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Influence of a Free Surface and Gradient Change of Material Properties on a Crack Behaviour
Ševčík, Martin ; Kohout, Jan (referee) ; Klusák, Jan (referee) ; Náhlík, Luboš (advisor)
This thesis was written under the supervision of Assoc. Prof. Luboš Náhlík, Ph.D. and Assoc. Prof. Pavel Hutař, Ph.D. The topic of this thesis is the study of a free surface effect and gradient change of material properties on a crack behavior. The common theme of the work is a fracture mechanics description of a crack behavior near a material nonhomogeneity. Here, the material nonhomogeneity can be understood either as a boundary of a body (interface between body and surrounding) or as a continuous change of material properties. The thesis is introduced by a review part where the state-of-the-art of the concerned topic is described. This part presents several stress state descriptions in the vicinity of some general singular stress concentrators, particularly a crack and a V-notch. The influence of the free surface on a fatigue crack front shape is discussed here. The review part follows with the fracture mechanics description of the graded materials. The problem formulation and the main aims of the thesis are stated in the following chapter. The core of this work is the fracture mechanical description of a crack propagating near the material nonhomogeneity. The thesis focuses on a stress field description near the free surface of the body where a change in a type of the singular stress field occurs. Methods used in generalized fracture mechanics are applied here to describe the stress field near the free surface. The difference between crack behavior in thin-walled and thick-walled structures is shown and supplied by relevant examples. Methods and procedures used are later utilized for estimation of a crack behavior in graded structures. The thesis is concluded by the discussion of obtained results in appropriate context.
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Fracture behavior of welded polyolefin pipes
Mikula, Jakub ; Kučera,, Jaroslav (referee) ; Ševčík, Martin (advisor)
Cílem této diplomové práce je řešit problematiku poškozování svařovaných polyolefinových potrubí. Prezentovaná práce je zaměřena na numerické modelování kvazi-křehkého porušování, které je přímým důsledkem šíření creepové trhliny. Svařování polyolefinových potrubí vnáší do oblasti svaru nehomogenní rozložení materiálových vlastností a tvar svarového výronku způsobuje vznik geometrických koncentrátorů napětí. Posouzení těchto vlivů na životnost svarového spoje a výpočet polyolefinových potrubí je hlavním cílem této práce. V práci je uveden popis několika experimentálních testů (především CRB a PENT) k určení materiálových vlastností popisujících kinetiku trhliny. Výsledky práce jsou prezentovány pro polyetylenové potrubí (PE 100 110 x 6.3 SDR 17.6). Polyetylen je jeden z nejčastěji používaných materiálů právě v oblasti potrubních systémů. Prezentované výsledky práce mohou být použity i pro jiný polyolefinový materiál. Predikce životnosti polyolefinových potrubí je založena na lineárně elastické lomové mechanice (LELM) a koncepci faktoru intenzity napětí. Diplomová práce doplňuje tuto metodologii výpočtu životnosti o výpočet svařovaných polyolefinových potrubí z pohledu lomové mechaniky s náznakem možnosti budoucího výzkumu.
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Numerical modeling of behavior of a particle composite with crosslinked polymer matrix
Máša, Bohuslav ; Ševčík, Martin (referee) ; Náhlík, Luboš (advisor)
The master's thesis deals with the determination of macroscopic behavior of a particulate composite with cross-linked polymer matrix under tensile load. The main focus of thesis is estimation of mechanical properties of a composite loaded by tensile loading using numerical methods (especially finite elements method). Investigated composite is composed of matrix in a rubbery state filled by alumina-based particles (Al2O3). Hyperelastic properties of the matrix have been modeled by the Mooney-Rivlin material model. Different compositions of particles, their different shape, orientation and different volume fractions have been considered. For all these characteristics of composite numerical models have been developed. The damage mechanisms of the matrix have also been taken into account. Results of numerical analyses have been compared with experimental data and good agreement between numerical models with damage mechanisms of matrix and experimental data has been found.
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Computational modelling of strain and stress in spur gearing using FEM
Ševčík, Martin ; Maňas, Pavel (referee) ; Vrbka, Martin (advisor)
This diploma thesis deals with computation modeling of spur gearing using FEM. The methodology of checking calculation was suggested for gears which are not possible to check only by using standard ČSN 01 4686. The analysis of influence of vicinity of stress concentration on stress distribution in tooth root was performed. It is possibility to use the gears with thinner rim than the standard ČSN 01 4686 recommends. The main part of this work is devoted to fatigue crack propagation study in gears with thin rim. Aiding T-stress the influence of constraint and stress multiaxiality on crack propagation was taking into account. The predicted fatigue crack propagation paths were compared with certain experimental data. It is demonstrated that the considering of influence of constraint improves estimation of consequent crack propagation direction and in some cases the influence of constraint can significantly change the estimated crack propagation path.
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Simulation of the HVS characterstics in Matlab
Ševčík, Martin ; Slanina, Martin (referee) ; Kratochvíl, Tomáš (advisor)
In theoretical part Diploma thesis deals with the model of human vision HVS (Human Visual System), which can be used for image quality assessment in TV technique area. It has been described calculations of selected JND (Just Noticeable Difference) metrics, used in evaluation of HVS. In practical part of the thesis it has been suggested and realized simulation model in Matlab, which may be used for evaluation of three JND metrics from color and grayscale images and evaluation in spatial a frequency domain. Results of JND models have been compared to another objective image quality evaluation metrics (MSE, NMSE, SNR and PSNR). For interpretation of dependencies it has been used images with different defined content.
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Comparison of freeware software for estimation of residual fatigue life of bodies
Hanáček, Jan ; Ševčík, Martin (referee) ; Pokorný, Pavel (advisor)
In the first part of this bachelor's thesis the fundamental laws of linear elastic fracture mechanics are described, especially state of stress in bodies with crack and stress intensity factor. Further, the propagation laws of fatigue crack are described and interaction effects of fatigue crack, especially retardation effects are explained. In the second part of this thesis the software NASGRO and AFGROW are described and compared. In the last part of this work the several examples are carried out to compare residual fatigue life obtained by no retardation approach and approach with consideration of retardation effects.
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Possibilities of estimation of selected fracture parameters under conditions of LEFM
Zouhar, Petr ; Hutař, Pavel (referee) ; Ševčík, Martin (advisor)
The presented bachelor´s thesis deals with possibilities of estimation of selected fracture parameters under conditions of linear elastic fracture mechanic (LEFM). The aim of this thesis is to quantify the accuracy of specific methods for the estimation of fracture parameters from a known stress and strain field obtained using numerical methods. Thesis is solved using finite element method. The work is divided into two logical parts. The first part of the thesis describes the basic concepts of LEFM and selected methods used for estimation of fracture parameters, namely the stress intensity factor, strain energy release rate and J-integral. A detailed description of the numerical model, a sensitivity analysis of the FEM mesh density and obtained results are described in the second part, focused on the solution of the problem,. The accuracy of each method is expressed by comparing with a known reference value of stress intensity factor. The applicability of selected methods for estimation of the stress intensity factor is shown on a practical example of three-dimensional body containing a crack.
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The influence of specimen dimensions on fatigue crack closure
Šebík, Marek ; Ševčík, Martin (referee) ; Poduška, Jan (advisor)
This study is focused on fatigue crack propagation, especially on plasticity-induced fatigue crack closure. The influence of specimen dimensions on fatigue crack closure is assessed by evaluation and consequent comparison of opening and closing stresses on two different fatigue specimen numerical models: compact tension specimen and center cracked plate tension specimen, using nonlinear material model. The numerical modeling of the phenomenon utilized finite element method. The models were created in Ansys programming language, APDL. The first part of the study deals with theoretical research of the subject. The second part consists of description and results of the numerical modeling.
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Numerical modeling of crack propagation under conditions of LEFM
Mikula, Jakub ; Hutař, Pavel (referee) ; Ševčík, Martin (advisor)
The presented bachelor’s thesis deals with numerical modeling of crack propagation in general planar body. A user macro was created in APDL programming language for estimation of crack propagation direction and collection of data. Macro allows selection one of the four criteria intended to estimate the direction of the crack and the subsequent listing of the data into a text document. The calculation is based on the finite element method (FEM) using a commercial system ANSYS. The conditions of linear elastic fracture mechanics (LEFM) are expected during the calculation. The first part of this thesis describes the basic concepts of LEFM, including a description of mentioned criteria. The second part is devoted to the created macro. It also describes the algorithm and the necessary conditions for correct processing of data. A comparison with available published data or with experimental data gives a summary of the accuracy and utilization of created macro. In conclusion, the example of estimation of the residual life by different criteria is shown, as its other application.
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