关键词： dynamics   vivo   regulating  

摘要： The spatiotemporal regulation of threedimensional(3D)genome dynamics has been implicated in various genome functions including gene transcription,DNA recombination,DNA replication,and DNA repair(Bickmore,2013).

关键词： 机械运输设备   三元边际   出口增长  

摘要： 本文利用2000-2017年UN Comtrade数据库中SITC五分位贸易数据,借助三元边际分解方法客观分析了中国向26国出口机械运输设备的扩展边际、数量边际和价格边际。在此基础上运用核密度估计刻画了三元边际动态演进,采用非参数检验对拉动出口增长的边际因素进行统计检验。研究结果表明:中国机械运输设备出口的增长动力主要是靠数量扩张,而扩展边际和价格因素对出口增长的拉动作用较小,贡献微弱,"低价扩张、数量拉动"传统出口模式亟待根本转变。

关键词： 机械功   热能与动力工程   应用和优化   分析和研究  

摘要： 机械功是物理中的一种定义,其可以简称为功,这种概念在热能与动力工程中是很常见的.应用范围也是很广泛的.机械功的概念是物体在力的方向上移动了一段距离,这就可以说明力对这个物体做了功.由于其概念,人们在发展过程中就会去思考如何省力或者让物体移动的距离短,这样也就能够更省力,是提高效率的一种方式.因此,本文就对机械功在热能与动力工程中的应用与优化进行分析和研究.

关键词： 运动力学   机械机构   设计   应用  

摘要： 机械制造业是推动我国工业及制造业发展的基石。为顺应工业产业及制造业的快速发展速度,机械制造技术需要重视技术升级及机构设计,巧妙使用运动力学原理,设计高质量、低能耗的机械产品。文章就运动力学在机械机构设计中的应用展开了研究。

关键词： 船用起重机   机械防摆   动力学分析   阻尼   摆动抑制   实验   张力   吊重  

摘要： 针对现有机械防摆方法对起重机本身作业造成干扰的问题,本文提出一种三绳牵引机械防摆装置。为了解决牵引索的张力设定问题,建立了所提机械防摆装置的几何关系模型和动力学模型;进行了静平衡分析,提出牵引索的张力约束条件;通过动力学分析并根据分析结果提出一种基于阻尼原理的张力设定方法。依据机械防摆装置原理,搭建了船用起重机防摆试验平台,吊重防摆实验结果表明:恒张力控制相对于无防摆时的面外角和面内角均分别减小达92%和69%以上,证明本文方法具有良好的吊重摆动抑制效果。

关键词： Penta-graphene   Molecular dynamics   Intrinsic mechanical properties   Fracture mechanism   Nanoindentation  

摘要： A new carbon allotrope called penta-graphene (PG) consisting of sp(2) and sp(3) hybridization has been confirmed to be dynamically, thermally and mechanically stable via ab initio calculations. In this work, we resort to the MD simulation to investigate the intrinsic mechanical properties and fracture mechanism of PG as well as graphene (GP) with only sp(2) during indentation. It is turned out that the PG film exhibits higher ductility than the GP as it has higher resistance force and larger critical fracture depth. The comparison of atomic configurations for the GP and the PG also proves that the PG film possesses a better capability to deform and withstand the damage since it has smaller cracks under the same loading condition. Furthermore, we probe into the fracture mechanism at the initial fracture point and reveal the fact that the crack is nucleated with the breakage of sp(3) bonds, and the broken bonds can recombine to form new sp(3) bonds which contributes to its high resistance to external force even after the crack. In addition, the typical force-displacement relationship is also obtained, from which its elastic modulus and breaking strength are determined as 579.46 GPa and 60.90 GPa, respectively.

关键词： Plasticizer   Polyamide   Migration   Diffusion   Evaporation   Ageing   Prediction   Safety factor  

摘要： Plasticizer migration is a major concern for plasticized polymers because it leads to unwanted changes in mechanical properties and, in many cases, contamination of the environment. In cases of slow migration, it is of practical importance to be able to perform accelerated testing and estimate migration rates from high temperature experiments. Despite the importance, a critical evaluation of different ways of extrapolating mass loss data has hitherto not been reported. Therefore, in this article, three different methods (involving for the first time a master-curve approach to mass loss data) to estimate low temperature migration from high temperature data are presented and critically evaluated. The system chosen was a plasticized polyamide 12 pipe, an important component in vehicle fuel-line systems. This system was challenging since the lower part of the temperature range in which the material is used overlaps with the glass transition region. All three methods (using data at 80-145 degrees C) over-estimated, although to different extents, the low-temperature mass loss rate (60 degrees C). The main reason for the over-estimation was the partial overlap with the glass transition region. Hence, there is a built-in safety factor when predicting plasticizer loss over glass transition regions, and the predictions are conservative. It was observed that plasticizer loss and annealing effects were the main reasons for changes in mechanical properties (increase in flexural stiffness/strength) during ageing. (C) 2019 The Authors. Published by Elsevier Ltd.

关键词： glasses   magnesium silicate glass   mechanical properties   molecular dynamics simulations   MgSiO3   network structure   tensile deformation   voids  

摘要： Molecular dynamics simulations of MgSiO3 glass have been carried out to study the structural and mechanical properties under uniaxial tension. The network structure consists of SiOx and MgOy units which link to others by corner-, edge-, and face-sharing bonds. The Si-rich and Mg-rich regions exist in MgSiO3 glass. The stress-strain curves exhibit the elastic and plastic characteristics at the various strain rates. The transformations of SiOx and MgOy units occur with increasing strain, at which the corner-, edge- and face-sharing bonds among SiOx and MgOy units change. The Si-rich and Mg-rich regions increase with increasing strain. Not only the Mg-O bond length is easier to stretch than the Si-O bond length but also the O-Mg-O bond angle of MgOy units is easier to vary than that of SiOx units with the strain. The porosity is analyzed via the voids. The voids gather into the clusters of voids. The radii of voids increase with increasing strain. The number of big Mg-voids increases under the tension and these voids coalesce to form large pores. These large pores do not coalesce to cause the fractured free surfaces. The formation of shear band was observed at the strain of 0.82.

关键词： Carbon nanorings   Elastic deformation   Mechanical properties   Small-scale effects  

摘要： Understanding of the elastic deformation behaviours of recently synthesised carbon nanorings (CNRs) is crucial in guiding their future applications, because the strain engineering provides an efficient means to modify their physical and chemical properties. In this paper, by using molecular dynamics simulations and nonlocal continuum mechanics models, we study the elastic deformations of CNRs with three different molecular structures, i.e., cycloparaphenylenes (CPPs), [4]cyclochrysenylenes and cyclacenes. Our results show that, compared to other two types of CNRs, CPPs have the smallest mechanical stiffness, which is attributed to the influence of numerous weak connecting carbon-carbon bonds existing between their component benzene rings. In addition to the molecular structure, the elastic deformation behaviours of CNRs are also found to strongly depend on the size. Specifically, the compressive stiffness of CNRs is found to increase as their size (radius) decreases. Meanwhile, the size reduction of CNRs can trigger the anisotropy of their compressive stiffness and can also aggravate the influence of small-scale effects on their elastic deformation behaviours, which can significantly reduce the compressive stiffness.

关键词： Differential geometry   Symplectic integrator   Symmetrical rigid body   Mechanical system   Dynamics  

摘要： The numerical integration plays a fundamental role in understanding the behaviour of many mechanical systems. In this paper a classical, still undisputed problem is presented. Some important aspects of the mechanical integrators on the dynamics of a mechanical system are studied. Therefore it is shown that if the Lie-Trotter integrator is obtained, in case of Euler equations for the dynamics of symmetric free rigid body, then it is a Poisson integrator. At the end of the paper some important remarks are presented. (C) 2019 Elsevier B.V. All rights reserved.