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Investigating ground state of nickel nitrides NiN and Ni2N with the help of quantum-mechanical calculations
Elstnerová, P. ; Friák, Martin ; Šob, Mojmír ; Neugebauer, J.
We have employed quantum mechanical calculations to identify ground-state structures of nickel nitrides NiN and Ni2N for which experimental data are lacking. In total 19 crystalline phases have been calculated for which not only thermodynamic but also structural and selected elastic properties have been determined. Employing density functional theory (DFT) methods, the total energies were calculated by means of a pseudopotential approach implemented in the VASP code and selected states were benchmarked by the full-potential linearized augmented plane wave (FP-LAPW) method implemented in the WIEN2k code. For the exchange-correlation energy the generalized gradient approximation (GGA) has been used.
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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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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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Evaluation of properties of pyrolysed resins
Halasová, M. ; Chlup, Zdeněk ; Strachota, Adam ; Černý, Martin ; Dlouhý, Ivo
The presented work describes mechanical properties of materials prepared by pyrolysis of polysiloxane resins. The polymeric precursors had different chemical composition. Materials under investigation are predetermined as a matrix for high temperature resistant long fibre composites. An instrumented hardness tester was employed for materials characterisation. The Vickers hardness, Martens hardness and indentation elastic modulus were the key parameters determined from the load-indentation depth curves. Influence of the mechanical properties on the ratio of T and D was established. Indents were observed by using confocal laser microscope.
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