Crack Propagation In Pipes Research Articles

Сравнение критериев моделирования разрушения трубопроводов методом конечных элементов

The work is devoted to the simulation of pipeline’s fracture. Application of modern viscous pipe steels does not completely solve the problem of brittle fracture of these structures, because the desire to increase the thickness of the wall and the operating pressure and the need to use a pipe in a cold climate shifts the fracture mode toward brittle – but, of course, does not make it completely brittle and require the application of nonlinear fracture mechanics. Simulation of the fracture by formation of large longitudial cracks should take into account the inertial forces of the pipe material coming into motion at crack propagation, and the possibility of a pressure drop in the pipe at the loss of its impermeability. These features of the problem can be modeled using finite element package LS-DYNA. However, LS-DYNA offers a limited set of a fracture criteria, which doesnot include the classic criteria of nonlinear fracture mechanics. The aim of the work is the selection of a criteria that is implemented in the LS-DYNA program and at the same time is in good agreement with the experimental data. As such a criterion equivalent plastic strain isconsidered – with the proviso that the plastic deformation near the crack tip is calculated using a weighted averaging procedure on some representative volume to exclude effect of the finite element sizes on results. Parameters of the weighting function are selected to ensure that the loads corresponding to the beginning of the crack growth are the same for the model of viscous fracture and for the nonlinear fracture mechanics. It is shown that the problem of parameter selection is ill-conditioned. However, the obtained values of parameters will subsequently allow to perform the dynamic calculation of longitudial crack propagation in pipes taking into consideration the effects treated by the LS-DYNA code – inertia forces of the material of the pipe wall, velocity of the perturbation wave in the gas or liquid filling the pipe and decompression.

Возможность использования ребер для защиты трубопроводов от распространения продольных трещин

This research aims to build a finite element model of oil or gas steel pipelines transmission. The model has been developed in that crack propagation in pipes can be reduced as possible to avoid the failure. The suggestion that is taken into consideration for this purposeis to stopping crack propagation as possibly, is by using ribs as arresters. Different thicknesses have been tested for the ribs used in this study. The first one was of 0.02 m, while as the second thickness was of 0.05 m. The dimensions of pipe are of 0.01 m thickness, and diameter of 0.5 m. The model of pipe consists of three parts. The first part has two different lengths 2 m and 3 m. The second, which is the rib, is of length of 0.15 m. The third part of pipe has the length of 1 m. So all the three parts are made up the case study that was studied in this work. The value of stress intensity factor K1 for different lengths of cracks has been calculated by the finite element model deve-loped by using ANSYS package. These calculations were applied with and without ribs. The results showed that when using the ribs, the stress intensity factor would be reduced significantly. This leads to reduce the possibility of failure.

A specimen for studying the resistance to ductile crack propagation in pipes

The paper describes a novel DCB-like specimen designed for studying the resistance of running axial cracks in pipelines. It is long enough for steady-state conditions to govern a large proportion of the crack growth, a condition monitored by direct observation of the crack tip opening angle (CTOA) using video cameras. To avoid possible inaccuracies due to pre-straining the material, realistic specimens must be cut from the pipe and tested without flattening. The geometry of the specimen design allows this, and also includes a measure of gauge-length thinning. Extra thickness due to loading side-plates accentuates two desirable features of gauge thinning, by providing directional stability in the advancing crack tip, and by increasing the level of constraint. Tests on specimens from API X80 grade pipe demonstrated a common pattern of CTOA development for 4, 8 and 10 mm gauge thickness material. All showed a reduction of CTOA with growth towards a constant, steady-state value, with the steady state being associated with slant fracture. The region of reducing CTOA started with flat fracture, and gradually developed increasingly definite slant fracture. The most accurate tests produced a steady-state mean value of CTOA of 11.2° and a standard deviation of 1°.

Crack propagation in pipes under cyclic axial loading : Carpinteri, A., Brighenti, R. and Spagnoli, A. Computational Methods Testing for Eng. Integrity (1996) 3–10 Conference: First International Conference on Computational Methods and Testing for Engineering Integrity, Kuala Lumpur, Malaysia (19–21 Mar. 1996)

The study of cyclic loading on saturated natural clay deposits is of great interest for the analysis of offshore and onshore structures. Recent experimental research in the field of offshore engineering has indicated that the mechanical behavior of monopiles is strongly influenced by the reconsolidation processes that take place during calm conditions. After the reconsolidation episodes, the soil-pile stiffness considerably increases. Unfortunately, the previous behavior is usually disregarded in the numerical analysis and design of monopiles. In this work, the behavior of monopiles subjected to multiple episodes of cyclic loading and reconsolidation is analyzed from a numerical point of view. For that purpose, the sophisticated hypoplastic model for clays by Mašín (2014) with intergranular strain by Niemunis and Herle (1997) is carefully evaluated under element test conditions and by back-calculating some centrifuge tests. For the calibration of the constitutive model, three undrained monotonic and three undrained cyclic triaxial tests were performed. At the end, the influence of reconsolidation stages on the design of monopiles and the model capabilities to reproduce this behavior are discussed.

A model for unstable shear crack propagation in pipes containing gas pressure

A tentative analysis of an unstable shear crack propagating axially in the wall of a long pipe under gas pressure is developed. Six processes known to be associated with crack propagation are treated numerically: (1) axial decomptession of the gas, (2) bulging of the pipe wall, (3) radial decompression of the gas, (4) local stress and strain intensification at the crack tip, (5) plastic deformation, and (6) ductile cracking. The treatment is quasi-static; dynamic effects in the pipe wall are ignored. Because the numerical descriptions included in the model are approximate and incomplete, several variants of the basic model are examined. The response of the model is evaluated for different line pressures, geometries, and material properties and compared with full-scale test data for 100% shear cracks. A wide range of speeds can be calculated for the limits within which the system parameters are specified including the speeds observed in practice. The bulging and decompression characteristics of the model cause the crack speed to be relatively insensitive to line pressure. Yet the calculated crack speeds are influenced by yield strength and toughness of the material. The model does not provide for nonaxial crack paths, nor does it adequately describe crack-arrest possibilities. The paper represents the first step in the analysis of a complex problem.

Crack Propagation In Pipes Under Cyclic AxialLoading

The propagation of a circumferential external surface flaw in a metallic round pipe under constant amplitude cyclic axial loading is examined through a two-parameter theoretical model. The surface flaw is assumed to present an elliptical-arc shape with aspect ratio a = a^i/beiThe relative depth 5 of the deepest point on the crack front is equal to the ratio between the maximum crack depth â i and the pipe wall thickness t. The edge flaw being considered is shown to follow preferred propagation patterns in the diagram of a against g.