Finite volume method and multigrid acceleration in modelling of rapid crack propagation in full-scale pipe test

Rapid Crack Propagation (RCP) along pressurised plastic pipes is by far the most dangerous pipe failure mode. Despite the economic benefits offered by increasing pipe size and operating pressure, both strategies increase the risk and the potential consequences of RCP. It is therefore extremely important to account for RCP in establishing the safe operational conditions. Combined experimental-numerical study is the only reliable approach of addressing the problem, and extensive research is undertaken by various fracture groups (e.g. Southwest Research Institute – USA, Imperial College – UK). This paper presents numerical results from finite volume modelling of full-scale test on medium density polyethylene gas pressurised pipes. The crack speed and pressure profile are prescribed in the analysis. Both steady-state and transient RCPs are considered, and the comparison between the two shown. The steady-state results are efficiently achieved employing a full multigrid acceleration technique, where sets of progressively finer grids are used in V-cycles. Also, the effect of inelastic behaviour of polyethylene on RCP results is demonstrated.     

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.     

Fatigue behavior of surface cracked filament wound pipes with high tangential strength in corrosive environment

The aim of this study is to examine the corrosion fatigue behavior of filament wound composite pipes with a surface crack under alternating internal pressure. The filament wound pipes are composed of multi-layered E-glass/epoxy composites with a [±75°]₃ lay-up. The surface notches were formed on the outer surface of the pipe along the pipe axis. Dilute (0.6 M) HCl acid was applied to the surface crack region by a corrosion cell mounted on the outer surface of the pipe. The results of an experimental investigation into the corrosion fatigue tests are conducted to observe the oil leakage failure and the crack propagation of the composite pipe subjected internal pressure loading with an open ended condition in which the pipe can be deformed freely in the axial direction. The internal pressure was generated by conventional hydraulic oil for fatigue loading. The fatigue tests are performed at 0.42 Hz frequency and a stress ratio of R = 0.05 in accordance with ASTM D-2992 standard. The oil leakage from the crack tip was observed after the crack propagation reached to the critical stress intensity level. The fatigue crack propagation behavior with the environment exposure was strongly dependent on the crack parameters such as crack-depth ratio and crack-aspect ratio. The micro structure of the fracture surface with the effect of environment and the fatigue loading were also observed.     

Dynamic responses and damages of water-filled pre-pressurized metal tube impacted by mass

An account is given of some principal observations made from a series of experiments in which three-span pipe beams were subjected to central impact by indenters with different nose shapes. These pipes were filled and pre-pressurized with water in order to identify the main effects produced by the fluid–structure interaction. In comparison, the impact experiments of the pipes with no water were also carried out. The perforation failure modes and corresponding critical impact energies were obtained in different test conditions. The experimental results indicated that the critical perforation energy and the deformation of the wall of the pipe were significantly influenced by the presence of the water and the pressure.     

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.     

Influence of ageing in the failure pressure of a GFRP pipe used in oil industry

This paper is concerned with the influence of ageing in the failure pressure of a glass fibre reinforced polymer (GFRP) pipes used for oil and gas transport. Burst tests were performed on pipes submitted to accelerated ageing combining hydraulic pressure and temperature (1 MPa and 80 °C). An alternative method, which does not require the immersion in a water bath or other fluid bath, was adopted for the ageing of the specimens. The experiments show that the burst pressure can be strongly affected by the ageing period. Tensile tests also have been performed, showing a brittle-elastic behaviour. For this particular composite, the stiffness of the tensile specimen is not significantly affected by the ageing time, but the ultimate tensile stress (UTS) is affected by the ageing time. A methodology to obtain analytic estimates of both UTS and failure pressure for a given ageing time is proposed. In order to obtain a lower bound of the failure pressure at a given operation time, besides the pipe geometry, it is only necessary to know the UTS of the composite obtained in a minimum of three tensile tests performed at different ageing times. The prediction error is less than 0.8% for the UTS and is less than 25% for the failure pressure.     

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.     

埋地PVC管纵向断裂失效预测概率模型

针对存在初始裂纹缺陷的埋地PVC管在荷载作用下的纵向断裂失效问题,基于管道环向受力特性和裂纹扩展经验公式建立失效预测模型,并在此基础上运用蒙特卡罗法(MC)对模型和参数的不确定性进行随机模拟,给出管道失效风险率随服役时间变化的规律。计算结果表明:MC数值模拟结果与实测值吻合良好,说明失效预测模型的合理性;裂纹增长是一个由慢到快的过程,初始裂纹越大,失效时间越短;失效风险率在开始的20年内达到最高,随后逐渐减小;所有参数都作为随机变量考虑时与只有初始裂纹为随机变量时相比,失效风险率提高近一倍;管道初始裂纹、内压、残余应力和壁厚是影响管道失效时间的四个重要因素,管道内压和残余应力增加都导致管道失效率变大,准确的给出管道初始裂纹的分布对PVC管纵向断裂失效预测尤为重要。能够减小PVC管初始裂纹缺陷的生产工艺和施工技术可以有效提高管道的使用寿命。     

客车空调压缩机排气管路失效分析与结构改进

采用ANSYS Workbench软件对客车空调压缩机排气管路进行失效分析和结构改进。以3种典型管路为研究对象,在压缩机端施加位移载荷进行管路失效分析,模拟结果与管路实际断裂位置一致。通过添加弯头、调整管路角度以及增加不锈钢波纹管长度等改进措施,可以降低管路的最大等效应力,延长管路的使用寿命。     

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.     

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.     

Criterion analysis of the fracture of cylindrical shell structures loaded by internal pressure induced by localized thermal impacts

We present experimental results of the investigation of the strength of cylindrical shells loaded by internal pressure formed as a result of a localized thermal impact caused by laser radiation. The types of fracture of shells recorded in the experiments are characterized either by the loss of gas-tightness as a result of the formation of cracks (or holes) or by the total fracture of a shell. We performed the criterion analysis of the experimental results by using the methods of the theory of similarity. The amount of conveyed radiant energy and the level of excess internal pressure are the main factors responsible for the fracture of shells. We suggest a formula which establishes a relationship between these factors and which can be regarded as a helpful criterion in comparing the results of testing of various models under different loading modes and application of these results to actual objects. Translated from Problemy Prochnosti, No. 5, pp. 51–57, September–October, 1997.     

A physical probabilistic model to predict failure rates in buried PVC pipelines

For older water pipeline materials such as cast iron and asbestos cement, future pipe failure rates can be extrapolated from large volumes of existing historical failure data held by water utilities. However, for newer pipeline materials such as polyvinyl chloride (PVC), only limited failure data exists and confident forecasts of future pipe failures cannot be made from historical data alone. To solve this problem, this paper presents a physical probabilistic model, which has been developed to estimate failure rates in buried PVC pipelines as they age. The model assumes that under in-service operating conditions, crack initiation can occur from inherent defects located in the pipe wall. Linear elastic fracture mechanics theory is used to predict the time to brittle fracture for pipes with internal defects subjected to combined internal pressure and soil deflection loading together with through-wall residual stress. To include uncertainty in the failure process, inherent defect size is treated as a stochastic variable, and modelled with an appropriate probability distribution. Microscopic examination of fracture surfaces from field failures in Australian PVC pipes suggests that the 2-parameter Weibull distribution can be applied. Monte Carlo simulation is then used to estimate lifetime probability distributions for pipes with internal defects, subjected to typical operating conditions. As with inherent defect size, the 2-parameter Weibull distribution is shown to be appropriate to model uncertainty in predicted pipe lifetime. The Weibull hazard function for pipe lifetime is then used to estimate the expected failure rate (per pipe length/per year) as a function of pipe age. To validate the model, predicted failure rates are compared to aggregated failure data from 17 UK water utilities obtained from the United Kingdom Water Industry Research (UKWIR) National Mains Failure Database. In the absence of actual operating pressure data in the UKWIR database, typical values from Australian water utilities were assumed to apply. While the physical probabilistic failure model shows good agreement with data recorded by UK water utilities, actual operating pressures from the UK is required to complete the model validation.     

Fatigue analysis of damaged steel pipelines under cyclic internal pressure

A methodology for fatigue analysis of damaged steel pipelines under cyclic internal pressure is proposed. This methodology employs stress concentration factors, which are commonly used to modify standard S–N curves of metallic structures under high cycle fatigue loadings. Experiments are accomplished to evaluate the strain behavior of small-scale steel pipes during denting and cyclic internal pressure. A nonlinear finite element model is developed to obtain stress concentration factors induced by plain dents on steel pipes under internal pressure. Afterwards, analytical expressions are developed to estimate stress concentration factors as function of the damaged pipe geometric parameters. Finally, fatigue tests are conducted to evaluate the finite life behavior of small-scale damaged pipes under cyclic internal pressure and to validate the proposed methodology of fatigue analysis.     

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.     

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.     

Assessment on failure pressure of high strength pipeline with corrosion defects

An accurate prediction on the failure pressure of line pipe is very important in the engineering design and integrity assessment of oil and gas transmission pipelines. This paper analytically investigates the failure pressure of line pipes with or without corrosion defects, and focus on the high strength steels. Based on von Mises strength failure criterion, a classic strength failure criterion, the failure pressure of end-capped and defect-free pipe p_M is theoretically deduced with the strain hardening material. In order to derive a general solution for corrosion defect assessment of high strength pipelines, an extensive series of finite element analyses on various elliptical corrosion defects was performed. Finally, a new formula for predicting the failure pressure of corroded pipe in the material of high strength steels is formulated, based on the FE models and p_M, and is validated using 79 groups of full-scale burst test data, which contain the low, middle and high strength pipeline. The results indicated that the proposed formula for predicting the failure pressure is closely matches the experimental data for the high strength steels.     

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.     

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.     

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.