关键词： molecular dynamics   mechanical activation   mass mixing   friction stir welding  

摘要： The paper presents a molecular dynamics study to investigate the behavior of materials under loading by friction stir welding (FPW). The loading is simulated by assigning constant angular and forward velocities to a certain group of atoms, being a FPW tool. The joined materials are two defect-free Cu crystallites, Cu and Fe crystallites, and two crystallites of the same solid solution structured as D16 (2024) alloy. It is found that as the tool passes along the weld line, the crystal structure of the materials is rearranged with subsequent mixing of their surface atoms. Under certain loading conditions, the crystal lattice after passage of the tool recovers its regular order. Also analyzed is the influence of vibrations additionally applied to the FPW tool. The simulation results provide a better understanding of the processes involved in mechanically activated diffusion.

关键词： hydride phase   thermal resistance   kinetics   thermal desorption spectroscopy  

摘要： With the aim to decrease temperature and improve the kinetics of decomposition of the MgH2 stoichiometric hydride, we study the possibility of its complex alloying with 3 at.% Al + 3 at.% Ni + 7 at.% Ti performed with the help of mechanochemical synthesis. It is shown that, as a result of the addition of Al, Ni, and Ti to magnesium, the thermal resistance of the MgH2 hydride phase of the mechanical alloy obtained by the reactive mechanical alloying decreases and, hence, the temperature of the onset of hydrogen desorption decreases from 320 degrees C (for MgH2 without Al, Ni, and Ti) to 280 degrees C. We do not detect any decrease in the equilibrium temperature of the onset of hydrogen desorption under its constant pressure of 1 bar in a reactor, which may serve as an indication of the loss of thermodynamic stability of MgH2 as a result of mechanical alloying. It is shown that the kinetics of hydrogen desorption of the MgH2 phase of the mechanical alloy is mainly improved by alloying with Ni and Ti, whereas the contribution of Al is negligible.

关键词： chitin   chitosan   acetyl group   fracture energy   stiffness   steered molecular dynamics (SMD) simulation  

摘要： Chitin fiber is the load-bearing component in natural chitin-based materials. In these materials, chitin is always partially deacetylated to different levels, leading to diverse material properties. In order to understand how the acetyl group enhances the fracture resistance capability of chitin fiber, we constructed atomistic models of chitin with varied acetylation degree and analyzed the hydrogen bonding pattern, fracture, and stress-strain behavior of these models. We notice that the acetyl group can contribute to the formation of hydrogen bonds that can stabilize the crystalline structure. In addition, it is found that the specimen with a higher acetylation degree presents a greater resistance against fracture. This study describes the role of the functional group, acetyl groups, in crystalline chitin. Such information could provide preliminary understanding of nanomaterials when similar functional groups are encountered.

关键词： UV-curable coating   Hyperbranched polymer   Curing   Viscosity   Physical/Mechanical properties  

摘要： A polyester-amide based hyperbranched polymer with hydroxyl end-groups is modified by the aid of acrylic functionalities. Two levels of modification, i.e. 40% and 65%, are considered. The unmodified hyperbranched polymer and the modified ones are added to UV-curable monomeric and oligomeric systems. Rheology measurements are conducted to investigate the effects of hyperbranched polymer on the rheological behavior. In order to assess the curing kinetics and physical/mechanical characteristics of the samples, photo-DSC and DMA experiments are exploited, respectively. The effect of viscosity and number of UV-curable functionalities of the hyperbranched polymer on the curing level and kinetics of the curing reactions are discussed. Dynamic mechanical analysis shows that the addition of hyperbranched polymer in the formulation increases a 25% in storage modulus of the glassy region and cross-linking density of the UV-curable coating. Furthermore, it is also seen that the existence of the hyperbranched polymer increases the viscosity of Trimethylolpropane triacrylate (TMPTA)-based system while it is decreasing the viscosity of oligomeric-based of the epoxy di-acrylate system. It is obtained that the hyperbranched polymer makes the curing level in the monomeric and the oligomeric system, improve up to 15% and 10% correspondingly. It is also observed that Tgs of the coatings containing the hyperbranched polymer increases up to 5.9 degrees C. (C) 2015 Elsevier B.V. All rights reserved.

关键词： THERMOPLASTICS   POLYETHYLENE   MOLECULAR interactions   NANOCOMPOSITE materials   DYNAMICS  

摘要： Because of their remarkable performance properties and technological promise, polymer nanocomposites reinforced with single-walled carbon nanotubes (SWCNTs) have attracted considerable attention in the engineering, applied physics, and materials science communities. Recent experimental and computational investigations have shown that the presence of nanoscale defects in CNTs can significantly impact their electrical, mechanical, and thermal properties. In this article, for the first time, we examine the effect of defective CNTs on the interfacial characteristics and mechanical properties of CNT/polyethylene (PE) nanocomposites. Our molecular dynamics simulations show that as few as five vacancy defects in each CNT in a high-volume-fraction CNT/PE nanocomposite can decrease the longitudinal Young's modulus of the nanocomposite by as much as 18%, and the shear stress at the CNT/polymer interface by as much as 38%. By accounting for nanoscale defects and their effect on the CNT/polymer interfacial mechanics, our findings provide a practical guide for designing nanocomposites that are capable of attaining a desired set of elastic performance properties. (C) 2014 Society of Plastics Engineers

关键词： 混合动力客车   机械电控自动变速器   能量管理   换挡规律   联合仿真  

摘要： 混合动力汽车作为传统汽车向纯电动汽车的过渡产品,逐渐在当今汽车市场中占据重要地位。混合动力汽车拥有两种或两种以上的动力源,目前混合动力系统的控制研究主要包括两类问题:一是稳态或动态过程中的能量分配问题;二是动态过程的动态协调控制问题,包括变速器的换档、工作模式的切换等。本文结合江苏省高校自然科学重大研究项目“混合动力电动汽车动力切换与协调控制”,以某配备机械电控自动变速器AMT的混合动力客车为例,在研究其能量管理策略的基础上,研究混合动力系统不同工作模式下的换挡规律。主要内容如下:首先,分析某型单轴并联式混合动力客车的结构特点,通过混合动力台架试验系统获得发动机、电动机的台架试验数据,分别建立了部件效率模型;并基于Cruise平台建立了混合动力客车的各部件模块和整车模型。其次,研究了该混合动力客车目前所沿用的逻辑门限能量管理策略,分析了混合动力客车的工作模式,并制定不同工作模式切换规则和转矩分配规则,在Matlab/Simulink中建立了仿真模型;针对该能量管理策略中转矩分配规则的不足之处设计了以动力源效率和尾气排放为优化目标的多目标优化算法,并对优化前后的控制策略进行仿真,仿真结果表明,优化后的控制策略能够提高车辆动力源效率并降低了尾气排放量。然后,基于上述优化后的能量管理策略,即确定了工作模式以及一定需求转矩下的转矩分配规则后,提出了混合动力系统综合效率的概念,并以该综合效率为换挡目标,选择发动机转矩、车速、电动机转矩为换挡参数,针对不同工作模式设计了三参数的效率换挡曲面,并采用模糊控制方法设计换挡模糊控制器。基于Cruise和Matlab在城市循环工况(UDC)下联合仿真,结果表明本文所设计的换挡规律能够提高车辆的燃油经济性。最后,根据已有的试验条件设计了该混合动力客车的转毂试验,将改进前的原型客车、采用优化能量管理策略后的原型客车以及采用本文设计的换挡规律的原型客车在中国典型城市公交循环工况(CCBC)中进行试验,获得了尾气排放曲线、发动机的燃油消耗以及电池的电能消耗,根据国家标准GB/T 19754-2005中电池电能的等效燃油消耗换算方法计算得到整车的综合燃油消耗。分析三次试验结果,获到如下结论:本文所设计的基于优化算法的换挡规律能够一定程度上提高车辆的燃油经济性并降低整车尾气排放。

ISBN: (纸本)9787030475473

关键词： 变结构机械臂   逆向运动学   误差补偿   多体动力学  

摘要： 变结构机械臂Variable Geometry Truss(VGT)是一种具有很高刚度和灵活性的高冗余机械臂。由于其灵活和高自由度,VGT机械臂可以完成一系列传统机械臂不能完成的任务。VGT在水下操作,空间操作和工业生产中有着广泛的用途,有着重要的研究意义。因此,本文研究一种具有镜像对称结构的八面体机械结构的高冗余机械臂的运动学和动力学控制问题。针对根据在实际控制时实时准确的定位需求,VGT的快速逆向运动学运算是非常重要和基础的问题。然而,由于VGT约束方程的非线性和每节机构偏转方向和位置的耦合,使得VGT逆向运动学的快速求解变得十分困难。所以,为了实现VGT机械臂的实时定位,本文提出一种解析的多节VGT逆向运动学算法。由于需要得到逆向运动学的解析解,本方法从揭示变结构机械臂的几何对称特性和简化计算开始,提出一种与传统欧拉角和D-H参数不同的旋转矩阵表示方法。随后,本文基于新的旋转矩阵的表示方式提出一种快速准确的逆向运动学算法。该方法利用镜像变换的几何特性与两个辅助坐标系,将多节变结构机械臂逆向运动学的非线性约束方程组拆分和简化为多个解析可解的方程。本文提出的方法相对于广泛应用的雅可比方法在计算速度和准确性上有大幅度的提高。在实际使用中,算法的实时性和准确性得到验证。同时在变结构机械臂中,由于装配误差,作动器定位精度不高以及结构弹性等因素的影响,实际的运动学与理论推导的数学模型会有着比较大的差别。这种模型误差是影响变结构机械臂定位精度的主要因素。为了减小结构误差带来的定位精度的降低,本文提出一种基于离散控制和的误差调节算法。首先,本文将误差调节控制转换为离散控制系统的形式,然后设计了一种补偿控制算法。补偿算法根据安装在机械臂头部的摄像机测量头部与目标位置和方向的差值,计算出作动器对误差的补偿量,经过多次调节最终使得头部对准期望的目标。这种误差补偿算法在实际实验过程中可以抵消模型不确定的误差,实现精确定位。最后,本文研究了VGT的多体动力学建模和控制。由于VGT结构的复杂性,其动力学学建模面临着较一般连杆机械臂更大的困难。所以为了满足动力学控制的需求,研究VGT动力学和控制是必不可少的。本文根据拉格朗日动力学,结合Simulink中simMechanics模块,对VGT的动力学模型经行建模和控制。最后,本文设计了一种基于PD控制律的速度控制系统,并通过SimMechanics仿真验证其可行性,为未来更加完善的动力学和控制研究打下基础。实验和仿真充分验证的本文提出的方法。快速逆向运动学方法和误差补偿算法均被运用在实验装置中。通过对仿真结果和实验测试结果的比较,本文的提出的快速逆向运动学方法被证明是正确有效的,成功实现VGT实时定位解算的功能。同时,实验也对误差补偿算经行了验证,本文的误差补偿算法可以在有限步之内将定位误差补偿为零,让VGT的末端操作机构达到毫米定位精度。最后,本文在SimMechanics软件中建立VGT多体模型,测试提出的PD控制器。仿真证明PD控制器可以精确跟踪给定的速度和位置指令。

关键词： 柔性关节机械臂   动力学建模   神经网络   反演控制   PID控制  

摘要： 传统的机械臂动力学模型都是建立在刚体动力学模型基础下进行控制器设计的,但是在实际的操作过程中精度和效率等方面存在一定局限性。而柔性关节机械臂具有质量轻、能耗低、效率高等众多优点,因此被大量使用于太空探索、海洋探索、高精度操作、恶劣环境下工作的等多种类型的机器人中。但是关节的柔性引起了机械臂的振动,增加了控制的难度。因此本文基于考虑关节柔性的动力学模型基础下对机械臂控制器进行设计,以提高机械臂的控制精度和负载能力。首先基于D-H参数法建立六自由度柔性关节机械臂的运动学模型,并进一步利用拉格朗日方程建立了完整的动力学模型,在MATLAB中进行了分析仿真。然后在上述动力学模型基础上分别设计了几种控制器。先以单自由度柔性关节机械臂为例,分别设计了传统PID控制器和神经网络PID控制器,并在MATLAB的simulink中进行数值仿真研究。从仿真实验的对比结果可以看出,神经网络PID控制相比于传统PID控制具有参数调整简单、响应速度快、控制精度高的优点,跟踪误差减少了50%,能够达到更好的控制效果。后又采用神经网络反演法对机械臂控制器进行了设计,并对控制器的跟踪控制性能进行仿真,结果表明该控制方法受模型参数变化影响较小。最后,为了验证上述柔性关节机械臂控制器的控制性能,搭建单自由度柔性机械臂实验平台,为后续实验研究做准备。

关键词： Differential scanning calorimetry  

摘要： In this article, poly(lactic acid)/metal-organic framework composites were prepared by melt-blending methods and the effects of MOFs on the non-isothermal crystallization, thermal degradation and mechanical property of poly(lactic acid) (PLA) were studied by differential scanning calorimetry (DSC), thermogravimetric analyses (TGA) and tensile testing, respectively. The Jeziorny and Mo methods were applied to describe the kinetics of the non-isothermal crystallization process. The results demonstrated that the presence of MOFs accelerated the crystallization rate and increased the crystallinity (X-c) of the PLA/MOF composites. The XRD results confirmed that the addition of MOFs decreased the crystallite size of PLA. The TGA results indicated that MOFs increased the initial thermal degradation temperature of PLA and decreased the thermal stability at high temperature regions. The tensile strength of PLA was increased from 66.23 MPa to 92.44 MPa, 96.03 MPa and 95.25 MPa with MOF loadings of 0.5 wt%, 1 wt% and 1.5 wt%, respectively, but the strain at break was decreased.