Dislocation structures introduced by high-speed deformation in bcc metals

Systematic experiments were carried out over a wide range of strain rate, 10⁰–10⁶ s⁻¹, so as to reveal the deformation mode in bcc crystals, especially at high strain rate. Dislocation structure showed heterogeneous distribution at low strain rates in all three bcc metals examined. At higher strain rates exceeding 10³ s⁻¹, distribution of dislocations was random, and the formation of small dislocation loops was observed in V and Nb. In Mo, small dislocation loops were not formed by deformation, even at high strain rates. However, post-deformation annealing of an Mo specimen that had been deformed by 20% at 5×10⁵ s⁻¹ produced dislocation loops. The inside–outside contrast method identified these loops to be of vacancy type. These results reveal that in Mo vacancy clusters are not formed directly from the interaction of dislocations, but by the aggregation of vacancies. In V and Nb, the same formation process is believed to occur at high strain rates. These results suggest that the different mode of plastic deformation at high strain rates accompanied by production of vacancies also occurred in bcc metals.     

Computationally efficient modelling of short fatigue crack growth using dislocation formulations

In an attempt to develop a computationally efficient model for simulation of fatigue crack growth of microstructurally short cracks two dislocation based models have been compared. In both models, the geometry of the boundary and the crack is described using dislocation dipole elements, whereas the plasticity is described either by discrete dislocations or by distributed dipole elements. The two models were found to agree qualitatively as well as quantitatively. It was concluded that modelling the plasticity by dipole elements becomes comparatively increasingly more time efficient with increasing grain size plastic zone size. However, plasticity modelling by dipole elements showed to provide somewhat higher crack growth rates.     

超声波振动对钛箔拉伸性能及位错分布的影响

以研究超声波振动条件下钛箔的塑性变形特征和位错分布为目的,通过超声波辅助单向拉伸实验和微观组织分析研究不同超声波振动施加方式对钛箔塑性拉伸变形过程中的应力-应变、伸长率及位错分布的影响规律。结果表明:超声波振动过程中钛箔的流动应力最大降幅可以达到约80%,材料的伸长率从未施加超声时的40.33%最大增加至54.46%。通过TEM可以发现超声波振动条件下位错呈现平行分布的趋势且无大量缠结出现,而未施加超声拉伸的试样中位错的分布则显得杂乱无章且缠结严重。     

Precipitation of Cu in Fe–Cu alloys by high-speed deformation

The differences between defect structures in Fe–Cu alloys deformed at the high (4.3×10⁵ s⁻¹) and the low strain rate (67 s⁻¹) were studied. Positron lifetime and coincidence Doppler broadening (CDB) measurements were carried out to investigate the formation of vacancy clusters and Cu precipitates. Both the size of vacancy clusters and the total amount of vacancy-type defects were larger after high-speed deformation at room temperature. Cu precipitation in the specimen deformed at the high-speed stopped for 10 h after annealing at 400 °C, while that in the specimen deformed at the low-speed continued for 100 h. Transmission electron microscopy (TEM) observations showed a heterogeneous distribution of dislocations in the case of low-speed deformation but a homogeneous distribution in the case of high-speed deformation. These results suggested that the sink efficiency for defects was higher in the specimen deformed at the high-speed.     

Optimization of structures under material parameter uncertainty using evidence theory

An evidence-based approach is developed for optimization of structural components under material parameter uncertainty. The approach is applied to evidence-based design optimization (EBDO) of externally stiffened circular tubes under axial impact load using an isotropic–elastic–plastic plasticity model to simulate dynamic material behaviour. Uncertainty modelling considers the changes in material parameters that are caused by variability in material properties as well as incertitude and errors in experimental data and procedure to determine the material parameters. Spatial variation of material parameters across the structural component is modelled using a field joint belief structure and propagated for the calculation of evidence-based objective function and design constraints. Surrogate models are used in both uncertainty propagation and solution of the optimization problem. The methodology and the solution to the EBDO example problem are presented and discussed.     

Evolution of the distribution of density,accumulated deformation,and topological features of powder cylindrical workpieces under the deforming broaching conditions. Communication 1. Modeling and analysis of the distribution of density and accumulated deformation in powder bodies formed using different schemes of the deforming broaching

The results are given of the investigation into the structure of powdered bodies subjected to plastic deformation by broaching. Different schemes of deforming broaching have been considered, which involve direct contact of the workpiece surface with a deforming element and broaching in a closed system. A mathematical model of the process is given and calculated patterns of the distribution of the density and accumulated deformation of powdered compacts at different moments of deforming are discussed. The structural manifestations of the application of both schemes of deforming broaching have been compared.     

基于有限元不同大塑性变形应变特征对比研究及组织分析

目的 探究往复挤压工艺(CEC)与往复挤扭镦工艺(SETE)的变形特点,并研究单道次变形后的金属流动行为和应变分布特征。方法 以低活化钢为材料,对CEC和SETE等2种工艺下的变形过程进行有限元模拟分析,获得3个道次变形后的CLAM钢等效应变云图,同时,开展相关试验以验证有限元模拟的准确性。结果 往复挤压工艺(CEC)与往复挤扭镦工艺(SETE)均能实现多道次累积应变,其中SETE下的单道次累积应变量更大,其各道次等效应变分别比CEC下的等效应变高2.47、5.06、7.84。0.5道次变形后,SETE下的平均硬度值比CEC下的高6.1HV。结论 在CEC与SETE下进行各道次变形后,边缘等效应变都高于心部等效应变,且1道次变形后应变均匀性相差不大,但随着道次的增加,SETE下的应变分布更加均匀。0.5道次变形后,CEC与SETE下的边缘晶粒尺寸均较心部晶粒尺寸细小,且SETE下的晶粒细化程度更高。     

Weighted least‐squares estimation of the shape parameter of the Weibull distribution

The purpose of this paper is to propose the weighted least‐squares procedure for estimating the shape parameter of the Weibull distribution. Results from simulation studies illustrate the mean‐squared error of the weighted least‐squares estimator is smaller than competing procedures in all cases considered. Copyright © 2001 John Wiley & Sons, Ltd.     

前后向时间序列模型联合估计的时变结构模态参数辨识

为提高时变结构模态参数辨识精度和抗噪声能力,提出一种前后向泛函向量时变自回归滑动平均（FS-VTARMA）时间序列模型联合估计的模态参数辨识方法。首先建立前后向FS-VTARMA模型联合估计的均方误差形式的费用函数,其次引入非平稳信号中前向模型和后向模型估计系数的近似共轭关系,再利用两步最小二乘法（2SLS）得到时变模型系数,最后把时变模型特征方程转换为广义特征值问题提取出模态参数。利用时变刚度系统非平稳振动信号验证该方法,结果表明：能有效地克服前向模型估计中模态参数一步延迟以及起始时刻无法准确获得,以及后向模型估计中模态参数一步超前以及终止时刻无法准确获得的缺点,具有更高的模态参数辨识精度和更强的抗噪声能力。     

Formation of vacancy clusters in deformed thin films of Al–Mg and Al–Cu dilute alloys

In the present study, vacancy clusters in elongated Al–Mg and Al–Cu thin films (Mg/Cu CONCENTRATION=0.05–1.70 at.%) were examined by electron microscopy. No dislocations were observed in these films. In Al–Mg thin films deformed at room temperature, a large number of stacking fault tetrahedra (sft) were observed alongside a few vacancy loops. The opposite was true for Al–Cu thin films, where well-grown loops predominated, and only a few sft were observed. The Al–Cu film results show that the majority of vacancies form loops larger than sft. We also deformed Al–0.05at.% (Mg or Cu) alloys in liquid nitrogen and cold-transferred to an electron microscope. In Al–Mg, a large number of dotted defects (possibly sft) were observed, while very few such defects were observed in Al–Cu. This indicates that loops observed in Al–Cu thin films deformed at room temperature, grew during/after deformation. The likely contribution of strain-induced vacancies in deformed Al thin films to the voiding in VLSI interconnect wires due to electro-migration were discussed.     

基于延时随机子空间方法的非白噪声环境激励结构模态参数识别

为了消除非白噪声环境激励在结构模态参数识别结果中产生的虚假模态,引入扩展状态模型,从状态空间方程的角度论证了虚假模态产生的原因;然后,针对一类具有典型性和代表性的（自相关函数在纵坐标轴附近一定范围内有非零值的）非白噪声环境激励信号,在传统随机子空间算法的基础上引入延时指标,建立延时随机子空间方法。数值算例表明延时随机子空间方法能够有效地剔除非白噪声环境激励在模态参数识别结果中产生的虚假模态,放宽了传统模态参数识别方法对环境输入的白噪声假设。     

Statistical distribution of the optical intensity obtained using a Gaussian Schell model for space-to-ground link laser communications

Abstract

Based on the characteristics of the laser device and the inevitable error of the processing technique, a laser beam emitted from a communication terminal can be represented by the Gaussian Schell model (GSM). In space-to-ground link laser communications, the optical intensity is affected by the source coherence parameter and the zenith angle. With full consideration of these two parameters, the statistical distribution model of the optical intensity with a GSM laser in both downlink and uplink is derived. The simulation results indicate that increasing the source coherence parameter has an effect on the statistical distribution of the optical intensity; this effect is highly similar to the effect of a larger zenith angle. The optical intensity invariably degrades with increasing source coherence parameter or zenith angle. The results of this work can promote the improvement of the redundancy design of a laser communication receiver system.     

Practical implementation of the method of weight functions for the construction of estimators of the distribution density function

Practical implementation of the method of weight functions for the construction of estimators of the distribution density function is demonstrated, an evaluation of the selection of adjusted parameters of the method is represented, and an estimator of the precision characteristics for a number of distributions is presented. Translated from Izmeritel'naya Tekhnika, No. 9, pp. 39–41, September, 2008.     

颗粒随机分布复合材料的细观构造对有效热传导系数的影响

结合建立的颗粒随机分布复合材料的微结构模型和基于均匀化理论统计的双尺度计算方法, 针对氮化硼颗粒增强高密度聚乙烯(BN/HDPE)复合材料, 研究了颗粒形状、体积分数和空间分布参数等微结构特征对复合材料有效热传导系数的影响。结果表明: 颗粒体积分数的增加将导致有效热传导系数升高; 球形颗粒的位置参数、长椭球颗粒的取向程度都对有效热传导系数有重要影响。数值试验表明, 材料的微结构特征对复合材料的有效热传导系数具有极大影响。     

Analysis of the Results of Mechanical Testing of Metals by Constructing the Diagrams of Structural States

For polycrystalline metals and alloys with bcc (Mo, Fe, and Fe–Si) and fcc (Ni and Cu) crystal lattices, it is shown that the application of the method of reconstruction of parabolic stress–strain diagrams on S –eⁿcoordinates, where S and e are the actual stress and strain, respectively, and n is the strain-hardening index, makes it possible to represent the results of mechanical testing in the form of a diagram of structural states plotted in the form of the dependences of actual stresses and strains on temperature. This diagram practically completely describes the changes in the strength and plastic characteristics of the materials and their structural states in the process of plastic deformation. We study specific features of the construction of these diagrams depending on the composition of the alloy, its structural state, the energy of stacking faults, etc. On the basis of the analysis of the character of correlations between the critical values of strains and stresses and the mechanisms of plastic deformation within broad ranges of temperature and structural states, we discuss the possibilities of scientific and practical application of the diagrams of actual stresses and strains vs temperature.     

Analysis of adaptive multi-rate FSO/RF hybrid systems using Málaga-ℳ distribution model in turbulent channels

Precise modeling of the optical channels with different conditions of atmospheric turbulence is an important prerequisite for analyzing the performance of Free Space Optical (FSO) communication systems. Statistical models which have been proposed to describe the optical channels, have some limitations to precisely model the channel. In this paper, The ? distribution which is a relatively new and comprehensive model, introduced by the University of Malaga, has been used to model and evaluate FSO/RF hybrid systems under the effect of atmospheric turbulence of the channels for two modes of operation, classic adaptive rate transmission, and the combination of adaptive rate transmission and automatic repeat request.The simulation results have been compared with the experimental results which have been reported by other researchers. It seems, using ? distribution model, yields more consistency between design objectives and implementation results.     

Minimizing makespan in a no-wait flowshop with two batch processing machines using estimation of distribution algorithm

This paper studies the problem of minimising makespan in a no-wait flowshop with two batch processing machines (comprised of a parallel batch processing machine and a serial batch processing machine), non-identical job sizes and unequal ready times. We propose a population-based evolutionary method named estimation of distribution algorithm (EDA). Firstly, the individuals in the population are coded into job sequences. Then, a probabilistic model is built to generate new population and an incremental learning method is developed to update the probabilistic model. Thirdly, the best-fit heuristic is used to group jobs into batches and a least idle/waiting time approach is proposed to sequence the batches on batch processing machines. In addition, some problem-dependent local search heuristics are incorporated into the EDA to further improve the searching quality. Computational simulation and comparisons with some existing algorithms demonstrate the effectiveness and robustness of the proposed algorithm. Furthermore, the effectiveness of embedding the local search method in the EDA is also evaluated.     

Reliability estimation by Bayesian method: definition of prior distribution using dependability study

In this article, we present an evaluation method of the system reliability using Bayesian statistics and data feedback. In particular, we deal with the problem of the prior definition. A method is proposed to build the prior distribution from the dependability study (risk evaluation study).     

True Stress vs Temperature Diagram of Structural States for Polycrystalline bcc Metals

For the first time, a new type of a diagram of structural states is constructed for polycrystalline bcc metals. This true-stress-vs-temperature diagram is a regular collection of temperature dependences of critical stresses corresponding to successive changes in the type of dislocation structure from the onset of plastic deformation to fracture within the temperature range from completely brittle fracture to the formation of a new grain structure. On the basis of analysis of true-stress-vs-temperature diagrams for metals and alloys (Mo, Fe, and Fe–3%Si) with the help of the thermal activation analysis of the temperature dependences of critical stresses, we establish the regularities and distinctive features of the formation and behavior of the temperature–force boundaries and domains of existence of certain structural states and identify the mechanisms of plastic deformation within the limits of each of these domains.