ABSTRACT. An analysis focused on capturing the phenomenon of quasibrittle fracture is presented. Selected parameters relevant for quasi-brittle fracture are evaluated and an assessment of their dependence on the size and shape of the test specimen is studied. Determination of these fracture characteristics is based on the records of the fracture tests on notched specimens, particularly from recorded loading diagrams. A method of separation of the energy amounts released for the propagation of the (effective) crack and that dissipated within the volume of a large nonlinear zone at the crack tip -the fracture process zone -is introduced and tested on selected data from experimental campaigns published in the literature. The work is accompanied with own conducted numerical simulations using commercial finite element code with implemented cohesive crack model. Results from three-point bending tests on specimens of different sizes and relative notch lengths are taken into account in this study. The proposed model has only two parameters whose values are constant for all specimen sizes and notch lengths.

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In the case of quasi-brittle material, there is a zone of non-linear behaving material, near the crack tip. A major part of this zone forms a socalled fracture process zone (FPZ), where mechanisms of material toughening take place. The main idea is to estimate the size of the fracture process zone under various types of load by using X-ray tomography. The estimation of the zone is supported by the theory of linear elastic fracture mechanics (LEFM) that could be a limit case of quasi-brittle mechanics. The size of the zone envelop is provided in X-ray snaps and compared with the theoretical size from LEFM.

The paper presents an analysis with an attempt to capture the phenomenon of quasi-brittle fracture based on the record of the fracture test on a notched specimen via separation the energy amounts released for the crack advance and dissipated within the volume of the sizeable nonlinear zone at the crack tip – the fracture process zone (FPZ). The described approach is tested on selected data of published experimental campaigns accompanied with own conducted numerical simulations.

The paper discusses creation of numerical models of the modified Compact Tension (CT) test configuration on specimens made of fine-grained cement composite. Numerical models of this test configuration are used for predictions of crack initiation and damage propagation and are important for the further evaluation of fracture parameters. To assemble the numerical models, ATENA FEM software was used. In this software, fracture of the structure/specimen caused by cracks, their initiation and progressive propagation throughout the loading process, can be modelled. In case of this study the material model for concrete based on cohesive law approach was used. The analysis is focused on evaluation of 2D and 3D numerical models created with real material properties obtained from experimental data.