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1
Shademan, Sassan Steven. "Mechanism-based models of fatigue crack growth /." The Ohio State University, 2000. http://rave.ohiolink.edu/etdc/view?acc_num=osu1488203857249745.
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俞立剛 and Ligang Yu. "Orthotropic damage models for fatigue crack initiation andpropagation." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1993. http://hub.hku.hk/bib/B31233995.
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馮錦生 and Kam-sang Fung. "Fatigue crack propagation with strain energy density approach." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1989. http://hub.hku.hk/bib/B31209713.
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Sabuncuoglu, Baris. "Fatigue Crack Growth Analysis Models For Functionally Graded Materials." Master's thesis, METU, 2006. http://etd.lib.metu.edu.tr/upload/2/12607024/index.pdf.
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The objective of this study is to develop crack growth analysis methods for functionally graded materials under mode I cyclic loading by using finite element technique. The study starts with the analysis of test specimens which are given in
ASTM standard E399. The material properties of specimens are assumed to be changing along the thickness direction according to a presumed variation function used for the modeling of functionally graded materials. The results of the study reveal the influence of different material variation functions on the crack growth
behavior.
In the second part, the growth of an elliptical crack which is a common case in engineering applications is analyzed. First, mode I cycling loading is applied perpendicular to the crack plane and crack growth profiles for a certain number of cycles are obtained for homogeneous materials. Then, the code is extended for the analysis functionally graded materials. The material properties are assumed to vary as an exponential function along the major or minor axis direction of the crack. The results can be used to examine the crack profile and material constants&rsquoinfluence for a certain number of cyclic loading.
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Yu, Ligang. "Orthotropic damage models for fatigue crack initiation and propagation /." [Hong Kong : University of Hong Kong], 1993. http://sunzi.lib.hku.hk/hkuto/record.jsp?B13570377.
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黃小華 and Siu-wah Wong. "Predicition of fatigue crack propagation using strain energy density method." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1989. http://hub.hku.hk/bib/B31209506.
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Scholey, Kenneth Erwin. "Heat tranfser and crack formation in water-cooled zinc fuming furnace jackets." Thesis, University of British Columbia, 1991. http://hdl.handle.net/2429/30078.
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In the zinc slag fuming process, zinc is extracted from lead blast furnace slag by reduction with a coal/air mixture injected into the slag through submerged tuyeres. The furnace is constructed of water-cooled jackets to contain the molten bath and freeze a protective slag layer. The slag layer greatly reduces vessel wear caused by the corrosive and violently agitated bath. However, the jackets are known to develop cracks in the working face panel that initiate on the slag face and propagate towards the water cavity. If the cracks reach the water cavity explosions may result should the molten slag come into contact with the water.
In this study an analysis of heat transfer in the jacket has been carried out using in-plant measurements and mathematical modelling. The working face of a water jacket was instrumented with thermocouples and positioned in a fuming furnace at the Trail smelter of Cominco Ltd. Measurements revealed the presence of large thermal transients or temperature "spikes" in the panel approximately 20 cm above the tuyeres. The transients were observed during charging and tapping of the furnace and are likely associated with slag fall-off due to surface wave action and gas injection effects when the bath level is low. Temperatures at the mid-thickness were seen to rise by as much as 180 °C above the steady-state level. Under these conditions large compressive stresses are produced in the panel that are sufficient to cause yielding. Over time, the transients lead to low-cycle fatigue of the working face panel with crack formation initiating at pre-existing surface flaws.
A mathematical modelling analysis of the transient freezing phenomena has been carried out using the finite element method. The results indicate that the temperature spikes are associated with the sudden removal of patches of slag and molten slag
coming into direct contact with the jacket. The temperature spikes are large enough to generate compressive stresses that cause yielding of the material in the exposed area. In order to reduce the damage caused by the removal of the slag shell an increased number of anchoring studs should be used in critical areas and a higher water circulation velocity should be employed to increase the size of the frozen slag layer and its strength.Applied Science, Faculty of
Materials Engineering, Department of
Graduate
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Nguyen, Olivier T. Ortiz Michael. "Cohesive models of fatigue crack growth and stress-corrosion cracking /." Diss., Pasadena, Calif. : California Institute of Technology, 2002. http://resolver.caltech.edu/CaltechETD:etd-12032004-161201.
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鄧偉 and Wei Deng. "Advances in interfacial crack/inclusion problems and constitutive models in solids." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1997. http://hub.hku.hk/bib/B31235748.
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Deng, Wei. "Advances in interfacial crack/inclusion problems and constitutive models in solids /." Hong Kong : University of Hong Kong, 1997. http://sunzi.lib.hku.hk/hkuto/record.jsp?B18404340.
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Argyrakis, Christos. "Models for designing pipe-grade polyethylenes to resist rapid crack propagation." Thesis, Imperial College London, 2010. http://hdl.handle.net/10044/1/5564.
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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
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Que, Norbert S. Civil & Environmental Engineering Faculty of Engineering UNSW. "Identification of cohesive crack fracture parameters using mathematical programming." Awarded by:University of New South Wales. School of Civil and Environmental Engineering, 2003. http://handle.unsw.edu.au/1959.4/19189.
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This thesis is concerned with the characterisation of the parameters governing the tension-softening relations of the cohesive crack model. Parameter identification is an important area in fracture mechanics as it enables the use of a fracture model for the simulation of fracture processes in structures. Research, however, has shown that such a task is not trivial and continues to pose challenging problems to experimentalists and analysts alike. This dissertation presents general and efficient indirect methods for the characterisation of mode I fracture parameters defining the cohesive crack model. The identification problem is formulated as a special type of inverse problem. The formulation is in the form of a constrained optimisation problem known as a mathematical program with equilibrium constraints characterised, in the present instance, by complementarity conditions involving the orthogonality of two-sign constrained vectors. The solution of such a mathematical program is computationally challenging as it is disjunctive and nonconvex by nature. A number of nonlinear programming based approaches are proposed, after appropriate reformulation of the mathematical program as an equivalent nonlinear programming problem. Actual experimental data are used to validate and determine the most suitable algorithm for parameter identification. It was found that the smoothing-based method is by far superior than other schemes. As the problem is nonconvex and the nonlinear program can only guarantee a local or stationary point, global optimisation procedures are introduced in order to verify the accuracy of the solutions obtained by the algorithm. Two evolutionary search methods capable of finding the global optimum are implemented for parameter identification. The results generated by the evolutionary search techniques confirm the reliability of the solutions identified by the best nonlinear programming algorithm. All computations carried out in the thesis suggest the suitability and robustness of the selected algorithm for parameter identification.
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Trask, David Anthony. "Experimental and numerical investigation into fatigue crack propagation models for 350WT steel." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0003/MQ31652.pdf.
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Larson, Mårten. "Thermal crack estimation in early age concrete : models and methods for practical application /." Luleå, 2003. http://epubl.luth.se/1402-1544/2003/20.
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Thomas, Brian Gordon. "Investigation of panel crack formation in steel ingots using mathematical and physical models." Thesis, University of British Columbia, 1985. http://hdl.handle.net/2429/25980.
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An investigation of panel crack formation in steel ingots was undertaken
to improve understanding of the mechanisms by which they develop and to evaluate possible solutions to the problem. The investigation revealed that two distinct types of panel cracks, both of which are partly caused by intermediate-temperature embrittlement of steel involving aluminum nitride precipitation, operate under different mechanisms. Isothermal, physical modelling experiments were conducted to determine the flow patterns, velocity profiles and flame geometry in a bottom-fired soaking pit and the resultant effects on heat transfer. An investigation involving comparison with analytical solutions determined the optimum numerical method to employ for the mathematical modelling of complex, two-dimensional, transient, heat-conduction problems. This method was formulated to calculate the temperature distribution in a steel ingot during the various processing stages from initial casting up to rolling and was verified with industrial measurements. A transient, elasto-visco-plastic, thermal-stress model employing the finite-element method was formulated, developed and verified using analytical solutions. Based on the temperatures calculated by the finite-element, heat-transfer model as input data, the transient, internal stress state of the ingot was calculated, taking into account the effects of phase-transformation volume changes and kinetics, creep, and temperature-dependent mechanical property behavior. The simulated stress histories were found to be directly linked to the progress of the phase-transformation front and were used to clarify the role of stress generation in panel crack formation. Finally, the results of a metallurgical investigation of steel ingot samples containing off-corner panel cracks were synthesized with the results of the physical and mathematical models to determine mechanisms and to suggest solutions for the formation of both mid-face and off-corner panel cracks. Mid-face panel cracks are apparently formed during air cooling when the mid-face surface is between the Ar₁ and 500 °C. Off-corner panel cracks appear to initiate internally during the early stages of reheating, but do not propagate to the surface until air cooling after removal from the soaking pit.Applied Science, Faculty of
Mining Engineering, Keevil Institute of
Graduate
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PIRES, FERNANDO SALLES DA SILVA. "NUMERICAL STUDY ON THE PREDICTION OF FRETTING FATIGUE CRACK NUCLEATION VIA MULTIAXIAL FATIGUE MODELS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2018. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=34764@1.
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PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIROCOORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
PROGRAMA DE EXCELENCIA ACADEMICA
Esta dissertação revisita quatro modelos multiaxiais de dano por fadiga (SWTD, SWT, Findley e Crossland) e discute a sua aplicação na previsão do risco de nucleação de trinca sob solicitações de fadiga por fretting segundo as abordagens local e não-local. As previsões do risco de nucleação de trinca obtidas através dos modelos estudados foram confrontadas com dados experimentais disponíveis na literatura para o contato cilindro/plano em aços AISI 52100 e AISI 1034 respectivamente. As melhores previsões foram obtidas através da versão desviadora SWTD. Segundo a abordagem local, o modelo SWTD fez previsões menos conservativas que os demais, sendo estas com erros conservativos inferiores a 15 porcento. Segundo a abordagem não-local, os quatro modelos geram previsões com erros inferiores a 15 porcento se os mesmos forem calibrados de forma independente. É importante mencionar que as melhores previsões obtidas através da abordagem não-local (que requer a calibração de um parâmetro adicional) são similares àquelas obtidas por SWTD local. Especificamente para os testes de fadiga por fretting com carga remota, o modelo SWTD não-local gerou previsões com 100 porcento de acertos (13 testes). Segundo a abordagem não-local, foi verificado que, para o conjunto de testes estudado, a precisão nas previsões é mais influenciada pela dimensão característica (que deve ser calibrada) que pelo modelo de fadiga multiaxial ou método de integração considerado. Por fim, verificou-se que previsões não conservativas podem ser obtidas ao se considerar uma dimensão característica assumida como parâmetro material (sem calibração) tal qual a distância crítica proposta por Taylor, definida em termos do método do ponto.
This work presents and discusses four multiaxial fatigue damage models (SWTD, SWT, Findley and Crossland) applied to crack nucleation prediction under fretting fatigue loadings according to the local and the non-local approaches. The predictions have been compared with experimental data available in the literature for the cylinder on flat configuration made of 52100 and 1034 AISI steel s respectively. In particular, for the data set analyzed, SWTD parameter (deviatoric version of SWT model) predictions were less conservative than the ones made by the other three models and have better correlated the experimental results. Considering the local approach, the error observed on the SWTD predictions was lower than 15 percent. On the other hand, for the non-local approach, it has been demonstrated that the four multiaxial fatigue models can provide crack nucleation predictions with similar level of accuracy (error lower than 15 percent) if the characteristic dimension is calibrated model-by-model. Note that this same level of accuracy was obtained with SWTD parameter defined in terms of the local approach, which requires one less parameter to be calibrated (characteristic dimension). In particular, considering only the fretting fatigue tests with bulk stresses, SWTD non-local (area method) prediction accuracy was 100 percent (13 tests). For the material studied, it has also been concluded that the non-local predictions accuracy depends more on the calibrated characteristic dimension than on the multiaxial model or integration method considered. Moreover, it has been verified that non-conservative predictions can be made if the characteristic dimension is assumed as a material property as proposed by Taylor on the critical distances theory (point method).
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Fredericks, Brandon. "Examination of flexural crack width prediction in concrete: comparison of analytical and numerical models." Master's thesis, Faculty of Engineering and the Built Environment, 2021. http://hdl.handle.net/11427/32675.
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The reliability of crack prediction methods in concrete design plays a role in the degree of confidence with which durability can be ensured. Bond failure between concrete and the embedded reinforcing steel exposes the steel surface within a crack path. This relative slip results from differential tensile strain between concrete and steel that allows harmful ions to reach and then react along the rebar length. A reliable prediction method should therefore account for the loss of bond in crack propagation. Researchers question the significance of the role played by surface cracks in structural deterioration, therefore casting doubt on the need for exhaustive crack analysis. The applicable fundamental theory of cracking, namely non-slip or slip determines the steel exposure and therefore the likelihood of reduced structural service life attributable to crack behaviour. However, while cracks originate at the surface of reinforcement through bond failure, simultaneously a cover distance away no cracks could appear on the concrete tension surface or they could be twice to ten times the crack width at the rebar level. Due to the heterogeneous composition of concrete, some commentaries state it impossible to accurately predict crack widths. Design standards therefore provide estimates of maximum crack widths to a degree of probability. This study examines the methods available for predicting cracks on the tension surface and the degree to which this is indicative of weakened bond between concrete and reinforcement. In this examination, it will be seen that concrete has ductility due to tension softening behaviour. The addition of steel to the tension area transforms the fracture process zone problem to the definition of a bond-slip relationship. Bond stresses generated at the rebar perimeter define the analytical relationships in design codes. These stresses control crack width at the tension surface. Results from the analytical code-based models are compared for increasing section depths, bar sizes and maximum spacings in the tension zone. A significant variation in the predicted maximum crack width is observed for deeper members. For very large concrete sections, the analytical models appear to provide unreasonable crack width values. The analytical equations in design codes focus on the bond relationship and ignore the size effect of concrete inherent in its microstructure. The concentration of flaws increases in larger members; hence size effect would play a greater role. Numerical modelling for crack prediction is therefore recommended for crack analysis in larger concrete members.
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Findley, Kip Owen. "Physically-based models for elevated temperature low cycle fatigue crack initiation and growth in Rene." Diss., Available online, Georgia Institute of Technology, 2005, 2005. http://etd.gatech.edu/theses/available/etd-04292005-092902/.
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Thesis (Ph. D.)--Materials Science and Engineering, Georgia Institute of Technology, 2006.McDowell, David, Committee Member ; Gokhale, Arun, Committee Member ; Saxena, Ashok, Committee Chair ; Johnson, Steven, Committee Member ; Sanders, Thomas, Committee Member.
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Araújo, Maycon de Sousa. "Propagação de trincas em meios desordenados submetidos à fadiga induzida por carregamento cíclico." Universidade de São Paulo, 2016. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-20092016-140901/.
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Neste trabalho desenvolveremos um modelo estatístico em uma escala micrométrica de interações entre as componentes do sistema que pretende descrever a propagação de trincas em materiais submetidos a tensões cíclicas. Apesar de sua extrema simplicidade, este modelo é capaz de reproduzir um resultado experimental bastante difundido entre engenheiros e especialistas, conhecido como lei de Paris, cujo enunciado estabelece que a taxa de crescimento de uma trinca sob carregamento cíclico é proporcional a uma potência da variação em seu correspondente fator de intensidade de tensões sendo largamente utilizada em aplicações práticas. Estamos particularmente interessados em estudar a introdução de desordem em determinados parâmetros associados ao material investigando as modificações impostas por este tipo de abordagem ao comportamento estatístico do modelo. Nossos principais resultados serão obtidos numericamente a partir de uma aproximação do tipo campo efetivo que ignora a correlação existente entre as diversas trincas que podem se formar ao longo do sistema durante o processo. Simulações numéricas do modelo serão igualmente consideradas ao analisarmos situações mais gerais do processo de propagação em que efeitos associados à regeneração de trincas podem desempenhar um importante papel na descrição do comportamento mecânico de um material.In this work we consider a statistical model in a micrometric scale of interactions between the components of the system which intends to describe the failure of materials subjected to cyclic-load fatigue. Although quite simple, this model is able to reproduce an important experimental result widespread among engineers and experts, known as Paris law, which states that the growth rate of a crack at subcritical load is proportional to a power of the change in its stress-intensity factor and it is largely used in engineering practice. We are particularly interested to study the introduction of disorder in some parameters of the material investigating the modifications caused by this kind of approach in the statistical properties of the model. Our main results will be obtained numerically assuming an effective-field like approximation which neglects the correlation between the different cracks emerging throughout the system during the breaking process. Numerical simulations of the model are also performed in order to describe more general situations of propagation where the effects of crack self-healing can play an important role in the material strength.
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Oikonomidis, Fokon Th. "Prediction of crack propagation in natural gas transmission pipelines by means of micromechanics and strain rate dependent models." Thesis, University of Bristol, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.541649.
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McKellar, Dougan Kelk. "A dislocation model of plasticity with particular application to fatigue crack closure." Thesis, University of Oxford, 2001. http://ora.ox.ac.uk/objects/uuid:45183b90-017f-4ac1-9550-94772a0ca88b.
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The ability to predict fatigue crack growth rates is essential in safety critical systems. The discovery of fatigue crack closure in 1970 caused a flourish of research in attempts to simulate this behaviour, which crucially affects crack growth rates. Historically, crack tip plasticity models have been based on one-dimensional rays of plasticity emanating from the crack tip, either co-linear with the crack (for the case of plane stress), or at a chosen angle in the plane of analysis (for plane strain). In this thesis, one such model for plane stress, developed to predict fatigue crack closure, has been refined. It is applied to a study of the relationship between the apparent stress intensity range (easily calculated using linear elastic fracture mechanics), and the true stress intensity range, which includes the effects of plasticity induced fatigue crack closure. Results are presented for all load cases for a finite crack in an infinite plane, and a method is demonstrated which allows the calculation of the true stress intensity range for a growing crack, based only on the apparent stress intensity range for a static crack. Although the yield criterion is satisfied along the plastic ray, these one-dimensional plasticity models violate the yield criterion in the area immediately surrounding the plasticity ray. An area plasticity model is therefore required in order to model the plasticity more accurately. This thesis develops such a model by distributing dislocations over an area. Use of the model reveals that current methods for incremental plasticity algorithms using distributed dislocations produce an over-constrained system, due to misleading assumptions concerning the normality condition. A method is presented which allows the system an extra degree of freedom; this requires the introduction of a parameter, derived using the Prandtl-Reuss flow rule, which relates the magnitude of slip on complementary shear planes. The method is applied to two problems, confirming its validity.
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Pant, Sudeep Raj. "Mathematical and physical modelling of crack growth near free boundaries in compression." University of Western Australia. School of Civil and Resource Engineering, 2005. http://theses.library.uwa.edu.au/adt-WU2005.0139.
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[Truncated abstract] The fracture of brittle materials in uniaxial compression is a complex process with the development of cracks generated from initial defects. The fracture mechanism and pattern of crack growth can be altered considerably by the presence of a free surface. In proximity of a free surface, initially stable cracks that require an increase in the load to maintain the crack growth can become unstable such that the crack growth maintains itself without requiring further increase in the load. This leads to a sudden relief of accumulated energy and, in some cases, to catastrophic failures. In the cases of rock and rock mass fracturing, this mechanism manifests itself as skin rockbursts and borehole breakouts or as various non-catastrophic forms of failure, e.g. spalling. Hence, the study of crack-boundary interaction is important in further understanding of such failures especially for the purpose of applications to resource engineering. Two major factors control the effect of the free boundary: the distance from the crack and the boundary shape. Both these factors as well as the effect of the initial defect and the material structure are investigated in this thesis. Three types of boundary shapes - rectilinear, convex and concave - are considered. Two types of initial defects - a circular pore and inclined shear cracks are investigated in homogeneous casting resin, microheterogeneous cement mixes and specially fabricated granulate material. The preexisting defects are artificially introduced in the physical model by the method of inclusion and are found to successfully replicate the feature of pre-existing defects in terms of load-deformation response to the applied external load. It is observed that the possibility of crack growth and the onset of unstable crack growth are affected by the type of initial defect, inclination of the initial crack, the boundary shape and the location of the initial defect with respect to the boundary. The initial defects are located at either the centre or edge of the sample. The stresses required for the wing crack initiation and the onset of unstable crack growth is highest for the initial cracks inclined at 35° to the compression axis, lowest at 45° and subsequently increases towards 60° for all the boundary shapes and crack locations. In the case of convex boundary, the stress of wing crack initiation and the stress of unstable crack growth are lower than for the case of rectilinear and concave boundary for all the crack inclinations and crack locations. The crack growth from a pre-existing crack in a sample with concave boundary is stable, requiring stress increase for each increment of crack growth. The stress of unstable crack growth for the crack situated at the edge of the boundary is lower than the crack located at the centre of the sample for all the crack inclinations and boundary shapes.
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Orifici, Adrian Cirino, and adrian orifici@student rmit edu au. "Degradation Models for the Collapse Analysis of Composite Aerospace Structures." RMIT University. Aerospace, Mechanical and Manufacturing Engineering, 2007. http://adt.lib.rmit.edu.au/adt/public/adt-VIT20080619.090039.
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Onifade, Ibrahim. "Development of Energy-based Damage and Plasticity Models for Asphalt Concrete Mixtures." Doctoral thesis, KTH, Byggnadsmaterial, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-198663.
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Characterizing the full range of damage and plastic behaviour of asphalt mixtures under varying strain-rates and stress states is a complex and challenging task. One reason for this is partly due to the strain rate and temperature dependent nature of the material as well as the variation in the properties of the constituent materials that make up the composite asphalt mixture. Existing stress-based models for asphalt concrete materials are developed based on mechanics principles, but these models are, however, limited in their application for actual pavement analysis and design since rate dependency parameters are needed in the constitutive model to account for the influence of the strain rate on the stress-based yield and evolution criteria. Till date, we are yet to arrive at simple and comprehensive constitutive models that can be used to model the behaviour of asphalt mixture over a wide range of strain-rate which is experienced in the actual pavement sections. The aim of this thesis is to develop an increased understanding of the strength and deformation mechanism of asphalt mixtures through multi-scale modeling and to develop simple and comprehensive continuum models to characterize the non-linear behaviour of the material under varying stress-states and conditions. An analysis framework is developed for the evaluation of the influence of asphalt mixture morphology on its mechanical properties and response using X-Ray CT and digital image processing techniques. The procedure developed in the analysis framework is then used to investigate the existence of an invariant critical energy threshold for meso-crack initiation which serves as the basis for the development of a theory for the development of energy-based damage and plastic deformation models for asphalt mixtures. A new energy-based viscoelastic damage model is developed and proposed based on continuum damage mechanics (CDM) and the thermodynamics of irreversible processes. A second order damage variable tensor is introduced to account for the distributed damage in the material in the different principal damage directions. In this way, the material response in tension and compression can be decoupled and the effects of both tension- and compression stress states on the material behaviour can be accounted for adequately. Based on the finding from the energy-based damage model, an equivalent micro-crack stress approach is developed and proposed for the damage and fracture characterization of asphalt mixtures. The effective micro-crack stress approach takes account of the material stiffness and a critical energy threshold for micro-crack initiation in the characterization of damage and fracture properties of the mixture. The effective micro-crack stress approach is developed based on fundamental mechanics principles and it reduces to the Griffith's energy balance criterion when purely elastic materials are considered without the need for the consideration of the surface energy and a crack size in the determination of the fracture stress. A new Continuum Plasticity Mechanics (CPM) model is developed within the framework of thermodynamics to describe the plastic behaviour of asphalt concrete material with energy-based criteria derived for the initiation and evolution of plastic deformation. An internal state variable termed the "plasticity variable" is introduced to described the distributed dislocation movement in the microstructure. The CPM model unifies aspects of existing elasto-plastic and visco-plastic theories in one theory and shows particular strength in the modeling of rate-dependent plastic behaviour of materials without the need for the consideration of rate dependency parameters in the constitutive relationships. The CPM model is further extended to consider the reduction in the stiffness properties with incremental loading and to develop a unified energy-based damage and plasticity model. The models are implemented in a Finite Element (FE) analysis program for the validation of the models. The result shows that the energy-based damage and plastic deformation models are capable of predicting the behaviour of asphalt concrete mixtures under varying stress-states and strain-rate conditions. The work in this thesis provides the basis for the development of more fundamental understanding of the asphalt concrete material response and the application of sound and solid mechanics principles in the analysis and design of pavement structures.En heltäckande karakterisering av skador och plastiska beteende hos asfaltblandningar under varierande belastningshastighet och spänningstillstånd är en komplex och svår uppgift. En orsak till detta är relaterat till materialets belastningshastighet- och temperaturberoende, såväl som variationen i materialegenskaperna hos de ingående komponenterna i den sammansatta asfaltblandningen. Befintliga spänningsbaserade modeller för asfaltbetongmaterial är utvecklade baserade på mekanikprinciper, men dessa modeller är begränsade när det gäller analys och design av verkliga asfaltsbeläggningar eftersom hastighetsberoende parametrar behövs i den konstitutiva modellen även med hänsyn till töjningshastighetens inverkan på kriterier för gränser och utveckling av spänningstillstånd. Det finns därför behov av att utveckla enkla men ändå heltäckande konstitutiva modeller som kan användas för att modellera beteendet hos asfaltmassan över ett brett spektrum av belastningshastigheter för olika av sektioner asfaltsbeläggningar. Syftet med denna avhandling är att öka förståelsen av hållfasthets- och deformationsmekanismer för asfaltblandningar genom multi-modellering. Målet är att utveckla enkla och heltäckande kontinuummodeller som karakteriserar materialets olinjära beteende under varierande spänningstillstånd och betingelser. Ett analysramverk har utvecklats för utvärdering av påverkan av asfaltmassans morfologi på dess mekaniska egenskaper och beteende med hjälp av röntgendatortomografi och digital bildbehandlingsteknik. Detta förfarande har sedan använts för att undersöka förekomsten av inneboende kritiska tröskelvärden för brottenergin för mesosprickinitiering vilket i sin tur ligger till grund för utvecklingen av en teori för modellering av energibaserade skador och plastisk deformation hos asfaltblandningar. En ny energidensitet baserad viskoelastisk skademodell utvecklas och föreslås utgå från kontinuum-skade-mekanik (CDM) och termodynamik för irreversibla processer. En andra ordningens skadevariabeltensor införs för att ta hänsyn till skadedistributionen i materialen i de olika principiella skaderiktningarna. På detta sätt kan materialets respons i drag- och tryckbelastning separeras och effekterna av spänningstillstånd i både drag och tryck kan beaktas på ett adekvat sätt. Baserat på resultaten från den energibaserade skademodellen utvecklas och föreslås en motsvarande metod för mikrosprickspänning gällande skade- och brottkarakteriseringen av asfaltblandningar. Metoden för den effektiva mikrosprickspänningen tar hänsyn till materialets styvhet och en kritisk tröskelenergi för mikrosprickinitiering för karakteriseringen av skador och brottegenskaper hos blandningen. Denna metod är utvecklad baserat på grundläggande mekanikprinciper och kan för rent elastiska material reduceras till Griffiths energibalanskriterium utan hänsyn till ytenergi och sprickstorlek vid bestämningen av brottspänningen. En ny termodynamikbaserad modell för kontinuumplasticitetsmekanik (CPM) utvecklas för att beskriva det plastiska beteendet hos asfaltbetongmaterial med energibaserade kriterier härledda för initiering och progression av plastisk deformation. En intern tillståndsvariabel kallad "plasticitetvariabeln" införs för att beskriva den fördelade dislokationsrörelsen i mikrostrukturen. CPM-modellen förenar befintliga elasto-plastiska och visko-plastiska teorier i en teori och visar sig vara särskilt effektiv i modelleringen av hastighetsberoende plastiskt beteende hos material utan att behöva beakta hastighetsberoende parametrar i de konstitutiva sambanden. CPM-modellen utvidgas ytterligare för att kunna beakta reduktionen av styvheten med stegvis ökad belastning och för att utveckla en enhetlig energibaserad skade- och plasticitetmodell. Modellerna är implementerade i ett finit element (FE)-analysprogram för validering av modellerna. Resultatet visar att de energibaserade modellerna för skador och plastisk deformation kan förutsäga beteendet hos asfaltbetongblandningar under varierande spänningstillstånd och töjningshastighetsförhållanden. Arbetet i denna avhandling utgör grunden för utvecklingen av mer grundläggande förståelse av asfaltbetongmaterialets respons och tillämpningen av sunda och robusta mekanikprinciper i analys och design av asfaltstrukturer.
QC 20161220
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Salih, Sarmed. "Rate-dependent cohesive-zone models for fracture and fatigue." Thesis, University of Manchester, 2018. https://www.research.manchester.ac.uk/portal/en/theses/ratedependent-cohesivezone-models-for-fracture-and-fatigue(d8bfee97-1a75-4418-8916-b5a7cf8cdfd9).html.
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Despite the phenomena of fracture and fatigue having been the focus of academic research for more than 150 years, it remains in effect an empirical science lacking a complete and comprehensive set of predictive solutions. In this regard, the focus of the research in this thesis is on the development of new cohesive-zone models for fracture and fatigue that are afforded an ability to capture strain-rate effects. For the case of monotonic fracture in ductile material, different combinations of material response are examined with rate effects appearing either in the bulk material or localised to the cohesive-zone or in both. The development of a new rate-dependent CZM required first an analysis of two existing methods for incorporating rate dependency, i.e.either via a temporal critical stress or a temporal critical separation. The analysis revealed unrealistic crack behaviour at high loading rates. The new rate-dependent cohesive model introduced in the thesis couples the temporal responses of critical stress and critical separation and is shown to provide a stable and realistic solution to dynamic fracture. For the case of fatigue, a new frequency-dependent cohesive-zone model (FDCZM) has been developed for the simulation of both high and low-cycle fatigue-crack growth in elasto-plastic material. The developed model provides an alternative approach that delivers the accuracy of the loading-unloading hysteresis damage model along with the computational efficiency of the equally well-established envelope load-damage model by incorporating a fast-track feature. With the fast-track procedure, a particular damage state for one loading cycle is 'frozen in' over a predefined number of cycles. Stress and strain states are subsequently updated followed by an update on the damage state in the representative loading cycle which again is 'frozen in' and applied over the same number of cycles. The process is repeated up to failure. The technique is shown to be highly efficient in terms of time and cost and is particularly effective when a large number of frozen cycles can be applied without significant loss of accuracy. To demonstrate the practical worth of the approach, the effect that the frequency has on fatigue crack growth in austenitic stainless-steel 304 is analysed. It is found that the crack growth rate (da/dN) decreases with increasing frequency up to a frequency of 5 Hz after which it levels off. The behaviour, which can be linked to martensitic phase transformation, is shown to be accurately captured by the new FDCZM.
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Lama, Salomon Abraham. "Digital State Models for Infrastructure Condition Assessment and Structural Testing." Diss., Virginia Tech, 2017. http://hdl.handle.net/10919/84502.
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This research introduces and applies the concept of digital state models for civil infrastructure condition assessment and structural testing. Digital state models are defined herein as any transient or permanent 3D model of an object (e.g. textured meshes and point clouds) combined with any electromagnetic radiation (e.g., visible light, infrared, X-ray) or other two-dimensional image-like representation. In this study, digital state models are built using visible light and used to document the transient state of a wide variety of structures (ranging from concrete elements to cold-formed steel columns and hot-rolled steel shear-walls) and civil infrastructures (bridges). The accuracy of digital state models was validated in comparison to traditional sensors (e.g., digital caliper, crack microscope, wire potentiometer). Overall, features measured from the 3D point clouds data presented a maximum error of ±0.10 in. (±2.5 mm); and surface features (i.e., crack widths) measured from the texture information in textured polygon meshes had a maximum error of ±0.010 in. (±0.25 mm). Results showed that digital state models have a similar performance between all specimen surface types and between laboratory and field experiments. Also, it is shown that digital state models have great potential for structural assessment by significantly improving data collection, automation, change detection, visualization, and augmented reality, with significant opportunities for commercial development. Algorithms to analyze and extract information from digital state models such as cracks, displacement, and buckling deformation are developed and tested. Finally, the extensive data sets collected in this effort are shared for research development in computer vision-based infrastructure condition assessment, eliminating the major obstacle for advancing in this field, the absence of publicly available data sets.Ph. D.
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Ahmed, Lamis. "Models for analysis of young cast and sprayed concrete subjected to impact-type loads." Doctoral thesis, KTH, Betongbyggnad, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-168211.
Full textAbstract:
The strive for a time-efficient construction process naturally put focus on the possibility of reducing the time of waiting between stages of construction, thereby minimizing the construction cost. If recently placed concrete, cast or sprayed, is exposed to impact vibrations at an early age while still in the process of hardening, damage that threatens the function of the hard concrete may occur. A waiting time when the concrete remains undisturbed, or a safe distance to the vibration source, is therefore needed. However, there is little, or no, fully proven knowledge of the length of this distance or time and there are no established guidelines for practical use. Therefore, conservative vibration limits are used for young and hardening concrete exposed to vibrations from e.g. blasting. As a first step in the dynamic analysis of a structure, the dynamic loads should always be identified and characterized. Here it is concluded that impact-type loads are the most dangerous of possible dynamic loads on young and hardening concrete. Shotcrete (sprayed concrete) on hard rock exposed to blasting and cast laboratory specimens subjected to direct mechanical impact loads have been investigated using finite element models based on the same analysis principles. Stress wave propagation is described in the same way whether it is through hard rock towards a shotcrete lining or through an element of young concrete. However, the failure modes differ for the two cases where shotcrete usually is damaged through loss of bond, partly or over larger sections that may result in shotcrete downfall. Cracking in shotcrete due to vibrations only is unusual and has not been observed during previous in situ tests. The study of shotcrete is included to demonstrate the need of specialized guidelines for cases other than for mass concrete, i.e. structural elements or concrete volumes with large dimensions in all directions. Within this project, work on evaluating and proposing analytical models are made in several steps, first with a focus on describing the behaviour of shotcrete on hard rock. It is demonstrated that wave propagation through rock towards shotcrete can be described using two-dimensional elastic finite element models in a dynamic analysis. The models must include the material properties of the rock and the accuracy of these parameters will greatly affect the results. It is possible to follow the propagation of stress waves through the rock mass, from the centre of blasting to the reflection at the shotcrete-rock interface. It is acceptable to use elastic material formulations until the strains are outside the elastic range, which thus indicates imminent material failure. The higher complexity of this type of model, compared with mechanical models using mass and spring elements, makes it possible to analyse more sophisticated geometries. Comparisons are made between numerical results and measurements from experiments in mining tunnels with ejected rock mass and shotcrete bond failure, and with measurements made during blasting for tunnel construction where rock and shotcrete remained intact. The calculated results are in good correspondence with the in situ observations and measurements, and with previous numerical modelling results. Examples of preliminary recommendations for practical use are given and it is demonstrated how the developed models and suggested analytical technique can be used for further detailed investigations. The modelling concept has also been used for analysis of impact loaded beams and concrete prisms modelled with 3D solid elements. As a first analysis step, an elastic material model was used to validate laboratory experiments with hammer-loaded concrete beams. The laboratory beam remained un-cracked during the experiments, and thus it was possible to achieve a good agreement using a linear elastic material model for fully hardened concrete. The model was further developed to enable modelling of cracked specimens. For verification of the numerical results, earlier laboratory experiments with hammer impacted smaller prisms of young concrete were chosen. A comparison between results showed that the laboratory tests can be reproduced numerically and those free vibration modes and natural frequencies of the test prisms contributed to the strain concentrations that gave cracking at high loads. Furthermore, it was investigated how a test prism modified with notches at the middle section would behave during laboratory testing. Calculated results showed that all cracking would be concentrated to one crack with a width equal to the sum of the multiple cracks that develop in un-notched prisms. In laboratory testing, the modified prism will provide a more reliable indication of when the critical load level is reached. This project has been interdisciplinary, combining structural dynamics, finite element modelling, concrete material technology, construction technology and rock support technology. It is a continuation from previous investigations of the effect on young shotcrete from blasting vibrations but this perspective has been widened to also include young, cast concrete. The outcome is a recommendation for how dynamic analysis of young concrete, cast and sprayed, can be carried out with an accurate description of the effect from impact-type loads. The type of numerical models presented and evaluated will provide an important tool for the work towards guidelines for practical use in civil engineering and concrete construction work. Some recommendations on safe distances and concrete ages are given, for newly cast concrete elements or mass concrete and for newly sprayed shotcrete on hard rock.QC 20150529
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Emdadi, Fard Maryam [Verfasser], Christine [Akademischer Betreuer] Müller, and Katja [Gutachter] Ickstadt. "Comparison of prediction intervals for crack growth based on random effects models / Maryam Emdadi Fard ; Gutachter: Katja Ickstadt ; Betreuer: Christine Müller." Dortmund : Universitätsbibliothek Dortmund, 2018. http://d-nb.info/1153200554/34.
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Abdulla, Warda Ibrahim. "FATIGUE BEHAVIOR AND SCALE EFFECTS IN RIVETED JOINTS." University of Akron / OhioLINK, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=akron161651595564376.
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De, Jager Charl. "A critical appraisal of existing models for nonlinear finite element analysis of reinforced concrete response." Thesis, Stellenbosch : Stellenbosch University, 2012. http://hdl.handle.net/10019.1/20052.
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Thesis (MScEng)--Stellenbosch University, 2012.ENGLISH ABSTRACT: This study entails the appraisal of the constitutive models available for the non linear finite element analysis of reinforced concrete, using the DIANA finite element package and following generally accepted guidelines for non linear finite element analyses. The constitutive models considered are plasticity and total strain based (fixed and rotating crack) models. The appraisal consists of the analysis of various experiments performed on reinforced concrete beams that are governed by compressive, shear and tensile dominated failures. The investigation is not limited to the accuracy of the results obtained using these models but also of the consistency of the results obtained with regard to various mesh types and sizes, as well as a study of the individual influence of several material parameters. The intention of the study was to provide the reader with an indication of the performance capacity (accuracy and consistency) of the available constitutive models, where the notion of the use of the results obtained from non linear finite element analyses for design purposes is considered. The results obtained were varied. The models performed reasonably well in the compressive and tension dominated studies, with the importance of accurate material parameters being emphasized especially for the more advanced cementitious materials investigated. The total strain rotating crack model also showed a proclivity of simulating incorrect failure modes as well as exhibiting reluctance towards stress redistribution. All models used for the shear dominated study yielded mostly inaccurate and inconsistent results, but it was found that the four node quadrilateral element with selective reduced integration performed the best. The plasticity model did not capture shear failure well, and convergence was often not attained. The constant shear retention factor of the total strain fixed crack model was found to yield more detailed response curves for the smaller mesh sizes. The results of the tension dominated beams inspired more confidence in the models as quite accurate values were attained, especially by the plasticity model used. The ability of the available models to simulate realistic structural behaviour under various failure modes is very limited, as is evident from the results obtained. The development of a more advanced and robust model is required, which can provide consistently accurate results and failure modes, and even ‘anticipate’ potential failure modes not considered by the user.
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Tanne, Erwan. "Variational phase-field models from brittle to ductile fracture : nucleation and propagation." Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLX088/document.
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Les simulations numériques des fissures fragiles par les modèles d’endommagement à gradient deviennent main- tenant très répandues. Les résultats théoriques et numériques montrent que dans le cadre de l’existence d’une pre-fissure la propagation suit le critère de Griffith. Alors que pour le problème à une dimension la nucléation de la fissure se fait à la contrainte critique, cette dernière propriété dimensionne le paramètre de longueur interne.Dans ce travail, on s’attarde sur le phénomène de nucléation de fissures pour les géométries communément rencontrées et qui ne présentent pas de solutions analytiques. On montre que pour une entaille en U- et V- l’initiation de la fissure varie continument entre la solution prédite par la contrainte critique et celle par la ténacité du matériau. Une série de vérifications et de validations sur diffèrent matériaux est réalisée pour les deux géométries considérées. On s’intéresse ensuite à un défaut elliptique dans un domaine infini ou très élancé pour illustrer la capacité du modèle à prendre en compte les effets d’échelles des matériaux et des structures.Dans un deuxième temps, ce modèle est étendu à la fracturation hydraulique. Une première phase de vérification du modèle est effectuée en stimulant une pré-fissure seule par l’injection d’une quantité donnée de fluide. Ensuite on étudie la simulation d’un réseau parallèle de fissures. Les résultats obtenus montrent qu’il a qu’une seule fissure qui se propage et que ce type de configuration minimise mieux l’énergie la propagation d’un réseau de fractures. Le dernier exemple se concentre sur la stabilité des fissures dans le cadre d’une expérience d’éclatement à pression imposée pour l’industrie pétrolière. Cette expérience d’éclatement de la roche est réalisée en laboratoire afin de simuler les conditions de confinement retrouvées lors des forages.La dernière partie de ce travail se concentre sur la rupture ductile en couplant le modèle à champ de phase avec les modèles de plasticité parfaite. Grâce à l’approche variationnelle du problème on décrit l’implantation numérique retenue pour le calcul parallèle. Les simulations réalisées montrent que pour une géométrie légèrement entaillée la phénoménologie des fissures ductiles comme par exemple la nucléation et la propagation sont en concordances avec ceux reportées dans la littératurePhase-field models, sometimes referred to as gradient damage, are widely used methods for the numerical simulation of crack propagation in brittle materials. Theoretical results and numerical evidences show that they can predict the propagation of a pre-existing crack according to Griffith’s criterion. For a one- dimensional problem, it has been shown that they can predict nucleation upon a critical stress, provided that the regularization parameter is identified with the material’s internal characteristic length.In this work, we draw on numerical simulations to study crack nucleation in commonly encountered geometries for which closed-form solutions are not available. We use U- and V-notches to show that the nucleation load varies smoothly from the one predicted by a strength criterion to the one of a toughness criterion when the strength of the stress concentration or singularity varies. We present validation and verification of numerical simulations for both types of geometries. We consider the problem of an elliptic cavity in an infinite or elongated domain to show that variational phase field models properly account for structural and material size effects.In a second movement, this model is extended to hydraulic fracturing. We present a validation of the model by simulating a single fracture in a large domain subject to a control amount of fluid. Then we study an infinite network of pressurized parallel cracks. Results show that the stimulation of a single fracture is the best energy minimizer compared to multi-fracking case. The last example focuses on fracturing stability regimes using linear elastic fracture mechanics for pressure driven fractures in an experimental geometry used in petroleum industry which replicates a situation encountered downhole with a borehole called burst experiment.The last part of this work focuses on ductile fracture by coupling phase-field models with perfect plasticity. Based on the variational structure of the problem we give a numerical implementation of the coupled model for parallel computing. Simulation results of a mild notch specimens are in agreement with the phenomenology of ductile fracture such that nucleation and propagation commonly reported in the literature
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Wagner, Benjamin. "Rotating Equipment Defect Detection Using the Algorithm of Mode Isolation." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/16230.
Full textAbstract:
Findings from a project involving rotating equipment defect detection using the Algorithm
of Mode Isolation (AMI) are presented. The prototypical system evaluated is a
rotating shaft, supported by hydrodynamic bearings at both ends, with one disk mounted
to the shaft. Shaft cracks and bearing wear are the two equipment defects considered.
An existing model of the prototypical system from the literature, termed the simplified
model. is modified to simulate the presence of a transverse shaft crack at mid-span. This
modified model is termed the standard model. Ritz series analysis, in conjunction with a
previously published description of the compliance related to the presence of a transverse
shaft crack, is used to describe the decrease in shaft stiffness associated with the crack.
The directional frequency response function (dFRF) is shown in the literature to provide
benefits over the standard frequency response function (FRF) in both system identification
and shaft crack detection for rotating equipment. The existing version of AMI is modified
to process dFRFs and termed Two-Sided AMI. The performance of Two-Sided AMI is
verified through system identification work using both the simplified model and a rigid
rotor model from the literature. The results confirm the benefits of using the dFRF for
system identification of isotropic systems. AMI and Two-Sided AMI are experimental modal
analysis (EMA) routines, which estimate modal properties based on a frequency domain
expression of system response. Eigenvalues and associated modal residues are the modal
properties considered in the present work.
Three defect detection studies are fully described. In the first, the simplified model is
used to investigate bearing wear detection. Various damage metrics related to the eigenvalue
and the residue are evaluated. The results show that residue-based metrics are sensitive
to bearing wear. Next, the standard model is used in an in-depth investigation of shaft
crack detection. When a shaft crack is present, the standard model is time-varying in both
the fixed and moving coordinate systems. Therefore, this analysis is also used to evaluate
performing EMA on non-modal data. In addition to continuing the evaluation of various
xiv damage metrics, the shaft crack study also investigates the effects of noise and coordinate
system choice (fixed or moving) on shaft crack detection. Crack detection through EMA
processing of noisy, non-modal data is found to be feasible. The eigenvalue-based damage
metrics show promise. Finally, the standard model is used in a dual-defect study. The
system is configured with both a shaft crack and a worn bearing. One defect is held
constant while the magnitude of the other is increased. The results suggest that AMI is
usable for defect detection of rotating machinery in the presence of multiple system defects,
even though the response data is not that of a time-invariant system. The relative merits
of both input data types, the FRF and the dFRF, are evaluated in each study.
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Patel, Surendra Kumar. "Experimental And Numerical Studies On Fatigue Crack Growth Of Single And Interacting Multiple Surface Cracks." Thesis, Indian Institute of Science, 2000. http://hdl.handle.net/2005/276.
Full textAbstract:
Design based on damage tolerance concepts has become mandatory in high technology structures. These concepts are also essential for evaluating life extension of aged structures which are in service beyond originally stipulated life. Fracture analysis of such structures in the presence of single or multiple three-dimensional flaws is essential for this approach. Surface cracks are the most commonly occurring flaws and development of accurate methods of analysis for such cracks is essential for structural integrity evaluation of newly designed or aged structures. The crack fronts of these surface flaws are usually approximated mathematically to be of either part-elliptical or part-circular in geometry. In this thesis, some of the issues related to fatigue crack growth of single and multiple surface cracks are studied in detail. Here emphasis is given to the development of simple and accurate post-processing techniques to estimate stress intensity factors for surface cracks, development and/or implementation of simple numerical methods to simulate three-dimensional single and multiple cracks in fatigue and their experimental verification.
Modified virtual crack closure integral (MVCCI) technique for estimation of strain energy release rates has been improved (chapter II) to deal with curved crack front and unequal elements across the crack front. The accuracy of this method is evaluated and presented in this chapter for certain benchmark surface flaw problems. The improved MVCCI is used in the investigation of interaction between multiple surface cracks in three-dimensional solids. The interaction effects are studied for both interacting and coalescing phases as observed to occur in the growth of multiple surface cracks. Extensive numerical work is performed to study the effects of various parameters such as aspect ratio, thickness ratio, interspacing on the interaction factors. These solutions are used in formulating empirical equations to estimate interaction factors. This facilitated the development of a simple semi-analytical method to study fatigue crack growth of multiple cracks.
The growth of surface cracks under fatigue loading in the finite width specimens of an aero-engine superalloy has been studied experimentally (presented in chapter III). Four configurations for single semi-elliptical cracks are considered. Fatigue crack growth is simulated by two models viz. two degrees of freedom and "multi degrees of freedom with ellipse fit'. These models are sometimes referred to as semi-analytical models as the crack growth is predicted by numerical integration combining Paris equation with an empirical form of stress intensity factor solution. In order to use two degrees of freedom model for fatigue crack growth prediction of semi-elliptical cracks, empirical solution for the Ml range of geometric parameters for stress intensity factor is required for the considered
configurations. The available Newman-Raju solution is useful for this purpose within a limited range of surface crack length to width (c/W) of the specimen. Based on the present finite element results, the empirical equations are developed for extended values of c/W. It is well understood that the fatigue prediction for two-dimensional crack can be improved by inclusion of crack closure effects. Usually, in semi-analytical models for growth of surface cracks under fatigue loading, the crack closure is included as a ratio of crack closure factor at surface and depth locations of semi-elliptical crack. In the present work, this ratio for the considered material of specimens is obtained by an experimental study. The difference in characteristics of preferred propagation path between semi-elliptical crack in a finite width plate and a wide plate is clearly brought out.
Current crack growth predictions for most of the structures are based on the presence of only a single crack. However, in structures several cracks may initiate simultaneously within a stress critical zone and may interact depending upon their geometry, spatial location, structure geometry and mode of loading. In this work various configurations of twin semi-elliptical cracks have been studied by experiments. The beachmarks created on the specimens during experiments are used in the investigation of crack shape progression during fatigue. A three degrees of freedom crack growth model for interacting and coalescing cracks has been proposed. The experimentally determined crack shape and lives have been compared with the corresponding values from numerical simulation.
The correlation of experimental results with numerical predictions was carried out through improved MVCCI for eight-noded brick elements. This has worked well in the configurations analysed. However, it is known in literature that there are benefits of using 20-noded singular elements. There could be special situations where the regular elements could fail, and singular elements could be essential. For this purpose, further development of MVCCI were carried out using 20-noded quarter-point elements (presented in chapter IV). Also a novel technique of decomposed crack closure integral (DCCI) was developed (presented in chapter V) for both regular and singular elements to represent the variation of MVCCI more accurately along the crack front.
It is well known that quarter-point elements at crack front produce the required singularity at the crack tip and give accurate stress distribution with fewer degrees of freedom than conventional elements. Thus to develop more efficient post-processing tools, the MVCCI expressions are formulated for 20-noded singular quarter-point element for various assumptions regarding stress and displacement distributions in the elements across the crack front. A comprehensive study is presented (chapter IV) on MVCCI for 20-noded singular brick element including various simplified expressions for three-dimensional part-through cracks in pure and mixed-mode state of deformation of fracture. The developed MVCCI expressions are also valid for 15-noded quarter-point Penta elements. The reduction in model size can further be obtained if 12-noded three-dimensional singular element is employed at the crack front and eight-noded elements are used away from the crack front. The MVCCI expressions are also developed for 12-noded singular element and their accuracy is evaluated by numerical solutions.
Presently, MVCCI, estimates the average stress intensity factor at the center of each element along the crack front. In this thesis, a Decomposed Crack Closure Integral (DCCI) is formulated to represent an assumed variation of stress intensity factor along the crack front in each element. The DCCI is formulated for 8-noded brick, 20-noded conventional brick and 20-noded singular brick elements. The numerical examples presented here deal with three-dimensional problems of patch repair technology and part-through cracks. The technique showed a major advantage for the patch repair problems where SIF variations along the crack front are of significance and large mesh sizes are computationally expensive. This along with MVCCI for 12-noded and 20-noded singular elements formed a part of the work on development of accurate and effective post-processing tools.
It is expected that the present work will be helpful in damage tolerance design and assessment of aerospace structures and the experimental work performed as a part of this thesis will enhance confidence in the damage tolerance analysis.
The thesis is concluded in chapter VI presenting the contributions of this thesis and projecting future lines of work possible in this area.
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Vaněk, David. "Analýzy damage tolerance s uvážením interakce zatěžovacích kmitů." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2016. http://www.nusl.cz/ntk/nusl-254285.
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Master’s thesis deals with study of fatigue crack growth retardation models. This document presents description, evaluation, verification and finally application of retardation models demonstrated on the damage tolerant structure. Analysis was provided on the L 410 NG aircraft in the location of the rear spar lower flange joint. Output of this thesis is comparison of the damage tolerance analysis with the current method using in the Aircraft Industry a. s., i.e. linear model without using load interaction models. The comparisons are fatigue crack growth curves, threshold and intervals of inspection program. AFGROW software was used for verification and application of retardation models.
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Vojtek, Tomáš. "Propagation of Fatigue Cracks under Shear Loading Modes II, III and II+III in the Near-Threshold Region." Doctoral thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2014. http://www.nusl.cz/ntk/nusl-234220.
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Práce je zaměřena na šíření únavových trhlin v módech II, III a II + III v prahové oblasti v kovových materiálech. Byly provedeny experimenty pro ARMCO železo, titan, nikl a austenitickou ocel na třech druzích vzorků pro zatěžování trhliny ve smykových módech. Byl použit zvláštní způsob vytváření iniciační trhliny, který eliminoval efekt zavírání trhliny na začátku smykového zatěžování. Ve všech zkoumaných materiálech byly efektivní prahové hodnoty v módu II přibližně 1,7 krát nižší než v módu III. Měřením úhlů lokálních odklonů a natočení lomových faset s použitím stereofotogrammetrie v SEM bylo usuzováno na tendenci trhliny k vyhýbání do lokálního módu I. Nejmenší úhly byly zjištěny u ARMCO železa pro trhliny v módech II i III, středně velké úhly u titanu a niklu a největší úhly (vyhnutí do čistého módu I) byly pozorovány u austenitické oceli. Tyto rozdíly lze vysvětlit různým počtem dostupných skluzových systémů v krystalových mřížkách daných materiálů. Poměry mezi naměřenými prahovými hodnotami v módu I, II a III byly srovnány s předpověďmi teoretických modelů. Byl navržen analytický vztah pro efektivní prahové hodnoty v módu II, který byl v dobré shodě s experimentálními výsledky. Pomocí efektivních prahových hodnot bylo vyčísleno kritérium pro vyhýbání do módu I, které vedlo ke kritickému úhlu odklonu 40° souvisejícímu s přechodem z lokálního smykového módu do otevíracího módu.
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Uribe, Suarez Diego Alejandro. "Combinaison d’éléments cohésifs et remaillage pour gérer la propagation arbitraire du chemin de fissure : des matériaux fragiles à l’analyse de fatigue thermique des petits corps du système solaire." Thesis, Université Côte d'Azur, 2021. http://www.theses.fr/2021COAZ4047.
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La présente thèse de doctorat a pour objectif d’améliorer la modélisation du phénomène de rupture dans les matériaux fragiles. Elle porte une attention particulière aux mécanismes de rupture des objets célestes. L’un des problèmes posant le plus de défis aux scientifiques spécialisés dans l’étude de la mécanique de la rupture est la propagation d’une fissure dans un maillage éléments finis, et ce pour des chemins arbitraires. Dans cette étude, ce problème est abordé en utilisant une technique de remaillage avancée utilisant des éléments finis cohésifs permettant la propagation de fissures suivant des directions arbitraires et indépendantes du maillage. La direction de la fissure est calculée suivant le critère du taux de restitution d’énergie maximal, implémentée à l’aide d’un modèle éléments finis et de la méthode Gθ. Les effets de différents paramètres numériques et physiques relatifs à la fissure ou à l’énergie libérée lors de la rupture sont investigués.Bien que différentes preuves de fissures et/ou fragments à la surface de corps célestes de notre système solaire induits par des variations cycliques de la température ont été détaillées, la compréhension de ces mécanismes de propagation dans des objets célestes reste très parcellaire. La fracturation thermique de roches en surface associée à l’impact de micro-météorites peutéventuellement conduire à la rupture complète de fragments de matière et à la production de régolithes. Cette dernière est définie comme la couche de matériau non consolidée qui recouvre la surface des planètes. Afin de comprendre ces mécanismes, l’étude s’attarde sur un exemple précis, celui de l’astéroïde (101955) Bennu. Pour ce faire, elle utilise un modèle thermoélastique couplé avec un modèle linéaire élastique de mécanique de la rupture permettant de considérer les variations cycliques de température liées aux alternances jour/nuit. En utilisant cette méthodologie, il a été observé que les fissures se propagent préférentiellement dans les directions : Nord vers Sud, Nord-Est vers Sud-Ouest et Nord-Ouest vers Sud-Est. Finalement, une analyse de fatigue est effectuée afin d’estimer la vitesse de croissance de la fissure.Les méthodes détaillées précédemment ont été implémentées dans Cimlib, une librairie C++ dévelopée au CEMEF. Au sein de cette librairie, une méthode permettant la propagation d’une ou plusieurs fissures, suivant des directions arbitraires, en 2D et au sein d’un environnement de calcul en parallèle est à présent disponible. Concernant l’extension de cette méthode à des problèmes 3D, une première approche a été mise au point. Elle permet de propager un front de fissure suivant une direction arbitaire. La structure développée permet d’ouvrir de nouvelles possibilités pour de nombreuses applications, telles que l’étude de la rupture de matériaux composites à l’échelle mesoscopiqueThe present PhD thesis aims at providing a better modeling of fracture phenomenon in brittle materials, with special attention focused on fracture processes taking place in astronomical bodies. One of the most challenging issues in computational fracture mechanics is the propagation of a crack through a finite element mesh for arbitrary crack paths. In this work, this problem is approached by means of an advanced remeshing technique that propagates a crack using cohesive elements through arbitrary directions (mesh-independent). The crack direction is computed using the maximal energy release rate criterion which is implemented using finite elements and the Gθ method. The effects of different numerical and physical parameters regarding the crack path and fracture energy have been investigated. Even though it has been shown that temperature cycles on airless bodies of our Solar System can cause damaging of surface materials (Thermal cracking), propagation mechanisms in the case of space objects are still poorly understood. Thermal cracking of surface rocks, in addition to the impact of micrometeorties, can eventually lead to rocks’ breakup and produce fresh regolith, the latter being the layer of unconsolidated material that covers planetary surfaces. For this reason, the present work combines a thermoelasticity model together with linear elastic fracture mechanics theory to predict fracture propagation in the presence of thermal gradients generated by diurnal temperature cycling and under conditions similar to those existing on asteroid (101955) Bennu. Using the implemented methodologies, it is found that in asteroid Bennu, cracks preferentially propagate in the North to South (N-S), in the North-East to South-West (NE-SW) and in the North-West to South-East (NW-SE) directions. Finally, thermal fatigue analysis was performed in order to estimate the crack growth rate.Aforementioned methodologies have been implemented in Cimlib, a C++ in-house finite element library developed at CEMEF. Inside Cimlib, a methodology allowing two-dimensional crack propagation through arbitrary directions with the option of handling multiple cracks in the domain and inside a parallel environment was developed. Regarding three-dimensional scenario, a first approach where a crack front was propagated through an arbitrary direction was achieved. Concerning numerical modeling of crack propagation, the developed framework opens new possibilities for various applications such as composites cracking at the meso-scale
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Perng, Jia-Der. "Analysis of crack propagation in asphalt concrete using a cohesive crack model." Connect to this title online, 1989. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1094737677.
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McMaster, Fraser J. "The effects of load interaction on fatigue crack growth and crack closure in aluminium alloys." Thesis, University of Bristol, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.297844.
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Gualandi, Gabriele. "Crack modeling and crack propagation in structures using damage model and extended finite element techniques." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2012. http://amslaurea.unibo.it/3931/.
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Zhou, Jun. "Numerical Modeling of Ductile Fracture." University of Akron / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=akron1384774266.
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Lee, Sam Lai. "A decohesion model of hydrogen assisted crack propagation /." The Ohio State University, 1986. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487322984316684.
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Coradi, Audrey. "Modélisation du comportement mécanique des composites a matrice céramique : développement du réseau de fissures." Thesis, Bordeaux, 2014. http://www.theses.fr/2014BORD0267/document.
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Les matériaux composites à matrice céramique (CMC) sont élaborés à partir de constituants fragiles. Le comportement mécanique et le développement de la fissuration dépendent des propriétés des constituants élémentaires des CMC. La connaissance de l’influence de ces propriétés sur l’évolution de la fissuration et du comportement mécanique fournit une aide au concepteur de matériaux composites.L’objectif de ce travail est de modéliser l’évolution du réseau de fissures au sein du CMC sollicité en traction, à l’échelle du fil et à l’échelle du composite tissé. L’approche proposée est une alternative aux principaux modèles de comportement des CMC.A l’échelle du fil, l’endommagement intervient d’abord sous forme de fissures matricielles accompagnées de décohésions à l’interface fibre/matrice. Les analyses de ces deux mécanismes ont permis d’exprimer leur évolution au sein du fil en traction. Le comportement en traction résultant de l’endommagement et l’ouverture de la fissure matricielle sont aussi exprimés semi-analytiquement.Les comparaisons avec un modèle numérique de zones cohésives et avec les essais expérimentaux montrent une bonne corrélation des résultats.Enfin ces expressions à l’échelle du fil sont utilisées pour modéliser l’endommagement du fil longitudinal au sein du composite tissé en traction. De plus, un outil numérique est développé pour modéliser la fissuration matricielle inter-fil dans le composite tisséCeramic matrix composite materials (CMC) are elaborated from fragile constituents. Their mechanical behaviour and crack growing depend on the properties of the CMC elementary constituent. Knowing the influence of these properties on crack development and mechanical behaviour provides support to the composite material designer.This work aims at modelling the crack networks development within the CMC under axial tension, at the yarn scale as well as at the woven composite scale. The proposed approach is an alternative to the main CMC behaviour models.At the yarn scale, matrix cracking with interfacial debonding between fiber and matrix first happen. Both mechanisms are analysed and their development are expressed. The mechanical behaviour resulting from damage and the crack opening displacement are also described using semi-analytical equations. Comparisons with numerical cohesive zone model and also with experimental testing shows good correlation between results.These semi-analytical expressions are then used for modelling damage within each yarns at the woven composite scale. In addition, a numerical tool is developed for matrix cracking and interfacial debonding between yarns of the woven composite
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De, Noronha Motta Carlos Henrique. "A fatigue crack growth model with mean stress effects." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp04/mq21163.pdf.
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KUNIEDA, Minoru, Keisuke KAWAMURA, Hikaru NAKAMURA, and Khoa K. TRAN. "QUANTITATIVELY EVALUATION OF CRACK PROPAGATION DUE TO REBAR CORROSION." 日本コンクリート工学会, 2010. http://hdl.handle.net/2237/20926.
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Reuschlé, Thierry. "Les fluides et l'evolution des proprietes mecaniques des roches." Université Louis Pasteur (Strasbourg) (1971-2008), 1989. http://www.theses.fr/1989STR13019.
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Le role des fluides dans l'evolution des proprietes mecaniques et de transport des roches est etudie. L'interaction chimique entre un fluide et la matrice rocheuse depend de la propagation lente de fissures et de la deformation des pores par transfert en solution. Un modele permet de comprendre l'evolution de la structure de porosite dans la croute terrestre. De plus dans le cadre de l'interaction mecanique on etudie l'expulsion des hydrocarbures dans les roches sources en mettant en evidence l'effet primordial de la proximite des fissures
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Villette, François. "Endommagement de milieux hétérogènes : Le papier en tant que matériau modèle." Thesis, Université Grenoble Alpes, 2020. http://www.theses.fr/2020GRALI062.
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La compréhension, la modélisation et la prévision de la rupture dans les matériaux hétérogènes sont des enjeux importants pour de nombreuses applications comme la résistance des structures de génie-civil ou les détachements de blocs rocheux par exemple. Actuellement, la modélisation de la fissuration dans les modèles d’endommagement fait intervenir une longueur interne qui n’est pas encore reliée explicitement aux longueurs caractéristiques du matériau. L’objectif de ce travail est d’étudier l’influence des hétérogénéités du matériau sur les processus de fissuration en utilisant le papier comme matériau d’étude. Ce matériau a en effet la propriété de révéler sa structure (fibres et agrégats de fibres) par transmission optique et permet ainsi de suivre l’évolution de l’endommagement au cours de la rupture à moindres coûts. Dans un premier temps, les propriétés structurales et mécaniques locales des agrégats de fibres ont été obtenues à partir d’images acquises par tomographie à rayons X et d’essais de tractions. Des essais de traction filmés ont ensuite permis de visualiser le développement de la zone d’endommagement et de relier ses dimensions au comportement post-pic de la courbe de traction. Sur la base de cette analyse, une méthode originale de calage de la longueur interne a été proposée sur un modèle d’endommagement continu non local. Le rôle des différentes longueurs caractéristiques du matériau a été mis en évidence par ces résultats qui ont été complétés par une étude de la statistique de propagation de fissure dans un matériau hétérogène en utilisant un Fiber Bundle Model (modèle à faisceaux de fibres), que nous avons dans le cadre de cette thèse étendu à des champs aléatoires de rupture corrélés dans l’espaceThe understanding, modeling and prediction of failure in heterogeneous materials are important issues for many applications such as the resistance of civil engineering structures or rock detachments for example. Currently, damage models involve an internal length that is not yet explicitly related to the characteristic lengths of the material. The objective of this work is to study the influence of material heterogeneities on cracking processes using paper as a model material. Indeed, this material has the property to reveal its structure (fibers and fiber aggregates) by optical transmission and thus allows following the evolution of the damage during the rupture at lower costs. In a first step, the local structural and mechanical properties of the fiber aggregates were obtained from images acquired by X-ray tomography and tensile tests. Filmed tensile tests were then used to visualize the development of the fracture process zone and to relate its dimensions to the post-peak behaviour of the tensile curve. On the basis of this analysis, a novel method of calibration of the internal length was proposed on a non-local continuous damage model. The role of the different characteristic lengths of the material was highlighted by these results which were complemented by a study of the crack propagation statistics in a heterogeneous material using a Fiber Bundle Model that we have extended to spatially correlated random fields of rupture
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Mühle, Volker. "Theoretische und numerische Untersuchungen zu morphologischen Übergängen beim Rißwachstum." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2000. http://nbn-resolving.de/urn:nbn:de:swb:14-993117114171-16728.
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In dieser Arbeit wird die Strukturbildung beim Risswachstum im stationären und instationärem Temperaturfeld im Rahmen der linear-elastischen Bruchmechanik analysiert und numerisch mittels der Methode der finiten Elemente (FEM) untersucht. Die beim langsamen Eintauchen eines heißen schmalen Glasstreifens in kaltes Wasser mit wachsender Temperaturdifferenz oder Eintauchgeschwindigkeit beobachteten Übergänge zwischen keinem, einem geraden und einem oszillierenden Riss werden in ein morphologisches Diagramm eingetragen und die Art des Überganges zur oszillierenden Rissausbreitung bestimmt. Die theoretischen Ergebnisse werden mit Experimenten verschiedener Autoren verglichen. Gleichartige Untersuchungen werden für die Ausbreitung mehrerer Risse durchgeführt. Beim Abschrecken einer erwärmten breiten Probe entstehen hierarchisch geordnete Risslängenstrukturen. Das Skalenverhalten der Rissdichte in Abhängigkeit von der Risslänge wird untersucht. Die Theorie liefert ohne Fitparameter eine sehr gute Übereinstimmung mit dem ExperimentThis paper investigates the formation of crack patterns in stationary and transient temperature fields analytically with linear elastic fracture mechanics and numerically with the finite elements method (FEM). In particular, we consider the experimental situation of a narrow thin strip of hot glass slowly lowered into cold water, with temperature difference and velocity as variable parameters. The parameter regions of no crack, one straight crack and one oscillating crack are determined. The type of phase transition related to the borderline between straight and oscillating crack is characterized. The theoretical results are compared with those of other authors. Similar investigations and comparisions are done for the propagation of multiple cracks. Quenching of a wide thin strip leads to a hierarchy of cracks whose scaling properties are analyzed. Without any fitting, theory and experiment agree surprisingly well
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VeÌlez, Juan Pedro Bercial. "Mathematical models of cracks interacting with boundaries and interfaces." Thesis, University of Liverpool, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.406823.
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Wang, Lei. "Investigations into deep cracks in rocket motor propellant models." Thesis, Virginia Tech, 1990. http://hdl.handle.net/10919/42146.
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Star grain configuration design has been widely used in solid rocket applications for several decades. Although a large number of surface cracks are detected in the rocket motor propellants, the mechanism of these cracks is sull not well known due to the complex geometry of the grain.
A stress-freezing photoelastic investigation has been performed to study the deep cracks which emanate from the fingertips of the star-shaped cutout cylinders. Using three-dimensional photoelasticity and proper algorithms in fracture mechanics, the stress intensity factors (SIF's) and the stress singularity orders along the crack front have been calculated. A surface effect on the dominant singularity order is observed and some analytical results are employed as a comparison.
Meanwhile, three-dimensional finite element solution to the circular cylinder is used to find the “equivalent” inner radius for the internal star cylinder and the variation of SIF's along the crack border shows a very similar trend to the experimental results once the "equivalent" radius is adopted.Master of Science
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Wang, Ying. "Failure modes of silicon nitride rolling elements with ring crack defects." Thesis, Bournemouth University, 2001. http://eprints.bournemouth.ac.uk/427/.
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High quality silicon nitride ceramics have shown some advantages for rolling element bearing applications. In particular hybrid bearings (silicon nitride rolling elements and steel races) have the ability to withstand high loads, severe environments and high speeds. However, the difficulties of both sintering and machining the material may result in surfacedefects,such as surface ring cracks. It is difficult to detect surface ring cracks during high volume production processes and hence it is crucially important to understand their influence and the fundamental mechanism of the failures they cause. The purpose of this study is to examine the contact fatigue failure modes of silicon nitride rolling elements with surface ring crack defects. In this study, new experimental and computational techniques are developed to measure and model the interaction of the surface with pre-existing crack defects. A rolling contact fatigue test method is devised for positioning the ring crack in the contact path. Rolling contact fatigue tests are conducted using a modified four-ball machine in a hybrid ceramic/steel combination. A three-dimensional boundary element model is used to determine the stress intensity factors and to carry out the crack face contact analysis. Research shows that the RCF life performance of silicon nitride bearing elements is dependent upon the crack location and fatigue spall happens only at a few crack orientations. The spalling fatigue failure is not only influenced by the original ring crack propagation but also strongly influenced by the subsequent crack face contact. Secondary surface cracks play an important role in the forination of a fatigue spall. The crack gap and crack face friction coefficients significantly affect the formation of secondary surface cracks. Numerical calculation results are consistent with the experimental observations. A quantitative three-dimensional boundary element model has been developed, which can be used to determine the geometry of acceptable surfacering cracks.