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Numerical Modelling of Fatigue Crack Closure
Oplt, Tomáš ; Růžička, Milan (referee) ; Horníková, Jana (referee) ; Hutař, Pavel (advisor)
This Ph.D. thesis was written under the supervision of Assoc. prof. Pavel Hutař, Ph.D., and Assoc. prof. Luboš Náhlík, Ph.D. The thesis is focused on the effect of plasticity induced crack closure, its characteristic and ways of numerical modelling. Premature fatigue crack closure has a significant effect on the fatigue crack propagation rate and therefore on the residual lifetime of a structure. A three-dimensional numerical model allows a detailed look at the stress and strain distribution along the crack front, and particularly it allows a local description of parameters along the crack front which governs the fatigue crack propagation rate. In the first part of the thesis, the study is focused on the influence of a singular stress field at the vicinity of the free surface on the crack front curvature without crack closure being involved. In the second part, a numerical model in 2D of plasticity induced crack closure was created and verified by experimental results. In the final part, a 3D numerical model is used to describe the influence of the crack closure on its fatigue propagation rate and explains typical crack front curvature. The suggested technique allows quantitative accuracy improvement of numerical simulation of the fatigue crack propagation and therefore, more reliable estimation of the residual lifetime of the cracked structure.
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Short fatigue crack propagation description
Trávníček, Lukáš ; Náhlík, Luboš (referee) ; Hutař, Pavel (advisor)
The presented master’s thesis deals with description of short fatigue cracks and can be divided to the several parts. In the first part, theoretical background of fatigue of materials and fracture mechanics parameters which can describe fatigue cracks is presented. Following part describes how to determine fracture mechanics parameters by finite elements method. Due to the shape of propagated fatigue crack, numerical model was established as a three-dimensional. Results obtained based on this model, was compared with literature with very good agreement. Than real shape of propagated short crack was used for description of the experimental data. It was shown, that plastic part of J-integral is suitable parameter for description of the short cracks.
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Influence of Structure Directionality on Fatigue Properties of Formed Al Alloy.
Jíša, David ; Mazal, Pavel (referee) ; Liškutín, Petr (advisor)
The main goal of this diploma thesis is the examination of the influence of structure directionality on fatigue properties of formed aluminium alloy 6082/T6. The main attention is focused on the study of the influence of structure directionality on kinetics of short fatigue cracks growth. The measurement of short fatigue cracks growth was performed on cylindrical samples. The samples were made in two different directions; one parallel with the forming direction and second perpendicular to the forming direction. Servo hydraulic machine MTS 880 was used for the cyclic loading. The samples were cycled at two different constant stress amplitudes. Cyclic loading was systematically interrupted in order to measure the length of short cracks by a light microscope. Tensile tests, measuring of cycling hardening-softening curves, observation of microstructure, observation of surface relief, measuring of microhardness and fractographical analysis of fracture surfaces were used for further examination of the influence of the structure directionality. Some of these measured characteristics did not show any influence of the structure directionality (microhardness, fatigue life curve, Young modulus). In other cases is this influence measurable, however insignificant (yield stress, ultimate stress, cyclic hardening-softening curves and kinetics of short fatigue cracks growth). It can be summarised that the material, though the directionality of its microstructure is apparent, shows relatively isotropic mechanical behaviour.
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Numerical Modelling of Fatigue Crack Closure
Oplt, Tomáš ; Růžička, Milan (referee) ; Horníková, Jana (referee) ; Hutař, Pavel (advisor)
This Ph.D. thesis was written under the supervision of Assoc. prof. Pavel Hutař, Ph.D., and Assoc. prof. Luboš Náhlík, Ph.D. The thesis is focused on the effect of plasticity induced crack closure, its characteristic and ways of numerical modelling. Premature fatigue crack closure has a significant effect on the fatigue crack propagation rate and therefore on the residual lifetime of a structure. A three-dimensional numerical model allows a detailed look at the stress and strain distribution along the crack front, and particularly it allows a local description of parameters along the crack front which governs the fatigue crack propagation rate. In the first part of the thesis, the study is focused on the influence of a singular stress field at the vicinity of the free surface on the crack front curvature without crack closure being involved. In the second part, a numerical model in 2D of plasticity induced crack closure was created and verified by experimental results. In the final part, a 3D numerical model is used to describe the influence of the crack closure on its fatigue propagation rate and explains typical crack front curvature. The suggested technique allows quantitative accuracy improvement of numerical simulation of the fatigue crack propagation and therefore, more reliable estimation of the residual lifetime of the cracked structure.
|
|
|
Short fatigue crack propagation description
Trávníček, Lukáš ; Náhlík, Luboš (referee) ; Hutař, Pavel (advisor)
The presented master’s thesis deals with description of short fatigue cracks and can be divided to the several parts. In the first part, theoretical background of fatigue of materials and fracture mechanics parameters which can describe fatigue cracks is presented. Following part describes how to determine fracture mechanics parameters by finite elements method. Due to the shape of propagated fatigue crack, numerical model was established as a three-dimensional. Results obtained based on this model, was compared with literature with very good agreement. Than real shape of propagated short crack was used for description of the experimental data. It was shown, that plastic part of J-integral is suitable parameter for description of the short cracks.
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|
|
Influence of Structure Directionality on Fatigue Properties of Formed Al Alloy.
Jíša, David ; Mazal, Pavel (referee) ; Liškutín, Petr (advisor)
The main goal of this diploma thesis is the examination of the influence of structure directionality on fatigue properties of formed aluminium alloy 6082/T6. The main attention is focused on the study of the influence of structure directionality on kinetics of short fatigue cracks growth. The measurement of short fatigue cracks growth was performed on cylindrical samples. The samples were made in two different directions; one parallel with the forming direction and second perpendicular to the forming direction. Servo hydraulic machine MTS 880 was used for the cyclic loading. The samples were cycled at two different constant stress amplitudes. Cyclic loading was systematically interrupted in order to measure the length of short cracks by a light microscope. Tensile tests, measuring of cycling hardening-softening curves, observation of microstructure, observation of surface relief, measuring of microhardness and fractographical analysis of fracture surfaces were used for further examination of the influence of the structure directionality. Some of these measured characteristics did not show any influence of the structure directionality (microhardness, fatigue life curve, Young modulus). In other cases is this influence measurable, however insignificant (yield stress, ultimate stress, cyclic hardening-softening curves and kinetics of short fatigue cracks growth). It can be summarised that the material, though the directionality of its microstructure is apparent, shows relatively isotropic mechanical behaviour.
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