关键词： biomaterials   differential scanning calorimetry   mechanical properties   thermal properties   thermosets  

摘要： In this contribution, first of all, the methoxy groups of organic solvent lignin (OSL) was converted to phenolic hydroxyl groups through demethylation reaction for the purpose of fabricating demethylated organic solvent lignin (DOSL). In addition, the resulting DOSL was utilized as a renewable material to synthesize a novel esterified lignin (EDOSL) by reacting with isobutyryl chloride for curing of epoxy resin. Finally, commercial liquid diglycidyl ether of bisphenol A was cured by EDOSL in the presence of 4-dimethylaminopyridine (DMAP) used as a catalyst based on dual-curing mechanism. Dual curing is a processing methodology based upon the alliance of two diverse and compatible polymerization reactions occurring sequentially or simultaneously. According to the FTIR spectra and H-1-NMR analyses, the demethylation of OSL, esterification of DOSL, and the curing reaction of epoxy resin with EDOSL were successfully conducted. The value of the phenolic hydrogen in the DOSL was approximately 4.89 mmol/g, which increased by 12.64% after demethylation. The thermal and mechanical performances of these cured epoxy samples were measured by DSC, DMA, TGA, and tensile testing. The epoxy system cured by 10%wt esterified lignin with 1%wt DMAP possessed the tensile strength of 71.54 +/- 7.50 MPa and the initial degradation temperature (T-5%) of 370 degrees C, which can compete fairly with commercial aromatic curing agents or other lignin-based agents studied currently for the curing of epoxy systems.

关键词： Calcium-silicate-hydrate   Molecular dynamics   Fe ions   Mechanical properties   Corrosion  

摘要： The adsorption of Fe ions in the calcium-silicate-hydrate (C-S-H) is of great importance as it involves the corrosion process of reinforced concrete structures. In this paper, molecular dynamics simulation was carried out to study the structure, dynamics and mechanical properties evolution of C-S-H induced by Fe ions. Through the analyses of local structure, atomic dynamics and bond stability, the results show that Fe ions are in tetrahedral coordination in the C-S-H and form 3 shells with neighbor atoms which are consistent with the extended X-ray absorption fine structure (EXAFS) results. In addition, the incorporation of Fe ions could reduce the polymerization of silicate chains by disordering the distribution of Os and Si atoms and weakening the strength of Si-O bonds. On the contrary, the connection between Caw (inter layer Ca atoms) and O atoms was slightly enhanced by Fe ions. The mechanical properties of C-S-H in three directions all decrease with the increasing Fe/Ca ratios (i.e., corrosion degree) as the silicate chains and interlayer connection were weakened by Fe ions. Hopefully, the properties of Fe ions adsorbed in the C-S-H could provide new insights on understanding and predicting performance of concrete structure under corrosion environment. (c) 2021 Published by Elsevier Ltd.

关键词： Elements   Deformation   Polymer particles   Energy   Polymers  

摘要： The objective of investigating macroscopic polymer properties with a low computing cost and a high resolution has led to the development of efficient hybrid simulation tools. Systems generated from such simulation tools can fail in service if the effect of uncertainty of model inputs on its outputs is not accounted for. This work focuses on quantifying the effect of parametric uncertainty in our coarse-grained molecular dynamics-finite element coupling approach using uncertainty quantification. We consider uniaxial deformation simulations of a polystyrene sample at T = 100 K in our study. Parametric uncertainty is assumed to originate from parameters in the molecular dynamics model with a nonperiodic boundary (the force constant between polymer beads and anchor points, the number of anchor points, and the size of the surrounding dissipative particle dynamics domain) and a parameter to blend the energies of particles and continuum (weighting factor). Key issues that arise in uncertainty quantification are discussed on the basis of the quantities of interest including mass density, end-to-end distance, and radial distribution function. This work reveals the influence of key input parameters on the properties of polymer structure and facilitates the determination of those parameters in the application of this hybrid molecular dynamics-finite element approach.

关键词： 农业   机械化   劳动力转移   VAR模型  

摘要： 基于粮食主产区河南省1989—2018年的数据,构建VAR模型,使用Granger检验、脉冲响应函数和方差分解分析等方法,对其农业机械化和农业劳动力转移的动态关系进行分析。结果表明:农业机械化和农业劳动力转移存在双向格兰杰因果关系,但双向互馈的影响程度不同,农业机械化水平提升对农业劳动力转移的作用显著高于其逆向作用。研究发现对于加快提高粮区农业机械化水平,助推农业劳动力有序流转,促进现代农业发展有重要的政策涵义。

关键词： 机械臂   旋量   动力学   Newton-Euler方法  

摘要： 机器人系统建模方法主要有D-H参数法和旋量法。旋量法建模具有物理意义清晰,表示无奇异等优点。基于旋量理论对6自由度的机械臂进行机构描述,采用Newton-Euler方法建立机械臂的动力学模型,然后进行动力学仿真实验。首先设置一段机械臂关节轨迹,利用逆动力学方程求解出相应的力矩,然后将力矩拟合函数和初始关节角位移和角速度代入到正动力学方程中,求得相应的运动轨迹。通过正、逆动力学仿真,可以看出正、逆动力学所得结果一致,验证了动力学模型的正确性。

关键词： 数学机械化   转子动力学   轴承系统   临界转速  

摘要： 随着近几年来人们对于科学技术的要求的不断提高,数学机械化被应用于越来越多的行业当中,而且人们近几年来在航天工程,汽车行业等诸多方面都取得了较大的发展,这也使得社会对于轴承系统的要求越来越高.在对转子轴承系统进行综合研究的过程当中,对转子动力学进行综合的探讨,对轴承系统当中的转子涡动或者是两元盘转子的临界转速等诸多方面进行综合的探讨,采用数学机械化的研究方式,将其作为研究对象,可以更好地实现对于轴承系统的综合探讨,而且还可以更好的了解轴承窝洞的各种参数,并对其进行综合的解析计算,更好地提高轴承转子稳定性.因此,本文对数学机械化在转的动力学研究中的应用进行简要探讨。

关键词： 多间隙   冲击振动   分岔   数值计算   动力学特性  

摘要： 建立含多重间隙弹性约束机械振动系统的力学模型.基于多参数耦合、多目标协同仿真分析,采用变步长Runge-Kutta法数值计算,研究系统在激励频率和间隙阈值的双参数平面内的周期冲击振动模式类型、分布规律和分岔特征.计算结果表明,基准参数条件下系统周期冲击振动的模式类型表现为复杂性和多样性特征.分析了系统相邻基本周期冲击振动的q/1周期振动经两种类型的Grazing分岔转迁为(q+1)/1周期振动的演化机理.揭示了间隙阈值取值较小时,系统表现为亚谐周期振动和混沌等复杂的动力学特性;且随激励频率的递减,系统因冲击次数足够大进而呈现出颤-冲击特性.研究该类振动系统激励频率和间隙阈值等关键参数与系统功能目标之间的映射关系,为系统动力学特性和功能目标协同优化的参数匹配规律及科学匹配范围提供依据和思路.

关键词： 机械臂   UUV回收   动力学模型   仿真分析  

摘要： 回收是舰艇装备UUV急需解决的一大难题。传统回收方式有着人力、缆绳及救生艇等诸多限制条件,机动性差、危险系数高,无法满足舰艇的紧急作战要求。基于舰艇发射管利用机械臂进行UUV的回收能解决传统回收方式的难题且更具隐蔽性,具有重大的军事意义。本文针对舰艇的回收需求,研究基于发射管的回收机械臂设计方案,利用牛顿-欧拉法建立动力学方程,然后在Matlab下利用七次多项式规划工作轨迹并进行动力学数值仿真。结果表明浮力、负载的重量和空间尺寸均对关节驱动力矩有一定影响,该结果可为后续回收机械臂的关节驱动校核、机械结构优化以及控制系统设计等工作提供参考。

关键词： Benzoxazine resin   Microcrystalline cellulose   Silanization   Kinetics   Mechanical properties   Thermal stability  

摘要： In the frame of developing sustainable, eco-friendly and high performance materials, microcrystalline cellulose modified through silane coupling agent (MCC Si) is used as a reinforcing agent of benzoxazine resin to manufacture composites at different loadings of 5, 10, 15, 20 wt%. The structural, morphological and crystallinity characterizations of the modified MCC were initially performed to scrutinize the changes and confirm the modification. Then, an investigation on the crosslinking process of the prepared composites was held through curing kinetic study employing isoconversional methods. The kinetic data revealed a decrease in the average values of activation energy and the pre-exponential factor, particularly for composite supplemented with 10% MCC Si, whereas all samples disclosed a tendency of an autocatalytic curing mechanism. Furthermore, the study of the dynamic mechanical properties and degradation features of the cured specimens, respectively, indicated a superior stiffness attributable to the good interaction between BA-a and MCC Si, and enhanced thermal stability for the composites compared to pristine resin. (c) 2021 Elsevier B.V. All rights reserved.

关键词： 农业机械   劳动力转移   劳动分工   农户健康  

摘要： 基于2016年中国劳动力动态调查数据(CLDS),运用工具变量法实证分析农业机械应用对农村劳动力非农转移的影响。结果表明:农业机械应用对非农就业具有显著正向影响,农业机械应用水平越高,农户家庭劳动力从事非农就业的可能性越大,非农就业人数比例越高,不考虑内生性问题将低估农业机械应用对农村劳动力转移的影响。进一步的机制分析显示,农业机械应用通过减少单位土地农业劳动力投入与改善农户健康状况来促进农户家庭劳动力非农转移。此外,户主年龄对农户家庭劳动力非农转移有倒“U”型影响;户主文化程度为高中、大专、本科以及户主健康状况和村庄交通状况对农村劳动力转移具有显著正向影响,而劳均耕地面积与是否获得农业补贴对农村劳动力转移具有显著负向影响。政府部门应多措并举强化政策支持力度,不断提高中国农业机械化水平,以进一步促进农村劳动力转移。