Suppression of vibration localization in non-axisymmetric periodic structures

In this paper, mistuned non-axisymmetric periodic structures are considered. In such a structure, vibration localization, which results in large vibrations in some components of the structure, can occur. Such a behavior is due to mistunings in the structure components, small damping, and weak coupling between components. The efficacy of a passive technique (previously proposed by the author for axisymmetric structures) in suppressing vibration localization in mistuned non-axisymmetric periodic structures is examined. The technique is based on adding small components between components of structures. It is first shown numerically that the added components suppress vibration localization in mistuned structures. Then, this conclusion is rigorously proved by using the singular perturbation method. Application of the technique studied in the paper to comb drives of micro electro-mechanical systems (MEMS) is given.     

Numerical modeling on concrete structures and steel–concrete composite frame structures

A steel–concrete composite fiber beam-column model is developed in this study. The composite fiber beam-column model consists of a preprocessor program that is used to divide a composite section into fibers and a group of uniaxial hysteretic material constitutive models coded in the user defined subprogram UMAT in ABAQUS. The steel–concrete composite fiber beam-column model is suitable for global elasto-plastic analysis on composite frames with rigid connections subjected to the combined action of gravity and cyclic lateral loads. The model is verified by a large number of experiments and the results show that the developed composite fiber model possesses better accuracy and broader applicability compared with a traditional finite element model. Although the fiber beam-column model neglects the slip between the steel beam and concrete slab, there are essentially no effects on the global calculation results of steel–concrete composite frames. The proposed model has a simple modeling procedure, high calculation efficiency and great advantage when it is used to analyze composite frames subjected to cyclic loading due to earthquake.     

Numerical and experimental investigations on the residual stresses of the butt-welded joints

Welding is a reliable and efficient joining process in which the coalescence of metals is achieved by fusion. Localized heating during welding, followed by rapid cooling, can generate residual stresses in the weld and in the base metal. Estimating the magnitude and distribution of welding residual stresses is important. This study applies thermal elasto-plastic analysis, using finite element techniques, to analyze the thermomechanical behavior and evaluate the residual stresses in butt-welded joints. The residual stresses at the surface of the weldments were measured by X-ray diffraction. The results of finite element analysis were compared with experimental residual stress data to confirm the accuracy of the method. The aim is to present data that may confirm the validity of currently employed fabrication processes in welded structures and even improve them.     

Study on vibration characteristics in periodic plate structures using the spectral element method

Periodic structures have great scientific potential because of their superior structural dynamic properties. The aim of this study is to analyze the dynamic behavior of periodic plate structures using the spectral element method (SEM). The spectral equations of the plate elements with two parallel sides that are simply supported are established. Then, the spectral dynamic stiffness matrix of the whole periodic plate structure is assembled. The frequency responses are obtained by the calculation of the spectral equations to illustrate the characteristics of the band gaps. From the results, it is seen that the SEM can be effectively applied to study the vibration properties of the periodic structures. Compared with the results calculated by the finite element method, it can be observed that more accurate results in high-frequency ranges can be achieved by the SEM. These results indicate that the band gap characteristics depend on both the material properties and unit cell numbers. Furthermore, the effects of the structure damping and type on the frequency responses are investigated.     

Optical properties of one-dimensional metal-dielectric photonic band-gap structures with low index dielectrics

We report on the design, fabrication and characterization of one dimensional metal-dielectric photonic band-gap structures (MDPBG) using low index dielectric materials. We describe design techniques that compensate for the shortcomings of the low dielectric material. The structures show high, flat, broad passband transmission within the visible spectrum and excellent out of band rejection in the infrared. We demonstrate one such MDPBG with five periods of Al₂O₃/Ag/Al₂O₃ that shows 53 ± 3% transmission over 150 nm within the visible spectrum and an angular variation smaller than 5% within a field-of view of at least 60°. These MDPBGs are thermally insensitive and show good environmental stability.     

周期局域共振蜂窝板的弯曲振动特性研究

基于声子晶体局域共振原理,设计了周期附加局域振子的蜂窝夹层板结构,利用双协调自由界面模态综合法,计算了周期局域共振蜂窝夹层板弯曲振动的频响特性,并给出了蜂窝夹层板的振型图。分析了局域振子刚度、质量等参数对振动带隙的影响特性,得出了带隙变化的定性规律。研究表明在振动带隙频率范围内,弯曲振动在此蜂窝夹层板能得到有效的抑制,该研究有望为复杂板壳振动控制提供一条新的技术途径。     

Numerical methods for analyzing the effects of uncertainties on the dynamics of periodic structures

Various approaches are proposed and compared for analysing the effects of structural irregularities and parameter uncertainties on the dynamics of finitely long, one-dimensional, mono-coupled, nearly periodic assemblies with and without structural damping. Computational concerns are raised in regard to the classical method of modal analysis, and alternative methods are formulated which allow for the numerically efficient determination of the exponential decay constant. While modal analysis and Cramer's rule require the solution of an eigenvalue problem, the matrix inversion and matrix partitioning approaches avoid costly eigenvalue computations and require only cost-effective algebraic manipulations. Therefore, implementation of either the direct inversion technique or matrix partitioning both simplifies the analysis and cuts computational costs substantially. © 1997 John Wiley & Sons, Ltd.     

Re-entrainment of particles from powder structures: experimental investigations

The effectiveness of re-entrainment of particles from powder structures into air flowing through a powder aliquot placed in channel of rectangular cross-section was investigated experimentally. The shear stream of the gas causes breakage of the powder structure and particles or their clusters move to the air stream with an efficiency that depends on airflow rates and air humidity. At higher flow rates, large agglomerates are re-entrained, especially if the powder was conditioned at higher air humidity. The modifications of particles surfaces through adsorption of surfactants or electrostaticaly active substances cause the screening of the short-range cohesive forces between particles and particle processing make the re-entrainment more effective. Reduced size of particles clusters re-entrained into the air stream is also observed for modified surfaces of powder particles.     

平面周期管系对轴向波与弯曲波的隔离特性

管路的振动小仅造成噪声污染,而且造成机器设备的损坏.将声子晶体布拉格带隙原理引入到管壁的结构设计中,将管壁设计成沿轴向交替排列的周期复合结构,采用传递矩阵法,计算此周期弯管中轴向波及弯曲波的传输特性,同时用MSC有限元软件验证了传递矩阵法计算的正确性.研究表明平面周期管路存在弯曲振动、轴向振动带隙特性,在带隙频率范围内,对相应的振动波传播具有很强的衰减作用.进一步,研究材料阻尼比对管路振动特性的影响.     

Numerical and experimental investigations on the use of mist flow process in refrigerated display cabinets

This study concerns the use of mist flow whereby fine water droplets are injected into the air curtain to improve the performance of Refrigerated Display Cabinets (RDCs). The deposition and evaporation of droplets on the surface of products partially compensate the radiative heat gained by the products by removing from it the amount of latent heat of the evaporated droplets.The experiments were carried out on an actual display cabinet. Numerical modelling was performed using Fluent Computational Fluid Dynamics (CFD) software. In two-phase flow, an Euler–Lagrange approach was adopted to predict the transport of droplets by the air curtain and their spatial distribution on the product surface of the RDC. An original numerical procedure was built in the CFD model in order to compute the deposited droplets while taking into account the evaporative flux of droplets on the product surface.The two-phase flow model was used to analyse the performance of the mist cooling process in terms of surface temperature decrease and the homogeneity of droplet deposition on the product surface of the RDC as a function of inlet droplet injection configurations.     

Theoretical and experimental investigations of single- and multilayer structures with SiGe nanoislands

The influence of geometrical and physical parameters of self-assembled SiGe/Si nanoislands on their energy has been investigated theoretically. The island energy minimum was shown to depend on the growth temperature and Si content in the islands. The results of the theoretical calculations are compared with experimental data obtained by atomic force microscopy and Raman spectroscopy (RS).     

Multiple fully stressed designs of steel frame structures with semi‐rigid connections

A fully stressed design of a frame structure, in which each member is performing to its maximum safe level under at least one loading case, is a common goal in structural design. When multiple fully stressed designs are possible, the designer must make some decisions regarding the desired load path because the outcome of the iterative fully stressed design computation depends upon the initial starting design selected. When steel frame structures are modelled as having semi‐rigid beam‐to‐column connections, the connection flexibility has a significant impact on the nature and diversity of potential fully stressed designs. This paper presents a computational study of how the variety of potential fully stressed designs is affected by connection flexibility. A numerical algorithm, which combines the stress ratio method and Newton's method to avoid problems associated with each method when used alone, is applied to generate sets of distinct fully stressed designs as connection flexibility is varied. It is found that as the connections become increasingly flexible, the variety of fully stressed designs diminishes. Load paths that are efficient with rigid connections are not realizable with semi‐rigid connections because of limited bending moment transfer capacity and, consequently, limited alternate load paths. Copyright © 2003 John Wiley & Sons, Ltd.     

Numerical and experimental investigations into ballistic performance of hybrid fabric panels

Strong, low density fibres have been favoured materials for ballistic protection, but the choice of fibres is limited for making body armour that is both protective and lightweight. In addition to developments of improved fibres, alternative approaches are required for creating more protective and lighter body armour. This paper reports on a study on hybrid fabric panels for ballistic protection. The Finite Element (FE) method was used to predict the response of different layers of fabric in a twelve-layer fabric model upon impact. It was found that the front layers of fabric are more likely to be broken in shear, and the rear layers of fabric tend to fail in tension. This suggested that using shear resistant materials for the front layer and tensile resistant materials for the rear layer may improve the ballistic performance of fabric panels. Two types of structure, ultra-high-molecular-weight polyethylene (UHMWPE) woven and unidirectional (UD) materials, were analyzed for their failure mode and response upon ballistic impact by using both FE and experimental methods. It was found that woven structures exhibit better shear resistance and UD structures gives better tensile resistance and wider transverse deflection upon ballistic impact. Two types of hybrid ballistic panels were designed from the fabrics. The experimental results showed that placing woven fabrics close to the impact face and UD material as the rear layers led to better ballistic performance than the panel constructed in the reverse sequence. It has also been found that the optimum ratio of woven to UD materials in the hybrid ballistic panel was 1:3. The improvement in ballistic protection of the hybrid fabric panels allows less material to be used, leading to lighter weight body armour.     

Numerical and experimental investigations of the influence of corner singularities on 3D fatigue crack propagation

In the case of surface breaking cracks, the typical square-root stress singularity is generally not sustainable and a 3D corner singularity in the vicinity of the intersection of crack front and free surface has to be considered. Only the crack front intersection under a special angle γ_r ensures a valid square-root stress singularity and the applicability of the classical SIF-concept. In this paper, the theory of the numerical determination of the intersection angle γ_r is briefly described and the influence of the 3D corner singularity on fatigue crack growth is experimentally investigated. Therefore, 3D fatigue crack propagation experiments under pure mode-I are performed. Transparent specimens of PMMA are used, in order to be able to observe and to document accurate sequences of real 3D crack front evolution profiles via in situ photographic measurement.     

Numerical investigations of the constant and periodic motions of the human vocal cords including stability and bifurcation phenomena

This paper presents a numerical approach to the systematic study of the behaviour of systems governed by autonomous nonlinear differential equations, such as the fifth-order system of equations of motion of the human vocal cords. Motions are traced, by changing the parameters, from steady state via Hopf bifurcations to periodic orbits

In certain intervals the character of the oscillations changes drastically when the parameters are varied slightly     

地铁引起的振动对框架结构的影响及隔振研究-以某教学楼为例

地铁振动对多高层框架结构有一定影响,以广州某教学楼为例进行了研究。首先测量了建筑场地的环境振动,发现场地z向振级大于水平振级,并超出限值要求;将场地最不利加速度时程作为一致激励进行了上部结构的振动响应分析,结果表明3.15-31.5 Hz频段的振动被放大,结构的z向振级逐层增大;对教学楼基底设置钢弹簧隔振装置后,分析结果表明该措施可有效降低23%的地铁振动,并满足限值要求。     

Numerical and experimental investigations in electromagnetic riveting with different rivet dies

In this work, the numerical simulations and electromagnetic riveting (EMR) experiments were conducted to investigate microstructure evolution and the forming mechanism of adiabatic shear bands (ASBs). And the effects of rivet dies on microstructure distributions in formed heads and mechanical properties of riveted structures were systematically explored. The impact velocity and deformation distribution results demonstrated that the proposed numerical method was accurate and reliable. The simulation results showed the slope angle of rivet dies notably affected the plastic flow of materials, and then determined the microstructure distribution in formed heads. The combined effects of inhomogeneous plastic flow and thermal softening were accounted for the forming of ASBs. The formed heads had two obvious ASBs (upper and lower ASB) for the 40° rivet die and flat rivet die. The formed heads only had the lower ASB and no clear upper for the 60° rivet die and 80° rivet die. The pull-out test results showed that the specific rivet die could improve the mechanical properties of the EMR joints, which contribute to the engineering applications of EMR riveted structures.     

Numerical and experimental investigations on new jar designs for high efficiency planetary ball milling

The internal shape of planetary ball mill jars was modified to increase the efficiency of the milling process. Four new jar designs are presented, where obstacles on the surface of a traditional cylindrical jar modify the ratio of normal-to-tangential transferred mechanical action, thus improving the comminution of the mill charge and reducing the process time. Multibody dynamics simulations, validated by operando video recordings of the process, were employed to investigate modified ball motion regimes promoting the increase of the number of high-energy impacts. Moreover, experimental grinding of calcium fluoride powder was performed to assess the effect of milling time and jar-to-plate velocity ratio, through the evaluation of size and microstrain of the end product deduced from X-ray diffraction line profile analysis.     

Correlations between the experimental and numerical investigations on the mechanical properties of erythrocyte by laser stretching

The deformation and mechanical properties of the erythrocytes are studied experimentally and numerically. For the experimental part, an osmotic swollen spherical erythrocyte was attached with a pair of silica beads, and then stretched at two opposite ends by a laser trap. The purpose of this experiment is to find the empirical correlation between the stretching force and the cell deformation in terms of the transverse strain, which is a measure of the change of radius in a spherical cell along its equator. Experimental results show the cell shape become more oblate, elliptic as the stretching force increases. On the numerical front, a physical model from the original work by Pamplona and Calladine for the lipsomes was extended to simulate the deformation of the cell membrane. Numerical analyses were performed to solve the nondimensionalized governing equations with proper boundary conditions imposed to simulate the experimental conditions. The simulated results indicate that at high tensile stiffness, the cell can deform into a spindle shape with negative curvature close to the ends of stretch. Finally, the experimental data and the simulated results were correlated through optimization by minimizing their discrepancy at various values of the shear stiffness. The optimal value of shear stiffness was found in the range of 2.35 approximately 4.29 10(-6) N.m(-1), which is comparable with those values reported in the literature.