Pavel Hutař

Determination of the Effect of Interphase on the Fracture Toughness and Stiffness of a Particulate Polymer Composite

The main objective of this paper is the numerical investigation of the fracture behavior of particulate polymer composites. The composite was modeled as a three-phase continuum consisting of a matrix, particles, and an interphase. Its mechanical response and the interaction between a microcrack propagating in the matrix and rigid particles covered with the interphase. Computations were performed with various material properties of the matrix and interphase by using the commercial finite-element code ANSYS. All calculations were based on the linear elastic fracture mechanics.

Life Time Estimation of the Multilayer Plastic Pipes

In the contribution solution of special fracture mechanics problems connected with multi-layer plastic pipes was investigated. The assumptions of linear elastic fracture mechanics were accepted. A complex three-dimensional numerical model of multi-layer pipe system consisting of main (functional) pipe and protective layers has been suggested and numerically solved by finite element method. Two basic problems connected with lifetime expectation of multilayer pipe system have been considered and discussed, namely: question of fracture mechanics description of multilayered pipe system and corresponding measurements of the material properties. The suggested approach can help for more accurate estimation of the multilayer pipe damage.

The Effect of the Singularity Induced by the Free Surface on Fatigue Crack Growth in Thin Structures

In many industrial applications is necessary to predict fatigue lifetime of thin structures, where the stress field near the crack front have a real three-dimensional nature. Due to the existence of vertex singularity in the point where the crack front touching free surface, crack propagation in 3D structures cannot be reduced to a series of plane strain or plane stress problems along the crack front edge. The paper describes the influence of vertex singularity on the distribution of the stresses around the crack front for three-dimensional body. The distribution of the stress singularity through the thickness of the specimen gives us indication of the crack behavior in thin structures. The estimation of the thickness of the specimen where the change of singularity plays an important role on fatigue crack growth rate (in dependence on Poisson’s ratio) is carried out. The results contribute to a better understanding of the crack behavior in thin structures, and can help to more reliable estimates of their residual fatigue life.

Crack Behaviour in Polymeric Composites: The Influence of Particle Shape

In this paper polymeric particulate composites are studied (especially polypropylene (PP) matrix stuffed by rigid mineral fillers). Presently, polymeric particulate composites are frequently used in many engineering applications. The composite was modeled as a three-phase continuum – matrix, interphase and particle. The properties of the particles (size, shape) have a significant effect on the global behaviour of the composite. On the basis of fracture mechanics methodology the interaction of micro-crack propagation in the matrix filled by rigid particles covered by the interphase was analyzed. The effect of the composite structure on their mechanical properties is studied here from the theoretical point of view.

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.

Numerical lifetime prediction of polymer pipes taking into account residual stress

In this paper a methodology for assessment of residual stress effects on crack behaviour in the polymer pipe is developed. For simplicitys sake, a linear distribution of residual stresses across the pipe wall is assumed. Linear elastic fracture mechanics is used for the fracture mechanics analysis of the cracked pipe. An approximate relation for the stress intensity factor estimation for a crack in a polymer pipe, with residual stress taken into account is suggested and discussed. The methodology presented can be helpful for a rapid lifetime estimation of polyolefin pipelines.

T -STRESS VALUES DURING FRACTURE IN WEDGE SPLITTING TEST GEOMETRIES: A NUMERICAL STUDY

The paper is focused on a detailed numerical analysis of the stress field in specimens used for the wedge splitting test (WST), which is a convenient alternative to the classical bending and tensile tests within the area of determination of the fracture-mechanical parameters 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. Particular attention is paid to the influence of usual variants of boundary conditions used for this kind of testing procedure on the stress field in the cracked body. The next part of the paper aims at investigation of how much the detailed description of the near-crack-tip stress field obtained by applying the two-parameter fracture mechanics approach is then utilizable for an estimation of the size and shape of the non-linear failure zone in quasi-brittle materials, i.e. the fracture process zone (FPZ). The results obtained with regard to the near-crack-tip stress field approximation are compared with data taken from the literature. An attempt is made to exploit the estimation of the FPZ extent within the determination of fracture-mechanical characteristics of cementitious composites.

Sensitivity of Fatigue Crack Growth Data to Specimen Geometry

The formulations of fatigue crack growth prediction are still mostly based on phenomenological models. A commonly used formula in the field of high cycle fatigue is the Paris- Erdogan law. For given experimental conditions (such as temperature, stress ratio or environmental conditions) the parameters C and m have to be experimentally determined and considered as material constants. Thus, for a given material, the fatigue crack growth rate (FCGR) depends only on the applied range of the stress intensity factor. In a threshold region a significant shift in the data of the fatigue crack propagation rate can be observed. The shift is induced by different test specimen geometry. To analyses it the authors will present their own laboratory fatigue crack growth rate test data measured on two different specimens with different levels of constraint and for different steels. It is demonstrated that fatigue characteristics (i.e. C, m and Kth) obtained from different specimen geometries are not only properties of the materials but depends on the specimen geometry.

Interaction of Creep and High Cycle Fatigue of IN 713LC Superalloy

The study deals with the interaction of creep and high cycle fatigue of cast polycrystalline nickel-based superalloy IN 713LC at high temperatures. Previous works indicated that creep lifetime of superalloy structures was un-affected or even slightly increased in the cases with superimposed vibrations. The reason for this behaviour was not well described up to now. Therefore, set of fatigue tests was conducted at high mean stresses level to observe this phenomenon. The mean stress was kept constant while the stress amplitudes were selected in order to measure wide range of conditions from pure creep to pure fatigue. Fractographic analysis by scanning electron microscopy (SEM) was done with the aim to identify governing damage mechanisms for particular test conditions as a preliminary evaluation of conducted tests.

Influence of Initial Inclined Surface Crack on Estimated Residual Fatigue Lifetime of Railway Axle

Railway axles are subjected to cyclic loading which can lead to fatigue failure. For safe operation of railway axles a damage tolerance approach taking into account a possible defect on railway axle surface is often required. The contribution deals with an estimation of residual fatigue lifetime of railway axle with initial inclined surface crack. 3D numerical model of inclined semi-elliptical surface crack in railway axle was developed and its curved propagation through the axle was simulated by finite element method. Presence of press-fitted wheel in the vicinity of initial crack was taken into account. A typical loading spectrum of railway axle was considered and residual fatigue lifetime was estimated by NASGRO approach. Material properties of typical axle steel EA4T were considered in numerical calculations and lifetime estimation.