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Determination of Fracture Mechanical Characteristics From Sub-Size Specimens
Stratil, Luděk ; Džugan, Jan (referee) ; Haušild, Petr (referee) ; Dlouhý, Ivo (advisor)
The standards of fracture toughness determination prescribe size requirements for size of test specimens. In cases of limited amount of test material miniature test specimens offer one from the possibilities of fracture toughness evaluation. Because of small loaded volumes in these specimens at the crack tip the loss of constraint occur affecting measured values of fracture toughness. In such cases the size requirements for valid fracture toughness characteristics determination are not fulfilled. These specimens can be even on limits of load range of test devices and handle manipulation by their small dimensions. The important task related to these specimens is, apart from methodology of their preparation and measurement of deformations, the interpretation of measured values of fracture toughness and their possible correction to standard test specimens. Moreover, in the upper shelf region of fracture toughness quantification and interpretation of size effects is still not resolved sufficiently. This thesis is by its aims experimentally computational study focused on evaluation of size effect on fracture toughness in the upper shelf region. The size effect was quantified by testing of miniature and large specimens’ sizes in order to determine J R curves. Two geometries of miniature test specimens, there point bend specimen and CT specimen, were used. The experimental materials were advanced steels developed for applications in nuclear and power industry, Eurofer97 steel and ODS steel MA956. Finite elements analyses of realized tests together with application of micromechanical model of ductile fracture were carried out in order to evaluate stress strain fields at the crack tip in tested specimens from Eurofer97 steel. By comparison of experimental results and numerical simulations of J R curves the mutual dependencies between geometry of specimens and element sizes at the crack tip were derived. On the basis of acquired relationships, the methodology of J R curve prediction for standard specimen size from limited amount of test material was proposed. Main contribution of thesis is description of effect of material’s fracture toughness level on resistance against ductile crack propagation in miniature specimens. For material where significant crack growth occurs after exceeding the limit values of J integral (Eurofer97), the loss of constraint is considerable and highly decreases resistance against tearing. Miniature specimens then show significantly lower J R curves in comparison with standard size specimens. This effect is the opposite to the behaviour of miniature specimens in transition region. In case of material with low toughness, in which significant crack growth occurs in the region of J integral validity (ODS MA956), the effect of constraint loss is small without large impact on resistance against tearing. In such case miniature specimens demonstrate comparable J R curves as specimens of larger sizes. Next important contribution is proposed methodology for prediction of J R curve from small amount of test material using micromechanical modeling.
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Advanced Composite Structural Steels for Applications in Heavy Liquid Metals
Husák, Roman ; Haušild, Petr (referee) ; Vilémová,, Monika (referee) ; Hadraba, Hynek (advisor)
Doctoral thesis was focused on preparing of new advanced ODS steels for use in heavy metal liquids enviroments. Possibility of new course for creating oxide dispersion in microstructure was verified by the course of internal oxidation of elements. By the internal oxidation method were prepared new ODS steels strengthened by complex oxides which were created by elements of IIIB and IVB group of elements. Based on analysis of damage ODS steels in LBE were designed surface protection of ODS steel by the oxide layer. The ODS steel protected by oxide layer was subjected to a corrosion test in LBE. For the experiments were chosen class of chromium steels: ferritic-martensitic steel 9Cr1WMnVTa and ferritic steels 14CrWTi and 17Cr1Mo. Steels without oxide dispersion and steel strenghtened by the dispersion based on Y-Ti-O oxides were prepared. On the steels were made series of mechanic tests which should reveal the effectivity of oxide dispersion on the strenght of steel prepared by the internal oxidation method and by the direct addition of oxide elements. It was found that significantly harder oxide material couldn't be fully disrupted through the mechanical alloying and fine oxide dispersion couldn't be created. There was verified fine oxide dispersion could be created by the internal oxidation method and by the direct adding of oxide elements. Same kind of steels strenghtened by new kind of complex oxides based on Y, Ce, Hf, La, Sc and Zr were prepared. The chemical analyses have proven that all added elements could created complex oxide by the reaction with yttrium. The computational analyses for observing of matrix influence and oxide phase influence on strenghtening of steels were performed. These computational analyses were based on microstructural analyses of ODS steels. There was found that the oxide particles could very effectively improve strenght of steels at room temperature and especialy at high temperature. Based on corrosion tests of 14Cr ODS steel in liquid Pb and LBE enviroment were designed surface protection of ODS steel. The effectiveness of protective layer was verified by the high temperature corrosion test of PM2000 in LBE. No damage of oxide layer was observed although Pb and Bi diffused through protective layer.
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Advanced aluminium alloys prepared by powder metallurgy and spark plasma sintering
Molnárová, Orsolya ; Málek, Přemysl (advisor) ; Haušild, Petr (referee) ; Vojtěch, Dalibor (referee)
Mechanical properties of aluminium alloys highly depend on their phase composition and microstructure. High strength can be achieved among others by introduction of a high volume fraction of fine, homogeneously distributed second phase particles and by a refinement of the grain size. Powder metallurgy allows to prepare fine grained materials with increased solid solubility which are favourable precursors for further precipitation strengthening. Gas atomization was used for the preparation of powders of the commercial Al7075 alloy and its modification containing 1 wt% Zr. A part of gas atomized powders was mechanically milled at different conditions. Mechanical milling reduced the grain size down to the nano-size range and the corresponding microhardness exceeded the value of 300 HV. Powders were consolidated by the spark plasma sintering method to nearly fully dense compacts. Due to a short time and relatively low temperature of sintering the favourable microstructure can be preserved in the bulk material. The grain size of compacts prepared from milled powder was retained in the submicrocrystalline range and the microhardness close to 200 HV exceeded that of the specially heat treated ingot metallurgical counterparts. The prepared compacts retained their fine grained structure and high...
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Study of cooperative dislocation phenomena in solids by the acoustic emission technique
Knapek, Michal ; Chmelík, František (advisor) ; Haušild, Petr (referee) ; Landa, Michal (referee)
Title: Study of cooperative dislocation phenomena in solids by the acoustic emis- sion technique Author: Michal KNAPEK Department: Department of Physics of Materials Supervisor: doc. RNDr. František Chmelík, CSc., Department of Physics of Materials Abstract: Plastic deformation of micron-scale crystalline materials differs con- siderably from bulk specimens, as it is characterized by random strain bursts. Three categories of metallic samples were investigated in this thesis: micron- scale copper micropillars with varied geometries, submillimeter-scale aluminum microwires, and aluminum and aluminum-magnesium salt-replicated foams. Very precise fabrication methods and sensitive measurement set-ups consisting of uni- axial compression and tensile tests with concurrent acoustic emission (AE) record- ing were developed. These fine methods allowed for investigations of effects re- lated to plastic deformation at micrometer scales, i.e. the dislocation dynamics associated with the stress drops. Size effects in plastic deformation, as well as clear correlations between the stress drops and the AE events, were found in mi- crosamples, confirming that dislocation avalanches are indeed responsible for the stochastic character of deformation processes also at microscales. Open-cell pure aluminum and aluminum-magnesium...
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Advanced Composite Structural Steels for Applications in Heavy Liquid Metals
Husák, Roman ; Haušild, Petr (referee) ; Vilémová,, Monika (referee) ; Hadraba, Hynek (advisor)
Doctoral thesis was focused on preparing of new advanced ODS steels for use in heavy metal liquids enviroments. Possibility of new course for creating oxide dispersion in microstructure was verified by the course of internal oxidation of elements. By the internal oxidation method were prepared new ODS steels strengthened by complex oxides which were created by elements of IIIB and IVB group of elements. Based on analysis of damage ODS steels in LBE were designed surface protection of ODS steel by the oxide layer. The ODS steel protected by oxide layer was subjected to a corrosion test in LBE. For the experiments were chosen class of chromium steels: ferritic-martensitic steel 9Cr1WMnVTa and ferritic steels 14CrWTi and 17Cr1Mo. Steels without oxide dispersion and steel strenghtened by the dispersion based on Y-Ti-O oxides were prepared. On the steels were made series of mechanic tests which should reveal the effectivity of oxide dispersion on the strenght of steel prepared by the internal oxidation method and by the direct addition of oxide elements. It was found that significantly harder oxide material couldn't be fully disrupted through the mechanical alloying and fine oxide dispersion couldn't be created. There was verified fine oxide dispersion could be created by the internal oxidation method and by the direct adding of oxide elements. Same kind of steels strenghtened by new kind of complex oxides based on Y, Ce, Hf, La, Sc and Zr were prepared. The chemical analyses have proven that all added elements could created complex oxide by the reaction with yttrium. The computational analyses for observing of matrix influence and oxide phase influence on strenghtening of steels were performed. These computational analyses were based on microstructural analyses of ODS steels. There was found that the oxide particles could very effectively improve strenght of steels at room temperature and especialy at high temperature. Based on corrosion tests of 14Cr ODS steel in liquid Pb and LBE enviroment were designed surface protection of ODS steel. The effectiveness of protective layer was verified by the high temperature corrosion test of PM2000 in LBE. No damage of oxide layer was observed although Pb and Bi diffused through protective layer.
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Advanced aluminium alloys prepared by powder metallurgy and spark plasma sintering
Molnárová, Orsolya ; Málek, Přemysl (advisor) ; Haušild, Petr (referee) ; Vojtěch, Dalibor (referee)
Mechanical properties of aluminium alloys highly depend on their phase composition and microstructure. High strength can be achieved among others by introduction of a high volume fraction of fine, homogeneously distributed second phase particles and by a refinement of the grain size. Powder metallurgy allows to prepare fine grained materials with increased solid solubility which are favourable precursors for further precipitation strengthening. Gas atomization was used for the preparation of powders of the commercial Al7075 alloy and its modification containing 1 wt% Zr. A part of gas atomized powders was mechanically milled at different conditions. Mechanical milling reduced the grain size down to the nano-size range and the corresponding microhardness exceeded the value of 300 HV. Powders were consolidated by the spark plasma sintering method to nearly fully dense compacts. Due to a short time and relatively low temperature of sintering the favourable microstructure can be preserved in the bulk material. The grain size of compacts prepared from milled powder was retained in the submicrocrystalline range and the microhardness close to 200 HV exceeded that of the specially heat treated ingot metallurgical counterparts. The prepared compacts retained their fine grained structure and high...
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Study of cooperative dislocation phenomena in solids by the acoustic emission technique
Knapek, Michal ; Chmelík, František (advisor) ; Haušild, Petr (referee) ; Landa, Michal (referee)
Title: Study of cooperative dislocation phenomena in solids by the acoustic emis- sion technique Author: Michal KNAPEK Department: Department of Physics of Materials Supervisor: doc. RNDr. František Chmelík, CSc., Department of Physics of Materials Abstract: Plastic deformation of micron-scale crystalline materials differs con- siderably from bulk specimens, as it is characterized by random strain bursts. Three categories of metallic samples were investigated in this thesis: micron- scale copper micropillars with varied geometries, submillimeter-scale aluminum microwires, and aluminum and aluminum-magnesium salt-replicated foams. Very precise fabrication methods and sensitive measurement set-ups consisting of uni- axial compression and tensile tests with concurrent acoustic emission (AE) record- ing were developed. These fine methods allowed for investigations of effects re- lated to plastic deformation at micrometer scales, i.e. the dislocation dynamics associated with the stress drops. Size effects in plastic deformation, as well as clear correlations between the stress drops and the AE events, were found in mi- crosamples, confirming that dislocation avalanches are indeed responsible for the stochastic character of deformation processes also at microscales. Open-cell pure aluminum and aluminum-magnesium...
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Determination of Fracture Mechanical Characteristics From Sub-Size Specimens
Stratil, Luděk ; Džugan, Jan (referee) ; Haušild, Petr (referee) ; Dlouhý, Ivo (advisor)
The standards of fracture toughness determination prescribe size requirements for size of test specimens. In cases of limited amount of test material miniature test specimens offer one from the possibilities of fracture toughness evaluation. Because of small loaded volumes in these specimens at the crack tip the loss of constraint occur affecting measured values of fracture toughness. In such cases the size requirements for valid fracture toughness characteristics determination are not fulfilled. These specimens can be even on limits of load range of test devices and handle manipulation by their small dimensions. The important task related to these specimens is, apart from methodology of their preparation and measurement of deformations, the interpretation of measured values of fracture toughness and their possible correction to standard test specimens. Moreover, in the upper shelf region of fracture toughness quantification and interpretation of size effects is still not resolved sufficiently. This thesis is by its aims experimentally computational study focused on evaluation of size effect on fracture toughness in the upper shelf region. The size effect was quantified by testing of miniature and large specimens’ sizes in order to determine J R curves. Two geometries of miniature test specimens, there point bend specimen and CT specimen, were used. The experimental materials were advanced steels developed for applications in nuclear and power industry, Eurofer97 steel and ODS steel MA956. Finite elements analyses of realized tests together with application of micromechanical model of ductile fracture were carried out in order to evaluate stress strain fields at the crack tip in tested specimens from Eurofer97 steel. By comparison of experimental results and numerical simulations of J R curves the mutual dependencies between geometry of specimens and element sizes at the crack tip were derived. On the basis of acquired relationships, the methodology of J R curve prediction for standard specimen size from limited amount of test material was proposed. Main contribution of thesis is description of effect of material’s fracture toughness level on resistance against ductile crack propagation in miniature specimens. For material where significant crack growth occurs after exceeding the limit values of J integral (Eurofer97), the loss of constraint is considerable and highly decreases resistance against tearing. Miniature specimens then show significantly lower J R curves in comparison with standard size specimens. This effect is the opposite to the behaviour of miniature specimens in transition region. In case of material with low toughness, in which significant crack growth occurs in the region of J integral validity (ODS MA956), the effect of constraint loss is small without large impact on resistance against tearing. In such case miniature specimens demonstrate comparable J R curves as specimens of larger sizes. Next important contribution is proposed methodology for prediction of J R curve from small amount of test material using micromechanical modeling.
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