基于颗粒流的花岗岩微观破坏机制研究

为研究岩石在单轴压缩条件下的微观破坏机制,利用岩石力学试验机进行花岗岩岩样的室内试验,采用颗粒流分析程序编程建立可靠的数值模型,通过分析试件压缩破坏过程的裂纹演化规律、裂纹数特征和能量耗散规律,讨论其微观破坏机制。结果表明,花岗岩岩样内部拉裂纹分布与宏观破坏面较为吻合,在裂隙发展中起着关键作用;峰值强度前,剪裂纹较拉裂纹发育显著,峰值强度后,拉裂纹数剧烈增加,造成岩样承载力的降低;岩样最终失效形式为沿对角形成宏观剪切面破坏,并有少量张拉襞裂破坏。     

水工钢闸门主梁模糊失效概率计算方法研究

为合理计算水工钢闸门主梁模糊失效概率,分别将主梁相对变形当作一个随机变量及三个变量的组合,采用积分法、当量随机化方法及蒙特卡罗法进行了计算。当相对变形为一个随机量时,采用积分法及当量随机化方法计算,两者的差别在于积分法用隶属函数描述模糊限值,当量随机化方法是将模糊限值当量作为一个随机量。当相对变形看作三个变量的组合时采用蒙特卡罗法进行计算,该方法考虑了三个变量的分布特性,更符合实际情况,模糊限值也用随机量表示。计算表明,积分法与当量随机化方法结果相近,验证了当量随机化方法的精度;蒙特卡罗法结果与相对变形服从正态分布时、用当量随机化方法计算的结果接近,故主梁相对变形服从正态分布更为合理。三种计算方法中,当量随机化方法计算失效概率相较于其它方法有计算过程简便,效率高的优点。     

印尼某火电厂Ａ２—Ａ４区人工边坡失稳机制研究

以印尼某火电厂涉外项目Ａ２—Ａ４区的问题人工边坡为研究对象,通过现场地勘、实际变形失稳过程、数值模拟等方法综合研究了该地人工边坡失稳机制。在此基础上,探讨了边滑坡加固措施的作用效果及合理性。所得主要结论:Ａ２—Ａ４区人工边坡变形失稳机制主要为软弱地层（煤层）切穿暴露所造成的组合滑移以及坡体沿土岩分界面所造成局部变形,变形破坏模式接近圆弧滑动,人工边坡中的软弱夹层是导致边坡失稳的主控因素;Ａ２、Ａ４区综合考虑桩锚＋坡面锚杆的方式进行加固,煤层和全风化泥岩及上部土层内的组合变形没有发展贯通趋势,边坡整体安全。Ａ３区在加固作用下存在沿二级坡脚＋煤层＋全风化泥岩＋上部土层内越顶组合滑移风险以及基坑开挖造成基坑侧壁坍滑风险,需要引起重视并加强监测。     

小浪底细砂岩单轴压缩声发射特征研究

采用岩石伺服试验机和声发射仪,对小浪底三叠系细砂岩进行单轴压缩声发射试验。基于试验结果,将试样变形过程划分为压密、弹性、塑性和破坏四个阶段,分析了不同阶段试样声发射幅值、峰值频率、振铃计数和能量的变化特征,在此基础上对比分析了试样声发射ｂ值以及累计振铃计数的演化规律,得出了岩石的破坏时间前兆特征。研究结果表明:压密和弹性阶段试样声发射幅值、振铃计数和能量处于较低水平、峰值频率频带数量依次增多;塑性阶段试样幅值增大、振铃计数与能量上升且整体处于较高水平、峰值频率数值增大且呈现多频带化,临近破坏阶段试样产生大量100ｄＢ的高幅值信号,振铃计数与能量持续急增,峰值频率降低,频带减少;声发射ｂ值的变化规律可以间接反映岩体内部微裂隙萌生、扩展的演化特征。     

Optimal tracking performance of MIMO discrete‐time systems with communication constraints

The optimal tracking problem for multiple‐input multiple‐output linear‐time‐invariant discrete‐time systems with communication constraints in the feedback path is studied in this paper. The tracking performance is measured by the energy of the error signal between the output of the plant and the reference signal. The objective is to obtain an optimal tracking performance, attainable by all possible stabilizing compensators. It is shown that the optimal tracking performance consists of two parts, one depends on the nonminimum phase zeros and zero direction of the given plant, as well as the reference input signal direction, and the other depends on the nonminimum phase zeros, unstable poles, and pole direction of the given plant, as well as the bandwidth and additive white Gaussian noise of the communication channel. It is also shown that, if the constraint of the communication channel does not exist, the optimal tracking performance reduces to the existing tracking performance of the control system without communication constraints. A typical example is given to illustrate the theoretical results. Copyright © 2011 John Wiley & Sons, Ltd.     

Observer‐based Reliable Control for Discrete Time Systems: An Average Dwell Time Approach

This paper deals with the problem of reliable control for discrete time systems with actuator failures. The actuator is assumed to fail occasionally and can recover over a time interval. During the time of suffering failures, the considered closed‐loop system is assumed unstable. Using an average dwell time method and under the condition that the activation time ratio between the system without actuator failures and the system with actuator failures is not less than a specified constant, an observer‐based feedback controller is developed in terms of linear matrix inequalities such that the resulting closed‐loop system is exponentially stable. An example is included to demonstrate the effectiveness of the proposed approach.     

Optimal Performance in Tracking Stochastic Signal Under Disturbance Rejection

The analysis method of optimal tracking performance is proposed for multiple‐input multiple‐output (MIMO) linear time‐invariant (LTI) systems under disturbance rejection. An H₂ criterion of the error signal between the output of the plant and the reference signal is used as a measure for the tracking performance. Spectral factorization is applied to obtain the optimal solution of the system tracking error. The explicit expressions are derived for this minimal tracking error with respect to random reference signals under disturbance rejection. It is shown that the nonminimum phase zeros, the zero direction, the unstable poles, the pole direction of a given plant, statistical characteristics of the reference input signal, and disturbance signal have a negative effect on a feedback system's ability to reduce the system error with disturbance rejection. The results show that the optimal tracking performance will further be damaged because of disturbance rejection. Some typical examples are given to illustrate the theoretical results.     

Perceived control over caregiving outcomes: Implications for child abuse.

We predicted low perceived caregiver control over caregiving failure to be related to (a) coercive or abusive parenting and (b) affective reactions to "difficult" children. On the basis of a multidimensional scaling analysis of the Parent Attribution Test (Study 1), we constructed a scale (PCF) that assessed perceived balance of control over caregiving failure (attributed control to caregivers vs attributed control to children). In Study 2, we found low PCF to predict abusiveness and nonabusive coerciveness among mothers in counseling at a child abuse agency. Additionally, we found low PCF to predict experienced annoyance/irritation among unrelated mothers interacting with children at relatively high risk for abuse (compared with their lower-risk siblings). We interpreted results as demonstrating the potential importance of low perceived control as a moderator of negative affect in response to difficult children. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

基于多模型变权重组合的软件可靠性仿真研究

研究提高软件可靠性预测精度问题,对软件可靠性研究已成为当前软件工程的一个研究热点,传统的单一软件可靠性模型由于使用的技术及提取的信息有限,软件可靠性预测精度不高。为提高软件可靠性预测精度,在建立多种单一软件可靠性预测模型的基础上,提出一种样本点的多模型变权重组合模型。将多种预测技术有效地聚合在一起,取长补短,在样本数据有限的情况下,不仅改善了样本内学习能力也增强了样本外的泛化能力,提高了综合预测精度。仿真验证模型无论在样本内还是样本外都较优于经过模拟退火算法优化的BP神经网络(SA-BP)及经过遗传算法优化的最小二乘支持向量机(GA-LSVM),说明变权重组合模型是一种精度更高的软件可靠性失效数据预测模型,具有较好的应用推广价值。     

浅析计算机网络通讯存在的问题及其改进策略

随着计算机技术取得的长足发展以及计算机的日益普及,计算机网络通讯已成为人们生活及工作中的不可或缺的组成部分。虽然计算机网络通讯极大的方便了人与人之间的沟通,但是仍然存在诸多问题,主要表现在网络通讯技术故障和网络通讯安全方面。网络故障问题主要是硬件连接和软件设置问题,网络通讯安全问题主要体现在非法窃听、恶意篡改、假冒以及对个人信息的非法操作等方面。本文简要剖析了计算机网络通讯当前存在的问题以及针对问题提出了相应解决方案。