Zbyněk Keršner

FraMePID-3PB software for material parameter identification using fracture tests and inverse analysis

Abstract Knowledge regarding the values of fracture-mechanical parameters is critical for the virtual failure modeling of elements and structures made of concrete. A key parameter in nonlinear fracture mechanics modeling is the specific fracture energy of concrete, and its variability. Three-point bending tests on notched-beam specimens are fundamental experiments for the determination of fracture-mechanical parameters. In the present paper, two basic approaches are applied to determine fracture-mechanical parameter values from these tests: (i) the effective crack model/work-of-fracture method, and (ii) inverse analysis using artificial neural networks and stochastic simulations. First, the paper describes suitable methods for the determination of fracture-mechanical parameters. Second, the FraMePID-3PB software tool, which has been developed in order to automate the whole time consuming process of inverse analysis, is described. Finally, the verification of methodology and software is presented using two illustrative examples.

Fatigue Parameters of Cement-Based Composites with Various Types of Fibres

The paper introduces the basic fracture mechanics parameters of advanced building material – cement-based composites with various types of fibres, prepared as high performance concrete/mortar developed by ZPSV, a.s. company for production of thin-walled panels/elements. To this end three-point bend specimens with starting notch were prepared and tested under static (load–deflection diagram, effective fracture toughness) and cyclic loading (fatigue parameter – Wöhler curve). The experimentally obtained results of cement-based composites are compared and the suitability of these types composites for its application are discussed.

Complex Evaluation of Fatigue Tests Results of Plain C30/37 and C45/55 Class Concrete Specimens

The aim of this paper is to summarize basic fatigue parameter values obtained for plain C30/37 and C45/55 class concrete specimens during dynamic tests. Selected approximation curves for relative mechanical-fracture parameter values over time − compressive cube strength, modulus of elasticity, effective fracture toughness and specific fracture energy − were used to determine the most accurate fatigue parameter values corresponding to the age of specimens at the time dynamic tests were performed. Three models are used for standard description of the S−N curve: the Weibull and Gumbel models developed by Castillo et al. as well as the power law equation. Fatigue properties were obtained from three point bending tests conducted on beam specimens (100×100×400 mm nominal size) with a central edge notch (depth 10 mm).

T -STRESS VALUES DURING FRACTURE IN WEDGE SPLITTING TEST GEOMETRIES: A NUMERICAL STUDY

The paper is focused on a detailed numerical analysis of the stress field in specimens used for the wedge splitting test (WST), which is a convenient alternative to the classical bending and tensile tests within the area of determination of the fracture-mechanical parameters of quasi-brittle building materials, particularly cementitious composites. The near-crack-tip stress field in the WST specimen is described by means of constraint-based two-parameter fracture mechanics in the paper. Particular attention is paid to the influence of usual variants of boundary conditions used for this kind of testing procedure on the stress field in the cracked body. The next part of the paper aims at investigation of how much the detailed description of the near-crack-tip stress field obtained by applying the two-parameter fracture mechanics approach is then utilizable for an estimation of the size and shape of the non-linear failure zone in quasi-brittle materials, i.e. the fracture process zone (FPZ). The results obtained with regard to the near-crack-tip stress field approximation are compared with data taken from the literature. An attempt is made to exploit the estimation of the FPZ extent within the determination of fracture-mechanical characteristics of cementitious composites.

Component Wedge-Splitting/Bending Test of Notched Specimens with Various Crack-Tip Constraint Conditions: Experiments and Simulations

The paper presents a fracture-mechanics study conducted on a novel test configuration based on a combination of the wedge splitting and the bending test geometry. Four variants of this configuration differing in parameters of the specimen shape and boundary conditions were considered in the study. These selected variants exhibit significantly different stress state conditions at the crack tip, or, more generally, in the whole specimen ligament. The variation of the crack-tip stress constraint is of a crucial importance for the study because the expected differences in the fracture process zone extent and the amount of energy dissipation related to this zone is of particular interest here. An extensive experimental campaign was conducted on concrete specimens made of the same mixture. Processing and evaluation of the test data using the standard work of fracture technique was accompanied with numerical simulations by means of finite elements with implemented cohesive crack model to correlate the amount of energy dissipation with the simulated damaged zone extent. Variations of the fracture energy and also the stress/inelastic strain distribution with the change of the crack-tip constraint parameter and the notch length are observed.

Acoustic Emission from Quasi-Brittle Failure of Cementitious Composites - Experimental Measurements and Cohesive Crack Model Simulations

Conducted loading tests of plain concrete specimens are briefly introduced in the paper together with description of the performed numerical simulations of these tests in ATENA 3D software. The simulation results of concrete failure are analysed in detail and compared with the experimental results recording failure processes with the help of acoustic emission. The simulation results are in good agreement with the recorded data.

Evaluation of Concrete Fatigue Measurement Using Standard and Non-Linear Regression Model

The paper deals with the problem of estimating the S-N curve for the cement based composites under fatigue loading. Selected approximation curves for mechanical-fracture parameter values derived from specific fracture energy in time are used to determine the most accurate fatigue parameter values corresponding to the age of specimens when dynamic tests were performed. For standard description of S-N curve the power dependence and Weibull model developed by Castillo et al. are used to discuss the problem under study and the suitability of using models as an input data for finite element modeling

Fractality of Simulated Fracture

The paper is focussed on numerical simulations of the fracture of a quasi-brittle specimen due to its impact onto a fixed rigid elastic plate. The failure of the specimen after the impact is modelled in two ways based on the physical discretization of continuum: via physical discrete elements and pseudo-particles. Advantages and drawbacks of both used methods are discussed. The size distribution of the fragments of the broken specimen resulting from physical discrete element model simulation follows a power law, which indicates the ability of the numerical model to identify the fractal nature of the fracture. The pseudo-particle model, on the other side, can successfully predict the kinematics of the fragments of the specimen under impact failure.

Modelling of interfacial transition zone effect on resistance to crack propagation in fine-grained cement-based composites

In this paper, the attention is paid to investigation of the importance of the interfacial transition zone (ITZ) in selected fine-grained cement-based composites for the global fracture behaviour. This is a region of cement paste around the aggregate particles which specific features could have significant impact on the final behaviour of cement composites with a crack tip nearby this interface under applied tension. The aim of this work is to show the basic interface microstructure by scanning electron microscopy (SEM) done by MIRA3 TESCAN and to analyse the behaviour of such composite by numerical modelling. Numerical studies assume two different ITZ thicknesses taken from SEM analysis. A simplified cracked geometry (consisting of three phases – matrix, ITZ, and aggregate) is modelled by means of the finite element method with a crack terminating at the matrix–ITZ interface. ITZ’s modulus of elasticity is taken from generalized self-consistent scheme. A few conclusions are discussed based on comparison of the average values of the opening stress ahead of the crack tip with their critical values. The analyses dealing with the effect of ITZ’s properties on the stress distribution should contribute to better description of toughening mechanisms in silicate-based composites.

Engineering properties of composite materials containing waste ceramic dust from advanced hollow brick production as a partial replacement of Portland cement

Waste ceramic dust originating in the advanced hollow brick production is applied as a supplementary cementing material replacing a part of Portland cement in concrete. The measurements of mechanical and fracture-mechanical properties, water vapor and liquid water transport parameters, thermal conductivity, specific heat capacity, and freeze/thaw resistance show that the ceramic dust application does not affect negatively the properties of the analyzed concretes over the whole studied Portland cement replacement range up to 40% by mass. The achievement of such a high limit for the ceramic dust application can be attributed, besides the pozzolanic reaction being initiated already during the time period of 7 to 28 days, to the positive effect of the excess ceramic dust in the mixes with a high volume of uniformly distributed air voids. The part of the ceramic additive which cannot participate in the hydration and pozzolanic reactions due to the lack of available Ca2+ acts, apparently, as fine aggregate partially filling the voids, thus contributing to the compaction of the hardened mixes and compensating, to a certain extent, the factual decrease of the amount of binder.