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Lomová analýza epoxido-hliníkových spojů vystavených teplotnímu a strukturnímu zatížení
Korbel, J. ; Klusák, Jan ; Knésl, Zdeněk
The paper presents a fracture mechanics approach to estimation of critical load (thermal or structural) of adhesive joints. Attention is focused on failure of test specimens prepared by gluing aluminum plates with a commercial thermosetting adhesive. Residual stresses caused by differences of thermal expansion of the adhesive and adherent are also examined. Classical fracture mechanics does not deal with behavior of a bi-material interface, and therefore an approach based on a generalized stress intensity factor and generalized fracture toughness is employed.
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Influence of stress concentrator geometry on fracture parameters values of quasi-brittle materials
Frantík, P. ; Keršner, Z. ; Klusák, Jan ; Řoutil, L. ; Seitl, Stanislav ; Veselý, V.
To assess the reliability of building structures, values of fracture mechanical properties of their materials have to be known. In the case of cement-based composite types the basic parameters are ascertained on the basis of evaluation of the response of specimens conducted to a test in a suitable testing configuration. Three-point bending specimens with a central starting notch are the most frequently used specimens for laboratory testing. The influence of the stress concentrator – starting central edge notch – in a three point bend specimen on the resulting values of fracture toughness was studied in author’s papers: a crack initiation at the notch tip was numerically simulated. It was shown that the values of fracture toughness obtained for specimens with larger widths of starting notch can be overestimated.
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Influence of vertical compresive force on level of constraint in cubical specimen for wedge splitting test
Seitl, Stanislav ; Klusák, Jan ; Dymáček, Petr ; Řoutil, L. ; Veselý, V.
The subject of the contribution is to gain new aspects of knowledge about the fracture behaviour of cementitious building materials by the wedge splitting test (WST). The particularly, the paper investigates the influence of the vertical compressive force Pv, that represents a reaction of the bottom support of the specimen, on the fracture parameters via tools of the two-parameter fracture mechanics. The effects of the vertical compressive force, Pv, on the plastic and fracture process zones are examined.
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Analýza porušení systému matrice – vlákno
Klusák, Jan ; Knésl, Zdeněk
Conditions of damage initiation in fibre composite materials are analysed in the paper. The main objective of the presentation is to quantify the influence of fibre-matrix material mismatch on failure critical stress under which a crack is initiated in the vicinity of a free fibre end. The considerations are derived from the knowledge of the stress state caused by the existence of the bi-material interface. The step change of material properties in the interface leads to singular peak of stresses, which may be responsible for crack initiation and consequently for failure of the structure. The singular stress state near the interface is analysed by means of generalized linear elastic fracture mechanics and conditions of crack initiation are assessed. On the basis of the results the question of interfacial debonding between the fibre and matrix is discussed.
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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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Estimation of generalized stress intensity factors by means of mean value of strain energy density factor
Klusák, Jan
The contribution deals with bi-material wedges whose stress field is characterized by two stress singular terms. The exponents of stress singularity are generally in the interval (0; 1). For these cases each of the singularities covers both normal and shear mode of loading. Because of the inherently combined loading mode, it is advantageous to utilize the strain energy density concept. A mean value of the strain energy density factor taken from a numerical solution of particular materials junction is compared to its analytical representation to ascertain values H1 and H2. These values enter to stability criterion to indicate if crack is initiated in the wedge tip.
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