|
|
Design of high-performance auxetic structure for energy absorption
Sobol, Vítězslav ; Hutař, Pavel (referee) ; Červinek, Ondřej (advisor)
Additive technologies enable the production of complex structures with high control over their geometric parameters. In the field of energy absorption, it is advantageous to use a structured material because they can safely absorb large amount of energy. For high-performance absorbers, it can be advantageous to use auxetic structures which, due to their unique internal geometry, provide, e.g. better energy redistribution. Compared to conventional structures, however, they do not achieve such high values of absorbed energy. Also, literature does not offer a detailed description of the mechanisms of absorbed energy increase, based on which the geometry of the auxetic structure could be effectively modified. This thesis dealt with the systematic design of the internal geometry of a 2D auxetic structure to increase the absorption performance. Five different arm geometries were tested as well as cells with reinforcements with stepped distance from the centre of the cell. Compression testing showed a low dependence of the arm geometry used and a significant benefit of the reinforcements on the energy absorbed. The DIC technology provided deformation maps of structures, which led to the clarification of the energy increase mechanism by the reinforcement implementation. The results obtained led to an auxetic structure that was able to absorb 70 % more energy per unit mass compared to the reference geometry.
|
|
|
Influence of underground installation conditions on crack propagation in polypropylene pipes
Tesař, Jakub ; Gratza, Roman (referee) ; Hutař, Pavel (advisor)
The thesis addresses the issue of buried polymer pipes and the impact of installation conditions on their lifetime. Based on data obtained from standards, the values of external loads acting on the polymer pipe are determined. To apply these values in numerical calculations, the conversion of individual load components is also provided. Various stress values, simulating different burial installation conditions, are applied to a 2D model. The numerical calculation of the 2D model includes the progression of tangential stresses and their comparison with the standard. Furthermore, the influence of installation conditions on the burial depth of the pipe is analyzed. The comparison of models is conducted for both 2D and 3D models against the standard. The comparison verifies that the models are equivalent and closely correspond to the basic calculations according to the standard. The 3D model is then extended to include a crack, and the propagation through the pipe wall under the most critical load conditions is modeled. The concept of linear elastic fracture mechanics is used to estimate the lifetime of the pipe.
|
|
|
Preheating of the base plate during WAAM production of magnesium alloy AZ61
Zeman, Stanislav ; Hutař, Pavel (referee) ; Slavíček, Jakub (advisor)
The aim of this thesis was to design and construct a heated table, which was subsequently used to verify the effect of preheating on parts made by WAAM technology using AZ61 magnesium alloy as filler wire. The effect of preheating in the range of 50°C-350°C on the basic geometry of the single-track weld was successively investigated. It has been shown that the effect of preheating reduces hight and widens the geometry of the single-track weld, while deepening the penetration. Based on this knowledge, an ideal preheating temperature of 200 °C was determined for the production of thin-walled specimens. Preheating stabilized the width of the layers in the thin-walled specimen and significantly increased its quality. Furthermore, preheating was used to reduce residual stress. Strain measurements showed up to 50% residual stress reduction within the thin-walled specimen. Finally, a suitable temperature range of 200°C - 250°C was determined for WAAM fabrication of AZ61 magnesium alloy.
|
|
|
Numerical modelling of residual stress in metallic materials
Rakušan, Jakub ; Vosynek, Petr (referee) ; Hutař, Pavel (advisor)
The presented master thesis is focused on residual stresses, more precisely on the implementation of residual stresses in simulations using the finite element method (FEM), experimental determination of residual stresses and the influence of residual stresses on fatigue crack propagation. In this thesis, the found approaches of implementing residual stresses in FEM are tested and the most suitable of the tested approaches is then used in the simulation experiment. By comparing the FEM simulation and experimentally measured data determined by the modified slotting method, the residual stress values on an induction hardened railway axle specimen are determined. This is followed by the verification of the determined residual stress values by X-ray diffraction and the determination of the effect of residual stress on the rate of propagation of long fatigue cracks.
|
|
|
Description of the slow crack growth in polymer materials
Trávníček, Lukáš ; Nezbedová,, Eva (referee) ; Padovec,, Zdeněk (referee) ; Hutař, Pavel (advisor)
Požadovaná životnost plastových trubek používaných v tlakových systémech je 50 let. Zároveň je však vyvíjen tlak na použití recyklovaných materiálů, které mají podstatně horší mechanické vlastnosti a samotné je pro výrobu tlakových polymerních potrubí nelze použít. Tyto dvě protichůdné podmínky jdou skloubit pomocí trubky vytvořené z více vrstev, kde pro kritické časti, kterými jsou vnější a vnitřní povrch, je použit moderní odolný materiál. Na druhou stranu pro střední vrstvu, jejíž mechanické vlastnosti nejsou tak důležité z pohledu dlouhodobé životnosti, je možno použit materiál recyklovaný. Hlavním cílem této práce je navrhnout geometrii popsaného vícevrstvého potrubí s využitím recyklovaného materiálu při zachování minimální požadované životnosti. Životnost polymerních trubek je poměrně náročné testovat za běžných provozních podmínek z časových důvodů. Je však možné ji odhadnout, neboť nejčastější mechanismus poškození je pomalé šíření trhliny. Metodika odhadu životnosti využívá parametrů lineární elastické lomové mechaniky pro popis šíření těchto trhlin. Důležitou součástí je pak simulace šíření trhliny v trubce metodou konečných prvků se zohledněním různých typů zatížení, kterým je trubka v provozu vystavena. Mezi tyto zatížení patří reziduální napětí a zatížení způsobené tíhou zeminy, dopravou nebo kameny, pokud je trubka zakopána. Pro určení reziduálního napětí byla vyvinuta metodika, která využívá kombinace experimentů a numerického modelování a která byla také použita. Případ zakopané trubky je také simulován pomocí metody konečných prvků. Část práce je věnována měření rychlosti šíření creepové trhliny pomocí CRB testů (cracked round bar) a následnému vyhodnocení experimentálních dat. Rychlost šíření creepové trhliny spolu s kombinací s výsledky numerického modelování bude použita pro stanovení odhadu životnosti vícevrstvé polymerní trubky obsahující recyklovaný materiál.
|
|
|
Design of support structures for parts made of Inconel 718
Kuhajdik, Matej ; Hutař, Pavel (referee) ; Koutný, Daniel (advisor)
This master thesis deals by the design of support structures for the production of parts made of nickel superalloy IN718 using selective laser melting (SLM). The aim of this work is to design a support structure so that technological problems (excessive deformation of the part due to high residual stresses and insufficient heat dissipation) are eliminated and replace the use of massive volume support, which is inefficient in terms of design and production time, consumed material and postprocessing. Mechanical behaviour of residual stresses of built component, design points of selected support structures (perforated block with pins, BCCZ with perforated contour) and design recommendations were quantified by using thermo-mechanical simulations of production and series of experiments. The functional sample was tested by designing specific support structures for the turbine wheel with subsequent production. The support structure ensured safe, successful production without potential problems and met the requirement to minimize material consumption through effective large-scale perforation of the structure.
|
|
|
Design of camera crane
Navrátil, Pavel ; Hutař, Pavel (referee) ; Koutný, Daniel (advisor)
This Master’s thesis deals with several variants construction design of camera crane. Two optimal variants are chosen with regard to practical usage. The thesis includes complete design of these variants with 3D models and strenght analyses. This analyses are processed by final element method. The first variant made based on technical documentation and it has worked for company Filmochod s.r.o..
|
|
|
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.
|
|
|
Influence of Friction to the Shear Stress State in the Crack-Front Vicinity
Vlach, Jiří ; Hutař, Pavel (referee) ; Horníková, Jana (advisor)
The aim of this master thesis is to explore the influence of frictional forces to the stress and strain on the straight crack tip loaded in shear modes. The first section summarizes the most important things from the field of fracture mechanics. Especially theoretical knowledge about fracture parameters (stress intensity factor, J – integral) and their determination in computational environment ANSYS Workbench. The second part deals with computational modeling. At the beginning, is created the model of material, the model of geometry, the model of loads, etc. Then it is possible to solve the direct problem outlined in the introduction. A result of this thesis is the assessment of how the friction influences the fracture parameters in shear modes II and III.
|
|
|
Description of the residual stresses in polymer pipes
Poduška, Jan ; Majer, Zdeněk (referee) ; Hutař, Pavel (advisor)
This thesis is dealing with the issue of residual stress present in the wall of a polymer pipe and the influence on its lifetime. Experimental data obtained by the ring slitting method are evaluated and the tangential residual stress in the wall of polypropylene pipe is determined. The evaluation is carried out using a new methodology based on the curved beam theory. The method is verified using numerical simulation. 3D numerical model is used to verify the behavior of the pipe, when there are both tangential and axial residual stresses present in the pipe wall, because the presence of axial stress causes a rise in the magnitude of tangential residual stress. A correction of the tangential stress values corresponding to the pipe length is then proposed. It is shown, that the distribution of the tangential residual stress does not depend significantly on the dimensions or ma-terial of the extruded pipe and a general equation is proposed to describe the distribution. This general distribution is then involved in the calculations of the pipe lifetime that are carried out using a method based on the linear elastic fracture mechanics. A significantly lower lifetimes are obtained when taking the residual stress into account.
|
guest ::
RSS feed.