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Stress state aspects of quasi-brittle fracture
Sobek, Jakub ; Němeček,, Jiří (referee) ; Keršner, Zbyněk (referee) ; Klusák,, Jan (referee) ; Veselý, Václav (advisor)
The presented dissertation thesis is focused, as the title suggests, on the analysis of stress state aspects of quasi-brittle fracture. That means the fracture of composite materials with cement matrix (such as concrete, mortar, plaster, etc.), ceramics and other composites. Used methods are based on the theory of multi-parameter linear elastic fracture mechanics, which highlights the importance of considering of several initial terms of Williams power series, approximating the stress and displacement fields in a cracked body, within conducted fracture analyses. Determination of values of coefficients of terms of this series, recalculated into the shape functions serving in most of the conducted stress state analyses, is performed via the so called over-deterministic method. Another tool for the problem solving is nonlinear fracture mechanics, represented primarily by the cohesive crack model, namely the crack band model implemented in the used ATENA software. For the backward reconstruction of stress field in the cracked bodies the application ReFraPro is used. The analytical part deals with various aspects of wedge-splitting test – from the boundary conditions, though various possibilities of nodal selection (required as input variables for the over-deterministic method) up to the advanced (automated) analysis of numerical model. Special chapter includes atypical test specimens designed for adjusting of various levels of constraint of stress and deformation at the propagating crack tip. The study of this geometry and also the subsequent detail analysis reveals important information for real experiments. Backward reconstruction of stress field presents analysis on suitable possibilities of nodal selections as inputs into the procedure of approximation of the crack tip fields and answers the question of the necessity of application of the multi-parameter linear elastic fracture mechanics for certain fracture analyses of specimens from quasi-brittle materials. The th
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Analysis of stress state and failure in test specimens used for determination of fracture-mechanical parameters of quasi-brittle materials
Holušová, Táňa ; Seitl,, Stanislav (referee) ; Veselý, Václav (advisor)
The thesis is focused on a test on determination of the fracture-mechanical parameters of quasi-brittle materials, especially concrete. What is referred to as the wedge-splitting test is considered, for which a variety of shapes of notched specimen can be used. This work is exclusively focused on the cylinder-shaped specimen of diameter 150 mm and breadth of 100 mm. The test is performed virtually using Atena 2D FEM software. Progress of failure is observed during loading of the specimen for various notch lengths. The amount of energy released for the development the failure outside of the tested cross-sectional area (weakened by the notch) is quantified and the size of the fracture process zone is investigated. The described analysis is performed for several material sets witch differ in cohesive properties of the quasi-brittle material expressed via the so-called characteristic length. Suitable proportions of the test specimen are sought, in order to avoid the failure and thus also the energy dissipation outside of the specimen ligament area during the experimental tests, which shall lead to more accurate estimates of fracture-mechanical parameters of the tested material.
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Multilevel Determination of Mechanical Fracture Parameters of Concrete from Wedge Splitting Tests
Lipowczan, Martin ; Šomodíková, Martina ; Lehký, David
The paper presents the results of a part of the research focused on the development of a complex experimental–computational methodology for determining the values of mechanical fracture parameters of concrete independent of the specimen size and geometry. For this purpose, laboratory wedge splitting tests were carried out on specimens of three different sizes with two different notch depths. Based on the experimentally obtained responses of the test specimens, the fracture mechanical parameters of all specimens were identified by inverse analysis. The obtained parameters were used in the numerical simulation of the tests and analysed in terms of their dependence on the size of the initial uncracked ligament.
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Stress state aspects of quasi-brittle fracture
Sobek, Jakub ; Němeček,, Jiří (referee) ; Keršner, Zbyněk (referee) ; Klusák,, Jan (referee) ; Veselý, Václav (advisor)
The presented dissertation thesis is focused, as the title suggests, on the analysis of stress state aspects of quasi-brittle fracture. That means the fracture of composite materials with cement matrix (such as concrete, mortar, plaster, etc.), ceramics and other composites. Used methods are based on the theory of multi-parameter linear elastic fracture mechanics, which highlights the importance of considering of several initial terms of Williams power series, approximating the stress and displacement fields in a cracked body, within conducted fracture analyses. Determination of values of coefficients of terms of this series, recalculated into the shape functions serving in most of the conducted stress state analyses, is performed via the so called over-deterministic method. Another tool for the problem solving is nonlinear fracture mechanics, represented primarily by the cohesive crack model, namely the crack band model implemented in the used ATENA software. For the backward reconstruction of stress field in the cracked bodies the application ReFraPro is used. The analytical part deals with various aspects of wedge-splitting test – from the boundary conditions, though various possibilities of nodal selection (required as input variables for the over-deterministic method) up to the advanced (automated) analysis of numerical model. Special chapter includes atypical test specimens designed for adjusting of various levels of constraint of stress and deformation at the propagating crack tip. The study of this geometry and also the subsequent detail analysis reveals important information for real experiments. Backward reconstruction of stress field presents analysis on suitable possibilities of nodal selections as inputs into the procedure of approximation of the crack tip fields and answers the question of the necessity of application of the multi-parameter linear elastic fracture mechanics for certain fracture analyses of specimens from quasi-brittle materials. The th
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INFLUENCE OF BOUNDARY CONDITIONS ON HIGHER ORDER TERMS OF NEAR-CRACK-TIP STRESS FIELD IN A WST SPECIMEN
Veselý, V. ; Šestáková, L. ; Seitl, Stanislav
A precise description of the stress and deformation fields in a cracked body is provided using multi-parameter fracture mechanics based on the approximation of the fields by means of the Williams' power series. This paper presents a detailed analysis of the stress field in a wedge-splitting test geometry specimen aimed at the calculation of coefficients of the higher order terms (up to 14) of the Williams' expansion. The numerical study is conducted with the use of a conventional finite element package; however, for processing of the results an over-deterministic method is employed. Special attention is paid to the influence of boundary conditions of the test geometry on the values of the coefficients of the higher order terms of the Williams' series. The results are compared to data from the literature; a strong effect of the boundary conditions is observed.
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Fracture Analysis of Cube- and Cylinder-shaped WST Specimens Made of Cementitious Composites with Various Characteristic Length
Řoutil, L. ; Veselý, V. ; Seitl, Stanislav
The paper is focused on finding reasonable proportions for both cube-shaped and cylinder-shaped silicate-based composite specimens subjected to wedge-splitting tests. The analysis is conducted using finite element method code with an implemented cohesive crack model. The aspect of the material's brittleness, related to the heterogeneity of the material and described by what is termed as the characteristic length of quasi-brittle material, is accented. The results yield some recommendations for the determination of parameters of nonlinear fracture models for cementitious composites by means of wedge splitting tests of laboratory specimens of the two standard shapes.
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Analysis of stress state and failure in test specimens used for determination of fracture-mechanical parameters of quasi-brittle materials
Holušová, Táňa ; Seitl,, Stanislav (referee) ; Veselý, Václav (advisor)
The thesis is focused on a test on determination of the fracture-mechanical parameters of quasi-brittle materials, especially concrete. What is referred to as the wedge-splitting test is considered, for which a variety of shapes of notched specimen can be used. This work is exclusively focused on the cylinder-shaped specimen of diameter 150 mm and breadth of 100 mm. The test is performed virtually using Atena 2D FEM software. Progress of failure is observed during loading of the specimen for various notch lengths. The amount of energy released for the development the failure outside of the tested cross-sectional area (weakened by the notch) is quantified and the size of the fracture process zone is investigated. The described analysis is performed for several material sets witch differ in cohesive properties of the quasi-brittle material expressed via the so-called characteristic length. Suitable proportions of the test specimen are sought, in order to avoid the failure and thus also the energy dissipation outside of the specimen ligament area during the experimental tests, which shall lead to more accurate estimates of fracture-mechanical parameters of the tested material.
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Numerical prediction of parasitic energy dissipation in wedge splitting tests on concrete specimens
Veselý, V. ; Holušová, Táňa ; Seitl, Stanislav
Undesirable energy dissipation taking place during wedge-splitting tests on cementitious composites and resulting in overestimation of the values of the determined fracture-mechanical characteristics of the tested materials is investigated in this paper via numerical simulations performed using a commercial finite element method tool with an implemented cohesive crack model. The rather broad range of cohesive behaviour of the studied materials was simulated through adjustments made to the corresponding characteristic length of the composite. The parasitic amount of energy is dissipated in fracture processes around the corners of the groove for the insertion of the loading platens, as these corners introduce rather strong stress concentrators to the specimen. This amount was extracted from simulated load-displacement curves and it was discovered that the amount considerably depends on the specimen proportions but its dependence on the level of material brittleness is not so significant.
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