Numerical investigation of the failure mechanism of cubic concrete specimens in SHPB tests

Cylindrical specimens are commonly used in Split Hopkinson pressure bar (SHPB) tests to study the uniaxial dynamic properties of concrete-like materials.In recent years,true tri-axial SHPB equipment has also been developed or is under development to investigate the material dynamic properties under tri-axial impact loads.For such tests,cubic specimens are needed.It is well understood that static material strength obtained from cylinder and cube specimens are different.Conversion factors are obtained and adopted in some guidelines to convert the material strength obtained from the two types of specimens.Previous uniaxial impact tests have also demonstrated that the failure mode and the strain rate effect of cubic specimens are very different from that of cylindrical ones.However,the mechanical background of these findings is unclear.As an extension of the previous laboratory study,this study performs numerical SHPB tests of cubic and cylindrical concrete specimens subjected to uniaxial impact load with the validated numerical model.The stress states of cubic specimens in relation to its failure mode under different strain rates is analyzed and compared with cylindrical specimens.The detailed analyses of the numerical simulation results show that the lateral inertial confinement of the cylindrical specimen is higher than that of the cubic specimen under the same strain rates.For cubic specimen,the corners are more severely damaged because of the lower lateral confinement and the occurrence of the tensile radial stress which is not observed in cylindrical specimens.These results explain why the dynamic material strengths obtained from the two types of specimens are different and are strain rate dependent.Based on the simulation results,an empirical formula of conversion factor as a function of strain rate is proposed,which supplements the traditional conversion factor for quasi-static material strength.It can be used for transforming the dynamic compressive strength from cylinders to cubes obtained from impact tests at different strain rates.     

Purchase and retrieval competition for seasonal produce

We study the competition problem of purchase and multiretrieval of perishable seasonal produce, where wholesalers purchase and stock their products in the first period, and then retrieve and sell them in subsequent periods. We first consider the duopoly case and assume that the prices are exogenous and fluctuate. In each period, after the price realization, the wholesalers retrieve some stock from their warehouses to satisfy their demands. One wholesaler's unsatisfied customers can switch to another and be satisfied by its left retrieved products. Any unsold retrieved stock has no salvage value and any unsatisfied demand is lost. The unretrieved stock is carried to the next period at a perishable rate. The wholesalers compete for the substitute demand by determining their own purchase and retrieval quantities. We show the existence and uniqueness of a pure-strategy Nash equilibrium, and that the Nash equilibrium strategy has the simple “sell-down-to” structure. We also consider the general N-person game and show the existence of the Nash equilibrium, and characterize the structure of the equilibrium strategy for the symmetric case. In addition, we consider the case with endogenous prices, and show that the problem reduces to a repeated newsvendor game with price and inventory competition. We derive the conditions under which a unique Nash equilibrium exists and characterize the equilibrium strategy. Finally, we conduct numerical studies to examine the impacts of the model parameters on the equilibrium outcomes and to generate managerial insights.     

国内外安全标准现状与发展及其对构建国防工业安全标准体系的启示

介绍了国外发达国家、先进组织机构安全标准及体系的现状与特点,分析了国内安全标准的制修订情况和危险化学品、防尘防毒安全生产标准体系的分类与构成。在分析国防科技工业安全标准现状的基础上,提出了构建国防工业安全标准体系的主要任务和实施要求。     

一种纯方位跟踪中的自适应滤波算法

针对纯方位被动目标跟踪中扩展卡尔曼滤波算法易发散的不足,提出了一种自适应的改进算法。该算法利用极大后验噪声估计器Sage-Husa对虚拟观测噪声进行实时在线估计,动态补偿线性化带来的误差。算法的对比仿真分析结果表明,AEKF较之EKF滤波效果有所改善,增强了稳定性,提高了精度,为水下纯方位被动目标跟踪的实现提供一种新的方法。     

叶片式磁流变液减振器三维磁场有限元分析

介绍了2种车用叶片式MRF减振器结构方案.利用ANSOFT电磁场分析软件对2种不同结构方案的减振器进行三维磁场有限元分析,得到磁场分布相对比较理想的一种方案,并对该结构方案进行了改进.仿真分析表明,针对所讨论的叶片式MRF减振器,线圈的安匝数1 500为最优;用电工硅钢替代电工纯铁作为铁芯材料,所产生的磁感应强度更大,在缝隙处能达到1.161 4～1.355 T.     

基于CCD技术的万能工具显微镜再制造

一些工厂和院校现存大量的20世纪七八十年代生产的旧式万能工具显微镜,这些仪器因自身结构的缺陷逐渐被弃用.但由于它们依然有很好的光学和机械系统,对这些旧仪器进行再制造,能得到性能更好的再制造仪器.介绍了采用CCD和光栅器件对旧式万能工具显微镜进行再制造的过程,所研发的基于图像技术的测量系统,通过螺纹的测量,证明了这项再制造技术的可行性和有效性.     

多层次灰色评价法在后勤综合保障能力评估中的应用

对部队进行后勤综合保障能力评估是一项困难但十分重要的工作,利用多层次灰色评价法对后勤综合保障能力进行评估是一种比较科学的方法。评估时,先建立后勤保障能力评价指标体系,并用层次分析法确定各指标的权重。然后在样本矩阵的基础上确定评价灰类并计算灰色评价系数、权向量和权矩阵,从而对指标做出综合评价得到最终评价结果。最后,通过对该评价方法稍加改进,可以取得更加实用的评价结果。     

基于方位数值拟合的目标机动改变判断

被动声纳和侦察雷达等传感器材对目标的探测仅能获取目标方位信息。虽然基于纯方位信息和探测平台机动位置的方位平差法,能够解算得出定向定速运动目标的要素,然而在实际作战应用中,目标可能会频繁地出现转向机动,使运动要素永远也不可能收敛。判明目标改变机动具有十分重要的战术价值。本文研究给出了一种基于时间方位拟合的目标转向运动判断方法,并结合典型态势进行了仿真分析,得出了拟合判断参数,研究结果为目标运动要素解算控制、潜艇查明水面舰艇巡逻线等作战应用提供目标转向判断依据。与方位滤波方法相比,这一方法概念清晰、算法简单、实用性强。     

圆极化GPS螺旋天线设计与分析

螺旋天线具有很宽的工作频带,天线主辐射方向辐射场易形成圆极化场.阐述了螺旋天线的辐射机理,根据某一GPS天线设计指标设计出相应圆极化螺旋天线,并给出实际尺寸.采用高频仿真软件CST进行仿真得到较好理论结果.讨论了天线反射板尺寸变化以及螺距角变化对天线性能带来的影响,并给出相关结论.     

岛屿封锁作战炮兵目标价值排序的优化模型

通过对炮兵目标价值分析与排序的指标体系进行系统分析,在岛屿封锁这一特定的作战样式中选取了六种属性作为目标价值排序的准则,然后根据对各属性的评分,在权重未知的情况下从最优化角度出发,以各方案与理想方案的距离为目标函数建立了目标价值排序的优化模型,并采用拉格朗日乘数法对模型进行求解,得到最终的目标打击顺序.