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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)
Fig. 2 - Modeling results for the refracted angle of 60°: 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)
Fig. 3 - Modeling results for the refracted angle of 70°: 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)
Fig. 4 - Modeling results for the refracted angle of 78°: 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)
5
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]
Normalized amplitude
[% screen]
2MHz 7 45
3,25MHz 0 (reference) 100
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]
Normalized amplitude
[% screen]
2MHz 2 79
3,25MHz 0 (reference) 100
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
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