关键词： 纳米机械加工   晶体铜   表层形成机理  

摘要： 晶体铜纳米机械加工技术前的发展受到机械加工理论及实际加工工艺、质量测量等诸多方面的制约,对加工表面形成机理及损伤的控制、测量等方面均有待理论和实践多方面的提高。表层形成机理是机械加工机理的重要一环,制约着纳米机械技术的提高。为此,文章基于分子动力学可信有用的模拟原理,从晶体铜纳米机械加工原理、表层损伤的检测控制等方面对晶体铜纳米机械加工技术的表层形成机理进行了分析。

关键词： 机械臂   非线性摩擦模型   动力学建模   Newton-Euler法   正-反向动力学   机械臂运动学   运动轨迹  

摘要： 现有研究大多采用分段线性组合而成的拟非线性摩擦模型来描述机械臂各关节摩擦力矩大小及其分布,忽略了低速区关节摩擦的实际非线性变化,而该非线性变化恰恰是影响机械臂轨迹精度及稳定性的关键因素之一。基于此,着重研究机械臂关节非线性摩擦模型及其对机械臂末端轨迹的影响。对多关节机械臂进行几何机构建模,对任意设定的机械臂理想运动轨迹,通过机构运动学分析得到各关节的角位移、角速度和角加速度。考虑非线性摩擦,建立高可靠性的关节非线性摩擦的一般模型。对给定的理想运动轨迹下的角位移、角速度和角加速度进行Newton-Euler反向动力学分析,从而得到各关节的非线性驱动力矩。基于一般非线性摩擦模型,将得到的驱动力矩代入正向动力学模型中得到机械臂末端实际的运动轨迹。通过上述分析步骤,深入研究各关节在不同类型驱动力矩组合条件下对机械臂末端运动轨迹、及各关节角位移的影响。研究结果表明机械臂各关节的不同摩擦形式对末端运行轨迹影响显著,各关节驱动力矩的变化对其相邻关节的角位移也有明显影响。

关键词： 机械式枣去核设备   动力主轴   工作原理   性能指标   计算分析  

摘要： 文章对机械式枣去核设备的设计、工艺及工作方式进行了介绍,同时开展了动力主轴的计算分析,并与现有技术进行了对比。经鉴定检验,该设备能够满足预期控制要求且运行良好,对今后枣去核设备的设计思路有一定的指导作用。

关键词： 重症肺炎   呼吸衰竭   俯卧位机械通气   血气分析指标   血流动力学  

摘要： 目的:探讨俯卧位机械通气对重症肺炎并呼吸衰竭患者血气分析指标、血流动力学的影响。方法:选取2017年12月~2019年12月收治的重症肺炎并呼吸衰竭患者64例为研究对象,采用随机数字表法分为研究组和对照组,各32例。两组均给予常规治疗,对照组予以仰卧位机械通气治疗,研究组予以俯卧位机械通气治疗,对比两组患者不同体位机械通气治疗前和治疗后1 h、6 h、7 d后血气分析指标、血流动力学指标,对比两组患者治疗前及治疗7 d后急性生理与慢性健康状况评分。结果:给予不同体位机械通气治疗后,两组血氧饱和度、氧合指数均有改善,与同组治疗前比较,差异有统计学意义(P<0.05);研究组氧合指数在治疗6 h、7 d后较对照组显著提升,差异有统计学意义(P<0.05);治疗前及治疗后两组血流动力学指标比较无显著性差异(P>0.05);治疗7 d后两组患者急性生理与慢性健康状况评分较治疗前均有下降,且研究组较对照组下降更明显,差异有统计学意义(P<0.05)。结论:俯卧位机械通气对重症肺炎并呼吸衰竭患者血气分析指标均有改善,对血流动力学无不良影响,能有效改善重症肺炎并呼吸衰竭患者的氧合及病情。

关键词： Molecular dynamics simulations   Mechanical properties   Stress-strain curve   alpha-graphyne nanotubes   alpha 2-graphyne nanotubes  

摘要： In this paper, molecular dynamics simulations are applied to study the mechanical properties of two types of graphyne nanotubes;including alpha-graphyne and alpha 2-graphyne nanotubes. The effects of the geometrical properties of the nanotubes on the stress-train curve, Young's modulus, fracture strain and maximum stress are studied. It is observed that Young's modulus of the alpha-graphyne nanotubes are significantly smaller than Young's modulus of the alpha 2-graphyne nanotubes with the same geometrical parameters. Similarly, the maximum stress tolerated by the armchair alpha 2-graphyne nanotubes are larger than the armchair alpha-graphyne nanotubes. However, alpha-graphyne nanotubes tolerate larger fracture strains than the alpha 2-graphyne nanotubes. It is shown that increasing the nanotube diameter leads to increase in Young's modulus of the alpha-graphyne nanotubes and alpha 2-graphyne nanotubes. However, increasing the nanotube lengths has an inverse effect on Young's modulus of the considered nanostructures.

关键词： Carbon nanotubes  

摘要： Although carbon nanotube (CNT) is an excellent additive in geopolymer, the effect of its chemical bonding at interfaces is not well understood. This paper explores the effects of interfacial bonding on the structure, dynamics and mechanical properties of CNT-geopolymer nanocomposites via molecular dynamics simulations. Our findings from the structural analysis reveal that the interfacial bonding decreases the thickness of the "interfacial van der waals (vdW) excluded volume" and interphase (inner) region, and condenses the interfacial structure. Remarkably, it is found that the interfacial bonding provides a new "restrictive mechanism" limiting the diffusion of components in the geopolymer matrix. A dynamical heterogeneous character is found as the atom mobility of the inner region is slower than that of the outer region. Furthermore, the pull-out test reveals about 15 times increase in the interfacial shear strength of CNT-geopolymer nanocomposite with 10.0% interfacial bonding concentration, which is beneficial for the enhancement of its mechanical properties and hinders crack formation under tensile loading. The tensile test reveals about 60% and 80% increase in the nanocomposite's Young's modulus and tensile strength, respectively. This study provides, for the first time, valuable insight into the role of the interfacial bonding on the performance of CNT-geopolymer nanocomposites. (C) 2020 Elsevier Ltd. All rights reserved.

关键词： Cylinder head   in-cylinder combustion   computational fluid dynamics analysis   transient thermal   thermo-mechanical fatigue  

摘要： A finite element analysis-computational fluid dynamics coupled analysis on the thermo-mechanical fatigue of cylinder head of a turbo-charged diesel engine was performed, and the complete simulation process is illustrated in this paper. In-cylinder combustion analysis and water jacket coolant flow analysis were conducted to provide heat transfer boundary conditions to the temperature field calculation of the cylinder head. Comparing with the conventional finite element analysis of cylinder head by which the heat transfer boundary conditions of the combustion and coolant sides are estimated, the present method coupled the three-dimensional combustion computational fluid dynamics and coolant computational fluid dynamics with the finite element analysis. Both computational fluid dynamics and finite element analysis obtain more accurate boundary conditions on their interface from each other, and thus, the present method improves accuracy of thermo-mechanical fatigue prediction. Based on the measured material performance parameters such as stress-strain curve under different temperatures and E-N curve, creep, and oxidation data material performance, the cylinder head-gasket-cylinder block finite element transient stress-strain field was calculated using ABAQUS. The thermo-mechanical fatigue analysis of cylinder head submodel was performed by using FEMFAT software that is based on the Sehitoglu model to predict the thermo-mechanical fatigue life of cylinder head. By comparing the measured and predicted temperatures of cylinder head, the temperature results showed a good agreement, and the error is less than 10%.

关键词： Microwave hybrid sintering   Alumina-zirconia composite   Fracture toughness   Stress-induced transformation toughening   Activation energy  

摘要： Structural ceramics such as Al2O3 and Al2O3-ZrO2 composites are widely used in harsh environment applications. The conventional sintering process for fabrication of these ceramics is time-consuming method that requires large amount of energy. Microwave sintering is a novel way to resolve this problem. However, to date, very limited research has been carried out to study the effects of different ZrO2 crystal structures on Al2O3-ZrO2 composites, especially on the sintering kinetics, when fabricated by microwave sintering. The microwave hybrid sintering of Al2O3 and Al2O3-ZrO2 composites was performed in this study. Tetragonal zirconia and cubic zirconia were used as two different reinforcements for an alpha-alumina matrix, and the mechanical and thermal properties were studied. It was found that Al2O3 experienced a remarkable increase in fracture toughness of up to 42% when t-ZrO2 was added. Al2O3-c-ZrO2 also showed increased fracture toughness. The sintering kinetics were also thoroughly investigated, and the average activation energy values for the intermediate stage of sintering were estimated to be 246 +/- 11 kJ/mol for pure Al2O3, 319 +/- 71 kJ/mol for Al2O3-c-ZrO2, and 342 +/- 77 kJ/mol for Al2O3-t-ZrO2. These values indicated that the activation energy was increased by the addition of either type of ZrO2, with the highest value shown by Al2O3-t-ZrO2.

关键词： Multifunctional composites   Environmental degradation   Mechanical testing   Extrusion  

摘要： Bilayer biodegradable, eco-friendly films were prepared by co-extrusion film blowing, coupling polylactic acid (PLA) and Bio-flex (R) (BIO). Furthermore, in the PLA layer, carvacrol (CRV) was added as a natural antimicrobial additive, whereas a nanoclay (D72T) was integrated to protect CRV from volatilization and to modulate release. The materials were analyzed by morphological, chemical-physical, mechanical testing. Furthermore, CRV release and degradation tests were performed. The results pointed out that coupling the two matrices allows gathering the stiffness of PLA with the ductility of BIO. Furthermore, the interlayer adhesion is promoted by CRV. D72T exerts a key-role in avoiding CRV volatilization, thus allowing more prolonged release. Degradation tests results highlight that bilayer films, while being particularly resistant in both acidic and neutral environments, showed a rapid degradation under alkaline conditions, which proved to be intermediate between those of the corresponding monolayers.