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Nízkocyklová únava pseudoelastické slitiny NiTi
Kaňová, Monika ; Hutař, Pavel (oponent) ; Pantělejev, Libor (vedoucí práce)
Tato práce se zabývá studiem mechanických vlastností slitiny NiTi, která vykazuje jev pseudoelasticity a paměťového efektu. V části zabývající se únavou je zkoumána oblast funkční a strukturní únavy. Cílem této práce bylo provést a vyhodnotit sérii únavových testů. Materiál k experimentům byl dodán ve formě drátu, který byl upnut do speciálních čelistí. V první části experimentu jsou vyhodnoceny tahové zkoušky, kterými lze dokázat reprodukovatelnost měření. Dále byla provedena série únavových zkoušek. Výsledky těchto testů byly vyhodnoceny společně s výsledky již dříve provedených zkoušek. Jedna část diskuse byla zaměřena na porovnání změn tvaru hysterezních smyček při cyklickém zatěžování v závislosti na rychlosti zatěžování. Ve druhé části byly vyneseny křivky únavové životnosti. Bylo zjištěno, že tyto křivky mají nestandardní tvar, který je v práci vysvětlen.
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Experimental data processing for calibration of isotropic hardening model
Chlaň, Jakub ; Kubík, Petr (oponent) ; Šebek, František (vedoucí práce)
The main aim of the present bachelor's thesis is to create a universally applicable script, which is used for the data processing of the fatigue test in order to calibrate the isotropic hardening model. The theoretical part is devoted to the phenomenon of cyclic hardening/softening, the mathematical models of hardening and last but not least, the practice of fatigue tests together with the possibilities of their evaluation. The rest of the work includes the algorithm creation procedure and demonstration of its functionality by processing data from fatigue tests of alloy AlCu4Mg1.
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Fatigue resistance and mechanisms of the fatigue damage in materials for high temperatures
Petráš, Roman ; Kohout,, Jan (oponent) ; Weidner, Anja (oponent) ; Polák, Jaroslav (vedoucí práce)
Superaustenitic stainless 22Cr25NiWCoCu steel designed for high temperature applications in power generation industry was investigated in terms of low cycle fatigue at room and elevated temperature. Individual specimens were subjected to different loading procedures in order to study the material response along with the mechanism of fatigue damage. Cyclic hardening/softening curves, Coffin-Manson and cyclic stress-strain curves were evaluated. Life-time behavior for various types of loading procedures was discussed with respect to the effective damage mechanisms developed under specific loading conditions. Standard isothermal low cycle fatigue tests at room and elevated temperature were conducted. Hysteresis loops recorded during cycling were analyzed by means of generalized statistical theory of hysteresis loop. The probability density distribution function of the internal critical stresses and its evolution during cycling straining was derived for different strain amplitudes. Evolution of the surface relief along with the internal dislocation arrangement for both temperatures was assessed in relation to the development of the probability density function of internal critical stresses. Surface relief evolution using SEM equipped with FIB revealed the early fatigue crack formation. Cyclic loading at room temperature resulted in the localization of the cyclic plastic strain into persistent slip bands and formation of surface persistent slip marking consisting of extrusions and intrusions. Deepening of an intrusion leads to the initiation of the fatigue crack from the tip of the deepest intrusion. Distinctive mechanism of the early crack formation was found in test at elevated temperature where the effect of environment is crucial. Rapid oxidation of the grain boundaries and subsequent cracking of the oxidized grain boundaries represent effective mechanisms of the nucleation of I-stage fatigue crack. Additional 10 minute tensile dwells implemented into the loading cycle led to the internal damage evolution. To reveal internal damage the longitudinal cross-sections parallel to the stress axis of the tested specimen were produced. The crack paths and their relation to the grain and twin boundaries were studied using electron back-scattered diffraction (EBSD) technique. The influence of dwells introduced in the loading cycle on fatigue life is assessed in relation to the evolution of the surface relief and internal damage. Specimens were also exposed to more complex loading procedures where the load along with the temperature varies with time simultaneously. In-phase and out-of-phase type thermomechanical fatigue (TMF) tests with or without dwells were conducted. Rapid cyclic hardening was observed in all tests regardless of the strain amplitude applied while tendency to saturation was found primarily in out-of-phase loading with/without dwells. Investigation of the surface relief by means of SEM along with FIB cutting revealed the preferential oxidization and cracking of the grain boundaries perpendicular to the loading axis. Dwells implemented in maximum tension resulted in the enlargement of the plastic strain amplitude and to the additional creep damage in the form of internal cracks. Intergranular crack propagation was observed for in-phase cycling with/without dwells. Damage evolution in out-of-phase cycling was found to be principally similar for straining with and without dwell.
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Zpracování napěťově deformační odezvy v průběhu cyklického zatěžování
Lehnert, Ondřej ; Slámečka, Karel (oponent) ; Horníková, Jana (vedoucí práce)
Tato bakalářská práce je zaměřena na zpracování dat ze zkušebního stroje a na stanovení cyklické křivky napětí-deformace s pomocí vrcholů saturovaných hysterezních smyček. Před samotnou analýzou dat je v této práci rozepsána rešeršní studie zabývající se obecně únavou materiálu, posléze zaměřena na změnu mechanických vlastností, popis hysterezních smyček a cyklické křivky. Poté následuje postup analýzy dat ze zkušebního stroje, jejímž cílem je porovnání cyklické křivky s tahovou křivkou a její matematický popis.
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Dynamic tensile experiments of hard rubber on instron test machine under finite deformations
Šulc, Petr ; Pešek, Luděk ; Bula, Vítězslav ; Šnábl, Pavel
This paper deals with the first experimental tests of hard rubbers under tensile harmonic loading on the axial-torsional servohydraulic Instron 8852 test machine. The objective of these experiments on cylindrical rubber specimens on this machine is to obtain stress-strain curves of rubber specimens, first considering uniaxial finite longitudinal strains for a single load cycle. Next, the hysteresis loops and their skeleton curves are compared for two selected strain measures as a function of their conjugate stress. It is shown how to obtain the dependence between elongation in the axial direction and constriction in the radial direction, which is used to determine the Cauchy stress in this configuration.
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Návrh výkonového zesilovače proudu pro induktivní zátěž
Nimrichter, Filip ; Steinbauer, Miloslav (oponent) ; Roubal, Zdeněk (vedoucí práce)
Tato bakalářská práce se zabývá Howlandovým zdrojem proudu, který by měl pracovat s čistě induktivní zátěží do frekvence 10 kHz. Tento zdroj proudu bude budit cívku s nelineárním magnetickým jádrem, takže by měl dodat minimálně 4 A. Práce se soustředí na stabilizaci, praktický návrh a měření reálných vlastností Howlandova zdroje proudu. Stabilita byla ověřena v programu PSpice a Micro-Cap. Praktický návrh byl sestaven v programu EAGLE.
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Vliv hydrologických extrémů na kvalitu povrchových vod v pramenných oblastech. Aplikace v modelovém povodí horní Rolavy.
Šajnarová, Lucie ; Matoušková, Milada (vedoucí práce) ; Havlíková, Petra (oponent)
Srážko-odtokové extrémy v pramenných oblastech toků mají výrazný vliv na změny v geochemismu povrchových vod, které jsou navíc ovlivňovány rašeliništi v povodí. Mezi hlavní cíle této práce patří rešerše odborné literatury na téma změn geochemismu povrchových vod v pramenných oblastech s rašeliništi v povodí v reakci na srážko-odtokové extrémy. Dále byla provedena analýza odtokových a srážkových poměrů v hydrologických letech 2018, 2019 a 2020 a analýza změn parametrů kvality vody v závislosti na srážko-odtokových extrémech pro období od roku 2013 do roku 2020. Vztah konduktivity a průtoku byl sledován pomocí hysterézních smyček. V práci byla využita data z experimentálního profilu KFGG PřF UK na Slatinném potoce a data o průtocích pro profil Chaloupky na Rolavě od ČHMÚ. Výsledky ukázaly, že vztah konduktivity a průtoku lze analyzovat pomocí hysterézních smyček zejména v obdobích při rychlém zvýšení průtoků a jejich tvar se mění v závislosti na prekondici povodí. Závislost na průtoku byla pozorována také u jiných parametrů, například u železa. Klíčová slova: hydrologické sucho, povodeň, kvalita povrchové vody, rašeliniště, elektrická vodivost, hysterézní smyčka
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Fatigue resistance and mechanisms of the fatigue damage in materials for high temperatures
Petráš, Roman ; Kohout,, Jan (oponent) ; Weidner, Anja (oponent) ; Polák, Jaroslav (vedoucí práce)
Superaustenitic stainless 22Cr25NiWCoCu steel designed for high temperature applications in power generation industry was investigated in terms of low cycle fatigue at room and elevated temperature. Individual specimens were subjected to different loading procedures in order to study the material response along with the mechanism of fatigue damage. Cyclic hardening/softening curves, Coffin-Manson and cyclic stress-strain curves were evaluated. Life-time behavior for various types of loading procedures was discussed with respect to the effective damage mechanisms developed under specific loading conditions. Standard isothermal low cycle fatigue tests at room and elevated temperature were conducted. Hysteresis loops recorded during cycling were analyzed by means of generalized statistical theory of hysteresis loop. The probability density distribution function of the internal critical stresses and its evolution during cycling straining was derived for different strain amplitudes. Evolution of the surface relief along with the internal dislocation arrangement for both temperatures was assessed in relation to the development of the probability density function of internal critical stresses. Surface relief evolution using SEM equipped with FIB revealed the early fatigue crack formation. Cyclic loading at room temperature resulted in the localization of the cyclic plastic strain into persistent slip bands and formation of surface persistent slip marking consisting of extrusions and intrusions. Deepening of an intrusion leads to the initiation of the fatigue crack from the tip of the deepest intrusion. Distinctive mechanism of the early crack formation was found in test at elevated temperature where the effect of environment is crucial. Rapid oxidation of the grain boundaries and subsequent cracking of the oxidized grain boundaries represent effective mechanisms of the nucleation of I-stage fatigue crack. Additional 10 minute tensile dwells implemented into the loading cycle led to the internal damage evolution. To reveal internal damage the longitudinal cross-sections parallel to the stress axis of the tested specimen were produced. The crack paths and their relation to the grain and twin boundaries were studied using electron back-scattered diffraction (EBSD) technique. The influence of dwells introduced in the loading cycle on fatigue life is assessed in relation to the evolution of the surface relief and internal damage. Specimens were also exposed to more complex loading procedures where the load along with the temperature varies with time simultaneously. In-phase and out-of-phase type thermomechanical fatigue (TMF) tests with or without dwells were conducted. Rapid cyclic hardening was observed in all tests regardless of the strain amplitude applied while tendency to saturation was found primarily in out-of-phase loading with/without dwells. Investigation of the surface relief by means of SEM along with FIB cutting revealed the preferential oxidization and cracking of the grain boundaries perpendicular to the loading axis. Dwells implemented in maximum tension resulted in the enlargement of the plastic strain amplitude and to the additional creep damage in the form of internal cracks. Intergranular crack propagation was observed for in-phase cycling with/without dwells. Damage evolution in out-of-phase cycling was found to be principally similar for straining with and without dwell.
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Testing of HPD model of dissipated energy of hard rubbers at torsional finite Deformations and temperatures
Šulc, Petr ; Pešek, Luděk ; Bula, Vítězslav ; Šnábl, Pavel
This paper deals with the temperature dependence of hard rubber behavior on torsional harmonic loading. This behavior was described by dissipated and deformation energy of the rubber. This knowledge was used to test the validity of the hyperelastic proportional damping (HPD) in dependence on temperature and finite deformations. From the experiments, the dissipated energy was obtained from the hysteresis loop in one period of harmonic load, depending on the temperature for the given frequency. The deformation energy was determined from the shear stress analysis on the surface of the cylinder considering hyperelasticity. The constants of these models were obtained by optimizing the stress-strain curve from the shear analysis on the cylinder surface. With this optimization, the Yeoh hyperelastic model was chosen.
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