工業革命後，在基礎工業建設中分布各式各樣的運輸管線，在公共安全的考慮下這些管線之安全乃極為重要。超聲導波技術可提供快速且長距離之管線腐蝕檢測，尤以扭矩模態導波T(0,1)更是被廣泛地應用於檢測被隱藏之管線區段，由於超聲導波技術可評估不易接近之管線使得此技術在管線非破壞檢測技術排行上穩居高位。然而，當欲檢測發生於管線焊接支撐座或柏油包覆底下之腐蝕時往往會出現問題，焊接支撐座所產生的大訊號將遮蔽住腐蝕的訊號而柏油包覆的存在則會大大地衰減腐蝕的訊號。因此，本研究藉由實驗與模擬方法來討論焊接支撐座與柏油包覆對導波的影響，並藉由連續小波轉換技術擷取出被遮蔽的腐蝕訊號。本研究設計五式實驗管線來進行實驗的研究。其中，實驗管線1為作為研究焊接支撐反射回波所用；實驗管線2則用來討論柏油包覆對導波之衰減效應；在量得實驗管線3、4與5上各種特徵之反射回波訊號後乃加以連續小波轉換處理。再者，使用線性立方體元素建構管線焊接支撐與管線包覆柏油等二種有限元素模型。由研究結果可得知，焊接支撐座對導波的影響包括波式轉換、訊號延遲出現、訊號拖曳與隨頻率變化之特性。當T(0,1)模態入射至焊接支撐座時，由於管壁為周向擾動所以將產生A0模態在支撐座裡繼續傳遞，而焊接支撐座所產生的反射回波乃由直接回波、延遲回波與拖曳回波所組成。在實驗與模擬結果中可知，當支撐座內A0模態在邊界反射後產生建設性干涉時，管線上支撐座之反射回波則會產生2組極大值，其頻率分別發生在20-22 kHz （頻率區間為0.0）與32-34 kHz（頻率區間為4.0）。針對柏油包覆而言，當分析實驗管線2在包覆柏油前、後所擷取的焊道與缺陷訊號，皆顯示柏油包覆對缺陷訊號之衰減以及增加檢測缺陷之困難度；由有限元素模型所預測得知的導波衰減曲線可瞭解到柏油包覆厚度的影響甚於其密度與阻尼因子的影響。在大部分案例中，隨著柏油包覆厚度的增加，同一操作頻率下衰減率將隨之升高。針對柏油包覆層厚度為5-mm時，其衰減曲線出現一最大值，當操作頻率低於此最大值相對應之頻率時，衰減率將隨頻率升高，操作頻率須避免落在此範圍以更有效地使用導波進行長距離管線檢測。被焊接支撐架或柏油包覆遮蔽住的腐蝕訊號經連續小波轉換後可呈現時頻分析圖。在圖上腐蝕訊號成功地被定義出來。在本論文中，藉由對於導波在支撐架或包覆管線上傳遞行為之瞭解，有助於回波訊號之解釋；更藉將連續小波轉換應用在導波訊號處理技術上，改善了導波對於發生於複雜管線幾何上腐蝕之檢測能力。

The safety of pipelines distributed in the infrastructure of many industries has become very important since the industrial revolution. The guided ultrasonic wave technique can provide the possibility for rapid screening in long pipelines with corrosion. Especially the torsional mode T(0,1) of guided waves has been used in the cases of the pipe in the hidden region substantially. The ability of evaluating the inaccessible areas of the pipe makes the guided ultrasonic wave technique sit high on the roster of non-destructive testing tool for pipe inspection. However, the problem arises when attempting to detect the corrosions at the welded support bracket or under the bitumen coating on the pipe. The signal reflected from the corrosion will be covered by a large signal induced by the welded support or attenuated by the bitumen coating seriously. Therefore, the effects of welded support and bitumen coating on the T(0,1) mode are investigated by the experimental and the simulative methods. The continuous wavelet transform analysis is the signal processing method to extract the hidden signal of corrosion in this dissertation. There are five test pipes in the experiments. The response of the normal welded support is studied on the #1 test pipe. The #2 test pipe is used for attenuation investigation. The reflected signals of the features on the #3, #4, and #5 test pipes are measured and processed by continuous wavelet transform during defect detection process. In addition, the linear hexahedron elements are used to build the finite element models of the 6-inch steel pipe with support bracket and the pipe with bitumen coating. It is found that the effects of support bracket on the reflection comprise mode conversion, delayed appearance, trailing echoes, and frequency dependent behavior. When the T(0,1) mode impinges on to the support bracket, it will convert into the A0 mode inside the support due to the circumferential disturbance on the pipe surface. The reflection of the support bracket is identified as three parts formed by the direct echo, delayed echo and the trailing echo. The constructive interference of the A0 mode reflecting from the boundaries inside the support causes that the reflection spectrum shows two maxima peak at around 20-22 kHz (frequency regime of 0.0) and 32-34 kHz (frequency regime of 4.0) from both the experimental and simulated results. For the bitumen coating, the data collected from the welds and defects under the bitumen coating on the #2 test pipe show the attenuation effect on guided wave propagation and the difficulty of minor corrosion detection. In the finite element model of coated pipe, the results of predicted attenuation curves of T(0,1) mode indicate that the attenuation effect on guided wave propagation is aggravated with the increased value of the thickness, density or damping factor of the coated layer. Especially, in the case of 5-mm, the predicted attenuation curve shows a maximum point. Before this point, the attenuation increases with the operating frequency. For long range pipe inspection, it is the best way to avoid choosing the operating frequency around the corresponding frequency of the point. The measured data of corrosion affected by the welded support or the coated bitumen layer was processed by continuous wavelet transform to form a time-frequency analysis. The corrosion signals were identified in the contour map of the wavelet coefficient successfully. The understanding of the guided wave propagation on the pipe welded with support or pipe coated with bitumen is helpful to interpret the reflected signals. The use of continuous wavelet transform on signal processing techniques can improve the ability of defect detection on pipe with complex geometries.

TABLE OF CONTENTS


Page
LIST OF TABLES…………………………………………………………………..iii
LIST OF FIGURES…………………………………………………………………..iv
ABSTRACT (CHINESE)…………………………………………………………..ix
ABSTRACT (ENGLISH)…………………………………………………………..xi
NOMENCLATURE…………………………………………………………………xiii


CHAPTER 1 INTRODUCTION……………………………………………………1
1.1 Objective and Scope of Research……………………………………………1
1.2 Literature Review of Guided Wave…………………………………………...3
1.3 Motivation……………………………………………………………………10
CHAPTER 2 THEORY OF GUIDED WAVES…………………………………..15
2.1 Analytical Solutions of Guided Ultrasonic Waves in Pipes………………….15
2.2 Generation of Guided Ultrasonic Waves in Pipes…………………………..19
2.3 Simulation of Guided Ultrasonic Waves in Pipes……………………………23
2.4 Advanced Signal Processing Method of Guided Ultrasonic Waves…………24
CHAPTER 3 THE EFFECT OF THE WELDED SUPPORTBRACKET ON THE PIPE FOR TORSIONAL GUIDED WAVE………………………….37
3.1 Dispersion Curves and Wave Structures……………………………………..37
3.2 Experimental Setup and Results……………………………………………39
3.3 Finite Element Model and Results…………………………………………41
3.3.1 Reflected Signal………………………………………………………42
3.3.2 Wave Propagation Visualization and Modal Analysis…………………44
3.4 Conclusion……………………………………………………………………45
CHAPTER 4 THE EFFECT OF COATED MATERIALS ON THE PIPE FOR TORSIONAL GUIDED WAVE…………………………………….57
4.1 Dispersion Curves of the Coated Pipe………………………………………57
4.2 Experimental Setup and Results……………………………………………59
4.2.1 The Attenuation of T(0,1) Mode Caused by Bitumen Coating………60
4.2.2 The Detection of Defects under Bitumen Coating……………………62
4.3 Finite Element Model and Results…………………………………………66
4.3.1 The Effect of Thickness………………………………………………67
4.3.2 The Effect of Density…………………………………………………68
4.3.3 The Effect of Damping Factor………………………………………68
4.4 Conclusion……………………………………………………………………69
CHAPTER 5 WAVELET TRANSFORM ANALYSIS OF GUIDED WAVE SIGNALS………………………………………………………….85
5.1 Corrosion at Welded Support………………………………………………85
5.1.1 The Reflection of Normal Support and Defective Support…………..85
5.1.2 The Reflection of Corrosive Support………………………………….88
5.2 Corrosions under Bitumen Coating…………………………………………90
5.2.1 The Reflection of Corrosions on the Bare Pipe………………………..90
5.2.2 The Reflection of Corrosions on the Coated Pipe…………………….92
5.3 Conclusion……………………………………………………………………94
CHAPTER 6 CONCLUSION AND FUTURE WORK……………………………109
6.1 Conclusions………………………………………………………………..109
6.2 Future Work………………………………………………………………..112


BIBLIOGRAPHY…………………………………………………………………113

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