Probabilistic life analysis of vital components considering multistage degradation under variable working conditions

The lifespan of a mechanical product is related to its working conditions; the product's performance typically shows a multistage degradation pattern throughout its life profile. The performance degradation is generally researched under constant test conditions, while the effects of different working conditions on life are seldom considered. This paper proposes a staged recursive derivation method for the multistage degradation under variable working conditions. The proposed method works by merging measured degradation data with an empirical degradation model. The measured degradation data of a new prototype are utilized to update the staged degradation model based on a Bayesian posterior probability analysis. The staged degradation model is derived stage by stage, and then the probabilistic life of the new prototype is predicted. The degradation data of a machine‐gun barrel are used as a case study to demonstrate and validate the proposed method. The results show that the probabilistic life of the test prototype can be predicted effectively in the case of relatively little measured degradation data at the product development stage. Furthermore, the proposed method appears to be especially suited to mechanical components requiring short test periods or low test costs.     

聚乳酸的合成与性能研究

以乳酸单体为原料,采用直接缩聚法合成了具有较高粘均分子质量的聚乳酸,用红外光谱、H—NMR等方法初步表征了聚乳酸的结构。研究了聚乳酸的亲水性和降解性。结果表明,聚乳酸的水接触角和吸水率分别为79．30和2．8％,说明聚乳酸具有一定的亲水性。聚乳酸在降解过程中表现为体型降解特点。粘均分子量和降解温度对聚乳酸的降解规律影响不大,但低分子量聚乳酸及聚乳酸在较高降解温度下的降解程度和初期降解速率明显大于高分子量聚乳酸及较低降解温度。本文还研究了聚乳酸粘均分子量在降解过程中的变化情况。     

层板复合材料疲劳性能的唯象学研究模型及其应用

介绍了层板复合材料在疲劳载荷作用下的剩余强度衰退模型、剩余刚度衰退模型及剩余强度和剩余刚度的关系模型 ,给出了模型的实验验证实例 ,讨论了这些模型的应用范围及需要进一步研究的问题     

Dynamic mechanical analysis and thermal properties of LLDPE/EVA/modified silica nanocomposites

The effect of hexamethylene disilazane modified nanosilica on the dynamic mechanical analysis (DMA), crystallization, melting and thermal degradation behavior of linear low density polyethylene/ethylene vinyl acetate copolymer (LLDPE/EVA) blends are explored.Detailed DMA analysis is carried out in order to investigate the reinforcing behavior of nanosilica adopting Kerner–Nielson model. Oxidative degradation and thermal stabilities of samples are also studied by the thermogravimetery analysis. The high content of nanosilica particles results in significant shift of degradation temperature to higher temperatures in the oxygen atmosphere. This behavior might be attributed to the barrier properties of nanoparticles against oxygen and gaseous degradation products. However, incorporation of modified nanosilica into LLDPE/EVA blend is decreased the onset of degradation temperature of the unfilled system. In nitrogen atmosphere, no changes are observed in the thermal degradation range and only a reduction is documented in the onset of degradation temperature. Considering important role of onset of degradation temperature, activation energy of starting of degradation temperature is calculated utilizing Kissinger-Ozawa model in both oxygen and nitrogen atmospheres. Results showed that activation energy of degradation reaction is decreased by ∼ 20 kJ/mol. This decrease is owing to the release of modifiers from the nanoparticles.     

不同相对分子质量聚乙二醇的热氧降解行为

3种不同相对分子质量的聚乙二醇(PEG)在80℃经过不同的老化时间,均发生了不同程度的热氧降解。采用红外光谱法、凝胶渗透色谱法和核磁共振波谱法研究了不同相对分子质量聚乙二醇热氧降解的行为及降解产物结构,定量表征了不同相对分子质量聚乙二醇的降解程度。结果表明,老化时间越长,3种PEG降解生成的小分子酯类化合物越多。其中PEG10000最稳定,不易发生热氧降解,PEG800降解程度最大。     

Applicability of boron-doped diamond electrode to the degradation of chloride-mediated and chloride-free wastewaters

The degradation of diphenylamine (DPA) in aqueous solution by persulfate is investigated. Effects of pH, persulfate concentration, ionic strength, temperature and catalytic ions Fe(3+) and Ag(+) on the degradation efficiency of DPA by persulfate are examined in batch experiments. The degradation of DPA by persulfate is found to follow the pseudo-first-order kinetic model. Increasing the reaction temperature or persulfate concentration may significantly accelerate the DPA degradation. Fe(3+) and Ag(+) ions can enhance the degradation of DPA, and Ag(+) ion is more efficient than Fe(3+) ion. However, the increase of either the pH value or ionic strength will decrease the rate of DPA degradation. N-Phenyl-4-quinoneimine, N-carboxyl-4-quinoneimine, 4-quinoneimine and oxalic acid are identified as the major intermediates of DPA degradation, and a primary pathway for the degradation of DPA is proposed. The degradation of DPA in surface water, groundwater and seawater is also tested by persulfate, and more than 90% of DPA can be degraded at room temperature in 45min at an initial concentration of 20mgL(-1).     

超高分子量聚乙烯热降解行为与冲击性能

利用热重分析法,差热分析法、粘度法研究了超高分子量聚乙烯（ＵＨＭＰＷＰＥ）在空气或Ｎ２气氛下的热降解行为,考察孤热降解行为对其冲击性能的影响,分析了材料的断口扫描电镜照片。结果表明,ＵＨＭＷＰＥ热降解过程程符合自由基连锁反应机理,其热降解行为受空气或Ｎ２气氛、升温速率和恒温时间的影响,而热降解导致ＵＨＴＭＰＥ微纤断裂是其冲击强度下降的主要原因。     

Degradation modeling and classification of mixed populations using segmental continuous hidden Markov models

With the market demands, the classification for highly reliable products becomes more and more significant. The degradation data can provide information about the degradation states and can be used to classify products to various classes according to the reliability attribute. In this paper, a temporal probabilistic approach, named segmental continuous hidden Markov model (SCHMM), is proposed to tackle the problem of degradation modeling and classification for mixed populations. Separate SCHMMs are built for each class of the mixed populations. The SCHMMs can directly depict the correspondence between actual degradation and the hidden states. A novel method called self‐training algorithm for the preprocessing of the original data from the mixed populations is proposed. Furthermore, the unknown parameters of the SCHMMs are estimated by the maximum likelihood method with the complete degradation data. The root mean square error of the estimated degradation value compared with the actual physical degradation value, as well as Akaike information criterion and Bayesian information criterion, is used for the evolution of the fitting accuracy and the selection of model topologies and discretization methods. Then the maximum posterior probability‐based classification criteria are developed. Degradation tests are designed for the data collection. To obtain the optimal classification policies, a cost function that consists of the degradation test cost and misclassification cost is constructed. A numerical example is used to illustrate the proposed method and demonstrate its advantages by comparing with other classification methods.     

A transformed gamma process for bounded degradation phenomena

Most of the stochastic models adopted to describe the evolution over time of degradation phenomena of technological units assume that their degradation level can increase indeterminately. However, these degradation phenomena are typically subjected to obvious bounds, if only because technological units have finite size. In fact, very often, this inconsistency does not significantly affect the effectiveness of unbounded degradation models, since degrading units are usually assumed to fail when their degradation level exceeds a failure threshold that is much smaller than the obvious bounds. Nevertheless, in some cases, due to the very nature of the underlying degradation mechanism, less obvious bounds could exist, which are not necessarily far from the failure thresholds. The question that arises is whether the use of a bounded degradation model, in this latter type of experimental situations, could be beneficial. For this purpose, since a bounded degradation process should necessarily have dependent increments, in this paper we investigate the potential of a new bounded transformed gamma (TG) process to adequately describe bounded degradation phenomena and predict their future evolution. Differently from other existing gamma process based bounded degradation models, here the upper bound is treated as an unknown parameter that has to be estimated from the available degradation data. A numerical example is presented where the parameters of the proposed model are estimated from simulated data. Then the model is applied to a set of wear measures of cylinder liners that equip a diesel engine for marine propulsion, which have also stimulated this study. Model parameters are estimated by using the maximum likelihood (ML) method. The fitting ability of the proposed new bounded process is compared to that of an unbounded gamma process, which was previously adopted to analyze the same liner wear data. Obtained results are critically discussed in the paper.     

A failure mechanism consistency test method for accelerated degradation test

Accelerated degradation test (ADT) is generally used to accelerate degradation processes in products to estimate their lifespan and to assess their reliability in a short period of time. How to perform the failure mechanism consistency test is crucial in the application of the ADT method. Existing failure mechanism consistency test methods assume that degradation rates among individual products are the same. However, these methods do not take degradation dispersions caused by manufacturing technologies into consideration. To address this issue, a failure mechanism consistency test method for ADT based on the activation energy invariant method and the likelihood ratio test is proposed. First, a degradation modeling method for ADT is introduced. Then, the logarithmic maximum likelihood function values of the degradation models are estimated based on the two-step maximum likelihood estimation (MLE) method. Finally, the decision rule is proposed based on the likelihood ratio test. The method mentioned above is, then, used on the real degradation data of carbon-film resistors and bullet O-rings, and its effectiveness is verified. Furthermore, based on the failure mechanism change point in RTV5370 siloxane rubbers, the simulated degradation data are degenerated to compare the proposed method with the method not considering individual differences in different ADT programs and degradation dispersions.     

Treatment of nitrophenols by cathode reduction and electro-Fenton methods

This study deals with the degradation of various nitrophenols by cathode reduction and electro-Fenton methods. Phenols (Poh), 2-nitrophenol (2-NP), 3-nitrophenol (3-NP), 4-nitrophenol (4-NP), and 2,4-dinitrophenol (2,4-DNP) are treated and different degradation sequences are obtained. The relationship between the structure and activities of nitrophenols is discussed. Using 4-NP as a model nitrophenol, the electrochemical behaviors on graphite cathode and Pt anode are analyzed by cyclic voltammetry. The contribution of different reactions to the degradation of 4-NP is investigated in divided cells. The degradation of 4-NP is much faster in the cathode cell than in the anodic cell. In the cathode cell, the degradation of 4-NP is significantly enhanced by the introduction of aeration and Fe(2+). Ultraviolet-visible (UV-vis) spectra reveal different reaction pathways for the degradation in the anodic cell and cathode cell. Treatment of high concentration of 4-NP in the undivided cell shows that more than 98% removal of 4-NP and about 13% removal of total organic carbon (TOC) are obtained for both processes, while the subsequent biodegradability test shows that electro-Fenton can eliminate the toxicity and improve the biodegradability of 4-NP. Negligible quantity of nitrate and nitrite ions detected in both processes indicates that there is no direct release of -NO(2) and -NO groups from 4-NP and its degradation intermediates. Intermediates such as hydroquinone and bezoquinone are detected by gas chromatography/mass spectrum (GC/MS). The degradation pathway of 4-NP in electro-Fenton process is proposed as the cathode reduction followed by hydroxyl oxidation.     

聚乳酸降解机理及其方法探讨

可降解材料聚乳酸是一种高分子聚酯树脂，在自然条件下可降解成二氧化碳和水，对环境十分友好。凡是能引起酯键断裂的因素都可以使聚乳酸发生降解，分析了聚乳酸间接降解的机理，归纳了引起聚乳酸降解的因素，并对聚乳酸的发展前景做了展望。     

晶体硅太阳电池光衰减现象研究的新进展

晶体硅太阳电池一直占据光伏市场的主导地位,关于其光衰减的研究也因此受到了广泛关注。综合评述了近年来国内外晶体硅太阳电池光衰减现象的研究进展,介绍了硼氧缺陷、铁硼对以及铜相关的缺陷导致光衰减的基本机制,着重阐述了硼氧缺陷的产生率、钝化率以及相应的激活能大小与硼氧含量的关系。最后介绍了减弱或避免光衰减的一些措施。     

Using degradation data to assess reliability: a case study on train wheel degradation

Degradation experiments are usually used to assess the lifetime distribution of highly reliable products, which are not likely to fail under the traditional life tests or accelerated life tests. In such cases, if there exist product characteristics whose degradation over time can be related to reliability, then collecting ‘degradation data’ can provide information about product reliability. In general, the degradation data are modeled by a nonlinear regression model with random coefficients. If we can obtain the estimates of parameters under the model, then the failure‐time distribution can be estimated. In order to estimate those parameters, three basic methods are available, namely, the analytical, numerical and the approximate. They are chosen according to the complexity of the degradation path model used in the analysis. In this paper, the numerical and the approximate methods are compared in a simulation study, assuming a simple linear degradation path model. A comparison with traditional failure‐time analysis is also performed. The mean‐squared error of the estimated 100pth percentile of the lifetime distribution is evaluated for each one of the approaches. The approaches are applied to a real degradation data set. Copyright © 2008 John Wiley & Sons, Ltd.     

Equivalence analysis of accelerated degradation mechanism based on stochastic degradation models

For an effective accelerated degradation test, it is important to ensure that the degradation mechanism under different stress levels remains unchanged. In this article, we are interested in the equivalence analysis of accelerated degradation mechanism based on degradation data rather than physical or chemical techniques. Under the assumption that products' underlying degradation follows stochastic degradation models, we first introduce the relationship between mechanism equivalence and parameters of stochastic degradation models based on the acceleration factor invariant principle. Then the necessary conditions for mechanism equivalence, which should be satisfied under different stress levels, are derived and tested by the proposed parameter equivalence test method based on the modified Bartlett statistic and T statistic. Next a novel selection method for stochastic degradation models is derived therefrom by comparing the variation of coefficients of acceleration factors. The accuracy of the necessary conditions and the parameter equivalence test method is demonstrated through a simulation study. In addition, an electrical connector example with real stress relaxation data is analyzed to illustrate the proposed method further.     

Monetite and brushite coated magnesium: in vivo and in vitro models for degradation analysis

The use of magnesium (Mg) as a biodegradable metallic replacement of permanent orthopaedic materials is a current topic of interest and investigation. The appropriate biocompatibility, elastic modulus and mechanical properties of Mg recommend its suitability for bone fracture fixation. However, the degradation rates of Mg can be rapid and unpredictable resulting in mass hydrogen production and potential loss of mechanical integrity. Thus the application of calcium phosphate coatings has been considered as a means of improving the degradation properties of Mg. Brushite and monetite are utilized and their degradation properties (alongside uncoated Mg controls) are assessed in an in vivo subcutaneous environment and the findings compared to their in vitro degradation behaviour in immersion tests. The current findings suggest monetite coatings have significant degradation protective effects compared to brushite coatings in vivo. Furthermore, it is postulated that an in vitro immersion test may be used as a tentative predictor of in vivo subcutaneous degradation behavior of calcium phosphate coated and uncoated Mg.     

A Bayesian Approach to Condition Monitoring with Imperfect Inspections

Degradation is a common phenomenon for many products. Because of a variety of reasons, the degradation rates of units from the same population are often heterogeneous. In addition, when the degradation process is monitored using dedicated sensors, the measurements are often inaccurate because of various noisy factors. To account for the heterogeneous degradation rate and the non‐negligible measurement errors, we model the degradation observations using a random‐effects Wiener process with measurement errors. Under the model, direct estimation of current degradation and prediction of future degradation are difficult. We thus develop a filtering algorithm that recursively estimates the joint distribution of the degradation rate and the current degradation levels. Based on the estimates, the distribution of the remaining useful life can be timely predicted. Our method is both computational efficient and storage efficient. Its effectiveness is demonstrated through simulation and real data. Copyright © 2013 John Wiley & Sons, Ltd.     

超声波降解油田含聚污水研究进展

随着聚合物驱、复合驱等三次采油技术在油田开采中的广泛应用,我国原油的采收率得到有效提高,但是伴随产生的含聚污水量也在成倍增加,油田含聚污水难降解问题也成为污水处理相关领域研究的热点。超声波降解技术具有氧化能力强、降解速率快且无二次污染等优点,在污水处理中的应用越来越受到重视。介绍了目前油田含聚污水的特性,并分析了超声波对污水中聚丙烯酰胺的降解机理以及对水包油乳状液的破乳机理,综述了国内外超声波降解技术在油田含聚污水处理的现状。结合不同类型油田含聚污水的特点,展望了未来超声波降解技术处理油田污水的发展趋势。     

Modelling and prediction of the chemical and physical degradation of fibre reinforced plastics

Modelling and prediction of the environmental degradation of fibre reinforced plastics (FRP) has been hindered by the complexity of the process. Published works are limited to effects and mechanism characterisation or partial models, most of the time empirical. In this article, an analytical approach is presented which resolves the degradation process into only three components: the chemical link density variation, the cohesion force variation and the stress state modification. The first two are referred to as chemical and physical degradation. Based on material science theories, the analysis demonstrates that in a constant environment an exponential function correlates the chemical and physical degradation to the environmental factors. It is also shown that the chemical and physical degradation rate in a real service environment can be determined in a laboratory in a constant environment based only on the variation of chemical link density. Laboratory experiments show that the model correlates excellently with the degradation process.     

Frequency dependence of degradation and breakdown of thin SiO2 films

Thin oxide MOS capacitors are subjected to bipolar voltage stresses of different amplitudes and frequencies. According to a previously proposed breakdown model, the evolution of the current with the stress time has been considered to be due to the degradation of the oxide, i.e. to the generation and partial occupation of electron traps. When log(J) is represented versus the stress time, the slope of the plot and the magnitude of the current (which tends to decrease during constant-voltage tests) are taken as indicators of the oxide degradation rate and degradation level, respectively. Our results suggest lower degradation rates, and consequently lower degradation levels for the same stress times, at high frequencies. This is consistent with the increase of time-to-breakdown with stress frequency observed by other authors, and confirms that, also for dynamic stresses, the relation between degradation and breakdown is fundamental to understand the physics of these phenomena. The slower degradation rates confirm the improvement of oxide reliability under dynamic AC stress conditions.