Risk of structural failure in concrete sewers due to internal corrosion

The purpose of the investigations was to find out the frequency of internal corrosion occurrence in concrete sewers, the thickness of corroded walls, and to develop the method of determining the risk of structural failure due to corroded pipes.The data on corroded concrete pipes were collected in CCTV (Closed Circuit Television) surveys performed at the Kielce University of Technology. The surveys covered the total of > 14 km of concrete sewers operating in numerous locations in Poland's cities and towns. The frequency and the thicknesses of corroded pipes in sanitary, stormwater and combined sewers were established. The sewers, in which corrosion was observed, were classified into the proposed categories of structural failure probability which differed in sewer wall loss relative to the full thickness of the sewer wall. Additionally, the method was proposed for determining the category of failure consequences and the risk of sewer failure due to internal corrosion.The results of surveys showed that, like in other countries, also in Poland, internal corrosion in concrete sewer poses a serious structural hazard to their safety. The proposed method gives an important tool that allows a proper management of sewer systems constructed from concrete pipes. The method makes it possible to eliminate, or significantly to reduce structural risk caused by internal corrosion in concrete sewers which can be done by scheduled CCTV surveys of sewers and trenchless renewal performed according to plans developed in advance.     

Development of a protected gas sensor for exhaust automotive applications

A /spl beta/-alumina-based gas sensor for automotive exhaust application (hydrocarbon, CO, NO/sub 2/ detection in 10-1000 ppm concentration range) has been developed by thick film technology (screen-printing) in the frame of a European project. The sensing device consists of a solid electrolyte (/spl beta/ alumina) and of two metallic electrodes having different catalytic properties, the whole system being in contact with the surrounding atmosphere to be analyzed. The detection principle is based on the chemisorption of oxygen which leads to a capacitance effect at the metal-electrolyte interface, resulting in a measurable difference of potential depending on nature and concentration of pollutants and on the sensor temperature. For application in exhaust pipe, a porous protective layer based on /spl alpha/-alumina for preserving the sensing material and the metal electrodes from contamination and deterioration was screen-printed on the sensing element. For limiting the possible interface interactions between the overlapped layers, a new concept of screen -printable ink was set up based on mixing the oxide powder and its gelly precursor without any inorganic binder addition. The performances of the sensor were tested both on laboratory and engine bench. The sensitivity is relevant for exhaust application, and the long-term stability is improved by the protective layer.     

Simulation of road salt corrosion in austenitic alloys for automotive exhaust systems

Abstract

Cyclic oxidation tests in air with intermittent salt spraying have been performed to simulate the conditions of road salt (NaCl–CaCl₂) enhanced corrosion in automotive exhaust systems.

Tests were carried out at 600 and 700°C on three austenitic alloys, including two stainless steels currently employed for exhaust components (AISI 316 Ti and AISI 302B) and a higher nickel heat resisting alloy.

The presence of salt causes internal corrosion, both along a regular front beneath an outer oxide scale and down alloy grain boundaries. An increase in temperature accelerates the corrosion rate and particularly enhances intergranular penetration.

The results of micrographic and microanalytical investigations are in general agreement with an active oxidation mechanism in which Na_xCl_x vapour species, and not only chlorine, appear to play an important role. The regions directly affected are depleted in chromium and iron and enriched in nickel.

Although internal oxidation of silicon is observed, a high silicon content (2%) does not necessarily ensure effective protection against this type of attack.     

A life prediction model for the chemical failure of FeCrAlRE alloys: preliminary assessment of model extension to lower temperatures

Abstract

FeCrAl alloys are being deployed increasingly for industrial applications at elevated temperatures, primarily because their environment protection derives from the formation of an alumina scale. However, such protection is limited ultimately by chemical failure of the scale resulting in catastrophic, non-protective corrosion rates. The accurate prediction of component failure, therefore, is essential for economic and safety reasons.

A model for the chemical failure of alumina scales has been developed, which predicts the lifetimes of commercial FeCrAlRE alloys in oxidising environments. This model is based on the consumption of aluminium in the alloy through scale growth and also takes into account the effect of mechanical scale failure/spallation accelerating the rate of aluminium depletion – over the temperature range (1100–1400°C) this model has been shown to be applicable, when the scale formed was α-Al₂O₃. Its growth and failure processes are essentially similar over the range, although, of course, rates are temperature dependent.

In other potential technological applications, for example automotive catalytic converters, service temperatures can be much lower, in some instances as low as 750°C, while component sections are considerably thinner than for the high temperature (≥1100°C) structural applications.

At these lower temperatures scale growth mechanisms are more complex, involving the formation initially of transitional aluminas, which in some instances transform with time into the more stable α alumina. Additionally, the role of the reactive element (RE) and also of the mechanical interaction between substrate and scale can vary over the lower temperature range.

Component lifetime prediction at these temperatures is vital also. So, to initiate this action, this paper will present a preliminary assessment of the ability of the existing high temperature model to predict the lifetime of FeCrAlRE alloys in oxidising environments at temperatures in the range 750–1050°C.     

A predictive fatigue life model for anodized 7050 aluminium alloy

The objective of this study is to predict fatigue life of anodized 7050 aluminum alloy specimens. In the case of anodized 7050-T7451 alloy, fractographic observations of fatigue tested specimens showed that pickling pits were the predominant sites for crack nucleation and subsequent failure. It has been shown that fatigue failure was favored by the presence of multiple cracks. From these experimental results, a fatigue life predictive model has been developed including multi-site crack consideration, coalescence between neighboring cracks, a short crack growth stage and a long crack propagation stage. In this model, all pickling pits are considered as potential initial flaws from which short cracks could nucleate if stress conditions allow. This model is built from experimental topography measurements of pickled surfaces which allowed to detect the pits and to characterize their sizes (depth, length, width). From depth crack propagation point of view, the pickling pits are considered as stress concentrator during the only short crack growth stage. From surface crack propagation point of view, machining roughness is equally considered as stress concentrator and its influence is taken into account during the all propagation stage. The predictive model results have been compared to experimental fatigue data obtained for anodized 7050-T7451 specimens. Predictions and experimental results are in good agreement.     

A periodic inspection and replacement policy for systems subject to competing failure modes due to degradation and traumatic events

This paper deals with the condition-based maintenance of single-unit systems which are subject to the competing and dependent failures due deterioration and traumatic shock events. The main aim is to provide a model to assess the value of condition monitoring information for the maintenance decision-making. A condition-based periodic inspection/replacement policy is developed and compared with a benchmark time-based block replacement policy.Numerical results show that it is indeed useful to follow closely the actual evolution of the system to adapt the maintenance decisions to the true system state to improve the performance of maintenance policies. The analysis of the maintenance costs savings can be used to justify or not the choice to implement a policy based on condition monitoring information and to invest in condition monitoring devices.     

A progressive failure model for mesh-size-independent FE analysis of composite laminates subject to low-velocity impact damage

An original, ply-level, computationally efficient, three-dimensional (3D) composite damage model is presented in this paper, which is applicable to predicting the low velocity impact response of unidirectional (UD) PMC laminates. The proposed model is implemented into the Finite Element (FE) code ABAQUS/Explicit for one-integration point solid elements and validated against low velocity impact experimental results.     

A model for prediction of time from corrosion initiation to corrosion cracking

Prediction of time to corrosion cracking is a key element in evaluating the service life of corroded reinforced concrete (RC) structures. This paper presents a mathematical model that predicts the time from corrosion initiation to corrosion cracking. In the present model a relationship between the steel mass loss and the internal radial pressure caused by the expansion of corrosion products is developed. The concrete around a corroding steel reinforcing bar is modeled as a thick-walled cylinder with a wall thickness equal to the thinnest concrete cover. The concrete ring is assumed to crack when the tensile stresses in the circumferential direction at every part of the ring have reached the tensile strength of concrete. The internal radial pressure at cracking is then determined and related to the steel mass loss. Faraday’s law is then utilized to predict the time from corrosion initiation to corrosion cracking. The model accounts for the time required for corrosion products to fill a porous zone before they start inducing expansive pressure on the concrete surrounding the steel reinforcing bar. The accuracy of the model is demonstrated by comparing the model’s predictions with experimental data published in the literature.     

Unidimensional model for pipe flow of a gas mixture allowing for condensation

Condensation kinetics is described with allowance for the dependence of the temperature of the phase transformation on the pressure and concentration of the condensing gas at the phase boundary.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 43, No. 5, pp. 727–733, November, 1982.     

Extension to finite life of a failure assessment diagram for contact fatigue loading

The paper presents an extension to finite life of a failure assessment diagram recently proposed for components subjected to rolling contact loading. The approach considers inherent defects, like inclusions, as potential sources of crack initiation and propagation. A predictive model based on the critical distance theory was proposed for this aim, allowing to define iso-life curves, represented by a quarter of ellipse in the diagram. For working conditions exceeding shakedown, an approach to calculate finite life due to ratcheting was also presented, by comparing cumulated shear strain with the critical one.The proposed approach was used to predict the number of cycles to failure of disk specimens made of SAE 5135 gear steel and compared with the results of previously carried out rolling contact fatigue tests. The agreement between predicted and experimental specimens life resulted good, showing the effectiveness of the approach.     

汽车尾气净化负载钯超微粒催化剂的研究

以复合稀土氧化物的γ-Al2O3为催化剂载体,将负载钯的超微粒子金属铁、钴、镍与γ-Fe2O3粉体混合成型焙烧制成催化刺.研究了复合稀土氧化物对γ-Al2O3表面积的影响,通过催化剂老化实验、丙烷化合物起燃温度检测和对模拟汽车尾气中丙烷、CO及NOx等物质的转化性能的研究,表明该型催化剂各性能较好.     

A mixed-Weibull regression model for the analysis of automotive warranty data

This paper presents a case study regarding the reliability analysis of some automotive components based on field failure warranty data. The components exhibit two different failure modes, namely early and wearout failures, and are mounted on different vehicles, which differ among themselves for car model and engine type, thus involving different operating conditions. Hence, the failure time of each component is a random variable with a bimodal pdf which also depends upon a vector of covariates that indexes the specific operating condition. Then, a mixed-Weibull distribution, where the pdf of each subpopulation (namely the ‘weak’ and ‘strong’ subpopulation) depends on the covariates through the scale parameter, is used to analyze the component lifetime. A Fortran algorithm for the maximum likelihood estimation of model parameters has been implemented and a stepwise procedure, in its backwards version, has been used to test the significance of covariates and to construct the regression model. The presence of a weak subpopulation has been verified and the fraction of weak units in the population has also been estimated. Finally, the adequacy of the proposed model to fit the observed data has been assessed.     

A comprehensive framework for evaluation of piping reliability due to erosion–corrosion for risk-informed inservice inspection

Mathematical models described by multivariable functions f(x) where x=(x₁,…,x_n) are investigated. If the model output is influenced mainly by low order combinations of input variables x₁,…,x_n, an attempt can be made to construct a low order approximation to the model using values of f(x) only.     

一种减小有限控制集模型预测控制开关状态计算时间的方法

有限控制集模型预测控制(finite-control-set model predictive control,FCS-MPC)依靠被控逆变器所具有开关状态对控制目标滚动寻优计算。为减小多电平逆变器中滚动计算开关状态所需时间,该文对经典有限控制集模型预测电流控制进行改进。首先,利用控制系统的离散数学模型和参考电流求得被控逆变器在此参考电流下所应输出的参考电压值。此时,引进SVPWM中的分扇区计算概念,判断参考电压所处扇区,最后将该参考电压所处扇区内所包含的开关状态来循环寻优计算。最后Matlab/Simulink仿真平台搭建二极管钳位型五电平并网逆变器系统模型,通过仿真结果,得以验证改进算法的正确性和实用性。     

A probabilistic physics-of-failure model for prognostic health management of structures subject to pitting and corrosion-fatigue

A combined probabilistic physics-of-failure-based model for pitting and corrosion-fatigue degradation mechanisms is proposed to estimate the reliability of structures and to perform prognosis and health management. A mechanistic superposition model for corrosion-fatigue mechanism was used as a benchmark model to propose the simple model. The proposed model describes the degradation of the structures as a function of physical and critical environmental stresses, such as amplitude and frequency of mechanical loads (for example caused by the internal piping pressure) and the concentration of corrosive chemical agents. The parameters of the proposed model are represented by the probability density functions and estimated through a Bayesian approach based on the data taken from the experiments performed as part of this research. For demonstrating applications, the proposed model provides prognostic information about the reliability of aging of structures and is helpful in developing inspection and replacement strategies.     

A new model for the strategic management of inventories subject to peaks in market demand

This research work is aimed at analysing a production system under dynamic conditions. The focus is on determining the strategic inventory, defined as the minimum initial inventory level needed to fulfil a sudden peak of finished products demand, as well as the minimum allowed lapse of time between two consecutive peaks to avoid stockout. A background of theoretical results and a comprehensive survey of the literature are provided, structured through a classification framework. Furthermore a new model has been developed which explicitly considers production capacity limits. It actually represents the only nonlinear model throughout the research body of knowledge in this field and, at the same time, it is characterised by an outstanding level of simplicity, which allows theoretical results in a closed analytic form, on behalf of the implementation in real-life manufacturing environments.