Papers by Author: Martin Ševčík

Abstract: In the paper the polymer pipes loaded by internal pressure with additional point load on the external surface are studied using numerical methods. It is shown that the additional external loading can significantly influence residual lifetime of the pressured pipe. In this study the effect of the pipe geometry is considered. The shape of the propagating crack is estimated based on a special algorithm and the stress intensity factor is evaluated by direct method. The relation between crack size and a geometric function YasYas for specific material and geometrical properties is found. The results of this paper should contribute to better understanding of the real pipe behavior and to prevent unexpected failure of the pressured pipe system due to non-homogenous distribution of the soil load.

Abstract: Fracture behaviour of a three-layer polymer pipe subjected to nonhomogenous distribution of external pressure induced by soil embedding is studied in this paper. Both long term and additional short term loading is considered. Such loading induces tensile stresses in the inner pipe wall which can lead to crack initiation and further slow crack propagation. The material interface between a protective layer and the base pipe can contribute to crack deceleration and can prolong the residual lifetime of the pipe. The paper presents three-dimensional numerical analysis of a commercial three-layer pipe containing an internal semi-elliptical crack. The effect of soil load on the fracture behaviour of the cracked pipe is quantified and discussed.

Abstract: As a result of the production process, there are axial and tangential residual stresses present in pressure pipes made of polymer materials such as polyethylene or polypropylene. The residual stress magnitude and distribution have a significant influence on the pipe lifetime. In this contribution the results from experiments focused on determining the tangential residual stress distribution in the walls of polypropylene pipes of different dimensions are compared. The experimental method used involves measuring the deformation of ring shaped specimens that were slit in the axial direction. Measured deformation of the ring specimen is a result of the tangential and axial stress superposition. However, the effect of the axial residual stress depends on the specimen axial dimension and tangential residual stress estimated basing on experimental data should be corrected according to axial dimension of the specimen used. The correction suggested in this article is determined based on three-dimensional FEM simulations of the experiment.

Abstract: The composites are widely used due to their advanced mechanical properties. Particulate composites with polymer matrix were studied with regard to micro-crack propagation in the matrix. A three phase numerical model was created. Linear elastic material properties of particle and matrix were taken from experiment. Using numerical simulations (based on the finite element method) the influence of interphase properties on micro-crack propagation was studied. The results of this paper can contribute to a better understanding of the micro-crack propagation in the particulate composites with respect to the interphase.

Abstract: The oxide dispersion strengthened (ODS) steels are widely studied as possible candidate material for fission and fusion technology. Due to application of the ODS materials, mainly low cycle fatigue is important. Therefore, small crack initiation and propagation for rather high stress amplitude are in focus. The main aim of the presented work is description of the small fatigue crack propagation for ODS-EUROFER steel. Due to limited quantity of ODS steel special miniaturized cylindrical specimens for fatigue testing were designed. Crack propagation law based on plastic part of J-integral is presented and applied for mentioned material. The resulting fatigue crack propagation rates for ODS-EUROFER steel and EUROFER 97 are compared and discussed.

Abstract: This paper presents a methodology suitable for estimation of residual lifetime of polymer pipes. The linear and non-linear distribution of residual stresses in the pipe wall is studied using FEM analysis. The approximate relation for the stress intensity factor calculation is presented. It is shown that the presented relation gives a stress intensity factor similar to the FEM analysis for the linear or non-linear distribution of residual stress. The suggested procedure produces a slightly conservative lifetime estimation where the accuracy of the procedure increases with increasing ratio between applied internal pressure level to residual stress level. The accuracy of the residual stress estimation and corresponding stress intensity factor is discussed and a final recommendation for lifetime determination based on simplified methodology is given.

Abstract: It has been shown that the service lifetime of buried pipes can be reduced especially in relation to stress concentration caused by external point loads. If the pipe is loaded locally the stress is concentrated there and a crack can initiate at this position or the existing crack can be affected by corresponding stress redistribution. In the paper, the effect of a hard indenter, material properties, and the hoop stress level on the stress intensity factor of the axial inner crack are evaluated. A simple relation for estimation of the stress intensity factor value without FEM simulations is proposed.

Abstract: 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.

Abstract: The paper presents a methodology for the lifetime assessment of welded polymer pipes. A fracture mechanics analysis of a butt-welded joint is performed by simulating radial crack growth in the nonhomogenous region of the pipe weld. It was found that the presence of material nonhomogeneity in the pipe weld caused by the welding procedure leads to an increase in the stress intensity factor of the radial crack and changes the usual failure mode of the pipe system. This can lead to a significant reduction in the lifetime of the pipe system.

Abstract: The most relevant loading conditions for real polymer pipe systems are not only internal pressure, but also loading caused by sand embedding including bending or different kinds of point loads. It has been shown that service lifetime of buried pipes can be reduced especially due to stress concentration caused by external point loads. If the pipe is loaded locally the stress is concentrated here and a crack can initiate at this position or the existing crack can be affected by corresponding stress redistribution. In the paper the effect of the hard indenter, Poissons ratio, hoop stress level and pipe wall thickness on the crack shape was estimated using numerical simulations of the creep crack propagation based on finite element method. Relation between crack length and crack width was found and expressed by simple relationship. A deeper understanding of the point load effect in order to prevent unexpected failure of the pipelines is of paramount importance for pipeline design.