关键词： Molecular dynamics   Polyvinylidene fluoride   Carbon nanotube   Mechanical property   Microstructure  

摘要： Molecular dynamics (MD) simulation was used to investigate the microstructure and mechanical properties of pure polyvinylidene fluoride (PVDF) and PVDF/carbon nanotube (CNT) composites with and without a polarization process. After annealing, the trans percentages of non-polarized PVDF and composites are improved by about 25% compared to the initial model of alpha-phase PVDF. For PVDF and composites with polarization process, the PVDF trans percentages can be further enhanced by around 10% compared to those without the polarization process. The mechanical properties of pure PVDF can be improved by blending the PVDF with CNTs and with the polarization process. The local shear strain analysis shows that the strength of PVDF around the CNTs have been enhanced, leading to higher brittleness of the composites at increasing CNT weight fractions. During the tensile process, the trans percentage of PVDF increases in the non-polarized cases with increasing strain, but are less sensitive to polarized cases.

关键词： Dynamic mechanical analysis   Molecular dynamics   Viscoelastic   Polyvinylidene fluoride   1-ethyl-3-methylimidazolium tetrafluoroborate  

摘要： Bucky gel actuator is a kind of ionic electroactive polymer that bends when stimulated by an electric field. The mechanical properties of bucky gel actuator play a crucial role in its performance, determining the amount of displacement and blocking force. Dynamic mechanical analysis is performed by molecular dynamics simulation for the electrolyte layer that contains polyvinylidene fluoride as polymer membrane and 1-ethyl-3-methylimidazolium tetrafluoroborate as ionic liquid to study viscoelastic properties behavior of electrolyte at different strain frequencies. Calculated storage modulus increases and viscosity decreases while strain frequency increases in both tensile and shear modes. This is consistent with experimental result for pure polyvinylidene fluoride. Therefore, the presented molecular dynamics simulation method could be utilized for further studies to understand the ion transportation and bend mechanism of the bucky gel actuator.

关键词： Hybrid graphene/hBN   Defects   Molecular dynamics   Mechanical properties   Circular defect   Square defect  

摘要： In this paper, by using molecular dynamics simulation, the mechanical properties of butt-joined hybrid graphene-boron nitride (BN) sheets in the presence of various defects have been investigated. For this purpose, two types of defects including circular and square holes have been created in the examined specimens. The effects of increasing the hole diameter, hole length (circular and square), number of holes and also the effect of different locations of such defects in hybrid graphene-boron nitride sheets on their mechanical properties have been analyzed and discussed. The findings indicate that the presence of holes in the considered structures reduces their mechanical properties including the failure strength and strain and Young's modulus;but the effect of such defects on Young's modulus is much less than that on the other two mechanical parameters. Also, it has been demonstrated that the circular holes have a less severe effect on mechanical properties than the square holes. For example, by making circular holes of 10 angstrom diameter in a hybrid graphene-boron nitride sheet in the left graphene region (GFL), its failure strength is reduced 20% relative to that of a defect-free structure;while the Young's modulus diminishes by 10% under the same conditions. Different cases of these defects in the structure of hybrid graphene-boron nitride nanosheets and their effects on mechanical properties have been explored for the purpose of using them in nanotransistors and nanodiodes. As a key result, it was observed that the hybrid sheets behave as a more ductile material when the density of the defects increases.

关键词： In situ crosslinkable hydrogel   Mechanics   Dual-mode release kinetics  

摘要： Michael addition is extensively utilized to fabricate hydrogels for biomedical applications, due their ability to undergo crosslinking reaction under physiological conditions without the need of a catalyst. Herein, mechanical properties and degradation behavior of in situ crosslinkable poly (ethylene glycol) diacrylate (PEGDA)-polyethyleneimine (PEI) hydrogels are explored. PEGDA and PEI, having acrylate and amine groups respectively, undergo crosslinking reaction via Michael addition to form hydrogels. The mechanical properties are controlled in a wide range by varying their concentrations and the molecular weight of PEGDA. In addition, the hydrogels are all shown to undergo degradation due to the expedited hydrolysis of ester linkages by the presence of unreacted amine groups on PEI. With this interesting combination of tunable mechanical properties and degradation, the PEGDA-PEI hydrogel system display a dual-mode drug release kinetics in which the drug release was governed by swelling-controlled and degradation-controlled mechanisms in sequence, which demonstrates the potential for drug delivery applications. (C) 2018 Elsevier Ltd. All rights reserved.

关键词： 油菜   收获脱出物   物理机械特性   空气动力学特性  

摘要： 研究了两个油菜品种(沣油737和中农油6号)收获脱出物的物理特性和空气动力学特性。结果表明:两种油菜收获脱出物的三轴尺寸存在较大的差异;两种油菜籽粒的含水率、千粒重和密度基本一致,而荚壳的千粒重、含水率、密度以及短茎秆的千粒重存在一定差异;两种油菜籽粒的恢复系数存在差异;两种油菜籽粒和荚壳的剪切模量都有明显差异;两种油菜收获脱出物的悬浮速度基本一致。

关键词： Kevlar (R)   Molecular dynamics simulations   Hydrogen bonds   Mechanical properties   Failure modes  

摘要： Effects of loading direction and hydrogen bonds on the anisotropic elastic and strength properties of p-phenylene terephthalamide (PPTA) crystal (i.e. Kevlar (R)) are studied using molecular dynamics simulations with reactive force field ReaxFF. Simulations of monotonic tension, compression and shear loadings of the crystal indicate that the elastic properties exhibit orthotropic symmetry but strengths in the various crystal directions are anisotropic and highly dependent on the direction of loading. Tensile strength in the chain axis of the crystal fails by chain scission at a stress level that is twenty times higher than the compression strength governed by chain buckling. To study the effects of hydrogen bonds, simulations are repeated by removing the hydrogen bonding interactions from the ReaxFF force field parameter file. Simulation results indicate that hydrogen bonds do not affect the axial properties that are dominated by covalent bonds in the chains. However, in the absence of hydrogen bonds, transverse tensile, compressive and shear moduli (40-44%) and strengths (47-69%) are reduced significantly. The absence of hydrogen bonding on changes in failure modes for the various loading directions are identified. The results presented in this study provide insight into moisture induced property degradation mechanisms of Kevlar (R) reported in the literature attributed to reduction in hydrogen bonding.

关键词： 动力机   故障识别   参数寻优   最小二乘支持向量机   布谷鸟搜索算法  

摘要： 目的针对包装机械设备中动力机轴承的故障诊断识别率低的问题,提出一种基于参数寻优的故障识别方法。方法首先通过主元分析算法对包装设备动力机的振动数据进行主成分特征提取,减少各数据间的相关性,然后采用LSSVM对各类数据样本进行故障识别。为了克服LSSVM惩罚因子和核函数参数易出现局部最优、收敛精度差等问题,提出一种ICS算法优化LSSVM的状态参数,提高包装机械动力机轴承故障诊断的识别率,以实测糖果厂包装机械振动数据为例验证所提方法的有效性。结果实验结果表明,在包装机械动力机轴承故障类别确定的情况下,算法能够高精度地识别各类动力机故障。结论该算法实现了分类器参数的自适应选择,为提高包装机械动力机轴承故障诊断的识别率提供了可靠的方法。

关键词： 农业机械总动力   农业劳动生产率   协整   格兰杰因果性   VAR分析  

摘要： 从全国农业机械总动力与农业劳动生产率关系的视角进行研究,主要采用实证分析法,以全国1996-2015年的二十组有关农业机械总动力和农业劳动生产率的数据为基础,借助Eviews8.0统计分析软件,通过简单的线性回归、协整、格兰杰检验、VAR分析,得出农业机械总动力提高了农业劳动生产率的结论,并对今后农业机械化的发展提出相关建议。

关键词： 工程船舶   动力机械   状态监测   故障诊断技巧  

摘要： 时代的快速进步与发展为我国不断带来新的机遇与挑战,就工程船舶方面,我国相比较几十年的起步阶段,已经取得了非常傲人的成绩,工程船舶与一般的运输船舶不同,对于我国水上水下工程有着非常重要作用。而作为驱动工程船舶运作的动力机械作为工程船舶的正常运作的核心,对其进行及时的状态监测和故障诊断必不可少。本文以此为出发点,对于工程船舶动力机械状态监测和故障诊断技巧进行深入分析。

关键词： 对靶喷施   喷洒模型   沉积覆盖区域   机械臂   风扰   动力学模型  

摘要： 为实现对靶标杂草的有效覆盖和提高对靶施药精度,减少药液浪费和生态污染,采用基于二自由度机械臂的对靶施药方式,优化了除草机器人的对靶喷施性能;除草机器人药液喷洒模型的建立是提高对靶喷施精度的关键,在考虑风扰的前提条件下,根据液滴粒子轨迹动力学方程,结合机械臂连杆和关节变量参数,建立液滴喷洒轨迹模型,并推导得到液滴理论沉积覆盖区域;在此基础上通过计算机数值模拟,分析了喷头位姿、车速和风速对液滴落地沉积覆盖区域的影响,并对靶标点进行了坐标重构;在室内搭建了小型开口直流低速风洞并进行药液喷洒验证试验,利用图像处理技术获取试验的液滴落地沉积覆盖区域,对比其与理论沉积覆盖区域,形心误差范围为4.1~7.2 mm,区域匹配误差范围为9.1%~17.8%。试验结果表明:理论沉积覆盖区域与试验测定的结果误差较小,两者具有较高的一致性。