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Šíření trhliny v okolí bi-materiálového rozhraní: aplikace na vícevrstvé trubky
Šestáková, Lucie ; Náhlík, Luboš ; Hutař, Pavel ; Knésl, Zdeněk
Layered materials are often used in praxis, primarily because of their better mechanical properties in comparison with properties of individual materials components. During the crack propagation process a configuration of a crack with its tip at the bi-material interface in the structure can occur. It is important to decide if the crack propagates into the second material in this case or not. The step change of material properties at bi-material interface causes that the classical linear elastic fracture mechanics cannot be used. A generalized approach has to be applied. It is to find suitable criteria for determination of the effective values of stress intensity factor for the crack terminating at bi-material interface. Knowledge of these values is important for assessment of service life time such structures.
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Computational modelling of fatigue crack propagation path in gears
Ševčík, Martin ; Vrbka, M. ; Hutař, Pavel ; Náhlík, Luboš
The paper deals with estimation of fatigue crack behavior for crack propagation from the tooth root. The conditions of linear elastic fracture mechanics are considered. The extension of LEFM – two-parameter LEFM is used for crack path estimation. The effect of the rim thickness on the path of fatigue crack was studied and procedure for more accurate description of the fatigue crack propagation at the gear rim is proposed in the work. Modified maximal tangential stress (MTS) criterion was used for an estimation of the fatigue crack path in the gear rim. Good agreement with experimental results was obtained. More accurate two-parameter description of the stress field around the crack tip can explain the discrepancies between experimental results and previous numerical calculations based on classical one parameter LEFM.
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Influence of particles shape on fracture toughness of PP-based particulate composite
Majer, Zdeněk ; Knésl, Zdeněk ; Hutař, Pavel
Polymeric particulate composites with thermoplastics are of great practical importance due to the possibility of modifying mechanical properties and reducing the price/volume ratio of resulting material. The properties of the particles themselves (size, shape, material properties) can have a significant effect on the global behaviour of the composite. Generally, the addition of rigid particles to a polymer matrix will have an embrittling effect on the composite. The presence of the particles significantly influences the cure reaction resulting in the formation of a third phase known as the interphase, which possesses a property distinct from those of the matrix and the particles. The particle-reinforced composite (PP-CaCO3) was modeled as a three-phase material with homogeneously distributed coated particles. The main aim of the article is estimate an effect of the interphase properties on microcrack behaviour.
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Measurement of mechanical properties of thin films by nanocompression
Kuběna, Ivo ; Kruml, Tomáš ; Hutař, Pavel ; Boháč, Petr ; Stranyánek, Martin ; Čtvrtlík, Radim ; Pánek, P. ; Vystavěl, T.
Mechanical properties of thin films are not easy to be measured. This is particularly true in the case of plastic properties as the yield point, the work hardening rate or the ultimate stress. Nevertheless, such parameters are needed e.g. in the design of integrated circuits where the thermal stresses may lead to mechanical failure of the component. We applied two modern experimental facilities, the focused ion beam and the nanoindentation for exact measurement of plastic properties of a Al-1.5%Cu thin film prepared by PVD, used for electrical connection of integrated circuits. By FIB milling, cylindrical specimens were prepared. The height of the specimens was equal to the film thickness (2 m) and their diameter was about 1.3 m. These specimens were subjected to the compressive loading using the nanoindenter equipped by a flat punch. Stress-strain curves of the film were obtained rather precisely.
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Výpočtová analýza růstu únavové trhliny v kole testovaném podle normy EN 13262
Hutař, Pavel ; Náhlík, Luboš ; Knésl, Zdeněk ; Kunz, Ludvík ; Matušek, P.
In this paper, the experimental set up corresponding to the test unit used for the full-scale fatigue test has been modeled and the numerical method for reliable estimation of residual fatigue lifetime of wheel web tested according to EN 13 262 Standard has been developed. Numerical calculations are based on finite element method utilizing the system ANSYS. The experimentally observed cracks in the wheel web have been taken as the initial crack configuration. Numerical calculations simulating the initial crack growth in the wheel web loaded by experimental set up have been performed. Under assumptions of linear elastic fracture mechanics the residual fatigue life of the cracked wheel has been predicted. The material properties necessary for numerical modeling were determined from the fatigue tests performed on small specimens taken from real wheel webs. The results obtained can be used for more reliable design of railway wheels.
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Stanovení kritické hodnoty pro šíření trhliny z ostrého v-vrubu
Majer, Zdeněk ; Hutař, Pavel ; Náhlík, Luboš ; Knésl, Zdeněk
The aim of this paper is to investigate the beginnings of crack propagation from sharp V-notches. Stress distribution around the tip of a V-notch is described on the basis of linear-elastic fracture mechanics. The V-notch is a singular stress concentrator with a singularity exponent dependent on the V-notch opening angle. The stability criteria are generalized for a stress singularity different from 0,5. Using FEM analysis the critical stress for crack initiation was estimated as a function of the V-notch angle. The results of numerical calculations were compared with experiments taken from the literature.
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Vliv rohové singularity na chování trhliny
Hutař, Pavel ; Náhlík, Luboš
The influence of corner singularity on crack behaviour for a threedimensional structure is described in this paper. The distribution of stress singularity through the thickness of a specimen provides us with an indication of crack behaviour close to a free surface. A decrease in fatigue crack propagation rate in the area influenced by the free surface was carried out. The change of fatigue crack propagation rate in the boundary layer, leads to different intersecting angles between crack front and free surface as a function of Poisson’s ratio.
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Vliv singularity způsobené volným povrchem na chování tenkých těles s v-vrubem
Majer, Zdeněk ; Hutař, Pavel ; Náhlík, Luboš
The influence of the singularity induced by the free surface on the behaviour of a body with a V-notch is investigated in this paper. Attention is focused mainly on the change of the singularity and the stress state through the thickness of a three-dimensional specimen. Numerical finite-element analysis of the tension loaded specimens with a V-notch were performed. The displacements and the stresses from the analysis were used to estimate the power of singularities using a log-log regression analysis along the crack front. It was found that in the middle of the specimen singularity of the V-notch is dominant this can be found using analytic equations describing two-dimensional stress distribution around the sharp V-notch. The thickness of boundary layer (region influenced by corner singularity) depends only on the Poisson's ratio of the material. The presented results can be used for a more reliable estimation of the free-surface effect on singular concentrator behaviour.
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Vliv plasticky indukovaného zavírání trhliny a bi-materiálového rozhraní na prahové hodnoty
Náhlík, Luboš ; Hutař, Pavel
The paper is devoted to fatigue crack propagation in layered materials. The influence of plasticity induced crack closure on threshold values for fatigue crack propagation through interfaces between different materials layers is studied. The main aim was to estimate the influence of the loading ratio R on threshold values for crack propagation through a bi-material interface as a function of the elastic mismatch of both materials. The finite element method (FEM) is used for numerical calculations. Results obtained for different loading ratios, materials, boundary conditions and magnitude of applied loading can be generalized and used for the design of composite bodies with different material layers.
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Influence of Bi-material Interface on Crack Propagation Direction in Multilayer Composite
Šestáková, Lucie ; Náhlík, Luboš ; Hutař, Pavel
A substantial number of experimentally based works indicate the changes in crack propagation direction on bi-material interface in composite laminate materials. The paper is focused on estimation of crack propagation direction after its passage through the interface between two materials. For determination of the consequent propagation direction the minimal strain energy density criterion was employed. More accurate results were obtained by means of jointed numerical and analytical solution. The values of crack propagation directions obtained for layered composite were compared with experimental data.
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