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Studies of structural and magnetic properties of iron nanowires from first principles
Zelený, Martin ; Šob, Mojmír ; Hafner, J.
In this work we study the structure and stability of iron ultrathin free-standing nanowires by first-principles density functional approach. For total energy calculations, we use the Vienna ab initio Simulation Package (VASP) in the projector augmented-wave (PAW) representation. The evolution of electronic structure and magnetism of iron chains as a function of compression and stretch is investigated. The forming of different types of structures is analyzed: simple linear chain, dimerized chain, ideal two-dimensional (2D) zig-zag structure relaxed in x and y directions, fully relaxed 2D structure, and three-dimensional (3D) structure with tetrahedral arrangements. It turns out that the structure of iron nanowires and types of interatomic bonds are strongly dependent on their stretch or compression.
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Studium tantalu pomocí potenciálů typu bond-order
Čák, Miroslav ; Mrověc, M. ; Vitek, V. ; Šob, Mojmír
We present a bond-order potential (BOP) study for the bcc transition metal tantalum. The BOPs are a real-space semiempirical scheme for the description of interatomic interactions based on the tight-binding approximation. The potentials are applied to the calculation of total energy profile along the tetragonal deformation path and gamma-surface of {110} plane. Our results are in a good agreement with the ab initio calculated values which indicates that BOPs are capable to describe properly the bonding in bcc transition metals.
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Electronic structure In-Sn alloys
Všianská, Monika ; Legut, Dominik ; Šob, Mojmír
The InSn system is interesting by the existence of a simple hexagonal phase for compositions from 72 to 87 at% Sn at 25 °C and from 73 to 85 at% Sn at -150 °C. These alloys are usually referred to as gamma–Sn. The InSn alloys are disordered in the whole concentration interval. In this contribution, energetics and electronic structure of InSn system is studied from first principles. A simplified version of virtual crystal approximation is employed to describe disorder. It turns out that the present approach is capable of describing phase composition of InSn system in the whole concentration interval. In particular, we are able to reproduce the existence of simple hexagonal phase around 80 at% Sn.
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Dynamic fracture toughness determination for inherently brittle materials
Flašar, Petr ; Chlup, Zdeněk ; Dlouhý, Ivo
Dynamic fracture toughness measurement of brittle materials using Charpy impact test. A method of dynamic fracture toughness evaluation of brittle materials using Charpy pendulum machine is presented. For low-speed loading rates a specimen damping technique was developed, in high-speed loading rates regime an eccentric onepoint bending test for DKC evaluation was used. A new technique of time to fracture tf measurement was designed using gold-layer deposition on specimen surface. Analytical and numerical analysis of studied problematic was carried out and compared to experimental data.
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Application of inverse FEM analysis for materials curve determination based on indentation test
Brumek, J. ; Strnadel, B. ; Dlouhý, Ivo
Study of instrumented indentation test is focused on prediction of the strain hardening behavior of the carbon steels. Indentation tests were applied on several steels samples. We proposed an improved technique to determine the plastic properties of material from the loaddisplacement curve from ball indentation test. The inverse finite element method was used for prediction of material properties. Results were found in good agreement with the data from conventional standard test. This non-destructive assessment of deformation behavior has good potential for the other materials.
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Tensile testing of high porous ceramic materials
Řehořek, Lukáš ; Chlup, Zdeněk ; Dlouhý, Ivo
In last years cellular materials are getting to focus of an interest in a number of industrial applications and science fields. These materials are often used for filtration of different mediums or in aerospace industry and even in tissue engineering. Hence, there is effort to understand and describe mechanical and physical properties of these materials. While behaviour of these materials in compression is well understand and published, tensile tests are not so frequent and concerning ceramic cellular materials they are highly rare. The methodology of evaluation of tensile properties of model ceramic open-pore foam material VUKAPOR((R))A used like filters in metallurgy was developed. This methodology is based on fixation of ceramic foam sample by specialised resin to a holder. Homogenous transfer of forces is ensured by this technique. Specimens of two sizes – 10x10x30mm and 15x15x40mm were tested. Ceramic foams with two pore densities – 10 PPI and 60 PPI have been investigated.
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Physical and chemical properties of FINEMET-type alloys
Zábranský, Karel
FINEMET-type alloys Fe80Nb3Cu1B16 (FM1), Fe80Nb3Cu1Si6B10 (FM2), and Fe33,5Ni40Nb3Cu1Si13, 5B9 (FM3) are investigated in the amorphous states. The aim of these studies is to follow an influence of variations in metalloid (Si, B) and metal (Fe, Ni) elements on the structural, magnetic, chemical, and mechanical properties.
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Kinetics of fatigue crack growth under notches in duplex steel
Kotecký, Ondřej ; Degallaix, S. ; Polák, Jaroslav
While under elastic fatigue loading of structural components with a stress concentration the material response is quite well known and the predictions are accurate, under elastic-plastic fatigue loading the predictions are poor and result in over-dimensioning the structure. The present work studies the effect of the stress gradient and the microstructure in an austenitic-ferritic (or duplex) 2507 stainless steel. Fatigue tests were performed on double edge notched specimens. 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 in-depth and surface crack growth. The effect of stress and strain gradients on the crack propagation kinetics is evaluated and discussed.
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Fracture mechanics parameters of bi-material body
Šestáková, Lucie
Multilayer pipes consisting of different materials are frequently used in praxis because of partial improvement of the properties of pipe systems. To estimate their service life the basic fracture parameters have to be determined. In this work finite element calculations are applied in order to estimate the stress intensity factor K and T-stress values. Transferability of the fracture mechanics parameters obtained from two proposed C-type specimens to pipe systems was discussed using a constraint-based two-parameter fracture mechanics approach.
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