焊接中热裂纹现象

在焊接过程中，避免热裂纹是一个主要的研究热点。最新采用的焊接方法是通过增加热裂纹的敏感性来控制裂纹，如采用电弧焊接奥氏体不锈钢时，接头处就不会产生裂纹。目前还没有完全了解清楚焊接中的热裂纹产生机理，虽然已经开发出一些软件来预测焊接母材和填充材料的抗热裂纹性，但是，由于不同的软件预测出的结果不一样，导致用软件模拟很难预测真实的焊接情况。本书收集了8个国家有关焊接热裂纹研究的最新进展，读者从中可以了解到各国对于焊接热裂纹产生机制的不同见解。     

Synchronization of two 3-scroll hyperchaotic attractors using wavelet transform

1 .INTRODUCTIONChaotic synchronizationis a key technology in cha-otic secure communication, the low di mensionalchaotic systems have only one positive Lyapunovexponent ,the performance of securityis not ideal ,hyperchaotic systems have two or more than twopositive Lyapunov exponents , the performance ofsecurityisi mproved greatly ,sothe synchronizationof hyperchotic systems has arisen great inter-est[1 ~4].N-scroll( N =3,4,5...)attractor is akind of hyperchaotic attractor ,the method of g…     

A distributed adaptive multi-hop certification authority schemefor mobile Ad Hoc networks

1.INTRODUCTION Amobileadhocnetwork(MANET)isanau tonomoussystemofmobilenodesconnectedbywire lesslinks,theunionofwhichformsacommunication networkmodeledintheformofanarbitrarycommu nicationgraph[1].Thesalientfeaturesofadhocnet worksposechallengesinachievingsecuritygoals.In wirednetworks,anadversarycannotattacknetworks untilitconnectswiredphysicallink.Bycontrary,an attackinwirelessnetworksmaycomefromanyloca tion.Thereforeadhocnetworksmustadoptdis tributedcertificationauthority(CA),otherwis…     

军用计算机技术的发展趋势

军用武器的研制即将进入灵巧与高性能武器、人工智能、2000年陆空战和星球大战的领域,因此需要完全以超大规模集成电路(VLSI)为基础的数字技术和更加先进的军用计算机。那么单靠超高速集成电路(VHSIC)计划能使军用数字电子设备与民用研制相比处于技术领先地位吗? 为了加速防御用的高性能VLSI的研制,     

Microstructural evaluation and nanohardness of an AlCoCuCrFeNiTi high-entropy alloy

An AlCoCuCrFeNiTi high-entropy alloy(HEA) was prepared by mechanical alloying and sintering to study the effect of Ti addition to the widely studied AlCoCuCrFeNi system. The structural and microstructural characteristics were investigated by X-ray diffraction(XRD), scanning electron microscopy(SEM), and transmission electron microscopy(TEM). The formation of four micrometric phases was detected: a Cu-rich phase with a face-centered cubic(fcc) structure, a body-centered cubic(bcc) solid solution with Cu-rich plate-like precipitates(fcc), an ordered bcc phase, and a tetragonal structure. The XRD patterns corroborate the presence of a mixture of bcc-, fcc-, and tetragonal-structured phases. The Vickers hardness of the alloy under study was more than twice that of the AlCoCuCrFeNi alloy. Nanoindentation tests were performed to evaluate the mechanical response of the individual phases to elucidate the relationship between chemical composition, crystal structure, and mechanical performance of the multiphase microstructure of the AlCoCuCrFeNiTi HEA.     

Effects of small additions of Zn on the microstructure,mechanical properties and corrosion resistance of WE43B Mg alloys

Zn is a commonly used alloying element for Mg alloys owing to its beneficial effects on mechanical properties. To improve the mechanical and corrosion properties of WE43B Mg alloys, the effects of 0-0.7wt% Zn addition on the microstructure and properties of sample alloys were investigated. Addition of Zn to as-cast WE43B alloy promoted the formation of the Mg₁₂Nd phase; by contrast, after T6 heat treatment, the phase composition of WE43B alloys with and without Zn addition remained mostly the same. A long-period stacking ordered phase was predicted by CALPHAD calculation, but this phase was not observed in either the as-cast or heat-treated Zn-containing WE43B alloys. The optimum temperature and duration of T6 heat treatment were obtained using CALPHAD calculations and hardness measurements. Addition of Zn resulted in a slight reduction in the average grain size of the as-cast and T6 heat-treated WE43B alloys and endowed them with increased corrosion resistance with little effect on their mechanical properties.     

数字式寻的制导——稳定性与系统性能的折衷处理

在导弹寻的制导系统中,可增添数字信号处理能力,以提供对付复杂威胁决策能力的广泛方法。数字处理或在地面上进行,或在导弹上进行。然而,无论在何处,都将把一些附带的动力学特性引入到制导回路中,因而可能降低系统性能。事实上,数字化的影响是如此之重要,以致在开始时就必须考虑,以保证制导系统的设计获得成功。数字化的最重要的影响之一是把采样和保持过程引入到姿态制导回路中。本文讨论数字处理对系统相对稳定性和性能的影响。介绍改善系统性能的各种方法,同时介绍折衷评定曲线,这些曲线表明应该如何确定主要的子系统参数,如导引头稳定回路增益及自动驾驶仪的时间常数。分析结果给出了当采用数字式制导时,制导子系统参数与可接受的系统性能鲁棒集之间的折衷处理。     

Variable Structure Controller Design of Singular Systems with Disturbance in the Case of Incomplete State Information

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高维代数簇的算术

求解不定方程是数学最古老的问题之一。从勾股方程到20世纪才证明的费马大定理都是典型的例子。为纪念希腊数学家丢番图，西方将不定方程称为丢番图方程。与通常的确定方程不同，丢番图方程没有统一的理论和解法。近年来许多高深的数学理论与方法的引人使这个领域获得飞速的发展，     

椭圆曲线密码学指导

椭圆曲线的研究可追溯到上世纪中叶，自从1984年H．Lenstra的基于椭圆曲线性质的整数分解算法出现，椭圆曲线被应用于密码学研究，由此产生了椭圆曲线密码学。经过20多年的成长，现在已被广泛应用于工业、商业、国防及政府行政等部门。