Realistic Finite Element Model Research Articles

Use of anisotropic modelling in electrical impedance tomography; Description of method and preliminary assessment of utility in imaging brain function in the adult human head

Electrical Impedance Tomography (EIT) is an imaging method which enables a volume conductivity map of a subject to be produced from multiple impedance measurements. It has the potential to become a portable non-invasive imaging technique of particular use in imaging brain function. Accurate numerical forward models may be used to improve image reconstruction but, until now, have employed an assumption of isotropic tissue conductivity. This may be expected to introduce inaccuracy, as body tissues, especially those such as white matter and the skull in head imaging, are highly anisotropic. The purpose of this study was, for the first time, to develop a method for incorporating anisotropy in a forward numerical model for EIT of the head and assess the resulting improvement in image quality in the case of linear reconstruction of one example of the human head. A realistic Finite Element Model (FEM) of an adult human head with segments for the scalp, skull, CSF, and brain was produced from a structural MRI. Anisotropy of the brain was estimated from a diffusion tensor-MRI of the same subject and anisotropy of the skull was approximated from the structural information. A method for incorporation of anisotropy in the forward model and its use in image reconstruction was produced. The improvement in reconstructed image quality was assessed in computer simulation by producing forward data, and then linear reconstruction using a sensitivity matrix approach. The mean boundary data difference between anisotropic and isotropic forward models for a reference conductivity was 50%. Use of the correct anisotropic FEM in image reconstruction, as opposed to an isotropic one, corrected an error of 24 mm in imaging a 10% conductivity decrease located in the hippocampus, improved localisation for conductivity changes deep in the brain and due to epilepsy by 4–17 mm, and, overall, led to a substantial improvement on image quality. This suggests that incorporation of anisotropy in numerical models used for image reconstruction is likely to improve EIT image quality.

Finite element evaluation of projectile nose angle effects in ballistic perforation of high strength fabric

The projectile nose angle is one of the parameters having a great influence on impact and penetration into high strength fabric. In order to study these effects, a realistic finite element model of TWARON fabric at yarn level is generated with eight node solid elements. LS-DYNA, a commercial finite element code is chosen as the solver. Projectile is assumed to be rigid in the simulations. The yarns, as major load carrying component of the fabric, are set to be transversely isotropic. To allow fracture in the yarns, an erosion algorithm was added to the yarn material. Eleven finite element models were built with nose angle of the projectile ranging from 30° to 180° with and difference of 15°. Simulations are performed and the effects of several parameters including energy absorption, size of the hole made by the projectile and von-Mises stress distribution for different nose angles and time steps are investigated.

An exploration of friction models for the chip–tool interface using an Arbitrary Lagrangian–Eulerian finite element model

A better understanding of friction modeling is required in order to produce more realistic finite element models of machining processes to support the goals of longer tool life and better surface quality. In this work an attempt has been made to explore and evaluate various friction models used in numerical metal cutting simulations. A finite element model, based on the ALE approach, was developed for orthogonal machining and used to study the conditions prevailing at the chip–tool interface for hardened steel. The ALE approach does not require any chip separation criteria and enables an approximate initial chip shape to smoothly evolve into a reasonable chip shape, while maintaining excellent mesh properties. The results, for a wide range of feed values, were obtained using different friction models and are compared to previously published experimental findings. A reasonable agreement was obtained between the measured and predicted forces with some discrepancy between the cutting and feed force depending on the friction model: if agreement with the cutting forces was good, then the feed force was underestimated; if the feed force agreed well, then the cutting force was overestimated. In all cases the chip thickness was well estimated but the chip–tool contact length was underestimated.

Improvement of the numerical modeling in orthogonal dry cutting of an AISI 316L stainless steel by the introduction of a new friction model

A better understanding of friction modeling is required to lead to more realistic finite element models (FEMs) of machining process. This work proposes to evaluate the performance of a new friction model depending on the local sliding velocity, compared to a standard approach considering a constant Coulomb friction coefficient. A finite element model, based on the Arbitrary Lagrangian–Eulerian (ALE) approach, was developed for orthogonal machining. This model has been applied to the investigation of an AISI 316L stainless steel in dry cutting. It has been shown that the material removal is very sensitive to the friction model. The flow of the workmaterial along the interfaces tool–workpiece–chip is very much disturbed by a variation of the friction model. The new friction model, previously published and introduced in this model, has shown its higher efficiency to reach reasonable data about the material removal phenomena.

Design of electrodes and current limits for low frequency electrical impedance tomography of the brain

For the novel application of recording of resistivity changes related to neuronal depolarization in the brain with electrical impedance tomography, optimal recording is with applied currents below 100 Hz, which might cause neural stimulation of skin or underlying brain. The purpose of this work was to develop a method for application of low frequency currents to the scalp, which delivered the maximum current without significant stimulation of skin or underlying brain. We propose a recessed electrode design which enabled current injection with an acceptable skin sensation to be increased from 100 muA using EEG electrodes, to 1 mA in 16 normal volunteers. The effect of current delivered to the brain was assessed with an anatomically realistic finite element model of the adult head. The modelled peak cerebral current density was 0.3 A/m(2), which was 5 to 25-fold less than the threshold for stimulation of the brain estimated from literature review.

Use of statistical parametric mapping (SPM) to enhance electrical impedance tomography (EIT) image sets

Use of statistical parametric mapping (SPM), which is widely used in analysis of neuroimaging studies with fMRI and PET, has the potential to improve quality of EIT images for clinical use. Minimal modification to SPM is needed, but statistical analysis based on height, not extent thresholds, should be employed, due to the 20–80% variation of the point spread function, across EIT images. SPM was assessed in EIT images reconstructed with a linear time difference algorithm utilizing an anatomically realistic finite element model of the human head. Images of the average of data sets were compared with those produced using SPM over 10–40 individual image data sets without averaging. For a point disturbance, a sponge 15% of the diameter of an anatomically realistic saline-filled tank including a skull, with a contrast of 15%, and for visual evoked response data in 14 normal human volunteers, images produced with SPM were less noisy than the average images. For the human data, no consistent physiologically realistic changes were seen with either SPM or direct reconstruction; however, only a small data set was available, limiting the power of the SPM analysis. SPM may be used on EIT images and has the potential to extract improved images from clinical data series with a low signal-to-noise ratio.

Inverse sensitivity to approximate dipolar source models

Abstract The current dipole is a widely used source model in forward and inverse EEG and MEG applications. Analytic solutions to the governing field equations have been developed for several approximations of the human head using ideal dipoles as the source model. Numeric approaches such as the finite element method have become popular because they allow the use of anatomically detailed head models. Although numeric methods can represent more realistic domains, the sources in such models are often an approximation of the ideal dipole. We previously compared the electromagnetic fields produced by alternative models of dipolar sources in finite element models of the head to the fields produced by infinitesimal dipolar sources in analytic models of the head. That study found that the practice of projecting current sources to the nearest nodes of a finite element mesh was inaccurate. The “subtraction approach” was found to be very accurate, but can be difficult to apply, especially with commercial solver codes. That work also presented an approximate formulation that provided reasonably good results, based on the Laplace equivalence between voltage and current sources. In this article we verify the applicability of the Laplace dipole approximation to inverse problems using a realistic finite element model of the head.

Numerical approaches for dipole modeling in finite element method based source analysis

In EEG/MEG source analysis, a mathematical dipole is widely used as the “atomic” structure of the primary current distribution. When using realistic finite element models for the forward problem, the current dipole introduces a singularity on the right-hand side of the governing differential equation that has to be treated specifically. We evaluated and compared three different numerical approaches, a subtraction method, a direct approach using partial integration and a direct approach using the principle of Saint Venant. Evaluation and comparison were carried out in a four-layer sphere model using quasi-analytical formulas.

3D modelling of sprayed-concrete-lined tunnels in clay

Realistic finite element (FE) modelling of the tunnel construction process has been restricted by the computational effort needed to incorporate the three-dimensional (3D) aspects of the tunnelling operation. Two-dimensional (2D) plane-strain models have therefore been widely adopted by practitioners and researchers in their studies of the effects of tunnelling. However, 2D modelling suffers the drawback whereby a volume loss must be assumed at the outset as the input for the FE model. This paper presents an analysis of measured surface ground movements induced by a sprayed concrete lined tunnel in London Clay. This was achieved using computationally efficient, non-linear, 3D modelling and Oasys LS-DYNA software. By modelling the problem using a non-linear small strain BRICK soil model, which incorporates anisotropic behaviour, good agreement with the actual Gaussian distribution of surface settlement was obtained. This was achieved without any initial assumptions regarding volume loss, but rather by realistically modelling the excavation sequence and time-dependent gain in lining strength. On the basis of this work it is considered that 3D modelling techniques can provide an effective means to estimate ground settlement and the associated effects on nearby structures, without dependence on empirical methods alone.

Experimental and finite element analysis to predict the dimensional changes of a cold-forged spur gear

The dimensional tolerance of spur gears for automobiles is within the range of several micrometres. Therefore, to manufacture these gears by cold forging, dimensions should be controlled in the same range. The dimensional changes by the various forging variables should be analysed and compensated into the forging tool. Computer-aided engineering (CAE) would be appropriate for investigating the effects of variables. However, the realistic finite element modelling (FEM) techniques should be confirmed and used to predict the dimension of the forged part in the order of micrometres. As realistic modelling considers the elastic characteristics of both workpiece and die at unloading and ejecting stage, threedimensional analysis for spur gears may often encounter difficulties in solving time and convergence. In this study, dimensional changes for cold-forged parts of spur gears were investigated by experimental and FEM analysis, after modelling to obtain desirable FEM results had been studied by closed-die upsetting. The dimensional changes of involute curve for the spur gear can be analysed from the FEM results during the whole forging stage; and give an index of a modified involute curve of the forging tool for improving gear profile accuracy.

A novel approach to compute the impedance matrix of a cochlear implant system incorporating an electrode-tissue interface based on finite element method

This paper proposes to use a new and realistic finite element model of a single turn human cochlea and cochlear implant electrode array with electrode-tissue interface to model the electric field imaging technique, which creates an impedance matrix. Due to discrepancy in the Warburg capacitor in the active and nonactive electrode contacts and electric power lost along the cochlea, the diagonal terms of the impedance matrix is larger than the off diagonal terms, which represent the impedance values of the nonactive electrode contacts. The impedance matrix from electric field imaging measurements is compared with the simulation results

A comparison of two models of electrodes for ECT simulations

This paper discusses some numerical aspects of the simulation of electroconvulsive therapy (ECT). A realistic finite-element model of the human head is used to discuss two approaches to modeling the electrodes applied to human head skin. The first approach models the electrode by a mixed-boundary condition, while the second one uses additional subdomain imitating electrode-to-skin contact for that purpose [three-dimensional (3-D) model]. An algorithm of grid modification used to add an external subdomain modeling the electrode contact resistance is presented. The authors examine the influence of the electrode model on the convergence speed of the iterative solver. The authors state that the 3-D model is better conditioned, and, thus, it converges faster

Identification of viscoplastic material parameters from spherical indentation data: Part I. Neural networks

In this paper, a new method for the identification of material parameters is presented. Neural networks, which are trained on the basis of finite element simulations, are used to solve the inverse problem. The material parameters to be identified are part of a viscoplasticity model that has been formulated for finite deformations and implemented in the finite element code ABAQUS. A proper multi-creep loading history was developed in a previous paper using a phenomenological model for viscoplastic spherical indentation. Now, this phenomenological model is replaced by a more realistic finite element model, which provides fast computation and numerical solutions of high accuracy at the same time. As a consequence, existing neural networks developed for the phenomenological model have been extended from a power law hardening with two material parameters to an Armstrong–Frederick hardening rule with three parameters. These are the yield stress, the initial slope of work hardening, and maximum hardening stress of the equilibrium response. In addition, elastic deformation is taken into account. The viscous part is based on a Chaboche-like overstress model, consisting of two material parameters determining velocity dependence and overstress as a function of the strain rate. The method has been verified by additional finite element simulations. Its application for various metals will be presented in Part II, [J. Mater. Res.21, 677 (2006)].

Subject-Specific Finite Element Model of the Pelvis: Development, Validation and Sensitivity Studies

A better understanding of the three-dimensional mechanics of the pelvis, at the patient-specific level, may lead to improved treatment modalities. Although finite element (FE) models of the pelvis have been developed, validation by direct comparison with subject-specific strains has not been performed, and previous models used simplifying assumptions regarding geometry and material properties. The objectives of this study were to develop and validate a realistic FE model of the pelvis using subject-specific estimates of bone geometry, location-dependent cortical thickness and trabecular bone elastic modulus, and to assess the sensitivity of FE strain predictions to assumptions regarding cortical bone thickness as well as bone and cartilage material properties. A FE model of a cadaveric pelvis was created using subject-specific computed tomography image data. Acetabular loading was applied to the same pelvis using a prosthetic femoral stem in a fashion that could be easily duplicated in the computational model. Cortical bone strains were monitored with rosette strain gauges in ten locations on the left hemipelvis. FE strain predictions were compared directly with experimental results for validation. Overall, baseline FE predictions were strongly correlated with experimental results (r2=0.824), with a best-fit line that was not statistically different than the line y=x (experimental strains = FE predicted strains). Changes to cortical bone thickness and elastic modulus had the largest effect on cortical bone strains. The FE model was less sensitive to changes in all other parameters. The methods developed and validated in this study will be useful for creating and analyzing patient-specific FE models to better understand the biomechanics of the pelvis.

ADDED VISCOUS DAMPERS IN SHEAR-TYPE STRUCTURES: THE EFFECTIVENESS OF MASS PROPORTIONAL DAMPING

In this paper the authors investigate the dynamic performances of multi-degree-of-freedom shear-type structures equipped with manufactured viscous dampers. The system of added viscous dampers that leads to a mass proportional damping (MPD) matrix is identified as the one providing the best overall performance in terms of minimum top-storey response variance to a Gaussian band limited white noise input within the class of classically damped systems. A performance index developed by the authors and based upon modal damping ratios confirms that, within the class of Rayleigh damping systems, the MPD limiting case provides the largest damping effect. For a 6-d.o.f. system, a comparison with results available in literature also shows that the MPD system provides better performance than optimised (as suggested by other research works) non-classical damping schemes. A physical explanation for the optimum damping performance offered by MPD systems is also provided herein, together with clear indications about how to implement MPD systems in real building structures. A check on the applicability of MPD systems to real structures and its effectiveness is also carried out successfully through the development and study of a realistic finite element model of an actual 18-storey building.

Image-Based Characterization and Finite Element Analysis of Porous SMA Behavior

Abstract Porous shape memory alloys (SMAs) are a relatively new group of materials that are of interest because of their potential use in the design of damping and shock mitigation systems. Benefits of the material include reduced weight, high level of energy absorption through phase transformation and possible increased energy absorption through wave scattering due to porosity. Essential to the use of these materials is an understanding of the structural and shock absorbing response of the material. Constitutive models that accurately represent these characteristics are necessary. The emphasis of this research is to develop a computational methodology that will bridge the mesostructural and macrostructural features of porous SMAs. The first step in the process involves the detailed characterization of the relevant mesostructure, i.e., information about pore shape, size, volume fraction and distribution. This representative characterization can be used to produce realistic image-based finite element models. Because the resultant models have large degrees of freedom they cannot be employed to analyze large-scale structural problems. However, simply designed boundary value problems such as the dynamic uniaxial compressive loading of a bar can be used as benchmarks for the verification of phenomenological macro-constitutive models, or models that are derived using averaging methods such as the Mori-Tanaka method or the self-consistent method. In this study, an attempt is made to analyze numerically porous SMA behavior under dynamic conditions based on the representative mesostructural features. Preliminary results are obtained for selected pore volume fractions and distinct trends in material behavior are observed.

Developing integrative computational models of pulmonary structure.

Integrative computational modeling of the pulmonary system aims to incorporate interactions between the lung's subsystems by means of a hierarchy of structural and functional models. This requires detailed imaging-based data, along with a wide range of functional information from experiments. Advances in computed tomography imaging technology ensure that high-resolution data are now readily available upon which the structure of these models can be based. We present methods for constructing anatomically realistic finite element models of interrelated pulmonary structures from such data. Segmented human lung lobe data are fit to high-order (cubic Hermite) volume elements. Meshes for the conducting airways and pulmonary arteries and veins are constructed within the lobe mesh, using a combination of fitting to imaging data and a bifurcating-distributive algorithm. The algorithm generates an airway-consistent mesh within a host volume, and this airway mesh is then used as a template for generating blood vessel models. The lung parenchyma is modeled as a space-filling three-dimensional (3D) Voronoi mesh, with generated geometry consistent with the alveolated airway structure. Pulmonary capillaries are generated over the alveolar model, as a 2D Voronoi mesh. These structural models have been compared extensively with morphometric data to verify that their geometry is representative of the pulmonary system. The models are designed to be integrative: they relate multiple structural systems within the same individual, and their use as computational meshes allows application of spatially distributed properties.

Analysis of the Worst Mistuning Patterns in Bladed Disk Assemblies

The problem of determining the worst mistuning patterns is formulated and solved as an optimization problem. Maximum resonant amplitudes searched across the many nodes of a large-scale finite element model of a mistuned bladed disk and across all the excitation frequencies in a given range are combined into an objective function. Individual blade mistuning is controlled by varying design parameters, whose variation range is constrained by manufacture tolerances. Detailed realistic finite element models, which have so far only been used for analyzing tuned bladed disks, are used for calculation of the forced resonant response of mistuned assemblies and for determination of its sensitivity coefficients with respect to mistuning variation. Results of the optimum search of mistuning patterns for some practical bladed disks are analyzed and reveal higher worst cases than those found in previous studies.

Performance of wall-stud cold-formed shear panels under monotonic and cyclic loading: Part II: Numerical modelling and performance analysis

The main components to provide earthquake performance of a light-gauge steel house are the shear walls. Understanding shear wall behaviour and finding suitable hysteretic models is important in order to be able to build realistic finite element models and assess structural performance in case of earthquake. As for any building structure expected to exceed its elastic behaviour-range in case of earthquake, the interaction of design capacity, load bearing capacity and structural ductility will influence the performance. In this paper alternative design methods and hysteretic modeling techniques are presented. Based on tests described in Part I, a numerical equivalent model for hysteretic behavior of wall panels working in shear was built and used in 3D dynamic nonlinear analysis of cold-formed steel framed buildings. Preliminary conclusions refer to the effect of over-strength and ductility upon possible earthquake load reduction in case of light-gauge shear wall structures.

Overview of Mathematical Computer Models of Striated Sphincter Muscles

The present paper compares current mathematical striated sphincter models. Current models are subdivided in four categories: (1) simple models, (2) implementations of the urethral resistance relation, (3) models with realistic muscle dynamics and (4) finite element models. In our research group a neural network model, representing Onuf' nucleus, the spinal motor nucleus that innervates the external urethral and anal sphincters, was developed. A realistic sphincter model is needed to test the neural network. To decide whether or not a model is applicable in our research two requirements should be fulfilled: (1) the presence of realistic muscle dynamics preferably by implementation of a Huxley type muscle model and (2) the model should consist of more than one muscle unit to form a more dimensional model. Reviewing the literature, if a myogenic sphincter is modelled, mainly the Hill-equation is applied. Moreover, single muscle unit models are published. In general a multi-unit muscle model of the sphincter is lacking, prohibiting the study of the inherent properties of sphincter muscles, which could give information on the realistic behaviour of elements in circular muscles. It is concluded that the functionality of current sphincter models is limited for our purpose.