Fracture toughness of multilayer pipes

Multilayer pipes composed of various materials improve partially the properties of a pipe system and are frequently used in service. To estimate the lifetime of these pipes the basic fracture parameters have to be measured. In the contribution a new approach to this estimation is presented. Special type of a C-shaped inhomogeneous fracture mechanics specimen machined directly from a pipe has been proposed, numerically analyzed and tested. The corresponding K values are calculated by finite-element method and fracture toughness values of polyethylene pipes material are obtained. __________ Translated from Problemy Prochnosti, No. 1, pp. 146–149, January–February, 2008.     

Application of volumetric method to the assessment of damage induced by action of foreign object on gas pipes

Gas pipes are submitted to three major types of damage: damages induced by impact of foreign objects, pits and craters produced by corrosion, weld defects produced during manufacturing. Damages induced by impact of foreign objects are considered as the most important problem for the gas pipes reliability and lead to dents, gouges or notches at the pipe failure. Geometrical effect of gauges or notches on brittle or elastoplastic failure of pipes submitted to internal pressure can be treated by the volumetric method. The volumetric method has been applied to a gas pipe submitted to internal pressure and exhibiting a longitudinal external and surface notch. We introduced a special procedure SINTAP, which allows one to compute the safety factor value. __________ Translated from Problemy Prochnosti, No. 4, pp. 109–117, July–August, 2006.     

Fatigue crack growth of filament wound GRP pipes with a surface crack under cyclic internal pressure

In this study, fatigue damage behavior of (±75₃) filament wound composite pipes with a surface crack under alternating internal pressure was investigated. The specimens were tested at room temperature and exposed to open ended fatigue tests in which the pipe can be deformed freely in the axial direction. The tests were carried out in accordance with the ASTM D-2992 standard. The alternating internal pressure was generated by conventional hydraulic oil. The low cycle tests were performed with 0.42 Hz frequency and R = 0.05 stress ratio. Glass reinforced polymer pipes (GRP) are made of E-glass/epoxy and have (±75₃) configuration. Surface cracks were machined in the axial direction of the pipes which have depth-to-thickness ratios a/t = 0.25–0.38–0.50 and depth to length ratio of a/c = 0.2. Tests were performed at three different loads of 50%, 40%, and 30% of ultimate hoop stress strength of unnotched pipes. The failure behavior of GRP pipes during the test was observed and fatigue test results were presented by means of (S–N) curves and delamination damage zone area-cycle (A–N) curves.     

Residual stress-strain state of the collector assembly of a steam generator in the process of press-fitting of heat-exchange pipes

The influence technological heredity connected with the manufacturing process on the strength and service life of the assembly of joints of the collector and heat exchange pipes of a steam generator proves to be one of the main causes of premature failures of steam generators of water-cooled reactors of nuclear power plants. We present the results of a numerical simulation of the process of press-fitting of heat-exchange pipes in the collector. We determine the residual stress-strain state of the collector assembly depending on the basic technological parameters of press-fitting (the initial gap between the pipe and the collector, the pressure of hydraulic press-fitting, and the boundaries of the region of application of pressure) and strength characteristics of the materials of the collector and pipes. We also estimate the influence of the additional mechanical flaring of pipes and redistribution of local stresses in the course of consecutive hydraulic press-fitting on the level of residual stresses in the collector assembly. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 66 – 86, March – April, 1998.     

Structural integrity analysis of HDPE pipes for water supplying network

A problem of reliability of polymer pipes manufactured with the high‐density polyethylene (HDPE) for drinking water supply was considered. The safety of the pipelines network was analyzed with taking into account two main factors: arising of sudden overpressure (water hammer phenomenon) and presence of defects on pipe surface. Based on elastoplastic fracture mechanics and using of J‐integral approach, the numerical model of cracked pipe was developed and verified by direct experimental test. The formula for calculation of crack initiation pressure, which is the function of pipe size and size of external surface defect, was derived. The validity of this relation was proved for pipes of diameter 450 to 800 mm. It was also shown that the pressure peaks of the water hammer phenomenon are quickly damped because of the high deformability of the polyethylene pipes and the defect depth that is equivalent to 40% of pipe wall thickness can be considered as acceptable. The applicability of developed tools was demonstrated under analysis in the real state of South Tunisian water supply network.     

The long-term behaviour of an aluminium-reinforced polyethylene pressure pipe

The long-term behaviour of an aluminium-reinforced polyethylene pressure pipe has been explored by undertaking stress rupture tests at 60 and 80 °C. The results of the tests showed these macrocomposite pipes have a time-dependent strength, such that with an increasing time under load the strength declined. In addition the pipes were weaker at 80 °C when compared to the 60 °C strength. The analysis of the influence of time and temperature on strength showed these multilayer pipes can be considered to behave as do conventional homogeneously structured plastic pipes, and that to describe the influence of time on the pipe strength, the accepted procedures developed for conventional plastics pipes can be applied. In addition the mode of failure of the pipes was examined. Pipe failure initiates by the strain-controlled failure of the reinforcing aluminium layer. The polyethylene layers then fail almost instantaneously in a ductile mode. This analysis of the mode of failure was supported by freeze-thaw cycling tests to – 25 °C and the 60 and 80 °C stress rupture tests.     

Fractographic study of high-density polyethylene pipe

High-density polyethylene (HDPE) pipe is now being used as an alternative to medium-density polyethylene (MDPE) for gas, water, sewage and waste-water distribution systems. Laboratory tests appear to show that HDPE is more able to suppress rapid crack propagation (RCP), whilst remaining sufficient resistance under the operational circumstances that lead to the type of slow crack growth observed in service failures. There have been many fractographic studies on MDPE pipe materials, actual pipe and fittings, but little on HDPE. A fractographic study of the type of HDPE pipe in current production has been undertaken. For these tests, whole pipe sections were subjected to either static or dynamic internal (water) pressurization fatigue loading. Failure mechanisms are discussed based on the fracture morphologies resulting from these tests. A further argument for good resistance of HDPE pipe to RCP is suggested. © 1998 Chapman & Hall     

Analysis of shape and location effects of closely spaced metal loss defects in pressurised pipes

Metal loss due to corrosion is a serious threat to the integrity of pressurised oil and gas transmission pipes. Pipe metal loss defects are found in either single form or in groups (clusters). One of the critical situations arises when two or more defects are spaced close enough to act as a single lengthier defect with respect to the axial direction, causing pipe ruptures rather than leaks, and impacting on the pressure containing capacity of a pipe. There have been few studies conducted to determine the distance needed for defects to interact leading to a failure pressure lower than that when the defects are treated as single defects and not interacting. Despite such efforts, there is no universally agreed defect interaction rule and pipe operators around the world have various rules to pick and choose from. In this work, the effects of defect shape and location on closely spaced defects are analysed using finite element analysis. The numerical results showed that defect shapes and locations have a great influence on the peak stress and its location as well as the failure pressure of pipes containing interacting defects.     

Pipe-defect assessment based on the limit analysis, failure-assessment diagram, and subcritical crack growth

We analyze the serviceability of a pipe containing a defect by the following three methods: limit analysis, failure-assessment diagram, and subcritical crack growth. The use of the proposed complex approach is explained by the fact that, despite the plasticity of pipe steels, they can suffer quasibrittle fracture (under the conditions of plane deformation) or fail as a result of plastic collapse. Moreover, the final fracture of a pipe with defect is preceded by the stage of stable crack growth. Therefore, in analyzing the serviceability of the pipe, one should also take into account the “leak-before-break” criterion. To take into account all these circumstances, it is necessary to use the indicated three methods, as in the case of evaluation of the serviceability of a pipe of API X52 steel containing a surface semielliptic notch. Defects of this sort reflect the influence of the surface gouges and dents responsible for the failures of pipes under the action of internal pressure. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 1, pp. 119–127, January–February, 2006.     

Analytical solution of the Brazier problem for thin-wall pipes with initial cross-sectional shape imperfection in the case of action of pressure

An analytical method is proposed for the solution of geometrically nonlinear Brazier problem for thin-mall pipes with initial cross-sectional shape imperfection in the case of action of pressure. Geometrical equations relating displacement components to strains and equilibrium equations taking into account change in the curvature of pipe cross section and axis have been derived. A solution in a first approximation for dimensionless flexibility parameter is presented, the exactness of which is illustrated by numerous examples. For the case of joint action of external bending moment and pressure, a limit curve of the critical moment value as a function of pressure value has been obtained. __________ Translated from Problemy Prochnosti, No. 3, pp. 100–123, May–June, 2008.     

Determination of material fracture toughness by a computational/experimental approach for rapid crack propagation in PE pipe

Based on an investigation of the Small Scale Steady State (S4) test, an integrated computational/ experimental approach has been developed in order to assess the fracture behaviour of polyethylene (PE) gas distribution pipe material during rapid crack propagation (RCP). This paper describes the use of the results obtained from the S4 test and program modified from PFRAC (Pipeline Fracture Analysis Code) to evaluate the fracture toughness of the material, G _d, which could not be directly obtained from the test, and to predict critical pressure, p _c, for RCP in a full scale PE pipe. The contact algorithms are developed to consider the opening pipe wall impact against a series of containment rings and the capabilities of PFRAC are also extended. Since G _d is evaluated, the investigations are made on it to the effect of temperature and wall thickness. In addition, procedures to evaluate the critical pressure for the S4 test pipe are also discussed.     

Diagnosis of the load-carrying capacity of pipes containing a longitudinal crack when loaded by an axial force and internal pressure

A method is developed for evaluating the load-carrying capacity and safe life of pipes with a through longitudinal crack when loaded by an axial force and internal pressure. The method is based on the use of previously developed nomograms of strain and limit load. The first nomogram is calculated on the basis of constitutive relations of the theory of plasticity, while the second is calculated in accordance with a fracture criterion for biaxial loading. The proposed method makes it possible to determine one of three parameters — critical crack length, limiting pressure, or limiting axial load — without preliminary calculations if the other two parameters are already known. As an example, nomograms are presented for a steel 15G2 pipe having a diameter of 1420 mm and wall thickness of 21.5 mm and containing a longitudinal crack. Translated from Problemy Prochnosti, No. 12, pp. 16–23, December, 1994.     

Kinetic Theory of Creep and Creep-Rupture Strength Analysis of Structural Components. Part 1. Stress-Strain State in Nonuniformly Heated Thick-Walled Pipes

The stress-strain state and the failure onset time are calculated by Rabotnov's kinetic theory of creep for a thick-walled nonuniformly heated pipe loaded by internal pressure. We analyze the influence of the temperature difference along the pipe radius on the stress intensity distribution and damageability parameter of material in the pipe cross section at various instants of time up to the rupture onset. Based on the comparative analysis, we proved incompatibility of the results obtained by the above-mentioned theory with those calculated by the similar theory of creep in the Kachanov's statement. __________ Translated from Problemy Prochnosti, No. 5, pp. 30 – 44, September – October, 2005.     

Centrifugal inertia effects in high-speed hydrostatic air thrust bearings

A modified Reynolds equation for compressible flow is used to model the dynamics of pressurised air bearings in a simplified axisymmetric geometry. The formulation incorporates the effect of centrifugal inertia for high-speed operation. A steady-state analysis is presented with a fixed rotor–stator clearance. The load-carrying capacity of the bearing is assessed for both inward and outward pressurisation and the air-flow characteristics are seen to depend on the level and direction of pressurisation. A critical shaft speed is identified that maintains no-net flow by balancing inertia and pressurisation effects. The nonlinear air–rotor–stator dynamics are investigated by modelling the axial stator position using a spring–mass–damper system coupled to the air-film dynamics. Solutions are presented to illustrate the effect of various frequencies and amplitudes of forcing for a range of rotation speeds. The time-averaged axial force and mass flow of air are used as characteristic measures of bearing performance, and further numerical results are presented as part of a parameter space analysis. Using the method of arc-length continuation key measures of the numerical solutions are tracked for changing values of the physical parameters. The minimum rotor–stator clearance is used to evaluate the limit of stable periodic operation without resonant stator dynamics and incorporating high operating speeds.     

Heat exchange in a circular pipe in modeling of turbulent air flow by oriented-fluid flow

Based on the anisotropic-turbulence model, the velocity profile and the temperature distribution in the flow in the case of steady-state air flow in a circular pipe have been obtained. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 1, pp. 155–161, January–February, 2006.     

Mathematical simulation of the strength of a pipe with three-dimensional defect

We develop a mathematical model of plastic fracture of a thin-walled pipe with three-dimensional part-through “smooth” defect and compute the degree of weakening caused by this defect. The defect is simulated by an elliptic (in plan) slot appearing in the pipe. It is necessary to find the level of internal pressure under which the material in the zone of the defect passes into the limiting state. or sufficiently deep defects whose sizes are much smaller than the radius of the pipe, the problem of determination of the stress-strain state in the zone of the defect under given internal pressure is reduced to the solution of a nonlinear system of two one-dimensional integral equations. This system is solved by using an original algorithm and the limiting pressure is found within the framework of the proposed mathematical model. Under certain assumptions concerning the distribution of stresses in the zone of the defect, we deduce a formula which expresses the limiting pressure via the sizes of the defect and the strength characteristics of the material. The results of theoretical analysis are compared with numerical data obtained by the method of finite elements. PNVP “Integrator”. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 1, pp. 65–70, January–February, 1998.     

Erosion rate correlations of a pipe protruded in an abrupt pipe contraction

Erosion is one of the most serious problems in various gas and liquid flow passages such as flow in pipes, pumps, turbines, compressors and many other devices. Sand presence causes loss of pipe wall thickness that can lead to pipe erosion, frequent failures and loss of expensive production time. The importance of this problem is mainly due to many related engineering applications, viz. heat exchangers. In order to reduce the frequency of such pipe erosions, caps in the form of replaceable pipes are protruded in the sudden contraction regions which are exposed to most of the serious erosion rates. In the present work, numerical investigation of the erosion of a pipe protruded in a sudden contraction is presented. The turbulent, steady, 2-D axi-symmetric flow inside an axi-symmetric abrupt contraction pipe with a pipe protrusion embedded in it was solved by steady-state time averaged conservation equations of mass and momentum along with two equation model for turbulence. Particles are tracked using Lagrangian particle tracking. An erosion model was employed to investigate the erosion phenomena for the given geometry. The influence of the different parameters such as the inlet flow velocity (3–10 m/s), the particle diameter (10–400 μm), the protruded pipe geometry (thickness T=1–5 mm and depth H=2–5 mm) and the pipe contraction ratio (Cr=0.25–0.5) on the erosion of pipe protrusion was investigated. Correlations for the influence of inlet flow velocity, depth and thickness of the protruded pipe on the erosion rate are presented.     

Failure analysis of natural gas pipes

Incident involving failures of 6 months old API 5L X42 (NPS8) and SDR 17, 125 mm medium density polyethylene pipe (MDPE) supplying natural gas to an industrial customer has caused serious 7 h supply disruption. Study was performed to identify the most probable cause of the pipes failures. The study conducted by reviewing the existing design and construction data, visual physical inspection, pipe material analysis, structural analysis using NASTRAN and Computational Fluid Dynamics analysis (CFD) using FLUENT. Investigations revealed that high pressure water jet from leaked water pipe had completely mixed with surrounding soil forming water soil slurry (high erosive properties) formed at a close vicinity of these pipes. Continuous impaction of this slurry upon the API 5L X42 pipe surface had caused losses of the pipe coating materials. Corrosion quickly ensued and material loss was rapid because of the continuous erosion of oxidised material that occurred simultaneously. This phenomenon explains the rapid thinning of the steel pipe body which later led to its failure. Metallurgical study using photomicrograph shows that the morphology of the steel material was consistent and did not show any evidence of internal corrosion or micro fractures. The structural and CFD simulation results proved that the location, rate and the extent of erosion failures on the pipe surfaces can be well predicted, as compared with actual instances.     

Heat transfer in complex-profile channels under different stabilized flow conditions of the medium and system search for higher-accuracy nonstationary temperature fields

By means of the double Laplace–Carson transform as integral averaging of the time function decreasing by the exponential law with weight and along a semi-bounded pipe, the nonstationary heat transfer equation under steady-state laminar or turbulent flow conditions is transformed into a boundary-value problem, which is solved by the method of orthogonal projection of the residual, where, as a finite element, the entire bounded domain of variation of elliptic coordinates is taken. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 6, pp. 1063–1080, November–December, 2008.