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Vliv šířky zářezu na lomovou odezvu ohýbaných betonových vzorků
Veselý, V. ; Keršner, Z. ; Seitl, Stanislav ; Klusák, Jan
An influence of the test specimen’s preparation – shape and width of a stress concentrator – on the values of parameters of nonlinear fracture models of quasi-brittle cement-based composites is analyses in the paper presented. The parameters – e.g. effective crack extension, effective fracture toughness, and specific fracture energy – can be calculated from results of loading tests performed on notched three point bend specimens. The effects are investigated by means of a comparison of numerically simulated fracture process in the cracked specimen with the specimens with the double V-notch. The numerical simulations of the fracture according to the linear elastic fracture mechanics were performed by using finite element method programs (ANSYS, and FRANC2D). The influence on the load-carrying capacity and the initial compliance of the cracked specimen are introduced and discussed.
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Influence of notch width and length on crack initiation in 3PB specimens
Seitl, Stanislav ; Klusák, Jan ; Keršner, Z.
The paper presents the influence of the various notch depths for the range of starting notch width on a crack initiation in three-point bending specimens. These specimens are the most frequently used for fracture laboratory testing, especially of quasi-brittle cement-based composites. It is shown that the values of fracture toughness obtained from the tests of specimens with larger widths with various length of the starting notch can be overestimated.
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Evaluation of notch geometry effect on behaviour of fracture toughness testing specimen
Seitl, Stanislav ; Klusák, Jan ; Keršner, Z. ; Knésl, Zdeněk
The basic fracture properties of cement-based composite are ascertained on the basis of test evaluations of the specimens in suitable test configurations. Three-point bending specimens with a central starting notch are the most frequently used specimen for laboratory testing. Through models of equivalent elastic cracks the critical crack length and effective fracture toughness (or toughness) can be calculated. The influence of the geometry of the starting notch in a three-point bend specimen on the resulting values of critical applied force and fracture toughness is studied in this paper. To this end, a crack initiation at the notch tip is numerically simulated. It is shown that the values of fracture toughness obtained for specimens with larger widths of starting notch can be overestimated.
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Concrete as a two-phase material: statistical/sensitivity modelling of failure
Keršner, Z. ; Náhlík, Luboš ; Knésl, Zdeněk
Concrete is represented as a two-phase material. Behaviour of a crack with its tip on the interface between two different materials ů hardened cement paste (matrix) and aggregate(particle) ů is modelled. Input variables of the model are considered as random variables using LHS method/FREET software. Then results of statistical/sensitivity analyses of output variable ů the critical stress for the intersection of coarse aggregate particle ů are presented.
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Fracture process zone of concrete: analysis of aggregate-crack interaction
Keršner, Z. ; Náhlík, Luboš ; Knésl, Zdeněk
Concrete is modelled as a two-phase material. Behaviour of a crack with its tip on the interface between two different materials - hardened cement paste (matrix) and aggregate (particle) - is studied. In contrast to cracks in homogeneous materials the stress singularity for a crack terminating at the interface is of the type r -lambda, where the value of the stress singularity exponent, lambda, is in the interval 0 < lambda < 1. Than classical fracture mechanics theory and criterion cannot be applied. In the paper a new stability criterion is used to estimate the critical stress for the intersection of coarse aggregate particles.
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Interaction of crack and aggregate in concrete
Keršner, Z. ; Náhlík, Luboš ; Knésl, Zdeněk
Concrete is represented as a two-phase material in this paper. Behaviour of a single macrocrack approaching to the interface between hardened cement paste (matrix) and aggregate (particle) is modelled. It is shown that in the case of where Young modulus of cement paste is greater then those of aggregates, the effective fracture toughness of concrete is predominantly given by fracture properties of aggregates.
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