master thesis - ::::: smart structures & systems in...

석 사 학 위 논 문

Master Thesis

접착성 결합 구조물의 검사 및 영상화를 위한

비접촉식 무기저 기법

A Reference Free and Non-Contact Method for Detecting and

Imaging Damages in Adhesive Bonded Structures

2017

Timotius Yonathan Sunarsa (孙迪羑)

한 국 과 학 기 술 원

Korea Advanced Institute of Science and Technology

석 사 학 위 논 문



2017

Timotius Yonathan Sunarsa

한 국 과 학 기 술 원

건설 및 환경공학과




위 논문은 한국과학기술원 석사학위논문으로

학위논문 심사위원회의 심사를 통과하였음

2017 년 06 월 20 일

심사위원장 손 훈 (인 )

심 사 위 원 정 형 조 (인 )

심 사 위 원 홍 정 욱 (인 )

A Reference Free and Non-contact Method for Detecting and

Imaging Damages in Adhesive Bonded Structures


Advisor: Hoon Sohn

A dissertation/thesis submitted to the faculty of

Korea Advanced Institute of Science and Technology in

partial fulfillment of the requirements for the degree of

Master of Science in Civil and Environmental Engineering

Daejeon, Korea

June 20, 2017

Approved by

Hoon Sohn

Professor of Civil and Environmental Engineering

The study was conducted in accordance with Code of Research Ethics1).

1) Declaration of Ethical Conduct in Research: I, as a graduate student of Korea Advanced Institute of Science and

Technology, hereby declare that I have not committed any act that may damage the credibility of my research. This

includes, but is not limited to, falsification, thesis written by someone else, distortion of research findings, and

plagiarism. I confirm that my dissertation contains honest conclusions based on my own careful research under the

guidance of my advisor.

초 록

접착제를 이용한 접착 구조물은 항공, 해양 산업과 자동차 산업에 널리 이용되고 있다.

조물 이음새에 발생하는 결함은 매우 복합적인 요소들로 인하여 발생하기 때문에,

효율적인 비용과 정확성을 가진 손상 감지 기술이 현 산업에 필요하다. 이 논문에서는

접착 이음새에 발생하는 손상을 감지하는 실험을 했고 이론적으로 분석했다. 이 손상

감지 기술은 공기 결합 트랜스듀서(ACT)를 이용해 완전 비접촉식으로 행해졌다. 공기

결합 트랜스듀서의 가진과 측정 위치 그리고 각도를 바꿔가며 시편을 통과한 신호와

시편에 반사되어 나온 신호를 얻었고 그 신호들을 분석해 보았다. 위에서 얻은 데이터를

분석하여 손상에 따른 측정 시간(time of flight)의 차이를 얻어냈고, 이를 비교 분석하여

시편의 손상 위치를 찾고 시각화하였다. 제안된 방법은 목표 구조물의 손상되지 않은

상태의 데이터를 참조 데이터로 필요하지 않고 작동할 수 있다. 알루미늄판 접착 시편과

트리플렉스 접착부의 접착 손상 부위를 측정하여 이 방법의 정확성을 확인하였다.

제안된 방법은 간단한 메커니즘과 ACT 의 적은 전력 소모라는 장점이 있기 때문에

비파괴 시험에 적용 가능성이 매우 크다.

키워드 접착 구조물, 공기결합트랜스듀서, 접착손상, 접착결함, 비파괴검사, reference-free,

in-situ

DCE

20154274

Timotius Yonathan Sunarsa. 접착성 결합 구조물의 검사 및 영상

화를 위한 비접촉식 무기저 기법. 건설 및 환경공학과. 2017년.

43+x 쪽. 지도교수: 손훈. (영문 논문)

Timotius Yonathan Sunarsa. A Reference Free and Non-Contact Method

for Detecting and Imaging Damages in Adhesive Bonded Structures.

Department of Civil and Environmental Engineering. 2017. 43+x pages.

Advisor: Hoon Sohn. (Text in English)

Abstract

Adhesive bonded structures have been used widely in aerospace, automobile, and marine

industries. Due to complex nature of the failure mechanisms of the joints, cost-effective and reliable

damage detection is crucial for the related industries. This thesis presents an experimental and

analytical candidate method for in-situ damage detection of adhesive-bonded joints. The fully non-

contact method utilizes air-coupled ultrasonic transducers (ACT). Various configurations of transmitter

and receiver including through transmission, pitch-catch scanning and probe holder angles are

attempted and the obtained results are analyzed. The method examines time of flight of the ultrasonic

waves over a spatial comparison to assess and visualize the presence of the critical damage and provide

the possibility of damage localization. The proposed method works without relying on reference data

obtained from the pristine condition of the target structure. Aluminum bonded plate and triplex

adhesive bond with debonding weakened bonding are used to confirm the accuracy of the method. The

proposed method is potentially suitable for in-situ non-destructive testing applications due to the

simplicity of the mechanism and little power consumption of the ACTs.

Keywords Adhesive bonded structure, air-coupled transducer, debonding, weakened bonding, non-

destructive testing, reference-free, in-situ.

i

Contents

Contents .................................................................................................................................................. i

List of Figures ........................................................................................................................................ ii

Chapter 1. Introduction ........................................................................................................................ 1

1.1 Motivation ..................................................................................................................................... 1

1.2 Research Objective and Uniqueness ............................................................................................. 4

1.3 Thesis Organization ...................................................................................................................... 4

Chapter 2. Air-Coupled Ultrasonic Transducer Scannning ............................................................. 5

2.1. Overview ...................................................................................................................................... 5

2.2 Wavelet Transform for TOF Extraction ........................................................................................ 7

2.3 Imaging Technique Algorithm ...................................................................................................... 8

Chapter 3. Experimental Validation: Adhesive Bonded Aluminum Plates with Debonding....... 11

3.1 Specimens Description ................................................................................................................ 11

3.1 Experimental Setup ..................................................................................................................... 12

3.3 Test Results ................................................................................................................................. 14

Chapter 4. Experimental Validation: Adhesive Bonded Triplex Layers with Weakened Bonding

.............................................................................................................................................................. 19

4.1 Specimen Description ................................................................................................................. 19

4.2 Experimental Setup ..................................................................................................................... 20

4.3 Test Results ................................................................................................................................. 23

Chapter 5. Conclusions and Future Works ...................................................................................... 31

5.1. Conclusion ................................................................................................................................. 31

5.2. Future Works ............................................................................................................................. 32

Bibliography ........................................................................................................................................ 33

Acknowledgement ............................................................................................................................... 35

CURRICULUM VITAE ..................................................................................................................... 36

ii

List of Figures

Chapter 1

Figure 1.1 The application of high performance adhesives in (a) aircraft [5] and (b) automobile. ........ 1

Figure 1.2 Three categories of damages typically found in adhesive joints; (a) inclusions, (b)

debonding, and (c) weakened bonding [4]. ........................................................................... 2

Figure 1.3 X-ray imaging technique for detecting: (a) debonding damages and (b) weakened bonding

in adhesive bonded triplex layers of a secondary barrier of a LNG cargo tank. The images

clearly shown the capability of this detection technique. Debonding damages has been

successfully detected while weakened bonding is unsuccessful. .......................................... 3

Chapter 2

Figure 2.1 Schematic picture of different ACT configurations for excitation and sensing (a) through

transmission and (b) pitch-catch. .......................................................................................... 6

Figure 2.2 Schematic illustration of the use of Snell’s law to determine the incident and sensing angles

of ACTs for pitch-catch configuration .................................................................................. 6

Figure 2.3 Illustration of the scanning direction for both through transmission and pitch-catch

configurations. By calculating a damage index for each spatial point through examining

the sensitive feature, damage visualization of the target structure can be achieved. ............ 9

Figure 2.4 Calculation of damage index for each spatial point in the scanning area [30]. ................... 10

Chapter 3

Figure 3.1 Material geometrical information: (a). Top view of the adhesive bond aluminum plate, (b).

Side view of the target specimen, (c). Debonding information of adhesive bonded

aluminum plate 1, (d). Debonding information of adhesive bonded aluminum plate 2...... 11

Figure 3.2 The appliance of adhesive in the aluminum plates in creating debonding damage; (a)

Adhesive bonded aluminum plate 1; (b) adhesive bonded aluminum plate 2..................... 12

Figure 3.3. (a) A schematic illustration of the through transmission experiment configuration on

adhesive bonded aluminum plates; (b) Bottom view of the sound blocker, the excitation

area denote a hole for transmitting the ultrasonic wave to the target structure. .................. 13

Figure 3.4 Schematic illustration of the experiment experimental arrangement. The generation of input

signal is shown with the red line, while the acquisition of the measured signal is shown by

the blue line. ........................................................................................................................ 14

Figure 3.5 Schematic illustration of the scanning area of adhesive bonded aluminum plates 1 (top) and

adhesive bonded aluminum plates 2 (bottom). ................................................................... 14

Figure 3.6 (a) Time domain signals of two representative signals; intact (blue solid line) and

weakened bonding (red solid line) (b) CWT of the time domain signal into time-frequency

domain; (left) time-frequency domain of intact signal; (right) time-frequency domain of

weakened bonding signal. (c)The extracted time of flight from the highest energy in the

CWT analysis. ..................................................................................................................... 16

Figure 3.7 (Top) C-Scan result of the adhesive bonded aluminum plate 1; (Bottom) Debonding

visualization of the adhesive bonded aluminum plate 1. .................................................... 17

Figure 3.8. (Top) C-Scan result of the adhesive bonded aluminum plate 2; (Bottom) Debonding

visualization of the adhesive bonded aluminum plate 2. .................................................... 18

Chapter 4

Figure 4.1 (a) The schematic illustration of the adhesive bonded triplex layers; (b) the detailed

geometrical information of the adhesive bonded triplex layer; .......................................... 20

Figure 4.2 (a) A Schematic illustration of pitch-catch configuration for detecting weakened bonding in

the adhesive bonded aluminum layers; (b) the scanning area for through transmission

configuration (30 mm × 180 mm) and pitch-catch configuration (40 mm × 180 mm). ...... 21

Figure 4.3 Schematic illustration of the experimental arrangement for measurement of the guide wave

iii

velocity to determine the incident angle using PZT. ........................................................... 22

Figure 4.4 (a) Time domain signals of transmitted wave, the wave with higher frequency travel with

higher speed; (b) Time of flight of the transmitted wave extracted from the central

frequency of the wave package.; (c) Wave velocity of the transmitted wave based on

dividing the distance between the transducer and receiver by the time of flight (d) Incident

angles for ultrasonic wave generation based on Snell’s law. .............................................. 23


weakened bonding (red solid line), the signal obtained from the through transmission

configuration. (b) CWT of the time domain signal into time-frequency domain; (left) time-

frequency domain of intact signal; (right) time-frequency domain of weakened bonding

signal (c) the extracted time of flight from the highest energy in the CWT analysis. ......... 24

Figure 4.6 (a) Weakened bonding visualization of adhesive bonded triplex layers with through

transmission configuration extracted by CWT analysis (b) weakened bonding visualization

of adhesive bonded triplex layers with through transmission configuration extracted by

Hilbert transform. ................................................................................................................ 25


weakened bonding (red solid line) (b) CWT of the time domain signal into time-frequency

domain; (left) time-frequency domain of intact signal; (right) time-frequency domain of

weakened bonding signal. (c) The extracted time of flight from the highest energy in the

CWT analysis. ..................................................................................................................... 27

Figure 4.8 (a) Weakened bonding visualization of adhesive bonded triplex layers with pitch-catch

configuration extracted by CWT analysis. (b) Weakened bonding visualization of adhesive

bonded triplex layers with pitch-catch configuration extracted by Hilbert transform. The

unusual high damage index values denote the location of the weakened bonding. The white

dashed line shows the location of the weakened bonding provided by Hyundai Heavy

Industries®. ......................................................................................................................... 28

- 1 -

Chapter 1. Introduction

The motivation of this thesis is explained in this chapter. The objective and

uniqueness of this study also explained. Lastly, the organization of this thesis is explained.

1.1 Motivation

To fulfill the demand for lighter and more resistant materials in aerospace, automobile, and

marine industries adhesive bonded structures have been widely adopted [1-3]. Figure 1.1 shows the

application of adhesives in aerospace and automobile industries. The utilization of adhesive bonded

structures become popular recently due to their ability in providing more uniform load transfer which

results in better damage tolerance and more importantly, fatigue life increase comparing to

conventional joining methods such as; mechanical fastening [4].

(a) (b)

Figure 1.1 The application of high performance adhesives in (a) aircraft [5] and (b) automobile.

However, problems such as manufacturing flaws or impact damage can seriously affect the

mechanical properties of the bonded structures [1]. Inclusion, debonding, and weakened adhesion are

three critical damage types that commonly exist in adhesive bonds due to the aforementioned reasons

[4], shown in Figure 1.2. Inclusion, defined as the existence of foreign material in the adhesive bond.

While debonding is the inclusion of air in adhesive bond. Debonding makes the bonded structures

subject to non-uniform load that can trigger failure of the structure [4]. Lastly, weakened bonding is

due to improper adhesion between the adhesive and the adherend or impact. Weakened bonding

causes less strong bonding condition between adhesives and adhered and leads to change of the local

- 2 -

mechanical properties of the structure [4]. The presence of weakened bonding is considered serious

threat for the bonded structure and incase of aircraft additional riveting is suggested to strengthen the

weakened area while the riveting itself adds to the weight and creates stress concentration areas on the

rivet holes [6]. Currently, reliable non-destructive methods to inspect adhesive bonded structures and

ensure the quality of the structure is still subject of intensive research. Additionally, a reliable

practical inspection method can lead to more efficient structures with lighter weight and cost

reduction [7].

Figure 1.2 Three categories of damages typically found in adhesive joints; (a) inclusions, (b) debonding, and (c)

weakened bonding [4].

Several damage detection methods have been developed and considered to address the

challenges regarding the inspection of the bonded structures over the past two decades. These

methods include; neutron imaging radiography [8], thermography [9-11] and ultrasonic [12-14]. The

methods such as X-ray and thermography have been confirmed successful for other types of damage

but seemed not very sensitive to the presence of weakened bonding [7]. Figure 1.3 shows the result of

X-ray imaging technique for debonding and weakened bonding in adhesive bonded structures.

- 3 -

(a) (b)

Figure 1.3 X-ray imaging technique for detecting: (a) debonding damages and (b) weakened bonding in adhesive

bonded triplex layers of a secondary barrier of a LNG cargo tank. The images clearly shown the capability of

this detection technique. Debonding damages has been successfully detected while weakened bonding is

unsuccessful.

Ultrasonic methods have been promising to detect different types of damages as well as

weakened bonding [15, 16]. However, the drawbacks of the methods such as requirement of

conductive materials only [13] or attachment of sensor and actuator on the surface of the structure to

be inspected [12, 17] limited the possibility of the real industrial applications. The laser ‘shockwave’

technique has also been adopted to detect weakened bonding in composite structures but the

shockwave can cause damage to the structure or the bonding [7, 18].

Amongst the ultrasonic approaches, the non-contact methods such as; using air-coupled

transducer (ACT) [19] and laser-based have shown more potential for in-situ applications. Ultrasonic

method using ACT has shown a promising track record to be utilized in non-destructive evaluation

and inspection applications [20, 21]. In a number of recent studies, ACT systems were applied in

damage detection and imaging for a number of structural components and materials [19, 20, 22].

This study presents a fully non-contact, baseline-free candidate method for in-situ damage

detection of adhesive-bonded structures using ACTs. Also different configurations of actuator and

sensor including through transmission, pitch-catch scanning and probe holder angles are presented

and examined. The method examines time of flight of the ultrasonic waves over a spatial comparison

to assess and visualize the presence of the critical damage and provide the possibility of damage

localization. Aluminum bonded plate and triplex adhesive bond with debonding and weakened

adhesion damage are used to confirm the accuracy of the method. The results confirmed the

effectiveness of the presented method to detect and localize various critical structural damages for

adhesively bonded structural components in aerospace, automobile and marine industries. The

significance of the developed system lies in; fully a non-contact method for various damages and

- 4 -

material, damage size estimation and localization, and the potential of the proposed technique to be

performed for in-situ application due to simplicity of operation.

This study initially introduces the methodology of the proposed method considering various

configuration of actuator and sensor also provides the study regarding the angular setup of the ACTs.

This is followed by the experimental setup, specimen description and results and discussion to

demonstrate the detection of weakened bonding and debonding damages for different specimens.

Finally, a conclusion is presented, summarizing the outcomes of the research study and considerations

for real industrial applications.

1.2 Research Objective and Uniqueness

This study aims to develop a debonding and weakened bonding detection on adhesive

bonded structures by using air-coupled ultrasonic transducer scanning system. The time of flight of

the propagated ultrasonic wave is chosen as the damage sensitive feature. A wavelet transform is

presented to extract time of flight of ultrasonic wave generated by the system. An imaging method is

developed by comparing extracted damage indexes in spatial domain. Then, the developed system can

finally visualize the damages exist in adhesive bonded structures.

The key achievement of this study is the detection of weakened bonding in adhesive bonded

triplex layers. No previous NDT method can visualize the weakened bonding exist in adhesive triplex

layers of a secondary barrier of a LNG Cargo Tank [4]. Therefore, a future rapid in-situ inspection

system can be developed prior to this study.

1.3 Thesis Organization

This thesis is organized with five chapters and summarizes as follows: In Chapter 2, the

theoretical background of linear ultrasonic and wave propagation is described. In Chapter 3, The

methodology of the fully non-contact ultrasonic scanning system is explained. Chapter 4 describes

experimental investigation for the proposed scanning system in laboratory scale. Finally, the summary

and future works are provided in Chapter 5.

- 5 -

Chapter 2. Air-Coupled Ultrasonic Transducer Scannning

This chapter explains the methodology adopted to obtain the experimental results for various

setup configurations. First, an overview of the method is provided then the detailed description of the

imaging algorithm used to visualize the damages is described.

2.1. Overview

Conventional ultrasonic techniques are used in the inspection of metals, fibre-reinforced

polymers[23], and adhesive bonded structures [24]. These techniques use water or coupling gel as

coupling medium. However, use of liquid may not be suitable for in-situ inspection since some

materials or structures might be contaminated or damaged due to the appliance of liquid as the

coupling. Therefore, the interest of using air as the coupling medium has been increasing.

Air-coupled transducer (ACT) is an ultrasonic transducer that utilizes air as the coupling

agent. One common ACT that is used widely is Piezoelectric based ACT [25]. It is made of a

piezoelectric disk and a thin low impedance layer. The disk function is to convert electrical voltage

into mechanical deformation. The mechanical deformation will generate desired ultrasonic wave. The

impedance layer function is to match the acoustic impedance of the generated ultrasonic wave in the

air. It enhances the efficiency of the generated ultrasonic waves in the air. The generation of

ultrasonic wave in ACT start by generating electrical voltage that will be converted into mechanical

deformation. The measurement process happen in the other way around, the generated ultrasonic

waves causing mechanical deformation of the impedance layer and the system will measure the

mechanical deformation by converting it into electrical voltage.

Generally, two ACTs are used for observing target structure condition. One for ultrasonic

wave generation and the other for sensing. These two ACTs can perform with several configurations.

Through transmission and pitch-catch are the most common configurations that has been reported in

the literature [20, 21, 26].

Through transmission configuration is illustrated in Figure 3.1(a). The transmitter and the

receiver are set on the front and back sides of the target structure. The ultrasonic wave generated from

the transmitter travels through the target specimen medium and measured by the receiver. In through

transmission configuration, two-sided access of the target structure is necessary to perform test.

Meanwhile in pitch-catch configuration, only single side access of the target structure is required

(Figure 3.1(b)).

- 6 -

(a) (b)

Figure 2.1 Schematic picture of different ACT configurations for excitation and sensing (a) through

transmission and (b) pitch-catch.

The generated ultrasonic waves in pitch-configuration are in the form of guided waves. The

guided waves travel in the thin plates have dispersive and multimodal character [21, 26]. Hence, to

excite a desired wave mode, the incident angle of ACT has to be set to a specific value by Snell’s law

[20]. Figure 3.2. gives the illustration of the use of Snell’s law to determine the incident and sensing

angles of ACTs in pitch-catch configuration.

Figure 2.2 Schematic illustration of the use of Snell’s law to determine the incident and sensing angles of ACTs for

pitch-catch configuration

Snell’s law describes the relationship between the angles of incidence and refraction when

the ultrasonic waves pass through a boundary between two different media. This concept is explained

in Equation 2.1.

Transmitter

Receiver

Spatial

Point

Receiver Transmitter

Spatial Point


𝜙 𝜙

𝑐𝑚𝑒𝑑𝑖𝑢𝑚 𝑐𝑎𝑖𝑟 𝑐𝑎𝑖𝑟

- 7 -

sin 𝜙

sin 𝜃=

𝐶𝑎𝑖𝑟

𝐶𝑚𝑒𝑑𝑖𝑢𝑚 (2.1)

where 𝜙 and 𝜃 denote the angles of incidence and refraction, and 𝐶𝑎𝑖𝑟 and 𝐶𝑚𝑒𝑑𝑖𝑎

denote the velocity of the ultrasonic waves in air and in the target medium, respectively. As the

ultrasonic waves propagate along the surface, 𝜃 = 90°, Equation 2.1 can be presented as Equation

2.2. Therefore the incident angle 𝜙 can be calculated through Equation 2.3.

sin 𝜙 =𝐶𝑎𝑖𝑟


𝜙 = sin−1𝐶𝑎𝑖𝑟


Since the ultrasonic waves travel in plate at different modes corresponding to different wave

velocities, choosing the appropriate measurement angle enables the ability to selectively measure a

distinct mode of the ultrasonic waves. If the information of the thickness and material properties are

known, the measurement of angle can be easily calculated using the theoretical dispersion curves.

Otherwise, the velocity of the interest mode can be measured experimentally.

2.2 Wavelet Transform for TOF Extraction

To evaluate the debonding and weakened bonding in the target structure, a unique feature

need to be extracted from the measured signal. As discussed in Chapter 3 and Chapter 4, two different

target specimens and two different experiment configurations are used in this study. Since the

thickness to the width ratio of all target structures are less than 0.1, both adhesive bonded aluminum

plates and adhesive bonded triplex layers can be categorized as thin plate structures [27].

For thin multilayered plate structure, the ultrasonic wave generated with through

transmission configuration will be reflected many times [28], thus it is challenging to extract the time

of flight of the propagated wave. Meanwhile, the guided waves measured with pitch-catch

configuration has dispersive characteristic [29]. Therefore, to estimate time of flight of the measured

signal, wavelet transform is adopted [30].

The wavelet transform maps a time function 𝑓(𝑡) into a two-dimensional function 𝜓𝑠,𝜏(𝑡),

Equation 2.4. defines the mapping:

𝜓𝑠,𝜏(𝑡) = 1

√𝑠𝜓(

𝑡 − 𝜏

𝑠) (2.4)

- 8 -

where 𝜓𝑠,𝜏(𝑡) is a family of wavelets created from a mother wavelet 𝜓(𝑡). The complex Morlet

wavelet, obtained as the product of a complex exponential by a Gaussian window, is used as the

mother wavelet 𝜓(𝑡). 𝑠 and 𝜏 denote the scale and translation parameters, respectively. 𝑠 scales

𝜓(𝑡) by compressing or stretching it and 𝜏 translate 𝜓(𝑡) along the time axis. For a given time

function 𝑓(𝑡), the continuous wavelet transform (CWT) (Equation 2.5.) at the scale 𝑠 and position 𝜏

is the convolution of the time function 𝑓(𝑡) with the wavelet function 𝜓𝑠,𝜏(𝑡) [27]:

𝐶𝑊𝑇(𝑠, 𝜏) = ∫ 𝑓(𝑡) ∙ 𝜓𝑠,𝜏(𝑡)+∞

−∞

𝑑𝑡 (2.5)

where 𝐶𝑊𝑇(𝑠, 𝜏) is a wavelet coefficient of 𝑓(𝑡) for the given 𝑠 and 𝜏 values, and it

depends on the values of 𝑓(𝑡) in the time-frequency domain where the energy of 𝜓𝑠,𝜏 is

concentrated. From this wavelet analysis, the time position and the scale of the maximum amplitude

wavelet coefficient can be obtained. Therefore, the time of flight and the group velocity of the

measured guided wave are estimated from the acquired signal. The time of flight is extracted at

different frequencies according to the given input signal. Equation 2.6. defines the mathematical

calculation of the feature:

Feature = ∑ 𝑇𝑂𝐹𝑓

𝑓

(2.6)

where TOF𝑓 is the time of flight of the guided wave at frequency 𝑓.

2.3 Imaging Technique Algorithm

A synchronized scanning strategy are developed to localize damage on the inspected target

structure. Figure 2.3 illustrates the scanning process that is worked for both through transmission and

pitch-catch configurations. A spatial point is defined as the sensing location in through transmission

configuration and as an inspected location in the middle of the transmitter and the receiver in pitch-

catch configuration, as defined in Figure 2.1. The spatial point is moved along the scanning sequences

in the scanning area. “Scan gap” is defined as the distance between the spatial points and it indicates

the scanning density in the scanning area. Based on the target structure and its damage type, the

distance between transmitter and receiver and scan gap can be adjusted to optimize the scanning

procedure.

To detect damage using the synchronized scanning strategy, features need to be extracted

- 9 -

from the acquired signal. Depend on the type of the target structure and type of damage, different

features can be extracted. As an example, Giurgiutiu et al. [31] measured the variation of the signal

amplitude, phase, velocity, and mode conversion of ultrasonic waves. These features are based on the

linear behavior of ultrasonic wave propagation. Nonlinear features are also used to detect micro

damage such as fatigue crack, fiber debonding and delamination since it provides more sensitivity to

those types of damage [32].

Figure 2.3 Illustration of the scanning direction for both through transmission and pitch-catch configurations. By

calculating a damage index for each spatial point through examining the sensitive feature, damage visualization of the

target structure can be achieved.

With the extracted features from the ultrasonic wave response, the damage index (DI) for

each spatial point can be calculated as in Equation 2.7.

𝐷𝐼𝑖 = ∑ 𝑊𝑖±𝑗 × 𝑓𝑒𝑎𝑡𝑢𝑟𝑒𝑖±𝑗, 𝑗 = 0,1,2,3, …

𝑗

(2.7)

𝑓𝑒𝑎𝑡𝑢𝑟𝑒𝑖±𝑗 denotes the feature value corresponding to the 𝑖𝑡ℎ spatial point or its spatially

adjacent 𝑖 ± 𝑗𝑡ℎ points, as shown in Figure 2.4. In the pitch-catch configuration, 𝑊𝑖±𝑗 is the weight

value for its feature and it is defined based on the overlapping degree between the wave propagation

distances related to the adjacent points. Equation 2.8 describes the determination of the weight value

𝑊𝑖±𝑗.

𝑊𝑖±𝑗 = {(1 −

𝑗 × 𝑑𝑠

0.5 × 𝑑𝑤)

2

𝑖𝑓 (𝑗 × 𝑑𝑠) < (0.5 × 𝑑𝑤)

0 𝑖𝑓 (𝑗 × 𝑑𝑠) ≥ (0.5 × 𝑑𝑤)

(2.8)

Where; 𝑑𝑤and 𝑑𝑠 are the distances of the wave propagation and the scan gap respectively.

Meanwhile in through transmission configuration, 𝑊𝑖±𝑗 is set to be 0, since there will be no

overlapping in the scanning process. The DI values calculated with Equation 2.8 can inspect the

structural condition of the target scanning area. When a damage exists in the scanning area, the DIs on

𝑋𝑔𝑎𝑝

𝑌 𝑔𝑎

𝑝

Target structure

: Spatial Point

Scanning area

- 10 -

the damage region will be different from the DIs in the intact region.

This imaging technique does not rely on any baseline data obtained from the pristine

condition of the target structure. The inspected area is marked through a spatial comparison from the

extracted DIs. Therefore, this imaging method is insensitive to environmental and operational

variation, such as temperature and loading changes. This is a significant advantage for in-situ

application.

Figure 2.4 Calculation of damage index for each spatial point in the scanning area [30].

𝑖 𝑖 + 1 𝑖 + 2 𝑖 + 𝑗 𝑖 − 1 𝑖 − 2 𝑖 − 𝑗

Wave propagation distance

Feature 𝑖 − 𝑗

Feature 𝑖 − 2

Feature 𝑖 − 1

Feature 𝑖

Feature 𝑖 + 1

Feature 𝑖 + 2

Feature 𝑖 − 𝑗

: Excitation point : Sensing point

- 11 -

Chapter 3. Experimental Validation: Adhesive Bonded Aluminum Plates

with Debonding

This chapter, presents the experimental validation of the proposed method in detecting

weakened bonding in adhesive bonded triplex layers. First, the target specimens are described. Then,

the experimental setup is explained. Lastly, the results of the proposed technique in detecting

weakened bonding in adhesive bonded aluminum plates are presented.

3.1 Specimens Description

Two adhesive bonded aluminum plates, with a length of 200 mm, a width of 20 mm, and

thickness of 2.1 mm, are bonded together using an adhesive layer. The aluminum is plate is made

from AlMg3 and the adhesive used for joining the two aluminum plates is composed of Betamate.

Figure 3.1 illustrates the detailed geometrical information about the target specimen. The adhesive

was pressed by having distance for the chosen debonding width. Adhesive bonded aluminum plate 1

has a line debonding damage in the middle of the specimen, while adhesive bonded aluminum plate 2

has a random-shaped debonding damage in the middle of the specimen.

Figure 3.1 Material geometrical information: (a). Top view of the adhesive bond aluminum plate, (b). Side view of

the target specimen, (c). Debonding information of adhesive bonded aluminum plate 1, (d). Debonding

information of adhesive bonded aluminum plate 2.

- 12 -

The debonding damage is introduced in the adhesive bond of two aluminum plates by not

applying the adhesive before the two aluminum plates were joined. Figure 3.2. shows the

methodology of how the debonding damages are introduced for each target specimen.

(a) (b)

Figure 3.2 The appliance of adhesive in the aluminum plates in creating debonding damage; (a) Adhesive bonded

aluminum plate 1; (b) adhesive bonded aluminum plate 2.

3.1 Experimental Setup

Two ACTs are used to inspect the adhesive bonded aluminum plates. The transmitter

(NCG200-D25-P76, The Ultran Group) and the receiver (MicroAcoustic BAT-1) are set based on the

through transmission configuration. The transmitter has a focused circular active transmitting area

with a diameter of 25 mm and a central frequency of 200 kHz with a focal distance of 76 mm. The

receiver has the capability to measure ultrasonic waves with a frequency band of 40 kHz to 2.25 MHz

with a 10 mm circular active sensing area. The transmitting and sensing distances are fixed at 60 mm

and 30 mm, respectively. They are fixed in a line, perpendicular to the target structure. Figure 3.3(a)

illustrates the details of the experiment configuration. A sound blocker, as described in Figure 3.3(b),

is used to prevent leaking waves and also functioned as a support of the specimen with a rectangular

opening for transmitting the ultrasonic waves.

- 13 -

(a) (b)

Figure 3.3. (a) A schematic illustration of the through transmission experiment configuration on adhesive bonded

aluminum plates; (b) Bottom view of the sound blocker, the excitation area denote a hole for transmitting the

ultrasonic wave to the target structure.

In this experiment, a 70 V, 5-cycle toneburst signal is transmitted from the AWG with a

frequency centered in 200 kHz. The source signal was generated by means of a 33220A Agilent

Arbitrary Waveform Generator and amplified up to 100 Vpp using a EPA-104 PIEZO Linear

Amplifier. This generated signals are monitored by LeCroy waveRunner 44Xi 400 MHz oscilloscope.

The signal received by the receiver is first amplified by a Q-Amp transimpedance preamplifier

(MicroAcoustic Instruments Inc.). All parts of the equipment are synchronized by a control system

consists of personal computer and junction box by PSV-400-3D Polytech. Figure 4.5 shows a

schematic representation of the experiment configurations.

Sensing distance Target structure

Sound blocker Transmitting distance

Receiver

Transmitter

Sound blocker

Excitation

Area

- 14 -

Figure 3.4 Schematic illustration of the experiment experimental arrangement. The generation of input signal is

shown with the red line, while the acquisition of the measured signal is shown by the blue line.

The scanning area is set to be 30 mm × 140 mm rectangular area. A total of 21 (3 × 7)

inspection points are assigned with a scan gap (center to center) equal to 10 mm in y-direction and 20

mm in x-direction within the scanning area as shown in Figure 3.5. To improve the signal to noise

ratio, the responses from the transmitted waves are measured 200 times and averaged in the time

domain during the scanning.

Figure 3.5 Schematic illustration of the scanning area of adhesive bonded aluminum plates 1 (top) and adhesive

bonded aluminum plates 2 (bottom).

3.3 Test Results

Arbitrary Waveform

Generator

(Agilent 33220A)

Power Amplifier

(EPA-104 Piezo System)

Oscilloscope

(waveRunner 44Xi)

Data Acquisition System

(PSV-400-3D Polytech

Data management system)

Experiment

Configuration

Synchronization

Scanning

area

Scanning area

30 mm

30 mm

140 mm

140

mm

- 15 -

Two representative signals obtained from the intact and debonding area are displayed in

Figure 4.10(a). The corresponding wavelet transform results are shown in Figure 4.10(b). and the time

of flight of the measured wave at each frequency is extracted in the time-frequency domain. In

Figure4.10(c). the time of flight of the measured signal is extracted at 150 kHz to 200 kHz and the

variation of the time of flight due the presence of debonding damage is presented. The extracted time

of flight shows that the ultrasonic wave travels through the debonding damage is coming slower than

the ultrasonic wave travels through the intact area. The wave travelling in the air with a velocity equal

to 340 𝑚/𝑠, where the wave travelling in the adhesive material with a velocity equal to 2610 𝑚/𝑠.

Since debonding is the inclusion of air in the adhesive as described in Chapter 1, it is reasonable to

have the the wave travels faster in the intact region.

- 16 -

Figure 3.6 (a) Time domain signals of two representative signals; intact (blue solid line) and weakened bonding

(red solid line) (b) CWT of the time domain signal into time-frequency domain; (left) time-frequency domain of

intact signal; (right) time-frequency domain of weakened bonding signal. (c)The extracted time of flight from the

highest energy in the CWT analysis.

Am

pli

tud

e (V

)

Time (s)

Intact Debonding

(a).

(b).

Frequency (Hz)

Tim

e o

f F

ligh

t(s)

Intact Weakened bonding

(c).

- 17 -

Figure 3.7 and Figure 3.8 show the visualization results obtained from the two adhesive

bonded aluminum plates. Here, all the DI values are normalized with respect to the highest DI values

of all DIs in each specimen. C-Scan images are provided in each figure to give a comparison of the

performed debonding detection. The visualization in Figure 3.7 and Figure 3.8 show the result

detected by the ACT scanning technique.

Figure 3.7 (Top) C-Scan result of the adhesive bonded aluminum plate 1; (Bottom) Debonding visualization of the

adhesive bonded aluminum plate 1.

Scan Area

Y-ax

is (

mm

)

0 -1

0

10

X-axis (mm) 0 -20 20 40 60 -40 -60

20 10

- 18 -

Figure 3.8. (Top) C-Scan result of the adhesive bonded aluminum plate 2; (Bottom) Debonding visualization of

the adhesive bonded aluminum plate 2.

Y-ax

is (

mm

)

0 -1

0

10

X-axis (mm) 0 -20 20 40 60 -40 -60

20 10

- 19 -

Chapter 4. Experimental Validation: Adhesive Bonded Triplex Layers with

Weakened Bonding

This section presents the experimental validation of the proposed method in detecting

weakened bonding in adhesive bonded triplex layers. First, the target specimen is described. Then, the

experimental setup is explained. Lastly, the results of the proposed technique in detecting weakened

bonding in adhesive bonded triplex layers is presented.

4.1 Specimen Description

Adhesive bonded triplex layers are part of secondary barrier of a LNG carrier cargo tank.

The adhesive bonded triplex layers consist of three sub-layers: (1) a flexible secondary barrier (FSB),

(2) an epoxy/urethane-based bonding layer, and (3) a resin-based rigid secondary barrier (RSB).

Figure 4.1(a) shows the adhesive bonded triplex layers.

Both FSB and RSB are composed of 0.7 mm thick sheet of aluminum with a layer of glass

cloth on either side attached using a resin system. This resin system later determines the rigid nature

of the triplex material [4]. The weakened bonding is created by introducing a very thin vinyl between

the adhesive and adherend interface. The thickness of the vinyl is much thinner than the thickness of

the adhesive layer. The adhesive bonded triplex layers validated in this experiment is shown in Figure

4.1(b) and 4.1(c).

(a) (b)

Aluminum sheet

Adhesive bonding

0.7

mm

Glass cloth

Aluminum sheet 0.7

mm

0.2- 1

Glass cloth

Glass cloth

Glass cloth

FSB

R

SB

Bo

nd

ing

450 mm

Weakened

bonding

damage

14

0 m

m

160 mm

Specimen 1

- 20 -

(c)

Figure 4.1 (a) The schematic illustration of the adhesive bonded triplex layers; (b) the detailed geometrical

information of the adhesive bonded triplex layers 1; (c) the detailed geometrical information of the adhesive

bonded triplex layers 2

4.2 Experimental Setup

Both through transmission and pitch-catch configuration are used to provide comparisons

between the obtained results since to the best of the authors’ knowledge, there is no effective non-

destructive method to detect weakened bonding type in this structure as discussed in Chapter 1. Pitch-

catch configuration has an advantage of its single sided access which can make this technique

applicable for in-situ inspection.

In the through transmission configuration, NCG200-S38 is used as the transmitter. This

transducer has an unfocused square active transmitting area with a size of 38 mm × 38 mm and a

central frequency of 200 kHz with a focal distance of 76 mm. Unlike the experiment performed in

Chapter 3, unfocused transducer is chosen due to the need for higher energy generation on the target

specimen. Besides the transmitter, all the parts of the equipment are identical with that for debonding

detection on adhesive bonded aluminum plates explained in the Chapter 3.

A 100 V, 5-cycle toneburst signal is transmitted from the AWG with a frequency centered in

255 kHz. The transmitting and sensing distance are fixed at 40 mm and 30 mm, respectively. A sound

blocker, made from a triplex board paper, is used to prevent leaking wave and also functioned as a

support of the specimen with a rectangular opening for transmitting the ultrasonic waves. The

scanning area is set to be a 30 mm × 180 mm rectangular area with 30 (3 × 10) inspection points

assigned within this scanning area, as shown in Figure 4.2(b). The active sensing area is 10 mm wide

with a scan gap (center to center) equal to 10 mm in the y-direction and 20 mm in the x-direction. To

450 mm

Weakened bonding

damage

14

0 m

m

160 mm

Specimen 2

230 mm

Weakened

bonding

damage

- 21 -

improve the signal to noise ratio, the responses from the transmitted waves were measured 1000 times

and averaged in the time domain during the scanning.

(a) (b)

(c)

Figure 4.2 (a) A Schematic illustration of pitch-catch configuration for detecting weakened bonding in the

adhesive bonded aluminum layers; (b) the scanning area for through transmission configuration (30 mm × 180

mm) and pitch-catch configuration (40 mm × 180 mm) of adhesive bonded triplex layers 1.; (c) the scanning area

for through transmission configuration (30 mm × 180 mm) and pitch-catch configuration (40 mm × 180 mm) of

adhesive bonded triplex layers 2.

For the pitch-catch configuration, all parts of the equipment are identical with that for

debonding detection on adhesive bonded aluminum plates. A 100 V 5-cycle toneburst signal is

transmitted from the AWG with a frequency centered in 200 kHz. Figure 4.2(a). gives a schematic

illustration of the experiment setup. The distance between transmitter and receiver is fixed to be as

close as possible to obtain the highest signal to noise ratio [33]. It is set at 60 mm, with a lift-off

distance equal to 40 mm for both transmitter and receiver. A sound blocker is put in the middle of the

transducer to prevent direct reflection and leaking wave from the transmitter. The blocker is made

from a triplex board paper identical with the sound blocker used in the experiment explained in

section 3.2.1. The scanning area is set to be a 40 mm × 180 mm rectangular area, as shown in Figure

4.2(b) and 4.2(c). A total of 40 (4 × 10) spatial points are observed within the scanning area. The scan

gap (center to center) is set to be 10 mm in the y-direction and 20 mm in the x-direction. To improve

40 mm

Target structure

Sound blocker

Wave propagation distance: 60 mm


40 mm 180 mm

Scanning area

30

/40

mm

Weakened bonding

180 mm

Scanning area

30

/40

mm

Weakened bonding

- 22 -

the signal to noise ratio, the responses from the transmitting waves were measured 1000 times and

averaged in the time domain during the scanning.

The incident and sensing angle of ACT are determined by conducting an experiment in the

target structure using piezoelectric transducers (PZT) with identical aforementioned parts of the

equipment. A 20 V, 5-cycle toneburst signal is transmitted with frequency input varies from 155 kHz

to 255 kHz with 10 kHz interval. The schematic of the experiment configuration is shown in Figure

4.3.

Figure 4.3 Schematic illustration of the experimental arrangement for measurement of the guide wave

velocity to determine the incident angle using PZT.

Figure 4.4. shows the measurement angle extracted from the arrival time of the wave

packet. The incident and angle lies between 5˚ to 8˚.

(a) (b)

Transmitter Receiver

Wave propagation distance: 100 mm

Adhesive bonded triplex layers

PZT PZT

Time of Flight

- 23 -

(c) (d)

Figure 4.4 (a) Time domain signals of transmitted wave, the wave with higher frequency travel with higher

speed; (b) Time of flight of the transmitted wave extracted from the central frequency of the wave package.; (c)

Wave velocity of the transmitted wave based on dividing the distance between the transducer and receiver by the

time of flight (d) Incident angles for ultrasonic wave generation based on Snell’s law.

4.3 Test Results

4.3.1 Visualization of Weakened Bonding Type with Through Transmission

Configuration

Two representative signals obtained from the intact and the weakened bonding areas are

displayed in Figure 4.5(a). The corresponding wavelet transform results are shown in Figure 4.5(b).

and the time of flight of the measured wave at each frequency is extracted. In Figure 4.5(c). the time

of flight of the measured signal is extracted at 150 kHz to 350 kHz and the variation of the time of

flight due the presence of weakened bonding is presented. The ultrasonic wave signal measured in the

weakened bonding area is slower than the ultrasonic wave signal measured in the intact area. The

delay in the weakened bonding area happened due to the existence of the thin vinyl inserted in the

adhesive layer in creating the weakened bonding. The presence of the thin vinyl layer adds an extra

layer to the structure and makes the ultrasonic wave transmitted in the weakened bonding region

travel through the extra layer. Different material properties cause the waves travel with different

velocities.

- 24 -


(red solid line), the signal obtained from the through transmission configuration. (b) CWT of the time domain

signal into time-frequency domain; (left) time-frequency domain of intact signal; (right) time-frequency domain of

weakened bonding signal (c) the extracted time of flight from the highest energy in the CWT analysis.

Time (s)

Am

pli

tud

e (V

)


(a).

Am

plit

ude (V)

(b).

Frequency (Hz)

Tim

e of

Fli

ght

(s)


(c).

- 25 -

Figure 4.6. shows the visualized inspected area of the adhesive bonded triplex layers. Figure

4.6(a). shows the visualization result of the extracted time of flight using CWT analysis while Figure

4.6(b). shows the visualization result of the extracted time of flight using Hilbert transform. The

visualization results show that the visualization through CWT analysis is more accurate. The

weakened bonding location is marked with the white dashed line. The unusual high DI values denote

the location of the weakened bonding. The white dashed line shows the location of the weakened

bonding provided by the manufacturer (Hyundai Heavy Industries®).

Figure 4.6 (a) Weakened bonding visualization of adhesive bonded triplex layers with through transmission

configuration extracted by CWT analysis (b) weakened bonding visualization of adhesive bonded triplex layers

with through transmission configuration extracted by Hilbert transform.

Y-a

xis

(m

m)

0 1

0

X-axis (mm)

0 40 -60 80 -40 60 20 -20 -80

15

-15

-10

Weakened bonding area

(a)

.

Y-a

xis

(m

m)

0 1

0

X-axis (mm)

0 40 -60 80 -40 60 20 -20 -80

15

-15

-10


(b)

.

- 26 -

4.3.2 Visualization of Weakened bonding Type with Pitch-catch Configuration

Two representative signals obtained from the intact and the weakened bonding areas are

displayed in Figure 4.7(a). The CWT results are shown in Figure 4.7(b). and the time of flight of the

measured wave at each frequency is extracted in the time-frequency domain.

In Figure 4.7(c). the time of flight of the measured signal is extracted at 150 kHz to 350 kHz and

the variation of the time of flight due the presence of weakened bonding is presented. The guided

waves propagated in the weakened bonding area is slower than the guided waves measured in the

intact area. Figure 4.7(b) shows the presence of several guided waves modes. Here, the time of flight

from each spatial point is extracted from the wave mode with the highest energy. The guided wave

measured from intact spatial points travel faster than the guided wave measured from spatial points in

the weakened bonding region.

Figure 4.8 presents the visualized inspected area of the adhesive bonded triplex layers.

Figure 4.8 (top) shows the visualization result of the extracted time of flight using CWT analysis

while Figure 4.8 (bottom) shows the visualization result of the extracted time of flight using Hilbert

transform. The weakened bonding location is marked with the white dashed line. This damage

location information is provided from the manufacturer, Hyundai Heavy Industries®.

- 27 -


(red solid line) (b) CWT of the time domain signal into time-frequency domain; (left) time-frequency domain of

intact signal; (right) time-frequency domain of weakened bonding signal. (c) The extracted time of flight from the

highest energy in the CWT analysis.

Time (s)

Am

pli

tud

e (V

)


(a).

(b).

Frequency (Hz)

Tim

e o

f F

light(

s)


(c).

- 28 -

Figure 4.8 (a) Weakened bonding visualization of adhesive bonded triplex layers 1 with pitch-catch

configuration extracted by CWT analysis. (b) Weakened bonding visualization of adhesive bonded triplex layers

with pitch-catch configuration extracted by Hilbert transform. The unusual high damage index values denote

the location of the weakened bonding. The white dashed line shows the location of the weakened bonding

provided by Hyundai Heavy Industries®.

Same as the through transmission, in pitch-catch configuration also, the high DI values signify

the location of the weakened bonding. Both CWT and Hilbert transform show promising results to

clearly detect weakened bonding. In general, with both configuration, the proposed method not only

successfully detect the damage but also clearly with good accuracy visualize the location of the

damage.

The outcomes of this experimental investigation on damage detection and imaging with ACT and

proposed methodology generally confirm the results of the previous research works to attempt for

characterization and inspection of adhesive bonds [6, 8, 15]. Specifically, that the feature responses

Y-a

xis

(m

m)

0 1

0

X-axis (mm)

0 40 -60 80 -40 60 20 -20 -80

20

-20

-10


(a).

Y-a

xis

(m

m)

0 1

0

X-axis (mm)

0 40 -60 80 -40 60 20 -20 -80

20

-20

-10


(b).

- 29 -

from various types of damage can be acquired reliably ultrasonic methods and the post-processing

stage provides more insight into feature extraction of the measured signals. Proper interpretation of

obtained data can lead to a clearer understanding of complex adhesive bonding structures. Many

recent research works have indicated that the industry demands for more reliable non-destructive

methods to inspect adhesive bonded structures and ensure the quality of the structure[8, 9, 12, 34].

Since a reliable practical inspection method can lead to huge cost reduction and improved safety of

the structures. The present research work is partially motivated by these expectations and represents a

study on experimental and analytical candidate method for in-situ inspection and damage detection of

adhesive-bonded structures.

- 30 -

Figure 4.9 (a) Weakened bonding visualization of adhesive bonded triplex layers 2 with pitch-catch

configuration extracted by CWT analysis. (b) Weakened bonding visualization of adhesive bonded triplex layers

with pitch-catch configuration extracted by Hilbert transform. The unusual high damage index values denote

the location of the weakened bonding. The white dashed line shows the location of the weakened bonding

provided by Hyundai Heavy Industries®.

(a). -60 0 60 120

X-axis (mm)

140

0

-10

10

Y-a

xis

(m

m)

20

-20

Weakened

bonding area

0

-10

10

Y-a

xis

(m

m)

20

-20


(b). -120 -60 0 60 120

X-axis (mm)

- 31 -

Chapter 5. Conclusions and Future Works

5.1. Conclusion

The research topic discussed in this thesis is mainly focused on the development of ACT for

examining adhesive bonding structures. A fully non-contact experimental and analytical damage

detection candidate method for in-situ inspection and damage detection of adhesive-bonded structures

is presented using ACT. A synchronized scanning strategy for damage visualization is developed for

both through transmission and pitch-catch configurations and prior incident angle calculation is

inspected through exciting and sensing guided waves with PZTs. The extracted time of flight by using

CWT analysis is defined as a damage sensitive feature for both debonding and weakened bonding

detection. The results confirm that the developed method is successful in detection and localization of

localizing (1) debonding damage in adhesive bonded aluminum plates and (2) weakened bonding in

adhesive bonded triplex layers. In particular, the flexibility of the proposed method to operate with

various configurations and also the fact that the method does not rely on reference data to assess the

presence of the critical damage which makes it independent of the variation of the environmental and

operational conditions make it a suitable choice for in-situ applications

- 32 -

5.2. Future Works

1) Further study on material properties.

To obtain the best result, a further study of the material properties need to be

performed. One example in this study is the adhesive bonded triplex layers. The complexity

of the material makes it challenging to understand the wave propagation behavior on the

material.

2) Algorithm development for assessing the state of health of structure.

In real application, a knowledge of the level of the adhesive bond condition is

important. For example, by knowing how severe is the debonding or weakened bonding

occurred in the structure, a critical decision can be taken.

3) Verification of the sensitivity of the proposed method to other specimens with similar

damage type.

The application of the proposed technique can be extended to other type of adhesive

bonded structures other than those structures used in this study. A future verification of other

materials might give us a better knowledge of the effectiveness of the proposed method on

examining adhesive bond condition.

4) Feasibility study on the potential of the proposed method to be integrated into in-situ

inspection system.

So far, a lab-scale verification on the proposed technique have been conducted. In

the real structure, different environmental conditions are exist at each specific time.

Therefore, a future test with different environmental condition might improve the detection

rate of the proposed method. A mobile inspection system also need to be developed.

- 33 -

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Acknowledgement

I would like to thank all the people who contributed in some way to the work described in

this thesis. First and foremost, I thank my academic advisor, Professor Hoon Sohn, for letting me into

his group. During my tenure, he contributed to a rewarding graduate school experience by giving me

intellectual freedom in my work, giving me the opportunities to involve in several projects, attend

conference, being a teaching assistant in his course, and demanding a high quality of work in all my

endeavors. Additionally, I would like to thank my committee members Professor Jung-Wuk Hong and

Professor Hyung-Jo Jung for their interest in my work.

The result described in this thesis was accomplished nonetheless with the help and support of

fellow labmates. Peipei Liu, Byeongjin Park, Jeon Ikgeun, Pouria Aryan, and I worked together on

several different phases of the whole research projects, and without their efforts, my job would

undoubtedly been more difficult. I would also like to thank all of my labmates, Yongtak Kim,

Seongheum Yoon, Gunhee Koo, Jun Lee, Kiyoung Kim, Ji Min Kim, Nazirah Ab. Wahab, Suyeong

Yang, Soonkyu Hwang, Jae-Mook Choi, Santhakumar Sampath, Jun-Yeong Chung, Jiho Park, Jinho

Jang, Gayoung Han, Qian Wang, Min-koo Kim, Byeongju Song, Jin-yeol Yang, Sangmin Lee,,

Seunghwan Jung and Hyung-Jin Lim. I believe, without the presence of all of you guys, the laboratory

would not be the same. Thank you for everything we have experience together, all the good things and

also the bad things, thank you for helping me whenever I need your help.

Finally, I would like to acknowledge friends and family who supported me during my time

here. First and foremost I would like to thank Mom, Dand, Ruthie, and Livia for their constant love

and support. Putu, Steven, Harry, and Eliz made my time here at KAIST a lot more fun. I would as

well thank all my Indonesian Fellow here in KAIST and also my church friends.

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CURRICULUM VITAE

Timotius Yonathan SUNARSA

M.Sc

Department of Civil and Environmental Engineering

Korea Advanced Institute of Science and Technology (KAIST)

291 Daehakro, Yuseong-gu, Daejeon, 34141, Republic of Korea

Phone: (82)+42-350-3665, Fax: (82)+42-350-3610

Email: [email protected]

RESEARCH INTERESTS

Structural Health Monitoring & Damage Detection, Non-destructive Testing, Sensing Technologies,

Laser Ultrasonics, Air-Coupled Transducer, Linear Ultrasonics, Statistical Pattern Recognition,

Probability & Statistical Analysis, Image Processing.

EDUCATION

2015 - 2017 M.S. Candidate, Civil and Environmental Engineering, Korea Advanced Institute

of Science and Technology (KAIST), Republic of Korea

2010 - 2014 B.S., Civil Engineering, Institut Teknologi Bandung (ITB), Indonesia

THESIS

1. Timotius Yonathan Sunarsa, “A Referece free and Non-contact Method for detecting and

imaging defects in adhesive bonded structures,” M.S Thesis, Department of Civil and

Environmental Engineering, Korea Advanced Institute of Science and Technology (KAIST), June,

2017.

2. Timotius Yonathan Sunarsa, “Vibration control of steel truss bridge under high-speed train

loading with semiactive tuned mass damper system,” B.S. Thesis, Department of Civil

Engineering, Institut Teknologi Bandung (ITB), October, 2014.

mailto:[email protected]

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JOURNAL PUBLICATIONS

* The corresponding author is underlined

1. Timotius Yonathan Sunarsa, Pouria Aryan, Ikgeun Jeon, Byeongjin Park, Peipei Liu, Hoon

Sohn, “A reference‐free and non‐contact method for detection and imaging of damages in

adhesive-bonded joints,” in preparation for a special issue in Materials, 2017.

CONFERENCE PROCEEDINGS

* The corresponding author is underlined

1. Timotius Yonathan Sunarsa, Byeongjin Park, Peipei Liu and Hoon Sohn, “Noncontact

debonding identification in adhesive using guided waves,” The 2016 World Congress on

Advances in Civil, Endbironmental, and Materials Research, Jeju, Sout Korea, August 28 –

September 1, 2016.

2. Peipei Liu, Timotius Yonathan Sunarsa, Hoon Sohn, “Damage visualization using synchronized

noncontact laser ultrasonic scanning,” SPIE Smart Structues/NDE, Las Vegas, US, March 20-24,

2016.

PARTICIPATED PROJECTS

1. Development of a noncontact monitoring system for online quality monitoring of industrial

products: Co-funded by EUROSTARS2 Program by the European Union Horizon 2020

Framework Programme and Korea Ministry of Trade, Industry and Energy (Funded: 750,000,000

KRW out of total 2,976,0000 KRW for 11/01/15 to 10/31/18).

EDUCATIONAL EXPERIENCES

Fall, 2015 Teaching Assistant, CE202, “Structural Mechanics,” KAIST.

master thesis - ::::: smart structures & systems in...

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