PLATE THICKNESS EVUALUATION USING

MODAL ACOUSTIC EMISSION (MAE)

APPROACH

MUHAMMAD FIRDAUS BIN OTHMAN

B. ENG (HONS.) MECHANICAL

ENGINEERING

UNIVERSITI MALAYSIA PAHANG

UNIVERSITI MALAYSIA PAHANG

DECLARATION OF THESIS AND COPYRIGHT

Authors full name : MUHAMMAD FIRDAUS BIN OTHMAN

Date of Birth : 22 JAN 1992

Title : PLATE THICKNESS EVALUATION USING MODAL

ACOUSTIC EMISSION (MAE) APPROACH

Academic Session : II 2014/2015

I declare that this thesis is classified as :

CONFIDENTIAL (Contains confidential information under the Official

Secret Act 1972)*

RESTRICTED (Contains restricted information as specified by the

Organisation where research was done)*

OPENACCESS I agree that my thesis to be published as online open

access (Full text)

I acknowledge that Universiti Malaysia Pahang reserve the right as follows:

1. The Thesis is the Property of Universiti Malaysia Pahang.

2. The Library of Universiti Malaysia Pahang has the right to make copies for the

purpose of research only.

3. The Library has the right to make copies of the thesis for academic exchange.

Certified By:

_________________________ ________________________

DR. MOHD HAFIZI BN

ZOHARI

Date: 19 JUNE 2015

920122-10-5903

Date: 19 JUNE 2015

PLATE THICKNESS EVUALUATION USING MODAL ACOUSTIC EMISSION

(MAE) APPROACH

MUHAMMAD FIRDAUS BIN OTHMAN

Thesis submitted in fulfillment of the requirements

for the awards of the degree of Bachelor of Mechanical Engineering

Faculty of Mechanical Engineering

UNIVERSITI MALAYSIA PAHANG

JUNE 2015

ii

SUPERVISORS DECLARATION

I hereby declare that I have checked this project and in my opinion, this project is

adequate in terms of scope and quality for the award of the degree of Bachelor of

Mechanical Engineering.

Signature :

Name of Supervisor : DR MOHD HAFIZI BIN ZOHARI

Position : LECTURER OF MECHANICAL ENGINEERING

Date : 19 JUNE 2015

iii

STUDENTS DECLARATION

I hereby declare that the work in this project is my own expect for quotations and

summaries which have been duly acknowledged. The project has not been accepted for

any degree and is not concurrently submitted for awards of other degree.

Signature :

Name : MUHAMMAD FIRDAUS BIN OTHMAN

ID Number : MA11116

Date : 19 JUNE 2015

iv

Every challenging work needs self efforts as well as guidance of elders especially those

who were very close to our hearts. With my humble effort, I dedicated to my loving

OTHMAN BIN JANTAN and NURLEHA BINTI PANOT

(Father&Mother)

I also would like to dedicate my thesis to my friends, lecturers and my siblings. Without

their patient, support and understanding, the compilation of the work would not have

been possible.

v

AKNOWLEDGEMENTS

Alhamdulillah, thank to god that give me opportunity for me to finished the Final Year Project (FYP) that is the final step of my long journey in obtaining my Bachelor Degree in Mechanical Engineering in Universiti Malaysia Pahang (UMP). I have been accompanied and supported by many people along my way to finish the Final Year Project. It is a pleasant way for me to give an opportunity to express my appreciation for all of them.

First of all, I would like to give a million thanks to my beloved Final Year

Project supervisor; Dr. Mohd Hafizi Bin Zohari for his great kindness, patience and efforts to help during my Final Year Project progress. Thanks for also guide me along my Final Year Project. With his support, I am able to finish up project and also thesis writing with lots of ideas that been given to me. Without his guidance and encouragement, I might not able to finish my Final Year Project very well.

I would also like to thank specially to my family and fellow friends that always

be together with me, give support and also ideas for me to complete my task. They were always being by my side whenever I went through the most difficult moment and help me directly or indirectly. It was completely meaningful experience that I had gained during the collaboration with all of you rather than finish this Final Year Project alone. Thank you so much.

vi

ABSTRACT

An original technique of acoustic emission (AE), analysis based on the waveform which is contain information source such as location, orientation, size and amount of damage structure. This project was carried out using the method of Modal Acoustic Emission (MAE) to determine the characteristic properties material (thickness) in thin Steel 347 (austenitic stainless) plate. MAE used to exploits the dispersion nature of Lamb Wave which gives a variety of modes and information the characteristics of dispersion and attenuation. This approach of Modal Analysis of AE shows that those signals propagate in the thin plate by two modes: the symmetrical mode called extensional and the unsymmetrical called flexural. The objective of this project is to acquire the acoustic emission signal using single channel acoustic emission tools and determine the thickness of stainless steels by using Modal Acoustic Emission approach. This experiment was conducted by using three difference of thickness of Steel 347 (austenitic stainless) plate which is 1.963mm, 1.99mm and 2.03mm. The source was recorded by using single piezoelectric sensor and assisted by AEwin software. A distance of 200mm from sensor to source location is used as the length of the location. Then, the signal analysis is conducted by using time domain (waveform), frequency domain (Fourier fast transform) and time-frequency domain (wavelet). Determination of arrival time of AE wave is measured based on the arrival of extensional and flexural wave modes using wavelet signal analysis. As a result, the plate thickness determination of steel plate can be observed significantly.

vii

ABSTRAK

Merupakan salah satu teknik asal pancaran akustik (AE), analisis berdasarkan bentuk gelombang yang mengandungi sumber maklumat seperti lokasi, orientasi, saiz dan jumlah struktur kerosakan. Projek ini telah dijalankan dengan menggunakan kaedah Pelepasan Akustik Modal (MAE) untuk menentukan sifat-sifat bahan (ketebalan) dalam papan besi nipis 347 (tahan karat austenit). MAE digunakan untuk mengeksploitasi Lamb Wave iaitu penyebaran semula jadi gelombang yang memberikan pelbagai mod dan maklumat ciri-ciri penyebaran dan pengecilan. Pendekatan Analisis Modal AE menunjukkan bahawa menyebarkan isyarat dalam papan nipis menghasilkan dua mod: mod simetri dipanggil pemanjangan dan assimetri yang dipanggil lenturan. Objektif projek ini adalah untuk memperoleh isyarat pancaran akustik menggunakan alat pancaran akustik dan menentukan ketebalan papan besi tahan karat dengan menggunakan pendekatan Pelepasan Akustik Modal. Eksperimen ini dijalankan dengan menggunakan tiga perbezaan ketebalan papan besi 347 (tahan karat austenit) iaitu 1.963mm, 1.99mm dan 2.03mm. sumber itu direkodkan dengan menggunakan sensor piezoelektrik tunggal dan dibantu oleh perisian AEwin. Jarak 200mm dari sensor ke lokasi sumber digunakan sebagai panjang lokasi. Kemudian, analisis isyarat dijalankan dengan menggunakan domain masa (gelombang), domain frekuensi (Fast Fourier Transform) dan domain masa (Wavelet Transform). Penentuan masa ketibaan gelombang AE dikira berdasarkan kedatangan mod gelombang pemanjangan dan lenturan menggunakan analisis isyarat wavelet. Akibatnya, penentuan ketebalan papan besi dapat dikira dengan ketara.

viii

TABLE OF CONTENTS

Page

SUPERVISORS DECLARATION ii

STUDENTS DECLARATION iii

DEDICATION iv

ACKNOWLEDGEMENTS v

ABSTRACT vi

ABSTRAK vii

TABLE OF CONTENTS viii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF SYMBOLS xiv

LIST OF ABBREVIATIONS xv

CHAPTER 1 INTRODUCTION

1.1 Overview 1

1.2 Important of study 2

1.3 Objective of the project 3

1.4 Project scope 3

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction and fundamental of Acoustic Emission (AE) 4

2.1.1 Acoustic Emission Testing 5 2.1.2 Type of Acoustic Emission 6 2.1.3 Source of Acoustic Emission 7

2.2 Lamb Waves Propagation Theory 7

2.2.1 Waves Propagation Modes 8 2.2.2 Theory of Lamb Waves 10 2.2.3 Dispersion Curve, Group and Phase Velocity 12 2.2.4 Waves Propagation Effect 12

ix

2.3 Source Location Technique 13

2.3.1 Types of Localization Method 13 2.3.2 Detection of Acoustic E mission 17

2.3.2.1 Sensors 17 2.3.2.2 Preamplifiers 18 2.3.2.3 Acquisition and Storage 18 2.3.3 Acoustic Emission Application 19 2.4 Signal Processing Technique 19

2.4.1 Signal Analysis Method 20 2.4.1.1 Time Domain 21 2.4.1.2 Frequency Domain 21 2.4.1.3 Time Frequency Domain 23

CHAPTER 3 METHOLOGY

3.1 Introduction 25

3.2 Project Flow Chart 26

3.3 Test Rig and Tool Preparations 27

3.4 Instrumentation of Acoustic Emission 29

3.5 Experiment Procedure 33

CHAPTER 4 RESULT AND DISCUSSION

4.1 Introduction 35

4.1.1 Experiment Test 35

4.2 Data Collect and Calculations 36

4.2.1 Determine frequency and arrival time of wave modes 36 4.2.2 Determine Group Velocity of modes and thickness 43

4.2.3 Comparison from Experiment and Theoretical 45 4.3 Discussion 46

CHAPTER 5 CONCLUSION AND RECOMMENDATION

5.1 Conclusion 47

5.2 Recommendation of study 47

x

REFERENCES 49

APPENDICES 50

A1 Dispersion Curve Steel 347 (Austenitic Stainless) of Group 50

Velocity and Phase Velocity at thickness 1.93mm

A2 Dispersion Curve Steel 347 (Austenitic Stainless) of Group 51

Velocity and Phase Velocity at thickness 1.99mm

A3 Dispersion Curve Steel 347 (Austenitic Stainless) of Group 52

Velocity and Phase Velocity at thickness 2.03mm

B Comparison Data between Experiment and Theoretical 53

C Coding Scripts generate using Matlab software in signal 54

Processing method

D1 Grant Chart during Final Year Project 1 (FYP 1) 55

D2 Grant Chart during Final Year Project 2 (FYP 2) 56

xi

LIST OF TABLES

Table No. Title Page 2.1 Waves types in solids 10 2.2 Type of signal processing 20 4.1 TOA from plate t = 1.93mm 38 4.2 TOA from plate t = 1.99mm 40 4.3 TOA from plate t = 2.03mm 42 4.4 Velocity flexural and thickness of plate t = 1.93mm 44 4.5 Velocity flexural and thickness of plate t = 1.99mm 44 4.6 Velocity flexural and thickness of plate t = 2.03mm 45

xii

LIST OF FIGURES

Figure No. Title Page 2.1 AE Testing 5 2.2 Type of AE signals 7 2.3 Movement of waves propagate 8

2.4 Type of wave modes 9 2.5 Schematic representation of plate and coordinates 11 2.6 Linear location technique 15

2.7 Zonal location technique 16 2.8 Point location technique 16 2.9 Schematic AE setup 17 2.10 AE sensor components 18 2.11 Signal processing 20 2.13 Signal analysis using Fast Fourier Transform 22 3.1 Experiment project flow chart 26 3.2 Experiment setup 27 3.3 Specimen plate with thickness 1.93mm, 1.99mm and 2.03mm 28 3.4 AE sensors attachment on plate 29 3.5 1283 USB AE NODE (DAQ) 30 3.6 Pre-amplifier AE sensor 30 3.7 Digital Vernier Caliper 31 3.8 Grease 31 3.9 Data analysis using MATLAB software 32 3.10 AEwin software control setting 33

xiii

4.1 Signal analysis using Fast Fourier Transform (FFT) for plate 37 thickness 1.93mm

4.2 Signal analysis using Wavelet Transform (WT) for plate 38 thickness 1.93mm 4.3 Signal analysis using Fast Fourier Transform (FFT). for plate 39 thickness 1.99mm 4.4 Signal analysis using Wavelet Transform (WT) for plate 40

thickness 1.99mm 4.5 Signal analysis using Fast Fourier Transform (FFT) plate 41

thickness 2.03mm 4.6 Signal analysis using Wavelet Transform (WT) plate 42

thickness 2.03mm 4.7 Comparison from experiment with theoretical 45

xiv

LIST OF SYMBOLS

f Frequency c Phase Velocity d Thickness w Angular frequency k Wave number cp Phase velocity cl Longitudinal wave velocity ct Transverse wave velocity vg Group velocity vp Phase velocity mm Millimetre ce Extensional Velocity cf Flexural Velocity t Thickness p Density l Length from sensor to source fA Frequency of Asymmetric fS Frequency of symmetric E Youngs modulus v Poissons ratio

xv

LIST OF ABBREVIATIONS

NDT Non Destructive Technique AE Acoustic Emission MAE Modal Acoustic Emission SHM Structural Health Monitoring FYP Final Year Project UMP Universiti Malaysia Pahang SSMAL Single Sensor Modal Analysis Location PLB Pencil Lead Breaking test NI DAQ National Instrument Data Acquisition system TOA Time of Arrival FFT Fast Fourier Transform WT Wavelet Transforms CRO Cathode Ray Oscilloscope DFT Discrete Fourier Transform STFT Short-time Fourier transform FIR Finite Impulse Response IIR Infinite Impulse Response CWT Continuous Wavelet Transform PC Personal Computer

CHAPTER 1

INTRODUCTION

1.1 OVERVIEW

The study of wave propagation through elastic solid plate can be conducted to

carry out non-destructive tests (NDT) of structures. These tests can be used to detect

and identify, in various cases, both the actual elastic properties and possible geometric

imperfections included characterisation in early warning failure detections. It will give

estimate on the safety operational life of the structure, reduce downtime for inspection

and ensure continued safe performance of the plant.

Lamb waves are also known plate wave that generally seen from plate like

structure which require during free lower and upper boundary to maintain its

propagation. From this testing, a comparison of a thickness using steel 347 (austenitic

stainless) plate with experimental are required to measured the accuracy using MAE

technique. The thickness of the plate can be measured by determine of waves modes

frequency and velocity using Lamb Waves Theory. This dispersion wave depends with

depth and propagates dispersive along the velocity variation in the media of

propagation. Moreover, in general surface wave propagates in several modes, which

means that the dispersion may be represented by two modes (a) the symmetric mode

(S0) @ extensional mode and; (b) the asymmetric mode (A0) @ flexural mode. Each

wave modes gives different signal features which can identify the important information

from the AE source.

Acoustic emission is the original technique that analysis based on the waveform

analysis in order to extract the information of source. The Modal Acoustic Emission is

technique was used to exploits the dispersive nature of Lamb wave. MAE source

2

location techniques involve determination of the arrival time of significant symmetric

mode called extensional and asymmetric mode called flexural wave mode components

and computing the velocity of modes by measuring their temporal separation. An

analysis of different parameters of the acoustic emission is conducted using MAE

technique to obtain gives information of thickness of the steel plate. Single Sensor

Modal Analysis Location (SSMAL) method is used based on the dispersive

characteristic of Lamb waves that propagate in plate like structure to measure the

flexural dispersion curve using MAE technique for the steel plate. SSMAL is methods

that allow determining of the transducer-source distance with only one sensor.

Piezoelectric transducers sensor is used to converts electrical signals into mechanical

vibrations (transmit mode) and mechanical vibrations into electrical signals (receive

mode).

In this study, the analysis of guided mode AE is used to extract the information

nature of the source and distinguishing different source of AE using signal analysis of

waveform.

1.2 IMPORTANT OF STUDY

This study is important to identify early warning failure detection and estimation

of the safety operation life structure of material. The important analysis of the

difference thickness of steel plate is to determine precious measurement between

experimental and theoretical. This research is important in order to validate the accuracy

the calculation predicted lamb wave and compared from theoretical. Beside that it also

provides reliable global inspection to detect and locate growth crack. Its an important

to the industry to implementing AE technique in their working routines can give a result

which is very reliable, precise, accurate and cost time without reduce down time for

inspection & safety operation life structure. This method is also a Non Destructive

Technique (NDT) that will not cause any damage to the structure which is being

inspected and will not interfere with the procedure.

3

1.3 OBJECTIVES OF THE PROJECT

a. To acquire the Acoustic Emission (AE) signal using single channel acoustic

emission tools.

b. To determine the thickness of Steel 347 (austenitic stainless) plate by using

Modal Acoustic Emission (MAE) approach.

1.4 PROJECT SCOPE

This research study is focussing on source location by using MAE technique with

single sensor source location (SSMAL) method by identifying and discriminating

signals from the difference sources of acoustic emission which able to determine the

thickness of the plate. Laboratory experiment such as Pencil Lead Breaking test (PLB)

is conducted to for simulating sources of acoustic emission in plate surface. After the

main source of sound wave is detected by piezoelectric transducers sensor, the one

channel National Instrument Data Acquisition system (NI DAQ) was used to acquire

the AE waveform. The acquired waveforms were then stored on a computer for further

analysis using MATLAB software to provide about the nature of source.

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION & FUNDAMENTAL OF ACOUSTIC EMISSION (AE)

The Acoustic Emission one of non-destructive technique (NDT) which study

and examining the behaviour applies used for detection source location of elastic waves

that generated by a material subjected from an external stress. Acoustic refers from

the acoustic source which are generates due to elastic wave. Acoustic emission method

are technique Structural Health Monitoring (SHM) which is able to detect damage of an

engineering structure by analyse acquired data that particular diagnostics overall

structure failure involves the observation, detection, location, identification and

assessment of faults. The material that used can be a solid, liquid, gas event plasma in

phase. This analysis is successfully by directly uses piezoelectric transducers on the

surface during testing and loading the structure.

Acoustic Emission technique has several advantages which are:

a. High sensitivity and accuracy

b. Cost reduction without close the operation process

c. Real time monitoring process

d. Early and rapid detection of structure failure such flaws and cracks

e. Defective are location only critical defects provide sustainable Acoustic

Emission sources

f. Easy and Minimize the time for examination plant which no need for stop

running production and the structure surface

5

2.1.1 Acoustic Emission (AE) Testing

Figure 2.1: AE Testing

Source: Murawin,2009

AE testing is a method used to examining and analysis the behaviour of material

deforming under stress. This testing refers to generation of transient elastic waves

during the rapid release of energy from localized sources within a structure material due

a sudden redistribution of stress. Once the structure is applied an external stimulus

(force), the source localized. When a structure is subjected to an external stimulus, the

source localized is determined by release a group of energy as stress waves, which

propagate along the surface of the plate. By detecting sources as small event as large as

brittle crack advance by the sensor, AE technology warns of danger, informs about

structural health and watches over costly and critical processes.

One of macroscopic source of AE is the crack propagation on the material.

Cracks as well as other discontinuities that occur during material centralize from

external stress to generate the elastic wave propagate. Cracks will followed along when

a higher distribution of potential energy, which is released from a minor surface area in

pattern of stress waves. Stress waves which are produce are occurring due to the rate of

yield stress does not delivery along with differences area of the medium. The stress

waves generated are elastic waves mostly but inelastic waves can be generated also

when stresses exceed yield limit. Acoustic emission is occurring from generation of the

elastic waves due to significant changes in the stress at the certain region on the

6

material. This significant change is because by the application of certain external

stimulus to the material during testing. The stress change must be rapid enough to

transmit some energy to the surrounding material and dissipates as an elastic wave.

The possibility distance from origin point of the source location can be

determine as soon as the elastic waves being generated and travel throughout along the

material. The elastic waves contain information on the variations and characteristic of

the acoustic path. This is happened due to structure of material of macroscopic and

microscopic generate energy which is attenuated by scattering and spreading on its

geometry. Therefore, as the elastic waves are generated from the source and travels via

acoustic path cause the formation of wave frequency contents. Hence, gives the

important information of the source location based on the elastic energy and the time of

arrival which is detected from the sensor on the structure of the material.

2.1.2 Type of Acoustic Emission (AE)

In Acoustic emission test, two mode of distinct qualitative type of acoustic

emission behaviour is divided which is burst and continuous. The main difference

between the two modes is temporal behaviour of the material.

Burst is a type of emission that involves an individual events occurring in a

material that result in discrete acoustic emission signals. During the burst mode AE,

energy is released in burst or pulses as the material undergoes plastic deformation. Burst

mode AE can be high amplitudes.

Continuous is a type of emission that related to time overlapping and successive

emission events from one or several sources that results in sustained signals. It will

occur when the changes in material are continuous, such as corrosion process.

Detection, ability to distinguish and analyze signals resulting from both emission types

is important for many acoustic emission applications.

7

Figure 2.2: Type of AE signals

Source: Achenbach, (1998)

2.1.3 Source of Acoustic Emission (AE)

Source of AE can be classified into two methods which are from acoustic wave

and thermal diffusion. However, a big difference between two methods which is

acoustic wave allows transfer energy from cooperative movement of many atoms

whereas in thermal diffusion an individual atom involves direct energy transfer.

Transmitting energy through a material is one of the non-electromagnetic methods on

Acoustic waves. However, the basic general acoustic waves from water and air is

known as the sound waves.

2.2 FUNDAMENTAL LAMB WAVE PROPAGATION

Propagation can be defined as movement through a medium. However, wave

can be refers as a disturbance (sound, radio waves, light) that moves through a medium

(air, water, vacuum). Lamb wave only propagate in solid plate which exits in thin

walled structure. Waves of different types propagate at different velocities and with

8

different oscillation directions. Moreover, passing through a medium, waves undergo

multiple changes due to attenuation, dispersion, diffraction, scattering, reflection from

boundaries, interaction with reflections and other.

According to the early studies of wave propagation in plate from Horace Lamb

(1917) and Rayleigh (1945), in term of lamb wave refers based on the characteristics

from waves propagate in plate. Lamb waves can be generated in plate with free

boundaries with an infinite number of modes which is within the layer. The infinite

number of modes exists for a specific plate thickness and acoustic frequency which are

identified by their respective phase velocities. During the waves are travelling in plate,

particles plate waves movement diverge to form two difference mode which is

asymmetric and anti-symmetric mode. The formation of plate modes has their own

phase and group velocities due to the stress and distribution of this waves particle

displacement around the plate thickness.

Figure2.3: Movement of waves propagate

Source: Horace Lamb (1917) and Rayleigh (1945)

9

Focusing on through medium of solids, the way of particle to oscillate from the

sound waves can propagate has four principle modes which is consists shear waves,

longitudinal waves, surface waves and plate waves. Basically, the two modes waves

which is shear and longitudinal waves are widely used in source location testing. Figure

2.3 shown an illustrated wave modes propagate

2.2.1 Wave Propagation Modes

Sound travels by the compression and refraction of air molecules at the direction

in air phase. However, during the solids phase, molecules can support vibrations in

other directions with a number of different types of sound waves are possible. The

propagation of waves is often described in terms of what are called wave modes

Sound travels by the compression and refraction of air molecules at the direction in air

phase. However, during the solids phase, molecules can support vibrations in other

directions with a number of different types of sound waves are possible. The

propagation of waves is often described in terms of what are called wave modes.

Figure 2.4: Type of wave modes

Source: Kuttruff, (1991)

10

Table 2.1: List type of wave modes possible in solids

Wave Types in Solids Particle Vibrations Longitudinal Parallel to wave direction Transverse (Shear) Perpendicular to wave direction Surface - Rayleigh Elliptical orbit - symmetrical mode Plate Wave - Lamb Component perpendicular to surface (extensional wave) Plate Wave - Love Parallel to plane layer, perpendicular to wave direction Stoneley(Leaky Rayleigh Waves) Wave guided along interface Sezawa Anti-symmetric mode

Based on Lambs wave theory, there are two common waves modes is produced

which is called asymmetrical and symmetric wave modes. Symmetrical Lamb waves

move in a symmetrical fashion about the median plane of the plate. This is sometimes

called the extensional mode because the wave is stretching and compressing the plate

in the wave motion direction. Wave motion in the symmetrical mode is most efficiently

produced when the exciting force is parallel to the plate. The asymmetrical Lamb wave

mode is often called the flexural mode because a large portion of the motion moves in

a normal direction to the plate, and a little motion occurs in the direction parallel to the

plate. In this mode, the body of the plate bends as the two surfaces move in the same

direction.

2.2.2 Theory of Lamb Wave

Pure Lamb waves are guided dispersive waves propagating in an elastic

isotropic plate with Traction free boundaries, Figure 1 show Lamb waves are formed by

interference of multiple reflections and mode conversion of longitudinal waves (P-

waves) and shear waves (S-waves) at the free surfaces of the plate .

11

Figure 2.5: Schematic representation of plate and coordinates

Source: Viktorov, (1967)

Lamb waves with particle displacement in both the x- and y-direction actually

represents a group of wave types including the bending wave, the Rayleigh wave, and

the quasi-longitudinal wave. Lamb (1917) derived the dispersion equation that can

handle the transitions between these types of waves. Harmonic wave propagation in the

x-direction is only possible for those combinations of frequency (f) and phase velocity

(c) corresponding to standing waves in the thickness y-direction. These waves must

obey the dispersion equation (Lamb, 1917), from which dispersion curves can be

calculated.

(

)

(

)=

() For symmetric modes (2.1)

(

)

(

)=

()

For Asymmetric modes (2.2)

Where given =

; =

and =

The parameter d, w, k, cp, cl, and ct are the diameter thickness, angular frequency, wave

number, phase velocity, longitudinal wave velocity and transverse wave velocity

respectively.

12

Equation 1 represents the dispersion relation for pure Lamb waves with particle

motion in both the x- and the y-direction. Although, the equation was derived long ago

and looks quite simple, calculating roots for dispersion curve generation can be

challenging, especially in some regions of wave number and frequency (Graff, 1975).

Therefore further insight into the practical dispersion curve calculation is given next.

The dispersion relation is a transcendental function and it not straightforward to

calculate a k value at any given frequency. A root searching technique has to be used to

find the right wave numbers at any given frequency. In general roots are complex, but if

only propagating waves are studied the imaginary component can be ignored

(Achenbach, 1998).

2.2.3 Dispersion Curve, Group & Phase Velocity

Dispersion describes the effect of dispersion in a medium on the properties of a

wave travelling within that medium. A dispersion relation relates the wavelength or

wave number of a wave to its frequency. From this relation the phase velocity and

group velocity of the wave have convenient expressions which then determine the

refractive indeed of the medium.

Group velocity can be defined as velocity from the propagation in a certain

group of waves which have a same package of frequency. Group velocity represented

as Vg=

. However, phase velocity is the velocity which occurs during the

propagation through media. The phase velocity can be defined by rewriting

equationVp =

.

2.2.4 Waves Propagation Effect

The following phenomena take place as AE waves propagate along the structure:

a. Attenuation: The gradual decrease in AE amplitude due to energy loss

mechanisms, from dispersion, diffraction or scattering.

13

b. Dispersion: A phenomenon caused by the frequency dependence of speed for

waves. Sound waves are composed of different frequencies hence the speed of

the wave differs for different frequency spectrums.

c. Diffraction: The spreading or bending of waves passing through an aperture or

around the edge of a barrier.

d. Scattering: The dispersion, deflection of waves encountering a discontinuity in

the material such as holes, sharp edges and cracks inclusions.

2.3 SOURCE LOCATION TECHNIQUE

All source localization techniques mainly depend on determination of the Time

of Arrival (TOA) and acoustic velocity for the AE signal at different arranged sensors.

There are two ways to measure TOA: traditional AE methods and alternative methods.

Traditional AE methods assume threshold value and consider the arrival time is at first

hit crossing the threshold. The alternative method, single sensor modal analysis

location, depends on MAE theory. This method is based on of lamb theory. As lamb

wave propagating in the plate has dispersive characteristics time deference between

arrival of the fundamental modes S0 and A0 at the sensor could lead to the source

location. Hence, signal processing such as Wavelet Transform (WT) and Fast Fourier

Transform (FFT) is required to determine the source location.

2.3.1 Type of Localization Method

Modal Analysis Technique

One of AE method which is occur based identification and asnalysis of AE wave

modes in thin plate using one sensor. The sensor can identify the two wave modes

(difference frequency) and velocities of the two modes known. Then, the time of arrival

between the two wave modes are measured to measure the length beetween a sensor and

source. This AE technique, broad to be used universal which is able to monitor the

major structure and wide area during testing.

14

Determination of AE source can be measured based on lamb wave modes which

is flexural and extensional mode at a sensor. Signal analysis search as Fast Fourier

Transform (FFT) which enable to identify peak frequency modes and Continuous

Wavelet Transform (CWT) that enable to determine the Tine of Arrival (TOA) was

conducted to calculated the desired distance between sensor and source AE

Multi-Channel Source Location Techniques

Determinations of source location are often used on acoustic emission (AE)

through an inspection since there is no damage through this AE analysis. During

conducting testing, the geometry properties can be obtained from this analysis. Multi-

channel source location technique is AE technique which is capable to record a single

AE hit using multiple channel/sensors during carried out testing. Therefore, AE hits is

recorded from the channel/sensors, the location of source can be obtain by determine

the velocity of the waves modes propagate on the material and differences on times of

arrival of hits. This variable can be measured by data acquisition system and computer

hardware. Hence, properly separate the sensor in relevant helps possibility to inspect the

entire structure.

Since the source location technique is based on the AE waves that travel on a

material at a constant velocity. Determination of wave modes can be obtained even

though there are sentences various effect may affected on expectation on velocity of AE

waves such as multiple mode and reflections also the accuracy on the technique.

Linear Location Technique

Linear location technique generally used to finding the location acoustic

emission source (AE) which is used time difference on linear structure of material likes

tubes, rods or pipes. This technique relate on the measured of time difference on arrival

of times from AE waves that recorded using two sensors. One of the commonly used

computed-source location techniques is the linear location principle shown to the right.

This technique is used to measure distance of source location in between midpoint on

multiple sensors. Since the source is located in midpoint on multiple sensors, the time

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arrival difference at the two sensors is calculate. To measure the length on source

location on midpoint of the multiple sensors, the arrival time difference is multiplication

with the wave velocity. Determination on the source location along either left or right is

based on hit from sensor first records obtain

Figure 2.6: Linear location technique

Source: Murawin, (2004)

Zonal Location Technique

Zone location is based on the principle that the sensor with the highest amplitude

or energy output will be closest to the source. Basically, this technique used to

determine the waves to a specific region or zone where sensors are separated to keep

away or attenuation effect from the material at multiple sensors.

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Figure 2.7: Zonal location technique

Source: Murawin, (2004)

Point Location

In order for point location to be justified, signals must be detected in a minimum

number of sensors: two for linear, three for planar, four for volumetric. Accurate arrival

times must also be available. Arrival times are often found by using peak amplitude or

the first threshold crossing. The velocity of wave propagation and exact position of the

sensors are necessary criteria as well. Equations can then be derived using sensor array

geometry or more complex algebra to locate more specific point of intercept.

Figure 2.8: Point location technique

Source: Murawin, (2004)

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2.3.2 Detection of Acoustic Emission

Detection of acoustic emission testing setup can be performed with the several

stationary laboratory setting and portable instrument. In briefly, the setup consist a

sensor, filter, preamplifier, hardware software, measurement tools and display (personal

computer and oscilloscope).

Figure 2.9: Schematic AE setup

Source: Kaphle,(2012)

2.3.2.1 Sensors

A sensor (transducer) defines as measure device that detect or records certain

type of input response from the acoustic wave that gives electrical signal during

stimulated. Several device from electromagnetic device that used to detect the generated

signal is piezoelectric devices, magnetostrictive devices, phonograph pickups, laser

interferometers and capacitive microphones which is use to identify the surface

displacement on each sample. This sensor is used to convert mechanical movement

(analog signal) into electrical signal (digital signal. Generally, this sensor requires

detecting typical frequency with range 30 kHz to 1 MHz. The exact relationship

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between the characteristics of the wave and those of the signal will depend on both the

sensor and the wave.

Figure 2.10: AE sensor components

Source: Mathew J. Yedlin, (2008)

2.3.2.2 Pre-amplifiers

Preamplifier is used to boosts the voltage and obtain gain and cable is to

increase the strength of the input signal. To minimize interference, a preamplifier is

placed close to the transducer; in fact, many transducers today are equipped with

integrated preamplifiers. Amplifying and frequency filtering are the two components of

signal conditioning.Then, elimination of low and high frequencies are obtained by the

filter which travels eventually to a computer for further analysis. Further filtering and

amplification are necessary based on noise conditions.

2.3.2.3 Acquisition and storage

Data acquisition is a device that performs analog-to-digital conversion of

signals, filtration, hits (useful signals) detection and it parameters evaluation, data

analysis and charting. This device obtains the measurement circuit by release the digital

pulse once the threshold voltage is fulfilled. The beginning of a hit shows significantly

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the first pulse until process has stopped. Most of acquisition design uses a hit-driven

architecture which is efficiently to calculate all detected signal and record digital

descriptions.

2.3.3 Acoustic Emission (AE) Application

Application of the acoustic emission as a diagnostic method, structural integrity

assessment tool is possible when a qualitative or quantitative relationship between

detected acoustic emission and material condition is established for a specific material

and structure. There are two major approaches to achieve this goal:

a. Determining experimentally a characteristic set (fingerprints) of acoustic

emission parameters and their characteristics that uniquely describe a material

condition, fracture stage, flaw type and etc.

b. Establishing a theoretical relationship between acoustic emission parameters and

their characteristics and material properties, fracture mechanics parameters and

etc.

2.4 SIGNAL PROCESSING TECHNIQUE

This techniques able to improve better understanding on information collected

that include fundamental theory, applications, algorithms, and implementations of

processing or transferring information contained in many different physical,

symbolic, or abstract formats broadly designated as signals and uses mathematical,

statistical, computational, heuristic, and linguistic representations, formalisms, and

techniques for representation, modelling, analysis, synthesis, discovery, recovery,

sensing, acquisition, extraction, learning, security, or forensics. Hence, signal

processing is able to excerpt all the information from the source location that are

recorded through the waveform during testing is conducted.

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Figure 2.11: Signal processing

Source: Murawin, (2007)

2.4.1 Signal Analysis Method

Signal analysis is a method which is used to extract the information and features

of from the recorded signal. This process very important to observe of AE signal

behaviour and determine the characteristic of the material structure.

Table 2.2: Type of signal processing method

Domain Parameter Information of the source Time domain variable Rate Rate of damage occurring

Peak amplitude Intensity of the source Relatives arrival times Source location Duration or count Energy of source Waveform Structure of source Energy Energy of source - damage type

Frequency domain variable

Frequency spectrum Nature of source

Time-Frequency domain variable

Spectrogram Energy distribution of source through time

Wavelet transform Energy distribution of source through time