Numerical investigation on stress concentration of corrosion pit

Localized (pitting) corrosion has been observed in steel and high-strength aluminium alloys in aqueous environments and has been identified as a potential origin for fatigue crack nucleation. In the present study, under uniaxial tension loading, stress distribution at the semi-elliptical corrosion pits has been investigated by conducting a series of three-dimensional semi-elliptical pitted models, systematically. Based on the finite element analyses, it is concluded that pit aspect ratio (a/2c) is a main parameter affecting stress concentration factor (SCF). An attempt has been made to construct simple equations to SCF depending on characteristic of pit parameters. At the bottom of hemispherical pit, contribution of secondary (premature) pit formation to SCF is very pronounced.     

Numerical modeling of corrosion pit propagation using the combined extended finite element and level set method

A sharp-interface Eulerian formulation for modeling the propagation of localized pitting corrosion is presented. This formulation allows for an accurate representation of the corrosion front independent of the underlying finite element mesh and handles complex morphological transitions such as pit merging without requiring remeshing or mesh-moving procedures. First, the governing equations of the moving interface problem associated with pit growth are derived for the two-phase (metal-solution) system. Next, the implementation of the combined extended finite element and level set method and the procedure to enforce interface conditions are discussed. Finally, the method is validated by conducting several benchmark numerical studies, and comparing the results with published experimental data and existing numerical studies. Simulation studies indicate that: during diffusion-controlled corrosion an isolated pit grows as a semi-circular shape, whereas closely spaced pits merge and grow into an elongated elliptical shape; and only during activation-controlled corrosion the initial pit morphology is preserved.     

Correlation between corrosion pits and stresses in Al alloys

Pitting corrosion is commonly observed in a wide range of aluminum alloys that are being used for aerospace applications. There is a need to study the stress environment around pits in order to predict the nucleation of cracks. The objective of this study is to investigate the correlation between pits and stresses in aluminum alloys. Corrosion experiments were carried out on aluminum 2024-T3 alloy samples and imaged through Atomic Force Microscopy (AFM) to obtain the pit profiles. An analysis procedure was developed using CAD and finite element analysis to predict stresses resulting from corrosion pits. Based on the analysis, it was observed that the stress distribution and levels on the corroded surface varied due to irregularity in the corrosion process. The results obtained indicate that the stress initially increases and reaches a plateau with increasing corrosion time. From these stresses it is possible to estimate the initiation of cracks, from which the life can be estimated for failure in the material.     

Challenges in modelling the evolution of stress corrosion cracks from pits

The evolution of stress corrosion cracks from pits is important in many industrial applications but continues to be a challenge in both measurement and prediction. Life prediction in these circumstances has to account for pit growth kinetics, the conditions for the transition from pits to cracks, and the growth rate of cracks in the short and long crack domain. An example of importance is the performance of steam turbine rotors in power generation. Although stress corrosion failures are comparatively rare, the consequences can be severe and occasionally catastrophic. Consequently, considerable effort is being focused on evaluating the effect of operational variables on pitting and crack growth and in developing an improved basis for structural integrity assessment. A preliminary mathematical model based on deterministic equations with statistically variable input parameters was developed for simulating the evolution of the pit depth distribution at different exposure times, and the transformation to stress corrosion cracks was based on the Kondo criteria. The model predicted some features of the damage well but recent novel measurements of the evolution of stress corrosion cracks from pits combined with finite element analysis of the strain distribution suggest that the transition from a pit to a crack is more complex than can be accounted for by the Kondo approach.     

Failure analysis of IEU drill pipe wash out

Many 127.0×9.19 mm IEU G105 drill pipe failures of wash out occurred after 2367 h of pure drilling time and 8726 m of penetration footage. This paper gives a detailed investigation on these failures and a systematic analysis is carried out on service and loading conditions of the drill pipes. Measurement and inspection were performed on configuration dimensions, chemical composition, mechanical performance, metallography, macro-fractography, micro-fractography, and corrosion products. Configuration stresses at the crack positions of the drill pipe were calculated by FEA. Crack extending velocity of the drill pipe material under corrosion medium was also measured. It is thought from test and analysis results that the drill pipe wash out or fracture accidents were premature corrosion fatigue failure accident. The failure courses were as the following: corrosion pits occurred first on the internal surface at the stress concentrating area of the drill pipe, and then fatigue cracks initiated in pit bottoms, and washed out or fractured subsequently as cracks penetrated through the wall thickness of the drill pipe. The reasons of drill pipe wash out were related to configuration, material quality, and load condition of the drill pipe string.     

点蚀损伤下海洋平台结构剩余强度的多尺度分析方法

均匀腐蚀损伤影响结构承载力的研究已趋于成熟,但点蚀损伤影响整体结构承载力的规律及其局部损伤的演化行为仍不明确。该文采用均匀分布的圆柱体点蚀损伤模型,探讨在宏观结构尺度中点蚀损伤的描述、多尺度区域的划分和跨尺度界面的连接以及结构非线性的求解方法,提出了研究考虑细观尺度点蚀损伤的海洋平台结构多尺度模型的构建和计算方法。计算结果表明:该文提出的多尺度模型构建策略有效地解决了细观尺度点蚀损伤在宏观结构尺度中的描述问题,且所建多尺度模型能够体现点蚀损伤削弱结构极限承载力的规律,为评估在役海洋平台结构的剩余强度提供了可行的方法。     

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.     

Limit Load Analysis of Pipes with Local Wall-thinning

The loss of metal in a pipe due to corrosion usually results in localized thinned areas with various depths and an irregular shape on its surface.In this paper,a number of numerical models of pipes with different size defects are established.The limit loads of these pipes are researched using the nonlinear finite element method.The effect of defect parameters of the local wall-thinning pipes on the limit load is discussed.The results show that limit loads decrease obviously when the depths and lengths of th...     

FATIGUE LIFE DISTRIBUTION AND GROWTH OF CORROSION PITS IN A MEDIUM CARBON STEEL IN 3%NaCl SOLUTION

Abstract— Statistical fatigue tests have been conducted on a structural medium carbon steel, S45C, in room air and in 3%NaCl solution, using five cantilever-type rotary bending fatigue testing machines which were specially manufactured for the purpose of the present study. Fatigue life distribution was examined at three and five stress levels in air and in 3%NaCl solution, respectively, and twenty specimens were allocated to each stress level. In room air, it was found that fatigue life distributions followed the three-parameter Weibull distribution, which were closely related to fracture morphology. In 3%NaCl solution, they also followed the Weibull distribution, but the scatter in fatigue life was smaller in comparison to that in air. It is suggested that the decrease in the scatter of fatigue life may be attributed to a smaller fraction of crack initiation life in 3%NaCl solution. The growth of corrosion pits was investigated using a laser microscope. The distribution of corrosion pit depths followed the log-normal distribution, and the corrosion pit depths increased with increasing time or the number of cycles. It was found that the growth of corrosion pits was accelerated by stress cycling and the depths increased with increasing stress level. Based on these results, a growth law of corrosion pits, including the effect of stress cycling, is proposed.     

Water pipeline failure due to water hammer effects

A numerical model has been established in order to simulate the propagation of pressure waves in water networks. The present model formulation is based on a system of partial hyperbolic differential equations. This system has been solved via the characteristics method. The current model provides the necessary data and the necessary damping of water hammer waves, taking into account the structure of the pipe network and the pressure loss. The numerical algorithm estimates the maximum pressure values resulting from the water hammer when closing valves in the network and consequently, the maximum stresses in the pipes have been calculated. In the case of simultaneous closing of several valves, the over pressure can exceed the admissible pressure. In this case, the severity of a defect such as a corrosion crater (pit) has been estimated by computing a safety factor for the stress distribution at the defect tip. This allows the applied notch stress intensity factor to be obtained. To investigate the defect geometry effects, semi‐spherical and semi‐elliptical defects are deemed to exist in up to one‐half of the thickness of the pipe wall. The outcomes have been introduced into the structural integrity assessment procedure (SINTAP) failure diagram assessment (FAD) in order to obtain the safety factor value. Conventionally, it is considered that a failure hazard exists if this safety factor is less than two.     

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.     

Evaluation of general corrosion on pipes using the guided wave technique

The torsional mode of a guided wave, T(0,?1), is capable of detecting features and defects in pipelines, especially in the cases of coated and buried pipes. However, it is hard to find or locate general corrosion, since the signals of shallow corrosion easily merge into the noise signal. The situation worsens when an inspector sets the array ring of the guided wave transducer directly on a general corrosion area of the pipe and serious energy attenuation of the detecting signal occurs. The goal of this study is to investigate the effects of the above-mentioned general corrosion on guided wave tests, as determined by the finite element method, experiments, and site inspection cases. The results show that the deeper the corrosion depth or the higher the operating frequency, the larger the attenuation rate of the guided wave; also, the higher the operating frequency, the fewer the coherent signals caused by the general corrosion. In the case of detecting localized severe corrosion inside a section of general corrosion on a pipe, the results show that a higher operating frequency reduces the coherent noise of general corrosion, and gives an obvious reflection signal of the local severe corrosion that leads to a good measurement being obtained.     

Explosive welding of stainless steel–carbon steel coaxial pipes

Bi-metallic corrosion resistant steel pipes were produced through explosive welding process. The weldability window of the stainless steel pipe (inner pipe) and the carbon steel pipe (outer pipe) was determined by the use of available semi-empirical relations. The impact velocity of the pipes as the most important collision parameter was calculated by the finite element simulation. Direct effect of the explosive mass reduction on the bonding interface of the pipes was studied. Optical microscopy study showed that a transition from a wavy interface to a smooth one occurs with decrease in explosive load.     

近海大气中耐候钢表面保护锈层残余应力分析

本文观察了锈层/耐候钢界面形貌以及锈层致密情况的变化,建立了锈层/基体界面模型,通过Ansys5.3有限元程序模拟计算,分析了锈层中应力分布情况.在低合金钢大气腐蚀过程中,表面在电化学电位低的地方形成腐蚀坑;在腐蚀坑腰部锈层中存在着应力集中.此外锈层首先开裂,加速此处基体腐蚀,加大了腐蚀坑曲率半径,有利于锈层/基体界面平直化,减弱应力集中程度.促进致密锈层形成.     

Finite element analysis of internal flows with heat transfer

This paper presents a finite element-based model for the prediction of 2-D and 3-D internal flow problems. The Eulerian velocity correction method is used which can render a fast finite element code comparable with the finite difference methods. Nine different models for turbulent flows are incorporated in the code. A modified wall function approach for solving the energy equation with high Reynolds number models is presented for the first time. This is an extension of the wall function approach of Benim and Zinser and the method is insensitive to initial approximation. The performance of the nine turbulent models is evaluated by solving flow through pipes. The code is used to predict various internal flows such as flow in the diffuser and flow in a ribbed channel. The same Eulerian velocity correction method is extended to predict the 3-D laminar flows in various ducts. The steady state results have been compared with benchmark solutions and the agreement appears to be good.     

Hyperbolic constitutive model to study cast iron pipes in 3-D nonlinear finite element analyses

Cast iron (CI) pipes were widely installed as water mains and service connections in the last century and still large numbers of CI pipes remain in service. However, many CI pipes are becoming aged and severely corroded, causing frequent pipe leaks and breaks. To better understand the failure mechanism of CI pipes, this paper provides an efficient numerical approach to analyse the behaviour of CI pipes under various loading conditions. The numerical investigation was realised by implementing a hyperbolic constitutive model (a simplified nonlinear stress-strain analysis) for CI materials into finite element analysis (FEA) in ABAQUS. Three-dimensional (3-D) FEAs were carried out for a series of uniaxial tensile and compressive tests, 3-point beam bending tests and ring bending tests on various CI pipe coupons. The stress-strain characteristics and load-deflection responses obtained from these numerical examples were validated by experimental results. The numerical results obtained from the proposed method are in good agreement with the measured data, which indicates that the mechanical performance of deteriorated CI pipes can be adequately modelled using the relatively simple nonlinear constitutive model implemented in 3-D FEA. As nonlinear behaviour has proven to be intrinsic to the widely used CI pipes, it is expected that the proposed 3-D FEA modelling technique will be of importance to the evaluation of the mechanical performance of CI pipes and, possibly, other CI structures.     

Birefringence analysis of segmented cladding fiber

We present a full-vectorial modal analysis of a segmented cladding fiber (SCF). The analysis is based on the H-field vectorial finite element method (VFEM) employing polar mesh geometry. Using this method, we have analyzed the circular SCF and the elliptical SCF. We have found that the birefringence of the circular SCF is very small (1.0×10(-8)). Birefringence of a highly elliptical SCF can be altered to some extent by the number of segments and duty cycle of segmentation in the segmented cladding. However, the change is not profound. The analysis shows that the circular SCF possesses low birefringence and that the segmented cladding does not add any significant birefringence in an elliptical fiber. This result strongly indicates that small deviations in the segmented cladding parameters arising from fabrication process do not significantly affect the birefringence of the fiber.     

复合材料管接头拉扭作用下胶层应力分析

发展了一种复合材料胶接管接头在拉伸和扭转载荷作用下胶层内应力分析模型。该模型利用管接头结构中缠绕法成形的管和套管材料、几何形状和所承受载荷的轴对称性,采用一阶层合板理论分析管壁的应力和变形。通过管接头中管、套管和它们之间胶层的位移协调性确定胶层内的应变和应力,从而建立了管接头结构的平衡方程。本模型计算了缠绕角为54°和26°管接头胶层内剥离应力、轴向剪切应力和切向剪切应力,计算结果与有限元分析结果相吻合。      

G105钻杆管体刺穿失效分析

某油田钻井公司使用的规格为φ101.6 mm×8.38 mm的G105钻杆先后发生两起管体刺穿失效事故,通过化学成分分析、力学性能测试、金相检验、X射线衍射等方法并结合钻杆的受力状态对钻杆刺穿原因进行了分析。结果表明:钻杆刺穿失效形式为腐蚀疲劳,在钻井液中腐蚀介质作用下,钻杆首先发生氧腐蚀和Cl~-加速腐蚀,使钻杆外表面产生许多腐蚀坑,并于腐蚀坑底部形成应力集中;随后在钻井过程的交变应力作用下,应力集中严重的腐蚀坑底部开始萌生裂纹,裂纹不断疲劳扩展直至穿透管壁;最终在钻杆内钻井液的冲刷作用下发生刺穿失效。     

复合材料管接头与钢管间摩擦力及其对管接头强度的影响———数值分析

通过定义接触单元, 建立了复合材料套管接头与钢管连接的有限元分析模型。分析了在钢管端部受到弯曲、压缩和扭转载荷条件下, 复合材料管接头的应力状态, 并采用Tsai-Wu 强度准则对复合材料管接头进行了强度分析。重点研究了随着摩擦系数的变化复合材料管接头与钢管间摩擦力的变化规律及其对复合材料接头强度的影响。结果表明, 随着摩擦系数的增大, 复合材料管接头与钢管间最大正应力减小, 最大摩擦力增大; 在以弯曲载荷为主的组合载荷作用下, 复合材料管接头的安全裕度增大。