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Interaction of crack and aggregate in concrete
Keršner, Z. ; Náhlík, Luboš ; Knésl, Zdeněk
Concrete is represented as a two-phase material in this paper. Behaviour of a single macrocrack approaching to the interface between hardened cement paste (matrix) and aggregate (particle) is modelled. It is shown that in the case of where Young modulus of cement paste is greater then those of aggregates, the effective fracture toughness of concrete is predominantly given by fracture properties of aggregates.
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Interaction between crack and circular inclusion
Náhlík, Luboš
The interaction between an inhomogeneity and a crack propagating near the circular inhomogeneity is studied under the assumptions of linear elastic fracture mechanics (LEFM) (we assume elastic behaviour of the matrix and the inhomogeneity as well). Three cases are considered: a crack located in front of the inhomogeneity, a crack situated arbitrarily near the inhomogeneity, and a special case when the crack tip lies at the interface between the matrix and the inhomogeneity . The main purpose of this paper is to find the most often configurations in materials with inhomogeneities.
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Study of the plastic zone shape and size in a bi-material vicinity
Klusák, Jan
Shape and size of a plastic zone in a vicinity of a bi-material notch are studied in the article. There are analytical relations shown for determination of the plastic zone and for evaluation of generalized stress intensity factors. Conditions of crack initiation from bi-material wedge are determined based on comparison of numerically determined stress intensity factor with its threshold value or comparing applied stress with critical stress. The results of the study contribute to better life-time evaluations of constructions containing a bi-material notch.
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Evaluation of the T-stress by the quarter-point elements
Hutař, Pavel
The three-dimensional (3D) finite element model of center cracked plate, CT-specimen... is used to study evaluation of T-stress along the crack front. $T$ is evaluated by direct methods and from wedge quarter-point element nodal dispacement. Results are compared and discussed from point of view of using in three-dimensional finite element models. This paper contains summary of relations for T-stress evaluation for quarter-point element nodal.
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Problems of calculation T-stress in solid bodies
Seitl, Stanislav
Contribution is about problems of calculation T-stress in solid bodies. The calculation is performed by using finite element method. The problem was solved on center cracked plate tension specimen (CCT). The constraint distribution is investigated along the crack front.
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MICROSTRUCTURE/PROPERTIES RELATIONSHIP IN ULTRAFINE GRAINED ALUMINIUM PROCESSED BY ECAP TACHNIQUE
Král, Petr ; Dvořák, Jiří ; Svoboda, Milan ; Sklenička, Václav
Experiments were conducted on pure ultrafine grained aluminium processed by severe plastic deformation using the equal ů channel angular pressing ů ECAP technique. By this technique bulk ultrafine grained material was prepared. The propertis of this material considerably differs from properties of material with standard grain size. It was found that microstructure of ultrafine grained aluminium after one pass by the ECAP die contains subgrains and these subgrains are changed by subsequent ECAP passes in grains separated by high angle grain boundaries. In this work special attention is given to the stability of microstructure and the relationship between microstructure and mechanical properties of ECAP material.
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Creep of intermetallic alloy TiAl at very low strain rates
Mareček, P. ; Kloc, Luboš ; Fiala, J.
Intermetallic alloys are very advanced materials for high-temperature applications. There are series of attractive properties exceed coventional up to now used creep resistance steels. The creep behaviour of TiAl alloy was investigated under an applied stress lower than 100 MPa at temperatures 700-850 °C. The strain rate was lower than 10-9 s-1 under testing conditions, which have importance for technical applications. In relation to results, the creep behavior of intermetallic alloy TiAl shows a transition to a mechanism with low stress exponent in the range of minimum strain rates, similar to creep resistance steels with evidence change of deformation mechanism under strain rate 10-10 s-1 . However, this transition in TiAl alloys is not as clear as in creep resistant steels.
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Vliv zatěžovací historie na creep 9% chromové oceli
Kloc, Luboš ; Sklenička, Václav
The creep life assessment is currently based on the phenomenological models mostly derived from the constant stress or constant load creep experiments. Since the loading history of the parts in industry used to be complicated and frequently unknown, the model data can be invalid. Stress change creep experiments were done on the P-91 type creep resistant steel at loading conditions corresponding to the low stress creep regime. The experiment was interrupted, that is cooled down and heated up again several times. The interrupts were done during the ``quasi-steady state'' when the creep rate is stabilized. The effect of both stress changes and temperature cycles on creep properties was analyzed. While the temperature cycle introduce some additional creep strain, the stress changes reduce the creep rate considerably. Taking into account the proportionality between the creep strain and creep damage nucleation, it is possible to conclude that small stress changes during primary stage can improve the creep strength of the steel, while the temperature changes have opposite effect.
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MECHANISMY CREEPOVÉ DEFORMACE U UTRAJEMNOZRNNÉHO HLINÍKU PŘIPRAVENÉHO METODOU ECAP
Dvořák, Jiří ; Sklenička, Václav ; Král, Petr
Extremely coarse-grained aluminium (99,99%) was subjected to severe plastic deformation (ECAP)for obtain an ultrafine-grained microstructure. Creep tensile and compression tests were conducted on such material and for comparison reasons also on coarse-grained aluminium. The results indicate an increased creep resistant of ultrafine-grained aluminium compared to its coarse-grained state. It was proposed, that creep of ultrafine-grained aluminium is probably control by the same creep mechanisms as coarse-grained one. However, the role of grain boundary sliding in creep considerably differs in both ECAP and coarse-grained materials
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