Identification of the effective grain size responsible for the ductile to brittle transition temperature for steel with an ultrafine grain size ferrite/cementite microstructure with a bimodal ferrite grain size distribution

This research investigated the mechanism responsible for the ductile to brittle transition temperature for the newly developed steels with a bimodal, ultrafine grain size, ferrite/cementite microstructure (UGF/C), which are produced by caliber warm rolling followed by annealing. The microstructure of the steel was characterised. Charpy impact tests were carried out in the temperature range from 373 K to 4.2 K and the fracture surfaces were analysed. The effective grain size responsible for the ductile-to-brittle transition temperature corresponded to the grain size of the large grain size regions. The mechanism of this phenomenon was attributed to the characteristics of the grain boundaries, as high angle grain boundaries are more effective in impeding cleavage crack propagation. The grain size of the large grain size regions was important in determining the DBTT because these grain boundaries were high angle grain boundaries, whereas the small gain size regions were dominated by the low angle grain boundaries.     

Distortion Regions near the Grain Boundary and Their Effects on Nanocrystalline Materials

1. IntroductionGrain boundaries are the most common extendeddefects in crystalline materials. They strongly affect,and to a great extent control, the physical and mechanical properties of materials. In nanocrystallinematerials, the fraction of atoms locating at grainboundaries becomes so large that the material can beregarded as a two-phase system consisting of a boundary component and a crystalline component. Many ofthe properties of nanocrystalline materials are influenced by the grain bound…     

Influence of absorption on nonlinear vibrations of gas in a closed pipe

We consider dissipative mechanisms involved in resonance vibrations of gas in a closed pipe. Using analysis of a resonance curve as an example, we show the existence of four regimes differing in the mechanism of dissipation. We determine their boundaries, as well as lay a foundation for the procedures used to calculate the amplitude of vibrations within these intervals. Comparison of calculating formulas with experiments conducted by various authors is made.     

Determining the control functions of wave processes in regions with mobile boundaries for a plasma cylinder of finite length expanding in a compressible medium

Exact analytical solutions of the wave equation in regions with mobile boundaries for the general case of cylindrical symmetry are obtained using an original method developed previously for solving inverse problems with allowance for the interaction of nonlinear arguments. The obtained solutions are universal and are valid for both inverse and direct problems. A method of eliminating logarithmic singularities, known to exist in the case of cylindrical symmetry, is proposed for the quantitative determination of the control functions of wave processes in regions with mobile boundaries, including the case of a plasma cylinder of finite length expanding in a compressible medium.     

A new method of integration over polynomal finite element boundaries

In this note we are developing integration techniques for triangular regions in which one side is curved. Specifically, we are considering (a) general polynomial boundaries and (b) various combinations of elliptical boundaries. Integration is performed over all of these explicitly. One definite and considerable improvement achieved is the introduction of a new method of integration over polynomial type boundaries. This new method is not only exact for polynomials or other analytic functions, but also reduces the computing time considerably; for example, if for a sixth order polynomial the number of operations necessary (using previously available methods) to perform the integration was n, the method introduced here requires a number proportional to n^1/6. Furthermore, coding time is also reduced considerably. It may be of value to mention that the availability of the results presented here could obviate the need for the use of isoparametric elements in some regions with curved boundaries. For completeness, explicit integration formulas for elliptic shaped regions are also listed.     

粘弹性变速运动梁稳定性的直接多尺度分析

研究粘弹性轴向加速运动梁横向振动的稳定性。将多尺度法直接应用于系统控制方程,导出了消去长期项的可解性条件。利用该条件得到了次谐共振和组合共振的稳定边界。给出的数值算例说明了粘弹性系数对稳定性边界的影响。     

A coupled-mode theory for periodic piezoelectric composites

A coupled oscillator model to calculate the resonance spectrum of a one-dimensional piezoelectric composite plate, used in ultrasonic transducers, is proposed. Two resonant modes, one produced by the elastic wave reflection on the plate boundaries (thickness resonance) and the other by the reflection on the periodic discontinuities (lateral resonance) are considered. A Kronig-Penney model is used to calculate the lateral resonances. The thickness resonance is obtained with an effective medium model. The coupling of these two modes is described by a biquadratic equation whose solutions are the resonant frequencies of the piezoelectric composite plate. A criterion for a distribution of phases to keep the spurious lateral resonances away from the thickness resonance vicinity is obtained.     

Arbitrary placement of local meshes in a global mesh by the interface‐element method (IEM)

A new method is proposed to place local meshes in a global mesh with the aid of the interface‐element method (IEM). The interface‐elements use moving least‐square (MLS)‐based shape functions to join partitioned finite‐element domains with non‐matching interfaces. The supports of nodes are defined to satisfy the continuity condition on the interfaces by introducing pseudonodes on the boundaries of interface regions. Particularly, the weight functions of nodes on the boundaries of interface regions span only neighbouring nodes, ensuring that the resulting shape functions are identical to those of adjoining finite‐elements. The completeness of the shape functions of the interface‐elements up to the order of basis provides a reasonable transfer of strain fields through the non‐matching interfaces between partitioned domains. Taking these great advantages of the IEM, local meshes can be easily inserted at arbitrary places in a global mesh. Several numerical examples show the effectiveness of this technique for modelling of local regions in a global domain. Copyright © 2003 John Wiley & Sons, Ltd.     

Conformal grid generation for high aspect ratio simply and doubly connected regions

Conformal co-ordinate transformations are used to map rectangular computational domains onto arbitrary simply and doubly connected regions with smooth boundaries. The efficient numerical schemes of Wegmann involving the solution of the inverse boundary correspondence function problems associated with the mapping of the unit disc or circular annulus onto simply or doubly connected domains, respectively, are employed. The numerical implementation of these schemes is emphasized. Examples are generated for regions with elliptic inner and outer boundaries. Additional examples are used to demonstrate the accuracy and convergence of the schemes and their practical limitations. The techniques are found to converge well if holomorphic functions are used to describe the boundaries. The use of preconditioning maps is also discussed.     

Resonance properties of two-dimensional photonic crystals

The resonance properties of two-dimensional photonic crystals, which are manifested by the localization of radiation inside these objects at frequencies corresponding to their bandgap boundaries, have been theoretically studied. This effect is compared to resonances in photonic crystals with violated internal structure, which are related to the generation of defect modes.     

The energy of fracture of a film of carbon monoxide adsorbed on a tungsten single crystal surface

The adsorption and desorption of carbon monoxide were studied on a tungsten single crystal in a field electron microscope. The most interesting effects were observed for the CO desorption from the crystal surface regions containing the {100} W cubic faces. In the course of desorption from these regions, the adsorbed electropositive film, retaining sharp boundaries, shrank toward the <100> poles. The desorption energy value (3.4±0.1 eV), determined using the Arrhenius plots for the boundary velocity, is indicative of a considerable strength of the adsorbed CO film. Behavior of the CO phase desorbing from the {100} W regions can be consistently explained by assuming that this phase comprises a network of adsorbed CO dipoles oriented perpendicularly to the crystal surface, with the carbon atoms facing vacuum.     

Multiparameter families of difference approximations for the first initial boundary value problem for the heat equation in an arbitrary region

Summary A symbolic technique is developed to automatically generate consistent multiparameter families of difference approximations to the heat equation with Dirichlet boundary conditions in arbitrary regions. A stencil of non-uniform step size, conformable along the spatial axes via spatial displacement parameters, is devised to handle the problem of irregular boundaries. Using this stencil as a basic building block, multiparameter families of difference schemes applicable without modification both in the interior and along the boundaries of arbitrary regions, are algorithmically generated. The technique is demonstrated in detail for the one-and two-dimensional heat operator. Necessary and sufficient conditions for the stability of these families are given in terms of their parameters. All existing six-and ten-point two-level schemes for the one-and two-dimensional cases are shown to form subclasses of these families.This work was supported in part by the UNIDEL Foundation at the University of Delaware and in part by grants to the Massachusetts Institute of Technology MATHLAB Project from the Advanced Research Projects Agency (ARPA) and the Department of Defense under Office of Naval Research Contract N00014-70-A-0362-001.     

Method of determining the control functions for wave processes in regions with mobile boundaries (Cylindrical symmetry)

Exact analytical solutions of the wave equation in regions with mobile boundaries for the general case of cylindrical symmetry are obtained using an original method previously developed for solving inverse problems with allowance for the interaction of nonlinear arguments. The obtained solutions are universal and valid for both inverse and direct problems. A method of eliminating logarithmic singularities, known to exist in the case of cylindrical symmetry, is proposed for the quantitative determination of the control functions of wave processes in regions with mobile boundaries.     

A REBIC and CL study of interfaces in a zinc oxide based varistor

Conductive mode (CM) and cathodoluminescence (CL) microscopy techniques were used to study grain boundary structures in a zinc oxide based varistor, doped with 0.5 mol % Bi₂O₃ and 0.5 mol % Sb₂O₃. By combining these two techniques specific details of the electrical and luminescence properties of individual interfaces could be characterised. CM imaging clearly showed the presence of potential barriers at the grain boundaries. The same grain boundaries were regions of strong CL emission. It is suggested that the dominant CL emission at grain boundaries in this material originates from self-excitation centres at impurities and/or defects within the structure rather than the direct recombination of electron-hole pairs across the forbidden band gap.     

Influences of shear stresses on the dynamic instability of exponentially graded sandwich cylindrical shells

The aim of present study is to investigate the dynamic instability of exponentially graded (EG) sandwich cylindrical shells under static and time dependent periodic axial loadings using the shear deformation theory (SDT). The modified Donnell-type dynamic instability equations of EG sandwich cylindrical shells based on the SDT are deduced. Then are reduced to Mathieu-Hill equation and by solving the expressions for the boundaries of instability regions of EG sandwich cylindrical shells are obtained. The similar expressions for EG single-layer shell, ceramic-rich shell and metal coated sandwich cylindrical shell on the basis of SDT and classical shell theory (CST) are obtained in a special case. The numerical illustrations concern the influences of compositional profiles of coating layers, shear stresses and geometrical parameters of sandwich cylindrical shells on the boundaries of instability regions. As a check on the accuracy of the present study, the values of the lower and upper boundaries of instability regions are compared with those in the literature.     

Hydrodynamics and Heat Transfer Analysis of Airflow in a Sinusoidally Curved Channel

For heat transfer enhancement in heat exchangers, different types of channels are often tested. The performance of heat exchangers can be made better by considering geometry composed of sinusoidally curved walls. This research studies the modeling and simulation of airflow through a units long sinusoidally curved wavy channel. For the purpose, two-dimensional Navier Stokes equations along with heat equations are under consideration. To simulate the fluid flow problem, the finite element-based software COMSOL Multiphysics is used. The parametric study for Reynolds number from to and the period of vibration P from to are observed. The surface plots, streamline patterns, contours, and graphs are presented for the velocity field magnitude, temperature, and pressure against the Reynolds number as well as period of vibration. The results are compared with various literature. It is found that due to the creation of periodic contraction regions the velocity magnitude of the flow is continuously increasing with the increase of Reynolds number, on the contrary the pressure is decreasing from inlet to outlet of the channel. Also, a periodic variation in the pressure distribution along the vibrating boundaries has been found with an average increase of for the high Reynolds number. A novel work was done by expressing the rotation rate per second in terms of local Reynolds number for the recirculating regions found due to the periodic oscillation of the boundaries. The average temperature near the outlet where a fixed temperature is imposed initially is decreasing with an increase in Reynolds number. The convection process is weakened due to an increase of periodic vibration of boundaries.     

Electromagnetic phenomenological study of photonic band structures

Abstract

It is shown, in the case of one-dimensional photonic crystals, that the transmission gaps are caused by the existence of resonance phenomena inside the layers which constitute the crystal. From a mathematical point of view, these resonances are associated with poles and zeros in the complex plane of the wavenumber k. Transmission gaps are located outside these resonance regions. A phenomenological formula allows us to represent quantitatively the transmission inside the gaps. Finally, a synthetic explanation of the properties of doped and non-doped crystals is proposed and it is shown that the transmission peaks inside the gaps of doped photonic crystals are caused by a shift of poles and zeros located inside the resonance regions of non-doped crystals.     

Plasmon‐Mediated Synthesis of Silver Cubes with Unusual Twinning Structures Using Short Wavelength Excitation

The plasmon‐mediated synthesis of silver nanoparticles is a versatile synthetic method which leverages the localized surface plasmon resonance (LSPR) of nanoscale silver to generate particles with non‐spherical shapes and control over dimensions. Herein, a method is reported for controlling the twinning structure of silver nanoparticles, and consequently their shape, via the plasmon‐mediated synthesis, solely by varying the excitation wavelength between 400, 450, and 500 nm, which modulates the rate of Ag⁺ reduction. Shorter, higher energy excitation wavelengths lead to faster rates of reaction, which in turn yield structures containing a greater number of twin boundaries. With this method, silver cubes can be synthesized using 450 nm excitation, which represents the first time this shape has been realized by a plasmon‐mediated synthetic approach. In addition, these cubes contain an unusual twinning structure composed of two intersecting twin boundaries or multiple parallel twin boundaries. With respect to their twinning structure, these cubes fall between planar‐twinned and multiply twinned nanoparticles, which are synthesized using 500 and 400 nm excitation, respectively.     

低速碰撞下复合材料叠层板的微裂纹演化及其对弹性模量的影响

采用双线性特性破坏模型研究了复合材料叠层板各层内部开裂裂纹的演化;通过引入弹性模量的裂纹影响系数表示,推导出裂纹影响系数与应变及应变率之间的微分关系,并得到裂纹耗散功率与裂纹影响系数变化率之间的关系。通过计算不同初始碰撞速度下复合材料叠层板的应变、应变率响应以及裂纹影响系数的演化,得到整个冲击过程中各层内任意点附近裂纹开裂情形及其对弹性模量的影响;通过检查界面各点处的裂纹影响系数是否发生改变,预测了碰撞完成之后复合材料叠层板中各层内微裂纹的分布区域位置与大小;并将该预测结果与其他破坏准则计算结果进行了比较。计算结果表明,在碰撞过程中各层内任意点处的应力值超过其屈服强度后,该点附近的弹性模量开始发生衰减,衰减大小随铁球初始碰撞速度的增大而增大。在四边夹支的边界条件下,复合材料叠层板的裂纹分布区域同样最先出现在碰撞点及边界中点位置,区域面积随初始碰撞速度的增大不断扩大     

Automatic generation of transitional meshes

Advances in commercial computer‐aided design software have made finite element analysis with three‐dimensional solid finite elements routinely available. Since these analyses usually confine themselves to those geometrical objects for which particular CAD systems can produce finite element meshes, expanding the capability of analyses becomes an issue of expanding the capability of generating meshes. This paper presents a method for stitching together two three‐dimensional meshes with diverse elements that can include tetrahedral, pentahedral and hexahedral solid finite elements. The stitching produces a mesh that coincides with the edges which already exist on the portion of boundaries that will be joined. Moreover, the transitional mesh does not introduce new edges on these boundaries. Since the boundaries of the regions to be stitched together can have a mixture of triangles and quadrilaterals, tetrahedral and pyramidal elements provide the transitional elements required to honor these constraints. On these boundaries a pyramidal element shares its base face with the quadrilateral faces of hexahedra and pentahedra. Tetrahedral elements share a face with the triangles on the boundary. Tetrahedra populate the remaining interior of the transitional region. Copyright © 2001 John Wiley & Sons, Ltd.