 71,3 30,23

Table 9 – maximum amplitude of detection and the corresponding angle

Fig. 16 – (A) B-scan, (b) A-scan and (c) C-scan of discontinuity B using 3,25MHz probe with 16

elements

Discontinuity C

Probes Angle (°) Maximum

amplitude [%]

2MHz 77,1 26,71

3,25MHz (13 elem) 70,8 35,91

3,25MHz (16 elem) 71 40,08

5MHz 72 24,48

Table 10 - maximum amplitude of detection and the corresponding angle

Fig. 17 - (A) B-scan, (b) A-scan and (c) C-scan of discontinuity C using 3,25MHz probe with 16

elements

The experimental results are according to

the modeling results, since all discontinuities

were detected and with similar amplitudes to

the modeling, and as such, this procedure is

considered validated. As expected, the

3,25MHz probe with 16 active elements is the

one that detects and sizes the discontinuities

with higher sensitivity and higher maximum

amplitudes.

The main objective of this work was

reached: develop a system of inspection that

covers the entire volume of a weld of low

thickness austenitic steel.

4. Conclusions

With the present work was possible to

understand the importance of an advanced

technique of UT as the Phased Array, it has to

overcome the problem of inspecting welded

joints of low thickness austenitic steel. By using

modeling it was possible to achieve

optimization of a probe, as well as the

inspection procedure for this type of joints. It

was also demonstrated the importance of the

modeling for the success of an inspection

procedure.

Using CIVA there were selected three

Phased Array probes to inspect the block under

study. The selected probes were linear with

frequencies of 2, 3,25 and 5MHz each having

respectively 32, 20 and 32 elements, and the

3,25MHz had mechanical focus. It was followed

by the simulation of the acoustic beam and

characterization of discontinuities, for each one

of the probes. From the analysis of these

results it has been concluded that the 3,25MHz

probe showed the best result. However, it was

found that it could still be optimized using

modeling and experimental tests.

From the modeling results it was possible to

establish the inspection configurations that

have been validated experimentally. With the

experimental validation it has been concluded

that the best solution for inspection of low

thickness welds of austenitic material is to use

linear PA probe of 3,25MHz focused

mechanically using as parameterization 16

10

active elements, generating longitudinal waves

with angles up to 78°. The use of creeping

probes allowed detect the discontinuities up

until 2,5mm deep, ensuring coverage of the

entire weld volume to inspect, reaching the

main objective of this work.

With the validation of the modeling results, it

is concluded that CIVA is reliable, by

withdrawing the following conclusions about the

results: to inspect low thicknesses of austenitic

welds the use of probes with intermediate

frequency is a good solution, because one

gains detection sensitivity in relation to lower

frequencies which are normally used for

austenitic materials and on the other hand

exhibit better results and generate less

attenuation compared to higher frequencies

which was presumed to be more suitable for

low thicknesses; mechanical focus also proved

to be an added value, to form a smaller focus

point which has a higher sound pressure,

thereby increasing the detection sensitivity. The

final conclusion is that passive aperture of the

elements also influences the acoustic beam,

and for the case in study, it was concluded that

it is necessary to minimize the passive aperture

and simultaneously ensure that occurs

constructive interaction of the individual beams

in the zones where it is intended to focus, being

that is intended that the beam stay as far

forward as possible, i.e. close to the weld.

Lastly, advanced techniques of automated

PA demonstrated advantages in inspection of

welds of LNG tanks compared to other

techniques namely radiography with X-rays.

In summary, this work presents an important

development in the area of NDT with UT, since

it enables applying advanced techniques of PA

at welds with low thicknesses and materials

with anisotropic acoustic properties, with the

advantage to do an inspection with automated

systems and data record.

References

[1] P. Moore and G. Booth, “Detecting weld

defects,” in The Welding Engineer’s Guide

to Fracture and Fatigue, Cambridge:

Elsevier Ltd, 2015, pp. 143–158.

[2] P. Shull, Ed., Nondestructive Evaluation:

Theory, Techniques, and Applications.

New York: Marcel Dekker, Inc., 2002.

[3] F. Pinto, J. Barata, and P. Barros, Ensaios

Não destrutivos. ISQ, 1992.

[4] Q. Ahsan, A. Haseeb, and N. Hussein, “9%

Nickel Steels and Their Welding Behavior,”

in Comprehensive Materials Processing,

Vol 6, Elsevier Ltd, 2014, pp. 135–149.

[5] C. Gueudre, L. Le Marrec, J. Moysan, and

B. Chassignole, “Direct model optimisation

for data inversion. Application to ultrasonic

characterisation of heterogeneous welds,”

NDT E Int., vol. 42, pp. 47–55, 2009.

[6] M. Nel, “Ultrasonic Phased Arrays : An

Insight into an Emerging Technology,” in

Proc. National Seminar on Non-Destructive

Evaluation, 2006, pp. 1–4.

[7] G. Neau and D. Hopkins, “The promise of

ultrasonic phased arrays and the role of

modeling in specifying systems Phased-

array principles,” www.bercli.net, 2006.

[8] B. W. Drinkwater and P. D. Wilcox,

“Ultrasonic arrays for non-destructive

evaluation: A review,” NDT E Int., vol. 39,

no. 7, pp. 525–541, 2006.

[9] Olympus NDT, Introduction to Phased

Array Ultrasonic Technology Applications:

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