The Pennsylvania State University
The Graduate School
Engineering Science and Mechanics Department
FURTHER DEVELOPMENT OF A LOW CHANNEL PHASED ARRAY
FOR MATERIAL DEFECT IMAGING
A Thesis in
Engineering Science
by
Mark Adam Bohenick
© 2008 Mark Adam Bohenick
Submitted in Partial Fulfillment of the Requirements
for the Degree of
Master of Science
May 2008
ii
The thesis of Mark Adam Bohenick was reviewed and approved* by the following: Bernhard R. Tittmann Schell Professor of Engineering Science and Mechanics Thesis Advisor Clifford J. Lissenden Associate Professor of Engineering Science and Mechanics Albert E. Segall Associate Professor of Engineering Science and Mechanics Judith A. Todd P. B. Brenneman Department Head
Department of Engineering Science and Mechanics *Signatures on file in the Graduate School
iii
Abstract
As lighter, stronger, and more affordable materials are available for industrial
applications in hazardous environments, the effect of the environment on the material
during operations must be studied. A material must perform adequately and degrade
gracefully. Sudden failure is unacceptable. Current methods to qualify materials are
expensive and do not provide real-time results. A method to monitor material
characteristics and degradation while the material is in the hazardous environment is
desirable.
Development of a Non-Destructive Evaluation device that can function inside a
high temperature, high pressure environment would provide a tool for monitoring a
material specimen in real-time. A limitation is that the number of inputs to the
environment should be minimized. The device should provide real-time data on the
specimen’s degradation. Surface and interior defects such as film growth, blistering,
cracking, and corrosion should be able to be characterized. In addition, geometric
changes such as shape, size, and warping should be monitored.
An ultrasonic linear phased array provides the capability to examine a specimen
without mechanical scanning. Linear phased arrays are essentially multiple ultrasonic
transducers that are arranged along a line. Electronic time-delays provide the ability to
scan a beam of acoustic energy across the specimen. Reflected pressure waves are
received by the array and are used to produce real-time images of the specimen. In order
to limit the number of inputs to the environment, a stepped linear phased array design
was used. A physical offset is designed into the device between multiple linear phased
arrays. Wire inputs to the environment connect an element on each step of the array.
iv
The physical offset allows for the signals of each step to be separated in time. This
allows additional sets of elements so that additional portions of a material specimen can
be investigated with the same number of wires.
Three prototype devices were manufactured. Each has four channels; four
piezoelectric elements on each step forming a linear phased array. Prototype 1 has four
steps and its elements have a central frequency of 2.25 MHz. Prototype 2 has four steps
and a central frequency of 10 MHz. Prototype 3 has six steps and a central frequency of
10 MHz.
The ability of the prototypes to monitor degradation of a material specimen is
examined through bench-top experiments modeling defects and geometric changes to a
material specimen. Beam steering and focusing were simulated using a ultrasonic phased
array simulation program, Field II, to determine optimum array dimensions.
The current prototypes cannot sufficiently monitor a specimen for defects. Large
surface defects can be detected but smaller individual surface defects cannot be
sufficiently resolved. The prototypes were unable to detect internal defects. Warping or
bending of the specimen in the vertical plane has been able to be examined by the
prototypes.
The stepped linear phased array design parameters can be optimized to develop a
device that can provide all the functionality desired. The higher frequency of Prototypes
2 and 3 did not show a marked increase in resolution or effectiveness. Sparse arrays and
data analysis may be required to further optimize the stepped linear phased array design.
v
Table of Contents List of Figures vii Acknowledgements ix Chapter 1 Introduction 1 Chapter 2 Literature Review and Theory 4 2.1 Piezoelectric Materials 4 2.2 Linear Phased Arrays 4 2.3 Array Element Spacing 6 2.4 Time of Flight 8 2.5 Near-Field and Far-Field 8 2.6 Beam Divergence 10 2.7 Frequency Considerations 11 2.8 Resolution 12 2.8.1 Lateral Resolution 12 2.8.2 Axial Resolution 13 2.9 Linear Phased Array Pressure Field Equations 14 2.10 Array Optimization 17 2.11 Additional Literature 19 Chapter 3 Methodology 20 3.1 Devices 20 3.1.1 Stepped Linear Phased Array 20 3.1.2 Omniscan MX System 23 3.2 Experiments 23 3.2.1 Experimental Setup 24 3.2.2 Thickness Measurement 25 3.2.3 Surface Absorbing Defect 26 3.2.4 Surface Scattering Defect 26 3.2.5 Single Surface Crater 26 3.2.6 Interior Defects 26 3.2.7 Bending of a Specimen 27 3.2.8 Effect of Temperature 28 3.3 Simulations 29 Chapter 4 Results 31 4.1 Experimental Results 31 4.1.1 Signals 31 4.1.2 Surface Absorbing Defect 34 4.1.3 Surface Scattering Defect 36
vi
4.1.4 Single Surface Crater 36 4.1.5 Interior Defects 39 4.1.6 Bending of a Specimen 39 4.1.7 Effect of Temperature 40 4.2 Simulations 41 4.3 Calculations form Linear Phased Array Theory 45 4.3.1 Inter-element Spacing 45 4.3.2 Application of Other Array Optimization Techniques 46 Chapter 5 Conclusions 47 Bibliography 50 Appendix A Non-Technical Abstract 52
vii
List of Figures Figure 1: Diagram of Array Element Spacing 6 Figure 2: Near-Field and Far-Field: Distance from Transducer vs. Nominal Amplitude.
9
Figure 3: Diagram of the Beam Divergence Angle of a Transducer. 11 Figure 4: Diagram of 1.5 and 2.5 Cycle Pulses. 13 Figure 5: Table of Calculated Axial Resolution 14 Figure 6: Diagram of Element and Point P(r, θ ) in the Pressure Field. 15 Figure 7: Photographs of Prototype 1. 20 Figure 8: Schematic Drawing of Stepped Array Device 21 Figure 9: Photograph of Prototype 2. 22 Figure 10: Photograph of Prototype 3. 22 Figure 11: Photograph of the Omniscan MX System. 23 Figure 12: Photograph of Experimental Setup for Prototype 1. 24 Figure 13: Photograph of Experimental Setup for Prototypes 2 and 3. 25 Figure 14: Bending Experiment Setup. 28 Figure 15: Effect of Temperature Experimental Setup. 29 Figure 16: Comparison of Full Array Waveform and First Step Waveform. 31 Figure 17: Comparison of Single Channel Signal and First Step, Single Channel Signal.
32
Figure 18: Signal from Channel 1 of Prototype 2. 33 Figure 19: Prototype 2 S-Scan of Pristine Sample. 34 Figure 20: Surface Absorbing Defects. 35 Figure 21: Surface Scattering Defects. 36
viii
Figure 22: Single Surface Crater, Prototype 1. 37 Figure 23: Single Surface Crater, Prototype 2. 38 Figure 24: Room Temperature Bending 39 Figure 25: Comparison of Bending at Room Temperature and 180°F. 41 Figure 26: Field II Simulation of 2.25 MHz Array. 42 Figure 27: Field II Simulation of 10 MHz Array. 43 Figure 28: Field II Simulations of Beam Steering. 44 Figure 29: Table of Calculated Critical Inter-element Spacing Values. 45
ix
Acknowledgements
I would like to thank Dr. Bernhard R. Tittmann for several years of guidance and
opportunities. I would also like to thank my fellow laboratory coworkers for their
contributions and camaraderie.
To my fiancé and family, thank you for your love and encouragement during my
collegiate years and in the years that lie ahead.
1
Chapter 1
Introduction
Material properties and defects can be investigated using Ultrasonic Non-
Destructive Evaluation (UNDE). This method uses high frequency pressure waves as a
means to examine a specimen. Ultrasonic waves are considered sound waves whose
frequency is higher than 40 kHz.
To generate these pressure waves, a piezoelectric transducer can be used. When a
voltage is applied to the piezoelectric material within the device, the material undergoes a
strain. This transduction produces a pressure wave. This wave can then be utilized to
interact with a specimen. As the pressure waves travel through the specimen, the
acoustic energy can be reflected, transmitted, or absorbed. The reflected pressure waves
can be detected using a piezoelectric transducer.
Reflected waves can be received by interacting with the same piezoelectric
transducer that was used to generate the waves. This is an A-Scan and the signal is
recorded as a series of data points of voltage amplitude and time. A time delay between
sending and receiving of a pressure wave allow for the device and electronics to
distinguish the received pressure wave from the generation of pressure waves.
Phased arrays are systems of piezoelectric transducers which work in unison to
perform scanning of a specimen. Working much like electromagnetic radar systems, the
piezoelectric elements of the array are pulsed by electronics in time intervals which allow
scanning and focusing to different points in space. The reflected ultrasonic pressure
waves received by each element of the array are then interpreted by the electronics to
create a composite electrical signal. The composite signals for different scan locations
2
can be combined, forming an image of the amplitude of reflected waves. This image is
considered a sectorial scan (S-Scan).
Understanding how materials behave in harsh environments is important so that
materials can be used in new applications. Corrosion, chemical composition changes,
and warping may cause unwanted property and dimension changes in the material. In
order to utilize new material technology, new inspection technologies need to be
developed to characterize the performance of these new materials.
The current methods of study of materials in harsh environments require a great
cost and lacks real-time information on material degradation. To analyze materials using
conventional methods requires removing the materials from the environment so that
laboratory equipment can be used to measure changes of a material that occurred in the
harsh environment. This method is costly and ignores the repercussions of cycling the
material between harsh environment and laboratory conditions in developing a study of
the effects of the environment on the material.
The performance of materials in high pressure and high temperature environments
is difficult to monitor in-situ. Current devices for UNDE are made of materials that
cannot withstand high pressures and, more importantly, high temperatures. The
transduction characteristics of many materials such as piezoelectricity and
magnetostriction are lost as the temperature rises. New smart materials are becoming
available for possible use in high temperature applications.
Another hindrance is the number of wires leading to a device within a harsh
environment. Penetrations into a harsh environment can be costly, and a way to
3
minimize these penetrations while maintaining the effectiveness of a phased array would
be beneficial.
Mechanical systems for scanning surfaces of materials are also unable to
withstand high temperatures and pressures. Phased arrays are able to perform scanning
without having any moving parts. By utilizing both new technologies in phased arrays
and piezoelectric materials, a phased array device may be designed to achieve in-situ
examinations of material changes in a harsh environment in real time.
A stepped linear phased array will be able to scan sections of a material specimen
for the effects of degradation while also having the ability to monitor bending or warping
of the overall specimen. The stepped design allows for a minimal number of wires
leading to the device which will be placed in the harsh environment with the material
specimen. This research work will examine the feasibility of a stepped linear phased
array design and will make recommendations for the advancement of the technology.
4
Chapter 2
Literature Review and Theory
2.1 Piezoelectric Materials
In some materials the influence of electrical energy causes a physical deformation
of the material. These materials convert electrical energy into mechanical energy via the
piezoelectric effect[1]. Piezoelectric materials have many promising applications and
have been used for many years in UNDE. By stimulating piezoelectric materials with
short electric pulses, a short mechanical response can be generated.
This mechanical response can propagate into a material in contact with the surface
of the piezoelectric material. This propagating mechanical wave can be considered an
ultrasonic wave, a pressure wave with frequency higher than ~40 kHz.
This ultrasonic wave can be utilized to interact with materials to characterize their
properties, locate defects, and monitor degradation.
Piezoelectric materials can also work in the reverse, converting mechanical
energy back into electrical energy. In this way, piezoelectric materials may be utilized to
receive the return signals that have been generated.
The piezoelectric effect can be optimized for UNDE purposes by including the
piezoelectric material within a device called a piezoelectric transducer. This device
improves the transmission and reception of mechanical waves and protects the
piezoelectric material from environmental conditions.
2.2 Linear Phased Arrays
Phased arrays are a network of piezoelectric elements which are excited
electronically in a specific time sequence to allow them to simultaneously direct
5
ultrasonic energy to points in space. The angle that an array is beaming ultrasonic energy
in is called the steering angle. This allows for maximum energy to interact at a specific
point, and any reflected waves that return to interact with the array can be recorded.
Time shifts are applied to the received signals similar to the time delays used in
the initial pulsing of the array elements. The time shifts correct the received signals so
that the array element signals received from specific points in the scan region are aligned
in space. A graphic can then be generated showing the amplitude of the received signal
from each point in space. This scanning and graphical representation can be conducted in
real-time, with no mechanical parts and without translation of the device.
In the 1980s, support of fracture mechanics study and the need to reduce
inspection time in hazardous environments were major driving forces to the development
of phased arrays[2]. Inspection techniques utilizing phased arrays were developed to take
advantage of the reduced inspection time associated with replacing mechanical scanning
with electronic scanning. In addition to academic fracture mechanics studies, scanning of
welds to ensure their integrity was vital to industry[2].
Phased arrays require complicated electronic systems to operate. Computing
power and data storage are important to fully utilize the capability of a phased array
system[3]. In the past, the computing power, data storage, and graphic display
capabilities were not available to fully implement high resolution phased array systems
for industrial applications. Higher resolution displays and high density computing power
and data storage have allowed phased arrays to become more viable for non-destructive
testing and evaluation.
6
While many phased arrays directly contact the target specimen, the array of
interest in this study was non-contact and couples the acoustic energy to the target
specimen via a water medium. Air-coupled phased array techniques have also been
studied[4]. A method using laser generated pressure waves has also been investigated[5].
This non-contact method produces the functionality of phased arrays by firing laser
pulses with time delays at the surface of a specimen. The targets on the surface coincide
to the location of phased array elements.
A linear phased array is a series of piezoelectric elements arranged along a line on
a plane. Typically they are the same size and have equal spacing. The spacing of the
elements affects the ability of the array to steer and focus, while the geometry of the
elements affects the frequency and beam divergence. These effects will be discussed in
the following sections.
2.3 Array Element Spacing
The spacing between the centers of elements is the inter-element spacing, d. The
width of the elements, w, must be less than this, otherwise elements would be
overlapping. Figure 1, below, illustrates inter-element spacing, d, width, w, and length, l.
d
l
w
Figure 1: Diagram of Array Element Spacing. Inter-element spacing, d, width, w, and length, l.
7
There is however a limit on the inter-element spacing relating to the wavelength
of the pressure wave in the medium, λ, and the maximum steering angle, θsteering, that is
required for the array’s application[6]. This calculation is shown in Equation (1) below.
( )steeringsteeringcr f
vdθθ
λsin1sin1 +
=+
= (1)
The width of the elements, w, must be less than the inter-element spacing, d,
which must be less than the critical inter-element spacing, dcr. This relation is shown in
Equation (2).
crddw << (2)
It is desirable to design the width of the elements, w, as close to the inter-element
spacing, d, as possible. However, this is difficult to manufacture and the closer the
elements are together, the more likely they may not maintain the ability to operate
independently; the electrical connections may interfere or there may be adverse operating
conditions degrade the electronics and lead to undesirable operations.
An alternative method of spacing elements of a linear phased array is creating a
sparse array. Sparse arrays eliminate some elements from a linear phased array, but
maintain the same spacing between elements and empty spaces where elements would be
placed in a linear phased array[7]. This type of array maintains some functionality of a
linear phased array with more elements and provides a cost savings over fully populated
linear phased arrays. Choosing which elements should be removed from a full linear
phased array and which should remain needs to be optimized for each design and
application[7].
8
2.4 Time of Flight
Ultrasonic waves are converted back to electrical waves, and then can be analyzed
by software or can be directly viewed on an oscilloscope. Typically a time versus voltage
graph is used as output in real time. This allows the user to see where in time reflections
of ultrasonic energy are received by the piezoelectric material.
Typically reflections occur at material interfaces due to mismatches in the
acoustic impedance of two materials. The reflected pressure waves associated with
material interfaces can be used to measure the thickness of a material layer. The time
difference between front wall and back wall reflections in the signal is directly correlated
to the time the ultrasonic wave takes to travel the distance between the front and back
surfaces of the specimen.
2.5 Near-Field and Far-Field
The amplitude of the ultrasound emitted by a transducer wildly oscillates in a
region close to the surface of the transducer. This occurs due to the interaction of many
pressure waves of slightly different frequencies and amplitudes emitted from different
locations on the surface of the transducer. After some axial distance, N, from the surface,
the waves develops into a wave front. The region before this distance is referred to as the
near-field, and the region past this distance is the far-field. The amplitude of the pressure
wave decreases exponentially in the far-field. It is desirable to maintain a distance
greater than N between the transducer face and the region of the target sample that is to
be evaluated. If the target is evaluated in the near-field region, results will neither be
useful nor accurate. Figure 2 illustrates how the amplitude of the pressure wave varies
with distance from the transducer[1].
9
Far-Field
Distance from the Transducer
Nominal Amplitude
Near-Field
N
Figure 2: Near-Field and Far-Field: Distance from Transducer vs. Nominal Amplitude. N is the distance to the point along the access of a transducer separating the near-field and the far-field[1].
For cylindrical pistons (transducers) emitting a continuous pressure wave, this
region, called the near-field, can be estimated to be related to the diameter of the emitting
surface, D, and the wavelength of the emitted pressure wave[1]. The wavelength is equal
to the velocity of sound, v, in the medium divided by the frequency of the continuous
pressure wave emitted, f. The distance, N, from the surface of the transducer that
separates the near-field from the far-field is calculated using Equation 3.
vfDDN
44
22
==λ
(3)
Although the near-field is easily calculated for continuous-wave cylindrical
transducers, the actual calculation is dependent on the bandwidth, central frequency, and
geometry of the transducer surface. Equation 3 can be used as a simple estimate of the
near-field region of the rectangular piezoelectric elements in the linear phased arrays.
10
When the elements of the array are used in unison to perform focusing and
scanning, they are unable to function as a linear phased array in the near-field of the
elements due to the amplitude oscillations that take place in this region. Linear phased
arrays should operate in the far-field of the piezoelectric array elements.
2.6 Beam Divergence
After the pressure wave passes by the near-field distance, the wave begins to
spread out. There is some angle, α, that relates the center axis of the beam and 6dB down
intensity level. The 6dB down intensity is about half the intensity of the center axis
intensity. For continuous-wave(CW), cylindrical transducers, α can be calculated from
the first root of the far-field pressure fields. Equation 4 is developed in Rose’s
Ultrasonic Waves in Solid Media[1].
⎟⎟⎠
⎞⎜⎜⎝
⎛= −
Dfv2.1sin 1α (4)
Beam divergence decreases as the dimension of the element increases because the
angle, α, decreases. Beam divergence is a factor in the decrease of the pressure wave
amplitude along the central axis of the transducer in the far-field. The pressure field
intensity spreads out over a larger region as the beam diverges. Figure 3 shows how the
beam diverges after entering the far-field.
11
Center Axis
Beam Divergence Angle, α
N
Near-Field
Far-Field
6 dB down
6 dB down“Natural Focus”
Figure 3: Diagram of the Beam Divergence Angle of a Transducer. The pressure field in the far-field spreads out at the beam divergence angle, α , which defines the region of the pressure field that is within 6 dB of the peak pressure along the center axis of the transducer in the far-field[1]. It is desirable for linear phased arrays to maintain pressure waves in the two-
dimensional plane outward from the line of array elements. To minimize losses to the
regions above and below this plane, the length, l, of the elements should be larger than
the width, w, of the elements. See Figure 1 for illustration of these array element
dimensions.
Similarly it is beneficial to have a smaller array element width because this allows
for more divergence in the scanning plane of the array. This allows for a larger angular
region of the plane to be scanned.
2.7 Frequency Considerations
Frequency plays a large role in the capability of an array. With a pulse excited
element, frequency is discussed in relation to the central frequency of the element. The
content of the pulse includes frequencies that are higher and lower than the central
12
frequency of the element, but to understand the overall performance of the array it is
useful to consider the central frequency as the frequency of the pulse and element.
Higher frequencies have a shorter wavelength and thus have a potential to interact
with smaller irregularities in a sample. This allows an array to detect smaller defects that
otherwise would appear not to be present. Determining the size of the anomalies that are
to be monitored (the resolution required) is an important step in choosing the central
frequency of the array elements.
While it is beneficial to increase frequency and thus increase resolution,
increasing frequency leads to many detrimental effects as well. As frequency increases,
the attenuation of the pressure wave increases in most materials. This means that the
amplitude of the pressure wave in the far-field decreases more rapidly with higher
frequency pulses. Increasing frequency decreases beam divergence and increases the
near-field distance.
2.8 Resolution
The resolution of a UNDE technique is dependent on the frequency of ultrasound,
the properties of materials in the system, the electronics used to operate the inspection
device, and the geometry of the device and system. In the next two sections, both lateral
resolution and axial resolution will be discussed.
2.8.1 Lateral Resolution
Lateral resolution is most dependent on the beam directivity of the elements. The
coverage of an individual element’s pressure wave on the surface of the sample is the
highest resolution that can be achieved. The near-field distance, N, is the point at which
13
the beam spot is the most focused. As the far-field pressure wave moves further through
the sample or medium, the larger the beam spot becomes.
Another factor affecting the lateral resolution is the number of elements in the
row of the array. 16 elements in a row can focus to a tighter spot than 4 elements can.
This is because a higher ratio of the total energy emitted can be concentrated on that spot,
and likewise a higher ratio of the reflected acoustic energy will reflect from the focus.
This is further expounded in Section 2.10.
2.8.2 Axial Resolution
Axial resolution determines the ability of the array to detect differences in depth.
Examples include the depth of a crater on the surface of the specimen, thickness of a
specimen, or a change in the surface position due to bending or warping. The axial
resolution is determined by the type of pressure pulse emitted by the elements of the
array. A short pulse of 1.5 cycles would lead to a resolution of 1.5λ. Figure 4 shows
examples of a 1.5 cycle pulse and a 2.5 cycle pulse.
2.5 cycle pulse 1.5 cycle pulse
Voltage Voltage
Time Time
Figure 4: Diagram of 1.5 and 2.5 Cycle Pulses. For pulses of the same frequency, a pulse with fewer cycles has a shorter time length.
Pulses this short can be achieved by providing sufficient electrical damping in the
equipment providing the pulse or within the array itself. The table, Figure 5, summarizes
14
the axial resolution that may be achieved by piezoelectric cylindrical pistons with
operating frequencies of 2.25 and 10 MHz.
Axial Resolution
Frequency 1.5 cycle, Water*
2.5 cycle, Water*
1.5 cycle, Copper*
2.5 cycle, Copper*
2.25 MHz 1.0 mm 1.7 mm 3.1 mm 5.2 mm 10 MHz 0.26 mm 0.38 mm 0.71 mm 1.2 mm
*Speed of Sound: 1500 m/s in Water; 4700 m/s in Copper. Figure 5: Table of Calculated Axial Resolutions. These values illustrate the ideal resolutions achievable for different frequency and cycle length pulses in water and copper. 2.9 Linear Phased Array Pressure Field Equations The pressure field produced by a phased array can be determined from
superimposition of the pressure fields associated with each array element. Similarly, the
pressure field of an individual array element can be calculated from integration over the
array element of infinitesimal elements. S.-C. Wooh and Y. Shi have extensively
documented work on developing pressure field equations and application to linear phased
array design[8-10].
15
Figure 6 below shows a single array element and a specific point in the pressure
field[8].
rθ
R
dx
x
a
P(r, θ )
Figure 6: Diagram of Element and Point P(r, θ ) in the Pressure Field. Here the width of the element is a, x is the distance from the top of the element to the differential element, dx is the width of a differential section of the element, R is the distance from the differential section to point P, r is the distance from the top of the element to point P, and theta is the angle from the element perpendicular to the line connecting point P to the top of the element[8]. The elements are estimated as line elements, with ‘a’ the width of the line
element. This simplifies equations minimizing the array calculations to a single two-
dimensional plane, and is acceptable because the length of the element is much larger
than the width, a, and is also good estimate for this application because the pressure field
of interest is in the plane perpendicular to the length direction of the line elements. In
addition, an estimate is made to arrive at an equation for the pressure field at the point, P.
For points, P, that are sufficiently far away from the array (where r/a >> 1), R can be
approximated as {r – x sin θ }[8].
The pressure field associated with an infinitesimal element can be approximated
as the Equation (5) below[8]:
([ ][ ]dxxrktjr
pdp O θω sinexp
21
−−⎟⎠
⎞⎜⎝
⎛≈ ) (5)
16
In Equation (5), po is the amplitude of the initial pressure wave generated at the
surface of the infinitesimal element, ω is the angular frequency of the initial pressure
wave, and k is its wave number. The angular frequency, ω , and wave number, k, can
also be written in terms of frequency, f, and sound speed, c, as in Equation (6) and
Equation (7) below.
fπω 2= (6)
cf
ck πω 2
== (7)
By integrating over the entire array element, the total pressure field generated by
the array element can be calculated[8]. The pressure field for an element is shown in
Equation (8) and Equation (9).
( ) ( )[ ][ ]∫∫ −−⎟⎠
⎞⎜⎝
⎛≈=
aO
element
dxxrktjr
pdptrp
0
21
sinexp,, θωθ (8)
( ) ( )( )( ) ( )[ ]krtjjka
kka
rptrp O −−⎟
⎠⎞
⎜⎝⎛−⎟
⎠⎞
⎜⎝⎛= ωθ
θθθ exp
2sinexp
2/sin2/sinsin,,
21
(9)
Again by superposition, the pressure field produced by the entire array of N
elements is the summation of the pressure fields associated with each element, as shown
in Equation (10) below.
( ) ( )∑=
=N
ii trptrp
1,,,, θθ (10)
17
By including time delays, τΔ , for the array element excitations and the inter-
element spacing, d, the pressure field can be written as Equation (11)[8].
( ) ( )( )( )
( )( )[ ]( )( )2/sinsin
2/sinsin2/sin
2/sinsin,,21
θτωθτω
θθθ
kdNkd
kka
rptrp O
−Δ−Δ
⎟⎠⎞
⎜⎝⎛=
( ) ( ) ([ krtjNkdjjka−−⎥⎦
⎤⎢⎣⎡ −
−Δ−⎟⎠⎞
⎜⎝⎛−× ωθτωθ exp1
2sinexp
2sinexp )] (11)
Here the time delay, τΔ , has been chosen as constant between adjacent array
elements and this simplifies the array pressure field equation. Time, t, was replaced with
[t - (i - 1) τΔ ] in the summation, Equation (10).
Time delays for dynamic linear phased arrays could be integrated into these
equations by replacing constant τΔ with a variable and evaluating the pressure field by
superposition rather than solving for the equivalent to Equation (11).
2.10 Array Optimization
S.-C. Wooh and Y. Shi have used their array pressure field calculations as a
method to optimize linear phased arrays for NDE applications. They note that
maximizing the effectiveness of beam steering will coincide with minimizing the main
lobe, eliminating grating lobes, and minimizing energy lost to side lobes[8].
The main lobe is the angular width of the beam traveling in the direction of the
steering angle. This width relates directly with the size of the beam spot that can interact
at the focus of the array. If the main lobe is sharp at the steering angle, the acoustic
energy is focused primarily in the steering direction. Main lobes widths at smaller
steering angles are less than larger steering angles.
18
Grating lobes can occur once the steering angle of an array is increased past the
maximum steering angle. Grating lobes occur at angles other than the steering angle and
have acoustic energy equivalent to the main lobe. This is a large drain on the energy in
the direction of the steering angle. The grating lobes acoustic energy can reflect back to
the array and cause spurious return signals. This is in addition to the decreased signal
from the intended steering angle reflections. From the pressure field equations of the
previous section, S.-C. Wooh and Y. Shi calculated a critical element spacing, dcr, that is
defined by the maximum steering angle and the wavelength of ultrasound in the
medium[8]. This is shown in Equation (1) in Section 2.3.
While grating lobes account for the maximum energy lost at steering angles
greater than the maximum steering angle, side lobes account for loss of acoustic energy at
all steering angles. Side lobes are smaller lobes of acoustic energy directed at angles
other than the main lobe. A design goal is to maximize the main lobe to side lobe
acoustic energy ratio. It was found that the method to increase this ratio was to increase
the number of array elements, N. Once N increases past 16, the effect of minimizing the
side lobe energy converged; increasing N past 16 was not beneficial to an array’s
effectiveness. The minimum side lobe amplitude converges to -13.5 dB of the main lobe
amplitude[8].
S.-C. Wooh and Y. Shi have also studied the effects of element width on array
characteristics. They found that element width had little or no effect on many phased
array characteristics. It is desirable to have the largest element width possible while
within the limits of the optimum inter-element spacing. This is because larger element
19
widths will produce a larger acoustic pressure. It is also reiterated that 16 elements is an
optimum number for linear phased array NDE applications[9].
In another article, S.-C. Wooh and Y. Shi used computer simulation methods to
further investigate linear phased array design. They provide a set of guidelines
summarizing their findings. One finding specifically important to linear phased arrays
with a limited number of elements is that as the inter-element spacing is increased past
the critical inter-element spacing the only way to avoid grating lobes is to decrease the
operating frequency of the transducer[10].
E. Kühnicke used different methods for array optimization. Using wave
scattering equations and considering array elements as point sources, arrays were
simulated to determine optimum characteristics for angle scanning and focusing[11].
Kühnicke also examined the effect of array elements arranged along a curve instead of a
flat plane[12]. This is applicable to non-contact linear phased arrays examining a fluid-
filled pipe from the inside. Kühnicke notes that the method of transient and time
harmonic sound fields is a faster technique to simulate arrays for optimization of array
parameters[12].
2.11 Additional Literature
K. Oliver documented early design work in The Design of a Unique Two
Dimensional Phased Array with Low Channel Applications for Imaging Defects on a
Metal Surface[14]. Early results from experiments were documented in Investigating a
stepped ultrasonic phased array transducer for the evaluation and characterization of
defects[15].
20
Chapter 3
Methodology
3.1 Devices
This section highlights some of the more important devices that are used n the
experiments which follow.
3.1.1 Stepped Linear Phased Array
The first prototype stepped transducer was built by Olympus NDT (formerly
known as RD Tech) of State College, Pennsylvania. It was built with specifications
determined by Kara Oliver2. Figure 7 shows Prototype 1 lying on its side on the left and
the upright on the right with an aluminum specimen.
Figure 7: Photographs of Prototype 1. Prototype 1 on its side (Left) and upright in experimental setup (Right) with an aluminum specimen. The housing provides an airtight seal for the interior electronics. The
piezoelectric elements are made of lead zirconate titanate (PZT). The elements lie under
the gold-colored surfaces in the photograph. PZT is commonly used in the manufacture
of piezoelectric transducers for UNDE.
21
Additionally, in order to show that using fewer wire inputs to the device is
feasible, the elements are connected in parallel, one on each step, with four per channel.
This allows for a minimal number of wires leading to the device; five wires, four
channels and one ground. The stepped design minimizes wire inputs which is important
for application in hazardous environments that have rules governing the number of
allowable throughways into the environment. Figure 8 below shows how the elements
are arranged in the array. The red lines show how the elements in a column are wired
together.
Front View Side View
Figure 8: Schematic Drawing of Stepped Array Device. Four elements are on the face of each step. A single electronic channel is connected to four elements. A wire connects one element on each step of the array along a column. The wiring is illustrated by the red lines. The elements on the same step act as an independent linear phased array.
22
Two additional prototypes were fabricated. Prototype 2 features the same 4 steps
as Prototype 1, but has different dimensions and its elements have a center frequency of
10 MHz. Prototype 3 has 6 steps and 10 MHz central frequency. In Prototypes 2 and 3,
the array elements lie under the white surfaces of the steps. In addition, these prototypes
are made for eventual testing in high temperature and pressure environments. Prototype
2 is shown below in Figure 9 and Prototype 3 is shown in Figure 10.
Figure 9: Photograph of Prototype 2. Prototype 2 has elements with a central frequency of 10 MHz and four steps, four elements on each step.
Figure 10: Photograph of Prototype 3. Prototype 3 has elements with a central frequency of 10 MHz and six steps, four elements on each step.
23
3.1.2 Omniscan MX System
An Omniscan MX system was obtained from the same company which built the
initial prototype, Olympus NDT. The system is commonly used in UNDE field work.
The software is not specific to an individual UNDE device such as an array or single
transducer but is intended to be versatile and useable by a technician. The device can be
seen in Figure 11.
Figure 11: Photograph of the Omniscan MX System. Manufactured by Olympus NDT (RD Tech), the portable system can interface with conventional transducers and phased arrays for Non-Destructive Testing. The goal of utilizing the Omniscan MX system is to prove that the stepped phased
array works as intended. Output can be observed in an image that shows the data
received in real time as absolute value of voltage received from reflections from the area
in time that is scanned by the software. The electrical signal that is received by the
Omniscan MX system is rectified to produce the image of the scanned area.
3.2 Experiments
Experiments were conducted to evaluate the ability of the first prototype to
characterize defects and material characteristics. The goal of the device is to characterize
surface defects, internal defects, and geometric changes of a specimen over time.
24
3.2.1 Experimental Setup
The holder for the specimen and Prototype 1 was built specifically for these
experiments. Prototype 1 and the specimen sit on the holder within cutouts that do not
allow them to move along the surface of the holder. This holder is placed at the bottom
of a glass beaker filled to a level above the top of the array with distilled water. Care is
taken to remove air bubbles from the surfaces of the specimen and array before
experiments are conducted. Prototype 1’s experimental setup can be seen below in
Figure 12.
Figure 12: Photograph of Experimental Setup for Prototype 1. This setup maintains the position of the prototype and specimen, allows for heating of the water and system with a hot plate, and translation of the top of the specimen. Prototypes 2 and 3 were built differently than Prototype 1. The wire leads come
out of a different side of the array and the holder for Prototype 1 experiments would not
be useful with Prototypes 2 and 3. Figure 13 below shows how Prototypes 2 and 3 were
fixed in place and their orientation with the specimen held in place in front of them.
25
Figure 13: Photograph of Experimental Setup for Prototypes 2 and 3. This setup maintains the position of the prototype and specimen and also allows for precise rotation of the specimen. The array prototypes have been manufactured to interface with the Omniscan MX
system. This system is used to operate the array and to receive and analyze signals for
the following experiments.
3.2.2 Thickness Measurement
The thickness of a specimen can be monitored by determining the speed of sound
in the material and monitoring the distance between front and back wall reflections from
the specimen. As discussed in Section 2.3, pressure waves can reflect at material
interfaces. The reflections of the front wall and the back wall of a specimen are separated
in time as the signals are received by the array. This time difference between reflections
is the additional time that the sound waves travel in order to interact with the back wall of
the specimen. This physical distance traveled is two times the thickness of the specimen.
In order to monitor this distance, one can measure the initial thickness of the specimen
conventionally and calculate the sound speed of the specimen, or the sound speed of the
26
known material can be read from available material literature or previously compiled
tables of material properties.
3.2.3 Surface Absorbing Defect
Some material defects that may arise on the surface will be chemical
transformations of the surface material. Such materials may have the tendency to absorb
sound energy and discharge this energy to some other form such as heat. These corrosive
films or deposits on the outer layer of a material may be important to monitor to
understand the life cycle of materials as they undergo chemical reactions.
3.2.4 Surface Scattering Defect
Scattering surface defects may occur when a specimen’s smooth surface becomes
dimpled and/or blistered with small growths or bubbling. This was simulated by shallow
drill holes on the surface of specimens.
3.2.5 Single Surface Crater
To determine the ability of the array to resolve a crater on its surface, a small drill
hole is made partway into the surface of specimens at various locations across from the
steps of the array.
3.2.6 Interior Defects
Interior voids caused by blistering within a material may lead to catastrophic
failure of a specimen. Mechanical characteristics could be severely hampered by such an
unseen defect growing within a material. Such defects may be caused by crack growth
and gaseous expansion.
27
The ability of the array to detect defects interior to specimens was simulated by
drilling holes through the side of samples. These holes could then be either plugged with
air or be allowed to fill with water.
3.2.7 Bending of a Specimen
Warping or bending of a specimen could occur with inhomogeneous stresses or
inhomogeneous expansion due to heating. These geometric changes could affect the
ability of a material to perform in a harsh environment.
To test the ability of the Prototype 1 to monitor the warping of a specimen, the
specimen’s top was moved horizontally using a precision translator, simulating the effect
of bending or warping without the need to bend the specimen with force. This allows for
precise movement of the specimen top while the bottom of the specimen rested in a grove
in the specimen holder, free to rotate but not to translate up and down. Figure 14
illustrates the movement of the specimen.
28
Fixed position but free to rotate
-7 mm to +7 mm
Figure 14: Bending Experiment Setup. The bottom of the specimen is fixed while the top of the specimen is precision translated to lean the specimen in relation to the stepped array. At every 0.2 mm movement of the top of the specimen, the time of the front wall
reflections for each step was recorded. This monitored how the position of the specimen
surface facing the array moved as the specimen bent away or towards the array.
Prototypes 2 and 3 could utilize their experimental setup to rotate the specimen by
angle instead of translation.
3.2.8 Effect of Temperature
The effect of raising the temperature of the water medium was also investigated.
To raise the temperature of the system, the beaker is placed upon a hot plate, and the
temperature is raised slowly to approximately 180 degrees F. The altered experimental
setup of the system can be seen in Figure 15.
29
Figure 15: Effect of Temperature Experimental Setup. The experiment is conducted on top of a hot plate and the temperature of the system is monitored with a thermocouple.
This temperature was chosen such that little boiling would occur in the water
medium. Gas bubbles would negatively affect the performance of the array because
sound waves have difficulty passing through a composite medium such as water and
gaseous water bubbles. The bubbles disrupt the pressure waves because of the
impedance mismatch between the bubble and the surrounding liquid.
At approximately 180°F, the temperature of the system was held constant and the
bending test was repeated, Section 3.2.7. The results were compared to determine the
effect that temperature had on the experiment.
3.3 Simulations
Computer simulations were conducted to determine how the next series of
prototypes would perform compared to the first prototype. The program used was Field
II, a program run through MatLab, by J. Jensen. Codes that were altered for 10 MHz
array simulations were initially developed by K. Oliver from codes packaged with Field
30
II[6]. Modifications were made to the codes to allow higher frequency ultrasonic waves
to be simulated.
31
Chapter 4
Results
4.1 Experimental Results
4.1.1 Signals
A better understanding of how the Omniscan MX system displayed the signals
from the entire prototype was desired. One step of the array was isolated by covering the
active surfaces with sound absorbing stick tack, effectively “turning off” the other steps.
The covered steps do not affect the signals received by the uncovered step, so the
received signals are only comprised of reflections due to the ultrasonic waves sent by the
uncovered step. The red waveform in Figure 16 below, overlayed on the black
waveform, is the waveform displayed when only the first step is left uncovered. The
black waveform is displayed when all four steps are in use.
Figure 16: Comparison of Full Array Waveform and First Step Waveform. The isolated waveform of the first step of the array is shown in red. The full waveform, with all four steps active, is displayed in black. The figure shows how the reflections associated with the first step appear in the full array waveform.
32
Figure 10 shows that echoes from the first step affect the waveforms associated
with successive steps. This means that with the current setup, array, and software, it is
difficult to generate reliable detection of defects and other sources of reflections. This is
especially true for reflections internal to the specimen.
For future work, an adapter was procured to activate individual channels of the
array prototype. When a single channel is operated using a pulser/receiver, the array
receives a similar signal as those received using the Omniscan MX system. This adapter
also allows the use of the array without the Omniscan MX system.
The adapter was used to replicate the simple results of the Omniscan MX system
with a conventional pulser/receiver. The graph below, Figure 17, compares the signals
of all four steps of the second channel active and only the first step uncovered.
Figure 17: Comparison of Single Channel Signal and First Step, Single Channel Signal. The isolated signal from a single element on the first step is shown in red. The full signal, with all four elements of the channel active, one on each step, is displayed in blue. The figure shows how the reflections associated with the single element on the first step appear in the full signal of the channel.
33
This confirmed that the signals seen on the Omniscan MX system were similar to
those that could be obtained using a pulser/receiver. This adapter will be useful to future
work when monitoring prototype element responses to different electronic pulses.
A pulse-echo signal of Prototype 2’s first channel was also captured for
comparison to Prototype 1. The waveform can be seen in Figure 18.
Prototype 2, 4 Step 10 MHzChannel 1
-4
-3
-2
-1
0
1
2
3
4
5
0.00000 0.00005 0.00010 0.00015 0.00020 0.00025 0.00030
Time (s)
Volta
ge
Figure 18: Signal from Channel 1 of Prototype 2. Prototype 2 has four steps and its elements have a central frequency of 10 MHz. This reflection signal of channel 1 shows significantly more noise and higher frequency content than the signals seen in Figure 17. This waveform shows that Prototype 2 behaves similarly to Prototype 1. the same
reflected front and back wall reflections can be seen, as well as additional echoes of the
reflections. The main difference that can be seen is that Prototype 2 has sharper minima
and maxima which are due to the higher central frequency of 10 MHz. There are many
more cycles in the pulse reflections. The pulse cycle length needs to be shortened to
34
several cycles in order to have any benefit of the increased central frequency of the array
elements. Figure 19 below also illustrates the poor quality of Prototype 2.
Figure 19: Prototype 2 S-Scan of Pristine Sample. This image shows a rectified waveform (top) and a S-Scan image of a defect-free specimen (bottom). Four front wall reflections from the four steps of Prototype 2 can be seen but back wall reflections are significantly lower in magnitude. Front wall reflections of Prototype 2 scans are large but back wall reflections are
almost nonexistent. The large pulse length of the front wall reflections also contributes to
an inability of Prototypes 2 and 3 to resolve surface and interior defects.
4.1.2 Surface Absorbing Defect
Absorbing surface defects were simulated by applying commercially available
stick tack, a rubbery adhesive, to the surface of specimens. This defect was applied in
front of individual steps of the array on a sample metal specimen and is circled in red in
Figure 20. The decreased amplitude of the signal received by the array is seen for the
step that has the sound absorbing stick tack in front of it.
35
Figure 20: Surface Absorbing Defects. Prototype 1 was used to show the applicability of the array to detect surface defects that absorb acoustic energy. A piece of stick tack was placed on the surface of the specimen in front of each step of the array. Each step was able to experience a decrease in the amplitude of the reflected wave associated with that step.
36
The stick tack decreases the amplitude of the reflected waves significantly. This
can be seen visually in the S-Scan produced by the Omniscan MX system and the
amplitude can be seen to decrease in the A-Scan signal as well.
4.1.3 Surface Scattering Defect
Surface scattering mock defects were made by drilling small shallow holes into
the surface of a specimen in front of a step of the array. An example of these mock
defects can be seen in the photograph on the right of Figure 21. On the left is the signal
associated with this defect.
Figure 21: Surface Scattering Defects. A surface defect that scatters pressure waves was produced on a specimen at the location across from the second step of Prototype 1. The defect lowered the amplitude of the reflected waves of the second step of the stepped array. The amplitude of the second step’s front wall reflection decreased significantly
due to the shallow divots on the specimen in front of the second step of the array.
4.1.4 Single Surface Crater
A single surface crater defect was made in a specimen. A larger (1/8” diameter)
shallow hole was drilled into the surface of a specimen in front of the first step of
37
Prototype 1. Figure 22 shows the output of a single surface crater in front of the center
of the first step of Prototype 1, and below this, a screenshot of a pristine specimen.
Figure 22: Single Surface Crater, Prototype 1. A single shallow drill hole was produced in the specimen surface, centered in front of the first step of Prototype 1. The effect in the front wall reflection (top) shows a shift of the front wall reflection, possibly reflecting off the bottom of the hole in addition to the surround surface. A defect-free specimen reflection image is also shown for comparison (bottom). While Figure 22 shows that a crater type defect can be detected by the system, it
is difficult to determine the dimensions of the defect. The depth that the crater penetrates
into the specimen and the width across the face of the specimen cannot be directly
correlated to the features of Figure 22.
38
Prototype 2 was also used to investigate single surface defects. Figure 23 below
shows an A-Scan of a single surface crater in front of the first step of Prototype 2. The
same method of covering steps 2 through 4 as was used in Section 4.1.1 was used to
isolate the signals of step 1.
Front wall
Bottom surface of crater defect
Figure 23: Single Surface Crater, Prototype 2. Similar to Figure 22, a shift of the front wall reflection can be seen with a similar defect with Prototype 2. The A-Scan shows a reflection from the surface of the specimen as well as a
reflection from the bottom surface of the crater. While the signal of the entire array had
more background noise and interferences that marred the ability to distinguish a single
surface crater, the crater could be seen in Figure 23. Prototype 2’s ability to resolve a
single surface defect was greater than Prototype 1.
Additional data analysis techniques would be beneficial to detecting surface
defects. Removing noise and spurious reflections would benefit defect detection and
characterization.
39
4.1.5 Interior Defects
Prototype 1 could not detect interior defects filled with air or water. No
reflections could be seen at the location of the defect within the depth of the specimen. In
addition, noise levels and interference with front and back wall reflections may have
obscured any reflections from an interior defect.
Prototype 2 similarly could not detect interior defects. Additional signal
processing techniques may provide the means to detect such internal defects.
4.1.6 Bending of a Specimen
As discussed in Section 3.2.7, it is desirable to monitor bending or warping of a
specimen in a hazardous environment. Figure 24 below shows how the position of the
front wall reflections moved in time as the specimen leaned away from Prototype 1.
Room Temperature Bending - 0 to 7 mm
0.000.501.001.502.002.503.003.504.00
0 2 4 6
Translation of Top of Specimen (mm)
Tim
e di
ffere
nce
from
zer
o de
flect
ion
(mic
rose
cond
s) 1st step2nd step3rd step4th step
Figure 24: Room Temperature Bending. Prototype 1 was used to monitor a specimen leaning away from the stepped array. The graph shows how the location in time of the front wall reflection of each step changed with 0.2 mm incremental movements of the top of the specimen.
40
As the specimen leaned away from Prototype 1, the distance the pressure waves
traveled to reflect off of the surface of the specimen increased. The fourth step’s distance
increased the most because the specimen was fixed closest to the first step. This
experiment shows that the stepped array is applicable for monitoring bending and
warping of a specimen along its vertical axis.
4.1.7 Effect of Temperature
The same experiment as Section 4.1.6 was conducted at an elevated temperature,
180°F. The system temperature was raised with a hot plate and held constant around
180°F. The specimen was again leaned in 0.2 mm increments and the results were
compared to the room temperature experiment conducted in Section 4.1.6. The results
can be seen in Figure 25. The open circles are the elevated temperature results and the
closed circles track the data obtained at room temperature.
41
Comparison of bending, 0 - +7 mm
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
0 1 2 3 4 5 6 7
Translation of Top of Specimen (mm)
Tim
e di
ffere
nce
from
zer
o de
flect
ion
(mic
rose
cond
s)1st step2nd step3rd step4th step1st step 180F2nd step 180F3rd step 180F4th step 180F
Figure 25: Comparison of Bending at Room Temperature and 180°F. Prototype 1 maintained functionality at both room temperature (solid circles) and 180°F (open circles). The results for leaning of the specimen show that Prototype 1 was effective at both temperatures. By overlaying the results of the bending experiment for both ambient and 180°F,
it can be seen that the bending can be monitored at either temperature. There is little
change in the times of the front wall reflections for the two experiments. The array
maintains its effectiveness at either temperature.
4.2 Simulations
Field II simulation results are provided in this section to provide insight in the
differences of the array prototypes. Prototype 1 has a center frequency of about 2.25
MHz and the first step is about 6 mm from the specimen. When the array focuses at 6
mm, the specimen plane, the simulation shows that the energy profile at the specimen
surface would look as follows in Figure 26.
42
x position [mm]
y po
sitio
n [m
m]
Pressure XY Cross Section Response
-20 -10 0 10 20
-30
-20
-10
0
10
20
30 -60
-50
-40
-30
-20
-10
0
Figure 26: Field II Simulation of 2.25 MHz Array. The image on the left shows the amplitude of the pressure wave in a 2-D plane corresponding to the surface of a specimen. On the right are the inputs to the simulation: the width of the elements, wth, height, ht, and space between elements, k_x. In addition, the number of elements is set at 4 in the next input, and the focus and axis capture (2-D plane to display) are set at 6 mm from the array. The central frequency of the elements is set at 2.25 MHz in the final input. The intensity of the pressure wave at the focus point is white in the figure. As the
intensity decreases in the figure, the representative color goes to yellow, red, and then
black. The graphic on the right is the user input boxes that are used to alter the
parameters of the program.
For comparison, a 10 MHz linear phased array was simulated for central focus at
a distance of 8 mm away from the specimen. Figure 27 shows the outputs of the
simulation for this case.
43
x position [mm]
y po
sitio
n [m
m]
Pressure XY Cross Section Response
-20 -10 0 10 20
-30
-20
-10
0
10
20
30
-80
-70
-60
-50
-40
-30
-20
-10
0
Figure 27: Field II Simulation of 10 MHz Array. The image on the left shows the amplitude of the pressure wave in a 2-D plane corresponding to the surface of a specimen. On the right are the inputs to the simulation: the width of the elements, wth, height, ht, and space between elements, k_x. In addition, the number of elements is set at 4 in the next input, and the focus and axis capture (2-D plane to display) are set at 8 mm from the array. The central frequency of the elements is set at 10 MHz in the final input. The beam spot can be seen to be much smaller. The increased frequency and
different array dimensions lead to a more focused, higher resolution beam. The width
and spacing of the elements were decreased for this simulation. These simulations show
that a higher frequency linear phased array has the potential to focus to a smaller beam
spot.
Beam steering can also be simulated using Field II. Grating lobes can be seen at
larger steering angles as discussed in Section 2.10. Figure 28 on the next page compares
beam steering of 2.25 MHz and 10 MHz linear phased arrays.
44
x position [mm]
z po
sitio
n [m
m]
Pressure XZ Cross Section Response
-20 -10 0 10 20
5
10
15
20
25
30
35
40
45-40
-35
-30
-25
-20
-15
-10
-5
0
x position [mm]
z po
sitio
n [m
m]
Pressure XZ Cross Section Response
-20 -10 0 10 20
5
10
15
20
25
30
35
40
45-45
-40
-35
-30
-25
-20
-15
-10
-5
0
(a) 2.25 MHz at 0° (b) 2.25 MHz at ~22°
x position [mm]
z po
sitio
n [m
m]
Pressure XZ Cross Section Response
-20 0 20
5
10
15
20
25
30
35
40
45
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
x position [mm]
z po
sitio
n [m
m]
Pressure XZ Cross Section Response
-20 0 20
5
10
15
20
25
30
35
40
45
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
(c) 10 MHz at 0° (d) 10 MHz at ~22° Figure 28: Field II Simulations of Beam Steering. These images were developed from similar inputs to those of Figure 26 and Figure 27. The 2-D planes that are shown are essentially looking down at the beam as it propagates from the array into space toward the specimen. The images show the differences between the beam steering of the 2.25 MHz array, (a) and (b), and the 10 MHz array, (c) and (d). The 2.25 MHz array can be seen to have a larger grating lobe at 22° (b) than the 10 MHz array (d).
45
The amplitude of the main beam is more intense relative to the surround side
lobes for the 10 MHz simulated array than the 2.25 MHz simulated array as seen in
Figure 28 (a) and (c). In addition, the possible grating lobe that appears in the simulation
of the 2.25 MHz array in (b) is larger in amplitude than the same lobe that appears in (d).
Array dimensions for a 10 MHz array simulated using Field II that optimize beam
steering and beam spot size were found to be 0.4 mm wide by 12 mm high elements with
inter-element spacing of 0.6 mm.
4.3 Calculations from Linear Phased Array Theory
4.3.1 Inter-element Spacing
From Equation (1), the critical inter-element spacing can be calculated. This
spacing maximizes the spacing of elements for an array to maintain a desired maximum
steering angle without grating lobes sapping energy from the main beam. The speed of
sound, v, of water (1500 m/s) is used in Equation (1) and Figure 29 below summarizes
some example calculations of critical inter-element spacing.
Frequency, f (MHz)
Maximum Steering Angle, steeringθ
Critical Inter-element spacing, dcr
2.25 15° 0.53 mm 2.25 30° 0.44 mm
5 15° 0.24 mm 5 30° 0.20 mm 10 15° 0.12 mm 10 30° 0.10 mm
Figure 29: Table of Calculated Critical Inter-element Spacing Values. These values are calculated from Equation (1). As the frequency and desired maximum steering angle are increased, the critical inter-element spacing decreases substantially. These calculations show that when increasing the frequency of array elements to
10 MHz, grating lobes will be more evident at larger steering angles. The width of
elements must be less than the critical inter-element spacing, and for 10 MHz arrays this
46
requirement is difficult meet during design and manufacture. A tradeoff should be made
to focus on development of a 5 MHz central frequency arrays in order to cover a larger
surface of the specimen without encountering grating lobes.
4.3.2 Application of Other Array Optimization Techniques
Prototype design for a minimal number of wire inputs to the array dictate a
minimal number of elements in the linear phased arrays of the prototype. A possible
solution to this problem may be to incorporate six array elements into a sparse array with
ten positions for array elements. This may allow for proper beam steering without
grating lobes and good beam focusing while still maintaining the resolution benefits of 5
MHz central frequency elements.
47
Chapter 5
Conclusions
The stepped linear phased array design has been developed to fill a need for a
method to monitor the degradation of materials in hazardous environments to support the
qualification of new materials for industrial applications. Throughout their life in service,
a material in a high temperature, high pressure environment must be able to maintain its
integrity and degrade gracefully. Corrosion and blistering, which may lead to sudden
cracking and failure, need to be studied in order to determine the operational
characteristics of a material.
Three prototypes were built to test the capabilities of the low-channel stepped
phased array design. Prototype 1 has four steps and four piezoelectric elements per step.
The central frequency of its elements is 2.25 MHz. Prototypes 2 and 3 have a central
frequency of 10 MHz. In addition, Prototype 3 has six steps.
The effectiveness of the prototypes to monitor material degradation was evaluated
through bench-top experiments. These experiments were conducted to determine the
ability of the prototypes to meet the objectives of detecting surface and interior defects
and changes in shape and size of a material specimen.
While Prototype 1 showed that it could detect surface defects and bending of the
specimen at room temperature and an elevated temperature, it remains to be seen whether
it can characterize individual surface and interior defects. Prototypes 2 and 3 were
similarly effective in detecting defects despite their higher central frequency of 10 MHz.
Interior defects were undetectable but surface defects were able to be detected.
48
Control of the electronic pulse in the Omniscan MX system is limited and is not
able to optimize the pressure waves emitted by Prototypes 2 and 3. A shorter electronic
pulse is needed to increase the axial resolution. Utilizing a different device to control the
phasing, data capture, and data analysis may be required.
The most important contribution of these results is to illustrate that the array
dimensions chosen by the manufacturer of Prototypes 2 and 3 were not optimum. In
addition, Prototype 2 and 3 array elements do not perform satisfactorily. The same long
pulses were seen with both Omniscan MX operation and a conventional pulser/receiver.
While the elements should respond similarly to pulsing, some produce weaker ultrasonic
pulses than others. When some elements of an array do not function correctly, the
advantages of an array cannot be fully utilized. These findings exemplify that increasing
frequency without considering the many other design parameters and limitations is
detrimental to the effectiveness of the stepped linear phased array.
Altering the prototype design to include interior electronics to provide the same
effect of turning off all but one step may be useful to more thoroughly examine the
specimen with only the active step. This would decrease the noise associated with the
operation of all the steps at the same time. However, such an electronic switch may
require additional wire inputs to the device from outside the harsh environment. The
switch itself would also have to remain operable in the harsh environment.
In addition sparse arrays and central frequencies below 10 MHz should be
evaluated for their application to future design of stepped linear phased array prototypes.
Sparse arrays provide some functionality of a similar array with more elements and
channels. Due to the input limitations of the harsh environment, this may be another
49
beneficial way to improve the scanning and focusing resolution of the array. Central
frequencies below 10 MHz may provide the best tradeoff between axial resolution and
array capabilities.
Despite the inadequacies of the current prototypes, overall the stepped linear
phased array device has shown promise for UNDE applications that require operations in
harsh environments. The prototypes have shown the ability to detect surface defects and
bending of a specimen. While the prototypes have not been able to resolve interior
defects, further research and design as described in this thesis should overcome the
difficulties of these three prototypes. The desired capabilities of this device are
attainable.
Future work should be done to investigate the applicability of sparse arrays.
Sparse arrays may provide additional optimization opportunities. In addition, working
closely with the manufacturer of future prototypes will be very beneficial. It is important
to ensure that the array is designed to specifications and that the final product is tested
and shown to perform as expected.
50
Bibliography
[1]Rose, J. L., Ultrasonic Waves in Solid Media. 1999, Cambridge: Cambridge University Press. [2]Komura, I., Nagai, S., Kashiwaya, H., Mori, T., Improved ultrasonic testing by phased array technique and its application. Nuclear Engineering and Design 87 (1985) 185-191. [3]McNab, A., Campbell, M. J., Ultrasonic phased arrays for nondestructive testing. NDT International Vol. 20, No. 6, December 1987. [4]Neild, A., et al. The radiated fields of focussing air-coupled ultrasonic arrays. Ultrasonics 43 (2005) 183-195. [5]Swift, C. I., Pierce, S. G., Culshaw, B., Generation of a steerable ultrasonic beam using a phased array of low power semiconductor laser sources and fiber optic delivery. Smart Mater. Struct. 16 (2007) 728-732. [6]Oliver, K. A., The Design of a Unique Two Dimensional Phased Array with Low Channel Applications for Imaging Defects on a Metal Surface. 2005, The Pennsylvania State University. [7]Yang, P., Chen, B., Shi, K.-R., A novel method to design sparse linear arrays for ultrasonic phased array. Ultrasonics 44 (2006) e717-e721. [8]Wooh, S.-C., Shi, Y., Optimum beam steering of linear phased arrays. Wave Motion 29 (1999) 245-265. [9]Wooh, S.-C., Shi, Y., Influence of phased array element size on beam steering behavior. Ultrasonics 36 (1998) 737-749. [10]Wooh, S.-C., Shi, Y., A simulation study of the beam steering characteristics for linear phased arrays. Journal of Nondestructive Evaluation, Vol. 18, No. 2, 1999. [11]Kühnicke, E., Plane arrays – Fundamental investigations for correct steering by means of sound field calculations. Wave Motion 44 (2007) 248 261. [12]Kühnicke, E., A fast algorithm for the optimization of arrays. 2005, IEEE Ultrasonics Symposium. [13]Oliver, K. A., The Design of a Unique Two Dimensional Phased Array with Low Channel Applications for Imaging Defects on a Metal Surface. 2005, The Pennsylvania State University.
51
[14]Bohenick, M. A., Blickley, E., Tittmann, B. R., and Kropf, M., Investigating a stepped ultrasonic phased array transducer for the evaluation and characterization of defects, Proceedings of SPIE, Volume 6532, Health Monitoring of Structural and Biological Systems, 2007.
52
Appendix A
Non-Technical Abstract
As lighter, stronger, and more affordable materials are available for industrial
applications, the effect of the harsh industrial environment on the material must be
determined. It must be certain that a material will not break for the time that it is
expected not to break. Sudden failure of a material is unacceptable. Current methods to
determine the effectiveness are expensive and time consuming. A method to provide
real-time results while testing a material in a harsh environment is needed.
Some harsh environments require that a minimal number of wires lead out from
the testing space to the exterior. This limitation rules out the use of complex devices with
many wires to perform the real-time evaluation of a material.
Defects on the surface of a material as well as defects inside of the material need
to be monitored. In addition changes in shape and size of a piece of material should be
recorded in real-time.
Ultrasonic transducers use high frequency sound waves to interact with a material.
The interaction can be interpreted and dimensions and defects can be examined. A linear
phased array provides the capability to examine a specimen without moving parts. Linear
phased arrays are essentially multiple ultrasonic transducers that are arranged along a
line. Electronic time-delays provide the ability to scan a beam of sound across the
specimen. Reflected sound waves are received back at the array and are used to produce
real-time images of the specimen.
In order to limit the number of wires leading into the harsh environment, a
stepped linear phased array design was used. The step provides additional distance for
53
the pressure waves to travel to separate the times that the electronics receive and record
the information. This allows the use of fewer wires while maintaining the effectiveness
of a device with more wires.
Three prototype devices were manufactured. Each has four channels; four
ultrasonic transducer elements on each step forming a linear phased array. Each prototype
has differences and these differences are compared. The ability of the prototypes to find
defects in a material is examined through bench-top experiments. Computer simulations
using Field II were also used to learn more about the design of a stepped linear phased
array.
The current prototypes cannot sufficiently monitor a specimen for defects.
Defects inside the material cannot be seen. The design of the stepped linear phased array
can be optimized to meet all the goals of the device. Further ideas have been proposed to
improve the design in the future.