This thesis is study about the capability of guided waves in the inspection of cracks in pipelines, and studying about the guided waves of mode conversion phenomenon caused by cracks. The generally used inspection methods in industry are all localized area inspection. It will cost expensively and spend lots of human resource and time consuming, if we want to inspect the whole area in pipelines in factory. Thus, guided waves are used to improve these shortcomings of traditional inspection methods. Guided Waves can propagate fast and long range along the pipelines without decay. With the ability that guided waves can incident at a single location then inspect the whole region of pipelines under efficiently propagating distance. In this thesis, with using laboratory equipment, 3 Toneburst Cycles, 400¡B500¡B600 kHz, and L(0,1)¡BL(0,2) incident mode set up for crack detective sensitivity experiment. By calculating the group velocity of the signals of crack, it is known that there are three separated modes L(0,1)¡BL(0,2) and F(1,2) from reflected waves by cracks. These modes are exactly mode converted by cracks. The results show that in the same crack circumferential length or crack depth, the longer length or the deeper depth cause the higher reflection coefficient. It means they are more sensitive on the inspection of cracks. Also the reflection coefficient increases with the raising cross section area loss of cracks. In two dimensional fast Fourier transform experiment (2-D FFT), with 3 Toneburst Cycles, 400¡B500¡B600 kHz, L(0,1)¡BL(0,2) incident mode, and 0.5 cm interval set up to gather 40-point signals for proceeding the experiment. It can separate signals with different group velocity which are mixing together in time domain. From results, there are three different modes L(0,1)¡BL(0,2) and F(1,2) mode separated at three-dimensional picture. Finally, using the guided waves inspection system with 3 toneburst, 90 kHz and T(0,1) incident mode set up to proceed the experiment as before. In inspecting crack sensitive experiment, the use of this system has the similar result with the use of laboratory equipment. It proves that the excellent capability of guided waves in inspection of cracks in pipelines. While in mode identification, this system can identify there is only T(0,1) and F(1,2) mode with the same group velocity exist. Then the same result is verified by 2-D FFT experiment. It shows that this system generate or receive the specific mode in detection cracks by phase construction or phase destruction of phase interference.

The width of an ultra high-power laser diode is greater than 50 £gm and more than 20 times of low-power laser diode.The core diameter of Erbium Doped Fiber Amplifier fiber (EDFA) is 4~6 £gm and it is a single-mode fiber (SMF).However¡Athe ultra high-power laser diode is multi-mode laser. Therefore¡Athe mismatch between high-power laser and SMF resulting in low coupling efficiency. We improve coupling efficiency by using a wedge-shaped graded-index fiber (GIF) tip spliced to a SMF then fused a fiber bragg grating (FBG) to form an external cavity laser. The GIF is a focusing action like a graded-index fiber. From near-field pattern (NFP)¡Awe find the best GIF length is 400 £gm. The coupling efficiency between ultra high-power laser diode and wedge-shape lensed fiber is only 5% .

Long-period fiber gratings (LPGs) with a period ranging from several hundred micrometers to a few millimeters can couple a core mode to discrete co-propagating cladding modes when the phase matching condition is satisfied. The rapid attenuation of cladding modes results in loss bands in the transmission spectrum. As the attenuation bands are sensitive to the LPG period and the fiber surrounding environment such as temperature, strain and ambient refractive index, LPGs can be used for sensing. However, traditional LPGs with gratings inscribed in the fibers can only sense a single point and cannot be used for distributed sensing. Although new ideas were proposed to use traveling LPG formed by a pulsed acoustic wave, the large attenuation of the acoustic wave in the fiber greatly limits the sensing range to only several meters. In this thesis, we proposed to use a traveling LPG formed by the interference of two high power co-propagating core modes, usually LP01 and LP11. The beating of the two modes will induce a refractive index grating due to the optical Kerr effect, and the grating is called optically induced long-period fiber grating (OLPG). Compared to the grating induced by acoustic waves, OLPG is able to travel for a long distance due to the small attenuation of the guided core modes. Mode conversion in the OLPG is numerically simulated and analyzed using the finite-difference beam propagation method (FD-BPM). The result shows full conversion for both core-core and core-cladding mode coupling under phase matching condition. Moreover, the sensitivity of OLPG to temperature, axial strain and ambient refractive index is investigated and analyzed. It is seen that the sensitivities of temperature and axial strain with OLPG are different from the traditional LPGs since the period variation in OLPG is caused by the effective index difference of the two core modes at the writing wavelength, while in the traditional LPGs it is directly induced by temperature or strain. For the refractive index sensitivity with a large cladding, OLPG behaves the same as a traditional LPG with only material contributions since the grating period remains unchanged. / Master of Science

There are three main wave types present in the Sun’s atmosphere: Alfvén waves and fast and slow magnetoacoustic waves. Alfvén waves are purely magnetic and would not exist if it was not for the Sun’s magnetic field. The fast and slow magnetoacoustic waves are so named due to their relative phase speeds. As the magnetic field tends to zero, the slow wave goes to zero as the fast wave becomes the sound wave. When a resonance occurs energy may be transferred between the different modes, causing one to increase in amplitude whilst the other decreases. This is known as mode conversion. Mode conversion of fast and slow magnetoacoustic waves takes place when the characteristic wave speeds, the sound and Alfvén speeds, are equal. This occurs in regions where the ratio of the gas pressure to the magnetic pressure, known as the plasma β, is approximately unity. In this thesis we investigate the conversion of fast and slow magnetoacoustic waves as they propagate from low- to high-β plasma. This investigation uses a combination of analytical and numerical techniques to gain a full understanding of the process. The MacCormack finite-difference method is used to model a wave as it undergoes mode conversion. Complementing this analytical techniques are employed to find the wave behaviour at, and distant from, the mode-conversion region. These methods are described in Chapter 2. The simple, one-dimensional model of an isothermal atmosphere permeated by a uniform magnetic field is studied in Chapter 3. Gravitational acceleration is included to ensure that mode conversion takes place. Driving a slow magnetoacoustic wave on the upper boundary conversion takes place as the wave passes from low- to high-β plasma. This is expanded upon in Chapter 4 where the effects of a non-isothermal temperature profile are examined. A tanh profile is selected to mimic the steep temperature gradient found in the transition region. In Chapter 5 the complexity is increased by allowing for a two-dimensional model. For this purpose we choose a radially-expanding magnetic field which is representative of a coronal hole. In this instance the slow magnetoacoustic wave is driven upwards from the surface, again travelling from low to high β. Finally, in Chapter 6 we investigate mode conversion near a two-dimensional, magnetic null point. At the null the plasma β becomes infinitely large and a wave propagating towards the null point will experience mode conversion. The methods used allow conversion of fast and slow waves to be described in the various model atmospheres. The amount of transmission and conversion are calculated and matched across the mode-conversion layer giving a full description of the wave behaviour.

It is usually to see a large number of pipelines separating around the refineries, chemical and petro-chemical plants. The corrosion and erosion defects are unavoidable to occur in transporting pipe line. Especially, the maintain stuff usually find out breakage pipe or leaking liquid at elbowing pipe line because of the corrosion and erosion defects. So it is essential to examine these pipelines with an efficient method. The use of guided waves method is very attractive to solve this problem since guided wave could be excited at one circle on the pipeline and propagate over considerable distance. To choose guided wave torsion mode T (0, 1) as excitation mode because its group velocity doesn¡¦t change with frequencies. And the research analyzes the mode conversion that occurred when T (0, 1) mode propagated after the elbow pipe. The research also discusses the signal difference in different depth, circumferential distribution and axial length defects on the elbow pipe. The erosion defect usually occurs in the elbow pipe line and it would change with fluid velocity, causticity of fluid and flow direction. Therefore, the research designs the defects according to the character of erosion defect by finite element method software and simulates T (0, 1) mode propagating in the pipe line. Then this research extracts and analyzes the reflection signals from defects. In this guided wave experiment, the research manufactures the defect on elbow pipe. Because the erosion defect could be usually found at outer side of elbow pipe, artificial defect would be set there. And the elbow pipe is manufactured with different depth, circumferential distribution and axial length defect. The research would discuss the relationship between change of defect and reflection signal. By elbow pipe defect signals of simulation and experiment consequence, the different depth, circumferential distribution and axial length defect signals could be still distinguished. The signals with different axial length defect that received from straight pipe and elbow pipe are similar and are affected by signal constructive and destructive interference. So the research could get maximum and minimum defect signal amplitudes from one-fourth wavelength axial defect and half wavelength axial defect. Therefore, the axial length defect of elbow pipe could be estimated from defect signals and this consequence could help judge the level of damaged elbow pipe. T (0, 1) mode has better sensitivity to outside of the pipe than inside of the pipe. So the bigger signal amplitude could be received from the notch at outside of the pipe. In the process of wave propagation simulation, there are overlapping waveforms and mode conversions occur at elbow pipe. This situation causes the defect signals were amplified at elbow pipe. In practical detection, the misjudgments of amplified defect signals should be attended to.

Using ultrasonic guided waves can achieve long range inspection along the pipeline rapidly. The presence of defect or other features on the pipe were identified by analyzing the reflected echoes as well as mode conversion phenomena. However, it is difficult for guided wave to find a minor corrosion, such as pitting. Therefore, a study of the reflection of torsional T(0,1) mode from pits on the pipe has been carried out and an advanced signal processing method wavelet transform is adopted to process the reflected echoes in this study. In order to understand that characteristic of the reflected echoes of pits, the propagation of guided wave T(0,1) through pits was simulated by the finite element method. The frequency response of the signal reflected from the pits with different sizes was discussed both by finite element method and experimental method. Then, we discuss two types of pitting including regular- distributed pitting and the random-distributed pitting. We not only discuss the relation between the axial length of regular pitting and wave length of the T(0,1) mode, but also the reflected singal of four random pittings. The experiments were performed on 3 inch carbon steel pipe for measuring the reflected signals from different pittings with different frequencies. The results of the simulation, indicate that the wave was easily scattered by pitting because the shape of geometry. It is the reason of reducing the amplitude of reflected signals. To receive a dominate signal reflected from pitting, the excitation with higher frequency was choosen within the frequency range of interest. The experimental results indicate that the signals would be too weak to be detected by guided waves when the estimated cross sectional loss of the pitting is less than 2 percent. However, the results after wavelet transform showed the feasibility of improving the abilities of detecting minor pitting. In the case of regular pitting, the maximu value of the reflected signal appeared when the axial length of the pitting equals to the 66 % of the wavelength. It is because the constructive interference. The mode conversion phenomena is another behavior of the reflected signal cased by the non-axissymetric geometry of the pitting. As for the random pitting, The reflected echo shows different behavior with the regular pitting. The amplitude of the signal is bigger with lower frequency we use. The different level of random pitting on the pipe were also identified successfully by wavelet transform. Understanding the phenomena of interaction between the guided wave and the pitting is helpful to the guided wave inspection.

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京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24004号 / エネ博第440号 / 新制||エネ||83(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 田中 仁, 教授 中村 祐司, 教授 長﨑 百伸 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

abstract: Composite materials are increasingly being used in aircraft, automobiles, and other applications due to their high strength to weight and stiffness to weight ratios. However, the presence of damage, such as delamination or matrix cracks, can significantly compromise the performance of these materials and result in premature failure. Structural components are often manually inspected to detect the presence of damage. This technique, known as schedule based maintenance, however, is expensive, time-consuming, and often limited to easily accessible structural elements. Therefore, there is an increased demand for robust and efficient Structural Health Monitoring (SHM) techniques that can be used for Condition Based Monitoring, which is the method in which structural components are inspected based upon damage metrics as opposed to flight hours. SHM relies on in situ frameworks for detecting early signs of damage in exposed and unexposed structural elements, offering not only reduced number of schedule based inspections, but also providing better useful life estimates. SHM frameworks require the development of different sensing technologies, algorithms, and procedures to detect, localize, quantify, characterize, as well as assess overall damage in aerospace structures so that strong estimations in the remaining useful life can be determined. The use of piezoelectric transducers along with guided Lamb waves is a method that has received considerable attention due to the weight, cost, and function of the systems based on these elements. The research in this thesis investigates the ability of Lamb waves to detect damage in feature dense anisotropic composite panels. Most current research negates the effects of experimental variability by performing tests on structurally simple isotropic plates that are used as a baseline and damaged specimen. However, in actual applications, variability cannot be negated, and therefore there is a need to research the effects of complex sample geometries, environmental operating conditions, and the effects of variability in material properties. This research is based on experiments conducted on a single blade-stiffened anisotropic composite panel that localizes delamination damage caused by impact. The overall goal was to utilize a correlative approach that used only the damage feature produced by the delamination as the damage index. This approach was adopted because it offered a simplistic way to determine the existence and location of damage without having to conduct a more complex wave propagation analysis or having to take into account the geometric complexities of the test specimen. Results showed that even in a complex structure, if the damage feature can be extracted and measured, then an appropriate damage index can be associated to it and the location of the damage can be inferred using a dense sensor array. The second experiment presented in this research studies the effects of temperature on damage detection when using one test specimen for a benchmark data set and another for damage data collection. This expands the previous experiment into exploring not only the effects of variable temperature, but also the effects of high experimental variability. Results from this work show that the damage feature in the data is not only extractable at higher temperatures, but that the data from one panel at one temperature can be directly compared to another panel at another temperature for baseline comparison due to linearity of the collected data. / Dissertation/Thesis / M.S. Aerospace Engineering 2012