Probabilistic risk analysis of corrosion associated failures in cast iron water mains

This paper proposes a method using probabilistic risk analysis for application to corrosion associated failures in grey cast iron water mains. External corrosion reduces the capacity of the pipeline to resist stresses. When external stresses exceed the residual ultimate strength, pipe breakage becomes imminent, and the overall reliability of a water distribution network is reduced. Modelling stresses and external corrosion acting on a pipe involves uncertainties inherent in the mechanistic/statistical models and their input parameters. Monte Carlo (MC) simulations were used to perform the probabilistic analysis. The reduction in the factor of safety (FOS) of water mains over time was computed, with a failure defined as a situation in which FOS becomes smaller than 1. The MC simulations yielded an empirical probability density function of time to failure, to which a lognormal distribution was fitted leading to the derivation of a failure hazard function. A sensitivity analysis revealed that the contribution of corrosion parameters to the variability of time to failure was more significant than the combined contributions of all other parameters. Areas where more research is needed are identified.     

响应面法在结构体系可靠度分析中的应用

一个失效模式由许多的失效单元构成,它是一个并联系统;而所有的失效模式构成一个串联系统。整个结构体系可看成是许多并联系统(失效模式)组成的一个串联系统。首先,利用基于响应面的随机有限元法来获得失效模式中各个单元的极限状态方程,这些方程都是二次多项式;第二步,利用结构可靠度分析中的几何法得到这些方程的等效线性化方程从而可逐步得到该失效模式的等效线性化方程;第三步,计算各失效模式间的相关系数;最后,由Ditlevsen界限法来计算结构的体系可靠度。算例表明,利用该方法来获得大型、复杂结构的体系可靠度具有高效、实用的特点。     

The rise and fall of cast iron in Victorian structures – A case study review

The Victorian era in English history is often referred to as a ‘golden age’, marked by unique and historic achievements in the field of engineering and technology. It was in Victorian Britain that new production methods first allowed cast iron to be produced in large enough quantities to be used in substantial building projects. However, a series of high profile structural failures sent shock waves through the engineering profession and general public, prompting one of the first ever systematic investigations into the failure of structures. To track these developments, this paper takes a retrospective view of a number of major cast iron structural disasters that occurred during this period of history. Reassessment of historic incidents will allow use of modern analytical techniques not available at the time of initial investigation. A specific case study analysis into the demise of the Tay Bridge is used to demonstrate the value of engineering lessons to be learnt from taking a retrospective view. Reconsideration of historic failure is shown to demonstrate incremental advances made in the understanding of the limits of materials available at that time.     

输电杆塔结构体系主要失效模式识别的计算程序研发

对大型复杂结构进行体系可靠性分析时,识别结构体系的失效模式是体系可靠性分析的首要核心工作。针对大型复杂结构体系失效模式识别的难点问题,利用ANSYS软件高效的可视化建模及结构有限元分析求解的功能和MATLAB软件强大的科学计算及易与其他程序接口的特点,基于搜寻结构体系主要失效模式的阶段临界强度分枝——约界法的基本原理,编制了适用于输电杆塔结构体系主要失效模式识别的计算程序,并将其成功应用于某实际工程输电杆塔结构体系主要失效模式的搜寻,结果表明,该程序可以准确高效地进行输电杆塔结构体系主要失效模式的识别,为进行输电杆塔结构体系的可靠度分析奠定了基础。     

Reliability analysis of pipelines carrying H2S for risk based inspection of heavy water plants

Reliability analysis of pipelines containing corrosion defects due to H₂S is presented. The pipeline carrying H₂S is more susceptible to the internal corrosion thereby reducing the pipeline's load carrying capacity. The objective of this study is to obtain the pipelines' failure probabilities that are required in establishing a Risk Based Inspection (RBI) programme for heavy water plants. The reliability assessment of pipelines involves the estimation of failure pressure and evaluating the limit state function. Several failure pressure models were studied for this purpose and it was found that the modified B31G failure pressure model is most suitable for the pipeline failure pressure modeling. Due to the presence of non-linearity in the limit state function and non-normal variables, the first order reliability method has been employed for carrying out the reliability analysis. The uncertainty of the random variables on which the limit state function depends is modeled using normal and non-normal probabilistic distributions. The failure probabilities and the categories of the pipelines connected to the first pair of first stage of exchange towers are presented. In addition, a sensitivity analysis was carried out on random variables involved in the problem. The results of sensitivity analysis are also presented.     

在役腐蚀管道动态非概率可靠性分析简

 腐蚀失效是压力管道失效的主要形式之一,研究腐蚀管道的可靠性具有重要理论意义和应用价值.在对腐蚀管道可靠性分析时,概率可靠性模型和模糊可靠性模型对于数据信息的要求较高.而在掌握不确定性信息很少情况下,为了充分利用管道的不确定性信息弥补原始数据的不足,可将腐蚀管道可靠性分析中的材料屈服强度、管道直径、缺陷深度和操作压力等不确定参数视为区间变量,基于区间模型建立一种在役腐蚀管道动态非概率可靠性模型,给出了腐蚀管道剩余寿命预测的简便方法.结合工程实例计算与分析,表明了文中所提出方法的可行性和合理性,并在此基础上,分析了管道的壁厚、缺陷深度、实际压力和腐蚀速率这些区间变量的不同变异系数对非概率可靠性指标的影响,分析结果表明非概率可靠性指标对管道壁厚的变异系数最为敏感.     

Failure of malleable cast iron sprinkler pipe end cap due to freezing of water within

Failures in end caps of sprinkler firewater systems frequently occur due to the freezing of the water inside the sprinkler systems on cold winter days. In these failures, a flat top of an end cap of a sprinkler system fails and completely separates from the threaded body of the end cap. In this study, metallurgical investigations including fractography, metallography, EDS analysis, and hardness measurement were performed for the failure analysis of the end cap. Finite element analysis was employed to identify both the maximum stress location and the maximum stress magnitude resulting from the freezing of and interaction between the ice and end cap when temperature dropped below the freezing point.Microstructure observations confirmed the ferritic malleable cast iron of the end cap. The hardness of the end cap was significantly below the hardness range specified for standard ferritic malleable cast iron. Small cracks formed during the casting of the end cap and the low strength of the end cap caused the final fracture. Failure stress determined by finite element analysis simulating the freezing of the inside water could accurately predict the failure stress and failure temperature. High tensile stress and poor manufacturing quality caused the crack to propagate in the cleavage mode and at the final stage in the ductile mode.     

Characterisation of the damaging micromechanisms in a pearlitic ductile cast iron and damage assessment by acoustic emission testing

The damaging micromechanisms in a pearlitic (EN‐GJS700‐2) ductile cast iron (DCI) are investigated by means of scanning electron microscope (SEM) analysis and acoustic emission (AE) testing. Monotonic uniaxial tensile tests are performed on microtensile specimens under strain control. SEM analysis is applied under in situ conditions by means of a tensile holder. The multiple damaging micromechanisms are identified, and their evolution along with the mechanical response is characterised. The traditional AE features are found to be qualitatively correlated to the onset of the fracture damage over the elastic behaviour. The information entropy of the AEs evaluated according to both Shannon and Kullback‐Leibler formulations is proven to be well correlated to the ongoing damage and the incipient failure. Tentative failure criteria are finally proposed. The assessment approach is found to be promising for structural health monitoring purposes.     

灾害荷载下结构体系失效模式的相关性及可靠度计算

本文通过引入荷载粗糙度指标,根据有关统计参数讨论了灾害荷载的特性,研究了灾害荷载下结构体系失效模式的相关性及可靠度的近似计算方法,得到了以下结论：灾害荷载下结构体系的失效模式近似完全相关,结构体系可靠度由结构的最弱失效模式决定。     

有限元法和退火进化算法相结合分析结构模糊可靠性

结构的失效除了具有随机性，还应具有模糊性。本文在介绍一种修正的联合概率密度函数的基础上，采用有限元法和退火进化算法相结合来研究结构的模糊可靠度。在每一模糊失效水平下，有限元法用来计算荷载效应项，并将荷载效应项代入原联合概率密度函数形成修正的联合概率密度函数。为了解决进化算法的早熟收敛问题，采用模拟退火算法与进化算法相结合，以保证更有效地搜索到最可能失效点(设计点)。解决不存在显式极限状态方程的大部分实际结构的可靠度研究的困难。数例结果表明该法可直接应用现有的确定性的有限元程序，并且具有很好的效率和精度。     

Failure investigation of lime coated grey cast iron moulds at TEMCO ferroalloy smelter

This case study outlines an investigation to identify the root cause for the failure of casting machine moulds, manufactured from grey cast iron, at the TEMCO Ferroalloy smelter in Bell Bay, Tasmania. A preliminary stress analysis shows that the thermal stresses developed in the moulds during operation are insufficient to cause failure in a non-defective mould. The failure can instead be attributed to high temperature oxidation that results from a combination of high mould surface temperature, decomposition of the hydrated lime release coating and the grey cast iron structure. Preventing the oxidation reaction would eliminate this material damage and increase the service life of the moulds. The formation of the high temperature oxide can be prevented by reducing the maximum surface temperature of the mould, using a coating that does not release water at operating temperatures or using moulds manufactured from a different type of cast iron.     

Fracture mechanics defect assessment diagram on pipe from steel P264GH with a notch

The design for a pipeline component is considered a tough challenge once defects in the base material develop. To address this issue, we present a gouge defect located in a pipe submitted to internal pressure. This problem is often encountered in the field of pipeline network and boiler components because of structural imperfection. To simulate the activity of such pressurized pipeline, a longitudinal cylindrical shell under pressure from a material of steel P264GH were involved in the research. Secondly, by this paper we propose a safety methodology to detect the boundary of failure assessment diagram (FAD), as a new solution to predict the service life of pipeline products. The methodology is connected to the modified FAD approach, whereas the specific parameters were settled using the Volumetric Method calculation, based on the notch Stress Intensity Factor, determined in the elasto-plastic field. This new modified FAD methodology entails an accurate solution, suitable to be used as expert tool for the assessment of workability and reliability of pipes network, in the case of corrosion defects presence.     

Assessment of the efficiency of Kriging surrogate models for structural reliability analysis

This paper presents an assessment of the efficiency of the Kriging interpolation models as surrogate models for structural reliability problems involving time-consuming numerical models such as nonlinear finite element analysis structural models. The efficiency assessment is performed through a systematic comparison of the accuracy of the failure probability predictions based on the first-order reliability method using the most common first- and second-order polynomial regression models and the Kriging interpolation models as surrogates for the true limit state function. An application problem of practical importance in the field of marine structures that requires the evaluation of a nonlinear finite element structural model is adopted as numerical example. The accuracy of the failure probability predictions is characterised as a function of the number of support points, dispersion of the support points in relation to the so-called design point and order of the Kriging basis functions. It is shown with the application problem considered that the Kriging interpolation models are efficient surrogate models for structural reliability problems and can provide significantly more accurate failure probability predictions as compared with the most common polynomial regression models.     

球墨铸铁煤气管道泄漏原因分析

通过宏观和微观分析等手段,对某QT400-18球墨铸铁煤气管道泄漏原因进行了分析。由宏观分析可知管道腐蚀泄漏是由局部腐蚀穿孔所致,腐蚀从内壁开始,由内向外直至穿孔泄漏;微观分析没有观察到微裂纹等缺陷,成分分析可知管道内壁腐蚀产物中含有大量硫元素,说明管道腐蚀泄漏的产生与管内介质中含有硫元素有关。综合分析结果表明:由于所输送煤气中含硫杂质超标,与管内壁冷凝水共同作用产生局部电化学腐蚀,从而导致了煤气管道的局部穿孔泄漏。建议在生产中将煤气中的有害杂质和水分含量控制在工艺要求范围内。     

Weibull analysis, extrapolations and implications for condition assessment of cast iron water mains

The applicability of Weibull statistics to the condition assessment of cast iron water distribution pipes has been considered. The effect of Weibull modulus, characteristic strength, sample size and mode of loading (tension or flexure) on the strength of cast iron water distribution pipes is investigated. The strength distribution of cast iron samples cut from sections of five different water distribution pipes recovered from the ground have been characterized. Strengths have been measured in flexure, at two different temperatures (ambient and 0 °C), and in tension at ambient temperature using two different sample sizes. It is shown that characteristic strength values in flexure decrease with increasing size of graphite flake and that there is no significant difference between the results at the two temperatures investigated. For samples of the same volume tested in tension and flexure, the reduced strength measured in tension is consistent with Weibull predictions. However, the strength of large samples tested in tension was not significantly different from the small samples, perhaps because the samples were of the same thickness and conventional Weibull scaling is not applicable. Finally, using a method which treats a large pipe as an assembly of small samples, the strength distributions from the small samples tested in tension are used to make a prediction of the strengths of 1 m span sections of pipe loaded in three‐point bending, which were reported in previous work. The predicted pipe strengths are close to the lower end of the measured pipe strength distribution. Overall, this work suggests that Weibull analysis is a useful tool to examine the strength distribution of removed from cast iron water pipes and so has the potential to contribute in the assessment of asset condition.     

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.     

Failure analysis of water supply system in the Polish city of Głogów

The aim of this work is to diagnose the operation (and faults) of the water supply system in Głogów, Poland. The failure analysis has been conducted basing on the operating data from 2007–2010.About 140 km of main and distribution conduits with diameters of 80–700 mm and 42 km of house connections with diameters of 25–200 mm have been investigated. The pipes are mainly made of grey cast iron, steel, and plastic. The analysis of the number of failures and failure rate λ in each year and on average in the whole inspected period involves a division into types of water-pipe networks as well as into the materials and diameters of particular conduits.On average, in 2007–2010, the values of indicator λ (fail./(km a)) were equal to 0.18 for mains, 0.31 for distribution pipes, and 0.49 for house connections. Failure rates of main and distribution conduits made of steel, grey cast iron and plastics were equal to 0.68, 0.25 and 0.08, respectively. The failure rate was almost two times higher for water mains and distribution conduits in winter (0.41) than in the rest of the year (0.23). As the literature and own investigations indicate, the technical condition of water network in Poland is improving, also in Głogów. Nevertheless, failure rate is higher than in other European countries. In Poland, the old grey cast iron and steel conduits are being modernized, which entails the increase of water-pipe reliability, the decrease of water losses, and the improvement of water quality.     

Reliability analysis of reinforced concrete grids with nonlinear material behavior

Reinforced concrete grids are usually used to support large floor slabs. These grids are characterized by a great number of critical cross-sections, where the overall failure is usually sudden. However, nonlinear behavior of concrete leads to the redistribution of internal forces and accurate reliability assessment becomes mandatory. This paper presents a reliability study on reinforced concrete (RC) grids based on coupling Monte Carlo simulations with the response surface techniques. This approach allows us to analyze real RC grids with large number of failure components. The response surface is used to evaluate the structural safety by using first order reliability methods. The application to simple grids shows the interest of the proposed method and the role of moment redistribution in the reliability assessment.     

Thermal seizures in automotive drum brakes

In automotive industry, drum brake system is used on two types of wheels: cast and spoke. Brake drum, brake panel and brake drum liner are important components of the brake system. Failures of these components observed during high-g braking on spoke wheels of a motorcycle are reported and systematically analyzed in this paper. The brake drum and the panel were found to have seized during high speed brake applications. Excessive wear on the drum liner made of cast iron was also observed. Metallurgical analysis (chemical analysis, hardness test and microstructure analysis) of the liner revealed that excessive wear on the liner was not due to any change in material properties. Hence, further steps were taken to investigate the problem. An experimental testing methodology was developed to simulate these failures. For the same material, testing conditions, and design specifications of cast and spoke wheels, no failure was observed in the cast wheels. This unusual failure was further investigated using three-dimensional steady state finite element analysis (FEA) of both cast and spoke wheels. The methodology adopted for determining the thermal and structural boundary conditions have been described in detail. Energy balance methodology was employed to determine the heat flux values on the drum liner. The structural boundary conditions are determined experimentally and validated with FEA. The predicted temperature from FEA for cast and spoke wheels compares reasonably well with the experiments. It was found that the failure of the brake system in the spoke wheels was due to excessive thermal expansion of the brake panel and the drum beyond the specified limit. An optimum range of labyrinth clearance between the brake drum and the brake panel was recommended for the brake system of cast and spoke wheels.     

Brittle Fracture of Hot-Forging Cast Dies

The site applications of cast hot-forging dies made from a new type cast steel were performed and several of the dies experienced brittle fracture. Analysis of the operating modes and evaluation of mechanical properties were used to establish relations among failure patterns and lives of dies. The results show that the dies experience elevated-temperature wear, plastic deformation and thermal fatigue and that brittle fracture was the predominant failure pattern. Brittle fracture of cast dies has two patterns: early brittle fracture and impact fatigue fracture. The early brittle fractures seldom occur but, when this failure mode occurs, the failure severely shortens the life of the cast dies. The early brittle fracture is attributed to abnormal heat-treatment and operation. Impact fatigue fracture is the dominant failure pattern in the cast dies. This failure mode usually initiates from sand inclusions, micro-cracks and/or thermal fatigue cracks. The specifications of the die material should include a requirement for impact toughness to assure that the dies are resistant to multiple impact loads. This means that strength-dominant toughness must be controlled and optimization of the heat-treatment processes must focus on this requirement.