Zbynek Kersner

Parallelization of lattice modelling for estimation of fracture process zone extent in cementitious composites

This paper is focused on the verification and validation of the developing technique for estimation of the extent (the size and shape) of the fracture process zone (FPZ) in quasi-brittle silicate-based specimens/structures during failure process (termed the ReFraPro -Reconstruction of Fracture Process - technique). Most experimental data published in the literature are incomplete for its sound validation; therefore, numerical simulations by means of physical discretization of continuum are used for supplementing the verification of the technique. A discrete spring network/lattice particle-type model formulated as a nonlinear dynamical system is utilized. Parallelized implementation within the CUDA environment helps to decrease the computational cost of the simulations to an admissible level. The conducted analysis demonstrates satisfactory agreement of the size and shape of the FPZ reconstructed by the ReFraPro technique with both the data of the performed simulations and selected experimental data from literature.

X-ray dynamic observation of the evolution of the fracture process zone in a quasi-brittle specimen

The aim of this work is the evaluation of the fracture process zone while loading a quasi-brittle concrete compound. The regularly used optical observation of the specimen surface does not provide accurate information regarding the fracture zone shape, particularly when this zone is tunnelled inside of the specimen body. Therefore, X-ray dynamic defectoscopy and computed tomography were employed as tools for an extended investigation of process zone evolution. A notched specimen manufactured from silicate composite was subjected to the three-point bending test in a special table-top loading device. On-line radiographic observation of the process zone during the loading experiment serves for overall evaluation, while a tomographic measurement - which is conducted during temporal loading interruption - provides information about the spatial distribution of the newly developed cracks.

Effect of Fibre Type in Concrete on Crack Initiation

Concrete is a traditionally used building material and its crack resistance can be improved by the addition of different types of fibres. Wedge splitting tests on specimens fabricated from five types of concrete mixture (six identical specimens from each mixture) were used to quantify the mechanical fracture parameters of these materials – a reference specimen without fibres, two others with steel fibres (Dramix, Tabix), and a final two with synthetic fibres (Enduro, Strux). Vertical load versus crack mouth opening displacement (P–CMOD) diagrams were recorded during the tests. The data points creating these diagrams were filtered first, subsequently processed and then evaluated using the double-K fracture model. Thus, values were obtained for e.g. Young’s modulus and fracture toughness. This paper is focused on quantification of the effect of the fibre type used in concrete on crack initiation, which corresponds to the beginning of stable crack growth in this sort of composite material.

Determine Parameters for Double-K Model at Three-Point Bending by Application of Acoustic Emission Method

As concrete is one of the most popular building materials, it is important to know its basic properties and behaviour especially at loading. When cracks occur in concrete, released fracture energy will be proportionally transformed into the energy contained in acoustic emission. According to this physical phenomenon, acoustic emission technique provides an effective monitoring method for fracture process of concrete through generated acoustic emission. However, such monitoring is limited in qualitative evaluation of fracture process in most occasions. Quantitative interpretation of fracture process is difficult to accomplish by simply acquiring the amount of acoustic emission generated or by conducting parameter-based acoustic emission analysis. This paper investigates the use of the double-K fracture model and acoustic emission for prediction of load at the beginning of stable crack propagation in three-point bending tests of concrete specimens. Double-K model combines the concept of cohesive forces acting on the effective crack increment with a criterion based on the stress intensity factor.

Virtual reliability control of precast prestressed concrete elements using parameters identification and statistical simulation

An approach for statistical and reliability analyses of concrete structures is presented. It describes the virtual simulation used on the way from assessment of experimental results to reliability analysis. The whole approach is based on randomization of nonlinear fracture mechanics finite element analysis of concrete structures. Efficient techniques of both nonlinear numerical analysis of concrete structures and stochastic simulation methods of Monte Carlo type have been combined in order to offer an advanced tool for assessment of realistic behaviour of concrete structures. The stochastic response requires repeated analyses of the structure with stochastic input parameters, which reflects randomness and uncertainties in the input values. The procedure utilizes statistical simulation of Monte Carlo type also for preparation of training set of artificial neural network used for fracture mechanical parameters identification. The application of approach to virtual statistical simulation of shear capacity of precast prestressed concrete elements SPIROLL is presented. Design shear capacities calculated according to design codes and alternative procedures are compared with simulation of reality representing virtual reliability control of elements production.

Pilot evaluation of a fracture process zone in a modified compact tension specimen by X-ray tomography

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.

Evaluation of Fracture Tests of Concrete Specimens via Advanced Tool for Experimental Data Processing

Cement-based composites are traditionally used building materials. Concrete is the basic representative of this type of materials which exhibit the so called quasi-brittle response. Quantification of mechanical fracture parameters is performed using fracture tests on specimens with a stress concentrator. Load versus crack mouth opening displacement (P–CMOD) diagrams are recorded during these tests. In order to correctly evaluate these diagrams, an advanced own developed software tool was used for the data filtering and appropriate modifications. In this paper, the programmed Java utility is generally introduced and its utilization demonstrated on the set of recorded P–CMOD diagrams, which are further evaluated using Double-K fracture model.