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Numerical Modelling of Fatigue Crack Closure
Oplt, Tomáš ; Růžička, Milan (referee) ; Horníková, Jana (referee) ; Hutař, Pavel (advisor)
This Ph.D. thesis was written under the supervision of Assoc. prof. Pavel Hutař, Ph.D., and Assoc. prof. Luboš Náhlík, Ph.D. The thesis is focused on the effect of plasticity induced crack closure, its characteristic and ways of numerical modelling. Premature fatigue crack closure has a significant effect on the fatigue crack propagation rate and therefore on the residual lifetime of a structure. A three-dimensional numerical model allows a detailed look at the stress and strain distribution along the crack front, and particularly it allows a local description of parameters along the crack front which governs the fatigue crack propagation rate. In the first part of the thesis, the study is focused on the influence of a singular stress field at the vicinity of the free surface on the crack front curvature without crack closure being involved. In the second part, a numerical model in 2D of plasticity induced crack closure was created and verified by experimental results. In the final part, a 3D numerical model is used to describe the influence of the crack closure on its fatigue propagation rate and explains typical crack front curvature. The suggested technique allows quantitative accuracy improvement of numerical simulation of the fatigue crack propagation and therefore, more reliable estimation of the residual lifetime of the cracked structure.
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Usage of kevlar fibres in asphalt mixtures
Mazáč, Jan ; Hegr, Zdeněk (referee) ; Hýzl, Petr (advisor)
Diploma thesis deals with topic of using aramid (Kevlar) fibres in road engineering in asphalt mixtures. Theoretical part of thesis is describing characteristics of aramid fibres and their technical usage. Thesis deals with research and application of aramid fibres abroad and also in Czech Republic. Practical part deals with influence of aramid fibres, namely FORTA FI fibres in mixture with modified binder for wearing course. An integral part of this thesis is a description of all methods that were used during the processing of the practical part. In the end of the thesis there are outcomes of tests and theoretical research.
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Determination of residual fatigue lifetime of railway axle after surface treatment
Pánek, Ondřej ; Poduška, Jan (referee) ; Náhlík, Luboš (advisor)
This master thesis deals with prediction of residual fatigue lifetime of railway axle. In the first part of this thesis, theoretical introduction to the issue based on research of available literature is elaborated. The second part is dedicated to numerical modelling of actual railway axle with crack, which serves to obtain the necessary parameters entering estimation of the residual fatigue lifetime. At first, a finite element model of cylindrical body with crack loaded by bending moment was created and compared with analytical solution for verification purposes. The second step was the creation of separate models of railway axle with crack for three types of loading – bending moment, press fit of wheel and residual stresses, respectively. Furthermore, the shape of various crack propagation stages was determined, according to the methodology developed at IPM CAS. Using the determined crack shape, stress intensity factors were calculated separately from all types of loads for various crack depths. Dependence of stress intensity factor on the crack length was then used to calculate residual fatigue life for real loading spectrum.
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Influence of the residual stresses on the contact failure of ceramic laminates
German, Roman ; Majer, Zdeněk (referee) ; Ševeček, Oldřich (advisor)
The presence of the compressive or tensile thermal residual stresses in layers of a ceramic laminate induced due to different volume change of each layer´s material during the cooling from the sintering temperature can considerably affect resistivity of ceramics against contact damage. Within this work 2D parametric FEM models were created, in order to study the effect of the surface layer thickness, residual stress values and indenting body dimension on the initiation and propagation of the cone crack in the surface layer of the laminate. For the analysis of the critical conditions for the crack initiation, the coupled stress-energy criterion was used and for the determination of the direction of crack propagation we used the maximum tangential stress criterion. The results show that compressive thermal stresses in the surface layer increase the critical force for the crack initiation, shorten the crack distance from the contact area and shorten the occurred crack itself. Moreover, the compressive stresses enlarge the angle of the crack declination during the propagation process which cause an earlier crack arrest. The tensile thermal stresses have exactly the opposite effect. Results of simulations were compared to experimental results but due to lack of available measurements, the verification is partially limited.
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PROPAGATION OF LONG FATIGUE CRACKS IN AUSTENITIC STEEL UNDER SHEAR MODES II AND III
Holáň, Libor ; Man, Jiří (referee) ; Polák, Jaroslav (referee) ; Pokluda, Jaroslav (advisor)
This work is focused on the realization of experiment allowing simultaneous loading under mode II and III in a single circular specimen. Proposed experiment allowed to minimize crack closure during the cyclic loading and obtained values of thresholds of stress intensity range can be considered to be very close to effective values. This was attained by means of an unique experimental devices and procedure of preparation of pre-crack of specimen with circumferential notch, which was made of stainless austenitic steel. The obtained values were compared with theoretical models with the support of molecular dynamics and ab-anitio calculation. Based on observation was found out, that fatigue crack propagation is controlled by decohesion model in austenitic steel. The morphology of fracture surfaces was studied by means of optical chromatographie and 3D stereophotogrammetry, which allowed a comparison of created morphology under shear modes II and III. Morphology of fracture surface formed (static and cyclic loading) by pre-crack was also studied by means of selected roughness parameters. The mechanism of deflection (kink) of crack growth under mode II was defined.
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Description of creep crack propagation in electrofusion sockets for polymer pipes connection
Helešicová, Petra ; Horníková, Jana (referee) ; Trávníček, Lukáš (advisor)
The presented bachelor’s thesis deals with crack propagation by the slow crack growth mechanism in electrofusion sockets used for welding of polymer pipes. The thesis can be divided into several parts. First part presents a research on the topic of polymer pipes, materials used for their production, types of failures and ways of connecting including detailed description of the process of electrofusion welding with electrofusion sockets. Next part is focused on fracture mechanics that is used for the practical part of the thesis. The theory of linear elastic fracture mechanics and its stress-intensity approach are described. Different ways of obtaining the stress intensity factor, which is the main parameter used to describe the stress distribution at the crack tip, are presented. The practical part of the thesis focuses on numerical modelling of crack propagation through an electrofusion socket wall and calculating values of stress intensity factor in dependence on the crack length. The creation of the numerical model for the crack propagation simulation is described. The direction of propagation is evaluated using MTS criterion. The whole simulation is carried out by a parametric macro created using the APDL language. The main goal of the thesis is to describe the influence of the socket geometry or joint configuration changes on the stress intensity factor values and calculated lifetimes. The stress intensity factor functions for the modified configurations are compared to the original. At the end of the thesis, lifetime estimations of the electrofusion sockets are also calculated using the stress intensity factor functions and compared.
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Propagation of inclined cracks to the interface of ceramic laminates
Novotná, Lenka ; Trunec, Martin (referee) ; Chlup, Zdeněk (advisor)
Composite materials with laminated structure provide advantages which are utilised during component design. Low density, temperature and chemical stability are the profitable properties predetermining application of ceramic laminates. The main obstacle for wide spread of ceramic materials is their inherent brittleness. Therefore, in this thesis, the crack propagation in ceramics laminates has been extensively studied. Laminated structures with various volume fractions of components (alumina and zirconia) were prepared by electrophoretic deposition. Evaluation of crack propagation through the interface and determination of basic mechanical properties was conducted on the basis of extensive literature search. Crack deflection originated in both presence of internal stresses and differences in elastic modulus during the crack interface passing was monitored. A special type of specimen geometry was employed with the aim to set arbitrary angle between crack and interface. It was experimentally found that the degree of crack deflection is dependent on entering angle and volume fraction of components. Higher crack deflection was already found in the bulk of the test piece comparing to the test piece surface. The 3D fracture surface reconstruction generated using laser confocal microscopy was used in this detailed crack propagation study. Further basic elastic and strength characteristics of laminates were determined and compared to those obtained from monolithic materials. The validity of the mix rule for elastic characteristics was confirmed by comparing of elastics modulus. The most reliable method for elastic modulus determination was marked the dynamic resonance method due to low scatter and consistency in measurement. The flexural strength of all laminates tends to be close to the flexural strength of the weakest component. Therefore the mix rule is not applicable for flexural strength estimation on the contrary of elastic characteristics. The change of component volume fraction leads only to change of flexural strength scatter. Thanks to gained knowledge about crack propagation and basic characteristic determination will be possible to design ceramic laminates more efficiently for given needs of application.
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STOCHASTIC CRACK PROPAGATION MODELLING USING THE EXTENDED FINITE ELEMENT METHOD
Nešpůrek, Lukáš ; Novák, Drahomír (referee) ; Hutař, Pavel (referee) ; Anthony Nouy, PhD., HDR (referee) ; Knésl, Zdeněk (advisor)
Tato disertační práce vychází z výzkumu v rámci francouzsko-českého programu doktorátu pod dvojím vedením na pracovišti Institut français de mécanique avancée v Clermont-Ferrand a na Ústavu fyziky materiálu AV v Brně. Úvodní výzkumný úkol na brněnském pracovišti se zabýval numerickou analýzou pole napětí v okolí čela trhliny v tenké kovové fólii. Zvláštní pozornost byla zaměřena na vliv speciálního typu singularity v průsečíku čela trhliny s volným povrchem. Těžiště disertační práce spočívá v numerickém modelování a stochastické analýze problémů šíření trhlin se složitou geometrií v dvojrozměrném prostoru. Při analýze těchto problémů se dříve zřídka používaly numerické metody, a to z důvodu vysoké náročnosti na výpočtový čas. V této disertaci je ukázáno, že aplikací moderních metod numerické mechaniky a vhodných technik v analýze spolehlivosti lze tyto problémy řešit s pomocí numerických metod i na PC. Ve spolehlivostní analýze byla využita lineární aproximační metoda FORM. Pro rychlost šíření trhlin se vycházelo z Parisova-Erdoganova vztahu. Pro parametry tohoto vztahu byl použit dvourozměrný statistický model, který postihuje vysokou citlivost na korelaci obou parametrů. Mechanická odezva byla počítána rozšířenou metodou konečných prvků (XFEM), která eliminuje výpočetní náročnost a numerický šum související se změnou sítě v klasické metodě konečných prvků. Prostřednictvím přímé diferenciace bylo odvozeno několik vztahů pro derivace funkce odezvy, čímž se dosáhlo lepší numerické stability a konvergence spolehlivostní analýzy a výrazného zkrácení doby výpočtu. Problém zatížení s proměnou amplitudou byl řešen aplikací transformace zatížení metodou PREFFAS. Využití distribuce výpočtů v síti PC umožnilo další zrychlení analýzy.
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Design of special asphalt mixtures SAL for the repair of cement concrete pavements
Kalfeřt, Martin ; Špaček, Petr (referee) ; Varaus, Michal (advisor)
In this thesis layer with increased resistance to crack propagation (SAL) are designed. The theoretical part describes the SAL layer, further the input materials, their production and design of various mixtures. The following is a description of the tests, which include tests of permanent deformation, crack propagation, low-temperature characteristics, test, flexural strength and relaxation. After the test results and conclusion are stated.
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Computational modelling and analysis of crack propagation in ceramic composites containing residual stresses
Černý, Hynek ; Majer, Zdeněk (referee) ; Ševeček, Oldřich (advisor)
The diploma thesis dealt with 3D computational modelling of crack propagation in particle and layered ceramic composites. In first part of thesis, all tools for fracture mechanics implemented in software ANSYS were explored for their suitability in given topic. In the next part, crack propagation in particle composites was analysed, where extra focus was applied on influence of different stiffness and thermal expansion of all components of the composite. First, the crack propagation in homogenous material model was set up, then the effect of different stiffness of the composite components was added, and in the last stage, the effect of the residual stresses was also added. From the achieved results, it was possible to describe the influence of the previously listed factors on the energy required for crack propagation. This created model should be applicable to the model of a real composite, thanks to which it should be possible to quantify whether the composite is more resistant to crack propagation then the homogenous material. In the last part of thesis, the crack growth through layered ceramic laminates was analysed, where emphasis was again placed on the different stiffness and thermal expansion of all components of the laminate. In this part, the great advantage of the ANSYS APDL, creating of macros, were used. Using macros, a parametric model was created, which, after the input data was entered, was able to create geometry of the computational model, enter boundary conditions, create a mesh of the computational model, set up the analysis and start the simulation. The created model can be used, for example, to determine the apparent fracture toughness of laminate, thanks to which it is possible to access how much the used configuration is more suitable than the use of a homogenous material.
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