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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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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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Influence of bimaterial interface on fatigue crack closure
Náhlík, Luboš
The article deals with the behaviour of a fatigue crack propagating across a bi-material interface. A stability criterion for the crack touching the bi-material interface, taking into account the residual stresses closing the crack faces is formulated. The linear elastic fracture mechanics is assumed to apply and the finite element method is used in the calculations. The criterion proposed is applied to determine the fatigue threshold stress for crack propagation across the interface. It is shown that the threshold values for crack propagation are influenced by the residual stresses closing the crack and by the specific combination of the elastic constants of the materials used. Two different geometries and two different loading ratios are considered. It is shown that the threshold values for crack propagation are influenced by the residual stresses closing the crack and by the specific combination of the elastic constants of the materials used.
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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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Numerical analysis of a surface crack in protective layers
Šestáková, Lucie ; Majer, Zdeněk ; Náhlík, Luboš ; Knésl, Zdeněk
Two elastic plane strain problems relevant to the cracking of protective layers bonded to dissimilar semi-infinite substrate materials are solved. In the first case a crack initiated at the free surface of the layer and propagating within the film is considered and the influence of its original orientation on cracking mode of the layer is studied. The second problem is that of the crack in the layer oriented perpendicularly to the layer/substrate interface and its influence on a debonding of the layer/substrate system is analysed. In both cases the influence of elastic mismatch of both components on the crack behaviour is discussed. The results provide the details making it possible to solve practical problems connected with damage of bodies with protective layers.
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Influence of T-stress on residual fatigue life of railway wheel set
Náhlík, Luboš ; Hutař, Pavel ; Knésl, Zdeněk ; Matušek, P.
Influence of T-stress on residual fatigue life of railway wheel set is analysed. The influence of constraint caused by different specimen geometry on fatigue crack growth is analyzed and a phenomenological model is proposed to correlate the fatigue crack propagation rate under different constraint levels. The approach is based on an application of two-parameter fracture mechanics and the corresponding calculations are performed according to the finite element method. The constraint level is quantified by the value of T-stress. The result presented makes it possible to relate experimentally measured data obtained from specimens with different geometries and thus contribute to more reliable estimates of the residual fatigue life of structures.
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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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