Stanislav Seitl

Multi-parameter crack tip stress state description for estimation of fracture process zone extent in silicate composite WST specimens

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.

Influence of Boundary Conditions on Higher Order Terms of Near-Crack-Tip Stress Field in a WST Specimen

A precise description of the stress and deformation fields in a cracked body is provided using multi-parameter fracture mechanics based on the approximation of the fields by means of the Williams’ power series. This paper presents a detailed analysis of the stress field in a wedge-splitting test geometry specimen aimed at the calculation of coefficients of the higher order terms (up to 14) of the Williams’ expansion. The numerical study is conducted with the use of a conventional finite element package; however, for processing of the results an over-deterministic method is employed. Special attention is paid to the influence of boundary conditions of the test geometry on the values of the coefficients of the higher order terms of the Williams’ series. The results are compared to data from the literature; a strong effect of the boundary conditions is observed.

Influence of Friction on Stress and Strain Distributions in Small Punch Creep Test Models

The FEM modeling of small punch tests on miniaturized thin discs (SPT) of two heat resistant steels was performed. The FE models did represent the creep SPT, i.e. tests with constant acting force. It was shown that different values of the surface friction coefficient used in the calculations have significant impact on the calculated stress and strain state and consequently on the deformed shape of the disc. Thus, the surface friction coefficient should be considered one of the key factors for any correct correlation of SPT and uniaxial creep test results. Proper attention must be paid to the friction conditions during the long term creep deformation. An attempt to define simple approach how to relate the SPT with uniaxial tests is suggested. Some of the calculated results are compared with experiment.

Accurate Description of Near-Crack-Tip Fields for the Estimation of Inelastic Zone Extent in Quasi-Brittle Materials

A description of stress and displacement fields by means of the Williams power series using also higher-order terms is the focus of this paper. Coefficients of this series are determined via the over-deterministic method from the results of conventional finite element (FE) analysis. A study is conducted into the selection of the FE node set whose results are processed in this regression technique. Coefficients up to the twelfth term were determined with high precision. The effect of the position of the FE node set on the accuracy of the values of the higher-order term coefficients is reported.

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.

Determining fracture energy parameters of concrete from the modified compact tension test

The modified compact tension (MCT) test, though not yet recognized as a valid test for determining fracture energy of concrete, is believed to represent a plausible and suitable alternative versus other well established procedures, such as the wedge-splitting test (WST) and the three point (3PB) or four point bending (4PB) tests, due to its simplicity and low cost. The aim of the paper is twofold: Firstly, to demonstrate the necessary correspondence between the experimental MCT test setup and finite element simulations and secondly, to initiate the way of establishing the desirable conversion between the fracture energy parameter values resulting from the MCT test and the standard conventional procedures. MCT tests are carried out and compared with the numerical results from 2-D and 3-D finite element calculations using the commercial codes ABAQUS and ATENA, the latter being specifically developed for applications on concrete structures and elements. In this way, the usability of the modified compact tension test for practical purposes is confirmed.

Fatigue Crack Propagation Rate in EUROFER 97 Estimated Using Small Specimens

The proposed paper describes fatigue damage evolution in Eurofer 97 reduced activation ferritic-martensitic steel. The short crack growth study was performed on small cylindrical specimens using an MTS 880 servohydraulic machine at constant strain amplitude. Based on the fatigue crack growth data obtained and corresponding 3D finite element analysis Paris law region of the fatigue curve were estimated. The results obtained were compared with standard determination of the fatigue crack growth rate according to ASTM using CT specimens. The presented results can help to transfer experimental data measured on small specimens to large structures and vice versa.

Effect of the Load Eccentricity on Fracture Behaviour of Cementitious Materials Subjected to the Modified Compact Tension Test

Fracture properties of quasi-brittle cementitious composites are typically determined from the load–displacement response recorded during a fracture test by using the work-of-fracture method or possibly other relevant fracture models. Our contribution is focused on a set of experimental tests which are used to study the fracture behaviour on notched dog-bone-shaped specimens made of cementitious materials. These specimens are subjected to modified compact tension (ModCT) test under a specific range of eccentricity of the tensile load. This type of test generates a stress state in the specimen ligament which combines a direct tension with a defined level of bending due to eccentricity of the tensile load. Several values of relative notch length are also considered. While the crack propagates, a variety of stress states, resulting in variations in the crack-tip stress and deformation constraint, appears in the ligament zone because of the changes in the eccentricity of the applied load, which influences the fracture behaviour of the investigated specimens. The K-calibration, T-stress, CMOD and COD curves for ModCT specimens are introduced and variations of these curves with varying load eccentricity are discussed.

Numerical Simulation of Modified Compact Tension Test Depicting of Experimental Measurement by ARAMIS

The paper is focused on the determination of mechanical fracture parameters from the modified compact tension test applied to the cement-based composites. The experimental measurement was carried out by means of the ARAMIS equipment. The numerical study is performed by ATENA 2D software (based on a cohesive law for crack propagation) taking the material parameters for numerical study from the standard compression test. The experimental and numerical results are discussed and compared with the help of basic fracture parameters and Load – COD (crack open displacement) diagrams.

Selecting a Suitable Specimen Shape with Low Constraint Value for Determination of Fracture Parameters of Cementitious Composites

The stress intensity factor and the T-stress describing the near-crack-tip fields for selected specimen shapes of a test geometry based on wedge splitting and three point bending tests with several variants of boundary conditions are computed using finite element software ANSYS. The test configuration in question is expected to be a convenient alternative to classical fracture tests (especially the tensile ones) for investigation of the quasi-brittle fracture of building materials, when low constraint is requested. These specimens are investigated within the framework of two-parameter fracture mechanics; near-crack-tip stress field parameters are determined and compared with those of the wedge splitting test due to their shape similarity. The sensitivity of the values of these parameters to the boundary conditions is also shown. Suitable choice of the shape of the specimens is discussed.