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Low cycle fatigue research and application in nuclear industry
Sehnal, Dominik ; Kuželka, Jiří (referee) ; Hůlka, Jiří (advisor)
Fatique life extension of nuclear powerplants lies in the search for project reserves. This work deals with the evaluation of low-cycle fatigue of nuclear installations of the VVER type and the assessment of the influence of the computational model level. Fatigue tests of austenitic steel using optical method of digital image correlation for which the evaluation procedure is designed and used is performed. Selected model of plasticity with kimenatic (Chaboche) and combinated hardening (Chaboche, Voce) are calibrated from the obtained data. Subsequently, the durability of the test specimen is determined by computational modeling for different material models. From the comparison of the results of fatigue tests with the calculation, the material models suitable for the description of fatigue life and their validity are determined.
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Damage mechanisms in nickel-based superalloy René 41 subjected to fatigue
Babinský, Tomáš ; Pippan, Reinhard (referee) ; Růžička,, Milan (referee) ; Polák, Jaroslav (advisor)
Předkládaná práce se zabývá vznikem poškození ve tvářené niklové superslitině René 41, která je podrobena únavovému zatěžování za pokojové a zvýšené (800 °C) teploty. Slitina byla zkoumána ve dvou termodynamických stavech: po rozpouštěcím žíhání a po stárnutí. Jednoosé zkoušky nízkocyklové únavy byly prováděny symetricky vzhledem k řízené veličině, kterou byla celková podélná deformace. Za účelem studia mechanismů poškození a jejich souvislosti s plastickou deformací byly aplikovány techniky elektronové mikroskopie doplněné o analýzu cyklické odezvy ve formě hysterezních smyček statistickým přístupem. Navíc byly provedeny přerušované únavové zkoušky za účelem studia vývoje povrchového reliéfu ve vztahu k dislokační substruktuře, k čemuž bylo opět využito elektronové mikroskopie. Porušení za pokojové teploty bylo povětšinou transkrystalické, únavové trhliny iniciovaly především uvnitř zrn z povrchového reliéfu. Srovnáme-li dva zkoušené stavy, stárnutý materiál vykazoval vyšší hodnoty cyklického napětí a při srovnatelném napětí dosahoval vyšších životností. Cyklická deformace byla od prvního cyklu lokalizována do persistentních skluzových pásů. Bylo zjištěno, že většina skluzových pásů vzniká na začátku únavového života, kvůli čemuž jsou koherentní precipitáty ‘ významně deformovány pouze na začátku únavového života. Skluz v persistentních skluzových pásech vedl k vytvoření povrchového reliéfu ve formě skluzových schodů, extruzí a intruzí. Vznik povrchového reliéfu byl podrobně diskutován s existujícími modely. Vysokoteplotní expozice měla za následek oxidaci povrchu, zejména na oslabených hranicích zrn, kde iniciovaly a následně se šířily únavové trhliny. Výsledné poškození bylo tudíž interkrystalické.
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Use of advanced kinematic hardening rules for prediction of multiaxial ratcheting
Klepač, Vilém ; Parma, Slavomír ; Feigenbaum, H. P. ; Marek, René ; Plešek, Jiří ; Svárovský, Jiří
The cumulation of plastic deformation due to cyclic loading, so called ratcheting, is considered as critical process for purpose of simulation. For this reason, many of advanced models of plasticity have been developed to capture the evolution of material during the load reversals. Historically, these models are based on concept of multicomponent back-stress, that provides the prediction of phenomenon of kinematic hardening (KH) [Chaboche 1979]. The key for accurate prediction is, besides well designed model, identification of model parameters. To present results on prediction of multiaxial ratcheting by FEA, the multicomponent Armstrong-Frederick with a threshold with r Modification was implemented via the UMAT interface of Abaqus Standart. Model parameters were identified on experimental data presented in [Hassan, Kyriakides 1992] and [Hassan et. al. 1992]. To prove the performance of the model subroutine, other numerical examples will be presented and their computational costs will be discussed.
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Low cycle fatigue research and application in nuclear industry
Sehnal, Dominik ; Kuželka, Jiří (referee) ; Hůlka, Jiří (advisor)
Fatique life extension of nuclear powerplants lies in the search for project reserves. This work deals with the evaluation of low-cycle fatigue of nuclear installations of the VVER type and the assessment of the influence of the computational model level. Fatigue tests of austenitic steel using optical method of digital image correlation for which the evaluation procedure is designed and used is performed. Selected model of plasticity with kimenatic (Chaboche) and combinated hardening (Chaboche, Voce) are calibrated from the obtained data. Subsequently, the durability of the test specimen is determined by computational modeling for different material models. From the comparison of the results of fatigue tests with the calculation, the material models suitable for the description of fatigue life and their validity are determined.
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Fatigue characteristics of Metal Matrix Composites (MMC)
Judas, Jakub ; Věchet, Stanislav (referee) ; Zapletal, Josef (advisor)
This bachelor thesis is focused on analysis of fatigue characteristics of hybrid composite based on magnesium matrix QE22 reinforced with Al2O3 fibres and SiC particles. Test specimens of the composite material prepared by infiltration method were subjected to quasi-static tensile and compression test and discovered data were afterwards compared with the magnesium matrix. Cyclic response and low-cycle fatigue life were studied on the material under stress-control mode. All of the experiments were conducted at room temperature. Cyclic curves were fit with regression analysis in order to measure fatigue parameters of the hybrid composite. Cyclic and fatigue parameters were discussed and compared to similar magnesium alloy AZ91 reinforced with SiC particles.
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Shape optimization of new circumferential steam turbine blade attachment type
Mívalt, Tomáš ; Petruška, Jindřich (referee) ; Polzer, Stanislav (advisor)
This thesis describes selection and shows calibration of material model, capable of describing cyclic softening of material. Stress-strain FEM analysis of circumferential blade attachment for last section of rotor blades of steam turbine is performed, expected lifetime of existing attachment is evaluated. Multi-parameter optimization of new-shape attachment was done, resulting in dimensions for new-shape attachment with longer lifetime. Improvements in strain amount in comparison with existing attachment were evaluated and possible RPM increase of turbine with new attachment type was calculated.
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Fatigue Behaviour of AZ31 and AZ61 Magnesium Alloys
Gejdoš, Pavel ; Lukáč, Pavel (referee) ; Kohout, Jan (referee) ; Podrábský, Tomáš (advisor)
This doctoral thesis deals with the fatigue behaviour of AZ31 and AZ61 magnesium alloy casted by advanced methods of casting squeeze casting. Based on the regression functions and measured fatigue data were determined fatigue characteristics of these alloys. The work also extends into the area of fatigue cracks in the AZ31 alloy. Were measured fatigue crack propagation rate on modified cylindrical specimens. The paper also outlines the possibilities for describing the fatigue behaviour of AZ61 alloy in extremely low-cycle fatigue.
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Low Cycle and High Cycle Fatigue Properties of Austempered Ductile Iron
Zapletal, Josef ; Věchet, Stanislav (advisor)
The thesis is focused on assessment of fatigue behaviour of austempered ductile iron with nodular graphite. Optimal period of transformation was determined based on the best combination of stress and strain characteristics established by tensile test. Cyclic response and low-cycle fatigue life were studied under both stress-control and longitudinal strain-control mode at room temperature. For both modes, shapes of cyclic hardening curves are dependent on stress amplitude. Cyclic deformation curves (CDC) were fitted by power regression function. Results were compared with CDC established by multiple step test in both modes with verification of the influence of cyclic creep (high stress levels, stress-control mode). Experimental data of S-N curves are in agreement with the Manson-Coffin and the Basquin law. Fatigue and cyclic parameters were compared. Fatigue life time in high-cycle fatigue region was determined. Experimental data were fitted by suitable regression functions. Regression parameters and fatigue limit were established by means of each regression function. Experimental data in low- and high-cycle fatigue regions were used to construct S-N curve and to determine relevant parameters. Discontinuity of experimental data was not observed. Low-cycle fatigue behaviour was predicted. Approximation of tolerance bands was realized in high-cycle and both high and low cycle fatigue regions.
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