关键词： MECHANICAL properties of polymers   NANORODS   MOLECULAR simulation methods   VISCOELASTICITY   NANOCOMPOSITE materials  

摘要： An understanding of the structure property relationship in polymer/nanorod (NR) nanocomposites is of fundamental importance in designing and fabricating polymer nanocomposites (PNCs) with desired properties. Here, we study the structural, mechanical, and viscoelastic properties of polymer grafted NR filled PNCs, using coarse-grained molecular dynamics simulation. The mechanical reinforcement efficiency is found to be determined by the NR/polymer interfacial properties, which are in turn modulated by the grafting density, the grafted chain length, and the graft matrix interaction strength. By systematically analyzing the evolution of the polymer-grafted NRs during mechanical deformation, we find that the NRs aligning side by side with each other do not contribute much to the mechanical reinforcement. Simulation results also indicate that the strain-dependent viscoelastic behavior (the Payne effect) originates from the failure of the local filler network and, especially, NR clusters constructed via site-to-site contacts. PNCs with low grafting density and short grafted chains are found to form NR aggregates, mostly through the site-to-site contact state, which leads to a more pronounced Payne effect as reflected in the slope of the storage modulus versus shear amplitude. Furthermore, for stronger graft matrix interactions, the NRs dispersed in the polymer matrix act as the temporary cross-linking points for a polymer shell layer-bridged NR network, accounting for the significant improvement in the mechanical property and the large increase in the Payne effect at high graft matrix interaction strengths. In general, higher grafting density, longer grafted chains, and moderate graft matrix interactions can effectively minimize the nonlinear viscoelastic behavior of PNCs.

关键词： failure   colloidal gels   rheology   light scattering   plasticity  

摘要： Material failure is ubiquitous, with implications from geology to everyday life and material science. It often involves sudden, unpredictable events, with little or no macroscopically detectable precursors. A deeper understanding of the microscopic mechanisms eventually leading to failure is clearly required, but experiments remain scarce. Here, we show that the microscopic dynamics of a colloidal gel, a model network-forming system, exhibit dramatic changes that precede its macroscopic failure by thousands of seconds. Using an original setup coupling light scattering and rheology, we simultaneously measure the macroscopic deformation and the microscopic dynamics of the gel, while applying a constant shear stress. We show that the network failure is preceded by qualitative and quantitative changes of the dynamics, from reversible particle displacements to a burst of irreversible plastic rearrangements.

关键词： 中国铁路   机械动力设备   管理工作   优化研究  

摘要： 中国铁路事业伴随着标准动车组时代的到来取得了新的突破性发展,铁路运营愈发应用到更多的高技术集成的动力设备与制造工艺。期间,铁路机械动力设备管理也面临着全新的挑战,业务单位与管理技术人员需要持续改进设备管理质量,确保铁路部门工作效能、经济效益与高速发展的铁路事业相匹配。针对此类情形,本文着重讨论了铁路机械动力设备管理工作的改进思路。

关键词： 双弧面凸轮   ATC机械手   刚柔耦合分析   数字化样机  

摘要： 针对双弧面凸轮式ATC机械手,完成了整机的三维建模;并基于机械系统动力学的理论基础建立了双弧面凸轮式ATC机械手多自由度刚柔耦合动力学模型;通过ANSYS对输入轴、拨叉轴、套筒和拨叉进行了柔性化处理,完成了换刀机构刚柔耦合数字化样机的建立;在此基础上基于ADMAS对刀臂上下和回转动作的动态性能进行了分析.此分析过程尽可能模拟了换刀机构在实际工作过程中的情况,其结果对今后换刀机构的设计、加工和刚柔耦合分析都具有参考价值.

关键词： 农业劳动力   农业机械化率   替代弹性   粮食产量   超越对数生产函数   粮食主产区  

摘要： 利用1978-2015年13个粮食主产区的省域面板数据,采用超越对数生产函数分析了中国农业劳动力与机械利用变迁对粮食生产的影响。并在测算出机械与劳动力替代弹性的基础上分析弹性变化的原因,论证农业机械对劳动投入与粮食生产的影响。研究表明:农业劳动力与农业机械均对粮食产量具有显著正向影响,但是两者对粮食产出的边际影响不显著。农业机械与劳动力之间存在显著的替代关系,农机具购置补贴不仅使农业机械对劳动的替代作用变强,而且显著提高了农业机械化水平。1978-2015年农业机械与劳动力的替代弹性呈现出四个较明显的阶段性特征,并与中国农业劳动力转移的阶段性特征相一致。但粮食主产区域内农业机械与劳动力的替代弹性变化存在显著差异性。为适应农业生产方式转变、保障国家粮食安全,粮食主产区应采取差异化的农村剩余劳动力转移政策,将农业机械化支持政策偏向农机具购置补贴与农业机械示范推广。

关键词： Time-delay   Stability   Frequency-domain approach   Control design   Vibration control  

摘要： A question of ongoing interest for linear Time-delay systems is to determine conditions on the equation parameters that guarantee the exponential stability of solutions. In recent works a new interesting property of time-delay systems was emphasized. As a matter of fact, the multiple spectral values for time-delay systems was characterized by using a Birkhoff/Vandermonde-based approach. Then, a multiplicity induced stability criteria were exhibited for reduced order systems;scalar delay-equations and a special class of second order systems. This work, further explores such a criteria and shows their applicability to the control of a mechanical system. (C) 2017 Elsevier Inc. All rights reserved.

关键词： 六自由度机械臂   ADAMS   动力学仿真   PID控制  

摘要： 介绍了一种六自由度机械臂装置,分析了六自由度机械臂的基本理论知识,在SolidWorks软件中建立了六自由度机械臂的模型。然后将模型导入ADAMS中,建立动力学仿真模型,通过对机械臂各部件运动的仿真分析,得出每个关节达到指定转速所需力矩,从而为各关节电机的选型提供依据。最后使用MATLAB与ADAMS进行联合仿真,在Simulink中建立控制方案,实现对机械臂各关节角的精确控制,为六自由度机械臂的发展提供设计参考及依据。

关键词： 空间机械臂   耦合   动力学   位姿   仿真  

摘要： SpiderFab型空间机械臂是一种典型的漂浮基空间机械臂系统。为了定量的认识机械臂在运动过程中对基体的扰动影响,并以此为依据设计浮动基体的位姿控制系统,建立了系统的运动学、动力学方程。根据系统动量守恒条件显式的表达出基体与机械臂之间的速度耦合关系,并由此说明基体的位姿变化不仅与机械臂实时关节速度有关,还与机械臂的历史关节路径有关。为了更进一步说明系统内存在的动力学耦合问题,针对一个漂浮基三自由度空间机械臂系统,设计了三组仿真试验。三组不同的关节速度曲线作为输入条件,求解基体关节所受的扰动量。仿真结果证明了计算方法的正确性。

关键词： 翻转课堂   嵌入式教学   教学策略   实践  

摘要： [目的／意义]设计基于翻转课堂的嵌入式教学模式，开展多门代表性课程的教学实践，以期提升嵌入式教学的效果和效率。[方法／过程]设计和实践课前一线上自主学习、课中一线下互动研讨、课后一线上下答疑辅导的嵌入式教学模式，并通过问卷调查对教学效果进行评估与分析。[结果／结论]实践表明，基于翻转课堂的嵌入式教学效果良好，大多数学生喜欢翻转课堂在嵌入式教学中的应用，认为翻转课堂教学方法有效，提出翻转课堂教学方法具有传统教学方法无法做到的优点，但未来学科服务团队还需在学生学习主动性提升、馆员教学能力提高、平台功能与技术应用强化等方面继续探索。

关键词： 动车组配电柜   装配装置   静力学分析   柔性多体动力学  

摘要： 动车组配电柜体积大、质量重,车内空间狭小,为此设计一款装配装置实现大角度翻转。而翻转过程中重心位置动态变化,翻转过程受力复杂。通过静力学分析所得的力矩曲线可知,翻转角度在0.224π和0.239π时,产生2类不利的前倾方式,并通过气动缓冲器和结构优化加以解决。对优化前后的翻转过程进行柔性多体动力学仿真分析,研究结果表明:优化后前倾力矩被有效抑制,配电柜可顺利翻转。输出柔性体关键部位的动态应力曲线,实现机械结构的动态设计。