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Vliv rohové singularity na tvar únavové trhliny
Hutař, Pavel ; Náhlík, Luboš ; Knésl, Zdeněk ; Seitl, Stanislav
Due to the existence of vertex singularity at the point where the crack intersects the free surface, stress distribution around the crack tip and the type of the singularity is changed. In the interior of the specimen the classical singular behaviour of the crack is dominant and can be described using generally known analytic equations. Contrary to this, at the free surface or in the boundary layer close to the free surface the vertex singularity is significant. The influence of vertex singularity on crack behaviour and crack shape for a three-dimensional structure is described in this paper. To estimate the influence of the free surface on fatigue crack growth rate the model of a general singular stress concentrator was applied. The estimated fatigue crack shape can help to provide a more reliable estimation of the fatigue life of the structures considered.
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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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The Influence of Multilayer Composite Interface on Crack Propagation Direction
Šestáková, Lucie ; Náhlík, Luboš ; Hutař, Pavel
The paper deals with crack propagation in multilayer composites. A substantial number of experimentally based works have indicated the changes in crack propagation direction on multilayer composite interfaces. Changes of this kind are mostly desirable for composite technical life extension. Crack direction modification in the composite interface is investigated through a numerical simulation of this problem. For determination of the consequent propagation direction the minimal strain energy density criterion is employed. The ascertained propagation direction of the crack in the multilayer composite is compared with experimental results.
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The influence of specimen geometry on plasticity induced crack closure
Náhlík, Luboš ; Hutař, Pavel
The paper investigates the effect of in-plane constraint on plasticity induced crack closure under conditions of small scale yielding. The growth of a sharp straight crack in middle tension specimen (MT) and compact tension (CT) specimen, under constant amplitude of cyclic loading, was analyzed with help of finite element method. The material constitutive behavior was represented by a purely kinematic hardening law. Obtained results are discussed from point of view of the influence of T-stress on plasticity induced crack closure. Significant difference between opening values for CT and MT specimens was found under conditions of plane strain.
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INFLUENCE OF MATERIAL PARAMETERS ON POLZNOM USED FOR MODIFICATION PARIS LAW
Seitl, Stanislav
In this paper sensitivity analysis of the effect of material properties (particularly a yield stress and hardening exponent) on the polynomial function used at modification of the Paris-Erdogan law was performed. The modification, which is based on the two-parameter fracture mechanics, describes fatigue crack propagation using a size of the plastic zone. The stress field around a crack tip is described by two parameters – stress intensity factor K and T-stress. Dependency of the plastic zone size and the parameters K, T is then formulated by using the polynomial function, which is found out in the contribution. The finite element method together with the method of modified boundary layer for the numerical simulations was used. The results should lead to safer transmission of the data from laboratory specimens to real structures and their components.
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Influence of geometry on fatigue crack growth
Hutař, Pavel ; Náhlík, Luboš ; Knésl, Zdeněk
The paper describes an application of constraint based fracture mechanics to the behaviour of fatigue cracks. This work is focused mainly on the area of high cycle fatigue (corresponding to small scale yielding conditions) and different constraint levels are quantified by T-stress value. It is suggested that the fatigue crack propagation rate is controlled by an effective value of the stress intensity factor depending on the nominal value of the stress intensity factor, the yield stress of the material and the T-stress value. It is shown that in some cases the effect of geometry has a significant influence on the fatigue crack propagation rate and on the threshold values. The T-stress effect on plasticity induced crack closure is discussed. The approach suggested is intended to contribute a more reliable transfer of laboratory data to engineering structures with different constraint levels.
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Plasticity induced crack closure for specimens with different geometries
Hutař, Pavel
The article is solving problem of plasticity induced crack closure for specimens with different geometries. The specimens with low level of constraint (MT-specimen) produced higher level of opening stress intensity factor than specimen with high level of constraint (CT-specimen). It was concluded that for better estimation of residual fatigue life of structures is necessary to consider effects of crack closure.
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