Interaction of point defects in ferroelectrics -parameter identification and simulation

Ferroelectric materials are used in a wide field of applications, where they are exposed to a high number of mechanical and electrical load cycles. This involves degradation of the material and a decrease of the electromechanical coupling capability, which is usually called electric fatigue. The causes are assumed to be ionic and electronic charge carriers, which interact with each other, with microstructural elements in the bulk and with interfaces. Accumulation of defects can lead to degradation, mechanical damage and dissociation reactions, for more details see e.g. [3]. In order to get a better understanding of the defect accumulation processes, a model based on material forces is used in [6] to simulate the interaction of defects in periodic and in infinite cells. Applying thermodynamically reasonable kinetic laws, defect migration is simulated in a deterministic way in order to understand the general tendency of defect formations. The transversally isotropic material is modelled with linear electromechanical coupling. Here, the defect parameters used in the continuum model are obtained by fitting the results of molecular dynamics (MD) simulations to the continuous spatial fields. Transferring data from the atomic to the continuum level is a field of active research and no unique solution can be presented. On the atomic level, Coulomb–interaction causes a displacement field incompatible to an elastic solution. To address this difficulty, the volume change of a domain around the defect is used to determine defect parameters. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase-field simulation of piezoresponse force microscopy in consideration of different environmental conditions

Piezoresponse force microscopy (PFM) is a feasible tool which is widely used for investigating information of the domain structures of ferroelectrics. Nevertheless, one drawback of the technique may be that environmental conditions could effect the very small signal which is detected from the displacement of the tip. The present contribution addresses the simulation of PFM in consideration of environmental conditions. We employ a continuum-mechanical model based on the phase-field method which accounts for the transversely isotropic symmetry of the underlying material. The goal of this contribution is to analyze the environmental effect on the tip-sample interaction. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Identification of Microstructural Components of Bitumen by Means of Atomic Force Microscopy (AFM)

Accounting for the large variation of asphalt mixes, resulting from variations of constituents and composition, and from the allowance of additives, a multiscale model for asphalt is currently developed at the Christian Doppler Laboratory for “Performance‐based optimization of flexible road pavements”. The multiscale concept allows to relate macroscopic material properties of asphalt to phenomena and material properties of finer scales of observation. Starting with the characterization of the finest scale, i.e., the bitumen‐scale, Atomic Force Microscopy (AFM) is employed. Depending on the mode of measurement (tapping versus pulsed‐force mode), the AFM provides insight into the surface topography or stiffness and adhesion properties of bitumen. The obtained results will serve as input for upscaling in the context of the multiscale model in order to obtain the homogenized material behavior of bitumen at the next‐higher scale, i.e., the mastic‐scale. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An investigation of the behavior of simulation response surfaces

This paper is part of a research stream whose purpose is to study the effect of simulation response surface behavior on the choice of appropriate simulation optimization search technique. This paper's research lays some groundwork by examining the behavior of simulation response surfaces themselves. The point here is not to criticize existing simulation-optimization techniques (such as Response Surface Methodology (RSM). Rather, one point is to emphasize the care and precision that must be used to invoke extant procedures properly, while another is to demonstrate the need for additional methods such as nonparametric approaches. In particular, this paper examines a simple, inventory-simulation model under various experimental conditions, including some factors under a user's control, and some not. Both point and region estimates of surface characteristics are determined and graphed while such factors as number of replications, simulation run length, and demand and lead-time variances are varied. It is found, for example, that even for this simple surface such optimization techniques as first-order RSM can be inappropriate over 21–98% of the feasible region, depending on the case. Four implications are noted from the research: the care that should be exercised with existing simulation-optimization techniques; the need for a simulation-optimization starter; the importance of examining global, nonparametric-metamodeling approaches to simulation optimization; and the desirability of investigating a multi-strategy approach to optimization. The paper concludes with a call for further research investigating these suggestions.     

受内反馈控制的Phase-Field系统的稳定性

该文证明了受内反馈控制的phase-field系统的解在某些条件下是稳定的.     

Multiphysics finite element model for the computation of the electro-mechanical dynamics of a hybrid reluctance actuator

ABSTRACT

In hybrid reluctance actuators, the achievable closed-loop system bandwidth is affected by the eddy currents and hysteresis in the ferromagnetic components and the mechanical resonance modes. Such effects must be accurately predicted to achieve high performance via feedback control. Therefore, a multiphysics electro-mechanical finite element model is proposed in this paper to compute the dynamics of a 2-DoF hybrid reluctance actuator. An electromagnetic simulation is adopted to compute the electromagnetic dynamics and the actuation torque, which is employed as input for a structural dynamic simulation computing the electro-mechanical frequency response function. For model validation, the simulated and measured frequency response plots are compared for two actuators with solid and laminated outer yoke, respectively. In both cases, the model accurately predicts the measurement results, with a maximum relative phase error of 1.7% between the first resonance frequency and 1 kHz and a relative error of 1.5% for the second resonance frequency..     

Simulation of Atomic Force Microscopy for investigating BaTiO3 and LiMn2O4 nanostructures based on Phase Field Approach

Atomic Force Microscopy (AFM) probes the surface features of specimens using an extremely sharp tip scanning the sample surface while the force is applied. AFM is also widely used for investigating the electrically non-conductive materials by applying an electric potential on the tip. Piezoresponse Force Microscopy (PFM) and Electrochemical Strain Microscopy (ESM) are variants of AFM for different materials. Both PFM and ESM signals are obtained by observing the displacement of the tip when applying electric fields during the scanning process. The PFM technique is based on converse piezoelectric effect of ferroelectrics and the ESM technique is based on electrochemical coupling in solid ionic conductors. In this work, two continuum-mechanical formulations for simulation of PFM and ESM are discussed. In the first model, for PFM simulation, a phase field approach based on the Allen-Cahn equation for non-conserved order parameters is employed for ferroelectrics. Here, the polarization vector is chosen as order parameter. Since ferroelectrics have highly anisotropic properties, this model accounts for transversely isotropic symmetry using an invariant formulation. The polarization switching behavior under the electric field will be discussed with some numerical examples. In the simulation of ESM, we employ a constitutive model based on the work of Bohn et al. [8] for the modeling of lithium manganese dioxide LiMn₂O₄ (LMO). It simulates the deformation of the LMO particle according to an applied voltage and the evolution of lithium concentration after removing a DC pulse. The modeling results are compared to experimental data. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Porous-media simulation of bone-cement spreading during vertebroplasty

The reinforcement of porous vertebral cancellous bone by the injection of bone cement is a practical procedure for the stabilisation of osteoporotic compression fractures and other weakening lesions. This contribution concerns the reproduction and prediction of the resulting bone-cement distribution during the injection procedure by means of numerical simulation. A detailed micromechanical (locally single-phasic) model exhibits the drawback that all geometrical and physical transition conditions of the individual parts of the complex aggregate have to be known. Therefore, we rather proceed from a macroscopic (and multi-constituent) continuum-mechanical model based on the Theory of Porous Media. In this regard, the homogenisation of the underlying micro-structure results in a model of three constituents: these are the solid bone skeleton, which is saturated by the liquid bone marrow that may be displaced by the injected liquid bone cement. The influence of the micro-architecture of the pore space on the spreading of the bone cement is considered by a spatial diversification of the anisotropic permeability tensors, obtained through image processing techniques applied to medical imaging data (µCT). The numerical investigation of the strongly coupled problem enables the study of vertebroplasty and allows for the comparison between the simulation results and the experimentally determined bone-cement distribution that were imaged during injections. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The integration of analysis and testing for the simulation of the response of hyperelastic materials

A family of hyperelastic finite elements capable of modeling arbitrarily large strains for axisymmetric and plane strain analyses has been developed. Constitutive behavior is determined by the selection of a strain energy density function for which user-supplied coefficients are required. Selective reduced integration for the volumetric strain energy terms allows for successful modeling of nearly incompressible materials. Available strain energy density functions are as follows: Mooney-Rivlin, Blatz-Ko, power law, and a nine-term Mooney expansion. The Ogden Strain Energy (OSE) law has also been implemented. The OSE law defines the strain energy relationship entirely in terms of the three principal components of stretch. This differs from the approach of other strain energy formulations, such as the Mooney law in which the strain energy is written as a function of strain invariants. The OSE law as implemented in this formulation is designed to facilitate the user's task of converting physical test data to the numerical (algebraic) form required for input. The family of hyperelastic finite elements has been integrated into ANSYS Revision 4.2 via the user element interface. Numerous verification solutions have been performed. As a representative example, a comparison with a closed-form solution for a Mooney-Rivlin type material is presented. Finally, the difficulties of obtaining test data in the form of user-supplied constants is discussed in the context of the comparison of experimental measurements and analytical simulation of an elastomeric test specimen.     

Multi response simulation and optimization of gas tungsten arc welding

In the fabrication of a pressure vessel, the successful bending operation (after welding) demands higher tensile strength of weld bead. Therefore, to achieve typical tensile strength and hardness is the primary objective of this paper. Stainless steel 304 is widely used material in almost all the industrial applications, hence it is selected as a candidate material for study of tungsten inert gas (TIG) welding process. In order to produce, a high quality and reliable welding, the welding process needs to be robust in performance. A recently developed popular experimental approach known as definitive screening design (DSD) is used for process improvement. These optimum variable settings necessary for consistent welding are obtained through the application of simulation by using central composite design. The typical values of tensile strength and hardness are obtained at a low value of purging gas flow rate, filler rod dia.; intermediate values of root gap, plate thickness; and at high values of electrode dia., current, and gas flow rate.     

Phase-field modeling of the effect of interfacial energy on pyrolytic carbon morphology in chemical vapor deposition

A conserved phase-field model is proposed to investigate the effect of interfacial energy on the morphological evolution of the pyrolytic carbon deposit in chemical vapor deposition. The equilibrium geometry of carbon deposit islands is analytically predicted, of which the contact angle was controlled through the boundary conditions of the phase-field parameter at the substrate surface according to the Young-Laplace equation. Simulations of deposit growth are carried out for single and multi island nucleation. It is clarified that the island morphology depends on the magnitude of the interface energy. It is also observed that high interface energy results in large island size fluctuation. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hospital preparedness during epidemics using simulation: the case of COVID-19

Central European Journal of Operations Research - This paper presents a discrete event simulation model to support decision-making for the short-term planning of hospital resource needs, especially...     

Boundary condition difficulties encountered in the simulation of bulging during the continuous casting of steel

In modelling the continuous casting of steel it is required to simulate the movement of the solidifying steel (containing liquid steel) through the guide rolls. This paper outlines some of the difficulties involved in forcing a modelled ‘single roll pitch’ to pass underneath the exit roll and describes a practical solution.     

Thermomechanical modeling and simulation of aluminum alloys during extrusion process

The purpose of this work is to simulate the microstructure development of aluminum alloys during hot metal forming processes such as extrusion with the help of the Finite Element Method (FEM). To model the thermomechanical coupled behavior of the material during the extrusion process an appropriate material model is required. In the current work a Johnson–Cook like thermoelastic viscoplastic material model is used. To overcome the numerical difficulties during simulation of extrusion such as contact problem and element distortion an adaptive meshing system is developed and applied. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical and experimental simulation of wave formation during explosion welding

The results of numerical and experimental simulation of wave formation under an oblique impact of metal plates during explosion welding are presented. The numerical simulation was carried out on the basis of the Maxwell relaxation model and by a molecular dynamics method. In the experiments, the impact of metal plates was investigated with X-ray radiography of phenomena in front of and behind the point of contact, and the preserved samples were studied metallographically. It is shown that the numerical simulation correctly reproduces the formation and evolution of waves on the contact boundary. Simultaneously, constraints are pointed out that prohibit the use of the elastoplastic model in the impact zone of the plates starting from the moment when the material of the plate in this zone is decomposed into thin jets. In this zone, the dependence of the specific deformation energy E on the tensor C _j ⁱ is no longer described by a convex function. It is this fact that motivated the transition to the molecular dynamics model.