bluuebunny/crossref_metadata_2025 huggingface dataset:
Parallel Simulations of CO2 Sequestration Using a Non-Isothermal Compositional Model
Authors: Delshad Mojdeh , Thomas Sunil G. , Wheeler Mary F.
Published: August 2009

This paper describes an efficient and parallel numerical scheme for multiphase compositional flow. The underlying theory is first presented followed by a brief description of the equation of state (EOS) and the two-phase flash implementation. An iterative “implicit-pressure and explicit-concentrations” (IMPEC) algorithm is then applied to enforce a non-linear volume balance (saturation) constraint. The pressure system is solved using a mixed finite element method, while the concentrations are updated explicitly in a manner that preserves local mass balance of every component. A major application of this scheme is in the modeling of field scale CO2 sequestration, as an enhanced oil recovery (EOR) process or for storage in deep saline aquifers. Thermal energy transfer also plays an important role in such problems since it can effect the phase properties dramatically. Hence, accurate and locally conservative methods are desirable to model the thermal effects. To this end, the paper also presents a time-split scheme for modeling thermal energy transfer which is sequentially coupled to flow. Finally, some numerical results are presented for challenging benchmark problems.
The Analysis for Relation of Fracture Toughness and Charpy Impact Energy of High Strength Pipeline Steel
Authors: Zhang Guang-Li , Zhao Xin-Wei , Luo Jin-Heng , Gong Shao-Tao , Zhang Hua , Zhang Yi
Published: August 2009

According to the linear regression analysis, a new empirical equations between the fracture toughness (KIc) and Charpy impact energy (CVN) of X80 high strength pipeline steel, were developed on the basis of experiment sample data. And the theory of statistical test was used to test the significance of the new equation. The test result indicated that the new empirical equation has very high significance with the sample data. Comparing with the empirical equation recommended by the API 579-2007, the empirical equation developed here was more suitable for the X80 high strength pipeline steel, and can make better estimation for the fracture toughness of X80. And for the estimation of fracture toughness of X100 high strength pipeline steel, the equation developed here is applicable too.
Nondimensional Analysis of Interfacial Stresses of Wavy CNT/Polymer Composites
Authors: Yazdchi K. , Salehi M.
Published: August 2009

In this paper, with introducing a new simplified 3-D Representative Volume Element (RVE) for a wavy carbon nanotube (CNT), an analytical model has been developed to study the stress transfer in single-walled carbon nanotube (SWNT) reinforced polymer composites (NRPCs). The model is capable of predicting axial as well as interfacial shear stresses, along a wavy CNT embedded in a matrix. Based on the pullout modeling technique, the effects of waviness, wavelength and matrix modulus on axial and interfacial shear stresses have also been analyzed in details also using the statistical multiple non-linear regression method, the best-fitted functions for the interfacial stresses of CNT/polymer composites are obtained. The results of the present analytical model are in good agreements when compared with the corresponding results for straight NTs.
On the Vibration of Layered Sandwich Elastic Plates
Authors: Olunloyo Vincent O. S. , Osheku Charles A.
Published: August 2009

Sandwich elastic plates have found increasing applications in civil, aerospace, military and offshore industries to enhance superior resistance to fatigue crack propagation, impact damage, local buckling and are very effective for vibration damping and noise reduction. Such structural application has significantly led to reduction in vulnerability of warships to blasts, ballistics, bomb and fire attacks. In engineering structures, one of the effective ways of damping vibration and noise attenuation, is to exploit the occurrence of slip at the interface of structural laminates where such members are held together in a pressurised environment. Recent analysis and experimental investigation of vibration characteristics and damping properties of layered sandwich structures, are mostly limited to elastic beams. This paper is an attempt to extend such analytical investigations to layered sandwich plates. By employing contact mechanics and laminated thin plate theory, the generalised equation governing the vibration of two layered sandwich plates that are held together in pressurised environment is presented. In particular, by invoking operational methods for the case of linear interface pressure distribution, closed form analytical results for the system natural frequency and dynamic response under external excitation are reported for design analysis and applications.
Stabilized Finite Element Methods for Coupled Solid-Deformation/Fluid-Flow in Porous Geomaterials
Authors: White Joshua A. , Borja Ronaldo I.
Published: August 2009

We present a finite element formulation for saturated and partially saturated porous geomaterials undergoing elastoplastic deformations. The deforming body is treated as a multiphase continuum, and the governing mass and momentum balance equations are solved in a fully coupled manner. The constitutive behavior of the solid is based on a critical state plasticity model that explicitly accounts for the effect of capillary pressures on the geometry of the yield surface in the partially saturated regime. It is well known, however, that mixed formulations of the type examined here may lead to unstable approximations unless the spaces chosen for the pressure and displacement interpolations satisfy stringent stability restrictions. Failure to choose stable combinations typically leads to non-physical pressure oscillations and poor convergence behavior upon mesh refinement. Unfortunately, many seemingly natural combinations—including equal-order interpolations—do not satisfy the necessary requirements. In this work, we instead employ a stabilized formulation, based on a relatively minor modification of the variational equations, which allows us to circumvent these restrictions and employ equal-order mixed elements. We demonstrate the appealing features of this approach with several boundary value problems taken from geotechnical applications, including modeling strain localization and failure initiation in variably saturated slopes.
Mechanical and Thermal Properties of Carbon-Nanofiber Infused Polyurethane Foam
Authors: Bhuiyan Md. Atiqur , Hosur Mahesh V. , Farooq Yaseen , Jeelani Shaik
Published: August 2009

In this study, thermal and mechanical properties of carbon nanofiber infused polyurethane foam were investigated. Low density liquid polyurethane foam composed of Diphenylmethane Diisocyanate (Part A) and Polyol (Part B) was doped with carbon nanofibers (CNF). A high-intensity ultrasonic liquid processor was used to obtain a homogeneous mixture of Diphenylmethane Diisocyanate (Part A) and carbon nanofibers (CNF). The CNF were infused into the Part A of the polyurethane foam through sonic cavitation. The modified foams containing nanoparticles were mixed with Part B (Polyol) using a high-speed mechanical agitator. The mixture was then cast into pre-heated rectangular aluminum molds to form the nano-phased foam panels. Flexure, static and high strain rate compression, and dynamic mechanical analysis (DMA) were performed on neat, 0.2 wt%, 0.4 wt% and 0.6 wt% CNF filled polyurethane foam to identify the effect of adding CNF on the thermal and mechanical properties. The highest improvement on thermal and mechanical properties was obtained with 0.2 wt% loading of CNF. Morphology of the samples was studied through X-ray diffraction.
Phase-Field Models of Gravity-Driven Unsaturated Flow in Porous Media
Authors: Cueto-Felgueroso Luis , Juanes Ruben
Published: August 2009

Existing continuum models of multiphase flow in porous media are unable to explain why preferential flow (fingering) occurs during infiltration into homogeneous, dry soil. We identify a relevant pattern-forming mechanism in the dynamics of the wetting front, and present a macroscopic model that reproduces the experimentally observed features of fingered flows. The proposed model reveals a scaling between local and nonlocal interface phenomena in imbibition, and does not introduce new independent parameters. The predictions based on this model are consistent with experiments and theories of scaling in porous media.
Modeling of Strain-Induced Phonon Thermal Conductivity Reduction in Thermoelectric Nanocomposites
Authors: Xu Yaoyao , Li Gang
Published: August 2009

In this paper, we study strain effects on the phonon thermal conductivity of 2-D Si/Ge nanocomposites. Lattice dynamics is employed for the calculation of the phonon scattering properties as a function of strain. Cauchy-Born rule is used to model the deformed configuration of the atoms. The effective thermal conductivity of the nanocomposite material is modeled by using a modified effective medium approximation (EMA) approach. The strain effects are incorporated into the modified EMA through the strain dependent phonon mean free path. The effective thermal conductivity of the strained nanocomposite material is calculated for different characteristic lengths of the Si component. The results show that a 2% tensile strain can reduce the effective thermal conductivity by more than 10%.
Design Optimization of Material Properties in a Particle-Filled Composite Material System
Authors: Gurrum Siva P. , Zhao Jie-Hua , Edwards Darvin R.
Published: August 2009

This work presents a methodology implementing random packing of spheres combined with commercial finite element method (FEM) software to optimize the material properties, such as Young’s modulus, Poisson’s ratio, coefficient of thermal expansion (CTE) of two-phase materials used in electronic packaging. The methodology includes an implementation of a numerical algorithm of random packing of spheres and a technique for creating conformal FEM mesh of a large aggregate of particles embedded in a medium. We explored the random packing of spheres with different diameters using particle generation algorithms coded in MATLAB. The FEM meshes were generated using MATLAB and TETGEN. After importing the nodes and elements databases into commercial FEM software ANSYS, the composite materials with spherical fillers and the polymer matrix were modeled using ANSYS. The effective Young’s modulus, Poisson’s ratio, and CTE along different axes were calculated using ANSYS by applying proper loading and boundary conditions. It was found that the composite material was virtually isotropic. The Young’s modulus and Poisson’s ratio calculated by FEM models were compared to a number of analytical solutions in the literature. For low volume fraction of filler content, the FEM results and analytical solutions agree well. However, for high volume fraction of filler content, there is some discrepancy between FEM and analytical models and also among the analytical models themselves.
A Stabilized B-Splines FEM Formulation for the Solution of an Inverse Elasticity Problem Arising in Medical Imaging
Authors: Rivas Carlos E. , Barbone Paul E. , Oberai Assad A.
Published: August 2009

Soft tissue pathologies are often associated with changes in mechanical properties. For example, breast and other tumors usually present as stiff lumps. Imaging the spatial distribution of the mechanical properties of tissues thus reveals information of diagnostic value. Doing so, however, typically requires the solution of an inverse elasticity problem. In this work we consider the inverse elasticity problem for an incompressible material in plane stress, formulated and solved as a constrained optimization problem. We formulate this inverse problem enforcing high order continuity for our variables. Driven by the requirements for the strong and weak solutions to this problem, we assume that our data field (i.e. the measured displacement) is in H2 and our parameter distribution (i.e. the sought shear modulus distribution) is in H1. This high order regularity requirement for the data is incompatible with standard FEM. We solve this problem using a FEM formulation that is novel in two respects. First, we employ quadratic b-splines that enforce C1 continuity in our displacement field, consistent with the variational requirements of the continuous problem. Second, we include Galerkin-least-squares (GLS) stabilization in the iterative optimization formulation. GLS adds consistent stability to the discrete formulation that otherwise violates an ellipticity condition that is satisfied by the continuous problem. Computational examples validate this formulation and demonstrate numerical convergence with mesh refinement.
Fluid-Structure Interaction Simulation With Free Surface Flows by Smoothed Particle Hydrodynamics
Authors: Farahani M. H. , Amanifard N. , Asadi H. , Mahnama M.
Published: August 2009

Simulation of the fluid-structure interaction (FSI) and free surface flows includes an area of extremely challenging problems in the computational mechanics community. In this paper, a newly proposed SPH algorithm is employed to simulate FSI problems with complex free surface flows. In this way, fluid and elastic structure continua are coupled using a monolithic but explicit numerical scheme. The proposed method is similar to so-called SPH projection method and consists of three steps. The first two steps play the role of prediction, while in the third step a Poisson equation is used for both fluid and structure to impose incompressibility constraint.
Modeling of Visco-Hyperelastic Behavior of Foams
Authors: Anani Y. , Asghari M. , Naghdabadi R.
Published: August 2009

In this paper, a new visco-hyperelastic constitutive law for describing the rate dependent behavior of foams is proposed. The proposed model was based on a phenomenological Zener model: a hyperelastic equilibrium spring, which describes the steady-state, long-term response, parallel to a Maxwell element, which captures the ratedependency. A nonlinear viscous damper connected in series to a hyperelastic intermediate spring, controls the ratedependency of the Maxwell element. Therefore, the stress is the sum of equilibrium stress on the equilibrium spring and overstress on the intermediate spring. In hyperelastic theory stress is not calculated directly as in the case of small-strain, linear elastic materials. Instead, stresses are derived from the principle of virtual work using the stored strain energy potential function. In addition, foams are compressible, therefore classic strain energy functions such as the Ogden strain energy function or the Mooney-Rivlin strain energy function are not suitable to describe hyperelastic behavior of foams. So, strain energy functions must include the effect of compressibility. That means the third principal invariant of the deformation gradient tensor F should enter in strain energy functions. For rate-dependent behavior of foams, history integral constitutive law is used. For the equilibrium spring and the intermediate spring, the same strain energy function is employed. In order to use this stain energy function in history integral equation, the kernel function of it is calculated. The effect of compressibility is considered in rate-dependent behavior of foams too. All material constants were obtained from the results of uniaxial tensile tests. Nonlinear regulation was used to find these constants. In these calculations, Average strain rate was employed to find material constants.
Fabrication and Characterization of Bio-Inspired Structural Composites
Authors: Allameh Seyed M. , Ogonek Tom , Cooper Paul D.
Published: August 2009

This paper presents the results of mechanical tests performed on bio-inspired structural composites. The details of synthesis process, loading configurations, testing conditions are discussed. Results of the tests clearly show the superiority of the biomimicked layered composites made from concrete and polymer, in terms of toughness over their monolithic counterparts. The implications of the results and their impact on construction technology will be elucidated.
Fractal Finite-Element Method for Evaluating Sensitivities of Fracture Parameters for Multiple Cracked Systems
Authors: Reddy R. M. , Rao B. N.
Published: August 2009

The sensitivities of fracture parameters in cracked structures provide useful information for the prediction of stability and arrest of a single crack, the growth pattern analysis of a system of interacting cracks, configurational stability analysis of evolving cracks, probabilistic fracture mechanics analysis and universal size effect model. In the case of multiple crack systems, for example, sensitivities of fracture parameters at one crack tip due to the growth of any other crack must be calculated to determine the strength of the interaction. In probabilistic fracture mechanics analysis of linear-elastic cracked structures, the first and second order reliability methods require accurate estimates of fracture parameters, their sensitivities. This paper presents a new fractal finite element method based continuum shape sensitivity analysis for evaluating sensitivities of fracture parameters in a homogeneous, isotropic, and two dimensional linear-elastic multiple cracked system subject to mixed-mode loading conditions. The method is based on the material derivative concept of continuum mechanics, and direct differentiation. Unlike virtual crack extension techniques, no mesh perturbation is needed in the proposed method to calculate the sensitivity of fracture parameters. Since the governing variational equation is differentiated prior to the process of discretization, the resulting sensitivity equations predict the first-order sensitivity of fracture parameters, more efficiently and accurately than the finite-difference method.
Modeling of Rate Dependent Finite Deformation Viscoelastic Behavior of Foams
Authors: Anani Y. , Asghari M. , Naghdabadi R.
Published: August 2009

The behavior of foams is typically rate-dependent and viscoelastic. In this paper, multiplicative decomposition of the deformation gradient and the second law of thermodynamics are employed to develop the differential constitutive equations for isotropic viscoelastic foams experiencing finite deformations, from a phenomenological point of view, i.e. without referring to micro-structural viewpoint. A model containing an equilibrium hyperelastic spring which is parallel to a Maxwell model has been utilized for introducing constitutive formulation. The deformation gradient tensor is decomposed into two parts: elastic deformation gradient tensor and viscoelastic deformation gradient tensor. A strain energy function is presented for the equilibrium spring as a function of the invariants of the left Cauchy-Green stretch tensor to obtain equilibrium stress components. Also, a strain energy function is presented for the intermediate spring as a function of the invariants of elastic deformation gradient tensor to determine overstress components. The constants of the strain energies are calculated by using nonlinear regulation numerical methods and by comparing with the experimental data obtained from uniaxial tension tests. The developed finite deformation constitutive equations are derived such that for every admissible process, the second law of thermodynamics is satisfied.
Modeling Unstable Flow Dynamics in Porous Media Associated With CO2 Sequestration
Authors: Riaz Amir , Cinar Yildiray , Tchelepi Hamdi
Published: August 2009

Multiphase flow in porous media is fundamentally a microscopic process that governs the behavior of geologic scale processes. The application of existing (standard) macroscopic models to problems of geologic scale multiphase flow has proved to be unsatisfactory within a wide range of governing parameters. Our objective is to develop the missing link between the fundamental physics of multiphase flow at the pore-scale and the phenomenological representation of dynamic behaviors across a hierarchy of geologic scales. An essential prerequisite to such an analysis is a qualitative understanding of the flow behavior in terms of flow structures that exist for various parameter combination within the regime of CO2 sequestration. An experimental study addressing these objectives is presented. Experiments are carried out at the laboratory scale in a vertical glass-bead pack, in the parameter range of sequestration flows. Experimental results are interpreted with the help of invasion percolation models.
Analyses of Deformation in Viscoelastic Sandwich Composites Subject to Moisture Diffusion
Authors: Joshi Nikhil P. , Muliana Anastasia H.
Published: August 2009

Sandwich composites with polymer foam core are currently used in load-bearing components in buildings and naval structures due to their high strength to weight and stiffness to weight ratios, excellent thermal insulation, and ease of manufacturing. During their service time, sandwich composites are exposed to various external mechanical and hygro-thermal stimuli. It is known that the constituent properties of the sandwich composites are greatly influenced by the temperature and moisture fields. Granville [1] conducted experiments to study the effect of moisture on structural, dimensional stability, weight gain and peel strength of sandwich composites. Morganti et al. [2] analyzed the effect of moisture on the dimensional stability of the sandwich composites and concluded that moisture affects the physical behavior of the composite directly by modifying its structural characteristics such as matrix degradation and microcracks between fiber and matrix etc. However, the effect of moisture on the deformations in the sandwich composite with the viscoelastic foam cores has not yet been studied. The time-dependent response of the sandwich composite (due to the viscoelastic foam core) is aggravated in the moist environment conditions. Thus, it becomes necessary to analyze the effect of moisture on the overall response of the sandwich composites.
On the Homogenization of Nonlinearly Viscoelastic Composite Materials
Authors: Levesque Martin , Barello Romina B.
Published: August 2009

This papers shows a new linearization procedure for nonlinearly viscoelastic composites that relies on the second moment of the stresses. The predictions of this model are compared with finite elements simulations of the same microstructure. Adequate predictions are obtained for the cases simulated.
Study of the Mechanical Properties of Single-Walled Carbon Nanotubes
Authors: Jaramillo Paola , Benaroya Haym
Published: August 2009

Carbon nanotubes are composed of C-C covalent bonds, which are the strongest bonds found in nature. Hence, carbon nanotubes are identified as the “ultimate fiber” due to their great strength in the direction of the nanotube axis and their ability to enhance the elastic properties of materials. The first indications of synthesizing carbon nanotubes date back to 1952. Russian scientists Radushkevich and Lukyanovich [1] were able to produce nanosized hollow carbon filaments. Nevertheless, it was until 1991 that multi-walled carbon nanotubes (MWCNTs) were discovered by Sumio lijima [2, 3] at NEC Corporation Lab, which was followed by his study and synthesis of single-walled carbon nanotubes (SWCNTs) in 1993. Since their discovery, there has been a constant pursuit to understand the properties and identify the optimal applications of these structures. The paper focuses on the importance of carbon nanotubes and their ability to enhance the mechanical properties of other materials due to their unique elastic properties. Additionally, carbon nanotubes can improve the capabilities and properties of other materials, like polymer composite. Currently, there is an ongoing process to accurately understand the fundamental characteristics of these structures, in particular, to develop the governing laws necessary to control, predict, and manipulate these properties. This will eventually have an impact on the bulk properties of materials where carbon nanotubes may be incorporated. The current research focuses on the ability to create simplified models that can accurately predict the response of carbon nanotube structures undergoing different types of loading conditions. In this way, the mechanical characteristics regarding single-walled carbon nanotubes (SWCNTs) through finite element modeling are computed. A simplified finite element model is created in ANSYS for different types of SWCNTs with varying input parameters. An input array for the elastic modulus and load is generated to control the physical effects of these parameters in the nanotube structure. The geometries of the nanotubes are altered through various thicknesses employed for the construction of the C–C bonds. The current work contributes to the generation of different model responses to monitor the stress distribution employing a wide range of parameter values. The ability to introduce variability in the parameters and boundary conditions without altering the capabilities and computational time in the model represents the main contribution of this work.
Contact of Rough Surfaces: Combined Elastic and Rate-Dependent Effects
Authors: Sepehri Ali , Farhang Kambiz
Published: August 2009

Approximate closed form equations are found for normal and tangential contact forces of rough surfaces in dry friction. Using a viscoelastic asperity behavior, mathematical formulae are derived for normal and tangential components of the contact force that depend not only on the separation of the two surfaces but also the rate of approach and relative sliding. The tangential force over a half-plane, corresponding to the moving direction, is found accounting for the directionality of the tangential component of asperity forces. A statistical approach is forwarded in which dependence of the asperity normal and tangential contact force on relative tangential velocity of two asperities can presented as corrective factors in the mathematical description of normal and tangential force components. These are force directionality corrective coefficient and force-velocity directionality corrective coefficient. Two sets of approximate equations are found for each of the normal and half-plane tangential force components. The simplest forms of the approximate equations achieve accuracy to within five (5) percent error, while other forms yield approximation error within 0.2 percent.
Stribeck Effect in the Frictional Viscoelastic Contact of Rough Surfaces
Authors: Sepehri Ali , Farhang Kambiz
Published: August 2009

Using a nonlinear model of a two disk brake system, coupled equations of motion are found for their frictional interaction. The mathematical formulation relates the tribological events at micron scale and the macroscopic scale vibration response of a two-disk brake system. This is accomplished by a viscoelastic account of interaction at the micron scale, its statistical quantification through the approximate analytical representation of the statistical expectation of contact force and the introduction of the contact force into the macroscale dynamics of the two-disk system. Steady-state analysis of the system establishes the relation between friction torque and speed and supports observed behavior of many mechanical systems with friction. It is shown that, as a result of coupling of the macrosystem’s dynamics and contact, there are combinations of parameters at the micro- and macroscale that yield negative slope in friction torque/sliding speed relation, a well known source of dynamic instability. This results in an effective negative damping that tends to decrease with decrease in the normal load and/or increase in structural damping of the system.
Static-Kinetic Friction Transition Using a Proposed Slip Function
Authors: Farhang K. , Sepehri A. , Segalman D. , Starr M.
Published: August 2009

This paper considers the contact between two nominally flat rough surfaces. The rough surface interaction is viewed as a multi-sphere elastic interaction which upon external application of a tangential force would exhibit partial or full slip depending on the magnitude of the applied tangential force. Constitutive relation proposed by Mindlin at small scale, governing asperity interaction, is used along with the method proposed by Greenwood and Williamson to obtain the large scale slip function through a statistical summation of asperity scale events. The slip function establishes the fraction of asperity contact in full slip. The complement of the slip parameter is a fraction of asperities in partial slip. Through slip function it is shown that it is possible to define a slip condition for the entire surface. The derivation of the slip function allows the account of transition between static friction and kinetic friction. The use of the slip function is demonstrated in the dynamic response of a block in friction contact at the rough surface interface with a flat wherein the static-kinetic friction transition is correctly addressed.
Tribological Friction and Wear Behaviour of Graphite-Carbon Short Fiber Reinforced Al-17Si Alloy Hybrid Composites
Authors: Ramesh C. S. , Prasad T. B.
Published: August 2009

Graphite and carbon short fiber (Copper coated) reinforced (2 wt%) hypereutectic Al-17%Si alloy composites were prepared by liquid metallurgy route. Room temperature friction and wear properties of as cast hypereutectic Al-Si alloy reinforced with copper coated graphite and short carbon fibers were investigated both before and after heat-treatment. Friction and wear tests were conducted using a pin-on-disc machine under dry conditions. The loads were varied from 10 to 50N respectively. Results reveal that coefficient of friction and wear rate of the hybrid composite are lower than that of the matrix alloy in both heat treated and un-heat treated conditions. The coefficient of friction of the matrix alloy and its hybrid composite decreased with increased load up to 30N and increased beyond this load. The wear rates of both the matrix alloy and its hybrid composite increased with the increasing load. However at all the studied, the developed hybrid composite exhibited a lower coefficient of friction and wear rates when compared with the matrix alloy.
A Finite Element Seta Model for Studying Gecko Adhesion
Authors: Sauer Roger A.
Published: August 2009

We present a hierarchical rod model which describes the deformation and adhesion of Gecko setae. These fine hairs form the microstructure of the Gecko toes and enable the Gecko to adhere to inclined and overhanging surfaces. The adhesion is modelled by the van der Waals interaction between the molecules of the seta tips and the surface. To bridge the gap between molecular and seta scale, an intermediate model is formulated for the seta tips, the so called spatulae. The hierarchical seta model is cast into a nonlinear finite element framework to study the pull-off behavior of a single seta from an underlying substrate.
Durability Analysis for Tires by Using CED and VCCT Method With Consideration of Friction
Authors: Kim Taewung , Hwang Sangyoon , Jeong Hyun-Yong
Published: August 2009

In order to find out a physical quantity which controls the fatigue life of a structure and to predict the fatigue life of tires, a finite element simulation methodology to use the cracking energy density (CED) and the virtual crack closure technique (VCCT) was proposed and applied to a tire. CED was calculated to predict the location and the plane of a newly created crack, and VCCT was used to obtain the strain energy release rate (SERR) at the tip of a newly created crack with consideration of friction. For the SERR calculation, the elements in front of and behind a crack should be split. Finite element simulations showed that as a tire rotated, SERR oscillated with its minimum occurring just before the center of the contact zone and its maximum occurring just after the center of the contact zone, and the amplitude of SERR could be determined from the simulation results. Then, the fatigue life was predicted based on Paris law using the amplitude of SERR, and it was compared with the durability test data of the tire. The comparison showed that the predicted fatigue life was in fair agreement with the test data.
Microwave Absorbing Properties of Fe-Filled Carbon Nanotubes/Epoxy Composite in 2-18GHz
Authors: Gui Xuchun , Wang Kunlin , Wei Jinquan , Lv Ruitao , Wu Dehai
Published: August 2009

Carbon nanotubes (CNTs) filled with ferromagnetic metals have potential applications of microwave absorption and electromagnetic interference shielding due to their high aspect-ratio of nanostructures that promises low percolation threshold and their inner cavities which could incorporate magnetic materials to improve the permeability and magnetic loss of composites. It has been reported that CNTs have high permittivity [1, 2]. And it could be used as dielectric microwave absorbers due to their high permittivity [3, 4]. Excellent microwave absorption should have better electromagnetic match of magnetic loss and dielectric loss. The microwave absorption could be significantly improved by matching the dielectric and magnetic contribution from respective components (e.g. CNTs and Fe) [5]. For CNTs, only dielectric loss contributes to the energy loss of microwave, while for ferromagnetic nanowires, the effect of magnetic loss becomes dominant over the dielectric loss. Either only the dielectric loss or magnetic loss may induce weak microwave absorption due to the imbalance of the electromagnetic match. Here we directly fabricated the microwave absorber by as-grown Fe-filled CNTs, which have the dielectric properties of CNTs and magnetic properties of the filled iron nanowires.
Performance Assessment of a 3D Cable-Driven Haptic Device
Authors: Rosati Giulio , Zanotto Damiano , Rossi Aldo
Published: August 2009

Haptic displays are special devices, able to provide the sense of force or moment feedback to the human operator by rendering a variable mechanical impedance. They are widely used in Virtual Reality, teleoperation and virtual-drive systems. In this paper, the problem of measuring the force-exertion capability of a spatial cable-driven haptic display is addressed. A new set of manipulability indexes that applies to all completely restrained cable-based spatial systems is developed, by generalizing a set of indexes previously introduced for planar devices. These indexes can be used to analyze the working space of cable-based devices and/or to optimize their parameters (e.g., the locations of cable attachment points). In the paper, the set of manipulability indexes is applied to the analysis of a desktop cable-driven haptic display, called the PiRoGa5. PiRoGa5 is a 5 degree-of-freedom haptic pen, driven by 6 cables that are arranged to form two opposite pyramids, whose bases lay in two parallel planes and whose vertexes coincide with the ends of the pen-like end-effector. Wires are wound onto pulleys, pass through fixed eye-bolts, and are alternatively fixed to the upper and lower ends of the end-effector. The kinematics of the system is derived first, and position accuracy is inspected. Secondly, the new manipulability indexes are employed to determine and optimize the working space of the haptic device. Finally, a method is presented to analytically compute the wire tensions for a given wrench.
A Haptic System to Enhance Stability of Heavy Duty Machines
Authors: Rosati Giulio , Biondi Andrea , Cenci Stefano , Rossi Aldo , Boschetti Giovanni
Published: August 2009

The operator’s safety is a very crucial issue in heavy duty machines. The dangerousness of the working environment forces the operator to consider several parameters at the same time, and a large amount of practice and concentration are usually required to operate the machine safely. In fact, man-machine interaction is often very poor in these kind of machines. With the aim of improving this interaction and enhancing stability, a stability control system is proposed, which is based on a simplified model of the machine and on a 2-degrees-of-freedom force-feedback joystick, and combines the use of a passive ad an active actuator. Active drives can generate high-fidelity sensations but, in general, they have a poor torque/size ratio, they are expensive and can become dangerous for the operator. On the other hand, passive devices (like brakes) can generate higher torques, are cheaper and safer, however they cannot provide energy to the system but only dissipate it; this feature reduces the range of sensations that can be rendered to the operator. In order to partially overcome this limitation, an elastic element is integrated in series with the brake. In this way, it is possible to store some potential energy that can be returned to the operator to give a better sensation of the proximity of an instability working condition for the machine. In the paper, the stability control system is introduced, and the design of the haptic joystick is presented. First experimental results on a simulator are finally illustrated.
A Novel Perspective in the Design of Cable-Driven Systems
Authors: Rosati Giulio , Zanotto Damiano
Published: August 2009

This paper deals with a novel approach to the design of cable-driven systems. This kind of robots possesses several desirable features that distinguish them from common manipulators, such as: low-inertia, cost-effectiveness, safety, easy reconfiguration and transportability. One key-issue that arises from the unilateral actuation is the design for workspace optimization. Most previous researches on cable-driven systems design focused their attention on workspace analysis for existing devices. Conversely, we introduce a new approach for improving workspace by design, introducing movable pulley-blocks rather than increasing the number of cables. By properly moving the pulley-blocks, the end-effector can be always maintained in the best part of the working space, thus enhancing robot capabilities without the need for additional cables. Furthermore, the eventuality of cable interference is strongly reduced. In this paper, the novel design concept is applied to different planar point-mass cable-driven robots, with one or more translating pulley-blocks. The maximum feasible isotropic force, along with the power dissipation and the effective mass at the end-effector are employed to compare the performances of different configurations.
Determination of Optimum Hollowness for Hollow Cylindrical Rolling Element Bearing
Authors: Darji P. H. , Vakharia D. P.
Published: August 2009

Hollow-cylindrical-roller bearings are intended for light load applications, including all types of grinding, machining, and milling spindles. They use hollow cylindrical rollers instead of solid rollers. These provide significant advantages over standard bearings, such as reduced vibration, low-radial runout, better radial stiffness, higher operating speeds, and lower operating temperatures. Investigators have proposed that under large normal loads a hollow element with a sufficiently thin wall thickness will deflect appreciably more than a solid element of the same size. An improvement in load distribution and thus load capacity may be realized, as well as contact stress is also reduced considerably by using a bearing with hollow rolling elements. Since for hollow rolling element no method is available for the calculation of contact stresses and deformation. The contact stresses in hollow members are often calculated by using the same equations and procedures as for solid specimens. This approach seems to be incorrect. Recently, the Finite Element Analysis (FEA) has been successfully used to evaluate contact problems for the roller bearings. Investigations have been made for hollow rollers in pure normal loading. Different hollowness percentages ranging from 0% and 30% to 80% have been analysed in FEA software to find the optimum percentage hollowness which gives minimum stress and finally longest fatigue life.
Modeling Grain Boundary Scattering in Nanocomposites
Authors: Minnich Austin , Vashaee Daryoosh , Chen Gang
Published: August 2009

Modeling the thermoelectric properties of nanocomposites is difficult due to the complex grain boundary scattering processes which scatter both electrons and phonons. In this work we introduce several models which characterize the grain boundary scattering rate. This scattering rate is incorporated into a code we developed which numerically calculates the electrical and thermal properties of bulk and nanocomposite thermoelectric materials using the Boltzmann equation under the relaxation time approximation. The code is capable of calculating all the relevant thermoelectric properties over a wide range of temperatures, doping concentrations, and compositions, allowing for a full characterization of the material. We formulate two grain boundary scattering rates and implement the commonly used Mayadas model. The code and all of the grain boundary scattering models are then validated on bulk data for SiGe and applied to nano-SiGe. All the models are able to fit the experimental data after a fitting parameter is suitably chosen, so we determine which model is most appropriate based on the physical assumptions made by each model.
A Multiscale Finite Element Framework for Reactive Contaminant Transport in Heterogeneous Porous Media
Authors: Nakshatrala K. B. , Valocchi A. J.
Published: August 2009

Mixing of chemical species across plume boundaries has a major influence upon the fate of the reactive pollutant in the subsurface. Small-scale heterogeneity leads to irregular plume boundaries which enhances mixing-controlled reactions through increasing the interfacial area of the plume. Therefore, it is crucial to capture this small-scale heterogeneity in order to properly model reactive transport. Unfortunately, computational limitations do not permit full resolution of the smallest scales of heterogeneity as the size of geomodels used for reservoir description typically exceeds by several orders of magnitude the capabilities of conventional reservoir simulators. Thus, it is necessary to use a coarse numerical grid, particularly for the cases with a large number of reactive species.
An Experimental Analysis on Rubber-Metal Contact Stress Corrosion in Seawater
Authors: Shen H. , Wen Q. , Lifer K. C.
Published: August 2009

Rubber components for liquid sealing are widely used in corrosive environments such as marine applications or food-processing equipment. Corrosion is found more frequently at the rubber-metal contact area or its adjacent area than non-contact area in certain situation. While the role of rubber components in corrosion is not fully understood, the deterioration of rubber in aqueous environments may release chloride or acid to accelerate the corrosion process. Contact stress and local liquid entrapment may also affect the corrosion process. In this work, corrosion of rubber parts in combination with different metals and different contact pressure in seawater is studied experimentally. Metals used include titanium, bronze, nickel, aluminum, stainless steel, and 4130 steel. O-rings with different cross section shapes are used to study the role of different contact area shapes in corrosion process. The contact area of rubber components and the metals are designed to have different contact stress. It is found that the corrosion of metals is accelerated by interaction with rubber and contact stress in seawater. The corrosion initiates from the contact area of metals with rubber O-rings. Different materials show different corrosion behaviors in seawater.
A New and Consistent Approach for Deriving Brinson’s 1-D Constitutive Equation for Shape Memory Alloys
Authors: Khandan Rasoul , Mahzoon Mojtaba , Fazelzadeh Seyed Ahmad , Hemmasizadeh Ali
Published: August 2009

One of the first 1-D constitutive equations for shape memory alloys was presented by Brinson L.C in 1993 that became the base of later works typically. In Brinson’s equation, several proposed functions are considered in order to simplify the model and obtain the constitutive equation for SMA. In a recent paper V. R. Buravalla and A. Khandelwal (2007) have shown certain anomalies in Brinson’s model and have tried to present a modified model which unlike Brinson’s model satisfies the compatibility condition. However, their formulation, besides being lengthy, lacks clarity and in particular does not address proper expressions for transformation tensors Ωs and ΩT. In the present work, Brinson’s constitutive equation is derived from fundamental relations using a simple, clear-cut and straightforward approach. Without any extra and unnecessary assumption the consistency of the model is confirmed.
Micromechanical Simulations on Waving and Kinked Natural Fiber-Reinforced Plastic Composites
Authors: Xue Yibin , Fletcher Scott A. , Wang Kunpeng
Published: August 2009

Micromechanics-based simulations were conducted to evaluate the linear and nonlinear properties of natural fiber-reinforced plastic composites with fibers in various waving and kinked forms. Natural fibers, such as woodfibers and fibers from plants, have length-aspect ratio of longitudinal and transverse at or greater than 20. At such high aspect ratio, the natural fiber normally presents in waving, bending, twisting, kinking morphology in the composites. This paper presents a series of micromechanical simulations to predict the elastic and nonlinear elastic behaviors of natural fiber-reinforced plastic composites (NF-PCs) considering the effects of fiber kinking, waving, and arrangements on the stress-strain relationship. A set of three-dimensional unit cells (UC) were developed to mimic various fiber morphologies with the fiber volume fraction of fifty percent, a typical fiber volume fraction for the natural fiber plastic composites. Periodic displacement boundary conditions were implemented on the UC to simulate a unidirectional strain field. The homogenized anisotropic stress-strain relations for NF-PCs were predicted by postulating nonlinear behavior of plastic matrix and perfect and imperfect interface between the NF and the matrix. Stress distributions in the natural fiber were presented as a function of the fiber aspect ratio and the fiber waving and kinking forms. Even though, the high fiber aspect ratio provides relatively high elastic modulus and nonlinear hardening, it also induces high stresses or stress concentration in the fiber that may result in earlier failure of the fiber when the composites undergone a relatively large deformations (> 4%).