Finite element simulations of ultrasonically assisted turning

Ultrasonically assisted turning is a promising machining technology, where high frequency vibration (f≈20 kHz) with an amplitude a≈10 μm is superimposed on the movement of the cutting tool. Ultrasonic turning yields a noticeable decrease in cutting forces, heat and noise radiation, as well as a superior surface finish, comparing to the conventional machining technology. The present study utilizes both experimental techniques and numerical (finite element) simulations to analyze the microstructural processes at the cutting tool–chip interface. High-speed filming of the chip–tool interaction zone during cutting and microstructural and nanoindentation analyses of the machined surfaces are used to compare process zones and deformation processes for both conventional and ultrasonically assisted technologies. The suggested finite-element (FE) model, which utilizes MSC Marc/Mentat general FE code, provides a transient analysis for an elasto-plastic material, accounting for the frictionless contact interaction between a cutter and workpiece as well as material separation in front of the cutting edge. A detailed analysis of cutting for a single cycle of ultrasonic vibration is carried out for isothermal conditions. Differences between conventional and ultrasonic turning in stress distribution in the process zone and contact conditions at the tool/chip interface are investigated.     

Finite element simulations of thin-film composite BAW resonators

A finite element method (FEM) formulation is presented for the numerical solution of the electroelastic equations that govern the linear forced vibrations of piezoelectric media. A harmonic time dependence is assumed. Both of the approaches, that of solving the field problem (harmonic analysis) and that of solving the corresponding eigenvalue problem (modal analysis), are described. A FEM software package has been created from scratch. Important aspects central to the efficient implementation of FEM are explained, such as memory management and solving the generalized piezoelectric eigenvalue problem. Algorithms for reducing the required computer memory through optimization of the matrix profile, as well as Lanczos algorithm for the solution of the eigenvalue problem are linked into the software from external numerical libraries. Our FEM software is applied to detailed numerical modeling of thin-film bulk acoustic wave (BAW) composite resonators. Comparison of results from 2D and full 3D simulations of a resonator are presented. In particular, 3D simulations are used to investigate the effect of the top electrode shape on the resonator electrical response. The validity of the modeling technique is demonstrated by comparing the simulated and measured displacement profiles at several frequencies. The results show that useful information on the performance of the thin-film resonators can be obtained even with relatively coarse meshes and, consequently, moderate computational resources     

Finite element simulations of poly-crystalline shape memory alloys based on a micromechanical model

We present a finite element implementation of a micromechanically motivated model for poly-crystalline shape memory alloys, based on energy minimization principles. The implementation allows simulation of anisotropic material behavior as well as the pseudo-elastic and pseudo-plastic material response of whole samples. The evolving phase distribution over the entire specimen is calculated. The finite element model predicts the material properties for a relatively small number of grains. For different points of interest in the specimen the model can be consistently evaluated with a significantly higher number of grains in a post-processing step, which allows to predict the re-orientation of martensite at different loads. The influence of pre-texture on the material’s properties, due to some previous treatment like rolling, is discussed.     

Finite element simulations of notch tip fields in magnesium single crystals

Recent experiments using three point bend specimens of Mg single crystals have revealed that tensile twins of \(\{10\bar{1}2\}\) -type form profusely near a notch tip and enhance the fracture toughness through large plastic dissipation. In this work, 3D finite element simulations of these experiments are carried out using a crystal plasticity framework which includes slip and twinning to gain insights on the mechanics of fracture. The predicted load–displacement curves, slip and tensile twinning activities from finite element analysis corroborate well with the experimental observations. The numerical results are used to explore the 3D nature of the crack tip stress, plastic slip and twin volume fraction distributions near the notch root. The occurrence of tensile twinning is rationalized from the variation of normal stress ahead of the notch tip. Further, deflection of the crack path at twin–twin intersections observed in the experiments is examined from an energy standpoint by modeling discrete twins close to the notch root.     

Effect of particle agglomeration on the magnetization reversal of fine powders

The magnetization reversal of loose low-coercivity fine powders of magnetite or iron is independent of the value of the particle packing fraction. However, when these powders are compacted under pressure in a solid matrix of Perspex, the coercivity of the samples increases logarithmically with the increasing powder density. We assume that in these powders small superparamagnetic particles are agglomerated into clusters. Supposing the magnetization process to be due to a nucleation mechanism, the critical field at which this nucleation starts appears to be a function of the number of particles agglomerated in a cluster. This number varies with the density of the powder compacted in the matrix. By the suggested theoretical model, we have estimated the diameter of the superparamagnetic particles of the experimentally investigated fine iron powder to be approximately 46 Å. Valuable information about the magnetic structure of these powders has been obtained from the studies of the hyperfine splitting in their Mössbauer patterns and from those of the changes in the line intensities caused by the application of an external magnetic field.     

Finite element crash simulations of the human body: Passive and active muscle modelling

Conventional dummy based testing procedures suffer from known limitations. This report addresses issues in finite element human body models in evaluating pedestrian and occupant crash safety measures. A review of material properties of soft tissues and characterization methods show a scarcity of material properties for characterizing soft tissues in dynamic loading. Experiments imparting impacts to tissues and subsequent inverse finite element mapping to extract material properties are described. The effect of muscle activation due to voluntary and non-voluntary reflexes on injuries has been investigated through finite element modelling.     

Micromagnetic simulation on dynamics of spin transfer torque magnetization reversal

I study the magnetization dynamics induced by spin transfer torque in CoFe/Ru/CoFe/Cu/NiFe nano-pillars using LLG micromagnetic simulations. The required current for spin transfer torque magnetization reversal was investigated with the switching speed and the temperature. The required current in rectangular nanopillars is much larger than that in elliptical nanopillers. The temperature dependence is pretty complicated. The antiparallel to parallel magnetization reversal at 77 K requires a smaller current than at 300 K. The parallel to antiparallel magnetization reversal at 77 K requires a smaller current than at 300 K only when the pulse duration time is very short.     

Peculiarities of the magnetization reversal in heterophase nanocomposite permanent magnets

We have studied the process of magnetization reversal in a thin-film Fe/Sm₂Co₇ exchange coupled bilayer structure under the action of an in-plane external field. An analysis of the local magnetization changes, as measured using the magnetooptical indicator film technique, showed that the magnetization reversal proceeds by inhomogeneous rotation of the magnetic moments in Fe and SmCo layers, both in plane and in the perpendicular direction. It is established that, because of the exchange interaction between layers, the magnetization reversal along the easy axis in the entire structure is determined primarily by the formation of exchange-induced spin helices and domain walls in the magnetically soft layer, whereas the magnetization reversal at an angle of α with respect to the easy axis plays a significant role in the magnetically hard layer and becomes dominating for α=90°.     

Finite element simulations of crack propagation in functionally graded materials under flexural loading

Cracks in stepped and continuously graded material specimens under flexural loading were investigated via finite element analysis. Calculation of mechanical energy release rates and propagation angles with crack-opening displacement correlation and the local symmetry (K_II = 0) criterion, respectively, provided results most efficiently and accurately, as compared with compliance and J-integral approaches and other deflection criteria. A routine was developed for automatic crack extension and remeshing, enabling simulation of incremental crack propagation. Effects of gradient profile and crack geometry on crack-tip stresses and crack propagation path are examined, and implications of these for optimal design of graded components against failure by fast fracture are discussed.     

Finite element simulations of elasto-plastic processes in Nb3Sn strands

The manufacturing of Nb₃Sn strands, with drawing and annealing of multifilamentary strands followed by a heat treatment at about 900 K to form the Nb₃Sn by reaction of tin and niobium, has the potential to create a complex internal stress system. The strain sensitivity of the Nb₃Sn superconducting properties makes prediction of the internal stresses a necessary step to understanding the performance of Nb₃Sn conductors under the magnetic load conditions experienced in a coil. An elasto-plastic one dimensional finite element model, including temperature dependent stress-strain curves, annealing and manufacturing process stresses, is used to derive the internal stresses of Nb₃Sn strands. The model is benchmarked against a range of experimental data, including stress-strain tensile tests, superconducting critical current-strain tests, and length changes through heat treatment and through a 4 K thermal cycle. The model can predict all the experimental features and shows a number of unexpected conclusions regarding the origin of the Nb₃Sn stresses.     

Effects of surface topography on magnetization reversal of magnetic thin films

The influence of the created surface roughness on the coercivity of magnetic thin films has been investigated. The magnetic thin films (CoFe and alternatively NiFe) are sputtered on top of smooth substrates that were previously covered with an array of considerably rougher lines with one of these materials Pt, Cu, CoFe, and NiFe. The lines have been patterned using optical lithography into arrays that are deposited with different thicknesses varying between 5 nm-15 nm. The lines have been designed to have a very rough edge and seated in two different angles relative to the wafer edge (zero and 45 degrees). Magneto-optic Kerr effect (MOKE) measurements showed two distinct switching fields in the hysteresis loops that are due to magnetic domain wall trapping created by the surface roughness. The magnetization reversal showed a strong dependence on the height, the orientation angle, and the material's type of the created surface roughness (the lines).     

Finite element methods for the non‐linear mechanics of crystalline sheets and nanotubes

The formulation and finite element implementation of a finite deformation continuum theory for the mechanics of crystalline sheets is described. This theory generalizes standard crystal elasticity to curved monolayer lattices by means of the exponential Cauchy–Born rule. The constitutive model for a two‐dimensional continuum deforming in three dimensions (a surface) is written explicitly in terms of the underlying atomistic model. The resulting hyper‐elastic potential depends on the stretch and the curvature of the surface, as well as on internal elastic variables describing the rearrangements of the crystal within the unit cell. Coarse grained calculations of carbon nanotubes (CNTs) are performed by discretizing this continuum mechanics theory by finite elements. A smooth discrete representation of the surface is required, and subdivision finite elements, proposed for thin‐shell analysis, are used. A detailed set of numerical experiments, in which the continuum/finite element solutions are compared to the corresponding full atomistic calculations of CNTs, involving very large deformations and geometric instabilities, demonstrates the accuracy of the proposed approach. Simulations for large multi‐million systems illustrate the computational savings which can be achieved. Copyright © 2003 John Wiley & Sons, Ltd.     

Cyclic dynamic magnetization reversal in system of magnetic dipoles

Dynamics of the magnetic moment of a circular ring system of spherical magnetic bodies has been numerically simulated. Conditions necessary for the cyclic magnetization reversal in this system by means of the sequential excitation of various oscillatory regimes (including a phase with zero total magnetic moment) are determined using bifurcation diagrams.     

High-pressure behaviors of carbon nanotubes

In this paper, we have reviewed the experimental and theoretical studies on pressure-induced polygonization, ovalization, racetrack-shape deformation, and polymerization of carbon nanotubes (CNTs). The corresponding electronic, optical, and mechanical changes accompanying these behaviors have been discussed. The transformations of armchair (n, n) CNT bundles (n = 2, 3, 4, 6, and 8) under hydrostatic or nonhydrostatic pressure into new carbons, including recently proposed superhard bct-C₄, Cco-C₈, and B-B₁A_L2R2 carbon phases have also been demonstrated. Given the diversity of CNTs from various chiralities, diameters, and arrangements, pressure-induced CNT polymerization provides a promising approach to produce numerous novel metastable carbons exhibiting unique electronic, optical, and mechanical characteristics.     

Finite element simulations of stretch-blow moulding with experimental validation over a broad process window

Injection stretch blow moulding is a well-established method of forming thin-walled containers and has been extensively researched for numerous years. This paper is concerned with validating the finite element analysis of the free-stretch-blow process in an effort to progress the development of injection stretch blow moulding of poly(ethylene terephthalate). Extensive data was obtained experimentally over a wide process window accounting for material temperature and air flow rate, while capturing cavity pressure, stretch-rod reaction force and preform surface strain. This data was then used to assess the accuracy of the correlating FE simulation constructed using ABAQUS/Explicit solver and an appropriate viscoelastic material subroutine. Results reveal that the simulation is able to give good quantitative correlation for conditions where the deformation was predominantly equal biaxial whilst qualitative correlation was achievable when the mode of deformation was predominantly sequential biaxial. Overall the simulation was able to pick up the general trends of how the pressure, reaction force, strain rate and strain vary with the variation in preform temperature and air flow rate. The knowledge gained from these analyses provides insight into the mechanisms of bottle formation, subsequently improving the blow moulding simulation and allowing for reduction in future development costs.     

Finite element analysis of a multilayer piezoelectric actuator taking into account the ferroelectric and ferroelastic behaviors

Ferroelasticity and ferroelectricity are the non-linear behaviors exhibited by piezoceramics, especially in the case of high electric field or stress. Many studies have focused on the role of ferroelastic and ferroelectric switching in fracture of actuators. However, engineering reliability analyzes are carried out with tools like finite element software that do not take into account these non-linear phenomena. To overcome such a problem, a simplified phenomenological constitutive law describing the non-linear behavior of piezoceramics has been developed and implemented in the commercial software ABAQUS. This finite element tool is used to study the effects of applied voltage on the electroelastic field concentrations ahead of electrodes in a multilayer piezoelectric actuator. The study lies on the experimental observations made by Shindo et al. [Y. Shindo, M. Yoshida, F. Narita, K. Horiguchi, Electroelastic field concentrations ahead of electrodes in multilayer piezoelectric actuators: experiment and finite element simulation, J. Mech. Phys. Solids 52 (2004) 1109–1124]. Electroelastic analysis on piezoceramics with surface electrode showed that high values of stress and electric displacement arose in the neighborhood of the electrode tip. Thus, the strain, stress and electric displacement concentrations were calculated and the numerical results showed that ferroelectric switching arose in the area of the electrode tip, causing a change in remnant polarization and remnant strain.     

Effective medium theory of magnetization reversal in magnetically interacting particles

We report an effective medium theory of magnetization reversal and hysteresis in magnetically interacting particles, where the intergranular magnetostatic interaction is accounted for by an effective medium approximation. We introduce two dimensionless parameters, /spl lambda/ and h/sub 0/, that completely characterize the hysteresis in a ferromagnetic polycrystal when the grain size is much larger than the exchange length so that the exchange coupling can be ignored. The competition between the anisotropy energy and the intergranular magnetostatic energy is measured by /spl lambda/, while the competition between the anisotropy energy and Zeeman's energy is measured by h/sub 0/. The hysteresis loop, magnetostatic energy density, and anisotropy energy density calculated by using this theory agrees well with micromagnetic simulations. The calculations also reveal that the subnucleation field switching due to the magnetic field fluctuation is important when the magnet is not very hard, and that has been accounted for by a probability-based switching model.     

碳纳米管增强镁基复合材料热残余应力的有限元分析

为了探寻Ni层厚度对镀镍碳纳米管增强AZ91D镁基复合材料(Ni-CNTs/AZ91D)中热残余应力的影响, 在实验基础上, 建立不同Ni层厚度时Ni-CNTs/AZ91D复合材料的有限元模型, 模拟了Ni-CNTs/AZ91D复合材料中热残余应力的分布。研究发现: 在碳纳米管表面镀镍能够明显降低Ni-CNTs/AZ91D复合材料中的热残余应力。Ni-CNTs/AZ91D复合材料中, 热残余应力在Ni层厚度为6nm时最小; Ni层厚度由2nm增至6nm时, 热残余应力随着Ni层厚度的增加而减小; 当Ni层厚度超过6 nm时热残余应力随着Ni层厚度的增加而增大。复合材料中热残余应力的最大值随碳纳米管表面Ni层厚度的增加向Ni层与基体的界面移动。      

Cavity-enhanced magnetooptical observation of magnetization reversal in individual single-domain nanomagnets

Optical studies of nanoscale magnets promise access to ultrafast magnetization dynamics but are challenging because of limited spatial resolution. We demonstrate that cavity enhancement of the magnetooptical Kerr effect increases the sensitivity in nanomagnetooptics significantly. Magnetization switching in individual single-domain magnets in both far-field and near-field Kerr microscopy is observed, and scaling laws are determined. Near-field signals remain nearly constant with reduced magnet diameter, indicating favorable scaling of near-field magnetooptics into the deep nanometer range.