Dissertations / Theses on the topic 'Crack in pipe'

In the oil and gas industry, threaded pipe connection is frequently used to connect the casing string, drill pipe strings or production and transportation risers and pipelines. The connection is normally preloaded in order to maintain a sealed and secure connection while in service and avoid leakage. Tapered thread are a common connection and in order to introduce preload to the threaded connection when they are assembled a certain make-up torque is going to be applied. The make-up torque plus external loads result in a multiaxial stress distribution over the connection, where the threaded connections act as stress risers. Environment such as waves and currents cause dynamic loads acting on the pipe line and offshore structures. The weakest point in offshore structure is the pipe connection because of fatigue crack initiated in the connection's threads. Researchers and engineers developed a variety of patented threaded pipe connection which all claiming to improve a connection's fatigue life. The experimental data for patented designs, available in literature, is limited. Most published studies usually comprise experiments on a single connection type. For detailed fatigue analysis those published studies cannot be used since there is no uniformity in testing setup, loading conditions and damage detection technique exist. Moreover, current design curves in codes and standards lead to overly conservative or inaccurate results. The aim of this work is to provide a better understanding of the fatigue mechanisms of threaded pipe connections and to study the effect of different design features on a connection's fatigue life. The final goal is to formulate guidelines for new fatigue resistant connection designs. API connection is used as a reference in this study. Several modifications and design features are applied to the connection type. To simulate the effect of these modifications, a parametric 2D axisymmetric finite element model, ABAQUS is used. 2D finite element result are compared with a 3D model to prove its validity for both make-up. In addition, the results of the 2D axisymmetric simulation are validated by static strain gauge measurements during a make-up test and an axial tension test. The validated model is then used to evaluated the influence of the connection properties and design features on the threaded connection's behaviour. Test rigs were designed to perform axial fatigue experiment on two scales: the small-scale experiments on 1" (33.4 mm outer diameter) connections are performed in axial fatigue testing, the medium scale tests on 4.5" (114.3 mm) connections are carried out under axial tension for which a setup is developed. The majority of the performed fatigue tests are small scale experiments. Several modified configurations are tested. The S-N curve is constructed, so that the effect of certain configuration on the connection's fatigue life can be quantified. The local modification of the threaded connection's geometry as well as the connection's contact condition's contact conditions can have an important influence on the fatigue life of the connection. A beach marking technique is used to visualized the crack fronts at different moments during the tests so that exact crack shape can be seen during post-mortem analysis. The result shown that a crack initiates at the root of the last engaged thread of the male part of the connection, and propagates slowly over a large segment of the circumference, forming a long shallow crack. When the crack penetrates the pipe wall, it rapidly increases in size along two crack fronts. The shape of crack observed in beach mark analysis do not have a semi-elliptical shape as commonly used in fracture mechanics. A fatigue crack growth analysis that considers the crack as an annular flaw, is effective in describing the crack growth behaviour. The experimentally obtained S-N curves and the result from the finite element simulations are combined in multiaxial damage evolution law. The observed trend in fatigue lives of the configuration are explained by using the fatigue analysis. Using a connection's thread load distribution as a measure for its fatigue life is proven to be inaccurate. The main reason for this is that the load distribution is related to axial stresses over the connection. The fatigue life of a threaded connection is determined by the local multiaxial stress distribution and strain range around the root of the last engaged thread. These local conditions are not only the result of the load distribution, but they are also affected by the hoop stress introduced during make-up, which can additionally be affected by a changed connection stiffness. The multiaxial damage evolution law is used to analyse the influence of several features on a connection's fatigue life. It is not for all patented modifications that an increased fatigue life is predicted when applied to the API connection. The final conclusion reached is that, in order to optimize a fatigue resistant connection, several design features must be combined together. The thread shape can be optimized to obtained a low stress concentration factor and reduce the local strains at the thread root. The connection's global geometry and make-up conditions can be optimized to improve the load distribution over the threads and reduce local stresses and strains at the threads.

Plastic pipeline systems have now become dominant for fuel-gas and water distribution networks. Although they have an impressive service record failures do occur, with Rapid Crack Propagation being characterised as the least probable but most potentially catastrophic one. This study investigates the effect of structural morphology and bulk residual strains on the RCP performance of polyethylene pipes, and proposes a new methodology for predicting a safe service envelope. During crack propagation in PE pipes, the fracture surface has two distinct regions; plane strain and plane stress. In addition to the Instrumented Charpy, Reversed Charpy, High Speed Double Torsion, Dynamic Mechanical Analysis and uniaxial tensile testing, S4 tests of extruded pipe specimens were employed in order to evaluate the structural and fracture parameters of pipe grade resins in these two fracture modes on pipe. A new experimental technique, which modified the pipe bore crystallinity without altering the residual strain field (as evaluated from slit ring tests) showed that the bore surface layer properties had much less influence on RCP than previously thought. Parallel with the experimental work, modeling of the fracture mechanisms was also undertaken. Using previous models in the field, such as the adiabatic decohesion model, the plane strain fracture toughness was evaluated while the plane stress fracture toughness was evaluated either from the Reversed Charpy or from the stability of adiabatic drawing in a tensile test. A mixed mode, temperature sensitive toughness was finally evaluated, leading to an overall fracture properties assessment for polyethylene pipes which could be compared directly to the crack driving force during RCP in pipe. By employing a new mathematical approach, which incorporated both the effects of residual strains and pipe stiffness behind the pressure decay length, a previous basic analytical RCP model was further developed and compared to more elaborate finite element and finite volume solutions. The new results were also compared to S4 experiments using high-speed photography and showed that the new methodology could be employed by the end user even when testing facilities are not directly available

Thesis (Ph.D.)--Case Western Reserve University, 2009
Title from PDF (viewed on 19 August 2009) Department of Macromolecular Science and Engineering Includes abstract Includes bibliographical references Available online via the OhioLINK ETD Center

The level of residual stresses in welded components is known to have a significant influence on their failure behaviour. It is, therefore, necessary to understand the combined effect of mechanical loading and residual stresses on the ductile fracture behaviour of these structures in order to provide the accurate structural safety assessment. Recently, STYLE (Structural integrity for lifetime management-non-RPV component) performed a large scale bending test on a welded steel pipe containing a circumferential through-thickness crack (the MU2 test). The purpose of this test is to study the impact of high magnitude weld residual stresses on the initiation and growth of cracks in austenitic stainless steels. This research presents the simulation part of the STYLE project which aims to develop the finite element model of MU2 test in ABAQUS to enhance the understanding and ability to predict the combined influence of mechanical loading and residual stresses on the ductile fracture behaviour of nuclear pressure vessel steels. This research employs both fracture mechanics principles (global approach) and Rousselier damage model (local approach) to study this behaviour including crack initiation and growth. In this research, the Rousselier model was implemented into ABAQUS via the user defined subroutines for ABAQUS/Standard and ABAQUS/Explicit modules, i.e. UMAT and VUMAT. The subroutines were developed based on the integration algorithm proposed by Aravas and Zhang. The validation of these subroutines was checked by comparing the FE results obtained from the implementation of these subroutines with the analytical and other benchmark solutions. This process showed that UMAT and VUMAT provide accurate results. However, the UMAT developed in this work shows convergence problems when the elements start to fail. Hence, only VUMAT was used in the construction of the finite element model of the MU2 test. As mentioned above, the results obtained from both fracture mechanics approach and Rousselier model are compared with the experimental data to validate the accuracy of the model. The results shows that both fracture mechanics approach and the Rousselier model predict similar final crack shapes which correspond closely to the test results in south direction. The other conclusions about the influence of residual stress on ductile fracture obtained from this work are also summarized in this thesis.

As recentes descobertas de petróleo e gás na camada do Pré-sal representam um enorme potencial exploratório no Brasil, entretanto, os desafios tecnológicos para a exploração desses recursos minerais são imensos e, consequentemente, têm motivado o desenvolvimento de estudos voltados a métodos e materiais eficientes para suas produções. Os tubos condutores de petróleo e gás são denominados de elevadores catenários ou do inglês \"risers\", e são elementos que necessariamente são soldados e possuem fundamental importância nessa cadeia produtiva, pois transportam petróleo e gás natural do fundo do mar à plataforma, estando sujeitos a carregamentos dinâmicos (fadiga) durante sua operação. Adicionalmente, um dos problemas centrais à produção de óleo e gás das reservas do Pré-Sal está diretamente associado a meios altamente corrosivos, tais como H2S e CO2. Uma forma mais barata de proteção dos tubos é a aplicação de uma camada de um material metálico resistente à corrosão na parte interna desses tubos (clad). Assim, a união entre esses tubos para formação dos \"risers\" deve ser realizada pelo emprego de soldas circunferenciais de ligas igualmente resistentes à corrosão. Nesse contexto, como os elementos soldados são considerados possuir defeitos do tipo trinca, para a garantia de sua integridade estrutural quando submetidos a carregamentos cíclicos, é necessário o conhecimento das taxas de propagação de trinca por fadiga da solda circunferencial. Assim, neste trabalho, foram realizados ensaios de propagação de trinca por fadiga na região da solda circunferencial de Inconel® 625 realizada em tubo de aço API 5L X65 cladeado, utilizando corpos de prova do tipo SEN(B) (Single Edge Notch Bending) com relações entre espessura e largura (B/W) iguais a 0,5, 1 e 2. O propósito central deste trabalho foi de obter a curva da taxa de propagação de trinca por fadiga (da/dN) versus a variação do fator de intensidade de tensão (ΔK) para o metal de solda por meio de ensaios normatizados, utilizando diferentes técnicas de acompanhamento e medição da trinca. A monitoração de crescimento da trinca foi feita por três técnicas: variação da flexibilidade elástica (VFE), queda de potencial elétrico (QPE) e análise de imagem (Ai). Os resultados mostraram que as diferentes relações B/W utilizadas no estudo não alteraram significantemente as taxas de propagação de trinca por fadiga, respeitado que a propagação aconteceu em condições de escoamento em pequena escala na frente da trinca. Os resultados de propagação de trinca por fadiga permitiram a obtenção das regiões I e II da curva da/dN versus ΔK para o metal de solda. O valor de ΔKlim obtido para o mesmo foi em torno de 11,8 MPa.m1/2 e os valores encontrados das constantes experimentais C e m da equação de Paris-Erdogan foram respectivamente iguais a 1,55 x10-10 [(mm/ciclo)/(MPa.m1/2)m] e 4,15. A propagação de trinca no metal de solda deu-se por deformação plástica, com a formação de estrias de fadiga.
Recent oil and gas discoveries in the Pre-Salt layer represent a huge exploration potential in Brazil, however, the technological challenges for the exploitation of these mineral resources are immense and therefore have motivated the development of studies looking for efficient methods and materials for their productions. The oil and gas pipellines, called risers, are elements that are necessarily welded and have fundamental importance in the production chain, since they transport oil and natural gas from the sea bed to the platforms and are subject to dynamic loads (fatigue) during operation. Additionally, one of the central problems in the production of oil and gas in the Pre-Salt reserves is directly associated with a highly corrosive media, such as H2S and CO2. A cheaper way to protect the pipelines from these medias is applying a protective layer of a corrosion resistant metal on the inner diameter of these pipes, creating a cladded pipe. Thus, a joining process of these pipes to form the risers must be carried out by the use of girth welds with a corrosion resistance material similar to the clad metal. As the welded structures are seen as potential location of \"crack like\" defects, to ensure the structural integrity of such component when subjected to repetitive loading conditions, it is necessary to know the fatigue crack growth rates for the girth weld. Therefore, in this work it was carried out fatigue crack propagation tests in the weld region of an API 5L X65 cladded pipe with Inconel® 625, girth welded using Inconel® 625 electrodes. From the welded region, Single Edge Notch Bending specimens, SEN(B), were removed with different thickness and width ratios (B/W= 0.5, 1, and 2). From the fatigue tests, the crack propagation rates (da/dN) as function of the variation of the stress intensity factor (ΔK), were determined for the weld metal, using different crack size measurement techniques: the elastic compliance (EC), electric potential drop (EPD) and image analysis (IA). The results showed that the different B/W ratios used in study did not modified significantly the fatigue crack growth rates, considering that crack propagation took place under small scale yielding conditions. The results of fatigue crack growth tests allowed to obtain the regions I and II of da/dN x ΔK curves for the weld metal. The ΔKth value obtained for the weld metal was around 11,8 MPa.m1/2 and the found values of the experimental constants C and m of Paris-Erdogan\'s equation were respectively equal to 1,55 x10-10 [(mm/cycle)/( MPa.m1/2)m] and 4.15. The micromechanism of fatigue crack growth took place by plastic deformation, with the formation of fatigue striations.

У дисертації запропоновано нове ізоляційне покриття та методи і засоби його нанесення, з метою суміщення протикорозійного захисту нафтогазопроводів з підвищенням їх опору механічному руйнуванню, в тому числі на стадії росту коротких втомних тріщин. Розроблено ізоляційно-композитне покриття на основі епоксидної смоли, армоване склотканиною, яке відзначається високими термостійкістю, міцністю, адгезією до сталевих труб та протикорозійними властивостями. Встановлено особливості росту коротких тріщин у сталі 17Г1С газопроводу та виявлено, що тривала експлуатація трубопроводу незначно зменшила циклічну тріщиностійкость сталі за пульсівної асиметрії навантаження, проте з її підвищенням прогнозується зростання негативного впливу експлуатації. Розроблено спосіб підвищення опору поширення коротких втомних тріщин у трубних сталях нанесенням плинної складової ізоляційного покриття, що штучно створює ефект закриття тріщини, чим суміщається протикорозійний захист з підвищенням циклічної тріщиностійкості металу. Досліджено напружений стан сталевого трубопроводу, вкритого тришаровим композиційним покриттям та встановлено, що ізоляційно-композитне покриття незначно зменшує напруження у бездефектній трубі та є відчутним у трубі з тріщиною.
В диссертации предложено новое изоляционно-защитное покрытие, а также методы и средства его нанесения, с целью совмещения противокоррозионной защиты нефтегазопроводов с повышением их сопротивления механическому разрушению, в том числе на стадии роста коротких усталостных трещин. Разработано изоляционно-композитное покрытие на основании эпоксидной смолы, армированное стеклотканью, назначенное для изоляции нефтегазопроводов диаметром 500 - 720 мм класса А и класса Б, которое отличается высокими термостойкостью, прочностью, адгезией к стальным трубам, изоляционными и противокоррозионными свойствами. Решена задача создания способа предупреждения развития трещиноподобных и коррозионных дефектов поверхности трубопроводов путем применения нового, разработанного автором состава многокомпонентного композитного покрытия и способа его нанесения. Способ обеспечивает высокую технологичность и простоту нанесения покрытия, поскольку монтаж данных покрытий не нуждается в достижении особенных контролируемых параметров, таких как очистка поверхности труб до металлического блеска и нагревание до определенной температуры. Способ эффективен в условиях, когда нагревание трубы при нанесении изоляции недопустимо. Защитное покрытие, которое получают согласно данному способу для защиты труб от повреждений, отмечается термостойкостью, прочностью, высокой адгезией к стальным трубам, высокими изоляционными и противокоррозионными свойствами. Ударная прочность полученного покрытия составляет 15 Дж, что отвечает усиленному покрытию класса В. Установлены особенности роста коротких усталостных трещин в стали 17Г1С газопровода и выявлено, что длительная эксплуатация незначительно уменьшила циклическую трещиностойкость стали при пульсирующем цикле нагрузки, но с ее повышением прогнозируется увеличение отрицательного влияния эксплуатации. По значениям порогового размаха коэффициента интенсивности напряжений вычислена граница усталости металла з короткими трещинами. Для нагрузки, близкой к пульсирующей, наблюдается увеличение границы усталости в узком диапазоне коротких трещин, что обусловлено эффектом закрытия трещин. Длительная эксплуатация газопровода проявилась в снижении границы усталости металла. Разработан способ повышения сопротивления распространению коротких усталостных трещин в трубных сталях нанесением на трубопровод под давлением текучей составляющей разработанного изоляционно-композиционного покрытия, что искусственно создает эффект закрытия трещин после ее затвердевания, чем совмещается противокоррозионная защита з повышением циклической трещиностойкости металла. Исследовано напряженное состояние стального трубопровода, покрытого трехслойным композиционным покрытием и определено, что оно незначительно уменьшает напряжения в бездефектной трубе, но ощутимо в трубе с трещиной. Определено концентрацию напряжений возле поверхностной трещины в трубе с покрытием при различных значениях ремонтного давления. Установлено, что разработанное покрытие уменьшает за счет навитой стеклопластиковой ленты напряжения в бездефектной трубе на 4,5 % и на 2,3 % соответственно при ррем=0 и при ррем=0,5р, а также уменьшает коэффициент интенсивности напряжений в окрестности вершины трещины на 12...25 %. Разработана конструкция автоматизированного устройства для намотки полимерно-композиционного бандажа на трубу с автоматизированным регулированием необходимого натяжения, плотностью прилегания к поверхности, необходимой толщиной намотки нахлесткой смежных витков, что позволяет достигать высокой прочности адгезионных покрытий с поверхностью металла.
The new isolative coating and the methods and facilities for its covering are proposed in the dissertation, with a goal of a combination of oil and gas pipelines anticorro-sion protection and an increase of its resistance to mechanical fracture, including on the stage of short fatigue crack growth. The isolative-composition coating is developed on the base of epoxy resin, reinforced by glass-fabric, which is distinguished by high thermal resistance, strength, adhesion with steel pipes and anticorrosion properties The peculiarities of short fatigue crack growth in gas pipeline 17G1S steel are established ant it is revealed that long-term service of the pipeline decreased slightly fatigue crack growth resistance of the steel at the pulsatile load asymmetry but the negative influence of service is predicted to be increase with a rise of asymmetry. The method of an increase of fatigue short crack growth resistance in pipe steels by a deposit of the fluid component of the isolative coating what creates artificial crack closure effect and it combines anticorrosion protection with an increase of fatigue crack growth resistance of metal. Stress state of steel pipeline, covered by three layers composite coating, is investigated and it is established that it decreases slightly stresses in defectless pipe but noticeably in pipe with crack.

Includes abstract.
Includes bibliographical references.
The Vertical Marine Mining technique is a lucrative method used by De Beers to recover bottom lying diamonds from the seabed, mainly off the coast of Namibia. The method employs a 200m length pipe-drilling structure rotated from the surface and acting as a conduit through which the disturbed sediment is transported to the surface for processing. However, cyclic operating stresses combined with corrosion have tended to cause circumferential fatigue cracks to develop on the outside surface of the drill pipe and propagate inwards. As no early warning system exists, these cracks propagate undetected until failure occurs. High operational costs and losses associated with down time, provide a strong motivation for a system to detect fatigue cracking prior to failure, especially given that fatigue crack growth rate and fatigue lifetimes can be estimated using Fracture Mechanics principles. Therefore what is required is an early warning of fatigue crack initiation with non-destructive testing (NOT). The eddy current method is an ideal NDT technique as it does not require contact with the test surface and is highly sensitive to fatigue cracks .. However, this method is generally only sensitive to surface and near surface defects. This proves to be a major limitation - the external drill pipe surface is obstructed by flanges and fittings such that continuous inspection of the external surface would be impossible. Therefore two specialised eddy current methods to allow the detection of external fatigue cracks from inside the drill pipe were reviewed, namely: Pulsed Eddy Current (PEC) and Remote Field Eddy Current (RFEC). It was found that unlike PEC, RFEC is highly sensitive to external circumferential planar defects in ferrous pipes. This, above all, highlighted the suitability of RFEC for drill pipe inspection.

Includes bibliographical references.
Water is an important and increasingly scarce resource in the world today. Unfortunately, a lot of water is lost through leakage since most distribution systems are deteriorating. Therefore research in leakage management is necessary in order to improve the utilization of water resources. Leakage may be reduced by managing the water pressure in water distribution systems. One of the important factors affecting the pressure-leakage relationship is pipe material behaviour (Van Zyl & Clayton 2007; Greyvenstein & Van Zyl 2007). The pressure – leakage relationship has been described by several relationships such as the Torricelli equation, the Fixed and Variable Area Discharge (FAVAD) concept and the conventional equation. Pipe material behaviour affects leakage parameters in the pressure-leakage relationship such as the leak area and the leak exponent (Cassa et al. 2010). For this project, the pressure-leakage relationships in High Density Polyethylene (HDPE) and Polyvinylchloride (PVC) pipes are investigated. HDPE and PVC are polymeric materials and therefore exhibit a viscoelastic response to applied stress and applied strain. Viscoelastic responses include creep, relaxation, hysteresis and time dependency. When these pipes experience stresses due to water pressures, failure and fracture may occur as leaks. The viscoelastic properties of these materials therefore affect how the leaks respond to pressure change. The effect of viscoelastic deformations in leaks was investigated using Finite Element Analysis (FEA) software, Abaqus. Round holes and longitudinal cracks were represented as individual leaks in HDPE and PVC pipes in Abaqus. Pressure was applied to each pipe model for different time periods, and the deformed leak areas were obtained. Further analysis was carried out to determine the effects of pressure on leak parameters such as the gradient of the leak area-pressure relationship, leak exponent and the leakage number. The analysis shows that viscoelastic deformations have an effect on the pressure-leakage relationship. A linear relationship exists between the leak area and pressure for all time periods investigated and therefore gradients could be obtained. Deformed leak areas, gradients and leak exponents all increased with time and therefore confirmed that the time dependency of viscoelastic materials affected the pressure-leakage relationship. The leakage exponents for both materials were found to vary between 0.5 and 1.5 for both HDPE and PVC. HDPE also exhibited higher leak exponents, gradients and larger deformed leak areas than PVC for the same leaks. It was also found that leakage in viscoelastic materials may be analysed using the leakage number, developed for elastically deforming materials by Van Zyl & Cassa (2013).

Дисертаційна робота присвячена розробці чисельних і аналітичних методів розрахунку коефіцієнта інтенсивності напружень, (КІН) для некласичних проблем механіки руйнування, зокрема, для тріщин складної форми та для тріщин в тонкостінних конструкціях з врахуванням геометричної нелінійності, (ГН). Представлена модифікація методу Вільямса, що враховує затухаючі на нескінченності члени. Досліджено межі та надані рекомендації щодо застосування даного методу. Розроблено аналітичний метод розрахунку КІН в трубах з довгими осьовими поверхневими тріщинами з врахуванням ГН в залежності від внутрішнього тиску. Наведено результати верифікації величин КІН з отриманими в роботі чисельними розв’язками МСЕ. Побудовані чисельні моделі для розрахунку КІН та розкриття берегів тріщин в ГН постановці для наскрізних тріщин в залежності від змінної величини осьової сили. Побудована апроксимаційна формула, що описує вплив ГН ефекту для узагальненої осьової сили та безрозмірної довжини тріщини. Показана значимість ефекту для реальних лабораторних експериментів. Розроблено універсальний напіваналітичний метод розв’язку інтегро-диференційного рівняння теорії пружності для плоских тріщин нормального відриву довільної форми. Для перевірки аналітичних результатів побудовані чисельні моделі МСЕ для тріщин різної форми, наприклад, для прямокутної, внутрішньої півелітичної тріщини, тощо, та проведено співставлення результатів. Проведені практичні розрахунки КІН в часі для елементів конструкцій АЕС для різних сценаріїв протікання аварійних ситуацій. Створено ряд імітаційних моделей з вбудованою тріщиною, для якої розрахунки КІН проводилися методами нелінійної механіки руйнування. Для тріщин, що проходять через наплавлений матеріал, де відбувається стрибки напружень, обґрунтовано застосування методу функцій впливу, для чого розроблено аналітико-чисельну процедуру, що використовує частково неперервні базові закони навантаження.
The dissertation is dedicated to the development of numerical and analytical methods for calculating the stress intensity factor (SIF) for nonclassical problems of fracture mechanics, in particular, for cracks of complex shape and for cracks in thin structures with taking into account geometric nonlinearity (GN) and their applications in various industries, in particular, for nuclear power plants, for calculation of the elements of the reactor unit. A modification of Williams's method is presented, which describes the stress state in the crack tip. For modification of the classical approach in the considered method the additional members were presented, which are infinite in the crack tip, but attenuate at infinity. The main idea of the method is to divide the whole area of the body into two separate parts – internal one, which embraces the tip of the crack, and the external one. In the inner area, only the classic Williams functions are used, and in the outer area both the classical members and additional ones are used. At the boundary between the selected subdomains, the conjugation conditions are to be fulfilled, the essence of which is to equalize here the stresses and displacements. The very high efficiency of this method is shown for bodies that have the shape of a circle, or are infinite, where almost exact values (up to 6-7 digits) are achieved when using no more than 12-16 members of the expansion. However, for a very elongated rectangular body the method gives much worse results, and the error can reach up to 2%. The practical significance of the proposed approach consists in the obtaining the simple and effective tool for testing the capabilities of commercial packages for calculating the parameters of fracture mechanics, which, as we know, do not always provide the correct results for case when load is applied on the crack surfaces. A thin-walled pipe with a long surface crack is considered. The pipe, surface of which may contain the initial deviation form ideal circle, is loaded by inner pressure. In first time the problem of determining the SIF was considered in a geometrically nonlinear formulation, when changes in the geometry of the body in the process of deformation are accounted for linear material behavior. Based on the Chen-Finnie method, which considers the crack as concentrated compliance, and on original solution for geometrically nonlinear behavior of curved initially distorted beam, the compact analytical formulas are obtained, which gives the value of SIF for each value of inner pressure. Comparison of the analytical results with those obtained by careful numerical FEM analysis, on the one hand, showed their good correspondence, and therefore the accuracy and efficiency of both the analytical and numerical procedures are confirmed. On the other hand, for the first time in the scientific literature, the value of SIF in a geometrically nonlinear formulation are obtained for surface cracks in slightly distorted cylindrical shell with surface long crack. It was shown that even for perfectly circular pipes having the cracks with depth up to half of the thickness of pipe walls, loaded by moderate level of inner pressure, the geometrical nonlinear values of SIF can be 10-15% less than those at linear approach application. This is very significant practical result. Another geometrically nonlinear problem is numerically investigated by known commercial FEM software for the through crack, which is loaded by a significant value of additional longitudinal force (the main factor of geometric nonlinearity considered) and small value of internal pressure (linear consideration). This statement distinguished this task from research conducted at NASA (USA), where the pressure and axial force were proportional as to pipe with closed ends. The geometric nonlinearity of the study is investigated through an incremental increase in loads and the corresponding recalculation of the SIF for the already taken into account changes in the deformed geometry (curvature). A number of tasks for determining the SIF for different pipe radii and dimensionless force values were calculated. Dimensionless parameters, which characterize the deviation of the values of SIF from the linear one, are numerically determined. Application of least square method allowed to suggest the simplified analytical formula for calculation of these deviations. These results are of great theoretical and practical importance. In particular, it is shown that for real pipes the neglecting by influence of axial force in usual approach may lead to 4-6% error of SIF determination. It is noted that additional account for large values of pressure, possible plastic deformations, may further reduce the actual values of SIF. Thus, the significance of the problems and the need for further research in this direction are shown. The necessity of analysis of flat cracks of non-canonical shape in three-dimensional bodies is noted. This is due to the fact that almost all existing solutions in the literature and reference results in normative documents are given for cracks that have shape of an ellipse or its part. However, real cracks detected by non-destructive testing are irregularly shaped cracks. Therefore, it is necessary to create the analytical methods that would allow to assess the impact of the crack shape, and to verify them with careful numerical procedures by FEM. For this purpose, flat internal cracks in infinite 3D body are considered. The formulation of problems for them is reduced to well-known integro-differential equation of the theory of elasticity. Note, that exact fundamental analytical solutions of which exist only for a circular crack and only for some simple laws of loading for an elliptic crack. A universal semi-analytical method for solving the integro-differential equation of the theory of elasticity for plane cracks of normal separation of arbitrary shape has been developed. It is proposed to consider the displacement of the crack edges as a product of a certain function of the crack shape, which satisfy to known asymptotic behavior of the displacement field near the crack front, and a certain polynomial series. As for the functions of the form, three variants of their choice were investigated - a) classical, which depends on the squares of ratio of radial coordinate of the considered point, and the corresponding coordinate of contour point; b) multiplicative, based on the product of equations describing the straight sections of the crack contour, and c) the original Oore-Burns function, which is an integral of the crack contour from the inverse square of the distance of the considered point to each point of the contour. The results showed that the greatest accuracy is achieved by application of hypersingular approach with Oore-Burns function of form. The verification of results was performed for elliptic crack, semielliptic inner crack, rectangular crack. The results were compared with numerical ones calculated by FEM. Practical calculations of SIF dependance with time for NPP reactor ant its elements for different scenarios of emergency situations are carried out. A number of simulation models with a built-in crack have been created, for which SIF calculations were performed by nonlinear FEM analysis. For cracks going through the cladding, where stress jumps occur, the method of influence functions is elaborated, and analytical-numerical procedure used piece-wise continuous basic laws of loading. Practical calculations were performed for the nozzle of reactor vessel, the cylindrical part of vessel, the core barrel and core baffle. These calculations were used to justify the extension of the service life of several units of Ukrainian NPPs.
Диссертационная работа посвящена разработке численных и аналитических методов расчета коэффициента интенсивности напряжений, (КИН) для неклассических проблем механики разрушения, в частности, для трещин сложной формы и для трещин в тонкостенных конструкциях с учетом геометрической нелинейности, (ГН). Представлена модификация метода Вильямса, что учитывает затухающие на бесконечности члены. Исследованы границы и даны рекомендации о применении данного метода. Разработан аналитический метод расчета КИН в трубах с длинными осевыми поверхностными трещинами с учетом ГН в зависимости от внутреннего давления. Результаты хорошо коррелируют с полученными в роботе численными решениями МКЭ. Построены численные модели для расчета КИН и раскрытия берегов трещин в ГН постановке для сквозных трещин в зависимости от переменной величины осевой силы. Построена апроксимационная формула, что описывает влияние ГН эффекта для обобщенной осевой силы и безразмерной длины трещины. Показана значимость эффекта для реальных лабораторных экспериментов. Разработан универсальный полуаналитический метод решения интегро-дифференциального уравнения теории упругости для плоских трещин нормального отрыва произвольной формы. Для проверки аналитических результатов построены численные модели МКЭ для трещин разной формы, например, для прямоугольной, внутренней полуэллиптической трещины, и др., и проведено сопоставление результатов. Проведены практические расчеты КИН во времени для элементов конструкций АЭС для разных сценариев прохождения аварийных ситуаций. Для этого создан ряд имитационных моделей со встроенной трещиной, для которой расчеты КИН проводили методами нелинейной механики разрушения. Для трещин, что проходят через наплавленный материал, где происходят скачки напряжений, обосновано применение метода функций влияния, для чего разработано аналитико-численную процедуру, что использует частично непрерывные базовые законы нагружения.

The thesis deals with the study of the fracture behaviour of the pipe grades of high density polyethylene (HDPE). Detailed study of current state of knowledge was performed in the first part. The focus was put on those types of polymers used for pipeline applications, the limit states of the plastic pipes, and the key parameters affecting the fracture behaviour of plastic pipes’ materials. The methods used for testing the plastic pipes and native pipeline materials were summarized. In the second part of this work, the relationship between the processing conditions, internal structure and fracture behaviour of the native types of HDPE pipe grades was evaluated. Than, the correlation between short-term alternative tests and the standardized accelerated full notch creep test (FNCT) was studied. An experimental basis for numerical model for evaluation of the fracture behaviour of multilayer pipes was established. Two types of non-homogeneous specimens for evaluating the resistance of multilayer pipes against slow crack growth (SCG) were suggested. This evaluation was based on the parameters of linear elastic fracture mechanics (LEFM) and elasto-plastic fracture mechanics (EPFM).

Glass-fibre reinforced plastic pipes are widely used to convey fluids for various purposes. They offer a number of distinct advantages over conventional metals, such as high specific strengths, high specific moduli, superior corrosion resistance and low coefficient of thermal expansion. However, their behaviour under lateral quasi-static and impact loading are still not well known. The research programme described in this thesis was designed to characterise the performance of 55° winding angle GRP pipes, subjected to lateral quasi-static and impact loading. Two approaches: experimental tests and finite element analysis, were used to investigate the behaviour of the GRP pipes. The experimental investigation was started with diametral compression of short GRP pipes to examine the structural behaviour and failure mechanisms. Subsequently, lateral indentation tests were conducted on rigid-foundation supported or simply supported specimens using two different indenter geometries: line-ended and flat-ended. Furthermore, low-velocity impact tests were performed under similar conditions as those for indentation tests in order to characterise the response of the GRP pipes and to identify the correlation between the two forms of loading. The pipes exhibited multi-mode failure mechanisms, resin cracks, delaminations and fibre breakage. It is found that delamination, which resulted in significant loss in stiffness and strength, was the most significant mode of failure for the GRP pipes. A good correlation in behaviour was identified between quasi-static indentation and its energy equivalent low-velocity impact when the global bending stiffness of the GRP specimens were high. Specimens with span S 10.5D i, where Di is the internal diameter of the pipe, are considered to have high bending stiffness, while simply supported specimens with S10.5D i have low bending stiffness. Irrespective of the support conditions and loading type, specimens with high bending stiffness followed a failure mechanism sequence: local resin failure, delamination and the fibre breakage. However, the large global bending experienced by low bending stiffness specimens resulted in a change of failure mechanism, only local damage and surface tensile cracks were observed.

The main goal of this work is to develop an estimation procedure for the and CTOD driving forces for circumferential surface cracks in pipelines under combined bending and internal pressure loading. It is intended that the methodology proposed here will be applicable to a significant range of pipe and crack geometries, material yielding and strain hardening characteristics as well as loading biaxiality levels. In particular, pipelines currently employed in the offshore oil and gas production industry constitute an important class of potential application for this kind of procedure, and thus the current structural integrity concerns involving the reeling of pressurized pipelines have served as a motivating theme and bridge with real-world application throughout this research. The central theoretical framework upon which the developments presented here are based is the driving force estimation scheme known as EPRI methodology. This traditional approach for estimating and CTOD relies on splitting the driving forces into elastic and plastic components. The elastic part is calculated directly from widely available stress intensity factor solutions, while the plastic part is determined using fully plastic solutions derived from power-law descriptions of material behaviour. In the first part of this work the EPRI methodology in its more conventional form is extended to cover the cases of interest, allowing the calculation of and CTOD for circumferential surface cracks in pipelines subjected to combined internal pressure and bending loadings. This is done by carrying out detailed finite element simulations of bending of pressurized cracked pipes, the results of which then allow the direct determination of non-dimensional functions that correlate (CTOD) with applied loading, consistent with the form of the original EPRI fully plastic solutions. In the second part of the work attention is given to certain drawbacks and limitations of the procedure developed in the first part. A new, alternative procedure is then proposed which aims to overcome these problems by combining the main ideas behind the EPRI methodology with concepts from strain based design. The theoretical framework underlying this development is laid out and the analytical derivation of the new driving force estimation scheme is presented. Finally, results are given in a form similar to that employed in the first part, and the two procedures are compared. While they are shown to be conceptually equivalent, the strain based methodology is argued to be more readily applicable to some important classes of real world problems. The work concludes with comments on the effects of load biaxiality over crack driving forces and with discussions on the quality, accuracy and physical meaningfulness of the non-dimensional scaling functions obtained which correlate crack driving forces to loads or strains.
O objetivo principal deste trabalho é o desenvolvimento de um procedimento para estimação das forças motrizes e CTOD para trincas circumferenciais superficiais em dutos submetidos a carregamento combinado de flexão e pressão interna. Pretende-se que a metodologia aqui proposta seja aplicável a uma ampla faixa de geometrias de duto e trinca, características de escoamento e encruamento de material e níveis de biaxialidade de carregamento. Em particular, dutos atualmente empregados na exploração submarina de óleo e gás constituem uma classe importante de aplicações em potencial para procedimentos dessa natureza. Por essa razão, a avaliação de integridade estrutural de dutos pressurizados submetidos a enrolamento em carretel serve como tema motivador e ponto de conexão com a aplicação real ao longo deste trabalho. A base teórica fundamental sobre a qual se assentam os desenvolvimentos aqui propostos é o procedimento de estimação de forças motrizes de trinca conhecido como metodologia EPRI. Este método tradicional de cálculo de e CTOD separa as forças motrizes em componentes elástica e plástica. A component elástica é calculada diretamente a partir de soluções para o fator de intensidade de tensões, que se encontram amplamente disponíveis. A componente plástica, por sua vez, é determinada a partir de soluções totalmente plásticas derivadas de um modelo de lei de potência para o comportamento do material. Na primeira parte deste trabalho a metodologia EPRI em sua forma tradicional é estendida para abranger os casos de interesse, permitindo assim a determinação de e CTOD para trincas circumferenciais superficiais em dutos carregados por flexão e pressão interna. Para isto empregam-se simulações computacionais por elementos finitos, os resultados das quais permitem a determinação direta de fatores adimensionais que correlacionam (CTOD) com o carregamento aplicado de maneira consistente com as soluções totalmente plásticas da metodologia EPRI original. Na segunda parte do trabalho a atenção se volta para algumas deficiências e limitações da metodologia desenvolvida na primeira parte. Um procedimento novo é proposto como alternativa, tendo por objetivo superar estas dificuldades a partir da combinação dos principais conceitos da metodologia EPRI com ideias derivadas do projeto baseado em deformações. Apresentam-se a base teórica subjacente à estes conceitos e a derivação analítica do novo procedimento de estimação de forças motrizes. Finalmente, resultados semelhantes aos obtidos na primeira parte são calculados e os dois procedimentos são comparados. Embora ambos sejam conceitualmente equivalentes, argumenta-se que o procedimento baseado em deformações é mais imediata e convenientemente aplicado a algumas classes importantes de problemas práticos. O trabalho encerra com comentários acerca dos efeitos de biaxialidade de carregamento sobre as forças motrizes de trinca e com discussões sobre a acurácia, relevância, e significância física das funções adimensionais que escalam as forças motrizes com cargas ou deformações.

In this study, hydrogen induced cracking (HIC) behavior of welded steels used in petroleum lines under sour petroleum environments was investigated. The testing environment in NACE TM0284-2003 standard was used in order to simulate sour petroleum environment. In order to investigate behavior of welding parameters, used in pipe production, on HIC, welds were done with different line energies. Two different API X-65 steels were used in welding operations. The specimens taken from welded zones were tested in testing environment. The specimens were examined metallographically. Crack lengths were measured with a computer program. The results obtained were discussed in view of metallurgical and welding parameters aspects. The result obtained from this investigation led to a general conclusion that, the metallurgical parameters of steels used in pipe production were more important than welding parameters regarding their effect on HIC. It was shown that the composition and microstructural grain size of steels were in direct relation to HIC.

Le soudage du bois est une technique d'assemblage sans adhésif de deux pièces de bois, leur soudure étant produite par friction mécanique sous pression des deux pièces. Ce procédé, applicable à des pièces de bois plates, d'essences identiques ou différentes, se prête à la fabrication de meubles et à la menuiserie. Cependant, le joint obtenu n'est pas de classe "extérieur", ce qui le réserve à un usage "intérieur". En effet, un joint destiné à une utilisation extérieure ou en milieu à humidité variable doit présenter une résistance élevée à l'eau. L'objectif principal de cette thèse est d'étudier la résistance à l'eau du bois soudé. A cet effet, des méthodes d'essais complémentaires et non-destructrices ont été utilisées, comme le scanner ou l'imagerie par résonance magnétique (IRM). L'influence des paramètres de soudage et des propriétés du bois sur, d'une part, la formation et la propagation des fissures dans la ligne de soudure, et sur, d'autre part, la densité et l'absorption d'eau de la soudure a été ainsi étudiée. Les expériences de cette thèse seront menées sur des échantillons de pin (Pinus sylvestris) de dimensions 200 mm x20 mm x 40 mm, coupés dans la direction longitudinale du fil du bois. La Norme Européenne EN 205 a servi de cadre pour déterminer la résistance des échantillons de pin en traction-cisaillement. Les méthodes d'essais (non-destructrices) ont été utilisées selon leur pertinence: le scanner a servi à étudier la formation et la propagation des fissures; l'imagerie par résonance magnétique (IRM) a permis quant à elle de caractériser la pénétration et l'infiltration d'eau dans le bois soudé.Le mécanisme d'adhérence du pin a été étudié grâce à la RMN MAS (spectrométrie à résonance magnétique nucléaire avec polarisation croisée et rotation à l?angle magique) du carbone13 et à la micro-densitométrie par rayons X. Ces différentes méthodes, non destructrices, offrent l'avantage d'une analyse non invasive et l'élimination de facteurs parasites liés à la préparation et à la coupe du bois. Voici en résumé les résultats obtenus les plus marquants: (1) Le scanner et l'imagerie par résonance magnétique (IRM) sont des méthodes de recherche particulièrement polyvalentes et adaptées à l'étude des bois soudés. (2) L'utilisation de bois de coeur, une pression de soudage de 1.3 Mpa et un temps de soudage de 1.5 s permettent d'augmenter la résistance à l'eau du pin soudé. (3) Des tests d'optimisation ont montré que la résistance du pin en traction-cisaillement est plus sensible aux variations de temps de soudage qu'au temps de refroidissement et qu'elle peut être optimisée à plus de 9.7 MPa en respectant une pression de 1.3 Mpa, un temps de soudage > 3.5 s et un temps de refroidissement < 60 s. (4) La résistance à l'eau du bois soudé peut être améliorée dans une certaine mesure en faisant varier paramètres de soudage et propriétés des essences, mais dans tous les cas, le recours à un imperméabilisant naturel et écologique reste nécessaire. (5) Le pin soudé possède une résistance à l'eau et en traction-cisaillement inhabituellement élevée, cela pouvant s'expliquer par une teneur en composés extractifs augmentée. (6) Des essais sous IRM ont montré que les causes de rupture du joint varient suivant l'essence: faible résistance à l'eau de la ligne de soudure dans le cas du hêtre soudé, retrait et expansion du bois dans le cas du pin soudé. (7) Les extractifs du pin améliorent nettement la résistance à l'eau du joint soudé, mais à un niveau qui ne lui permet cependant pas la certification "extérieur" sans protection. En revanche, il peut être certifié "semi-extérieur" avec protection
Wood welding is a mechanical friction process allowing the assembly of timber without any adhesives. The process consists of applying mechanical friction, under pressure, alternately to the two wood surfaces to be welded. This process can be applied to weld two flat pieces of timber, originating from the same or different tree species, and can be used in the manufacture of furniture and wood joinery. The only limitation is that the joint is not exterior-grade, but only suitable for interior joints. Exterior use, or use in an environment with varying humidity demands water resistance of the welded joints. The main objective of this thesis is to study the water resistance of the welded wood. This is complemented with special attention to non-destructive test methods such as X-ray Computed Tomography (CT-) scanning and Magnetic Resolution Imaging (MRI). The influence of welding parameters and wood properties on crack formation and crack propagation in the weldline was investigated. The influence of these parameters on weldline density and water absorption in the weldline were also studied. Investigations in this thesis are based on welded samples of Scots pine (Pinus sylvestris) of the dimensions 200 mm × 20 mm × 40 mm which were cut in the longitudinal direction of the wood grain. The tensile-shear strength of the welded Scots pine samples were determined using European standard EN 205. Different non-destructive methods such as X-ray Computed Tomography (CT-) scanning to study crack formation and propagation, and magnetic Resolution Imaging (MRI) to characterize water penetration and the distribution mechanism in welded wood were used. Solid state CPMAS 13C NMR spectrometry and X-ray microdensitometry investigations were carried out to study the mechanism of adhesion in Scots pine. These various non-destructive methods offer the advantage of non-invasive analysis and the elimination of any artifacts present due to preparation and sectioning. The most important results are summarized as follows: (1) X-ray Computed Tomography (CT-) scanning and Magnetic Resolution Imaging (MRI) are versatile research methods applicable to investigations of welded woods. (2) Water resistance of welded Scots pine can be increased using heartwood, a welding pressure of 1.3 MPa, and a welding time of 1.5 s. (3) Optimization tests showed that the tensile-shear strength of Scots pine was more sensitive to welding time changes than holding time and could be optimized to more than 9.7 MPa using 1.3 MPa welding pressure, > 3.5 s welding time, and < 60 s holding time. (4) Changing welding parameters and wood properties can increase water resistance of welded wood to some extent, but treating the weldline with certain natural and environmentally-friendly water repellents is still necessary. (5) Welded Scots pine shows unusually high water resistance and tensile-shear strength. This may be explained by there being more extractives compounds in Scots pine. (6) MRI experiments showed that the origin of the joint failure in welded beech is poor water resistance of the weldline, while swelling and shrinkage of wood are the main reasons for joint failure of welded Scots pine. (7) Extractives in Scots pine dramatically improve water resistance of the welded joint, but not to a level to classify the joint as an unprotected exterior grade. However, it can qualify as a joint for protected semi-exterior application

La crescente attenzione verso un utilizzo attento, sostenibile ed economicamente efficiente della risorsa idrica rende di primaria importanza il tema delle perdite idriche e della gestione efficiente dei sistemi idrici. La richiesta di controlli dell’uso dell’acqua è stata avanzata a livello mondiale. Il problema delle perdite idriche nei Paesi industrializzati è stato così affrontato con specifiche normative e procedure di best practice gestionale per avanzare una valutazione delle perdite idriche e una limitazione degli sprechi e degli usi impropri. In quest’ambito, la pressione gioca un ruolo fondamentale nella regolazione delle perdite reali. La regolazione delle pressioni nelle diverse ore del giorno consente, infatti, di poter agire su queste ultime perdite, che aumentano all’aumentare della pressione secondo una cosiddetta legge di potenza. La motivazione della presente tesi è originata dalla necessità di quantificare il livello di perdita idrica in un sistema acquedottistico in relazione alla pressione all’interno del sistema stesso. Per avere una stima realistica che vada al di là della legge della foronomia, si vuole valutare l’influenza della deformabilità della condotta in pressione fessurata sull’entità delle perdite idriche, con particolare attenzione alle fessurazioni di tipo longitudinale. Tale studio è condotto tramite l’introduzione di un semplice modello di trave alla Winkler grazie al quale, attraverso un’analisi elastica, si descrive il comportamento di una generica condotta fessurata longitudinalmente e si valuta la quantità d’acqua perduta. I risultati ottenuti in condizioni specifiche della condotta (tipo di materiale, caratteristiche geometriche dei tubi e delle fessure, etc.) e mediante l’inserimento di opportuni parametri nel modello, calibrati sui risultati forniti da una raffinata modellazione tridimensionale agli elementi finiti delle medesime condotte, verranno poi confrontati con i risultati di alcune campagne sperimentali. Gli obiettivi del presente lavoro sono, quindi, la descrizione e la valutazione del modello di trave introdotto, per stabilire se esso, nonostante la sua semplicità, sia effettivamente in grado di riprodurre, in maniera realistica, la situazione che si potrebbe verificare nel caso di tubo fessurato longitudinalmente e di fornire risultati attendibili per lo studio delle perdite idriche. Nella prima parte verrà approfondito il problema della perdite idriche. Nella seconda parte si illustrerà il semplice modello di trave su suolo elastico adottato per l’analisi delle condotte in pressione fessurate, dopo alcuni cenni teorici ai quali si è fatto riferimento per la realizzazione del modello stesso. Successivamente, nella terza parte, si procederà alla calibrazione del modello, tramite il confronto con i risultati forniti da un’analisi tridimensionale agli elementi finiti. Infine nella quarta parte verrà ricavata la relazione flusso-pressione con particolare attenzione all’esponente di perdita, il cui valore risulterà superiore a quello predetto dalla teoria della foronomia, e verrà verificata l’effettiva validità del modello tramite un confronto con i risultati sperimentali di cui è stata fatta menzione in precedenza.

Face à la complexité croissante des trajectoires et des conditions opérationnelles des forages pétroliers et géothermiques, le phénomène de fatigue est devenu la cause principale de rupture des garnitures de forage. La fatigue des tiges est essentiellement liée à leur flexion cyclique due à leur rotation dans une section courbe du puits. L'objectif de ce travail est d'élaborer une méthodologie ainsi que les modèles numériques nécessaires pour évaluer la fatigue des tiges au cours du forage de puits à trajectoire complexe. Pour ce faire, nous proposons d'abord de choisir parmi les approches existantes de prévision de la durée de vie en fatigue d'une structure celles qui nous ont semblé les plus pertinentes pour le problème de fatigue des systèmes de forage. Puis, ces approches (qui comprennent les théories de la fatigue et de la rupture ainsi que des lois empiriques), et des logiciels de calcul de structures sont ensuite intégrés dans des algorithmes de calcul incrémental de la fatigue d'un système en fonction de l'évolution de l'opération du forage. Du fait que les contraintes dans les tiges restent souvent dans le domaine élastique, deux modèles de fatigue des tiges sont développés : un premier est basé sur le calcul du cumul de fatigue et un second sur le calcul de la propagation de fissure par fatigue. Ces deux modèles peuvent être utilisés dans la phase de conception de la trajectoire du puits et de la garniture pour le forer, de même qu'en opération pour prédire les risques de rupture par fatigue du train de tiges. Ceci permet à l'opérateur de planifier la gestion des tiges et leurs inspections en fonction de l'historique de leur utilisation
Facing the growing complexity of trajectories and operating conditions of oil and geothermal drillings, the fatigue phenomenon has become the main cause of drill-string failure. The fatigue of drill-pipes is essentially due to their cyclic bending caused by their rotation in a curved section of the well. The objective of this work is to develop a methodology and the necessary numerical models to assess the fatigue of drill-pipes during drilling operations of complex trajectory wells. For this purpose, we propose firstly to choose among the available approaches for structure fatigue life prediction those that seem most relevant to drill-string fatigue problem. Then, these approaches (which include the fatigue and fracture theories as well as empirical laws), and structural calculation software are then integrated into incremental computation algorithms of drill-pipe fatigue in function of drilling operation evolution. Since the stresses in drill-pipes remain often within the elastic domain, two fatigue models for drill-pipes are developed: the first one is based on the cumulative fatigue calculation and the second one on the fatigue crack growth calculation. These models can be used in the well and drill-string design, or in real time during drilling to predict the fatigue failure in the drill-string. This allows the drill operator to plan the management of drill-pipes and their inspections depending on their usage history

Cílem této diplomové práce je řešit problematiku poškozování svařovaných polyolefinových potrubí. Prezentovaná práce je zaměřena na numerické modelování kvazi-křehkého porušování, které je přímým důsledkem šíření creepové trhliny. Svařování polyolefinových potrubí vnáší do oblasti svaru nehomogenní rozložení materiálových vlastností a tvar svarového výronku způsobuje vznik geometrických koncentrátorů napětí. Posouzení těchto vlivů na životnost svarového spoje a výpočet polyolefinových potrubí je hlavním cílem této práce. V práci je uveden popis několika experimentálních testů (především CRB a PENT) k určení materiálových vlastností popisujících kinetiku trhliny. Výsledky práce jsou prezentovány pro polyetylenové potrubí (PE 100 110 x 6.3 SDR 17.6). Polyetylen je jeden z nejčastěji používaných materiálů právě v oblasti potrubních systémů. Prezentované výsledky práce mohou být použity i pro jiný polyolefinový materiál. Predikce životnosti polyolefinových potrubí je založena na lineárně elastické lomové mechanice (LELM) a koncepci faktoru intenzity napětí. Diplomová práce doplňuje tuto metodologii výpočtu životnosti o výpočet svařovaných polyolefinových potrubí z pohledu lomové mechaniky s náznakem možnosti budoucího výzkumu.

The lifetime of plastic pipes for water supply and other applications is demanded to exceed at least 50 years. Such a long lifetime is difficult to prove by standard testing methods like the hydrostatic pres-sure test. However, it is possible to calculate an estimation of the lifetime, as the most frequently oc-curring mechanism of failure of plastic pipes is the creep crack propagation and subsequent failure. The method is based on describing the crack propagation by parameters of the linear-elastic fracture mechanics. An important part of this method is a finite element simulation of crack propagation in a pipe loaded by various types of loads. Residual stress, a side product of solidification after extrusion, is one of these loads. This thesis begins with an introductory part and literature review of the relevant topics – most of all the typical material properties of the pipe materials, mechanisms of failure, methods of residual stress determination suitable for plastic pipes and their results, methods of testing the materials and calculating lifetime. After the introduction, the problems to be solved are defined. The description of the residual stress state in the wall of various plastic pipes is the main topic. Both tangential (hoop) and axial residual stress in pipes of different dimensions and materials are determined using a combination of experiments and numerical simulations. Also, a simplified method of tangential residual stress is designed that can provide a sufficiently precise description of the tangential resid-ual stress state and is not difficult to carry out. A method to include the residual stress in the lifetime calculations and its influence on the lifetime is also dealt with. Apart from residual stress, the influence of soil loads in case of a buried pipe is studied. The residual stress can also influence the experimental determination of crack growth rate. If the CRB (cracked round bar) test is used to measure the crack growth rate, the crack can propagate asymmetrically due to the presence of residual stress in the specimens, which affects the results. Based on a finite element simulation of crack propagation in a CRB specimen, the severity of the influence is assessed.

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Este trabalho apresenta um estudo prático da aplicação do conceito Leak Before Break (LBB), usualmente aplicado em usinas nucleares, em uma tubulação fabricada a partir de aço AISI 316LN soldada com a utilização de eletrodo revestido AISI 316L. O LBB é um critério fundamentado em análises de mecânica da fratura, que considera que um vazamento proveniente de uma trinca, presente em uma tubulação, possa ser detectado por sistemas de detecção de vazamento, antes que esta trinca alcance um tamanho crítico que implique na falha da tubulação. Na tubulação estudada, foram realizados ensaios mecânicos de tração e análises de Ramberg-Osgood, bem como ensaios de tenacidade à fratura para a obtenção da curva de resistência J-R do material. Os ensaios foram realizados considerando o metal base, a solda e a zona termicamente afetada (ZTA), nas temperaturas de operação de uma planta nuclear. Para as propriedades mecânicas encontradas nos ensaios foram realizadas análises de carga limite para se determinar o tamanho da trinca que cause um vazamento detectável e, também, o seu tamanho crítico que cause a falha por colapso plástico. Para o tamanho crítico de trinca encontrado na solda, região que apresentou a menor tenacidade, foram realizadas análises de Integral J e de módulo de rasgamento T, considerando falha por rasgamento dúctil. Os resultados demonstram um comportamento bem definido entre o metal base, a ZTA e a solda, onde o metal base apresenta um comportamento altamente tenaz, a solda um comportamento pouco tenaz e a ZTA apresentou propriedades mecânicas intermediárias entre o metal base e a solda. Utilizando o software PICEP, foram determinadas as curvas de taxa de vazamento versus tamanho de trinca e também o tamanho crítico da trinca, considerando análise por carga limite. Observou-se que, após certo tamanho de trinca, a taxa de vazamento do metal base é muito maior do que para a ZTA e solda, para um mesmo comprimento de trinca. Isso ocorre porque é esperado que a trinca cresça de forma mais arredondada no metal base, devido à sua maior tenacidade. O menor tamanho crítico de trinca foi encontrado para o metal base para trincas circunferenciais. Para as análises de Integral J realizadas na solda, foi demonstrado que a falha por rasgamento dúctil não ocorrerá nas condições consideradas e essa hipótese foi sedimentada pela análise de mecânica da fratura elasto-plástica (MFEL) com o uso do diagrama J/T. Dessa forma, pode-se concluir que a tubulação estudada estaria apta a ser empregada em um circuito primário de um reator que utilizasse o critério de LBB, nas condições de carregamento e geometria consideradas. Adicionalmente, concluiu-se que nessas condições apenas o modo de falha por colapso plástico é esperado.
Dissertação (Mestrado em Tecnologia Nuclear)
IPEN/D
Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

A new methodology of polymer pipe lifetime estimation taking into account residual stresses is described in this thesis. Engineering equations derived based on numerical simulations of a hydrostatic pressure test are proposed. Residual lifetime calculations were performed for different loading conditions using experimental data of a creep crack propagation in studied material and stress distribution in the pipe wall. The effects which significantly influence lifetime estimation were quantified with special focus on residual stresses.

Le problème du rendement de la distribution des eaux potables a récemment fait l’objet de nombreux travaux de recherche. En effet, les ruptures des canalisations et les fuites sont des phénomènes assez fréquents en milieu urbain. Afin de réduire au maximum les risques de fuite à long terme, près de 60 % des réseaux d’eau potable sont construits ou renouvelés avec des conduites en polyéthylène de troisième génération, PE100. De par ses caractéristiques, c’est un matériau de choix pour les réseaux d’adduction d’eau. Mais la présence d’un défaut superficiel peut générer la rupture de la conduite sous l’effet d’écoulement transitoire. Afin d’étudier ce problème, notre étude est décomposée en deux parties. La première est consacrée à l’étude de l’effet dynamique du comportement viscoélastique du matériau sur l’écoulement transitoire à l’aide d’un modèle mathématique que nous avons développé dans cette thèse. Nous avons démontré que le comportement viscoélastique du PE100 joue un rôle primordial dans l’amortissement du phénomène du coup de bélier et de résonance. En cas de présence de couplage de jonction, l’étude de l’interaction fluide-structure est nécessaire pour définir correctement les fréquences propres d’un système de conduites. Ensuite, dans la seconde partie, nous avons étudié le comportement à la fissuration des conduites en PE100 présentant un défaut. A l’aide d’essais expérimentaux et de modélisations par éléments finis, nous avons démontré que le concept de la mécanique élasto-plastique de rupture, l’intégrale J, peut définir avec une précision acceptable le comportement à la fissuration d’une conduite en PE100. Nous avons cherché à déterminer l’intégrale J à l’aide d’un modèle semi-empirique afin de définir une relation entre la pression d’amorçage de fissure, la taille de défaut (fissure semi-elliptique) et la géométrie de la conduite
The efficiency of potable water distribution systems has been recently the subject of much research. Leakages or a ruptures, occurring in pipes, are quite a frequent phenomenon in urban areas. In order to minimize long-term risks of leakage, about 60% of potable water networks are built or renewed with the third generation of polyethylene pipe, PE100. It is a material with various features, which makes it a widely used material in the water supply networks. However, under transient conditions and with a surficial defect in the pipe, a failure occurs. To investigate this issue, our study is divided into two main parts. The first part is studying the dynamic effect of the viscoelastic material under transient flow conditions using a developed mathematical model. We have demonstrated that the viscoelastic behavior of PE100 plays a primordial role in the damping of water hammer and resonance phenomenon. In case of presence of coupling junction, a study of the fluid structure interaction is mandatory to correctly define the natural frequencies of a piping system. In the second part, we have investigated the crack behavior of PE100 pipes with defects. Using the experimental tests and finite elements results, we have demonstrated that the concept of elastic-plastic fracture mechanics, the J-Integral, define with reasonable accuracy the behavior of cracked PE100-pipe. At the end, we sought to determine the J-Integral with a semi-empirical model in order to define a relationship between the crack initiation pressure, the dimension of semi-elliptical surface cracks and the geometry of the pipe

The work deals with design of load-bearing structure of watertower. The solution include design and assessment of all main parts of watertower according to ČSN EN 1992 in the ultimate limit state and the serviceability limit state (crack limitation).

A procedure to evaluate the leaching properties of radionuclides from irradiated graphite waste has been developed by combining ANSI 16.1 (USA) and NEN 7345 (Netherlands) standardised diffusion leaching techniques. The ANSI 16.1 standard has been followed to the acquire the leachates and to determine the leach rate/ diffusion coefficient and NEN 7345 standard technique has been used to determine the diffusion mechanism of radionuclides. The investigation employs simulated Drigg groundwater as a leachant using semi-dynamic technique for the production of leachate specimens. From gamma spectroscopy analysis the principal radionuclides present in terms of activity were 60Co, 137Cs, 134Cs, 155Eu, 133Ba and 46Sc. The dominant radionuclides are 60Co, 134Cs and 133Ba which together account for about 91 % of the total activity. The 91 % can be broken down into 73.4 % 60Co, 9.1 % 134Cs and 8.1 % 133Ba. Analysis of total beta and total beta without tritium activity release from Magnox graphite was measured using liquid scintillating counting. Preliminary results show that there is an initial high release of activity and decreases when the leaching period increases. This may be due to the depletion of contaminants which were absorbed by the internal pore networks and the surface. During the leaching test approximately 275.33 ± 18.20 Bq of 3H and 106.26 ± 7.01 Bq of 14C was released into the leachant within 91 days. Irradiation induced damages to the nuclear graphite crystal structure have been shown to cause disruption of the bonding across the basal planes. Moreover, the closures of Mrozowski cracks have been observed in nuclear graphite, the bulk property are governed by the porosity, in particular, at the nanometre scale. Therefore, knowledge of the crystallite structure and porosity distribution is very important; as it will assist in understand the affects of irradiated damage and location and the mechanism of the leaching of radionuclides. The work reported herein contributed several key findings to the international work on graphite leaching to offer guidance leading toward obtaining leaching data in the future: (a) the effective diffusion coefficient for 14C from graphite waste has been determined. The diffusion process for 14C has two stages resulting two different values of diffusion coefficient, i.e., for the fast and slow components; (b) the controlling leaching mechanism for 3H radionuclide from graphite is shown to be surface wash–off; and for that of 14C radionuclide the initial controlling leaching mechanism is surface wash-off following by diffusion which is the major transport mechanism ; (c) The weight loss originates from the open pore structure which has been opened up by radiolytic oxidation; at the higher weight losses much of the closed porosity in the graphite has been opened. The investigation indicates that weigh loss has a major influence on the leaching of elements from the irradiated graphite; and (d) the analysis of the pores in nuclear graphite can be categorised into three types. These three types of pores are: (1) small pores narrow which are slit-shaped pores in the binder phase or matrix, (2) gas evolution pores or gas entrapment pores within the binder phase or matrix and (3) lenticular pores which are large cracks within the filler particles. It is shown in this thesis that by using tomography to study the morphology of the different pores coupled with the distribution of impurities an understanding of the role of porosity in leaching is possible.

Monitoring the mechanical integrity of critical structures is extremely important, as mechanical defects can potentially have adverse impacts on their safe operability throughout their service life. Structural defects can be detected by using active structural health monitoring (SHM) approaches, in which a given structure is excited with harmonic mechanical waves generated by actuators. The response of the structure is then collected using sensor(s) and is analyzed for possible defects, with various active SHM approaches available for analyzing the response of a structure to single- or multi-frequency harmonic excitations. In order to identify the appropriate excitation frequency, however, the majority of such methods require a priori knowledge of the characteristics of the defects under consideration. This makes the whole enterprise of detecting structural defects logically circular, as there is usually limited a priori information about the characteristics and the locations of defects that are yet to be detected. Furthermore, the majority of SHM techniques rely on sensors for response collection, with the very same sensors also prone to structural damage. The Surface Response to Excitation (SuRE) method is a broadband frequency method that has high sensitivity to different types of defects, but it requires a baseline. In this study, initially, theoretical justification was provided for the validity of the SuRE method and it was implemented for detection of internal and external defects in pipes. Then, the Comprehensive Heterodyne Effect Based Inspection (CHEBI) method was developed based on the SuRE method to eliminate the need for any baseline. Unlike traditional approaches, the CHEBI method requires no a priori knowledge of defect characteristics for the selection of the excitation frequency. In addition, the proposed heterodyne effect-based approach constitutes the very first sensor-less smart monitoring technique, in which the emergence of mechanical defect(s) triggers an audible alarm in the structure with the defect. Finally, a novel compact phased array (CPA) method was developed for locating defects using only three transducers. The CPA approach provides an image of most probable defected areas in the structure in three steps. The techniques developed in this study were used to detect and/or locate different types of mechanical damages in structures with various geometries.

The subject of this diploma thesis is the design of the underground parts of the building. Part of this thesis is to comapare alternative designs and subsequentli evaluate the interaction of the upper structure with subsoil and pile fundantion. A detailed static assessment and implementation documentation is then prepared for the selected variant. The design of the base plate and reinforced concrete walls of the underground section are designed with respect to the crack width according to the principles for designing the white box. The theoretical part contains principles for designing and implementing a white box and evaluating design variants.

The prevalence of crack use among illicit drug users has dramatically increased in Canada over the past decade. The sharing of crack pipes and other crack use paraphernalia is common among users of crack cocaine and is associated with unique negative health harms and costs (Haydon & Fischer, 2005). This thesis explores the phenomenon of crack pipe sharing among crack users in Victoria, British Columbia. The study uses data from in-depth interviews with thirteen self-reported crack users who regularly share crack pipes. Interviews explored the experiences of participants around crack pipe sharing, focusing on contextual, social and environmental factors that influenced the sharing of pipes. Crack pipe sharing is presented as a largely social act around which shared meanings have emerged. The findings illustrate the social context of crack pipe sharing, which is mediated by informal rules and etiquette, as well as distinct sanctions and consequences for deviating from the generally accepted norms around sharing pipes. Further, three distinct dimensions of crack pipe sharing are proposed - mutual, distributive and receptive sharing - each associated with various costs and benefits, and framed by relations of status and power. The results of this study also demonstrate that crack pipe sharing serves a number of real and distinct purposes in crack users’ lives, providing economic, control and social functions. My findings illustrate that, despite the various health and social harms related to crack pipe sharing, sharing pipes makes sense in the reality and lived experience of the participants.

Thesis (M. Sc.)--University of Alberta, 2009.
Title from pdf file main screen (viewed on July 16, 2009). "A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Science, Department of Mechanical Engineering, University of Alberta." Includes bibliographical references.

Stress corrosion cracking (SCC) from the external surface of a buried pipeline is a serious matter and can cause significant economic and environmental losses. Despite of many research works which have been done on the understanding of crack initiation and propagation mechanisms, these mechanisms are still being debated. This research studied the crack growth behaviour of different pipeline steels including two types of X65, one X52 and one X80 pipeline steels in near-neutral pH environments. Crack growth behaviour of all steels has been found to be consistent with that of true corrosion fatigue. Crack growth rates were correlated with (K)2Kmax/f0.1. It was revealed that cracking behaviour of pipeline steels in near neutral pH environments is material dependent. Highest crack growth rate was seen in the steel which highest amount of hydrogen atoms could be generated and stored in its microstructure to contribute in cracking procedure due to hydrogen embrittlement effect.
Materials Engineering

Concrete structures can often be modeled as plates, for example, bridges, tunnel walls and pipes. Near-surface damage in concrete structures mostly takes the form of cracking. Surface-breaking cracks affect concrete properties and structural integrity; therefore, the nondestructive evaluation of crack depth is important for structural monitoring, strengthening and rehabilitation. On the other hand, material damping is a fundamental parameter for the dynamic analysis of material specimens and structures. Monitoring damping changes is useful for the assessment of material conditions and structural deterioration. The main objective of this research is to develop new methodologies for depth evaluation of surface-breaking cracks and the evaluation of damping in concrete plates. Nondestructive techniques based on wave propagation are useful because they are non-intrusive, efficient and cost effective. Previous studies for the depth evaluation of surface-breaking cracks in concrete have used diffracted compressional waves (P-waves). However, surface waves exhibit better properties for the characterization of near surface defects, because (a) surface waves dominate the surface response, they carry 67% of the wave propagation energy, and present lower geometrical attenuation because the propagating wave front is cylindrical; and (b) the penetration depth of Rayleigh waves (R-waves) depends on their frequency. Most of the R-wave energy concentrates at a depth of one-third of their wavelengths. The transmission of R-waves through a surface-breaking crack depends on the crack depth; this depth sensitivity is the basis for the so-called Fourier transmission coefficient (FTC) method. R-waves only exist in a half-space (one traction-free surface); whereas in the case of a plate (two traction-free surfaces), Lamb modes are generated. Fundamental Lamb modes behave like R-waves at high frequencies, because their wavelengths are small relative to the plate thickness. Lamb modes are not considered in the standard FTC method, and the FTC method is also affected by the selected spacing between receivers. The FTC calculation requires the use of an explicit time window for the identification of the arrival of surface waves, and the selection of a reliable frequency range. This research presents theoretical, numerical and experimental results. Theoretical aspects of Lamb modes are discussed, and a theoretical transfer function is derived, which can be used to study changes of Lamb modes in the time and frequency domains as a function of distance. The maximum amplitude of the wavelet transform varies with distance because of the dispersion of Lamb modes and the participation of higher Lamb modes in the response. Numerical simulations are conducted to study the wave propagation of Lamb modes through a surface-breaking crack with different depths. The surface response is found to be dominated by the fundamental Lamb mode. Using the 2D Fourier transform, the incident, transmitted and reflected fundamental Lamb modes are extracted. A transmission ratio between the transmitted and incident modes is calculated, which is sensitive to crack depths (d) normalized to the wavelength (λ) in a range (d / λ) = 0.1 to 1/3. A new wavelet transmission coefficient (WTC) method for the depth evaluation of surface-breaking cracks in concrete is proposed to overcome the main limitations of the FTC method. The WTC method gives a global coefficient that is correlated with the crack depth, which does not require time windowing and the pre-selection of a frequency bandwidth. To reduce the effects of wave reflections, which are present in the FTC method because of the non-equal spacing configuration, a new equal spacing configuration is used in the WTC method. The effects of Lamb mode dispersion are also reduced. In laboratory tests, an ultrasonic transmitter with central frequency at 50kHz is used as a source; the 50kHz frequency is appropriate for the concrete plate tested (thickness 80mm), because the fundamental Lamb modes have converged to the Rayleigh wave mode. The new method has also been used in-situ at Hanson Pipe and Precast Inc., Cambridge, Ontario, Canada, and it shows potential for practical applications. In general, the evaluation of material damping is more difficult than the measurement of wave velocity; the dynamic response and attenuation of structural vibrations are predominantly controlled by damping, and the damping is typically evaluated using the modal analysis technique, which requires considerable efforts. The existing methods based on surface waves, use the Fourier transform to measure material damping; however, an explicit time window is required for the spectral ratio method to extract the arrival of surface wave; in addition, a slope of the spectral ratio varies for different frequency ranges, and thus a reliable frequency range needs to be determined. This research uses the wavelet transform to measure material damping in plates, where neither an explicit time window nor the pre-selection of a frequency bandwidth are required. The measured material damping represents an average damping for a frequency range determined by source. Both numerical and experimental results show good agreement and the potential for practical applications.

碩士
淡江大學
機械工程學系
87
Title of Thesis: Page: 107 Mechanical Analysis and Design of Structural Pipes with Crack Name of Institute: Graduate Institute of Mechanical Engineering, TamKang University Graduate Date: July, 1999 Degree Conferred: Master Name of Student: Shih-Jeng Chen Advisor: C.J.Shih 陳 世 徵 Abstract: A statically indeterminated structural pipe with crack in this thesis has been used as the analytical and design problem. The mathematical model of the mechanics has been built up to analyze the mechanical behavior of the failure. To analyze the cracked- pipes deformation due to earthquake,the limit analysis theory and elastic-plastic fracture mechanics are combined to obtain the relation between the torsional angle-deformation and cracked angle-deformation under crack propagation,The analy-tical process and result allows one to understand the plastic collapse behavior of the structural pipe with crack. For improving the character of loading ability due to ductil unstable fracture,the J-R curve theory has been adopted tostudy the dJ/da which is the key factor of crack. Further more, the character of ductil unstable fracture can be applied to findthe critical dJ/da. For estimating the time required along the crack propagation,the stress corrosion cracking theory has been applied to compute the life estimation from the crack propagation tocollapse of pipe. The equation of collapsed load and deformation are used as constraints of optimal design. The goals are the minimization of volume and dJ/da value. Not only the structural fracture can be successfully analyzed,but also this presented optimization method can improve the crack behavior.

碩士
國立清華大學
動力機械工程學系
88
Based on the dc-pd (direct-current-potential-drop) technique, an efficient detection procedure is developed to identify the existence of unknown cracks in a pipe. By this procedure, the electric potential on the perfect pipe needs to be calculated first by finite element method. The proposed defect influence factor, which is defined as the electric potential of the defective pipe divided by that of the perfect one, is then employed to reveal the effect of unknown cracks on the electric potential. By depicting the constant contours of the defect influence factor with sufficient resolution, not only the position, but also the shape and size of cracks in the pipe can be identified accurately by the detection criterion devised in this work. Those cracks detected include circumferential crack, inclined crack, and multiple cracks. Good identified results show the merits of the procedure developed for the identification of unknown cracks in the pipe structure.

The derivation of the classical equation for flow through an orifice assumes a fixed orifice area. However, pipe materials exhibit expansion behaviour with increasing pressure, which alters the size of orifices and results in greater leakage rates. The purpose of this investigation was to study the behaviour of round holes and cracks in pipes through theoretical and experimental work. The results of the study include equations derived for increased flow through round holes in pressurized cylindrical shells and pipes. The theoretical models explain the increased flow experienced due to the leak area increasing. The models incorporate material properties, shell geometry and fluid properties for both uni-axially and bi-axially stressed pipe sections. Analytical results are compared with previous finite element investigations. In addition, an experimental study into the effects of pressure on a round hole in a class 6 uPVC pipe was conducted. Conclusions are made relating to the influence of material expansion to increased flow rate through openings in pressurised cylinders. The results compared positively with those of the theoretical equations. Conclusions are made relating to the influence of round hole or crack expansion to an increased flow rate through openings in pressurised pipes. Results include the effects of geometrical and material variables on the expansion of round holes. Better explanation of the increased flow through orifices, documented by practical observations, is presented. Results indicate that round hole area is linearly related to pressure. However, testing on longitudinal cracks resulted in a non-linear relationship between crack area and internal pipe pressure. Results indicate the expansion of round hole area is minimal. Leakage however is greatly affected over extended time periods by even the smallest increase in defect area. Critical pressures before brittle fracture obtained from testing on longitudinal cracks were compared to theoretical formulation. Results show a close relationship between current theory and experimentation.
Prof. K. van Zyl

D.Ing.
This study is a numerical investigation into the behaviour of cracks in uPVC pipes under pressure. This study is a continuation of a Masters dissertation which showed that leakage exponents vary significantly from the theoretical orifice exponent of 0.5 for cracks in pipes for different materials. This study looks at the behaviour of cracks in more detail and specifically with regard to the parameters of the pipe and crack. Using Finite Element Analysis the relationship between the pressure head and the leak area in pipes with longitudinal, spiral and circumferential cracks was investigated. It was found that the longitudinal, spiral and circumferential crack areas increase linearly with pressure. The slope of this linear relationship depends on various parameters, including loading state, pipe dimensions and pipe material properties. The effect that the individual pipe parameters had on the pressure-area slope was investigated. These parameters included the material properties of the pipe (Young’s modulus, Poisson’s ratio and longitudinal stress), the geometry of the pipe (internal diameter and wall thickness) as well as the geometry of the crack (length of the crack and the width of the crack). Once the effect of the pressure-area slope m is known, the link between the conventional leakage exponent N1 and the pressure-area slope m was further investigated and the effect of each parameter on the leakage exponent N1 was found. Using various data techniques the above data was combined and processed to find mathematical relationships that give reasonable descriptions of the pressure-area slopes of longitudinal, spiral and circumferential cracks. Once these equations for the pressure-area slopes were determined it was possible to obtain three new relationships for leakage from longitudinal, spiral and circumferential cracks.

碩士
國立虎尾科技大學
飛機工程系航空與電子科技碩士班
106
When a pipeline is subjected to a periodic loading for a long period of time, cracks initiated on the inner wall of the pipeline may cause safety concern, which is important when cracks occur in the region near pipe elbow where stress distribution is rather complex. This paper studies the impact of the bent angle of a pipe elbow and the location of a crack in crack growth process. The finite element method is used to analyze different elliptical cracks in a curved pipe wall, and to solve stress intensity factors and energy release rates of elliptical cracks. The cracks are assumed to keep in elliptic shape when they propagate. Two different crack growth models are studied in the present work: the constant crack growth rate mode and the non-constant crack growth rate mode. The length of the long axis and the short axis of the elliptic crack increases with the same amount every computational step in the former mode. In the latter mode, the crack of the short axis advances at a preset propagation rate, therefore, fixed crack length increase every step, and the crack length increase of the long axis is determined by the crack propagation rate, which depends on the local stress intensity factors. The behaviors of stress intensity factors, energy release rates and fatigue loading cycles of cracks were investigated under the constant crack growth rate mode and the non-constant crack growth rate mode. The results show that the stress intensity factor is larger when the position angle of crack is smaller, i.e. outer side of the pipe elbow with smaller curvature. When the pipe elbow bends, the stress intensity factor will increase until the bent angle reaches about 15 degree, after which the stress intensity factors reduces a little and the level up. Therefore, it can be concluded that the fracture position angle is minimal and the elbow bending angle is easily affected by the crack at 15 degrees. In both crack growth modes, stress intensity factors and energy release rates of the initial long axis are smaller than that of the short axis, however, the variation of the stress intensity factors and energy release rates of the long axis is larger than those of the short axis, and the stress intensity factors and the energy release rates of the long axis increase at faster rates than that of the short axis. In the constant crack growth rate model, the long axis and the short axis of crack are fixed at length 0.1mm. Under the two crack growth modes, the initial aspect ratio of two axes has little effect on the fatigue cycle life of the long axis. The short axis will have a shorter fatigue cycle life as the initial short axis ratio is smaller. In the non-constant crack growth rate mode, the crack length of the long and short axes of the elliptic crack has a direct effect on the local stress intensity factors, which cause the crack growth rates of the crack at different location are different along the crack front. However, as the crack advances, the elliptic aspect ratio of the crack will approaches to 0.88 under the present study.

One of the most prominent factors affecting the performance and longevity of risers is vortex induced vibration (VIV), which can cause severe fatigue damage, especially in risers used in deep waters. The available approaches for analyzing VIV induced fatigue in risers mainly focus on the VIV aspect of the problem; indeed less attention has been paid on the effect of VIV on a riser’s fatigue life and in prediction of fatigue life using various models. This dissertation first demonstrates how one can characterize fatigue of pipes and risers using an equivalent plate specimen as opposed to using a pipe specimen, thereby simplifying the task, yet obtaining good accuracy. Actual variable amplitude loadings (VAL) are used to study the fatigue crack growth in risers’ material with a focus on the various influencing parameters. Extensive experimental investigations are performed, followed by analytical and computational nonlinear finite element analyses. It is shown that the higher harmonics do cause significant fatigue damage, thus their influence should not be ignored. The influence of load interaction effects is also investigated, focusing on the fatigue crack growth retardation effects due to tension overloads, as well as the acceleration effects due to compression underloads. The crack closure concept is then used to explore into both the fatigue retardation and acceleration effects within a VAL scenario. An effective method for calculation of the stress intensity factor is proposed, which considers only the tensile portion of the stress range, while proposing another effective approach for accounting for the influence of compressive stress cycles. Moreover, a two-parameter approach is used in this dissertation, relating the fatigue crack growth rate (FCGR) to the crack tip opening displacement (CTOD). It is shown that the CTOD provides adequate information for calculating the FCGR under VAL, and it can be effectively used to account for the influence of the compressive stress cycles. The experimental investigation also considers the retardation effect resulting from the applied peak tensile overload cycles (TOLC) and the influence of various so-called “clipping” levels, demonstrating the significant influence of the TOLC on crack growth retardation in VAL.