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Influence of grain orientation on threshold values of short fatigue cracks
Náhlík, Luboš ; Knésl, Zdeněk
The influence of a grain boundary on the propagation of a fatigue crack is studied. To this aim the grain boundary is considered as an interface between two different materials with their elastic constants depending on the orientation of the adjacent grains. The main goal is to estimate the threshold value of a microstructurally short fatigue crack propagating through the grain boundary. The corresponding threshold values are calculated as a function of material parameters of grains. Corresponding modelling have been performed by combination of analytical and numerical approach. For numerical calculations the finite element system ANSYS has been used. The model describes only one of many aspects of short fatigue crack behaviour and the results obtained contribute to a better understanding of microstuctural aspects of short fatigue crack growth.
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Influence of material characteristics on the fatigue crack growth through bi-material interface
Náhlík, Luboš
This contribution describes a study of the behaviour of a fatigue crack growing in one material and penetrating through the bi-material interface into a second material. Dependence of the stress singularity exponent on Poisson's ratio of both materials is studied. The study especially aims to quantify the influence of the interface on the values of fatigue threshold. It is suggested that the controlling variable for the behaviour of a crack approaching to the interface is the size of the plastic zone existing near the tip of the crack. The results of the contribution makes it possible to decide whether the crack will stop at the interface or continue growing into the second material. The results contribute to a better understanding of the damage of composite materials.
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Estimation of the Critical Stress for Failure of a Protective Layer
Náhlík, Luboš ; Knésl, Zdeněk
This contribution describes a study of the behaviour of a crack growing in a protective layer and penetrating through an interface into a substrate. Special attention is devoted to a crack touching the interface. Conditions for the stability of a crack terminating at the interface between two materials are analyzed. The problem is studied under the assumptions of linear-elastic fracture mechanics. A new tentative criterion of stability based on application of the strain energy density concept is formulated and applied to the problem. The suggested criterion is used to estimate the critical applied stress for the failure of a substrate caused by a crack growing through the protective layer. It is shown that the critical stress may strongly depend on the fracture toughness of the substrate and on the bi-material parameters.
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An analysis of stress distribution around singular stress concentrators
Knésl, Zdeněk ; Náhlík, Luboš ; Hutař, Pavel
Problems of determination of stress distribution around singular stress concentrators are discussed in the paper. Procedure for dependable estimation of stress field in the vicinity of singular stress concentrator is suggested. The procedure is based on combination of numerical (FEM) and analytical approaches. Presented procedure facilitates appraisal of influence of the singular stress concentrators on lifetime of engineering structures.
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Interakce trhliny a kovového substrátu v plasmových keramických nástřicích
Náhlík, Luboš ; Knésl, Zdeněk ; Kroupa, František
Plasma-sprayed ceramic coatings contain a high density of intrasplat microcracks which are responsible for small YoungŽs moduli and low fracture toughness. The extension of an initial surface crack in the direction to the interface, where the crack is repelled by the metal substrate with higher YoungŽs modulus, is studied using the methods of fracture mechanics. It is shown that high tensile stresses induced by the crack in the interface can lead to a local decohesion along the interface so that the crack can deviate into the interface
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Vliv rozhraní mezí matricí a inkluzí na šíření únavových trhlin
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). 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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Modelling of crack propagation through bi-material interface
Náhlík, Luboš
Under conditions of Linear Elastic Fracture Mechanics is studied crack behaviour in front of circular particle. The typical behaviour of crack approaching the interface between matrix and particle is showen. Results of numerical calculations and applications of earlier suggested tentative fracture criteria for crack terminating at the bi-material interface are presented.
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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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