Two-parameter fracture mechanical analysis of a near-crack-tip stress field in wedge splitting test specimens (original) (raw)
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Applied and Computational Mechanics, 2009
The paper is focused on a detailed numerical analysis of the stress field in specimens used for the wedge splitting test (WST) which is an alternative to the classical fracture tests (bending, tensile) within the fracture mechanics 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. Different levels of constraint in the vicinity of the crack tip during fracture process through the specimen ligament are characterized by means of the T-stress. Two basic shapes of WST specimen-the cube-shaped and the cylinder-shaped one-are investigated and the determined near-crack-tip stress field parameters are compared to those of compact tension (CT) specimens according to the ASTM standard for classical and round geometry. Particular attention is paid to the effect of the compressive component of the loading force (complementing the splitting force) acting on the loaded side of the specimen and its reaction from the opposite part of the specimen on the stress field in the cracked body. Several variants of boundary conditions on the bottom side of the specimen used for this kind of testing procedure are also considered. The problem is solved numerically by means of the finite element method and results are compared with data taken from the literature.
Wedge Splitting Test: Displacement Field Analysis by Multi-parameter Fracture Mechanics
Transactions of the VŠB – Technical University of Ostrava, Civil Engineering Series
Multi-parameter fracture mechanics is nowadays quite extensively applied when cracked structures/specimens are investigated. The reason was that it has been shown that it can be helpful and bring results that are more accurate when for describing of fracture processes a larger region around the crack tip is used. This can be typical for material like concrete or other materials with quasi-brittle behaviour. Various relative crack length configurations were chosen in order to investigate the importance of the higher-order terms of the Williams expansion (WE) on the crack-tip stress field distribution in Wedge splitting test specimen. The higher-order terms were calculated by means of the over-deterministic method from displacements of nodes around the crack tip obtained by a finite element analysis in different radial distances from the crack tip. The effect of the constraint level (second member of WE) was investigated. Although the third and higher terms of the Williams series are very often neglected, their influence on the opening stress values was investigated and discussed.
Proceedings of the 9th International Conference on Fracture Mechanics of Concrete and Concrete Structures, 2016
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. 1 INTRODUCTION, MOTIVATION A novel method for evaluation of fracturemechanical parameters of quasi-brittle cementitious composites was introduced recently [1] by the Brno University of Technology (BUT) partner of the joint research presented in this paper. The method should eliminate drawbacks of most of the established models for determination of those characteristics, which is the dependence of values the fracture properties determined from fracture tests on laboratory specimens on the specimen size, geometry and its boundaries. Capturing these effects shall be achieved via a detailed incorporation of the fracture process zone (FPZ) existence into the evaluation
Determination of Fracture Paramaters of Effective Crack Model by Wedge-Splitting Test
2018
Although the cracked beams have been widely utilized in fracture mechanics of concrete, there have been some advantages of the cubical/cylindrical specimens such as compactness and lightness. In the present work, the wedge-split-tension tests on cubical specimens with different cement contents and water/cement ratios were initially performed for the effective crack model. Finally, some relationships based on regression between the fracture parameters and the strength properties of concrete were derived. The results of the split-tension cube tests look viable and very promising.
Theoretical and Applied Fracture Mechanics, 1995
The wedge splitting test is performed on notched shaped specimens that enables the determination of energies for large fracture surfaces and material exhibiting brittle behaviour. A stability condition is deduced and found to depend on the Young's modulus and the R-curve behaviour. The latter is defined by the fracture toughness KR and fracture energy R c, both of which depend on the crack length. A stable crack propagation is enhanced by high ratios of K'R/K R and RJK 2. The wedge loading tends to behave like raising the rigidity of the testing machine. The results are applied on an example with a special geometry.
Avoiding fracture instability in wedge splitting tests by means of numerical simulations
2017
In this paper, unstable fracture propagation obtained in a in-house performed experimentalWedge Splitting Test (WST) is simulated by means of the FEM and fracture- based zero-thickness interface elements. In order to obtain a specimen geometry suitable for a stable WST without modifying the remaining significant parameters of the test (machine stiffness and control parameter), additional simulations were performed varying the length of the specimen notch, until a load-COD (Crack Opening Displacement) curve without snap-back was obtained. Finally, a new experimental WST with the modified geometry was carried out leading to a stable load-COD curve. In the simulations, elastic continuum elements were used to represent the rock, the steel loading plates and the test- ing machine compliance via an “equivalent spring”, whereas interface elements were used for the notch and along the potential crack path. The interface elements representing the notch were equipped with linear elastic const...
Frattura ed Integrità Strutturale, 2016
The fracture mechanical properties of silicate based materials are determined from various fracture mechanicals tests, e.g. three-or four-point bending test, wedge splitting test, modified compact tension test etc. For evaluation of the parameters, knowledge about the calibration and compliance functions is required. Therefore, in this paper, the compliance and calibration curves for a novel test geometry based on combination of the wedge splitting test and three-point bending test are introduced. These selected variants exhibit significantly various stress state conditions at the crack tip, or, more generally, in the whole specimen ligament. The calibration and compliance curves are compared and used for evaluation of the data from pilot experimental measurement.
Multi_parameter crack tip stress state description for estimation of fracture p.PDF
For wedge splitting test specimens, the stress and displacement fields both in the vicinity and also in larger distance from the crack tip are investigated by means of numerical methods. Several variants of boundary conditions were modeled. The stress intensity factor K, T-stress and even higher-order terms of William series were determined and subsequently utilized for analytical approximation of the stress field. A good fit between the analytical and numerical solution in dependence on the distance from the crack tip was shown, compared and discussed. Presented approach is considered as suitable for estimation of the fracture process zone extent in silicate composite materials.
Review of the splitting-test standards from a fracture mechanics point of view
Cement and Concrete Research, 2001
This article analyzes by means of fracture mechanics the current splitting-test standards for concrete. The cohesive crack model, which has shown its utility in modeling the fracture of concrete and other cementitious materials, has been used to assess the effect of the specimen size, the specimen shape and the width of the load-bearing strips on the conventional splitting tensile strength, f st. The results show that, within the ranges recommended in the standards, the values of the splitting tensile strength can differ by up to 40%, and, consequently, f st can hardly be assumed to be a material property. Empirical formulae for concretes with different compressive strength (10 ± 80 MPa) and maximum aggregate size (8 ± 32 mm) have been used to show that f st is nearly specimen independent only for certain compositions, such as highstrength concretes, or when the aggregate size is under 16 mm. New closed-form expressions for f st are given in this paper to incorporate the effect of material properties, and some recommendations are drawn to minimize the influence of the width of the load-bearing strips.
Impact of specific fracture energy investigated in front of the crack tip of t.PDF
Presented study is focused on the analysis of the dependence of the specific fracture energy value on the assumed work of fracture in threepoint bending tests. Specimens of different sizes and relative notch lengths are assumed in this study, in order to take into account the size effect. The three-point bending test of cracked specimens is simulated numerically by means of commercial software based on the finite element method with implemented cohesive crack model. Three levels of the specific fracture energy are considered.