极低频信号的一种离散频谱校正新方法

对于振动工程中常见的极低频、极短时、极高频(接近奈奎斯特频率)等极端频率信号,常用的离散频谱分析与校正方法存在较大误差.对极端频率信号的典型情形进行了分析,针对极端频率信号中的极低频信号,提出了一种计及负频率成分干涉影响的离散频谱校正新方法.该方法基于Blackman窗,利用局部谱峰附近的三条谱线,建立包含正负频率贡献的离散频谱校正模型,通过对模型的求解获得频率、幅值和相位校正公式.采用频段内扫描的方式对频谱校正公式进行了仿真验证,结果表明所提方法有效降低了负频率成分的干涉影响,对极低频信号的频率、幅值和相位校正有较高的精度.     

铸态BeAl合金的低频阻尼特性研究

利用动态热机械分析仪研究了铸态BeAl合金的低频阻尼性能.研究结果表明,在小位移振幅下(A＜10μm)铸态BeAl合金的内耗值在30～200 ℃范围内基本不变,且与频率无关,温度高于200 ℃时,随温度的升高而急剧增大,并随频率增大而降低,分析认为,界面滑移、热错配应变和位错脱钉是铍铝合金阻尼值急剧变化的原因.得到了BeAl合金阻尼的线性拟合公式,利用该公式可以估算此合金的阻尼大小及行为,并揭示BeAl合金的阻尼机理.     

离散频谱四点能量重心校正法及抗噪性能分析

在对比离散频谱能量重心法采用不同点数时的频率、幅值和相位校正精度的基础上提出了4点能量重心校正法,推导了高斯白噪声背景下单频率谐波信号采用4点能量重心法进行频率、幅值和相位校正的统计方差公式,通过仿真计算验证了其正确性.分析对比了采用不同点数进行能量重心校正时的优缺点,建议在工程中采用Han-ning窗4点能量重心法进行谐波信号离散频谱校正.     

数字信号源频率校准及测量不确定度评定研究

以广泛应用的数字信号源为研究对象,将传统测量系统分析(MSA)方法与田口方法相结合进行了数字信号源频率测量系统分析和校准公式的推导,并进行具有交互效应场合下的数字信号源频率多变量误差分析和测量不确定度的评定.研究表明,可以通过测量数字信号源的典型值得到校准公式,由此推导出全量程任意点估计值,方便仪器的校准;可以使用正交表和SN比方法相结合分析测量仪器误差及进行测量不确定度评定.     

带边墙的三维光栅石墨烯表面激元辐射的色散特性研究北大核心CSCD

主要推导了平行运动电子注激励下,三维介质光栅的石墨烯表面等离子体激元的色散方程和电场的幅值公式,数值计算结果表明,电子注与色散曲线交点对应的频率与电场幅值最大值的频率一致,证明了理论推导公式的正确性。分析了介质光栅宽度对色散方程与电场幅值的影响,发现宽度变小时,电场幅值变小,工作点的频率变高,并且色散曲线的初始频率变大,即出现了截止频率,因此三维介质光栅的石墨烯表面等离子体激元无法工作在较低频段。本文还提出石墨烯的化学势变高,层数增加时,色散曲线上移,对应工作频点升高。通过求解电场方程,可以清楚看到石墨烯的表面等离子体激元以及转化的太赫兹辐射。     

带边墙的三维光栅石墨烯表面激元辐射的色散特性研究

主要推导了平行运动电子注激励下,三维介质光栅的石墨烯表面等离子体激元的色散方程和电场的幅值公式,数值计算结果表明,电子注与色散曲线交点对应的频率与电场幅值最大值的频率一致,证明了理论推导公式的正确性。分析了介质光栅宽度对色散方程与电场幅值的影响,发现宽度变小时,电场幅值变小,工作点的频率变高,并且色散曲线的初始频率变大,即出现了截止频率,因此三维介质光栅的石墨烯表面等离子体激元无法工作在较低频段。本文还提出石墨烯的化学势变高,层数增加时,色散曲线上移,对应工作频点升高。通过求解电场方程,可以清楚看到石墨烯的表面等离子体激元以及转化的太赫兹辐射。     

预应力CFRP布加固RC梁动力特性的试验研究

进行了预应力CFRP布加固完好RC梁的动力特性试验,结果表明:预应力加固梁的固有频率随着预张力的增加而增加。同时,考虑了相关的影响因素,从理论上推导了预张力与加固梁固有频率之间的定量关系,再利用一阶频率的试验值对公式进行拟合,得到频率影响系数。而后将二阶频率的试验值与理论值进行比较,发现理论值基本能反映频率随预张力的变化趋势,两者最大误差为0.173(在预张力为0kN时),试验值与理论值吻合较好。     

预应力CFRP布加固损伤RC梁的动力特性研究

从理论上推导了预张力与损伤RC梁固有频率之间的定量关系。同时进行了预应力CFRP布加固RC梁的动力特性试验,测定不同预张力条件下,完好梁与损伤梁的一阶频率值。而后利用ANSYS软件建立钢筋混凝土完好梁的有限元模型,根据损伤梁的动力测试结果,运用优化分析的方法得到损伤梁混凝土刚度折减系数;利用一阶频率的试验值对公式进行线性拟合,得到频率影响因素及损伤梁频率计算公式。最后将加固损伤梁一阶频率的理论值与试验值进行比较,发现在低预应力作用下,理论计算结果基本能反映出试验值随预张力变化的趋势,试验值与理论值吻合较好。     

配置直线不等长预应力筋简支梁自振频率研究

基于结构振动的基本理论,利用梁的自由振动方程,推导出了直线不等长预应力筋简支梁自振频率的理论计算公式。以某桥为背景,对该桥2片预应力混凝土空心板梁进行了模态试验,并用有限元软件ANSYS对该试验梁进行模态分析。结果表明:实测值、有限元分析值和理论公式计算值三者吻合良好,证明了所推导的公式对计算直线不等长预应力筋简支梁前两阶振动频率是合理准确的。最后结合理论计算公式和有限元模型分析了预加应力和预应力筋偏心距对不等长预应力筋简支梁自振频率的影响。     

两个密集频率成分重叠频谱的校正方法

在离散频谱分析中当频率成分比较密集时，各频率成分的谱线相互重叠，产生干涉，使谱线的幅值和相位产生误差，此时无法直接使用比值校正法对频谱的幅值、频率和相位进行校正。该文针对具有两个相互重叠频率成分的密集频谱，分析了两个重叠频率成分在复频域的叠加情况，在不增加采样长度的条件下，通过向量分解和单频率成分的识别、校正方法，推导出其校正公式。仿真研究表明，这种校正方法简单易行，可以对谱峰间距大于０．０１个频率分辨率的两个有重叠干涉的频率成分进行精确的幅值、频率和相位的校正，幅值误差小于０．０２‰，频率误差小于０．００００２个频率分辨率，相位误差小于０．００３度。     

Insertion of triangulated surfaces into a meccano tetrahedral discretization by means of mesh refinement and optimization procedures

In this paper, we present a new method for inserting several triangulated surfaces into an existing tetrahedral mesh generated by the meccano method. The result is a conformal mesh where each inserted surface is approximated by a set of faces of the final tetrahedral mesh. First, the tetrahedral mesh is refined around the inserted surfaces to capture their geometric features. Second, each immersed surface is approximated by a set of faces from the tetrahedral mesh. Third, following a novel approach, the nodes of the approximated surfaces are mapped to the corresponding immersed surface. Fourth, we untangle and smooth the mesh by optimizing a regularized shape distortion measure for tetrahedral elements in which we move all the nodes of the mesh, restricting the movement of the edge and surface nodes along the corresponding entity they belong to. The refining process allows approximating the immersed surface for any initial meccano tetrahedral mesh. Moreover, the proposed projection method avoids computational expensive geometric projections. Finally, the applied simultaneous untangling and smoothing process delivers a high‐quality mesh and ensures that the immersed surfaces are interpolated. Several examples are presented to assess the properties of the proposed method.     

From Industry 4.0 to Research 4.0. Development of High Performance Fibers in the Light of Digitization

We associate a variety of innovations with the term ＂Industry 4.0＂. The pioneer of many 4.0 modifications forms the basisfor the trend towards the integrated di...     

The Research of Anti-Wear Properties of Organomolybdenum Compounds in the Engine Oils

A four-ball tester was used to evaluate the anti-wear performance of three kinds of organomolybdemun compounds in the engine oils, i. e., molybdenum dialkyldithiophosphate (MoDDP), molybdenum dialkyldithiocarbamate ( MoDTC), and sulphur and phosphorus freeorganomolybdeum (Molybdate). The results indicate that a low concentration of MoDDP doesn' t improve the anti-wear properties of the commercial engine oils, but a high concentration of MoDDP can obviously improve the anti-wear properties and the load-carrying capacity of the engine oils. MoDTC doesn' t improve the antiwear properties of the engine oils, but worsens the anti-wear properties of the oils. Signifi can timprove ment of frictional and wear characteristics is obtained with Molybdate added in the commercial engine oils and the formulated oils.     

Roman iron and steel: A review

ABSTRACT

The production of ferrous metal increased during the Roman Late Republican period, Principate and Empire. The direct bloomery process was used to extract the metal from its ores using slag-tapping and slag-pit furnaces. The fuel was charcoal and an air blast was introduced by bellows-operated tuyères. Iron formed as a bloom, often as a spongy mass of metal, which contained impurities from the smelting process, including unreacted ore, fuel, slag and fragments from the furnace walls, while the metal was often inhomogeneous with varied carbon contents. Blooms were either smithed directly into bars or ingots or they were broken up, which also allowed the removal of gross impurities and a selection of pieces with similar properties to be made. These could then be forge-welded together and formed into characteristically shaped ingots. Making steel in the furnace seems to have been achieved: it depended on the ore and the furnace and conditions within it. Surface carburization was also carried out. Iron and steel were used extensively in construction and for tools and weapons. Fire welding was often used to add pieces of steel to make the edges of tools and weapons, which could be heat-treated by quenching to harden them.     

Certification Institutions Expected to Update and Transfer Information on Present Standards

Standards are the basis for production enterprises to organize production, ex-factory inspection, trade (delivery) and technical exchanges, product certification, quality arbitration and supervision.……     

Flow calorimeter for enthalpy increment measurements on condensed gases

A flow calorimeter for enthalpy increment measurements on condensed gases is presented. A better knowledge of the properties of the liquefied natural gas is needed, and therefore a liquid loop has been designed for our flow calorimeter. The fluid loop in the calorimeter is designed in order to avoid the two-phase region, since two phases would give compositional disturbances in the measurements. The avoidance of the two-phase region is made possible by increasing the pressure of the test fluid after the measurement section, then heating the fluid at super-critical pressure past the critical point. Finally, the fluid is throttled to the low-pressure gas state at the inlet condition of the compressor that circulates the fluid. To perform the pressure increase, a new cryogenic pump has been designed. To evaluate the new equipment, measurements were taken on liquid ethane over the temperature range 146–256 K at pressure between 0.9 and 5.1 MPa.     

Synthesis of Alumina-Zirconia Nanocomposites by Solgel Process

Al₂O₃-ZrO₂ nanocomposites were developed starting with the solgel process. Composite alumina-zirconia nanopowders were synthesized from metallorganic precursors (Aluminium secondary butoxide and zirconium Iso propoxide) using the solgel process. The parameters affecting the synthesis—solvent, concentration of precursor, R/H ratio (i.e., dilution of water in solvent)—were varied as also the temperature and pH. BET and TEM were used to measure nanosize. Diffuse reflectance spectroscopy and also qualitative optical absorption led to identical particle size estimate. The variation of process parameters was used to study the effect and interdependence of process parameters. Artificial Neural Networks was used to rigorously analyze the process. Although this led to confirmation of interdependence of parameters, the presence of a single overwhelming solvent variable was also established. Then the optimal process was used to synthesize more nanopowder. To produce bulk nanocomposite the nanopowders were sintered by varying the temperature and time period. The sintered lithoids were probed with a vickers hardness tester to measure elastic modulus, hardness, and fracture toughness. The results showed high elastic modulus, modest hardness, and very high fracture toughness.     

Certification Becomes Important for the I Quality; and Safety Supervision of the Chinese Liquor Sector

The end of 2007, over 200 unit products of more than 80 Chinese firms have passed the quality grade certification for liquor products. These products involve distilled spirits, beer, wine, yellow wine, fruit syrup wine and others, and cover over 80% of the national top-branded liquor products.……     

Deposition and Coating Properties on CVD Tungsten

Surface characterization and microstructure studies are performed on chemical vapor deposited (CVD) tungsten coating. There is about 2 μm thickness diffusion layer of tungsten in the molybdenum substrate. The thermal shock test shows tungsten coating has good adhesion with molybdenum substrate, but the elements of oxygen and carbon in the tungsten coating have the bad affection to the adhesion. The result of high-temperature diffusion experiment is the diffusion rate from molybdenum substrate to tungsten coating is faster.