engleski

NUMERICAL AND EXPERIMENTAL STUDY OF CONCRETE FRACTURE UNDER DYNAMIC LOADING

The behaviour of concrete structures is known to be strongly influenced by the loading rate. However, the knowledge with regards to the response of concrete subjected to dynamic loading is still incomplete. Numerical and experimental studies show that with increase of loading rate there is increase of resistance, change of failure mode, crack pattern and crack propagation velocity, while the influence of the loading (strain) rate on the tensile strength and fracture energy of concrete is not yet fully understood. Therefore, it is one of the major subjects of intensive investigation in the scientific community over many years. The research reported in this thesis is aimed to better understand dynamic fracture behaviour of concrete and the phenomenon of progressive increase of tensile strength and fracture energy, experimentally observed at high loading rates. Both experimental and numerical approaches have been employed in order to achieve the same. The experimental investigation performed on L-specimens subjected to dynamic loading has confirmed that loading rate plays significant role in the crack propagation direction of such concrete structures. For quasistatic load, the crack tends to propagate horizontally, perpendicular to the loading direction, while with increase of the loading rate the crack propagation tends to get vertical, parallel to the loading direction. A second aim of this study is to examine the validity of explicit 3D FE code for predicting the complex phenomena related to dynamic fracture of concrete according to the results of the experimental investigations. To this end, experiments on L- shaped concrete specimens under different loading rates are numerically simulated. Additionally, to clarify the correct evaluation procedure for tensile strength and fracture energy under dynamic loading, the experiments reported in literature on dynamically impacted notched concrete beams are also numerically investigated. The comparison between the results of experiments and numerical analyses show that presented relatively simple modeling approach based on continuum mechanics, rate dependent microplane model and standard finite elements is capable to realistically predict the performance of concrete under dynamic loading including the complex phenomena related to high loading rates such as rate dependent resistance, rate dependent failure mode, crack branching and crack velocity. Finally, it is found that progressive increase in dynamic strength and fracture energy is controlled mainly by inertial effect and it should be interpreted ii as structural rather than a material property. Therefore, experimental data on dynamic fracture of concrete has to be taken with caution.

concrete fracture; dynamic load; rate sensitivity; experiments; finite elements; microplane model; failure mode; crack branching; crack velocity; tensile strength; fracture energy

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