关键词： Polychloroprene   Strain-induced crystallization   X-ray  

摘要： Polychloroprene rubber is often quoted as an example of strain-crystallizing elastomer;however, this fundamental property, known to impart good mechanical properties and fatigue resistance, has received much less attention in this class of compounds than natural rubber. The present paper relates systematic investigations combining mechanical and X-ray diffraction measurements. Crystallization kinetics are first investigated based on tensile impact tests. An approximately logarithmic time-dependence is found for the crystalline content and the time-constants are estimated at various temperatures. The impact of strain-induced crystallization on stress-strain curves under mechanical cycling conditions is further assessed. It is demonstrated that one main effect of strain-induced crystallization is a partial relaxation of the strain experienced by the amorphous fraction, although stress-hardening is simultaneously observed. Based on the stress-optical law, the contribution of the amorphous phase to the stress is shown to level off after crystallization onset, and stress-hardening is attributed to crystallite networking. The crystallite network rapidly disintegrates at the beginning of recovery, and the stress-optical law shows that the amorphous fraction becomes the only contributor to the stress at still non-negligible crystalline contents. (C) 2018 Elsevier Ltd. All rights reserved.

关键词： Mechanical vertical drilling tool   torsional vibration   eccentric block   gravity sensitivity   non-linear analysis  

摘要： In present, the drilling industry have to face the reality of low oil and gas prices, fully mechanical vertical drilling tool is likely to become the best choice for vertical drilling in the future. Without electric measurement and control system, the mechanical vertical drilling tool has a mechanically stable platform with gravity sensitivity. The stable platform included an eccentric block which is not only sensitive to gravity but also severely affected by torsional vibration. In this article, we reveal the dynamics of eccentric block under the effect of torsional vibration, which is the key element of the well deviation control accuracy in mechanical vertical drilling tool. The dynamic model of eccentric block is established using Lagrangian equation and Coulomb friction, and the law of dynamic characteristics under the effect of different amplitudes and period of torsional vibration is found. The results show that, with the deviation angle and eccentric torsion moment of eccentric block is increased, the resistance to torsional vibration increases. While the frequency of torsional vibration decreases, a subordinate period gradually appears after the rotation speed value of omega is less than I rad/s, the torsional vibration with large amplitude and high frequency is detrimental to the stability of the eccentric block. In addition, the largest Lyapunov exponent is estimated using the time series of simulation, the dynamics of eccentric block under the effect of torsional vibration can be switched four types, and the maximum and minimum values of largest Lyapunov exponent are 3.99 X 10(-2) and -2.98 X 10(-2), which means with the state of torsional vibration changes and the response of eccentric block would be transformed to chaos.

关键词： High entropy alloy   Mechanical alloying   Sintering   Diffusion  

摘要： This work correlates the densification behaviors of high entropy alloy powders with the alloying order. Mechanically alloyed powders (CoFeNi, CoFeNiCr, CoFeNiCrMn and CoFeNiCrMnAl alloys) were non-isothermally sintered using dilatometer. It was demonstrated that when the alloying order or the number of alloying elements were increased from 3 elements to 6 elements, sintering kinetics were changed. Materials exhibit sluggish grain growth, reducing densification rates and increasing activation energies of sintering, with increasing configurational entropy. Increasing trend of micro-hardness also indicates the better mechanical properties of such alloys. (C) 2018 Elsevier B.V. All rights reserved.

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： 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： 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： 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.