线粘弹性材料中三维裂纹问题的加料有限元法

将加料有限元法扩展应用于线粘弹性材料三维断裂问题.为了反映裂纹尖端的奇异性,在裂尖附近的奇异区采用若干八节点六面体加料单元和过渡单元,非奇异区采用常规八节点六面体单元.三种单元分区混合使用形成求解域网格划分.基于Boltzmann叠加原理,推导了粘弹性材料的增量型本构关系,进而获得了增量加料有限元列式,并通过附加自由度计算粘弹性介质中裂纹应变能量释放率.数值算例验证了方法的正确性和有效性.     

军用飞机航材配送中心选址方法与模型

航材保障是装备保障工作的重要组成部分,直接影响部队作战和训练任务的完成。深入开展军用飞机航材配送中心选址方法研究,探索新的航材配送中心选址方法与手段,对于配送中心的科学选址,提高航材保障工作效益具有重要意义。针对军用飞机航材保障特点,综合考虑任务成功率、经济性和战时道路损毁等因素,提出了军用飞机航材保障配送中心选址方法,建立了配送中心的选址模型。最后,给出了具体算例,验证所建模型的科学性和有效性。     

压力喷嘴常温下雾化特性实验研究

为了满足工程中对喷嘴雾化特性测量的高精度、低成本要求,建立了喷嘴雾化特性实验系统,以自来水为雾化工质,对蒸发冷却压力喷嘴在常温、常压下的雾化特性进行了实验研究。用容积法测量了喷嘴的流量,用最小二乘法对测量数据进行回归分析,得到了喷嘴流量、雾化压力和喷孔直径之间的关联式与流量系数;用机器视觉方法测量了喷嘴的雾化锥角与雾滴尺寸,得到了雾化锥角与雾滴尺寸随雾化压力变化的规律。结果表明:随着雾化压力的增大,喷嘴雾化锥角不断增加,雾滴尺寸不断减小;随轴向距离的增大,D0.632、D0.5、D32有不同幅度的增大,而D0.9则基本不变。     

Bounding the optimal burn‐in time for a system with two types of failure

Burn‐in is a widely used method to improve the quality of products or systems after they have been produced. In this paper, we consider the problem of determining bounds to the optimal burn‐in time and optimal replacement policy maximizing the steady state availability of a repairable system. It is assumed that two types of system failures may occur: One is Type I failure (minor failure), which can be removed by a minimal repair, and the other is Type II failure (catastrophic failure), which can be removed only by a complete repair. Assuming that the underlying lifetime distribution of the system has a bathtub‐shaped failure rate function, upper and lower bounds for the optimal burn‐in time are provided. Furthermore, some other applications of optimal burn‐in are also considered. © 2004 Wiley Periodicals, Inc. Naval Research Logistics, 2004     

Research on the influences of confining pressure and strain rate on NEPE propellant: Experimental assessment and constitutive model

In order to study the influences of confining pressure and strain rate on the mechanical properties of the Nitrate Ester Plasticized Polyether (NEPE) propellant, uniaxial tensile tests were conducted using the self-made confining pressure system and material testing machine. The stress-strain responses of the NEPE propellant under different confining pressure conditions and strain rates were obtained and analyzed. The results show that confining pressure and strain rate have a remarkably influence on the mechanical responses of the NEPE propellant. As confining pressure increases (from 0 to 5.4 MPa), the maximum tensile stress and ultimate strain increase gradually. With the coupled effects of confining pressure and strain rate, the value of the maximum tensile stress and ultimate strain at 5.4 MPa and 0.0667 s⁻¹ is 2.03 times and 2.19 times of their values under 0 MPa and 0.00333 s⁻¹, respectively. Afterwards, the influence mechanism of confining pressure on the NEPE propellant was analyzed. Finally, based on the viscoelastic theory and continuous damage theory, a nonlinear constitutive model considering confining pressure and strain rate was developed. The damage was considered to be rate-dependent and pressure-dependent. The constitutive model was validated by comparing experimental data with predictions of the constitutive model. The whole maximum stress errors of the model predictions are lower than 4% and the corresponding strain errors are lower than 7%. The results show that confining pressure can suppress the damage initiation and evolution of the NEPE propellant and the nonlinear constitutive model can describe the mechanical responses of the NEPE propellant under various confining pressure conditions and strain rates. This research can lay a theoretical foundation for analyzing the structural integrity of propellant grain accurately under working pressure loading.     

在柴油机放热曲线计算中两个关键问题的解决方法

对影响柴油机放热曲线精度计算的因素进行了讨论 ,并对影响放热率计算的主要问题如动态上止点修正、示功图光顺处理两个主要环节进行了分析研究 ,提出了解决方法 .     

局域网传输系统可用性分析

采用可用性概念 ,分析局域网络系统的工作情况 ,对系统的平均故障间隔时间、中断率等参数进行评估 ,为工程设计和系统维护提供参考依据     

防空C4ISR系统指挥引导成功率的一种算法

基于防空C4ISR系统试验和评估,给出了一种指挥引导成功率的计算方法.依据指挥引导高度误差的不同要求,分别定义了二维指挥引导成功区和三维指挥引导成功区.通过对指挥引导散布现象的分析,说明了指挥引导点的分布特性,并且应用概率论和数理统计理论给出了指挥引导成功率的计算方法.通过仿真计算,证明了该算法的正确性和实用性.     

战术数据链的建设与发展

通过回顾战术数据链的建设与发展历程 ,提出了战术数据链定义、类型。介绍了战术数据链的国际现状 ,给出了欧美国家现有战术数据链。分析了美军战术数据链建设情况 ,着重研究了 16号链及 JTIDS的特点、运用和发展情况 ,就JTIDS的相对导航原理和对作战效能的影响进行了分析。指出了俄军战术数据链状况 ,说明了某型俄制飞机空空战术数据链的应用方法。最后依据本国实际提出了我军战术数据链建设和发展的意见和建议 ,提供了研究与开发的技术途径。     

Log‐concave and concave distributions in reliability

Nonparametric classes of life distributions are usually based on the pattern of aging in some sense. The common parametric families of life distributions also feature monotone aging. In this paper we consider the class of log‐concave distributions and the subclass of concave distributions. The work is motivated by the fact that most of the common parametric models of life distributions (including Weibull, Gamma, log‐normal, Pareto, and Gompertz distributions) are log‐concave, while the remaining life of maintained and old units tend to have a concave distribution. The classes of concave and log‐concave distributions do not feature monotone aging. Nevertheless, these two classes are shown to have several interesting and useful properties. We examine the closure of these classes under a number of reliability operations, and provide sharp reliability bounds for nonmaintained and maintained units having life distribution belonging to these classes. © 1999 John Wiley & Sons, Inc. Naval Research Logistics 46: 419–433, 1999