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Pressure pipe damage: Numerical estimation of point load effect
Zouhar, Michal ; Hutař, Pavel ; Ševčík, Martin ; Náhlík, Luboš
The most relevant loading conditions for real polymer pipe systems are not only internal pressure, but also loading caused by sand embedding including bending or different kinds of point loads. It has been shown that service lifetime of buried pipes can be reduced especially due to stress concentration caused by external point loads. If the pipe is loaded locally the stress is concentrated here and a crack can initiate at this position or the existing crack can be affected by corresponding stress redistribution. In the paper the effect of the hard indenter, Poisson's ratio, hoop stress level and pipe wall thickness on the crack shape was estimated using numerical simulations of the creep crack propagation based on finite element method. Relation between crack length and crack width was found and expressed by simple relationship. A deeper understanding of the point load effect in order to prevent unexpected failure of the pipelines is of paramount importance for pipeline design.
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Studium vlivu segregovaných nečistot na magnetismus hranic zrn a povrchů v FCC niklu a kobaltu
Všianská, Monika ; Vémolová, H. ; Šob, Mojmír
Na základě výpočtů elektronové struktury z prvních principů studujeme vliv segregovaných sp nečistot (Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb a Te) na hranici zrn (GB) 5(210) a na volném povrchu (FS) (210) fcc niklu a kobaltu. Provádíme přitom úplnou relaxaci atomové konfigurace GB a FS bez nečistot i se substitučně a intersticiálně vloženými nečistotami. Analyzujeme vliv studovaných nemagnetických nečistot na rozložení magnetického momentu, geometrické uspořádání atomů v okolí GB a FS a hustoty elektronových stavů. Vysvětlujeme také, proč při segregaci studovaných nečistot mohou být magneticky mrtvé vrstvy přítomny v niklu, ale nikoliv v kobaltu.
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Vliv reziduálních napětí v keramickém laminátu na šíření trhliny
Máša, Bohuslav ; Zouhar, Michal ; Náhlík, Luboš
The main focus of this contribution is the fracture behaviour of layered ceramics composite – Al2O3/ZrO2. Different ratios of the layers were considered. The residual stresses given by sintering of the composite were identified and were taken into account. Assumptions concerning linear fracture mechanics and small scale yielding are considered. Stress components and displacements in the vicinity of the crack tip were obtained by a numerical modelling using finite element method implemented to the commercial software Ansys. For the determination of the stress intensity factor analytical approach was involved. The paper presented can contribute to the better understanding of the crack propagation through the individual layers.
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Studie tvárného mechanismu porušování oceli Eurofer97
Stratil, Luděk ; Hadraba, Hynek ; Dlouhý, Ivo
V příspěvku je zkoumán tvárný mechanismus poškození oceli Eurofer97. Pro vytvoření různých podmínek triaxiality napjatosti byly zkoumány hladké a vrubované tahové tělesa. Přelomené poloviny tahových těles byly fraktograficky a metalograficky analyzovány a byla provedena kvantitativní studie mechanismu iniciace a nukleace dutin. Nukleace dutin se děje přednostně dekohezí rozhraní matrice-částice sekundární fáze. Podstatná fáze poškození oceli je následně tvořena postupným růstem dutin a konečná fáze poškození je završena velmi rychlým procesem koalescence dutin. Podmínky větší napjatosti uprostřed vrubovaných tahových těles způsobují potlačení nukleace dutin ale podporují jejich růst a také snižují úroveň lomové deformace.
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Modelling of ductile fracture for sub-sized three-point-bend geometry
Stratil, Luděk ; Hadraba, Hynek ; Kozák, Vladislav ; Dlouhý, Ivo
The contribution deals with the simulation of R-curve using complete Gurson model of ductile fracture. The R-curve was experimentally determined for a Eurofer97 steel on sub-sized three-point-bend geometry in previous study. To apply complete Gurson model the parameters describing the voids´ behaviour and characteristic length parameter need to be determined. The nucleation parameters were identified by single specimen method of smooth tensile test specimen and from metallographic examination of fracture micro-mechanism. The characteristic length parameter was derived by fitting load versus deflection curves of sub-sized specimens. The simulations of the tests were carried out by software ABAQUS in Standard and Explicit modules. The identification was supported by parametric studies. Comparing experimental and simulated R-curve the ductile tearing was not successfully achieved. Insufficient calibrated parameters as a result non-uniqueness problem of single specimen method were found.
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A simulation of R-curve by complete Gurson model
Stratil, Luděk ; Hadraba, Hynek ; Kozák, Vladislav ; Dlouhý, Ivo
The contribution deals with the simulation of R-curve using completed Gruson model of ductile fracture. The R-curve was experimentally determined for a Eurofer97 steel in previous study. The simulations of the tests were carried out by FEM software ABAQUS 6.11 in Standard and Explicit modules. The Gurson parameters were identified by single specimen method of smooth tensile test specimen and from metallographic examination of fracture micro-mechanism. The identification was supported by parametric studies. Comparing experimental and simulated R-curve the ductile tearing was not successfully achieved. Insufficient calibrated parameters as a result non-uniqueness problem of single specimen method were found.
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On a critical geometry of a crack arrested at a bimaterial interface
Ševčík, Martin ; Zouhar, Michal ; Máša, Bohuslav ; Hutař, Pavel ; Náhlík, Luboš
Surface crack propagation in a thin soft protective layer on a massive stiffer substrate is analyzed using generalized linear elastic fracture mechanics. The growth of the initial crack is considered in both forward and sideways directions and the influence of the interface between the protective layer and massive substrate on the final crack configuration is investigated. It is shown that, depending on the elastic mismatch, the part of the crack front can be arrested at the interface protective layer/substrate and the rest of the crack grows continuously sideways only. The effective value of the stress intensity factor is used in order to predict the conditions under which the crack will propagate through the interface into the second material. Corresponding calculations have been made by finite element method.
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