Equivalent Material Parameters Research Articles

Modelling and Simulation of Rectangular Bundle of Single-Walled Carbon Nanotubes for Antenna Applications

This paper aims to present an effective electromagnetic (EM) modelling approach for rectangular bundle of single-walled carbon nanotubes (RB-SWCNTs), based on the electrical conductivity, relative complex permittivity and linear distribution impedance by applying General Ohm’s law for this bundle. The equivalent single conductor material (ESCM) model for personification the RB-SWCNTs is present in this paper. The main target of this modeling approach is to estimate and investigate the EM properties of RB-SWCNTs using common EM engineering tool solver CST (MWS). For this purpose, the RB-SWCNTs and ESCM dipole antennas will be designed and implemented using CST (MWS). The equivalent conductivity model, relative complex permittivity and other parameters of the RB-SWCNTs will be derived in this paper and considered as an equivalent material parameters for the ESCM. This modeling technique is expected to provide new avenues for designing different antenna structures.

Modified foundation modelling of dowel embedment in glulam connections

This paper examines the embedment behaviour of single-dowel connections in Scandinavian Spruce Glulam by means of experimental and numerical investigations. First, the experimental results of a series of single-dowel tests on samples of different geometry and grain directions are presented. The evolution of local strain concentrations around the fastener at increasing levels of bearing deformation, is reported in detail by means of non-contact field strain measurements and its implications are discussed. Detailed Finite Element simulations are also carried out and subsequently employed to highlight the main features of the response of doweled connections in glulam. A foundation model, initially developed for Douglas-fir (Pseudotsuga menziesii) timber, is upgraded and adapted for Scandinavian Spruce Glulam (Picea abies) elements subjected to loads acting perpendicular and parallel to the grain direction. The proposed model is based on the definition of equivalent material parameters for the crushing region around the dowel hole. To this end, relationships for the estimation of material characteristics as a function of the crushing volume are suggested. The validity and accuracy of the proposed modified foundation models are examined against the experimental results. It is shown the improved foundation model is able to simulate the embedment stiffness, capacity and inelastic behaviour of single-dowel connections on glulam with reasonable accuracy for strains of up to 8%, and can therefore be used for design and assessment purposes.

Determination of electromagnetic properties of steel for prediction of stray losses in power transformers

This paper introduces a method for determination of equivalent linear electromagnetic parameters (constant complex permeability and electrical conductivity) of nonlinear magnetic steel, which can be used in a time-harmonic finite-element simulation to yield the same losses in the volume of that material as the measured ones. The conductivity and the static hysteresis loop of the steel have been measured, from which complex permeability as a function of flux density has been extracted. The indirect measurement of losses in various samples of nonmagnetic and magnetic steel has been carried out using a physical model of a transformer core with a coil. The 3D model of the core has been made with finite-element software and combining it with evolutionary optimization the equivalent constant complex permeability and conductivity of each sample have been found, which yield the equality of measured and calculated losses in the sample. Thus, calculated equivalent material parameters have been implemented in 3 finite-element models of transformers of various power ratings in order to determine the share of hysteresis losses in the total amount of losses in structural parts of transformers. The results have been compared with the measurement and the reasons for discrepancies have been explained.

Artificial magnetism at terahertz frequencies from three-dimensional lattices of TiO_2 microspheres accounting for spatial dispersion and magnetoelectric coupling

We employ the generalized Lorentz-Lorenz method to investigate how both magnetoelectric coupling and spatial dispersion influence the artificial magnetic capabilities at terahertz frequencies of the representative case of a metamaterial consisting of a three-dimensional (3D) lattice of TiO2 microspheres. The complex wavenumber dispersion relations pertaining to modes supported by the array, traveling along one of the principal axes of the array with electric or magnetic field polarized transversely and longitudinally (with respect to the mode traveling direction), are studied and thoroughly characterized. One mode with transverse polarization is dominant at any given frequency for the analyzed dimensions, proving that the 3D lattice can be treated as a homogeneous medium with defined electromagnetic material parameters. We show, however, that bianisotropy is a direct consequence of magnetoelectric coupling, and the dyadic expressions of both effective and equivalent material parameters are derived. In particular, we analyze the effect of spatial dispersion on the effective parameters relative to a composite material made by a 3D lattice of TiO2 microspheres with filling fraction around 30% and near the first Mie magnetic dipolar resonance. Finally, we homogenize the metamaterial in terms of equivalent index and impedance, and by comparison with full-wave simulations, we explain the presence of the unphysical antiresonance permittivity behavior observed in previous work.

Simulation Methodology for the Assessment of Field Uniformity in a Large Anechoic Chamber

The paper presents a simulation methodology for assessing the properties of a generic anechoic chamber, taking into account wall and floor coatings, measurement table, and antenna. Unlike previous approaches, the absorbing walls of the chamber are modeled as infinitely thin sheets. This leads to considerable improvement of the simulation performance. To this aim, a technique for extracting equivalent surface impedance material parameters from wall reflectivity measurements is presented. The large electrical size of the problem, its multiscale character, given by the large dimensions of a typical chamber on one hand and the fine details of the antenna on the other hand, and the complicated material parameters represent the challenges of such a simulation.

Analysis on the Analogue Slab Methods for Hollow Floor

This paper presents the results of numerical analysis of hollow floor supported by the column under vertical load and horizontal load. The validity of three different analogue methods were examined and compared. The macroscopic elastic constitutive method (MEC) gave satisfactory predictions but involves an elaborate procedure for determining the equivalent elastic material parameters. The equivalent thickness approach (ETA) gave moderately higher values for the section moment than the exact values. An alternative simplified approximate method based on the effective elastic modulus (EEM) is recommended by the specification. Numerical calculation for section moments by the proposed method were on the safe side and close to exact results. The proposed method could also be appropriate for the column supported structure and the horizontal load.

Study on Double Split-Rectangular Resonators (SRRs) with Negative Permeability

Metamaterial-composite structured materials have abnormal electromagnetic capabilities. Presently these electromagnetic capabilities can be realized by periodical resonant structure such as SRRs,SR, and fractal resonator. Based on the SRRs structure, double split rectangular resonators can be constructed by connecting two SRRs. This type of resonators exhibites negative permeability and is more compacted. The detailed design process and retrieved process of equivalent material parameters are presented and the effect of negative permeability can be analysied. Compared to SRRs, double split rectangular resonator is smaller in size for the same resonance frequency and more convenient for miniaturization applications .

Research on Homogenization of Composite Materials

A numerical model is given to identify equivalent parameters of composite materials, using BP neural network algorithm. Taking Filament-wound composite pressure vessels as the research object, finite element models are first constructed .Getting node displacements as network training samples, the mechanical parameters as output information of network for effective training, the equivalent material parameters can be obtained. The satisfactory numerical validation is given and results show that the proposed method can identify the equivalent modulus and the equivalent Poisson’s ratio of the Filament-wound composite pressure vessels with precision. The computational efficiency is improved with BP neural network.

Indentation stress in multi-layer delaminated thin films induced by a microwedge indenter

Indentation stresses in single- and multi-layer delaminated thin films made of elastic–perfectly plastic materials in microwedge indentation delamination tests are analyzed via finite element calculations with different wedge angles and other geometrical and mechanical parameters. Based on the formula for a single-layer thin film under indentation loading [Zhao et al. J Mater Res 2009;24:1943] and by introducing the equivalent material parameters, we developed simple analytical formulae for the loading indentation stress (as well as the energy release rate) in each layer of the multi-layer thin film, in terms of the residual stress, elastic modulus, Poisson’s ratio, yielding strength, and thickness of each layer. Our analytical solution is validated by the finite element calculations and should be useful to thin-film delamination tests.

A Simple Anisotropic Fiber Reinforced Hyperelastic Constitutive Model for Woven Composite Fabrics

Based on fiber reinforced continuum mechanics theory, a simple hyperelastic constitutive model is developed to characterize the anisotropic nonlinear material behaviour of woven composite fabrics under large deformation during forming. The strain energy function for the hyperelastic model is additively decomposed into two parts nominally representing the tensile energy from weft and warp yarn fiber stretches and shearing energy from fiber-fiber interaction between weft and warp yarns, respectively. The proposed material characterization approach is demonstrated on a balanced plain weave composite fabric. The equivalent material parameters in the hyperelastic constitutive model are obtained by matching experimental load-displacement data of uni-axial tensile and picture frame tests on the woven composite fabric. The development of this anisotropic fiber reinforced hyperelastic model is critical to the numerical simulation and optimization of woven composites forming.

Intermediate vibronic coupling in sexithiophene single crystals

A new approach is proposed to describe intermediate-to-strong linear vibronic coupling in an infinite molecular crystal. The Hamiltonian, transformed to the Lang-Firsov representation, is approximated by disregarding the terms involving more-than-two-particle excitations and block-diagonalized by the Fourier transformation. The spectroscopically relevant block corresponding to zero wave vector is further simplified by introducing a cutoff in the off-diagonal matrix elements and reduced to a manageable size by truncating the basis set, which enables one to diagonalize it numerically. The parametrization, based on independent experiments or theoretical estimates, is aimed to represent the sexithiophene crystal. The results, compared to those obtained for a finite cluster with equivalent material parameters, highlight the favorable convergence properties of the infinite-crystal approach.

New transmission lines of negative refractive index based on periodically loaded negative-impedance-converted inductors and capacitors

A new class of negative refractive index (NRI) transmission line is proposed. Differential operational amplifiers are used to form the required lumped elements, i.e., negative-impedance-converted (NIC) inductors and capacitors. When the gain of the differential operational amplifiers in the NIC circuits is very large over the entire frequency band, the equivalent material parameters are non-dispersive, and the phase response of the new NRI transmission line can be complimentary to that of a conventional transmission line over the entire frequency band. As an application example, a broadband 1:4 power divider based on the proposed NRI transmission line is studied.

Equivalent material parameter extraction of double strip loaded waveguide

Equivalent material parameters of a periodic waveguide structure loaded with double strips printed on dielectric has been extracted. It shows a passband where the relative permittivity and permeability are simultaneously negative also called as double negative (DNG) passband. Full-wave Hybrid MoM-Immittance Approach is employed for the material parameter extraction validated by independent Ansoft High Frequency Structure Simulator (HFSS) extracted results.

Imaging with lithium niobate/epoxy composites

Lithium niobate, LiNbO 3, is a very promising material for high temperature applications in non-destructive testing because of its high Curie temperature. However, for commercial applications LiNbO 3 has often been neglected because of its low electromechanical coupling coefficients. This paper explores the potential of LiNbO 3 composites, by means of room temperature characterization and measurements, for further use in operation in high temperature transducers. LiNbO 3 composites of 1–3 connectivity in an epoxy matrix with volume fractions of LiNbO 3 of 33% and 54% were fabricated. The composites were characterized by electrical impedance measurements and the results were compared with modelled impedance characteristics. Many parameters were predicted accurately, including an improvement of more than 75% in thickness mode electromechanical coupling coefficient, from k T=0.17 for bulk LiNbO 3 to k T=0.32 for composite material. Some large discrepancies between simulation and experiment were also identified when a conventional one-dimensional model was used to calculate equivalent composite material parameters; however, finite element modelling was more accurate. After characterization, the composite was configured for use as a linear array. Functional measurements were conducted on steel blocks with a side drilled hole to represent a crack tip. This was detected by the array and visualized in time-of-flight diffraction B-scan images.

Planar negative refractive index media using periodically L-C loaded transmission lines

Recent demonstrations of negative refraction utilize three-dimensional collections of discrete periodic scatterers to synthesize artificial dielectrics with simultaneously negative permittivity and permeability. In this paper, we propose an alternate perspective on the design and function of such materials that exploits the well-known L-C distributed network representation of homogeneous dielectrics. In the conventional low-pass topology, the quantities L and C represent a positive equivalent permeability and permittivity, respectively. However, in the dual configuration, in which the positions of L and C are simply interchanged, these equivalent material parameters assume simultaneously negative values. Two-dimensional periodic versions of these dual networks are used to demonstrate negative refraction and focusing; phenomena that are manifestations of the fact that such media support a propagating fundamental backward harmonic. We hereby present the characteristics of these artificial transmission-line media and propose a suitable means of implementing them in planar form. We then present circuit and full-wave field simulations illustrating negative refraction and focusing, and the first experimental verification of focusing using such an implementation.

A simple model for avalanche multiplication including deadspace effects

A simple Monte Carlo model (SMC) using single effective parabolic valleys and accurately accounting for deadspace effects is presented for calculating the avalanche process. Very good agreement is achieved with a range of measured electron and hole multiplication results from GaAs p/sup +/-i-N/sup +/'s with i-region thicknesses, /spl omega/, from 1 /spl mu/m down to 0.025 /spl mu/m and with the excess noise factors down to 0.05 /spl mu/m. While the results are insensitive to the precise values of input parameter for structures with /spl omega//spl ges/0.2 /spl mu/m, this is not the case in thinner structures where the deadspace represents a significant fraction of the device. For /spl omega/<0.2 /spl mu/m, the energy dependence of the ionization rate becomes increasingly important. The SMC model is tested against a full-band Monte Carlo model (FBMC) by comparing the mean distance between ionization events and the probability density functions, which are effectively the histograms of distances between ionization events, for equivalent material parameters. The good agreement between these suggests that the SMC, with a relatively small number of fitting parameters and much faster calculation times than the FBMC, is a useful tool for device simulation and interpreting experimental results.