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Surface modification of glass fibers for polymer composites
Knob, Antonín ; Marek,, Aleš (referee) ; Burša, Jiří (referee) ; Čech, Vladimír (advisor)
The doctoral thesis is aimed at preparation of glass fiber reinforced polymer composites with controlled interphase formed by plasma-polymerized tetravinylsilane and tetravinylsilane/oxygen thin films. The thin polymer films of specific physico-chemical properties and thickness were deposited to improve interfacial adhesion of glass fiber/polyester composites. The fiber surface modification was performed by using plasma enhanced chemical vapor deposition in low-temperature RF plasma operating in an various effective power range and different treatment time. Test results were examined in relation to the interlayer thickness and different treatment conditions. The prepared interlayers were analyzed to evaluate physico-chemical composition and properties (XPS, RBS, ERDA, FTIR and spectroscopic elipsometry). Selected mechanical properties were evaluated by AFM. Mechanical response of plasma interlayers was evaluated by short beam shear test and direct method of testing the interfacial shear strength using microindentation. The interphase shear failure was controlled by the shear strength at the interlayer/fiber interface as follows from experimental and model data.
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Literature search of possibilities of computational modelling of brain aneurysms
Lipenský, Zdeněk ; Polzer, Stanislav (referee) ; Burša, Jiří (advisor)
This bachelor thesis is focused on gathering general information about a cerebrovascular disorder called Cerebral Aneurysm (or also Brain Aneurysm) which is a medical condition consisting in an abnormal bulging outward of one of the arteries in the brain. The initial attention is given to the human cardiovascular system, a description of which is fatal for understanding the problem of brain aneurysm. The next attention is focused on the solution of eliminating the tension within the aneurysm by using knowledge gained in lectures "Flexibility and Strength II" during the sixth semester. The last chapter shows a summary of three works dealing with computational modeling of cerebral aneurysm.
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Design of cylindrical apparatus for determination of directions of reinforcing fibres in aorta in its deformed state
Král, Martin ; Vaverka, Jiří (referee) ; Burša, Jiří (advisor)
This bachelor thesis deals with the design of cylindrical apparatus, which allows the cont- rolled biaxial deformation of a cylindrical sample of the aorta. This apparatus is designed to determine the direction of collagen fibers in the deformed aorta. The theoretical part of the thesis is focused mainly on description of the structure and mechanical properties of the wall of the aorta. In the design part, attention is paid to the design of the cylindrical apparatus. In the conclusion, the deformation of the cylindrical sample of the abdominal aorta from the pig is carried out on the apparatus and its strain is measured.
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Computational modelling of heart contraction
Vaverka, Jiří ; Polzer, Stanislav (referee) ; Burša, Jiří (advisor)
This thesis aims to determine the impact of slowed myocardial conduction velocity and depressed myocyte contractility on the duration of isovolumic contraction time (ICT) of the left ventricle by carrying out simulations using finite element method. A 3D finite element model enabling to simulate both physiological and pathological states of myocardium was created. The model is based on simplified ellipsoidal geometry and accounts for anisotropic behavior of myocardium, its asynchronous contraction and variations in the arrangement of muscle fibers. Slowing of conduction velocity to a half of its physiological value resulted in prolongation of ICT by 27 %; slowing of shortening velocity of myocytes by the same percentage prolonged ICT by 73 %. It is therefore concluded that ICT can be much more prolonged due to depressed contractility than due to conduction slowing. The presented results give an idea of the extent to which ICT can be prolonged due to depressed contractility and conduction slowing and therefore can be useful in identifying the causes of decreased myocardial performance in heart disease.
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Constitutive Modelling of Composites with Elastomer Matrix and Fibres with Significant Bending Stiffness
Fedorova, Svitlana ; Kotoul, Michal (referee) ; Menzel, Andreas (referee) ; Burša, Jiří (advisor)
Constitutive modelling of fibre reinforced solids is the focus of this work. To account for the resulting anisotropy of material, the corresponding strain energy function contains additional terms. Thus, tensile stiffness in the fibre direction is characterised by additional strain invariant and respective material constant. In this way deformation in the fibre direction is penalised. Following this logic, the model investigated in this work includes the term that penalises change in curvature in the fibre direction. The model is based on the large strain anisotropic formulation involving couple stresses, also referred to as “polar elasticity for fibre reinforced solids”. The need of such formulation arises when the size effect becomes significant. Mechanical tests are carried out to confirm the limits of applicability of the classical elasticity for constitutive description of composites with thick fibres. Classical unimaterial models fail to take into account the size affect of fibres and their bending stiffness contribution. The specific simplified model is chosen, which involves new kinematic quantities related to fibre curvature and the corresponding material stiffness parameters. In particular, additional constant k3 (associated with the fibre bending stiffness) is considered. Within the small strains framework, k3 is analytically linked to the geometric and material properties of the composite and can serve as a parameter augmenting the integral stiffness of the whole plate. The numerical tests using the updated finite element code for couple stress theory confirm the relevance of this approach. An analytical study is also carried out, extending the existing solution by Farhat and Soldatos for the fibre-reinforced plate, by including additional extra moduli into constitutive description. Solution for a pure bending problem is extended analytically for couple stress theory. Size effect of fibres is observed analytically. Verification of the new constitutive model and the updated code is carried out using new exact solution for the anisotropic couple stress continuum with the incompressibility constraint. Perfect agreement is achieved for small strain case. Large strain problem is considered by finite element method only qualitatively. Three cases of kinematic constraints on transversely isotropic material are considered in the last section: incompressibility, inextensibility and the double constraint case. They are compared with a general material formulation in which the independent elastic constants are manipulated in order to converge the solution to the “constraint” formulation solution. The problem of a thick plate under sinusoidal load is used as a test problem. The inclusion of couple stresses and additional bending stiffness constant is considered as well. The scheme of determination of the additional constant d31 is suggested by using mechanical tests combined with the analytical procedure.
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Stress-strain analysis of a sclerotic artery during angioplasty
Trtík, Adam ; Fuis, Vladimír (referee) ; Burša, Jiří (advisor)
This bachelor thesis focuses on an analytical calculation of maximum stress and deformation in arterial wall suffering from atherosclerosis during angioplasty and compares this value to the stress in wall of both healthy and sclerotic artery loaded by normal systolic pressure. Finally, the results are compared with corresponding model using the finite element method. In the theoretical part, the histology of arteries, their biomechanical properties and pathogenesis of atherosclerosis itself are briefly described.
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Computational modelling of mechanical tests of composites "rubber - steel fibre"
Jarý, Milan ; Profant, Tomáš (referee) ; Burša, Jiří (advisor)
This diploma thesis focuses on realization of a computational model of fibre composite with elastomer matrix and on homogenization of properties of this composite. The work deals with computational modelling of strain-stress states which arise in mechanical tests of composites. The composites investigated by mechanical tests comprise of hyperelastic rubber matrix and steel reinforcing fibres. Computational modelling is carried out at two levels of the model. First, with three-dimensional modelling of fibres and matrix as two different materials and, second, using a homogenized model of composite; this constitutive model describes the composite as a homogeneous anisotropic material. It means that properties of fibres are encompassed into strain energy density by the mathematical formulation of the constitutive model. Further, the work deals with computational modelling of mechanical tests of hyperelastic isotropic materials used for identification of their material parameters and for verification of the selected constitutive model of material. For particular hyperelastic material, simulations of tests were carried out, namely of uniaxial tension, biaxial tension, uniaxial compression, biaxial compression, pure shear and uniaxial tension with constrained transversal strain (planar tension). Parameters of the constitutive model were determined of experimental input data. Verification of the constitutive model was carried out by comparison of the data acquired by experiments with the results of simulations of mechanical tests in FE program system Ansys. Then the authentic constitutive model of material was used for description of matrix behaviour in models of mechanical tests of composite material and results were compared with experimental data. Principal objectives which I want to attain are following: • to acquaint with the constitutive models of hyperelastic isotropic and anisotropic materials and identification of their perameters on base of mechanical tests. • to create computational models of testing specimens of composite “ rubber – steel fibre“ for different fibre arrangements and to use the created computational models in simulations of chosen tests. • to test the possibilities of computational modelling of composites with application of homogenized properties and to compare the results of both approaches. Results which were attained: • the computational models were created with the fibres modelled; the strain – stress characteristics are qualitatively corresponding to experiments, and quantitative difference is 20% - 40% (see (4.3)). • the computational models based on homogenization of properties were tested and gave results corresponding to the models with modelled fibres (see (4.4)) with a good accuracy.
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Stress-strain analysis of artery with atheroma
Maša, Marek ; Návrat, Tomáš (referee) ; Burša, Jiří (advisor)
The main goal of this diploma thesis was the stress-strain analysis of iliac artery with atheroma.This problem was solved using finite element method (FEM).For the calculation purposes three two-dimensional models were created. The geometry was gained from transversal sections through the iliac artery with ateroma. This geometry is educed from used literature review. The main calculation process was run by ANSYS 11.0 program system.
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Computational modelling of mechanical tests of animal cell
Orlová, Lucie ; Fuis, Vladimír (referee) ; Burša, Jiří (advisor)
Předkládaná diplomová práce se zabývá stavbou živých živočišných buněk a jejich odezvou na mechanické zatěžování. Zobecněným zaměřením práce je popis mechanického chování buňky nejenom ve fyziologickém, ale i v patologickém stavu. Výchozím předpokladem pro úspěšné řešení zadané úlohy je vysoce interdisciplinární přístup kombinující výpočtové přístupy mechaniky těles (v~tomto případě metodu konečných prvků) s lékařským výzkumem. Nejdůležitějším bodem při tvorbě výpočtového modelu, pomocí něhož je možné aproximovat chování živé buňky při zatížení, je zejména identifikace mechanicky významných komponent a~jejich materiálových parametrů. V tomto případě jsou jako mechanicky význačné identifikovány spojité součásti jádro, membrána a cytoplazma, které jsou nově propojeny s prvky diskrétními (mitochondriální sítí) v hybridním modelu, jehož platnost je ověřena pomocí experimentálních dat. Tento model slouží jako podklad k vyhodnocení míry vlivu mitochondrií na celkovou tuhost buňky.
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