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231.
Huan-huan Sun Yi-bo Ren Yang Feng Hao-qiang Ba Dong Chen Chun-juan Xia Hao-wei Wang 《防务技术》2021,17(3):1062-1070
Lightweight hybrid structures of Al MMCs and titanium alloy dissimilar materials have great prospect in the defence industry application. So, it is necessary to join Al MMCs with Ti metal to achieve this structural design. In this work, in-situ TiB2/7050 composite and TA2 were firstly attempted to join by TIG welding-brazing technique. The result was that the intact welding-brazing butt joint was successfully fabricated. The joint presents dual characteristics, being a brazing on TA2 side and a welding on TiB2/7050 side. At brazing joint side, ER4043 filler metal effectively wets on TA2 under TIG heating condition, and a continuous interfacial reaction layer with 1—3μm is formed at welded metal/TA2 interface. The whole interfacial reaction layers are composed of Ti(AlSi)3 intermetallic compounds (IMCs), but their morphologies at the different regions present obvious distinguishes. The microhardness of the reaction layers is as much as 141—190 HV. At welding joints side, the fusion zone appears the equixaed crystal structure, and the grain sizes are much smaller than those of welded metal, which is attributed to the effect of TiB2 particulates from the melted TiB2/7050 on acceleration formation and inhibiting growth for the new crystal nucleus. The tensile test results show that average tensile strength of the optimal welding-brazing joint is able to achieve 138 MPa. The failure of the tensile joint occurs by quasi-cleavage pattern, and the cracks initiate from the IMCs layer at the groove surface of TA2 and propagate into the welded metal. 相似文献
232.
分别给出了DS/FH混合扩频系统中窄带干扰的两种变换域处理方法。具体分析了基于频域FFT和时频域M_PWD分析的被干扰谱线检测与抑制算法 ,为避免谱泄漏 ,在变换处理之前加窗处理。给出了不同处理算法情况下系统误码率的数字仿真结果 相似文献
233.
针对前向阵的杂波分布特性 ,给出了一种空时自适应处理 (STAP)结合多重中脉冲重复频率(MPRF)的处理方案 ,研究了重复频率的选择以改进系统性能 ,并分析计算了三路多普勒通道联合处理STAP方法在一组中脉冲重复频率下的改善因子和距离 /速度二维盲区分布图。理论分析与仿真结果表明 ,该方案可以大大改善机载前向阵雷达的全程检测性能 相似文献
234.
Beatrice Heuser 《战略研究杂志》2017,40(1-2):225-262
The concept of the command of the sea has its roots in medieval notions of the sovereignty of coastal waters, as claimed by several monarchs and polities of Europe. In the sixteenth century, a surge of intellectual creativity, especially in Elizabethan England, fused this notion with the Thucydidean term ‘thalassocracy’ – the rule of the sea. In the light of the explorations of the oceans, this led to a new conceptualisation of naval warfare, developed in theory and then put into practice. This falsifies the mistaken but widespread assumption that there was no significant writing on naval strategy before the nineteenth century. 相似文献
235.
针对相控阵天线阵面备件配置存在的冗余性强、批量送修、多级维修等现实问题,综合考虑备件费用、维修能力以及库存策略之间的关系,建立了基于定期补给的两级备件优化配置模型。给出了系统的故障件维修周转过程和维修备件的定期补给过程,在分析备件、库存、维修能力之间关系的基础上,结合成批到达的排队理论,建立了系统的供应可用度模型。以备件配置费用最小为目标、以系统供应可用度为约束条件,建立了系统的备件优化配置模型,并通过边际效益分析法对模型进行了求解。通过算例仿真与分析对模型进行了验证。结果表明:构建的备件配置能够较好地解决相控阵天线阵面的备件配置问题,具有一定的优越性。 相似文献
236.
237.
Samir Puri 《Small Wars & Insurgencies》2017,28(1):218-232
Politics is critical to making sense of Pakistani successes and failures in dealing with non-state armed groups. This includes domestic political currents; regional political currents; and the global impetus of the post-9/11 era. How these currents overlap renders to any reading of insurgency in Pakistan real complexity. This article engages with this complexity rather than shirking from it. Its hypothesis is that while the insurgency bordering Afghanistan has been an epicentre of Pakistani military efforts to fight the Taliban, this theatre is in of itself insufficiently inclusive to grasp the nature of Pakistan’s security challenges and its consequent responses. 相似文献
238.
239.
The economic growth effects of military expenditure have been the subject of a large literature in defence economics. Theories on the economic impacts of military expenditure greatly differ and include arguments that they either enhance economic growth or crowd out productive investments. Empirical literature on defence expenditure and economic growth nexus generally employs linear specifications to investigate the impact of defence expenditures on economic growth. Although it is now well established that many economic variables may have a non-linear data-generating mechanism, it seems that this reality has long been neglected in empirical work on defence–growth nexus. This paper attempts to fill this gap by employing non-linear panel data models to examine the effects of military expenditures on economic growth for Middle Eastern countries and Turkey, for the time period 1988–2012. Results show that the effect of military expenditure on economic growth is nonlinear such that the state of the economy actually determines the effect of the former on the latter. This is important not only in showing asymmetric relationship between these variables but also in revealing the reasons of mixed results of earlier literature. 相似文献
240.
Linda Darkwa 《Contemporary Security Policy》2017,38(3):471-482
Declared operationally ready in 2016, the African Standby Force (ASF) has not been deployed in its originally designed form. This is not for the lack of opportunities but rather a demonstration of the power of the Regional Economic Communities/Regional Mechanisms (RECs/RMs) – Africa’s sub-regional security structures – over matters of peace and security. Experience gathered from its short existence suggests that the ASF may never be deployed in its current form. It may instead evolve into a robust framework, adaptable mainly by the RECs/RMs, for addressing varied security challenges. Four things are critical to enhancing the utility of the ASF: the political willingness of the RECs/RMs, the strategic interest of the member states, predictable and sustainable financing, and clarity on the role of the African Capability for Immediate Response to Crisis, the temporary battlegroup that was created to provide the African Union with a rapid response capability, pending the ASF’s operationalization. 相似文献