NDTnet 1998 March,
ULTRASONIC INSPECTION of COPPER CANISTERS
USING PHASE ARRAYS
T. Stepinski, P. Wu, E. Martinez *
In collaboration with SKB (Swedish Nuclear Fuel and Waste
(This is the abstract of a paper that will be presented at the 7th ECNDT in Copenhagen.
The article presented here introduces the same work)
Ultrasonic arrays offer a number of advantages in NDE applications, the most important is electronic beamforming and scanning enabling generation of the desired field distributions and defect insonifica-tion from different directions resulting in improved defect detection, and characterization.
The applicability of linear array technique for inspection of copper lined canis-ters for nuclear waste fuel has been investigated recently at Uppsala University.
The objective of this project is development of ultrasonic array technique for assessing the integrity of the circumferential electron beam (EB) weld between the lid and walls of copper lined canisters developed by SKB (Swedish Nuclear Fuels and Waste Management Co.) for encapsulation of nuclear waste.
Due to the radiation emitted by the nuclear waste encapsulated in the canisters the inspection must be carried out completely automatically. In order to achieve a very high level of confidence in the NDT results of copper canisters, the inspection system should be capable of inspecting 100% of cimcurferential weld zone in thick section copper. An ultrasonic array system consisting of a linear ultrasonic array, made of piezoelectric composite material, and a computer controlled multi-channel electronics is proposed as a solution to this problem.
Modern array technique which has become recently commercially available for NDT applications has a great potential in this application. The array sys-tem chosen for this application enables electronic focusing and rapid electronic scanning eliminating the use of complicated mechanical scanner. Secondly, it is capable of reducing the influence of grain noise by using a combination of focused sound field and signal processing. Thirdly, it is characterized by inherent flexibility enabling 100% inspection of the weld zone even if the weld is not parallel to the surface.
Our present research activity in this project addresses two issues: the development of software tools for efficient beam-forming, and a laboratory verification of the performance of ultrasonic array system on real EB welded canister samples. A special attention is paid to ultrasonic grain noise scattered by the internal copper structure. A method for grain noise suppression by electronic filtering B-scan images is verified in this application. Here we present some recent results obtained during the inspection of EB welded copper test blocks containing both artificial and natural defects.
The objective of this project is to develop a fully automated
ultrasonic technique for assessing the integrity of the
circumferential weld between the lid and wall of copper lined
canisters for nuclear waste fuel. The aim of this research was to
evaluate performance of linear ultrasonic arrays used for the
inspection of the electron beam (EB) weld in copper canisters,
and in particular:
- to evaluate of the ultrasonic array system Allin using
samples of canisters with EB weld
- to develop software for modeling elastic fields in
- to investigate ultrasonic noise (backscattering)
resulting from copper structure
Scope of work
Our research consists of a theoretical and an experimental
part. The theoretical part concerns development of modeling tools
enabling efficient beam-forming of the ultrasonic arrays. The
experimental part includes inspection of various canister samples
using the ultrasonic array system ALLIN from NDT Systems and the
linear ultrasonic array from Imasonic. The research activity in
the project include the following tasks:
- Extensive laboratory tests . The
experiments with the ultrasonic array in immerse mode
performed using mechanical scanner on copper blocks with
natural defects provided by SKB. A number of B- and
C-scan images has been acquired for evaluation of overall
performance of the array technique.
- Beam-forming for the linear ultrasonic array.
Software package for modeling sound field propagated by
an ultrasonic array into a two layer medium
(liquid-solid) has been developed and tested. The cases
of oblique and concave array surface have been included
into the simulation program.
- Grain noise problem. The ways of grain
noise estimation using ultrasonic array have been
investigated using available copper samples. Electronic
noise reduction methods have been adopted for the
ultrasonic array and investigated experimentally.
Experimental set up
A number of copper blocks (canister parts) was inspected in
immersion using the phase array technique. The experimental set
up which has been used for the experiment consisted of Allin
array system, linear phase array made of pizocomposite material
and the precision mechanical scanner with water tank, see Figure
All measurements have been performed using 128
channel ultrasonic array system Allin from NDT System which was
connected to 3 MHz linear array from Imasonic. The array was made
of piezoelectric composite material and had 64 elements with
dimensions 1mm by 33.5 mm. The array was fast focused in one
plane (cylindrical focus with focal distance 190 mm in water) and
waselectronically focused in the other plane. The Allin enabled
focusing both in the emission and reception modes and electronic
scanning using an aperture up to 32 elements. Besides scanning
and beam-forming the Allin performed also data acquisition and
steering of the mechanical scanner used for guiding the array.
The Allin enables storing all the measured data in real-time,
which means that all RF signals included in the acquired B-scans
can be stored. Thus when inspecting a 2D surface a sequence of
B-scans is stored forming data cube. The stored data
can then be processed and presented off-line and the desired
C-scans can be calculated. This feature has been used extensively
during the experiments.
The experiments were performed with the array radiating from the
top of the block into the copper and the EB weld located approx.
50 mm below the top surface (see Figure 2 for details).
Detecting artificial and natural defects
Sensitivity to the artificial defects has been
investigated using the blocks provided with various side drilled
and flat bottom holes in the weld zone as well as natural
Based on the inspection of artificial holes, we
can say that the ultrasonic array technique is capable of
detecting artificial defects (drilled holes) in the weld zone of
the copper canister in the diameter range of 1 mm to 3 mm
depending on the type of defect. Based on this we can estimate
that it would be capable of detecting single pores with diameter
>2 mm. Extensive evaluation of the sensitivity and resolution
is performed using other blocks made of various grades of copper.
Grain noise suppression
Metals with distinct coarse grains are difficult to inspect
ultrasonically due to the attenuation and backscattering from the
grain boundaries. This effect becomes very distinct for the
ultrasonic waves with lengths in the same range as the grain
size. In pulse echo inspection small reflections scattered from
the grain boundaries appear in the received signal as a kind of
noise often referred to as grain noise. Grain noise is different
from thermal noise since it does not vary with time. This means
that the for each transducer position one particular noise
realization is obtained, which is observed as a constant signal
on the screen. Due to this fact the grain noise cannot be reduced
by averaging in time but it can be reduced by averaging over
different transducer positions, the procedure called spatial
diversity. Another way to reduce the grain noise is to apply
frequency diversity algorithms, referred to as split spectrum
processing (SSP). The SSP is based on the analysis of narrow band
signals obtained by splitting the transducer frequency band in a
number of small frequency bands by means of FFT (fast Fourier
transform). Several SSP algorithms have been proposed with
different schemes of analysis of split signals . Their common
disadvantage is sensitivity to parameters set by the operator and
degraded temporal resolution of the output signal. A number of
more robust schemes has been proposed such as cut spectrum or
consecutive polarity spectrum , and more recently new
algorithms based on a concept of noncoherent detection were
developed . The latter algorithm was tested on ultrasonic data
obtained from copper block with coarse grain structure.
Signal Processing Procedure
The noncoherent detector (NCD) is a nonlinear filter used for
detecting transients embedded in a background noise in radar and
communication applications. It has been adopted to ultrasonic
noise suppression by M. Gustafsson, the details have been
presented in , .
The idea is to construct a filter processing the original
ultrasonic signal to yield a signal in which the echoes are more
likely to be detected. The filter suppresses the ultrasonic
clutter (grain noise) and maximizes the probability of detecting
certain family of transients. The noise suppression is based on
the estimated noise covariance matrix and the detector is
designed given a prototype transient. The NCD is more advanced
than an ordinary matched filter since it aims at detecting a
member of a predefined transient family. In our case the family
has been defined as a set of transients with given envelope and
center frequency but unknown phase.
The NCD algorithm has been implemented using Matlab(r)
(from MathWorks) so that it can be used for processing B-scan
images in data cube acquired by our array system. When all
B-scans in the data cube have been processed C-scans can be
extracted from the processed data, example is shown in Fig. 4.
Beam-forming of ultrasonic arrays in
Ultrasonic beam-forming is usually performed by means of
ultrasonic arrays [6,7]. Arrays can be differently shaped and of
different dimensions. Most commonly used are the one-dimensional
arrays, like linear arrays consisting of equally spaced elemental
radiators (or elements for abbreviation) laid out in a straight
line, concave and convex arrays made up of elements aligned
around a curve, usually a cylinder. A linear array can be used as
a phased array provided that the hardware is furnished with a
complicated time delay device to steer beams. Two-dimensional
arrays have also been put into use, such as planar arrays
composed of elements oriented on a geometric grid in a plane. The
arrays of our interest are linear, phased and concave arrays.
To make best use of the ultrasonic array system ALLIN for NDT
of materials, it is necessary to get knowledge of how waves from
an array propagate in the materials inspected. With this purpose,
we started the theoretical study of wave propagation in solids,
and up to now, we have developed a software package for modeling
the wave propagation. The model is implemented on personal
computers, and can simulate acoustic fields in fluids, and
longitudinal wave (LW) and shear wave (SW) in immersed solids
(with planar interfaces) from linear, phased, and concave arrays.
The fields simulated can be time-harmonic or transient, can be in
far or near regions [1,6,12-14].
The theories for ultrasonic array beam-forming used in NDT
[1,6,7] are, in the case of homogeneous fluid medium and in
far-field range, well established, and similar to those well
established in phased array antennas  and microwave phased
arrays . They treat the array elements as point-like
radiators. However in NDT applications, the problems of interest
are often in the near-field range, and the physical size of the
array elements must be taken into account on calculating sound
fields because it is not small compared to the wave length. As
presented in previous sections, the linear array is used to the
immersion test of metals. Elastic fields radiated by the array
into copper block, therefore, are of our interest. To this end,
we make advantage of the angular spectrum approach (ASA) and
developed a software package for modeling elastic fields in
immersed solids from linear, phased and concave arrays excited by
time-harmonic and transient waves. The model has shown to be
effective and efficient for treating elastic fields in immersed
solids both in near- and far-field ranges. Here we present the
basic theory of array and the model for elastic fields in the
case of phased arrays.
Basic Theory of Beam-Forming
by an Phased Array [6-9]
Geometry and notation for investigation of elastic fields in immersed solids from phased array.
Geometry and notation for investigation of elastic fields from curved source in immersed solids.
An ultrasonic phased array used in NDT usually consists of a
number of linearly aligned rectangular elements such as the one
shown in Figure 5, where the has N' elements and the
elements are rectangular with the dimension of 2ax2b, spaced with
d and located in a fluid at plane z = 0. In
homogeneous fluids and in far-filed, the conventional theory of
array applies [6,7]. The far-field beam-pattern of an array is an
important factor used to evaluate the performance of the array
used in certain specified situation [8,9]. A rectangular
piston-like radiator has a beam-pattern of two-dimensional sinc
function . By means of the array theory, the beam pattern can
be obtained which is the product of the beam-pattern of a
linearly-aligned point element array and that of a rectangular
piston source [6,10]. Since the elements can be excited
separately by either time-harmonic or pulse waves, the phases (or
delay times in the pulse case) and amplitudes of exciting waves
to the elements can be given separately. Specifying the phases in
different ways, the array can generate different kinds of beams,
such as steered and focused beams. For details of the basic
theory, refer to references [6-9].
A Model for Elastic Fields in
Immersed Solids from Phased Arrays [6,12-14]
The above introduced basic theory can only be used to treat
beam-patterns in the far-field range of a homogeneous fluid.
While our interest is mostly in the immersion test of block in
near-field. As is well known, when a beam radiated by the array
impinges on the fluid/solid interface, it will experience mode
conversion and refraction. The mode conversion results in
different modes' waves simultaneously propagating in the solid,
for example, longitudinal wave (LW) and shear wave (SW), which
are the two most commonly used modes. The refraction brings about
the deflection of the incident beam in the solid. This means that
the evaluation of such an elastic field is not an easy job. To
deal with near-fields from arrays both in homogeneous and in
layered media, we have developed a model based on the ASA.
A general procedure of applying the ASA to the calculation of
elastic fields in immersed solids involves two main steps. The
first step is the analysis of a source or a field from the
source at a certain plane in a fluid, that is, the decomposition
of the source or the field into a set of plane waves; and the
second step is the synthesis of fields in the immersed
solids. A field at point (x, y, z) in an
immersed solid is synthesized in such a way that the decomposed
plane waves propagating from the fluid into the immersed solid at
point (x, y, z) are superposed with
consideration of mode conversion at the fluid/solid interface.
Since the ASA represents a field from a source as a set of plane
waves, the solutions of fields from finite sources with different
shapes reduce to the solutions to the plane wave problems. This
is very helpful to solve wave problems involving layered media.
Solution of elastic fields in immersion measurement is one of the
For planar sources (e.g. phased arrays and tilt linear
arrays), the sources can be directly decomposed by means of the
2-D spatial Fourier transform (see Figure 5). For curved sources
(e.g. concave arrays), however, the Fourier transform can not be
directly applied (see Figure 6). Thus a field, here called
initial field, from a concave array is first calculated in a
certain plane (which is normally close to the source and parallel
to the fluid/solid interface), and then decomposed into an
angular spectrum of plane waves by means of the Fourier
transform. The latter method can be applied to the sources with
arbitrary shape. The initial field can be thought of as a
secondary source, a type of unbaffled planar source, equivalent
to the primary curved source. In this sense the latter case is
unified to the former case.
Fig 7: Magnitudes of the LW angular spectra of the nonfocused phased array in copper in the cases of steering angles (a) 0°, and (b) 10°, respectively.
Fig 8: Magnitude of LW velocity field of the nonfocused phased array for steering angles (a) 0°, and (b) 10°, respectively.
Fig 9: Magnitude of the LW velocity field of the nonfocused phased array with focusing the LW for steering angles (a) 0°, and (b) 10°, respectively. The focal point is supposed to lie in copper on the beam axis at a distance of 80 mm from the array.
Examples of Elastic fields in
Here are some examples given to the LW fields radiated by a
phased array with N' = 32, d = 0.5 mm, 2a =
0.4 mm, and 2b = 12 mm. The array is positioned in the x-y
plane at z = 0 and has its center in the origin of the
coordinates above a flat surface copper block immersed in water
(see Figure 5). The densities and the sound speeds used for
calculations are 1000 kg/m3 and 1500 m/s,
respectively in water, and 8960 kg/m3, 4660 m/s (LW) and
2260 m/s (SW), respectively in copper. The array is excited by a
time-harmonic wave with frequency 3 MHz and unit amplitude. The
calculation of the angular spectrum is implemented according to
the method proposed in [12-14]. In contrast to the fluid case,
the angular spectra of the array in immersed copper are derived
from the corresponding angular spectrum in the fluid by using the
transmission coefficients at the fluid/solid interface . The
simulation of elastic fields is performed from based on the
method in .
The examples of the LW beam-patterns, the LW nonfocused and
focused elastic fields in immersed copper are shown in figures
7-9, respectively. Figure 7 shows the angular spectra of the
nonfocused array at steering angles of 0 and 10 degrees, which
are presented as color scaled 2-D images. Figure 8 shows the
nonfocused LW field radiated by the array at steering angles of 0
and 10 degrees. Figure 9 shows the focused LW field radiated by
the array at steering angles of 0 and 10 degrees. The fields are
both in the y=0 plane.
Comparatively observing figures 7 and 8, we can find the
corresponding lobes of the field in figure 8 to those of the
beam-pattern in figure 7. This demonstrates that a beam-pattern
in an immersed solid can be easily determined from the angular
spectra of the array in the solid. By comparing figures 8 and 9,
it can be seen that the main lobe of the focused field in figure
9 is narrower in width and higher in maximal amplitude than that
of the nonfocused field in figure 8. This illustrates that the
focusing increases the lateral resolution and echo strength
around the focal zone.
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- A.B. Day, An Investigation into Optimisation of NDT of
Spent Nuclear Canisters - Phase III, report TWI 220343/1/95,
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Supression Using Noncoherent Detector Statistics and Split
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Nuclear Copper Canisters, SKB Projektrapport 97-08, August
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was born in Szczecin Poland, in 1950. He obtained his M.Sc. degree in electrical engineering from the Technical University of Szczecin, and a Doctor of Technology degree from the Technical University of Warsaw, Poland in 1973, and 1983, respectively.
From 1973 to 1984 he was with the Institute of Industrial Automation in the Department of Electrical Engineering of the Technical University of Szczecin, where he served as a lecturer teaching courses on the subjects of systems identification and modelling, automatic control, industrial control systems and electronic circuits.
In 1984 he joined Swedish company AB Sandvik Bergstarand where he was actively involved in developing eddy current systems for non-destructive testing of hot steel and in research on the application of modern signal processing methods to non-destructive evaluation. Since 1988 he has been with Uppsala University where he is Associate Professor at the Department of Material Science. He heads a research group working with the application of modern signal processing to NDE. His research interests are in the area of signal processing, pattern recognition and neural networks applied to ultrasound and eddy current data.
Dr. Stepinski is a member of IEEE, Acoustical Society of America, ASNT, he is a chairman of the Committee for New Techniques and Research in FOP (Swedish Society for NDT).
Signals and Systems Group
P O Box 528
SE-751 20 Uppsala, Sweden
+46 - 18 - 4711076
+46 - 18 - 555096
received the B.S. degree in electrical engineering in 1982 from Nanjing Aeronautic and Astronautic University, Nanjing, China, and the M.S. and Ph.D. degrees from Jiaotong University, Xi'an, China, in 1987 and 1991, respectively, in the fields of biomedical ultrasonics and ultrasonic engineering. From 1992 to 1994, he worked as post-doc in the Fraunhofer Institute for Biomedical Engineering, St. Ingbert, Germany, in the area of ultrasonic medical image processing and ultrasound tissue characterisation. Since late 1994 when he joined the Department of Technology at Uppsala University, Uppsala, Sweden, he has been working on elastic wave propagation and scattering and on ultrasonic imaging with applications to NDT.
E- Mail: Ping.Wu@signal.uu.se
Eider Martinez, M.Sc., lab research assistant.
Copyright © Rolf Diederichs,
email@example.com 1. Mar 1998
/DB:Article /AU:Stepinski_T /AU:Wu_P_ /AU:Martinez_E /CN:SE /CT:UT /CT:transducer /CT:array /ED:1998-03