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Deformation and stress fields at the front of shear cracks with complicated geometry
Žák, Stanislav ; Hutař,, Pavel (oponent) ; Kunz,, Jiří (oponent) ; Horníková, Jana (vedoucí práce)
This thesis summarizes the extend of research on the shear-modes loaded cracks done in the period of four years of doctoral studies of Ing. Stanislav Žák (Central European Institute of Technology, Brno University of Technology). The actual research aims to improve the scientific view on advanced fracture mechanics, specifically for the shear loading of cracks combined with cases when the geometry of the crack flanks and front doesn’t match with widely used and standardized models. This means proposal of new modelling approaches of geometrically complicated cracks and description of changes in the local stress intensity factors along such cracks. Initial part of the text presents a review of current approaches in the fracture mechanics and numerical methods used in further research. This part emphasizes theoretical methods in general fracture mechanics and also specific shear modes loading, particularly use of the two types of specimens – a special specimen dedicated to mode II loading (compact-tension-shear specimen) with a possibility of mixed-mode I+II loading, and widely used cylindrical specimen with circumferential notch and crack which is able to comprehend mode II and III loading. The second, more extensive part of this thesis concentrates on actual evaluation of conditions along the real-like (tortuous) crack fronts for two types of specimens under the remote shear mode loading. Theoretical solution of fracture parameters for both mentioned specimens is compared to the experimental results obtained in the frame of the follow-up projects. The cylindrical specimen is reviewed from two points of view. The first one is solely about the numerical methods and the simplification of complex numerical models. In the second one, the influence of small asperities along the crack front on local k2 inducement for specimens under the remote mode III loading is observed. The results are directly connected to the experimental measurement of mode III fatigue thresholds values for metallic materials where the local mode II advances of crack front are quantified. On fractures in compact-tension-shear specimen various configurations of crack front and flanks tortuosity are investigated. The global decrease of KII with increasing crack tortuosity is observed and used for correction of mode II fatigue threshold values of metallic materials. Furthermore, the conditions along one particular asperity are investigated. The results confirm that the shear loaded cracks (which were not described suficiently so far) are affected by the crack front and flanks microstructure. Therefore, this work extends the knowledge on roughness-induced shielding of shear loaded cracks.
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Deformation and stress fields at the front of shear cracks with complicated geometry
Žák, Stanislav ; Hutař,, Pavel (oponent) ; Kunz,, Jiří (oponent) ; Horníková, Jana (vedoucí práce)
This thesis summarizes the extend of research on the shear-modes loaded cracks done in the period of four years of doctoral studies of Ing. Stanislav Žák (Central European Institute of Technology, Brno University of Technology). The actual research aims to improve the scientific view on advanced fracture mechanics, specifically for the shear loading of cracks combined with cases when the geometry of the crack flanks and front doesn’t match with widely used and standardized models. This means proposal of new modelling approaches of geometrically complicated cracks and description of changes in the local stress intensity factors along such cracks. Initial part of the text presents a review of current approaches in the fracture mechanics and numerical methods used in further research. This part emphasizes theoretical methods in general fracture mechanics and also specific shear modes loading, particularly use of the two types of specimens – a special specimen dedicated to mode II loading (compact-tension-shear specimen) with a possibility of mixed-mode I+II loading, and widely used cylindrical specimen with circumferential notch and crack which is able to comprehend mode II and III loading. The second, more extensive part of this thesis concentrates on actual evaluation of conditions along the real-like (tortuous) crack fronts for two types of specimens under the remote shear mode loading. Theoretical solution of fracture parameters for both mentioned specimens is compared to the experimental results obtained in the frame of the follow-up projects. The cylindrical specimen is reviewed from two points of view. The first one is solely about the numerical methods and the simplification of complex numerical models. In the second one, the influence of small asperities along the crack front on local k2 inducement for specimens under the remote mode III loading is observed. The results are directly connected to the experimental measurement of mode III fatigue thresholds values for metallic materials where the local mode II advances of crack front are quantified. On fractures in compact-tension-shear specimen various configurations of crack front and flanks tortuosity are investigated. The global decrease of KII with increasing crack tortuosity is observed and used for correction of mode II fatigue threshold values of metallic materials. Furthermore, the conditions along one particular asperity are investigated. The results confirm that the shear loaded cracks (which were not described suficiently so far) are affected by the crack front and flanks microstructure. Therefore, this work extends the knowledge on roughness-induced shielding of shear loaded cracks.
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Comparison of calibration functions for short edge cracks under selected loads
Seitl, Stanislav ; Miarka, Petr ; Malíková, Lucie ; Krejsa, M.
Attention to the fatigue cracks in steel structures and bridges has been paid for long time. In spite to efforts to eliminate the creation and propagation of fatigue cracks throughout the designed service life, cracks are still revealed during inspections. Note, that depending on location of initial crack, the crack may propagate from the edge or from the surface. The theoretical model of fatigue crack progression is based on linear fracture mechanics. Steel specimens are subjected to various load (tension, three- and four-point bending, pure bending etc.). The calibration functions for short edge cracks are compared for various load and the discrepancies are discussed.
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