关键词： Crystallization kinetics   Crystalline morphology   Mechanical property  

摘要： Distribution of nanoparticles in nanocomposites is one of the most important roles to design unique morphologies and properties;however, up to now, it is still very difficult to control distribution of nanoparticles in polymer matrix. Here we show a simple approach to fabricate ferroferric oxide (Fe3O4) nanoparticles with controllable chain-like distribution in poly(e-caprolactone) (PCL) by a magnetic self organization. Randomly distributed Fe3O4 nanoparticles can be polarized and the interparticle dipole dipole attraction drives their assembly into linear chains in an external magnetic field. The accelerated PCL crystallization happens at low Fe3O4 loadings, while the retardation to crystallization happens at high Fe3O4 loadings, especially for the samples with chain-like structures. Furthermore, PCL spherulites are also arranged into chain-like structures by the Fe3O4 nanoparticle chains. As a result, the PCL/Fe3O4 nanocomposites with chain-like structures exhibit higher storage modulus, Young's modulus and tensile strength than that of the corresponding nanocomposites with random distribution of Fe3O4 nano particles, indicating mechanical enhancements. (C) 2017 Elsevier Ltd. All rights reserved.

关键词： ferroelectrics   metamaterials   nonlinear waves   phase field   phase transitions  

摘要： A mechanical metamaterial, a simple, periodic mechanical structure, is reported, which reproduces the nonlinear dynamic behavior of materials undergoing phase transitions and domain switching at the structural level. Tunable multistability is exploited to produce switching and transition phenomena whose kinetics are governed by the same Allen-Cahn law commonly used to describe material-level, structural-transition processes. The reported purely elastic mechanical system displays several key features commonly found in atomic- or mesoscale physics of solids. The rotating-mass network shows qualitatively analogous features as, e.g., ferroic ceramics or phase-transforming solids, and the discrete governing equation is shown to approach the phase field equation commonly used to simulate the above processes. This offers untapped opportunities for reproducing material-level, dissipative and diffusive kinetic phenomena at the structural level, which, in turn, invites experimental realization and paves the road for new active, intelligent, or phase-transforming mechanical metamaterials bringing small-scale processes to the macroscopically observable scale.

关键词： boronic acids   dynamic covalent bonds   Maxwell elements   self-healing   stress relaxation  

摘要： Hydrogels play a central role in a number of medical applications and new research aims to engineer their mechanical properties to improve their capacity to mimic the functional dynamics of native tissues. This study shows hierarchical mechanical tuning of hydrogel networks by utilizing mixtures of kinetically distinct reversible covalent crosslinks. A methodology is described to precisely tune stress relaxation in PEG networks formed from mixtures of two different phenylboronic acid derivatives with unique diol complexation rates, 4-carboxyphenylboronic acid, and o-aminomethylphenylboronic acid. Gel relaxation time and the mechanical response to dynamic shear are exquisitely controlled by the relative concentrations of the phenylboronic acid derivatives. The differences observed in the crossover frequencies corresponding to pK(a) differences in the phenylboronic acid derivatives directly connect the molecular kinetics of the reversible crosslinks to the macroscopic dynamic mechanical behavior. Mechanical tuning by mixing reversible covalent crosslinking kinetics is found to be independent of other attributes of network architecture, such as molecular weight between crosslinks.

关键词： colloids   small-angle X-ray scattering   drying   solidification   fracture   shear bands  

摘要： Charged colloidal dispersions make up the basis of a broad range of industrial and commercial products, from paints to coatings and additives in cosmetics. During drying, an initially liquid dispersion of such particles is slowly concentrated into a solid, displaying a range of mechanical instabilities in response to highly variable internal pressures. Here we summarize the current appreciation of this process by pairing an advection-diffusion model of particle motion with a Poisson-Boltzmann cell model of inter-particle interactions, to predict the concentration gradients in a drying colloidal film. We then test these predictions with osmotic compression experiments on colloidal silica, and small-angle X-ray scattering experiments on silica dispersions drying in Hele-Shaw cells. Finally, we use the details of the microscopic physics at play in these dispersions to explore how two macroscopic mechanical instabilities-shear-banding and fracture-can be controlled. This article is part of the themed issue 'Patterning through instabilities in complex media: theory and applications'.

关键词： Phenolic resins   Thermosetting polymer   Molecular dynamics simulation   Mechanical property  

摘要： An atomistic molecular dynamics simulation was performed for crosslinked phenolic resins which were constructed from phenols and crosslinkers using a pseudo-reaction algorithm, in order to understand the structure-mechanical property relationships therein from an atomistic perspective. The tensile modulus was characterized from the linear elastic region of the stress-strain curves under uniaxial tensile deformation. Analysis of the relationships between the moduli and interatomic interactions indicated that bond interaction, especially bond orientation for the elongation axis, dominantly affects the tensile modulus in a range of strain 0 <= epsilon <= 0.05, while stress-concentration in specific domains and long-range interactions including hydrogen bonding do not. (C) 2017 Elsevier Ltd. All rights reserved.

关键词： Mechanical response   Steered molecular dynamics   Type IV pilus   Coarse-grained model  

摘要： Type IV pili are long filamentous structures on the surface of bacteria, which can be rapidly assembled or disassembled with pilin subunits by molecular motors. They can generate force during retraction and are involved in many bacterial functions. Steered molecular dynamics simulations with coarse-grained MARTINI models are carried out to investigate the mechanical behaviors of pili under tension. Our study is the first to report a Young's modulus of 0.80 +/- 0.07 GPa and a spring constant of 1294.6 +/- 116.5 kJ mol(-1) nm(-2) for pilus. Our results show the mechanical responses of pili are different from those described by the worm-like chain model and the van der Waal's interactions play a critical role in the mechanical responses. Moreover, the effects of pulling rates and virtual spring constants of pilus on Young's modulus are studied and two distinct morphological stages with the conformational changes appear during the extension of pilus are observed. This work provide insight into the mechanics and the deformation mechanism of pilus assembly. (C) 2017 Elsevier Ltd. All rights reserved.

关键词： 机械手   动力学   力矩   仿真分析  

摘要： 运用软件Pro/e建立了某工业机械手的三维模型,设定机械手构件参数如杆长和转角等,再用Joseph-Louis Lagrange法建立机械手的动力学方程。按照仿真流程在软件Adams中对机械手进行动力学仿真,得到机械手各个关节的角度曲线、角速度曲线和角加速度曲线,以及大臂旋转关节各个方向的力矩曲线。最后再利用软件Matlab对机械手动力学方程进行求解并生成各个方向的力矩曲线,经与Adams仿真曲线和力矩值进行对比,发现两者力矩曲线走势相近,且力矩值偏差甚小,最大时仅为7.5 N·m,在合理范围内,从而验证了Adams动力学仿真的准确性。

关键词： 七连杆双自由度并联机构   虚拟样机   运动仿真   结构参数优化  

摘要： 由于汽车驾驶机器人换挡机械手结构的复杂性,使得机械手运动学与动力学分析的具体数学描述较为困难,而计算机虚拟仿真技术,为换挡机械手的特性分析提供了依据。为解决原换挡机械手驱动机构的横摆问题,通过增设选挡推杆及挂挡推杆,改进原七连杆二自由度并联机构。在ADAMS/View中建立了改进后换挡机械手机构的虚拟样机模型并进行了运动仿真,仿真结果验证了改进后的换挡机械手解决原换挡机械手驱动机构横摆问题的有效性。在上述基础上,以选挡推杆及挂挡推杆的长度为设计变量,以换挡机械手的最大驱动力最小为优化目标,利用ADAMS/View中Design Evaluation模块对挡机械手进行了结构参数优化,取得了较好的优化效果,为实际工程运用提供了依据。

关键词： 中外合作办学   教学管理   高校  

摘要： 截至目前，全国经审批机关批准设立或举办的中外合作办学机构有2545家，已经成为我国教育对外开放事业的重要组成部分，但也面临着一些新的挑战。中外合作办学项目的基层教学管理，是高校教学管理的新课题。文章以河南理工大学机械与动力工程学院为例，首先分析了中外合作办学模式下基层教学管理存在的问题，随后提出了解决此类问题的思路和办法。

关键词： MMET   Capture mathematical model   Ideal velocity for capture   Capture velocity error   Dynamics  

摘要： The dynamics of motorized momentum exchange tether (MMET) during and after payloads capture is studied. The ideal velocities of payloads with same mass for capture are analyzed, and a mathematical model of capture is proposed. If the MMET captures the payloads at the ideal velocity, the MMET's orbit and motion state are unchanged. However, considering that capture velocity error exists in practice, and analyzing the velocity error in relation to the payload approaching from the earth as an example, the impact on the MMET's orbital parameters and state motion are investigated during payload capture for different capture velocity errors, including errors in magnitude and direction. The simulation results show that if the payload velocity has only magnitude error, the perigees remain overlapping and the semi-major axes of the changed orbits also continue to coincide after capture;however, the changes in other orbital parameters are approximately proportional to the error. If the payload velocity has both magnitude and direction errors, then all of the orbital parameters change, including the perigee and the direction of the semi-major axis. However, the normal velocity increment caused by the capture's normal velocity error changes the orbital parameters far less than the tangential velocity increment. (C) 2017 COSPAR. Published by Elsevier Ltd. All rights reserved.