描述机械合金化过程的理论模型

本文综述了几种重要的描述机械合金化过程的理论模型，模型分析了机械合金化过程中球的机械运动，以及粉末在碰撞时的变形，断裂和焊合，碰撞能量的转化和粉末温升等重要问题，在一定程度上反映机械合金过程的趋势。     

描述机械合金化过程的理论模型

本文综述了几种重要的描述机械合金化（ＭＡ）过程的理论模型。这些模型分析了机械合金化过程中球的机械运动，以及粉末在碰撞时的变形、断裂和焊合，碰撞能量的转化和粉末温升等重要问题，在一定程度上反映机械合金化过程的趋势。     

机械合金化的原理及在磁性材料研究中的应用

介绍了机械合金化的原理和描述机械合金化过程的理论模型。综述了机械合金化在磁性材料研究中的应用，并对目前研究中的存在的问题及发展前景进行了分析。     

机械合金化制备Cu—Nb纳米弥散强化铜合金的研究

介绍了机械合金化制备Cu-Nb纳米弥散强化铜合金的研究情况，重点论述了机械合金化的制备原理、过程参数和后续处理工艺对合金综合性能的影响，并简单介绍了材料的应用前景。     

机械合金化制备MOSi2基复合材料

总结了近年来国内外大于机械合金化制备MoSi2基复合材料的研究成果，重点讨论了MoSi2的概械合金化合成过程和机理，以及机械合金化制备的MoSi2基复合材料的力学性能。机械合金化是一种合成MoSi2及其复合材料的行之有效的方法，其合成过程和机理与球磨条件有关，系统地研究了不同增强体对MoSi2的机械合金化合成过程和机理的影响是必要的，利用微合金化，合金化和复合化三方面的综合作用将成为改善MoSi2基复合材料性能的新发展趋势。     

Fe-Ni机械合金化过程中马氏体的形成及相变

研究了Ni含量、机械合金化工艺参数对Fe-Ni机械合金化过程中马氏体相变的影响及其机理.结果表明:在Fe-Ni机械合金化过程中存在着马氏体相变,但继续机械合金化马氏体是否会发生逆转变主要由Ni含量决定.当Ni≤30%(质量分数,下同)时,机械合金化引起的材料局部温度未达到形变促使马氏体相变逆转变开始温度,因此继续机械合金化马氏体不转变.对于Fe-35Ni,形变促使逆转变的开始温度低于局部温升,马氏体将向奥氏体转变.当Ni含量为35%时,随着机械合金化时间的延长、球磨速度和球料比的提高,机械合金化可以提供的相变驱动力增大导致奥氏体的量逐渐增多.     

机械合金化制备Cu-Nb纳米弥散强化铜合金的研究

介绍了机械合金化制备Cu-Nb纳米弥散强化铜合金的研究情况,重点论述了机械合金化的制备原理、过程参数和后续处理工艺对合金综合性能的影响,并简单介绍了材料的应用前景.     

机械合金化的机制

机械合金化是一种制备平衡态和亚稳态材料的新兴技术。机械合金化的热力学与动力学不同于常规的固态反应，而且相转变方式与合金系统及球磨条件密切相关。着重评述了机械合金化过程中非晶态，金属间化合物，过饱和固溶体及纳米晶形成的特点及机制。     

机械合金化和粉末冶金法制备Fe-Mn-Si基形状记忆合金的研究进展

铁基形状记忆合金由于价格低廉、强度高、加工性能好、可焊接等优点引起广泛重视。机械合金化(MA)和粉末冶金(PM)作为制备材料的新工艺,可以用来制备性能优越的形状记忆合金。本文详述了机械合金化和粉末冶金工艺在制备Fe-Mn-Si基形状记忆合金过程中对合金相变、组织与性能的影响,以及此类合金在新领域的应用。最后提出了现阶段在研究MA/PM工艺制备Fe-Mn-Si基SMA中有关工艺参数、相变机制以及回复应力和低温应力松弛所存在的问题。     

机械合金化制备Co-Cu纳米晶过饱和固溶体

根据Miedema半经验理论建立Co-Cu二元系机械合金化过程的热力学模型,讨论机械合金化法制备的Co-Cu(原子分数为20%)纳米晶过饱和固溶体的热力学和动力学特点。结果表明:采用机械合金化法可以获得Co-Cu纳米晶过饱和固溶体;热力学计算结果显示,Co-Cu二元系不具有形成固溶体的热力学驱动力,Co-Cu合金经机械合金化制备纳米晶过饱和固溶体的驱动力主要来源于动力学。     

ARMA模型用于多变量随机过程的数字仿真

本文研究采用ARMA模型对多变量随机过程进行数字仿真问题,导出了将模型的AR和MA参数分开计算的参数确定算法,从而减少了所需求解的联立方程的个数.算例表明,采用由所提出的算法导出的ARMA模型可很好地实现对多变量随机过程的仿真.     

Time-series analysis if data are randomly missing

Maximum-likelihood (ML) theory presents an elegant asymptotic solution for the estimation of the parameters of time-series models. Unfortunately, the performance of ML algorithms in finite samples is often disappointing, especially in missing-data problems. The likelihood function is symmetric with respect to the unit circle for the estimated zeros of time-series models. As a consequence, the unit circle is either a local maximum or a local minimum in the likelihood of moving-average (MA) models. This is a trap for nonlinear optimization algorithms that often converge to poor models, with estimated zeros precisely on the unit circle. With ML estimation, it is much easier to estimate a long autoregressive (AR) model with only poles. The parameters of that long AR model can then be used to estimate MA and autoregressive moving-average (ARMA) models for different model orders. The accuracy of the estimated AR, MA, and ARMA spectra is very good. The robustness is excellent as long as the AR order is less than 10 or 15. For still-higher AR orders until about 60, the possible convergence to a useful model will depend on the missing fraction and on the specific properties of the data at hand.     

On Model Selection for Autocorrelated Processes in Statistical Process Control

Using traditional control charts to monitor autocorrelated processes is not beneficial, because it will lead us to misleading detections in the processes. One of the methods used to deal with the control charts for autocorrelated process is the model‐based approach. It uses an adequate time series model that fits the process and uses the residuals as monitoring statistics. For the said purpose, it is important to pick a suitable model that can adequately be used for different designs of control charts under specific time series model. This study intends to do the same for three popular types of charts namely Shewhart, exponentially weighted moving average, and cumulative sum. The models covered in this study include AR(1), MA(1), and ARMA(1,1) as the potential models to fit the process of interest. We have focused on two performance aspects namely efficiency and robustness. Average run length is used as a performance measure for different in‐control and out‐of‐control states of the autocorrelated processes under varying levels of autocorrelation. An application example based on a real data set is also included in the study to highlight the importance of the study proposals. Copyright © 2016 John Wiley & Sons, Ltd.     

Modified Durbin Method for Accurate Estimation of Moving-Average Models

Spectra with narrow valleys can accurately be described with moving-average (MA) models by using only a small number of parameters. Durbin's MA method uses the estimated parameters of a long autoregressive (AR) model to calculate the MA parameters. Probably all the pejorative remarks on the quality of Durbin's method in the literature are based on suboptimal or wrong choices for the method of AR estimation or for the order of the intermediate AR model. Generally, the AR order should considerably be higher than the order of the best predicting AR model, and it should grow with the sample size. Furthermore, the Burg estimates for the AR parameters give the best results because they have the smallest variance of all the AR methods with a small bias. A modified Durbin MA method uses a properly defined number of AR parameters, which was estimated with Burg's method, and outperforms all the other known MA estimation methods, asymptotically as well as in finite samples. The accuracy is generally close to the Cramer-Rao bound.     

Automatic identification of time-series models from long autoregressive models

Identification is the selection of the model type and of the model order by using measured data of a process with unknown characteristics. If the observations themselves are used, it is possible to identify automatically a good time-series model for stochastic data. The selected model is an adequate representation of the statistically significant spectral details in the observed process. Sometimes, identification has to be based on many less than N characteristics of the data. The reduced statistics information is assumed to consist of a long autoregressive (AR) model. That AR model has to be used for the estimation of moving average (MA) and of combined ARMA models and for the selection of the best model orders. The accuracy of ARMA models is improved by using four different types of initial estimates in a first stage. After a second stage, it is possible to select automatically which initial estimates were most favorable in the present case by using the fit of the estimated ARMA models to the given long AR model. The same principle is used to select the best type of the time-series models and the best model order. No spectral information is lost in using only the long AR representation instead of all data. The quality of the model identified from a long AR model is comparable to that of the best time-series model that can be computed if all observations are available.     

主余震序列作用下非线性单自由度体系的增量损伤分析

同时采用真实主余震序列和人工构造主余震序列作为输入,开展了非线性单自由度体系在主余震序列作用下的增量损伤分析。挑选75组真实主余震序列,并基于相同的主震记录,分别采用重复法、随机法和衰减法构造不同的人工主余震序列作为输入。分别采用理想弹塑性模型、考虑屈服后刚度的双线型模型和三线型模型,开展主余震序列作用下非线性单自由度体系的反应分析。以修正的Park-Ang损伤指数作为结构损伤指标,对主余震序列作用下的结构增量损伤进行研究,并对强度折减系数、恢复力模型及主余震序列人工构造方法对增量损伤的影响进行研究。结果表明:余震造成的结构增量损伤不可忽视。基于重复法和随机法合成的人工主余震序列相比真实主余震序列的余震强度偏大,因此会高估余震对结构造成的增量损伤。基于衰减法合成的人工主余震序列的余震强度偏小,会低估余震对结构造成的增量损伤。结构特性对增量损伤的影响很小,体现为不同强度折减系数及不同恢复力模型对结构增量损伤的影响均不明显。     

包装设计流程的活性分析模型研究

在资讯化社会快速发展的背景下,通过使用活性系统分析法及工具分析复杂的商业包装设计流程,尝试构建了通用包装设计流程的概念模型,并提出了如何改良的建议和想法。根据改良后的概念模型,开发出了一套集知识管理信息为一体,具有简易操作界面设计的软件工具。利兹大学设计学院的硕士生们,通过对奶酪乳制品进行包装设计的实践创作对软件进行了测试,测试反馈的结果为完善软件的设计提供了一定的支持和帮助。     

A memetic algorithm for multi-objective flexible job-shop problem with worker flexibility

In existing scheduling models, the flexible job-shop scheduling problem mainly considers machine flexibility. However, human factor is also an important element existing in real production that is often neglected theoretically. In this paper, we originally probe into a multi-objective flexible job-shop scheduling problem with worker flexibility (MO-FJSPW). A non-linear integer programming model is presented for the problem. Correspondingly, a memetic algorithm (MA) is designed to solve the proposed MO-FJSPW whose objective is to minimise the maximum completion time, the maximum workload of machines and the total workload of all machines. A well-designed chromosome encoding/decoding method is proposed and the adaptive genetic operators are selected by experimental studies. An elimination process is executed to eliminate the repeated individuals in population. Moreover, a local search is incorporated into the non-dominated sorting genetic algorithm II. In experimental phase, the crossover operator and elimination operator in MA are examined firstly. Afterwards, some extensive comparisons are carried out between MA and some other multi-objective algorithms. The simulation results show that the MA performs better for the proposed MO-FJSPW than other algorithms.     

Thermodynamic analysis of NiTi formation by mechanical alloying

Disordered B2-NiTi intermetallic phase was produced from a mixture of Ni and Ti powders by mechanical alloying (MA). X-ray technique was used for phase analysis. The results indicated that Ni(Ti) solid solution can be formed earlier and changed to disordered B2-NiTi intermetallic phase after 60 h of MA. A thermodynamic analysis of the process was then carried out using Miedema model. The results showed that there is a thermodynamic driving force in Ni–Ti binary to form solid solution at all compositions and amorphous phase in the composition range X_Ni: 0.05–0.95 where X_Ni is mole fraction of Ni. However, the stable phase which has the minimal Gibbs free energy is solid solution compared to amorphous phase at all compositions. The results of MA were compared with thermodynamic analysis and it was indicated that the product of MA is the most stable phase in Ni–Ti binary system.     

商用客机对核安全壳撞击破坏效应的数值模拟分析

为研究商用客机撞击下新型核反应堆安全壳安全防护性能,该文建立新舟600飞机和安全壳精细化有限元模型,采用LS-DYNA软件对撞击全过程进行数值模拟,分析安全壳结构位移响应和局部损伤破坏情况。分析表明:新舟600撞击力时程曲线中的峰值荷载是因引擎和机翼撞击安全壳所致,撞击力时程曲线与Phantom F4战斗机和Boeing 707客机撞击力时程曲线的形状大体相似,但撞击持续时间、峰值荷载大小有较大差别;在正常巡航速度撞击下核安全壳局部破坏较为严重,局部变形量超过规范许可值,密闭性能受到影响;采用在直接施加撞击荷载的计算方法不能反映安全壳真实的响应和损伤破坏。