Repeated low-velocity impact responses of SiC particle reinforced Al metal-matrix composites

This study aimed to investigate experimentally the repeated low-velocity impact behaviors of SiC reinforced aluminum 6061 metal-matrix composites for different volume fractions and energy levels. In addition, the hardness variations were measured by the Vickers hardness tests from the impacted and impact-free cross-sections of the particle reinforced metal-matrix composites. Low-velocity impact tests were applied to composite samples manufactured by powder metallurgy (in 10, 20, and 30% volume fractions) at two total energy levels (15 and 60 J as single) and in repetitions equal to the sum of these energy levels (5 + 5 + 5 and 20 + 20 + 20 J as repeated). As a result, in increasing the impact number for all volume fractions, the total contact time was shortened and the peak contact force increased, whereas both the permanent central deflection and the absorbed energies reduced. Hence, these variations obtained under repeated impacts (5 + 5 + 5 and 20 + 20 + 20 J) revealed that metal-matrix composites showed a tougher behavior with an increase in the impact numbers from 1st to 3rd, particularly because of the strain hardening effect. Furthermore, an increase in volume fraction from 10 to 30% resulted in an increase in the impact strength under all repeated and single impacts despite changing deformation and damage mechanisms due to increasing the strain hardening effect and particle fractures. The hardness was affected by the volume fraction and increased as the volume fraction increased in both the impacted and impact-free zones. The repeated impact increased the impacted zone hardness more than the single impact for all volume fractions. Additionally, the hardness of the impacted zone under 20 + 20 + 20 J repeated impact was measured as the highest value in the 30% volume fraction. Therefore, metal-matrix composites can behave harder with the strain hardening effect under repeated impacts.     

在轨维修后端模块接口机构的研究

在轨维修可以大大的延长空间科学仪器的使用寿命,节省大量的经济成本。为了实现空间望远镜后端模块的在轨操作和更换,设计了一套与之对应的接口机构,能够解决在轨快速定位与安装。根据321运动学定位准则,详细介绍了该接口机构的内部组成和工作原理;然后把该接口机构与后端模块进行组件级有限元仿真,仿真结果表明一阶模态远高于整机基频可以有效地避免发射时候的共振;设计了一套平面内的工装来模拟后端模块,利用等效质量法对整个机构进行重力卸载;搭建实验平台,利用激光跟踪仪来测量整个后端模块的重复定位安装精度,实验数据表明,X,Y,Z 3个方向的重复平移定位误差分别为±5.58μm,±3.24μm及±3.63μm,优于总体指标±10μm;热实验结果表明整个机构可以完全释放由于温度变化产生的形变,具有很高的热稳定性;使入射光线和靶面的相对位置持续稳定,保证了较高的成像质量。为其他空间在轨维护装置提供强有力的参考价值。     

重复采动覆岩裂隙率空间分布相似模拟研究——以陕北矿区为例

由于水资源短缺,陕北矿区利用采空区储存矿井水进行再利用,采空区储水量与采动裂隙空间分布密切相关,为准确预测采空区储水量,基于工作面上覆岩层断裂分带划分结果,确定煤矿地下水库的主要储水空间为垮落带与断裂带的裂隙空间,建立了采动覆岩裂隙率空间分布的计算方法,采用相似模拟试验得出了柠条塔矿1^-2煤和2^-2煤重复采动后上覆岩层的下沉曲线,计算出重复采动覆岩裂隙率在垂直和水平方向的空间分布情况。结果表明:沿垂直方向,随着高度增加,覆岩裂隙率逐渐减小,1^-2煤开采后岩层裂隙率由1.87%逐渐下降为1.18%,2^-2煤开采后,裂隙率由12.85%下降为4.11%;沿水平方向,垮落带、断裂带和弯曲下沉带的裂隙率在采空区边缘附近都会产生2个峰值,并且峰值位置逐渐向采空区中央移动,1^-2煤开采后,中间压实区的最大裂隙率为1.21%,两边离层区的最大裂隙率为2.19%,2^-2煤开采后,压实区和离层区最大裂隙率分别为16.1%和6.54%;重复开采后,上覆岩层的裂隙率显著增加,有...     

宋式带“昂”斗栱节点力学性能试验研究

古建筑中宋式斗栱节点最为显著的特征之一就是带有“下昂”。为研究具有这种特征的斗栱节点的受力性能,制作 1∶3.52的缩尺模型试件进行竖向荷载试验和水平低周反复荷载试验,依据宋《营造法式》和实际建筑的 “有昂无梁栿”和“有昂有梁栿”两种类型的宋式斗栱节点模型,设计并制作了以柱头斗栱节点为对象的试验模型。研究结果表明：在竖向荷载作用下,栌斗始终作为薄弱构件首先破坏;下昂与梁栿虽打断了斗栱节点分层叠加,但其依旧能保持良好的整体性;水平荷载作用下单个斗栱节点的倒三角形构造易发生转动,但加入梁栿后,节点的转动受到约束而发生滑移,此时斗栱表现良好的耗能效果,由此说明斗栱节点良好的抗震性能并非来自单个斗栱节点,而是由其通过相互约束协同工作而产生;底部暗销有较强的锚固作用,斗栱节点水平承载力主要由底部暗销的锚固力和柱与斗栱节点接触面的摩擦力组成。     

Aggregate effects and measuring regional dynamics

Empirical models of regional adjustment often control for aggregate effects when estimating the impact of region-specific shocks on local economies. It is, however, difficult to filter out the effects of aggregate shocks—such as oil shocks, uncertainty shocks, or national recessions—because the incidence of these shocks varies across space and time. We propose an improved econometric method to control for this form of spatiotemporal heterogeneity, thereby yielding more accurate estimation of the effects of local shocks on regional economies. Applying the method to US states, we find that labour demand shocks are mostly absorbed through changes in participation; the migration response to these shocks is limited; and recoveries are highly protracted     

随机反复荷载下钢筋混凝土受弯构件变形的计算及可靠度分析

对于承受反复荷载作用的钢筋混凝土受弯构件 ,设计中除要考虑产生的瞬时变形外 ,还要考虑产生的疲劳残余变形。介绍了随机反复荷载作用下钢筋混凝土受弯构件长期变形的计算和可靠度分析方法 ,并以一吊车梁为例进行了分析和计算     

某交叉重叠隧道建造技术的研究与应用

本文结合工程实例,阐述了交叉重叠隧道建造技术的要点,重点介绍施工监控量测的重要性,并制定了有效的施工措施。     

路基土回弹模量应力依赖性分析及预估模型

通过室内重复加载三轴试验研究4种常用路基土的回弹特性,分析回弹模量对体应力、侧向应力、偏应力的依赖关系。试验结果表明,回弹模量值是两个应力变量的函数,且当其中一个变量保持不变时,回弹模量值随另一变量呈非线性变化,函数关系应为幂函数形式。在此分析基础上,参考Uzan模型表达式,以偏应力和体应力为变量回归得出相应的路基土回弹模量预估模型,拟合结果良好。模型的建立,有利于更好地理解路基土的回弹特性,并为路基模量设计值的取用与路面结构力学响应计算提供可靠的技术依据。