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

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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].

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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.

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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

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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].

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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.

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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.

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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.

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[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.

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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.