Finite Element Method Program Research Articles

Optimization of Crack-Free Polytypoidally Joined Dissimilar Ceramics ofFunctionally Graded Material (FGM) Using 3-Dimensional Modeling

Crack-free joining of Si N and Al O using 15 layers has been achieved by a unique approach introducing Sialon polytypoids as a functionally graded materials (FGMs) bonding layer. In the past, hot press sintering of multilayered FGMs with 20 layers of thickness 500 µm each has been fabricated successfully. In this study, the number of layers for FGM was reduced to 15 layers from 20 layers for optimization. For fabrication, model was hot pressed at 38 MPa while heating up to 1700 C, and it was cooled at 2 C/min to minimize residual stress during sintering. Initially, FGM with 15 layers had cracks near 90 wt.% 12H / 10 wt.% Al O and 90 wt.% 12H/10 wt.% Si N layers. To solve this problem, FEM (finite element method) program based on the maximum tensile stress theory was applied to design optimized FGM layers of crack free joint. The sample is 3-dimensional cylindrical shape where this has been transformed to 2-dimensional axisymmetric mode. Based on the simulation, crack-free FGM sample was obtained by designing axial, hoop and radial stresses less than tensile strength values across all the layers of FGM. Therefore, we were able to predict and prevent the damage by calculating its thermal stress using its elastic modulus and coefficient of thermal expansion. Such analyses are especially useful for FGM samples where the residual stresses are very difficult to measure experimentally.

고주파용 디커플링 임베디드 캐패시터에 관한 연구

본 논문에서는 전극들이 동일한 평면상에 놓이고, Gap에 의하여 유전간격을 형성한 새로운 구조의 임베디드 캐패시터(EC)를 제안하였다. 제안된 EC의 이름을 Gap type EC라고 하고, 유한요소법으로 그 특성을 평가하였다. Cap type EC의 공진주파수는 기존의 EC에 비하여 고주파 대역으로 이동되었다. 또한 공진주파수는 전극의 크기와 두께에 따라 변화되었다. Gap type EC는 Gap size가 <TEX>$50{\mu}m$</TEX>일 때 <TEX>$55pF/cm^2$</TEX>의 정전용량을 나타내었다. 이 값은 기존의 EC가 나타내는 <TEX>$25pF/cm^2$</TEX>에 비하여 높은 값이다. 따라서 본 논문에서 제안한 Gap type EC는 고주파 디커플링 용도로 충분히 사용될 수 있을 것이다. We proposed an embedded capacitor with the unique electrode structure, which electrodes are located on the same plane and dielectric gap was formed by electrodes. We named it 'Gap type EC', and it was analyzed by the FEM(Finite element Method) program tool. The resonant frequency of Cap type EC was obtained at more higher frequency region. Also, resonant frequency was changed with the magnitude and thickness of electrodes. The Gap type EC with the dielectric gap of <TEX>$50{\mu}m$</TEX> showed capacitance density of <TEX>$55pF/cm^2$</TEX>. This value is the higher than that of conventional EC. So, we concluded that the Gap type EC can be a good candidate for high frequency decoupling.

Numerical Simulation and Experimental Study of Residual Stresses in Compression Molding of Precision Glass Optical Components

Compression molding of glass optical components is a high volume near net-shape precision fabrication method. Residual stresses incurred during postmolding cooling are an important quality indicator for these components. In this research, residual stresses frozen inside molded glass lenses under different cooling conditions were investigated using both experimental approach and numerical simulation with a commercial finite element method program. In addition, optical birefringence method was also employed to verify the residual stress distribution in molded glass lenses. Specifically, optical retardations caused by the residual stresses in the glass lenses that were molded with different cooling rates were measured using a plane polariscope. The measured residual stresses of the molded glass lenses were compared with numerical simulation as a validation of the modeling approach. Furthermore, a methodology for optimizing annealing process was proposed using the residual stress simulation results.

Three-dimensional thermal stress analysis of the mould during high-speed casting of steel

On the basis of the calculated mould temperature, a three-dimensional thermal stress model of the mould is established. A corresponding finite element method program is developed with six-face and eight-node isoparameter elements. The Newton method is used to solve the stress equations. Making using of the stress model, the thermal distortion and stress distribution of the mould are obtained when the thickness of the mould wall is different. Experimental data have been used to validate the model. From the result, it is proved that the mould stress is less than its yield strength and does not result does in residual deformation.

Modeling and FEM Simulation of Shape Memory Microactuators

This article reports on two models for the shape memory effect and explains, how they are implemented in a finite element method program. The first model uses a phenomenological approach. For the example of a microgripper, the performance prediction of real actuators made of polycrystalline materials is demonstrated. In the second model, the martensite-austenite phase transition is treated as a thermodynamically activated process. Thermodynamic laws, like e.g. the minimization of the Gibbs free energy, are used for the formulation. To simplify the model, it is primarily intended to describe the behavior of single crystals. By comparing the simulated bending characteristic of a cantilever beam with experimental data, the applicability to polycrystalline material is tested. Due to the physics based formulation, this model gives more insight into the structural processes involved. This is very useful, e.g., for physical extensions needed for the simulation of the magnetic shape memory effect. It is shown, how the model can be extended to predict the behavior of actuators made of ferromagnetic Ni-Mn-Ga single crystals in a magnetic field.

A novel hybrid numerical technique to determine mass transport properties in supercritical fluid extraction processes

AbstractEffective diffusion coefficients (Def) and mass transfer coefficients (km) in essential oil extraction processes from vegetal porous matrices using high‐pressure carbon dioxide have been determined. The experimental data available for these calculations are extracted mass curves as functions of time for the extraction process of lemongrass essential oil (Cymbopogon citratus Staupf) using supercritical carbon dioxide as solvent extracting fluid. The mathematical model employed is based on Fick's second law, which has been solved using the finite element method and computer programs specially designed to evaluate the transport properties Def and km. A set of routines based on the modified Marquardt method to evaluate the optimum values of such properties were written and coupled with the finite element method code. The results show that the model employed and the computer programs developed are adequate to evaluate the transport properties and the extraction curves. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Environmental effects induced by excavation

Based on 3D Biot’s consolidation theory and nonlinear Duncan-Chang’s model, a 3D FEM (finite element method) program is developed considering the coupling of groundwater seepage and soil skeleton deformation during excavation. The comparison between the analysis result considering the variation of water head difference and that without considering it shows that the porewater pressure distribution of the former is distinctly different from that of the latter and that the foundation pit deformations of the former are larger than those of the latter, so that the result without considering the variation of water head difference is unreliable. The distribution rules of soil horizontal and vertical displacements around the pit and excess porewater pressure are analyzed in detail in time and space, which is very significant for guiding underground engineering construction and ensuring environment safety around the pit.

Topology Optimization for Heat Convection Problems Including Design-Dependent Effects

In this paper, a topology optimization method is constructed for thermal problems with generic heat transfer boundaries in a fixed design domain that include design-dependent effects. First, the topology optimization method for thermal problems is briefly explained using a homogenization method for the relaxation of the design domain, where a continuous material distribution is assumed, to suppress numerical instabilities and checkerboards. Next, a method is developed for handling heat transfer boundaries between material and void regions that appear in the fixed design domain and move during the optimization process, using the Heaviside function as a function of node-based material density to extract the boundary of the target structure being optimized so that the heat transfer boundary conditions can be set. Shape dependencies concerning heat transfer coefficients are also considered in the topology optimization scheme. The optimization problem is formulated using the concept of total potential energy and an optimization algorithm is constructed using the Finite Element Method and Sequential Linear Programming. Finally, several numerical examples are presented in order to confirm the usefulness of the proposed method.

Effects of heating and loading histories on post-fire cooling behaviour of concrete-filled steel tubular columns

A finite element method (FEM) program is developed and used in this paper to analyse the behaviour of concrete-filled steel tubular (CFST) columns during the entire stage of fire exposure, including: loading at ambient temperature, heating, cooling to the ambient temperature and post-fire loading to failure. The emphasis of this paper is on CFST column behaviours during the cooling and post-fire stages because these behaviours are affected by the loading and heating histories, but they have not previously been studied. This paper will present the mechanical property models for these different loading and heating stages. To validate the FEM program, some experimental data, including fire resistance, axial deformation and ultimate strength of CFST columns are compared and it is found that the FEM program can predict the test results with good accuracy. Using the FEM program, a parametric study is then conducted to investigate the influences of ambient temperature loading and heating history on the cooling and post-fire behaviours of CFST columns. It is concluded that various parameters (such as load ratio and elevating temperature time ratio etc.) affect the residual strength of CFST columns severely. Finally, this paper proposes a set of formulas which can be used to predict the residual strength of CFST columns after going through the whole fire exposure process.

Contact problem and numeric method of a planetary drive with small teeth number difference

This paper deals with a theoretical study on contact problem and numeric analysis of a planetary drive with small teeth number difference (PDSTD). A mechanics model and finite element method (FEM) solution are presented in this paper to conduct three-dimensional (3D) contact analysis and load calculations of the PDSTD through developing concepts of the mathematical programming method [T.F. Conry, A. Serireg, A mathematical programming method for design of elastic bodies in contact, Transactions of ASME, Journal of Applied Mechanics 38 (6) (1971) 387–392] and finite element method [S. Li, Gear contact model and loaded tooth contact analysis of a three-dimensional, thin-rimmed gear, Transactions of ASME, Journal of Mechanical Design 124 (3) (2002) 511–517; S. Li, Finite element analyses for contact strength and bending strength of a pair of spur gears with machining errors, assembly errors and tooth modifications, Mechanism and Machine Theory 42 (1) (2007) 88–114] to solve a more complex engineering contact problem. FEM programs are developed through many years’ efforts. Contact states of teeth, pins and bearing rollers of the PDSTD are made clear through performing contact analysis of the PDSTD with the developed FEM programs. It is found that there are only four pairs of teeth in contact for the PDSTD used as research object when it is loaded with a torque 15 kg m. It is also found that these four pairs of teeth are not located in the offset direction of the external gear. They are located at an angular position of 20–30° away from the offset direction. Loads shared by teeth, pins and rollers have big difference. The maximum load shared by the teeth is much greater than the ones shared by pins and rollers. This means that strength calculations of the teeth are more important than the ones of pins and rollers for the PDSTD. It is also found that all pins share loads while only a part of rollers share loads.

Numerical characteristics of peer‐to‐peer (P2P) internet online hybrid test system and its application to seismic simulation of SRC structure

AbstractThe peer‐to‐peer (P2P) Internet online hybrid test system has been developed for the seismic simulation of a structure. In this study, the stability and accuracy of the system are investigated analytically by studying the spectral radius of the recursive matrix of the test scheme featuring a two‐round quasi‐Newton test scheme. The applicability of the system is further examined by exploring the seismic responses of a complex structure, a steel‐encased reinforced concrete (SRC) structure with a steel tower on the top. The structure is divided into two numerical substructures and one tested part for hybrid test. The numerical substructures are simulated by sophisticated finite element method (FEM) models with material nonlinearities to capture local plastifications. Two types of FEM programs, namely OpenSEES and ABAQUS, which are suitable for the SRC part and the steel tower, respectively, are employed. The results demonstrate that the P2P system is able to simulate complex structures with significant nonlinearities. As compared with the previous study in which two elastic numerical substructures were considered, increase in the number of iterations in this study is not significant, because the associated nonlinearities are limited due to the small time interval adopted in the test. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Elasto-Plastic Analysis of 3-D Inner Cracks Using the Finite Element Alternating Method

The finite element alternating method (FEAM) was extended to obtain fracture mechanics parameters and elasto-plastic stress fields for 3-D inner cracks. For solving a problem of a 3-D finite body with cracks, the FEAM alternates independently the finite element method (FEM) solution for the uncracked body and the solution for the crack in an infinite body. As the required solution for a crack in an infinite body, the symmetric Galerkin boundary element method formulated by Li and Mear was used. For elasto-plastic numerical analysis, the initial stress method proposed by Zienkiewicz and co-workers and the iteration procedure proposed by Nikishkov and Atluri were used after modification. The extended FEAM was examined through comparing with the results of commercial FEM program for several example 3-D crack problems.

Electrical impedance tomography through constrained sequential linear programming: a topology optimization approach

Electrical impedance tomography (EIT) is an imaging method that estimates conductivity distribution inside a body. In EIT, images are obtained by applying a sequence of low intensity electrical currents through electrodes attached to the body. Although in EIT there are serious difficulties to obtain a high-quality conductivity image, for medical applications this technology is safer and cheaper than other tomography techniques. The EIT deals with an inverse problem in which given the measured voltages on electrodes and a finite element (FE) model, it estimates the conductivity distribution, which are parameters of the FE model. In this work, the topology optimization method is applied as a reconstruction algorithm to obtain absolute images in EIT. It is an optimization method that has been applied successfully to structural mechanical applications and consists of systematically finding a conductivity distribution (or material distribution) in the domain that minimizes the difference between measured voltages and voltages calculated by using a computational model. This algorithm combines the finite element method and sequential linear programming (SLP) to solve the inverse problem of EIT. The SLP allows us to easily apply some regularization schemes based on included constraints in the topology optimization problem. Constraints based on image tuning control and weighted distance interpolation (WDI) are proposed, while a material model is applied to ensure the relaxation of the optimization problem. A new formulation to analytically perform the sensitivity analysis is proposed, using Maxwell's reciprocity theorem. To illustrate, the implemented algorithm is applied to obtain conductivity image distributions of some 2D examples using numerical and experimental data.

수치해석을 이용한 배전용 변압기 권선 고장시의 전자력 계산방법 연구

본 논문에서는 전력계통에서 발생되는 surge나 고장전류에 의해서 변압기의 고압측 권선에 선간단락이 발생할 경우 유도되는 전자력의 크기를 유한요소 전자계해석 프로그램을 이용하여 해석하는 방법을 제안하였다. 연구에서 사용된 배전용 변압기 model은 1[MVA], 22,900/220[V] 단상 외철형 Cable 권선형 변압기로서 권선간 단락을 모의하는 방법을 제안하고, FEM 프로그램(FLUX2D)을 이용하여 단락전류와 각방향의 전자력을 계산하고 분석하였다. 누설자속분포, 권선단락 시 전압 및 전류파형, 권선간에 작용하는 힘의 변화와 분포를 계산하였다. Simulation 방법의 타당성을 검증하기 위하여 이론치와 Pspice 프로그램을 이용하여 계산한 결과와 5(%) 이내의 오차율로 아주 잘 일치함을 확인하였다. 선간단락전류는 정격전류의 약 400배이며, 전자력도 20-200배 정도로 급증하였다. 본 연구의 결과는 배전용 변압기의 신뢰도 향상을 위하여 변압기 코일과 절연구조 설계에 유용한 정보를 제공하게 될 것이다.

Investigation of electron trajectories of an x‐ray tube in magnetic fields of MR scanners

A hybrid x-ray/MR system combining an x-ray fluoroscopic system and an open-bore magnetic resonance (MR) system offers advantages from both powerful imaging modalities and thus can benefit numerous image-guided interventional procedures. In our hybrid system configurations, the x-ray tube and detector are placed in the MR magnet and therefore experience a strong magnetic field. The electron beam inside the x-ray tube can be deflected by a misaligned magnetic field, which may damage the tube. Understanding the deflection process is crucial to predicting the electron beam deflection and avoiding potential damage to the x-ray tube. For this purpose, the motion of an electron in combined electric (E) and magnetic (B) fields was analyzed theoretically to provide general solutions that can be applied to different geometries. For two specific cases, a slightly misaligned strong field and a perpendicular weak field, computer simulations were performed with a finite-element method program. In addition, experiments were conducted using an open MRI magnet and an inserted electromagnet to quantitatively verify the relationship between the deflections and the field misalignment. In a strong (B >> E/c; c: speed of light) and slightly misaligned magnetic field, the deflection in the plane of E and B caused by electrons following the magnetic field lines is the dominant component compared to the deflection in the E X B direction due to the drift of electrons. In a weak magnetic field (B < or = E/c), the main deflection is in the E x B direction and is caused by the perpendicular component of the magnetic field.

Simulation of rutting profiles using a viscoplastic model

The development of high speed computers has encouraged researchers to increasingly utilize the finite element method (FEM) to simulate more realistic pavement structures and loading conditions in their efforts to predict the performance of asphalt pavem ents. Analysis of pavement materials using mechanistic constitutive materials models is an integral part of these efforts. An elastic viscoplastic continuum model based on Perzyna’s viscoplastic theory and the Drucker-Prager yield function is proposed to simula te the permanent deformation of a hot mix asphalt (HMA) mixture at WesTrack. Compressive strength tests were performed at differen strain rates and confinement pressures to obtain the elastic-viscoplastic constitutive material parameters. The test temperature was 60 °C based on the critical temperature for permanent deformation at 12.5 mm of the WesTrack replacement section. Permanent deformation of the replacement section was simulated by the three dimensional FEM program, ABAQUS. The same trend of rutting profiles was obtained for both the measured and the simulated rutting profiles.

A 3-D Cell Method Formulation for Coupled Electric and Thermal Problems

A finite formulation is presented for the solution of coupled problems of steady-state electric and transient thermal conductions in 3-D regions. The model is based on the cell method approach and takes advantage of the very agile algebraic formulations that it can provide for field theories. Dual barycentric cell complexes are used for both space and time domains, the latter inducing a Crank-Nicolson time integration scheme. An example of application is given, consisting in the simulation of a treatment for the thermal ablation of liver cancer. Comparisons are given with a commercial finite-element method program

Topology Optimization for Thermal Problems Based on Assumed Continuous Approximation of Material Distributions

In structural designs considering thermal loading, maximization of temperature diffusivity in structures is one of the most important essential factors in reducing operating temperature and maintaining product durability, in addition to the usual maximization of stiffness that optimal designs achieve. In this paper, a topology optimization method is constructed for thermal problems considering generic heat transfer boundaries, including heat convection boundaries, based on the homogenization method. First, the topology optimization method for thermal problems is discussed using a homogenization method that assumes a continuous material distribution. Next, a new objective function that can take into account heat transfer boundaries, such as temperature-constant and heat convection boundaries, is proposed, based on the concept of a total potential energy maximization problem in the structural problem, and an optimization problem is formulated using the proposed objective function. An optimization algorithm is constructed using the Finite Element Method (FEM) and Sequential Linear Programming (SLP). Finally, several numerical examples are presented in order to confirm the usefulness of the proposed method.

Preliminary Design of an Augmented Railgun: Influence of the Dimensions of the Outer Rails on the Forces Acting on the Projectile and the Rails

For the preliminary design of an augmented railgun, the electromagnetic repulsive forces on the rails and the self and mutual inductance gradients were determined by simulation. This design is meant for the realization of an augmented railgun at the French-German Research Institute in Saint-Louis, France. The simulated railgun has a 25 mm × 25 mm square calibre and both the inner and outer rails have a rectangular cross-section. In our study the geometry of the inner rails is fixed while the dimensions of the outer rails were altered. A 2D transient analysis for the determination of the magnetic forces on the rails was carried out with the finite element method program ANSYS. The EM force on the projectile is analytically calculated with the force-equation based on values of the inductance gradients determined with 3D AC-analyses. The influence of the ratio between the currents in the inner and outer rails on the electromagnetic forces has also been investigated. We have shown that the EM force acting on the projectile increases when the height of the outer rails and the distance between the inner and outer rails decrease. However this also leads to higher repulsive forces between the rails. For the theoretically best geometry, based on the electromagnetic forces, a 3D transient analysis was carried out. The electromagnetic force determined directly with the 3D transient analysis is then compared with the one obtained with the analytical method. A good agreement was found.

일체형 주상용 몰드 변압기의 온도분포 및 특성 비교

The mold transformers have been widely used in underground substations of large building and have some advantages when compared with oil-transformer. Those advantages are low fire risk, environmental compatibility, compact size and high reliability. The mold transformer is generally known to have cooling duct between low voltage and high voltage coil. To achieve better compact structure and low loss, mold transformers made by one body molding method has been developed. Nevertheless, such kinds of transformer need better cooling method because heat radiation between each winding is still of problem. The life of transformer is significantly dependent on the thermal behavior in windings. Many designers have calculated temperature distribution in transformers and hot spot point by finite element method(FEM) to analyze winding temperature rise. In this paper, the temperature distribution analysis of 100 kVA pole mold transformer for power distribution were investigated by FEM program and the thermal analysis results were compared with temperature rise test.