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Effect of segregated nonmagnetic sp-impurities on the properties of grain boundaries and surfaces in nickel
Všianská, Monika ; Šob, Mojmír
We present an ab initio study of segregation of 12 nonmagnetic sp impurities (Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb and Te) at the Sigma5(210) grain boundary (GB) and (210) free surface (FS) in fcc ferromagnetic nickel and analyze their effect on structure, magnetic and mechanical properties. We determine the preferred segregation sites of the impurity atoms, their segregation enthalpies and strengthening/embrittling energies with their decomposition into the chemical and mechanical components. We find that the value of mechanical component is dependent on the type of the stable segregation position and is nearly constant in each period. On the other hand the chemical component changes strongly within each period and is determined by the electronegativity of impurity atoms. Magnetically dead layers found at the impurity segregated GB and FS are caused by a strong hybridization of sp states of the impurities with the d states of nickel.
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Numerical modelling of damage of a particulate composite with polymer matrix
Máša, Bohuslav ; Ševčík, Martin ; Náhlík, Luboš
The main focus of this contribution is estimation of mechanical properties of a composite loaded in tension using finite element method. Investigated composite is composed of matrix in a rubbery state filled by alumina-based particles (Al2O3). Hyperelastic properties of the matrix have been modelled by the Mooney-Rivlin material model. The damage mechanisms of the matrix have also been taken into account. Results of numerical analyses have been compared with experimental data and good agreement between numerical models with damage mechanisms of matrix and experimental data has been found.
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The influence of residual stresses on the polymer pipe lifetime
Luky, R. ; Zouhar, Michal ; Hutař, Pavel
The aim of this article is study the influence of residual stresses on the pipe lifetime. The two dimensional numerical model of the pipe with inner crack was created and the creep crack growth in the pipe with and without residual stress is modelled. The results of K-kalibration curves are presented. After that the Paris law is used for solving the lifetime. The results of time to failure in dependency on the hoop stress for pipe with and without residual stress were compared. The existence of residual stresses decrease the lifetime of the pipe. Combination of the creep crack growth tests and numerical simulations can be a very powerful tool for lifetime estimation of plastic pipes under different loading conditions.
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Stress state analysis of sub-sized pre-cracked three-point-bend specimen
Stratil, Luděk ; Kozák, Vladislav ; Hadraba, Hynek ; Dlouhý, Ivo
The contribution deals with finite element analysis of stress state in sub-sized pre-cracked three-point bend specimen (KLST). A studied material was Eurofer´97 steel. The true stress-true strain curve was obtained from tensile test. The three point bend tests were carried out on plain and side-grooved specimens at ambient temperature and were simulated using ABAQUS 6.10. The stress state in KLST specimen was compared with the stress state of standard specimen. The analysis confirmed the stress state in KLST specimens at the crack tip does not correspond to conditions of standard fracture toughness determination because loss of constraint.
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Determination of R-curves from sub-sized three-point-bend geometry
Stratil, Luděk ; Hadraba, Hynek ; Dlouhý, Ivo
Submitted article deals with determination of R-curves from sub-sized three-point-bend geometry. R-curves were measured by single and multi-specimen method on KLST samples fabricated from 25 mm sheet of Eurofer´97 steel. The applicability of single specimen method for given tested geometry and steel was verified. The curves determined by this are well comparable. However results obtained by single specimen method just from one specimen could be biased by material variability. Measured values are in good agreement with published results. The side-grooved specimens show lower values of J-integral and weaker crack-length effect.
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Structure and magnetism of clean and impurity-decorated grain boundaries in nickel from first principles
Všianská, Monika ; Šob, Mojmír
We present a detailed theoretical study of segregation of sp-elements from the 3rd-5th period (Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb and Te) at the Sigma5(210) grain boundary (GB) in fcc FM Ni. Whereas there is a slight enhancement of magnetization at the clean GB and FS with respect to bulk nickel (3–7% and 24%, respectively), the studied impurities entirely kill or strongly reduce ferromagnetism at the GB so that magnetically dead layers are formed. We determine the preferred segregation sites at the GB for the impurities studied, their segregation enthalpies and strengthening/embrittling energies. We find interstitially segregated Si and P and substitutionally segregated Al as a GB cohesion enhancer, and interstitially segregated S, Ge, As, Se and substitutionally segregated Ga, In, Sn, Sb and Te as GB embrittlers in Ni. As there is very little experimental information on GB segregation in nickel most of the present results are theoretical predictions which may motivate future experimental work.
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3D modelling of dislocation processes
Záležák, Tomáš
Article presents a 3D model which describes a motion of discrete dislocations in a crystalline material at high temperatures. The dislocation curves are represented by straight line segments. The driving forces are determined by a Peach-Koehler formula which considers the self-stress and applied stress. The segment velocity is approximated by a temperature-dependent linear relation to the Peach-Koehler force. The model incorporates spherical precipitates. The model is also capable of adaptive adjustment of a time integration step and remeshing of the straight segment network. Topological changes (i. e. annihilation) are also included. The numerical integration takes advantage of the model symmetry to speed up the simulation process, save computer memory and reduce numerical errors. The model is applied to a system of coaxial dislocation loops in a field of spherical particles.
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Methods for evaluation of stability of crack between two isotropic/orthotropic materials
Damborský, Petr
This article deal with problems of cracks in bodies with bimaterial interface. The approaches based on the stress intensity factor (SIF) and the energy release rate (ERR) concepts are presented for isotropic and orthotropic materials. A relation between the mixity of the two fracture modes (I and II) in accordance with the two approaches mentioned is presented. The last part deals with the use two states ψ- integral to determine generalized stress intensity factor.
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