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Embrittlement of EUROFER´97 steel due to thermal exposition
Hadraba, Hynek ; Dlouhý, Ivo
Ferritic-martensitic reduced activation steel Eurofer´97 is leading candidate for structural material for in-vessel and first wall components of proposed fusion reactor DEMO. The utilization of the Eurofer´97 steels is limited up to a temperature of about 550°C. The aim of the work is to investigate the influence of long term thermal ageing on fracture properties of Eurofer´97 steel. Short term thermal ageing of the steel was simulated by step cooling treatment. Long term thermal ageing was simulated by isothermal ageing at 550 °C/5000 h. No evident changes in Charpy impact energy have been registered when comparing the properties of the steel in as-received and in step-cooled state. Outstanding embrittlement of the steel was observed after long term isothermal ageing: transition temperatures shift by about 5 °C. Risk of grain boundary weakening by phosphorus segregation is very low while the carbide coarsening could led to outstanding embrittlement of the Eurofer´97 steel.
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Evaluation of KQ toughness and Kefc fracture toughness of hypoeutectoid steel R7T
Holzmann, Miloslav ; Jurášek, L.
Interpretation of the characteristic KQ using linear-elastic fracture mechanics coupled with Irwin´s concept of the effective crack length. The effective yield stress Ref as a basic parameter controlling the level of KQ and its scatter. The use of Irwin´s concept of effective crack length for determining fracture toughness Kefc. Validity of Irwin´s concept up to certain ratio of Fmax/FGY. The scatter of the fracture toughness Kefc, i. e. probability distribution of fracture toughness Kefc diagram. This diagram can be used in the case of application of probabilistic fracture mechanics for determining cumulative fracture probability of the given crack.
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Transgranular and intergranular failure competition as a manifestation of steel degradation
Dlouhý, Ivo ; Hadraba, Hynek ; Strnadel, B.
Main purpose of the work was to quantify causes and characteristics that control the intergranular fracture initiation and occurrence of this fracture micromechanism in competition with cleavage one. A CrNi steel of commercial quality and the same steel with increased content of impurity elements, Sn and Sb, was used. Step cooling treatment was applied in order to induce intergranular embrittlement and brittle fracture initiation in both steel melts. For fracture toughness determination standard bend specimen and pre-cracked Charpy type specimens both tested in three point bending were applied. Relation of cleavage fracture stress and critical stress for intergranular failure has been followed showing capability of this parameter for quantification of the transgranular/intergranular fracture. In order to characterise the quantitative morphological differences in fracture surfaces fractal analysis was applied.
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Zacelování povrchových trhlin u nanokompozitní keramiky typu Al2O3/SiC
Flašar, Petr ; Dlouhý, Ivo ; Chlup, Zdeněk ; Ando, K.
Indentation strength and self-crack-healing ability were systematically studied using beams containing a semi-elliptical surface crack made by Vickers indentor. Beams of Al2O3/SiC ceramic nanocomposite with a good self-crack-healing ability predetermined for four-point and three-point bending test were prepared by ceramographics techniques. Flexural strength and fracture toughness were determined using three- and four-point bend method. Thermal aging at 1573 K for 1h was applied to heal the samples with indentation pre-cracking. Fractographical analyses using an optical and/or scanning electron microscopy has been carried out to investigate the surface crack behaviour. Very good self-crack healing ability has been proved for surface microcracks having length up to 250 μm and depth no higher than 100 μm.
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