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Vysokocyklová únava IN 713LC
Kunz, Ludvík ; Lukáš, Petr ; Mintách, Rastislav ; Konečná, R.
High-cycle fatigue strength of Ni-base superalloy IN 713LC was experimentally determined for symmetrical loading and for loading with tensile mean stresses at a temperature of 800 °C in air. An analysis of the influence of microstructure on fatigue crack initiation and related fatigue strength was performed. Statistical method of the largest extreme value has been applied to characterize the casting defect distribution.
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Vývoj poškození u protlačovacích zkoušek na miniaturních discích při konstantní rychlosti deformace
Dymáček, Petr ; Milička, Karel ; Hrubý, V.
In this contribution a set of three steels of various ductility has been selected and tested using small punch technique. The set of specimens was loaded in incremental way, in order to determine at which load level the crack initiates. The location of crack initiation was evaluated and compared with the equivalent plastic strain distributions in the FEM model of the specimen. The findings should help for better comprehension of the fracture mechanisms that can occur in SP testing and recognizing the main differences between the conventional and SP testing.
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Lomové chování oceli EUROFER´97 po tepelném stárnutí
Hadraba, Hynek ; Dlouhý, Ivo
Eurofer´97 steel is candidate structural material for in-vessel components of proposed fusion reactors. The use of the steel is limited up to a temperature about 550 °C. On the other hand the efficiency enhancement of the fusion reactors to the level suitable for energy production is predetermined by an increase of temperature in reactor. The long term exposition of the steel at high temperatures leads to microstructural changes. The aim of the work was to investigate the influence of short and long term thermal ageing on fracture properties of Eurofer´97 steel. Short term thermal ageing of the steel was simulated by step-cooling treatment and long term thermal ageing was simulated by isothermal annealing treatment at 550 °C/5000 h. No evident changes in impact properties have been registered comparing the properties of the steel in as-received state and in state after step-cooling. Outstanding embrittlement of the thermally aged steel was observed after long term isothermal ageing.
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Růst krátkých trhlin z vrubů ve dvoufázové austenitcko-feritické oceli
Kotecký, Ondřej ; Degallaix, S. ; Polák, Jaroslav
The effect of microstructure and stress gradient in a two-phase austenitic-ferritic steel in specimens with confined plasticity is studied. Direct optical observations and potential drop technique calibrated via the finite element method are used to measure the surface crack size and its depth and to determine the surface crack growth. The effect of stress and strain gradients on the crack propagation kinetics and crack shape is evaluated and dicussed.
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Vliv rohové singularity na tvar únavové trhliny
Hutař, Pavel ; Náhlík, Luboš ; Knésl, Zdeněk ; Seitl, Stanislav
Due to the existence of vertex singularity at the point where the crack intersects the free surface, stress distribution around the crack tip and the type of the singularity is changed. In the interior of the specimen the classical singular behaviour of the crack is dominant and can be described using generally known analytic equations. Contrary to this, at the free surface or in the boundary layer close to the free surface the vertex singularity is significant. The influence of vertex singularity on crack behaviour and crack shape for a three-dimensional structure is described in this paper. To estimate the influence of the free surface on fatigue crack growth rate the model of a general singular stress concentrator was applied. The estimated fatigue crack shape can help to provide a more reliable estimation of the fatigue life of the structures considered.
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Odhad zdánlivé lomové houževnatosti keramických laminátů založený na zobecněném faktoru hustoty deformační energie
Náhlík, Luboš ; Šestáková, Lucie ; Hutař, Pavel
The paper presented deals with fracture behavior of ceramic laminates. Assumptions of the linear elastic fracture mechanics and small scale yielding are considered. In this frame the residual stresses in individual layers of Al2O3/5vol.%t-ZrO2 (ATZ) and Al2O3/30vol.%m-ZrO2 (AMZ) are determined. The procedure based on generalization of Sih's strain energy density factor to the case of a crack touching the interfaces between two dissimilar materials is used for determination of effective values of stress intensity factor at material interfaces.Important increase of the fracture toughness at AMZ/ATZ interface was predicted in comparison to the fracture toughness of individual material components. Predicted values were compared with data available in literature and mutual good agreement was found. Procedure suggested can be used for estimation of resistance to crack propagation of multilayered structures and its design.
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Efekt velikosti částic na lomové chování polypropylenového kompozitu
Nezbedová, E. ; Knésl, Zdeněk ; Hutař, Pavel ; Majer, Zdeněk ; Veselý, P.
The addition of mineral fillers to polypropylene (PP) can profoundly change the mechanical properties of polymer system. The properties of the particles themselves (size, shape, and material properties) can have significant effect on the global behaviour of composite, particularly on its toughness and stiffness. This contribution is focused on effects of particles size and their density on fracture toughness of polyprophylen particulate composite. The effect was studied from the experimental as well as from the theoretical point of view. The change of fracture toughness was estimated with utilisation of linear elastic fracture mechanics approach. The composite was modelled as a three-phase material with homogeneously distributed coated particles and numerically simulated using the finite element program ANSYS.
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