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Design and realization of experimental equipment for determination of residual stress of bent samples
Novotný, Lukáš ; Návrat, Tomáš (referee) ; Krejčí, Petr (advisor)
The master thesis is focused to design and realization of experimental equipment for researching of residual stress determination for specimen with non-uniform distribution stress along the depth. Literature search study of problematic was performed and it was used for elaboration of chapters about origin of residual stress, their classification and chapters about methods, which are used for measuring of residual stress and their determination. The following part explains the design of experimental equipment and stress-strain analysis of selected parts of this equipment. Then the final design of equipment is presented, including complete production drawings, and it’s explained final assembling of experimental equipment. Final chapter is devoted to examinable loading of specimen.
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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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Analysis of uncertainties in the calculation of residual stresses
Dubravec, Kristián ; Houfek, Lubomír (referee) ; Návrat, Tomáš (advisor)
This thesis is about analysis of uncertainties in the calculation of residual stresses, which are homogeneous and measured by the hole-drilling method. Stochastic approach is explained using models of beams. Theory of residual stresses is included. Source code for evaluating residual stresses deterministically and stochastically is created using the Python language. Sensitivity analysis of input parameters is performed.
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Description of Crack Propagation in the Fields of Strongly Non-Homogeneous and Residual Stresses
Štegnerová, Kateřina ; Materna, Aleš (referee) ; Malíková, Lucie (referee) ; Náhlík, Luboš (advisor)
This Ph.D. thesis was written under the supervision of Assoc. Prof. Luboš Náhlík, Ph.D. and Assoc. Prof. Pavel Hutař, Ph.D. Thesis is focused mainly on application of generalized linear elastic fracture mechanics, which allows description of crack behaviour propagating from general singular stress concentrators, such as material interfaces or sharp V-notches, and verification of validity of used fracture criteria. The obtained results were used in the next part of the thesis, which deals with the issue of crack propagating in ceramic composites, where the stress distribution field is strongly influenced by the existence of material interface and presence of residual stresses, that arise during manufacturing process of composite.
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Determination of residual stresses from strain field around drilled hole
Horák, Richard ; Návrat, Tomáš (referee) ; Halabuk, Dávid (advisor)
The master’s thesis focuses on determining homogeneous residual stresses using the hole-drilling method. The optical method of digital image correlation (DIC) is used for measuring released strains. In order to evaluate residual stresses using the entire strain field, it is necessary to know the functional dependencies of calibration constants on this field. Therefore, a significant part of the master’s thesis includes finding these dependencies in relation to the dimensions of the drilled hole and coordinates of the measured points. Another part of the thesis deals with the evaluation of homogeneous residual stresses based on provided experimental data obtained from the measurement of uniaxially loaded steel flat bar with a through-hole. At the end of the thesis, a sensitivity analysis is performed, which examines the influence of input variables in the calculation algorithm in relation to computed residual stresses.
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Impact of residual stresses and uncoiling of arterial specimen on its mechanical response
Sochor, Ondřej ; Lisický, Ondřej (referee) ; Burša, Jiří (advisor)
This thesis examines the impact of residual stresses in the carotid artery wall and the influence of specimen uncoiling on its mechanical response. The introductory part provides basic knowledge about the anatomy and the pathology of blood vessels, as well as overview of common methods used for testing artery samples. The main body of the thesis is divided into two sections. The first section focuses on the impact of specimen uncoiling on its mechanical response during the equibiaxial tensile test. It begins by describing the method used for sample testing and the method used for obtaining material constants from the mechanical response. These constants are then used as input for FEM simulation of the uncoiling and subsequent equibiaxial tensile test simulation on idealized model of geometry. The mechanical behaviour of the uncoiled model is compared to behaviour of the stress-free planar model to determine the influence of uncoiling. In the second section, the influence of residual stresses in the artery wall on its mechanical response is explored. This section includes a description of two methods used to show residual stress release, which are then used for specimen testing. The experiments are used to evaluate input parameters per layer for the model of geometry and subsequent FEM analysis. By comparing results from three levels of modeling, the impact of residual stresses on the artery’s mechanical response is determined.
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Numerical simulation of the hole drilling method for residual stress measurement
Halabuk, Dávid ; Ganev, Nikolaj (referee) ; Pástor,, Miroslav (referee) ; Návrat, Tomáš (advisor)
The doctoral thesis is focused on the investigation of various cases that may occur in the measurement of residual stresses by hole-drilling method and which deviate from the ideal state for which hole-drilling method is derived. In order to assess various stress states, geometries or material properties of the measured body, a computational model simulating the hole-drilling method was created by the finite element method. The first investigated case deals with cylindrical bodies and errors that may occur when the hole-drilling method is used to measure residual stresses in bodies with various surface radii. In addition to the evaluation of errors for different stress states, a procedure for the calculation of uniform residual stresses in cylindrical bodies has been proposed. The next part of the thesis is focused on investigating the influence of the residual stress placed in the perpendicular direction to the measured surface on the error of evaluated residual stresses located in a plane parallel to the surface of a measured body. The last and largest part of the thesis deals with cases in which plastic deformations form in the area around the drilled hole during residual stress measurement. After examining of various parameters influencing the formation of plastic deformations, a correction procedure which is capable to correct the uniform residual stresses for various hole diameters and various strain gauge rosettes independently of the stress state or material properties was proposed. The proposed correction procedure was thoroughly tested to ensure its satisfactory results. Based on the obtained results published in this thesis, it is possible to estimate the influence of various conditions deviating from the ideal case on the accuracy of the evaluated residual stresses and in some cases minimize this influence by the proposed procedures.
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Processing of Inconel 939 alloy using laser powder bed fusion at elevated temperatures
Hertl, David ; Dočekalová, Kateřina (referee) ; Malý, Martin (advisor)
The thesis deals with the problem of Inconel 939 alloy processing by SLM technology using base plate preheating. The main objective of the work is to verify the effect of base plate preheating on residual stresses. The problem was solved by two approaches: process simulation, and subsequent experimental verification. The process simulation with preheating temperature of 400 °C achieved the highest agreement with the experiment with a deviation of 4.1 %. Based on the experiment, a suitable base plate preheating temperature of 100 °C was determined to reduce the deformation and residual stresses. By processing Inconel 939 using preheating temperatures of 100 °C and 400 °C, static mechanical properties comparable to those of the standard components processed by SLM technology from Inconel 939 after the heat treatment were achieved. These results offer the potential for significant reductions in overall production time and costs associated with manufacturing Inconel 939.
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