Národní úložiště šedé literatury Nalezeno 6 záznamů.  Hledání trvalo 0.00 vteřin. 
Crack initiation in austenitic stainless steel sanicro 25 subjected to thermomechanical fatigue
Petráš, Roman ; Škorík, Viktor ; Polák, Jaroslav
Thermomechanical fatigue experiments were performed with austenitic stainless Sanicro 25 steel. Several amplitudes of mechanical strain in a wide temperature interval (250-700 °C) were applied to the specimens. Mechanical response was recorded and fatigue lives were obtained. Scanning electron microscopy combined with FIB technique was used to study the mechanism of crack initiation in in-phase and in out-of-phase thermomechanical cycling. Different mechanisms of the crack initiation were found in these two types of loading. During in-phase loading fatigue cracks start in grain boundaries by cracking of the oxide. Cracks grew preferentially along grain boundaries which resulted in rapid crack initiation and low fatigue life. In out-of-phase loading multiple cracks perpendicular to the stress axis developed only after sufficiently thick oxide layer was formed and cracked in low temperature loading half-cycle. The cracks in oxide allowed localized repeated oxidation and finally also cracking. The cracks grow transgranularly and result in longer fatigue life.
GRAIN REFINEMENT EFFECT ON FATIGUE PROPERTIES OF AUSTENITIC STAINLESS STEEL WITH DEFORMATION INDUCED MARTENSITE FORMATION
Chlupová, Alice ; Man, Jiří ; Polák, Jaroslav ; Škorík, Viktor ; Karjalainen, J.
Fatigue properties of metastable austenitic 301LN steel with coarse and ultrafine-grained microstructures were investigated at ambient temperature and the effect of grain refinement was evaluated. Two different microstructural states of material were introduced by different annealing conditions during special thermo-mechanical treatment. Fatigue life curves were obtained and compared for both coarse-grained and ultra-fine-grained states of austenitic stainless steel. Magnetic measurements before, during and after cyclic loading were performed to reveal structural changes, i.e. formation of deformation-induced martensite. Relationship between microstructure refinement, phase composition and fatigue properties was discussed.
Termomechanická únava superslitin na bázi niklu
Pech, Filip ; Škorík, Viktor (oponent) ; Šulák, Ivo (vedoucí práce)
Hlavním cílem této bakalářské práce je zpracovat literární rešerši na téma termomechanické únavy superslitin na bázi niklu. Ty byly vyvinuty pro práci za vysokých teplot, kde musí být zajištěna dostatečně velká pevnost, odolnost proti oxidaci, creepu a dalším nepříznivým vlivům uplatňujících se ve vysokoteplotních aplikacích. Tyto slitiny jsou i přes snahu o nalezení alternativ stále intenzivně používané a proto je nutná dobrá znalost jejich vlastností a limit. Na základě odborných publikací je v předkládané práci zpracován základní přehled niklových superslitin. Jejich dělení, chemické složení spolu s přehledem nejdůležitějších fázi ovlivňujících vlastnosti superslitin. Jejich struktura a popis jednotlivých fází. V další kapitole jsou popsány základy izotermické únavy materiálů, její definice, základní popis mechanismu a přehled nejdůležitějších křivek únavového života. Předposlední kapitola literární rešerše pojednává o termomechanické únavě, přičemž struktura textu je obdobná jako u izotermické únavy. V poslední kapitole je přehled termomechanické únavy jednotlivých druhů superslitin na bázi niklu.
Thermo-mechanical and isothermal fatigue behavior of austenitic stainless steel AISI 316L
Škorík, Viktor ; Šulák, Ivo ; Obrtlík, Karel ; Polák, Jaroslav
Many structural components of nuclear power plant systems are made of austenitic stainless steels. These structures undergo degradation by thermo-mechanical fatigue (TMF) caused by simultaneous cyclic straining and temperature cycling, particularly during start-up, shut-down and transient operations. The present work reports the cyclic deformation behavior and fatigue damage of austenitic stainless steel AISI 316L during TMF and isothermal fatigue (IF) testing in air. Total strain controlled in-phase TMF loading in the temperature range 200 - 600 °C and isothermal fatigue (IF) at 600 °C were performed. Hardening/softening curves, cyclic stress-strain response and fatigue life diagrams were obtained both for TMF and IF tests. Fatigue damage was documented using surface relief and fracture surface observations. Mean stress evolution and fatigue degradation data are employed to discuss the fatigue behavior of 316L steel both in TMF and IF regimes.
EFFECT OF HIP ON LOW CYCLE FATIGUE OF MAR-M247 AT 900°C
Šulák, Ivo ; Obrtlík, Karel ; Škorík, Viktor ; Hrbáček, K.
Polycrystalline nickel-base superalloy MAR-M247 is used for high temperature applications requiring excellent combination of fatigue properties, creep resistance and surface stability. These superior high temperature characteristics derive from the microstructure which habitually consists of face centred cubic matrix γ and precipitate γ´ (L12 type ordered structure). In the present work, the high temperature low cycle fatigue behaviour of cast nickel-base superalloy MAR-M247 in as received condition and in hot isostatically pressed (HIP) condition was studied. The microstructure of the materials is characterized by dendritic grains, carbides and casting defects. Distribution and size of defects in both materials were studied. Isothermal low cycle fatigue (LCF) tests were performed on cylindrical specimens under total strain control at 900°C in air. Cyclic stress–strain response and fatigue life of both materials were assessed. Beneficial effects of HIP process on cyclic stress-strain and fatigue life curves are discussed.
Grain refinement effect on fatigue properties of austenitic stainless steel with deformation induced martensite formation
Chlupová, Alice ; Man, Jiří ; Polák, Jaroslav ; Škorík, Viktor ; Karjalalinen, L.P.
Fatigue properties of metastable austenitic 301LN steel with coarse and ultrafine-grained microstructures were investigated at ambient temperature and the effect of grain refinement was evaluated. Two different microstructural states of material were introduced by different annealing conditions during special thermomechanical treatment. Fatigue life curves were obtained and compared for both coarse-grained and ultrafine- grained states of austenitic stainless steel. Magnetic measurements before, during and after cyclic loading were performed to reveal structural changes, i.e. formation of deformation-induced martensite. Relationship between microstructure refinement, phase composition and fatigue properties was discussed.

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