GEOMECHANICS AND TUNNELLING, Issue 4 2009
Entwurfskonzept für eine hybride U-Bahnstation Union Square/Market Street in San Francisco
Abstract The new Central Subway extension through downtown San Francisco consists of three underground stations and 2.7 km TBMdriven twin tunnel. This paper provides a description of the preliminary analyses and design of the ground support and final lining for the Union Square\Market Street Station (UMS) along Stockton Street. This station will serve the Union Square Shopping District and connect to the BART Powell Street Station. Due to shortage of space above ground and to minimize surface disruption, the UMS station design requires a complex hybrid method consisting of a 20 m deep braced cut-and-cover box with a mined enlargement bulb below it with a height of 9.3 m and a width of 17.8 m. The majority of the UMS station will be excavated in saturated alluvial deposits. Undifferentiated old bay deposits will be encountered in the invert, underlain by dense marine sands. The groundwater varies from 5 to 10 m below ground level, so uplift of the combined bulb/box structure has to be taken into account. The Finite Element (FE) analysis of the UMS station cavern reflects the separate construction phases of the station platform box and the bulb to account for soil-structure interaction and load-sharing effects. FE analyses are used to estimate support requirements including ground improvement and to predict surface settlements. Die Erweiterung der Central Subway durch die Innenstadt von San Francisco beinhaltet drei Stationsbauwerke und 2,7 km maschinell vorgetriebene Doppelröhrentunnel. In diesem Artikel erfolgt eine Beschreibung der Voruntersuchungen und Vorbemessung der Stützmaßnahmen sowie der Innenschale der Union Square\Market Street Station (UMS) im Verlauf der Stockton Street. Diese Station soll dem Union Square Shopping Distrikt dienen und zur BART Powell Street Station verbinden. Aufgrund der beengten Platzverhältnisse und zur Minimierung der Beeinträchtigung der Oberfläche ist ein "hybrides" Konzept der UMS-Station erforderlich. Dieses besteht aus einer 20 m tiefen ausgesteiften Baugrube (Box) und einer darunterliegenden bergmännisch hergestellten Kaverne (Bulb) mit 9,3 m Höhe und 17,8 m Breite. Der Großteil der UMS-Station befindet sich in gesättigten alluvialen Ablagerungen. Undifferenziert werden alte Bucht-Ablagerungen und dichte marine Sande in der Sohle vorgefunden. Der Grundwasserspiegel variiert in einer Teufe zwischen 5 bis 10 m unter der Oberfläche, aus diesem Grund ist der Auftrieb des kombinierten Bauwerks bestehend aus Bulb und Box zu berücksichtigen. In Finite Element (FE) Berechnungen der UMS-Station werden die einzelnen Bauphasen des Stationsbauwerks, sowohl von Box als auch Bulb, modelliert, um die Wechselwirkungen von Baugrund-Bauwerk und die jeweiligen Lastumlagerungen zu berücksichtigen. Mittels FE-Berechnungen werden schließlich die notwendigen Stützmaßnahmen , diese beinhalten auch Bodenverbesserungsmaßnahmen , und die Oberflächensetzungen festgelegt. [source]

From mixed finite elements to finite volumes for elliptic PDEs in two and three dimensions

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 3 2004
Anis Younes
Abstract The link between Mixed Finite Element (MFE) and Finite Volume (FV) methods applied to elliptic partial differential equations has been investigated by many authors. Recently, a FV formulation of the mixed approach has been developed. This approach was restricted to 2D problems with a scalar for the parameter used to calculate fluxes from the state variable gradient. This new approach is extended to 2D problems with a full parameter tensor and to 3D problems. The objective of this new formulation is to reduce the total number of unknowns while keeping the same accuracy. This is achieved by defining one new variable per element. For the 2D case with full parameter tensor, this new formulation exists for any kind of triangulation. It allows the reduction of the number of unknowns to the number of elements instead of the number of edges. No additional assumptions are required concerning the averaging of the parameter in hetero- geneous domains. For 3D problems, we demonstrate that the new formulation cannot exist for a general 3D tetrahedral discretization, unlike in the 2D problem. However, it does exist when the tetrahedrons are regular, or deduced from rectangular parallelepipeds, and allows reduction of the number of unknowns. Numerical experiments and comparisons between both formulations in 2D show the efficiency of the new formulation. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Modeling of Active Noise and Vibration Control with Finite Elements and Boundary Elements

PROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2009
Stefan Ringwelski
A recently developed coupled finite element-boundary element modeling scheme for the design of active noise and vibration control of multi-coupled structural-acoustic systems is presented. The approach allows the computation of structural vibrations and resulting sound fields. By means of an example, the paper describes the theoretical background of the coupled approach. In order to show the performance of the developed approach, test simulations are carried out in the frequency domain. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Two-dimensional finite element thermal modeling of an oil-immersed transformer

EUROPEAN TRANSACTIONS ON ELECTRICAL POWER, Issue 6 2008
Jawad Faiz
Abstract Finite element (FE) modeling of a typical transformer indicates that the hot spot position is always on the top most part of the transformer. The hot spot temperature of winding depends on the load and the type of loading and is changed by loading. A number of the generated magnetic flux lines of windings close to their paths perpendicular to the internal channel of the windings and therefore the flux density in the middle of the channel is considerably larger than the beginning and ending of the winding. Two models of windings are employed and different temperature distributions are obtained. The computation results show that the time constant of high voltage (HV) winding is lower than that of the low voltage (LV) winding. A good agreement between the test and computed results has been achieved. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Assessment of acceleration modelling for fluid-filled porous media subjected to dynamic loading

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 2 2008
B. Lenhof
Abstract The purpose of this paper is to examine the importance of different possible simplifying approximations when performing numerical simulations of fluid-filled porous media subjected to dynamic loading. In particular, the relative importance of the various acceleration terms for both the solid and the fluid, especially the convective contribution, is assessed. The porous medium is modelled as a binary mixture of a solid phase, in the sense of a porous skeleton, and a fluid phase that represents both liquid and air in the pores. The solid particles are assumed to be intrinsically incompressible, whereas the fluid is assigned a finite intrinsic compressibility. Finite element (FE) simulations are carried out while assuming material properties and loading conditions representative for a road structure. The results show that, for the range of the material data used in the simulations, omitting the relative acceleration gives differences in the solution of the seepage velocity field, whereas omitting only the convective term does not lead to significant differences. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Finite element and finite volume simulation and error assessment of polymer melt flow in closed channels

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 11 2006
M. Vaz Jr.
Abstract This work aims at evaluating the discretization errors associated to the finite volume and finite element methods of polymer melt flow in closed channels. Two strategies are discussed: (i) Richardson extrapolation and (ii) a posteriori error estimation based on projection/smoothing techniques. The numerical model accounts for the full interaction between the thermal effects caused by viscous heating and the momentum diffusion effects dictated by a shear rate and temperature-dependent constitutive model. The simulations have been performed for the commercial polymer Polyacetal POM-M90-44. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Finite element of slender beams in finite transformations: a geometrically exact approach

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 5 2004
Frédéric Boyer
Abstract This article is devoted to the modelling of thin beams undergoing finite deformations essentially due to bending and torsion and to their numerical resolution by the finite element method. The solution proposed here differs from the approaches usually implemented to treat thin beams, as it can be qualified as ,geometrically exact'. Two numerical models are proposed. The first one is a non-linear Euler,Bernoulli model while the second one is a non-linear Rayleigh model. The finite element method is tested on several numerical examples in statics and dynamics, and validated through comparison with analytical solutions, experimental observations and the geometrically exact approach of the Reissner beam theory initiated by Simo. The numerical result shows that this approach is a good alternative to the modelling of non-linear beams, especially in statics. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Finite element and sensitivity analysis of thermally induced flow instabilities

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 10 2010
Jean-Serge Giguère
Abstract This paper presents a finite element algorithm for the simulation of thermo-hydrodynamic instabilities causing manufacturing defects in injection molding of plastic and metal powder. Mold-filling parameters determine the flow pattern during filling, which in turn influences the quality of the final part. Insufficiently, well-controlled operating conditions may generate inhomogeneities, empty spaces or unusable parts. An understanding of the flow behavior will enable manufacturers to reduce or even eliminate defects and improve their competitiveness. This work presents a rigorous study using numerical simulation and sensitivity analysis. The problem is modeled by the Navier,Stokes equations, the energy equation and a generalized Newtonian viscosity model. The solution algorithm is applied to a simple flow in a symmetrical gate geometry. This problem exhibits both symmetrical and non-symmetrical solutions depending on the values taken by flow parameters. Under particular combinations of operating conditions, the flow was stable and symmetric, while some other combinations leading to large thermally induced viscosity gradients produce unstable and asymmetric flow. Based on the numerical results, a stability chart of the flow was established, identifying the boundaries between regions of stable and unstable flow in terms of the Graetz number (ratio of thermal conduction time to the convection time scale) and B, a dimensionless ratio indicating the sensitivity of viscosity to temperature changes. Sensitivities with respect to flow parameters are then computed using the continuous sensitivity equations method. We demonstrate that sensitivities are able to detect the transition between the stable and unstable flow regimes and correctly indicate how parameters should change in order to increase the stability of the flow. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Finite Element Analysis of the Proximal Femur and Hip Fracture Risk in Older Men,

JOURNAL OF BONE AND MINERAL RESEARCH, Issue 3 2009
Eric S Orwoll
Abstract Low areal BMD (aBMD) is associated with increased risk of hip fracture, but many hip fractures occur in persons without low aBMD. Finite element (FE) analysis of QCT scans provides a measure of hip strength. We studied the association of FE measures with risk of hip fracture in older men. A prospective case-cohort study of all first hip fractures (n = 40) and a random sample (n = 210) of nonfracture cases from 3549 community-dwelling men ,65 yr of age used baseline QCT scans of the hip (mean follow-up, 5.6 yr). Analyses included FE measures of strength and load-to-strength ratio and BMD by DXA. Hazard ratios (HRs) for hip fracture were estimated with proportional hazards regression. Both femoral strength (HR per SD change = 13.1; 95% CI: 3.9,43.5) and the load-to-strength ratio (HR = 4.0; 95% CI: 2.7,6.0) were strongly associated with hip fracture risk, as was aBMD as measured by DXA (HR = 5.1; 95% CI: 2.8,9.2). After adjusting for age, BMI, and study site, the associations remained significant (femoral strength HR = 6.5, 95% CI: 2.3,18.3; load-to-strength ratio HR = 4.3, 95% CI: 2.5,7.4; aBMD HR = 4.4, 95% CI: 2.1,9.1). When adjusted additionally for aBMD, the load-to-strength ratio remained significantly associated with fracture (HR = 3.1, 95% CI: 1.6,6.1). These results provide insight into hip fracture etiology and demonstrate the ability of FE-based biomechanical analysis of QCT scans to prospectively predict hip fractures in men. [source]

Experimental and model determination of human intervertebral disc osmoviscoelasticity

JOURNAL OF ORTHOPAEDIC RESEARCH, Issue 8 2008
Y. Schroeder
Abstract Finite element (FE) models have become an important tool to study load distribution in the healthy and degenerated disc. However, model predictions require accurate constitutive laws and material properties. As the mechanical properties of the intervertebral disc are regulated by its biochemical composition and fiber-reinforced structure, the relationship between the constitutive behavior of the tissue and its composition requires careful consideration. While numerous studies have investigated the annulus fibrosus compressive and tensile properties, specific conditions required to determine model parameters for the osmoviscoelastic model are unavailable. Therefore, the objectives of this study were (1) to complement the existing material testing in the literature with confined compression and tensile tests on human annulus fibrosus and (2) to use these data, together with existing nucleus pulposus compression data to tune a composition-based, osmoviscoelastic material constitutive law. The osmoviscoelastic material constitutive law and the experimental data were used to describe the fiber and nonfiber properties of the human disc. The compressive material properties of normal disc tissue were Gm,=,1.23 MPa, M,=,1.57, and ,,=,1.964,×,10,16 m4/Ns; the tensile fiber material parameters were E0,=,77.0 MPa; E,,=,500 MPa, and ,,=,1.8,×,103 MPa,s. The goodness of fit ranged from 0.88 to 0.96 for the four experimental conditions evaluated. The constitutive law emphasized the interdependency of the strong swelling ability of the tissue and the viscoelastic nature of the collagen fibers. This is especially important for numerical models to further study the load sharing behavior with regard to disc degeneration and regeneration. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:1141,1146, 2008 [source]

Motion visualization of human left ventricle with a time-varying deformable model for cardiac diagnosis

COMPUTER ANIMATION AND VIRTUAL WORLDS (PREV: JNL OF VISUALISATION & COMPUTER ANIMATION), Issue 2 2001
Soo-Mi Choi
Abstract We present a time-varying deformable model to visualize and analyze the motion of the left ventricle from a time series of 3-D images. The model is composed of a non-rigid body that deforms around a reference shape obtained from the previous time step. At each time step, the position and orientation of the left ventricle are extracted from the feature points of images. This information gives the position and orientation of the coordinate system attached to the non-rigid body. To compute a dense non-rigid motion field over the entire endocardial wall of the left ventricle, we introduce a 3-D blob finite element and Galerkin interpolants based on 3-D Gaussian, and use a physically based finite element method and a modal analysis. Then, cinematic attributes are visualized in pseudo colors on the reconstructed surface in order to help medical doctors in their interpretation of the data. Using the presented model, we estimate clinically useful quantitative parameters such as regional wall motion and ejection fraction. Experimental results are shown in a time series of X-ray angiographic images. Copyright ©2001 John Wiley & Sons, Ltd. [source]

Magnetostatic analysis of a brushless DC motor using a two-dimensional partial differential equation solver

COMPUTER APPLICATIONS IN ENGINEERING EDUCATION, Issue 2 2001
A. Kostaridis
Abstract A finite element, magnetostatic analysis, of a brushless direct current motor containing non-linear materials and permanent magnets is presented. The analysis is performed with PDEaseÔ, a low cost, two-dimensional partial differential equation solver. The descriptor file is remarkably short and easy to understand, enabling students to focus on the application and not on the finite element method. © 2001 John Wiley & Sons, Inc. Comput Appl Eng Educ 9: 93,100, 2001 [source]

Uncertainty and Sensitivity Analysis of Damage Identification Results Obtained Using Finite Element Model Updating

COMPUTER-AIDED CIVIL AND INFRASTRUCTURE ENGINEERING, Issue 5 2009
Babak Moaveni
The shake table tests were designed so as to damage the building progressively through several historical seismic motions reproduced on the shake table. A sensitivity-based finite element (FE) model updating method was used to identify damage in the building. The estimation uncertainty in the damage identification results was observed to be significant, which motivated the authors to perform, through numerical simulation, an uncertainty analysis on a set of damage identification results. This study investigates systematically the performance of FE model updating for damage identification. The damaged structure is simulated numerically through a change in stiffness in selected regions of a FE model of the shear wall test structure. The uncertainty of the identified damage (location and extent) due to variability of five input factors is quantified through analysis-of-variance (ANOVA) and meta-modeling. These five input factors are: (1,3) level of uncertainty in the (identified) modal parameters of each of the first three longitudinal modes, (4) spatial density of measurements (number of sensors), and (5) mesh size in the FE model used in the FE model updating procedure (a type of modeling error). A full factorial design of experiments is considered for these five input factors. In addition to ANOVA and meta-modeling, this study investigates the one-at-a-time sensitivity analysis of the identified damage to the level of uncertainty in the identified modal parameters of the first three longitudinal modes. The results of this investigation demonstrate that the level of confidence in the damage identification results obtained through FE model updating, is a function of not only the level of uncertainty in the identified modal parameters, but also choices made in the design of experiments (e.g., spatial density of measurements) and modeling errors (e.g., mesh size). Therefore, the experiments can be designed so that the more influential input factors (to the total uncertainty/variability of the damage identification results) are set at optimum levels so as to yield more accurate damage identification results. [source]

Assessment of Protection Systems for Buried Steel Pipelines Endangered by Rockfall

COMPUTER-AIDED CIVIL AND INFRASTRUCTURE ENGINEERING, Issue 5 2005
Bernhard Pichler
First, a gravel-based protection system (GBPS) is investigated, that is, a pipeline buried in sandy gravel is considered. To assess the load-carrying behavior of this structure when subjected to rockfall, a finite element (FE) model has been developed. The development and the validation of this structural model are strictly separated, that is, they are based on two physically and statistically independent sets of experiments. Subsequently, scenarios of rockfall onto a gravel-buried steel pipe are analyzed considering different boundary conditions and structural dimensions. Following the conclusions drawn from these numerical analyses, an enhanced protection system (EPS) is proposed. It consists of gravel as an energy-absorbing and impact-damping system and a buried steel plate resting on walls made of concrete representing a load-carrying structural component. The potential and the limitations of both protection systems are discussed in detail. [source]

An experimentally calibrated finite element study of maxillary trauma

DENTAL TRAUMATOLOGY, Issue 5 2007
Michael J Casas
Abstract,,, A baseball injury to an instrumented human cadaver maxillae was simulated with a regulation (142 g) baseball traveling at 14 m s,1. Measurements of strain were obtained with three-axis strain gauge rosettes located at the medial palate and both canine fossae. A three-dimensional finite element (FE) model of a dentate human maxilla was constructed from computed tomography scans of the skull of an adolescent. This three-dimensional mathematical model of the maxilla was deemed geometrically accurate by convergence testing when the model's degrees of freedom approximated 74 000. The simulated load case involved a transient dynamic impact to the medial maxilla with boundary conditions imposed at skeletal buttresses of the model. The model was calibrated through direct comparison with the displacements and principal strains gathered from experimental and epidemiological data. The comparison of experimental and calculated principal strains as a result of the simulated impacts revealed a 1.7,11.4% difference. [source]

Kinematic response functions and dynamic stiffnesses of bridge embankments

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 11 2002
Jian Zhang
Abstract Recognizing that soil,structure interaction affects appreciably the earthquake response of highway overcrossings, this paper compares approximate analytical solutions and finite element results to conclude on a simple procedure that allows for the estimation of the kinematic response functions and dynamic stiffnesses of approach embankments. It is shown that the shear-wedge model yields realistic estimates for the amplification functions of typical embankments and reveals the appropriate levels of dynamic strains which are subsequently used to estimate the stiffness and damping coefficients of embankments. The shear-wedge model is extended to a two-dimensional model in order to calculate the transverse static stiffness of an approach embankment loaded at one end. The formulation leads to a sound closed-form expression for the critical length, Lc, that is the ratio of the transverse static stiffness of an approach embankment and the transverse static stiffness of a unit-width wedge. It is shown through two case studies that the transverse dynamic stiffness (,spring' and ,dashpot') of the approach embankment can be estimated with confidence by multiplying the dynamic stiffness of the unit-width wedge with the critical length, Lc. The paper concludes that the values obtained for the transverse kinematic response function and dynamic stiffness can also be used with confidence to represent the longitudinal kinematic response function and dynamic stiffness, respectively. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Constituent Particle Break-Up During Hot Rolling of AA 5182,

ADVANCED ENGINEERING MATERIALS, Issue 1-2 2010
Nicolas Moulin
Aluminum sheet is currently used for body panels on a number of mass-produced vehicles, in particular for closure panels. AA5xxx alloys always contain coarse inter-metallic particles (Alx(Fe,Mn)ySi, Mg2Si) after casting. In the present work inter-metallic particle break-up during hot reversible rolling of AA5182 alloy sheets has been analyzed. The sizes and shapes of inter-metallic particles in as-cast and industrially hot rolled AA5182 alloys sheets were characterized by 3D X-ray tomography observations. The relation between particle break-up and particle morphology was then analyzed statistically and by a micromechanical finite element (FE)-based model. The essential outcomes of the statistical approach may be summarized as follows. The inter-metallic particle population may be described by five morphological parameters. Secondly the comparison of the particle morphology in as cast and industrially rolled sheets leads to the definition of five classes. The evolution of each particle class as function of the rolling strain is provided. The statistical analysis shows which particles break-up. The stresses and strains in inter-metallic particles, embedded in an elasto-viscoplastic aluminum matrix submitted to plane strain compression, were analyzed by an FE model. A new failure criterion was proposed. The essential outcomes of the mechanical approach are as follows: a precise description of stress concentration mechanisms in nonconvex particles, a close description of the parameters controlling particle break-up, and finally a simplified classification of the failure behavior. [source]

Fatigue crack growth and life prediction of a single interference fitted holed plate

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 10 2010
T. N. CHAKHERLOU
ABSTRACT To understand the different aspects of fatigue behaviour of complex structural joints it will be much helpful if the effects of different parameters are studied separately. In this article, to study the isolated effect of interference fit on fatigue life a pined hole specimen is investigated. This specimen is a single-holed plate with an oversized pin which force fitted to the hole. The investigation was carried out both experimentally and numerically. In the experimental part, interference fitted specimens along with open hole specimens were fatigue tested to study the experimental effect of the interference fit. In the numerical part, three-dimensional finite element (FE) simulations have been performed in order to obtain the created stresses due to interference fit and subsequent applied longitudinal load at the holed plate. The stress distribution obtained from FE simulation around the hole was used to predict crack initiation life using Smith,Watson,Topper method and fatigue crack growth life using the NASGRO equation with applying the AFGROW computer code. The predicted fatigue life obtained from the numerical methods show a good agreement with the experimental fatigue life. [source]

Damage analysis of laminated composites using a new coupled micro-meso approach

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 7 2010
A. FARROKHABADI
ABSTRACT In this study, the simplicity and strong physical meaning of micromechanics approach and capability of mesomechanics approach for damage analysis of structures with complex loadings are employed to develop a new micro-meso approach. For this purpose, a new micromechanics model is developed to predict the matrix cracking initiation and evolution in laminated composites. These damage initiation and evolution are replaced with the damage criteria and flow rule in the continuum damage approach, respectively. The results of this procedure are used in the FEM damage analyses of laminated composites to predict constitutive response of layered composites. It is shown that, the predicted stress distribution and strain energy in a lamina unit cell are in good agreement with the finite element results. Furthermore, it is shown that the predicted stress,strain behaviours are in good agreement with the available experimental results for various laminates with different lay-ups. [source]

Numerical investigation on J -integral testing of heterogeneous fracture toughness testing specimens: Part I , weld metal cracks

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 8 2003
Y.-J. KIM
ABSTRACT Based on extensive two-dimensional (2D) finite element (FE) analyses, the present work provides the plastic , factor solutions for fracture toughness J -integral testing of heterogeneous specimens with weldments. Solutions cover practically interesting ranges of strength mismatch and relative weld width, and are given for three typical geometries for toughness testing: a middle cracked tension (M(T)) specimen, single edge cracked bend (SE(B)) specimen and (C(T)) specimen. For mismatched M(T) specimens, both plane strain and plane stress conditions are considered, whereas for SE(B) and C(T) specimens, only the plane strain condition is considered. For all cases, only deep cracks are considered, and an idealized butt weld configuration is considered, where the weld metal strip has a rectangular cross section. Based on the present solutions for the strength mismatch effect on plastic , factors, a window is provided, within which the homogeneous J estimation procedure can be used for weldment toughness testing. The effect of the weld groove configuration on the plastic , factor is briefly discussed, concluding the need for further systematic analysis to provide guidance to practical toughness testing. [source]

Deformation analysis of notched components and assessment of approximate methods

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 11 2001
Y. Jiang
Finite element modelling was conducted on notched members subjected to proportional and non-proportional loading. A recently developed cyclic plasticity model capable of accurately describing cyclic material behaviour was implemented into a finite element code. A plate with a central hole and a shaft with a circumference groove were studied. Approximate methods for the notched problems were critically evaluated using the finite element results. [source]

The validation of some methods of notch fatigue analysis

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 5 2000
Taylor
This paper is concerned with the testing and validation of certain methods of notch analysis which the authors have developed theoretically in earlier publications. These methods were developed for use with finite element (FE) analysis in order to predict the fatigue limits of components containing stress concentrations. In the present work we tested and compared these methods using data from standard notches taken from the literature, covering a range of notch geometries, loading types, R -ratios and materials: a total of 47 different data sets were analysed. The greatest predictive success was achieved with critical-distance methods known as the point, line and area methods: 94% of these predictions fell within 20% of the experimental fatigue limits. This was a significant improvement on previous methods of this kind, e.g. that of Klesnil and Lucas [(1980) Fatigue of Metallic Materials, Elsevier Science]. Methods based on the Smith and Miller [(1978) Int. J. Mech. Sci. 20, 201,206] concept of crack-like notches were successful in 42% of cases; they experienced difficulties dealing with very small notches, and could be improved by using an ElHaddad-type correction factor, giving 87% success. An approach known as ,crack modelling' allowed the Smith and Miller method to be used with non-standard stress concentrations, where notch geometry is ill defined; this modification, with the same short-crack correction, had 68% success. It was concluded that the critical-distance approach is more accurate and can be more easily used to analyse components of complex shape, however, the crack modelling approach is sometimes preferable because it can be used with less mesh refinement. [source]

Communication between overpressured compartments

GEOFLUIDS (ELECTRONIC), Issue 4 2001
M. Wangen
Abstract Overpressure build-up in compartments, and communication between overpressured compartments across faults are studied with simple analytical and numerical models. It is shown that the excess pressure in a (vertical) one-dimensional, one-compartment model can be written as the sum of the excess pressure generated in the seal above the compartment, and a second part, which is due to the expulsion of fluid from the compartment and the rocks below. The one-compartment model is generalized to a two-compartment model, which accounts for the fluid communication between the compartments through a fault zone. The volume rates of flow through the seals and the fault zone are shown to be the weighted mean of the volume rates of the one-dimensional, one-compartment model. The normalized weights are given by dimensionless numbers, called fault,seal numbers, which control the communication between the compartments. A fault,seal number much less than unity implies that the fault is a stronger barrier for the fluid flow than the seal. A fault,seal number larger than unity implies the opposite: that the seal is a stronger barrier than the fault. The conditions for isolated compartments and other regimes are identified in terms of the fault,seal numbers. It is discussed how the compartment fault,seal numbers can be computed when the permeability is given in the fault zone. The results given by the analytical compartment models are demonstrated and validated with two-dimensional numerical (finite element) simulations. [source]

FE modelling of excavation and operation of a shield tunnelling machine.

GEOMECHANICS AND TUNNELLING, Issue 2 2009
FE-Modellierung des Ausbruchs und Betriebs einer Schildvortriebmaschine
Abstract A shield tunnelling machine is driven forward by applying mechanical jack forces behind the shield machine tail while excavating the soil in front of the shield machine with its cutting face. In this study, the advance and excavation processes of the shield tunnelling operation are modelled using the finite element method in order to investigate the effect of these construction processes on the ground response. An excavating finite element is introduced, which models the disturbed soil in front of the cutting face. The operation of shield advance and of soil excavation is simulated using the finite element re-meshing technique at each time step of the analysis. The proposed modelling techniques of shield tunnelling construction are applied to simulate a deep and a triple-face shield tunnelling project in Tokyo and the numerical results are compared with the field measurements. Eine Schildvortriebsmaschine wird durch Vorschubzylinder am Maschinenende vorangetrieben, während der Schneidkopf an der Vorderseite der Maschine den Boden abbaut. In dieser Studie werden der Vortrieb und der Bodenabbau beim Betrieb einer Schildmaschine mittels der Methode der Finiten Elemente modelliert, um den Einfluss dieser Vorgänge auf die Reaktion des Baugrunds zu untersuchen. Hierzu wird ein Ausbruchelement eingeführt, das den gestörten Boden vor der Maschine modelliert. Dabei wird das FE-Netz für jeden Zeitschritt angepasst. Die vorgestellte Modellierungstechnik für den Schildvortrieb wird eingesetzt, um einen tiefen Schildvortrieb und den Einsatz einer Dreikopf-Schildmaschine zu simulieren. Die numerischen Ergebnisse werden mit den Resultaten der vor Ort Messungen verglichen. [source]

Fabrication and Electromechanical Characterization of a Piezoelectric Structural Fiber for Multifunctional Composites

ADVANCED FUNCTIONAL MATERIALS, Issue 4 2009
Yirong Lin
Abstract The use of piezoceramic materials for structural sensing and actuation is a fairly well developed practice that has found use in a wide variety of applications. However, just as advanced composites offer numerous benefits over traditional engineering materials for structural design, actuators that utilize the active properties of piezoelectric fibers can improve upon many of the limitations encountered when using monolithic piezoceramic devices. Several new piezoelectric fiber composites have been developed; however, almost all studies have implemented these devices such that they are surface-bonded patches used for sensing or actuation. This paper will introduce a novel active piezoelectric structural fiber that can be laid up in a composite material to perform sensing and actuation, in addition to providing load bearing functionality. The sensing and actuation aspects of this multifunctional material will allow composites to be designed with numerous embedded functions, including structural health monitoring, power generation, vibration sensing and control, damping, and shape control through anisotropic actuation. This effort has developed a set of manufacturing techniques to fabricate the multifunctional fiber using a SiC fiber core and a BaTiO3 piezoelectric shell. The electromechanical coupling of the fiber is characterized using an atomic force microscope for various aspect ratios and is compared to predictions made using finite element modeling in ABAQUS. The results show good agreement between the finite element analysis model and indicate that the fibers could have coupling values as high as 68% of the active constituent used. [source]

Elasto-plastic analysis of block structures through a homogenization method

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 3 2010
G. de Felice
Abstract The paper describes the development and numerical implementation of a constitutive relationship for modeling the elasto-plastic behavior of block structures with periodic texture, regarded at a macroscopic scale as homogenized anisotropic media. The macroscopic model is shown to retain memory of the mechanical characteristics of the joints and of the shape of the blocks. The overall mechanical properties display anisotropy and singularities in the yield surface, arising from the discrete nature of the block structure and the geometrical arrangement of the units. The model is formulated in the framework of multi-surface plasticity. It is implemented in an finite element (FE) code by means of two different algorithms: an implicit return mapping scheme and a minimization algorithm directly derived from the Haar,Karman principle. The model is validated against analytical and experimental results: the comparison between the homogenized continuum and the original block assembly shows a good agreement in terms of ultimate inelastic behavior, when the size of the block is small as compared with that of the whole assembly. Copyright © 2009 John Wiley & Sons, Ltd. [source]

An extended finite element framework for slow-rate frictional faulting with bulk plasticity and variable friction

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 13 2009
Fushen Liu
Abstract We present an extended finite element (FE) approach for the simulation of slow-rate frictional faulting in geologic media incorporating bulk plasticity and variable friction. The method allows the fault to pass through the interior of FEs without remeshing. The extended FE algorithm for frictional faulting, advocated in two recent articles, emanates from a variational equation formulated in terms of the relative displacement on the fault. In the present paper we consider the combined effects of bulk plasticity and variable friction in a two-dimensional plane strain setting. Bulk plasticity is localized to the fault tip and could potentially be used as a predictor for the initiation and propagation of new faults. We utilize a variable velocity- and state-dependent friction, known as the Dieterich,Ruina or ,slowness' law, formulated in a slip-weakening format. The slip-weakening/variable friction model is then time-integrated according to the generalized trapezoidal rule. We present numerical examples demonstrating the convergence properties of a global Newton-based iterative scheme, as well as illustrate some interesting properties of the variable friction model. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Numerical modelling of regional faults in land subsidence prediction above gas/oil reservoirs

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 6 2008
Massimiliano Ferronato
Abstract The stress variation induced by gas/oil production may activate pre-existing regional faults. This may enhance the expected land subsidence due to the generation of mechanically weak points close to the producing field. A class of elasto-plastic interface elements (IE), specifically designed to address the mechanical behaviour of faults over a regional scale, is integrated into a finite element (FE) geomechanical model and used to investigate the role exerted by active faults in anthropogenic land subsidence. The importance of regional faults depends on a variety of factors including depth of the depleted reservoir, fault number, orientation and size, geomechanical properties of porous medium, pore pressure drawdown induced by fluid production, etc. With the aid of some representative examples, a useful indication is provided as to where and how fault activation may influence both magnitude and extent of the land subsidence bowl above producing gas/oil reservoirs, pointing to a generally limited impact on the ground surface. The simulation of a real faulted gas reservoir in a complex 3-D setting shows that the proposed IE can be simply and efficiently incorporated into a FE geomechanical model, thus improving the quality of the stress and displacement prediction. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Solution of the unsaturated soil moisture equation using repeated transforms

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 15 2001
S. G. Fityus
Abstract An alternative method of solution for the linearized ,theta-based' form of the Richards equation of unsaturated flow is developed in two spatial dimensions. The Laplace and Fourier transformations are employed to reduce the Richards equation to an ordinary differential equation in terms of a transformed moisture content and the transform variables, s and ,. Separate analytic solutions to the transformed equation are developed for initial states which are either in equilibrium or dis-equilibrium. The solutions are assembled into a finite layer formulation satisfying continuity of soil suction, thereby facilitating the analysis of horizontally stratified soil profiles. Solution techniques are outlined for various boundary conditions including prescribed constant moisture content, prescribed constant flux and flux as a function of moisture change. Example solutions are compared with linearized finite element solutions. The agreement is found to be good. An adaptation of the method for treating the quasilinearized Richards equation with variable diffusivity is also described. Comparisons of quasilinear solutions with some earlier semi-analytical, finite element and finite difference results are also favourable. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Finite element analysis of land subsidence above depleted reservoirs with pore pressure gradient and total stress formulations

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 4 2001
Giuseppe Gambolati
Abstract The solution of the poroelastic equations for predicting land subsidence above productive gas/oil fields may be addressed by the principle of virtual works using either the effective intergranular stress, with the pore pressure gradient regarded as a distributed body force, or the total stress incorporating the pore pressure. In the finite element (FE) method both approaches prove equivalent at the global assembled level. However, at the element level apparently the equivalence does not hold, and the strength source related to the pore pressure seems to generate different local forces on the element nodes. The two formulations are briefly reviewed and discussed for triangular and tetrahedral finite elements. They are shown to yield different results at the global level as well in a three-dimensional axisymmetric porous medium if the FE integration is performed using the average element-wise radius. A modification to both formulations is suggested which allows to correctly solve the problem of a finite reservoir with an infinite pressure gradient, i.e. with a pore pressure discontinuity on its boundary. Copyright © 2001 John Wiley & Sons, Ltd. [source]