bluuebunny/crossref_metadata_2025 huggingface dataset:
Variational Estimates for the Elastoplastic Response of Particle-Reinforced Metal-Matrix Composites
Authors: Li Guoan , Ponte Castan˜eda P.
Published: April 2009

Ductile solids reinforced by aligned elastic spheroidal inclusions, with overall transversely isotropic symmetry, are examined analytically in this paper. Estimates for the effective constitutive behavior of this class of composite materials are obtained in terms of simple optimization problems for general loading conditions, as functions of the particle stiffness, concentration and shape. In particular, explicit expressions are obtained for the yield functions of the composites. The results apply to composites with inclusion shapes ranging from continuous fibers (or needles in the limit of vanishingly small concentration), to approximately spherical, to continuous flat layers (or disks). As an example, we investigate a model composite of the type used in many structural applications, namely, 2124 Al–SiC which is made of a ductile matrix phase (Al) reinforced by hard brittle particles (SiC). The predicted stress-strain responses for these composites are compared with available experimental measurements and numerical calculations. Thus, it is shown that the constitutive model developed in this work predicts fairly accurately the uniaxial tensile experiments of Christman et al. (1989). In addition, the constitutive model is in good agreement with the periodic finite-element calculations of Tvergaard (1990) and Hom (1992), also for uniaxial loading conditions. A significant advantage of the analytical model proposed herein is that it can provide the constitutive response of composites under arbitrary loading conditions, without requiring complex numerical computations.
Laboratory Observations of Gravity Wave, Critical Layer Flows Using Single and Double Wave Forcing
Authors: Delisi Donald P. , Dunkerton Timothy J.
Published: April 2009

Laboratory measurements of gravity wave, critical layer flows are presented. The measurements are obtained in a salt-stratified annular tank, with a vertical shear profile. Internal gravity waves are generated at the floor of the tank and propagate vertically upward into the fluid. At a depth where the phase speed of the wave equals the mean flow speed, defined as a critical level, the waves break down, under the right forcing conditions, generating small scale turbulence. Two cases are presented. In the first case, the wave forcing is a single, monochromatic wave. In this case, the early wave breaking is characterized as Kelvin-Helmholtz breaking at depths below the critical level. Later wave breaking is characterized by weak overturning in the upper part of the tank and regular, internal mixing regions in the lower part of the tank. In the second case, the wave forcing is two monochromatic waves, each propagating with a different phase speed. In this case, the early wave breaking is again Kelvin-Helmholtz in nature, but later wave breaking is characterized by sustained overturning in the upper part of the tank with internal mixing regions in the lower part of the tank. Mean velocity profiles are obtained both before and during the experiments.
Utilizing Low-Dimensional Dynamical Systems Models to Guide Control Experiments
Authors: Corke Thomas C. , Glauser Mark N. , Berkooz Gal
Published: April 2009

In this paper we describe an effort in which the proper orthogonal decomposition (see Lumley 1967) based low-dimensional dynamical system of Glauser et al (1992) is utilized to guide control experiments for the jet. These ideas are then applied using the experimental setup of Corke and Kusek (1993). The experiment uses a closed-loop feed-back to control the flow field, including the possibility of converting the instabilities from convective to global. In the dynamical systems simulations, cascade phenomena are observed where streamwise wavenumbers act as trigger modes to excite various helical modes, in particular azimuthal modes 4, 5 and 6. When these higher helical modes were excited through the experimental control scheme, similar effects, such as mode competition and switching have been observed. These results clearly show the utility of using low-dimensional systems models to guide control experiments.
Hypervelocity Flow Simulation
Authors: Hornung Hans , Wen Chihyung , Germain Patrick
Published: April 2009

Many of the flow problems associated with flight vehicles designed to reach or return from space can not be solved computationally. It is essential to address them by experiment, in particular, by ground simulation of the flow. The requirements and most successful simulation techniques are described, and their important limitations are discussed. Two selected examples are then presented from the free-piston reflected shock tunnel T5 at Caltech: Dissociating flow over spheres and transition from laminar to turbulent flow on a slender cone.
Myths, Math, and Physics of Free-Surface Vorticity
Authors: Rood Edwin P.
Published: April 2009

The boundary equations for the production of vorticity are examined in the case of a free surface. Mathematical manipulation of the equations leads to interesting results which are given physical interpretation through examination of three flows: a surface wave, an accelerated free-surface channel flow, and the vortex ring interaction with a free surface. The predicted phenomena are unusual, and require abandonment of preconceptions regarding the notion that vorticity is angular momentum. The flux and appearance of vorticity in free-surface flows is shown to be a consequence of necessary viscous accelerations at the boundary even for otherwise irrotational subsurface flow. The deformation of the free surface is the critical factor separating the free-surface problem from the vortex image analogy.
Numerical Simulation of Free-Surface Turbulence
Authors: Dommermuth Douglas G. , Mui Rebecca C. Y.
Published: April 2009

Direct numerical simulations and large-eddy simulations of turbulent free-surface flows are currently being performed to investigate the roughening of the surface, and the scattering, radiation, and dissipation of waves by turbulence. The numerical simulation of turbulent free-surface flows is briefly reviewed. The numerical, modeling, and hardware issues are discussed.
Compressibility Effects on Turbulence Growth in High-Speed Shear Flows
Authors: Sarkar S.
Published: April 2009

Compressibility effects on the evolution of turbulence are obtained from direct numerical simulation of homogeneous shear flow. It is found that when the gradient Mach number - a parameter based on the mean shear rate, integral length scale and speed of sound - increases, the growth of turbulent kinetic energy is inhibited. The reduced ‘efficiency’ of production is shown to lead to the inhibited growth of turbulent kinetic energy. Implications for inhomogeneous shear flows are discussed.
Physics and Prediction of Supersonic Jet Noise
Authors: Tam Christopher K. W.
Published: April 2009

Both the large turbulence structures and the fine scale turbulence of the flows of supersonic jets are sources of turbulent mixing noise. At moderately high supersonic Mach numbers especially for hot jets, the dominant part of the noise is generated directly by the large turbulence structures. The large turbulence structures propagate downstream at supersonic velocities relative to the ambient sound speed. They generate strong Mach wave radiation analogous to a supersonically travelling wavy wall. A stochastic instability wave model theory of the large turbulence structures and noise of supersonic jets has recently been developed. The theory can predict both the spectrum and directivity of the dominant part of supersonic jet noise up to a multiplicative empirical constant. Calculated results agree well with measurements.
Methods for Enhanced Turbulence Mixing in Supersonic Shear Flows
Authors: Gutmark E. J. , Schadow K. C. , Yu K. H.
Published: April 2009

Supersonic shear layers have inherent low mixing rates due to compressibility effects. Their mixing rate relative to subsonic shear layers can be up to 5 times lower. Several important technological applications which require intense mixing of supersonic flows gave impetus to research aimed to develop methods to enhance mixing in compressible flows with minimal performance penalty. This paper reviews some of these methods applied to both planar shear layers and jets. The methods are arranged in several categories: passive and active control of shear layer instabilities, three dimensional jets, generation of axial vorticity and shock interaction with shear layers. The paper concludes by discussing the importance of the wide range of length-scales in which turbulent mixing occurs.
Mechanics of Contact Fatigue
Authors: Keer Leon M.
Published: April 2009

Contact fatigue between typical machine elements such as gears, roller followers, bearings and other components involves many complex interacting features. There are the effects of geometry, mechanical properties, material properties and surface chemistry. The present discussion will center around analytical prediction techniques that are concerned only with the mechanical aspects of contact fatigue between two elements. Aspects related to the initiation of a crack under repeated loading will be discussed. The application of an approach developed by Mura, analogous to Griffith’s criterion for fracture, will be used to show how estimates of initiation life can be made and how these estimates are related to currently used ones. Once a crack has been initiated, then issues related to crack propagation become important. Some fracture mechanics based methods developed to calculate crack growth will be described along with estimates of crack propagation life.
Tribology of Solid Contacts in Magnetic Recording Devices
Authors: Bhushan Bharat
Published: April 2009

The demand for close proximity of magnetic head to the medium during operation of magnetic recording devices, high surface smoothness, and high reliability requires that friction and wear of the head-medium interface be minimized. Friction and wear are minimized by suitable selection of interface materials and lubricants, optimization of surface roughness, and operating conditions. This invited paper presents the state of the art of the characterization of surface roughness and modelling of dry and wet solid contacts which are critical to the design of the modern magnetic storage devices.
Wear Mechanisms in Ceramic Machining
Authors: Farris T. N. , Chandrasekar S.
Published: April 2009

Ceramic materials are finished primarily by abrasive machining processes such as grinding, lapping, and polishing. In grinding, the abrasives typically are bonded in a grinding wheel and brought into contact with the ceramic surface at relatively high sliding speeds. In lapping and polishing, the ceramic is pressed against a polishing block with the abrasives suspended in between them in the form of a slurry. The material removal process, here, resembles three body wear. In all of these processes, the mechanical action of the abrasive can be thought of as the repeated application of relatively sharp sliding indenters to the ceramic surface. Under these conditions, a small number of mechanisms dominate the material removal process. These are brittle fracture due to crack systems oriented both parallel (lateral) and perpendicular (radial/median) to the free surface, ductile cutting with the formation of thin ribbon-like chips, and chemically assisted wear in the presence of a reactant that is enhanced by the mechanical action (tribochemical reaction). The relative role of each of these mechanisms in a particular finishing process can be related to the load applied to an abrasive particle, the sliding speed of the particle, and the presence of a chemical reactant. These wear mechanisms also cause damage to the near ceramic surface in the form of microcracking, residual stress, plastic deformation, and surface roughness which together determine the strength and performance of the finished component. A complete understanding of the wear mechanisms leading to material removal would allow for the design of efficient machining processes for producing ceramic surfaces of high quality.
Shear Band Analysis for Lubricants Based on a Viscoelastic Plasticity Model
Authors: Lee Y. K. , Ghosh J. , Bair S. , Winer W. O.
Published: April 2009

Shear banding is an outstanding problem in lubricant rheology where a thin film of lubricant is under high pressure and high shear stress. To study such a phenomenon, a viscoelastic-plastic model is proposed. It is postulated that shear bands appear when the character of the field equations changes from elliptic to hyperbolic. The model is derived based on a rate formulation which combines a Maxwell fluid model and a compressible rate-independent plasticity model. The model gives the constitutive relation of a compressible viscoelastic-plastic fluid for which the assumption of Stokes condition becomes unnecessary. It also accounts for the elastic coupling effect of both the viscous and the rate-independent behavior of a lubricant. It provides a novel feature that the development of shear bands can be tracked rather than being determined after they are fully developed. Hence, the process of shear banding may be articulated. The analysis focuses on identifying necessary and sufficient conditions which, if achieved in a deformation, would produce a change in character of the governing field equations. Through the use of Drucker-Prager criterion as the component that governs the rate-independent behavior of a lubricant, the analysis gives results that are in accord with experimental observations. Parametric results are given graphically.
Macroscopic Modeling of Viscous Suspension Flows
Authors: Brady John F.
Published: April 2009

Shear-induced particle migration in viscous suspension flows is shown to lead to intrinsic concentration variations in inhomogeneous shear flow. A recently proposed suspension balance model is discussed that explains this migration as resulting from the requirement that the macroscopic suspension pressure be constant perpendicular to the direction of mean motion. The results of this model are shown to compare well with Stokesian Dynamics simulations of pressure-driven channel flow.
Shear-Induced Size Segregation Phenomena in Bidisperse Suspensions
Authors: Krishnan Gokul P. , Leighton David T.
Published: April 2009

The behavior of tracer particles in a concentrated suspension of non-colloidal spheres sheared in a parallel plate viscometer at low Reynolds number was investigated. The tracer particles were shown to migrate radially, with the tracer particles larger than those making up the suspension migrating radially outward. The migration velocity of the large tracer particles was measured, and found to be consistent with a shear-induced migration scaling (Leighton and Acrivos, 1987b), however the direction of the migration was the reverse of that suggested by the experiments of Abbot, et al. (1991) in a wide gap Couette device. The particle migration and segregation described here are interpreted in terms of the curvature of the flow geometry.
Rapid Granular Flow Down Inclines
Authors: Jenkins James T.
Published: April 2009

As an example of the activity in the field of rapid granular flow, we sketch an analysis of a rapid granular flow of identical frictionless spheres that is driven by gravity down an incline. The flow is assumed to be dense, collisional, steady, and fully developed. Because we employ conditions at the base of the flow that are appropriate for a bumpy, frictionless boundary, the analysis is slightly more complicated than that of Savage (1983a, in Theory of Dispersed Multiphase Flow, RE Meyer (ed), Academic Press, New York, 339-358). Because we restrict our attention to dense flows, it is somewhat simpler than that of Richman and Marciniec (1990, J Appl Mech57, 1036-1043). It is essentially that of the dense collisional regime considered by Anderson and Jackson (1992, J Fluid Mech241, 145-168). We outline the determination of the profiles of the mean velocity, fluctuation velocity, and concentration through the depth of the flow and indicate how the boundary conditions provide relations between the depth of the flow, the angle of inclination, the fluctuation velocity at the base of the flow, and the mean velocity at the free surface.
Imperfection Sensitivity of Stiffened Cylindrical Shells Under Interactive Buckling
Authors: Sridharan Srinivasan
Published: April 2009

In this paper some recent advances made in the understanding of the phenomena and computational modelling of the interaction of local and overall instabilities in stiffened cylindrical shells will be reviewed. These relate to two distinct categories of problems: (1) Axially compressed stringer-stiffened shells and (2) Ring-stiffened cylinders subjected to hydrostatic compression. The former have been analyzed with a novel methodology which employs finite elements in which the local buckling information is embedded. Comparisons of the results of the new technique with Abaqus - a well established nonlinear analysis program - reveals the validity of the underlying concepts of the new technique and efficacy of the new approach. It is shown, that provided all the key local buckling modes triggered in the interaction are considered and the modulation of local buckling amplitudes is accounted for, it is justifiable to neglect the mixed second order stresses and strains in the analysis. Imperfection-sensitivity of a stringer stiffened cylindrical shell structure is illustrated. In the case ring-stiffened cylinders subjected to hydrostatic pressure, it is shown that the amplitude modulation is the key factor in the interaction; it performs the function of capturing the contributions of several neighboring modes of the same longitudinal description as the fundamental local mode, but with differing circumferential wave numbers. An examination of the potential energy function indicates that the amplitude modulation is solely responsible for the presence of the nonvanishing cubic terms, which are dominant over the quartic terms. Once again, mixed second order fields evaluated with appropriate orthogonality conditions have little influence on the interaction and can be safely neglected. An example of an orthotropic layered shell under coincident and well separated critical stresses is presented.
Model for the Micro-Buckling/Micro-Kinking Compressive Response of Fiber-Reinforced Composites
Authors: Chung Ilsup , Weitsman Y. Jack
Published: April 2009

This article focuses on the effects that non-uniform fiber spacings have on the compressive response of composites. The above type of imperfection is included in a model which incorporates initial fiber misalignments and accounts for non-linear shear response of the matrix and shear-deformable fibers. It is shown that the disparate response of fibers embedded within matrix layers of different thicknesses results in reduced levels of compressive strengths and leads to failure modes which contain discontinuities in the fiber shear strains.
Molecules, Mechanics, and Migration of Cells
Authors: Lauffenburger Douglas A.
Published: April 2009

A mechanistic model for migration of mammalian blood and tissue cells over two-dimensional substrata has been developed in order to understand effects of interactions of cell surface adhesion receptors with extracellular matrix ligands and intracellular cytoskeletal components. A central prediction of the model — that cell migration speed depends in biphasic (increasing, then decreasing) manner on the ratio of cell/substratum adhesive force to intracellular motile force — has been demonstrated consistent with experimental comparisons of smooth muscle cell migration on fibronectin and collagen, and of endothelial cell migration on fibronectin in the absence and presence of the soluble receptor-binding competitor echistatin. Further, two crucial underlying assumptions of the model have received experimental support: that there is a front vs. rear asymmetry in the physical strength of ligand-receptor-cytoskeleton linkage which can be modulated by chemical changes; and, that the physical force needed to disrupt ligand-receptor or receptor-cytoskeleton linkages varies with the chemical affinity of those linkages. Thus, this model relates biochemical and biophysical properties of molecular components involved in cell migration to a measurable feature of this behavioral function. A central concept emerging from this work is that biochemical and biophysical aspects of molecular interactions are closely related, and that alterations in one can influence the other in useful ways for modulating this, and likely other, cell functions.
Mechanics as a Basic Component of Future Engineering Curricula
Authors: Drucker Daniel C.
Published: April 2009

A consensus is beginning to emerge that the time has come for a radical change in all of our engineering curricula to make each of them far more responsive to the needs of today and tomorrow. However, there is profound disagreement on what directions of change are appropriate. It is essential that you, the mechanics community in the universities, industry, government, and private practice, enter immediately and vigorously into these ongoing debates on the future of undergraduate engineering education. Similar controversial issues are emerging at the graduate level and also call for your early input. You appreciate that mechanics in all its aspects is but a fraction of the totality of engineering education. However, you best understand why education in mechanics and education through mechanics, and the many other branches of fundamental engineering science, are among the essential building blocks for all engineers. You owe it to your colleagues in the specialized engineering disciplines to reach consensus among yourselves first, and then share your insight on the appropriate role of mechanics in the future before they commit themselves and you to any course of action.
Nonlinear Thinking in Mechanics and Design
Authors: Moon Francis C.
Published: April 2009

While the spread of computer aided design tools in the last two decades has been revolutionary, much of its analytical basis in elasticity, vibrations, thermal systems etc, rests on linear models. These new ideas have found application in many areas of applied science and are now just beginning to find their ways into practical devices. (See eg, Moon (1992)). As these ideas mature, it is natural to ask if they can be introduced into the undergraduate curriculum. The Author argues that creative solutions to design problems often result from using nonlinear models and concepts. Nonlinearity, often seen as something to be avoided, can sometimes offer alternative solutions that linear models cannot do. The Author also argues the case for introducing nonlinear thinking into the engineering curriculum in mathematics, mechanics of materials and dynamics using both computational and experimental laboratories. This paper describes a program at Cornell University to introduce nonlinear dynamics concepts to mechanical engineering undergraduates through an NSF sponsored grant for curriculum development.
Revised Mechanics Curriculum in the Department of Aeronautics and Astronautics at MIT
Authors: McManus Hugh L.
Published: April 2009

The mechanics curriculum in the Department of Aeronautics and Astronautics at MIT is being revised as part of a broad revision of the entire research and teaching program of the department. The traditional MIT approach of fundamental engineering science taught with an emphasis on its practical application has not changed, but it has been complemented by some important innovations. The mechanics curriculum has been revised and updated from the ground up. It has been coordinated with the rest of the aeronautical engineering curriculum, assuring that students understand the place of mechanics in the overall engineering of an aerospace system. Material science has been integrated throughout the mechanics curriculum. Finally, mechanics education plays a large role in teaching the “implicit curriculum”, a formalization of the skills and knowledge that a student is expected to gain outside of the stated curriculum.
Multiphase Flow Quantitative Visualization
Authors: Georgiadis John G.
Published: April 2009

Full-field quantitative visualization of multiphase flows requires the introduction of high resolution noninvasive methods. Two such methods are presented: Scanning Confocal Microscopy (SCM), and Magnetic Resonance Imaging (MRI). SCM has higher resolution, contrast, and depth discrimination than conventional light microscopy. A modern SCM system operating in reflection mode performs optical sectioning of 3D surfaces with submicron resolution at video rates, and this suggests its use in reconstructing evolving interfaces. MRI is a versatile tool for mapping the distribution of liquids (primarily water) in 3D space and for performing multicomponent velocity measurements. MRI is the only practical solution in systems that are strongly refracting or opaque to visible light. SCM is employed (for the first time) to image frost growing under ambient conditions, and MRI is used to visualize phase change and to measure local velocity in natural convection in water-saturated porous media. These problems reflect the research interests of the author but also serve to show the potential of the techniques in probing multiphase flows containing complex interfaces.
Computational Modeling of Material Failure
Authors: Needleman A.
Published: April 2009

Analyses of fracture are discussed where the initial-boundary value problem formulation allows for the possibility of a complete loss of stress carrying capacity, with the associated creation of new free surface. No additional failure criterion is employed so that fracture arises as a natural outcome of the deformation process. Two types of analyses are reviewed. In one case, the material’s constitutive description incorporates a model of the failure mechanism; the nucleation, growth and coalescence of microvoids for ductile fracture in structural metals. In some analyses this is augmented with a simple characterization of failure by cleavage to analyze ductile-brittle transitions. The other class of problems involves specifying separation relations for one or more cohesive surfaces present in the continuum. The emphasis is on reviewing recent work on dynamic failure phenomena and the discussion centers around issues of length scales, size effects and the convergence of numerical solutions.
Problems in Modeling Disperse Two-Phase Flows
Authors: Hunt J. C. R. , Perkins R. J. , Fung J. C. H.
Published: April 2009

This review touches on some of the fundamental problems in this subject and some practical solutions, viz: (i) the force on a small rigid particle, solid or gaseous; a general heuristic expression is presented for a spherical particle based on combining the limiting cases of inviscid non-uniform flow and simple viscous flow; (ii) how the lift and acceleration forces produce non-uniform distributions of bubbles in non-uniform turbulent pipe flows inclined at different angles to the horizontal; computer simulations are presented using the results of (i); (iii) the relative contributions of the spatial and temporal fluctuations to the difference between the diffusivities of solid and fluid particles; an idealised model of small inertial particles in turbulent motion gives useful insight; (iv) the differences in the spectra of their velocities; hypotheses based on (iii) tested by computing the trajectories of particles in velocity fields that simulate turbulence (Kinematic Simulation); (v) how low concentrations of particles interact between each other and affect the average flow field.
Dynamics of a Small Spherical Particle in Steady Two-Dimensional Vortex Flows
Authors: Lasheras Juan C. , Tio Kek-Kiong
Published: April 2009

The equation of motion of a small spherical particle in an isolated Rankine vortex is analyzed using an asymptotic scheme valid for the limiting case of small Stokes number St. The effects of particle inertia and added mass, gravity, the acceleration of the fluid, viscous drag, and the Basset history force are taken into consideration. For the case of an isolated Rankine vortex, the analysis shows that in the region where the fluid velocity is large enough, the viscous drag constrains the particle to move with a velocity equal to that of the fluid plus a perturbation of order St. This perturbative term incorporates the effects of gravity, the density difference between the particle and the fluid, and the local acceleration of the fluid. In the region where the fluid velocity is small, the particle moves with a velocity equal to the sum of the fluid velocity and the rising/settling velocity of the particle in still fluid, the effects of particle inertia and fluid acceleration appearing as small perturbations. Throughout the whole region of the flow, the effect of the Basset force always appears at higher order than the other forces acting on the particle and may, consequently, be neglected. The analysis also shows that a particle with a mass density greater than that of the fluid always escapes from the central region of the vortex, but a buoyant particle may be trapped by the equilibrium point located there.
Simulation of Interactions Between Microbubbles and Turbulent Flows
Authors: Maxey M. R. , Chang E. J. , Wang L. -P.
Published: April 2009

Microbubbles formed by small air bubbles in water are characterized as spherical inclusions that are essentially rigid due to the effects of surfactants, and respond to the action of drag forces and added-mass effects from the motion relative to the surrounding fluid. Direct numerical simulations of homogeneous, isotropic turbulence are used to study the effects of the small-scale, dissipation range turbulence on microbubble transport and in particular the average rise velocity of microbubbles. It is found that microbubbles rise significantly more slowly than in still fluid even in the absence of a mean flow, due to a strong interaction with the small-scale vorticity. The way in which microbubbles might modify the underlying turbulence by the variations in their local distribution is discussed for dilute, dispersed systems and some estimates for the enhanced viscous dissipation given.
Particle Induced Turbulence
Authors: Tsuji Y. , Tanaka T. , Yonemura S.
Published: April 2009

A strong turbulent field produced by particle clouds was simulated in a numerical calculation based on the DSMC method. The flow studied was a gas-solid flow in a vertical channel. Particles were initially distributed homogeneously. Under the effects of particle-fluid and particle-particle interactions, the particle distribution gradually becomes inhomogeneous and finally leads to formation of particle clouds. These clouds are a kind of large obstacles for fluid motion, and produce large scale fluid turbulence.
Concrete Plasticity: Recent Developments
Authors: Chen W. F.
Published: April 2009

The present state of some key aspects of development of concrete plasticity is summarized. Emphasis is placed on the representation and interpretation of the inelastic deformation of concrete materials in the post-elastic range. The development of plasticity-based models in prepeak stress range is first summarized, emphasizing the fundamental concepts peculiar to concrete materials. A general formulation of softening behavior by the plasticity theory combined with the fracturing (damage) theory is then discussed. Next, recent studies relating macrospace features and microscopic events are presented. Finally, the role of cement-paste-sand interfaces in providing a deeper understanding of the concrete materials at the microscale are explored. Directions for further research are indicated.
Cohesive Crack Model for Geomaterials: Stability Analysis and Rate Effect
Authors: Bazˇant Zdeneˇk P. , Li Yuan-Neng
Published: April 2009

The cohesive (or ficticious) crack model, characterized by softening stress-displacement relations, provides a good description of fracture of quasibrittle materials such as conrete, rock, or tough ceramics. The cohesive crack model is formulated in terms of compliance influence functions and the failure is analyzed as a stability problem. The size effect is determined by means of an eigenvalue problem. In this problem, the structure size for which a given relative crack length yields the maximum load is the eigenvalue. The model is further generalized to time dependence. The opening displacement is considered as a function of the cohesive stress and the opening rate of the crack. Finally, applications to rock and concrete are discussed.
A Novel Fully Compliant Planar Linear-Motion Mechanism
Authors: Hubbard Neal B. , Wittwer Jonathan W. , Kennedy John A. , Wilcox Daniel L. , Howell Larry L.
Published: April 2009

A new fully compliant linear-motion mechanism, called the XBob, is presented. The mechanism is based on the pseudo-rigid-body model (PRBM) of a system of Roberts approximate straight-line mechanisms combined in series and parallel. It can be fabricated in a single plane and has a linear force-displacement relationship. Symmetry and compliance compensate for the structural error inherent in the Roberts mechanism, resulting in precise straight-line motion. The device is designed and its motion and force-displacement relations are predicted by the PRBM. The design is validated using finite element analysis and experimental results.
Comparison of PD-Based Controllers for Robotic Manipulators
Authors: Ouyang P. R. , Zhang W. J.
Published: April 2009

PD control is widely used in industrial robotic manipulators because of its simple structure and acceptable performance. In this paper, the PD-based control schemes for the trajectory tracking of the robotic manipulators are addressed. The fixed gain PD control, the nonlinear gain PD (NPD) control, the adaptive PD learning control (PD-LC), and the adaptive NPD learning control (NPD-LC) are applied for the trajectory tracking of both serial and parallel robotic manipulators. The PD-LC and NPD-LC controllers can be used to improve the tracking performance for the repeatable tracking tasks in an iterative mode. The PD-LC and NPD-LC consists of a PD/NPD control as the basic feedback control and an additional feedforward control term directly inherited from the previous iteration of the same control task. A comparative study of four PD-based controllers is conducted to understand how different control schemes will affect the trajectory tracking performance, and the results are shown in this paper. Case studies are presented to demonstrate the validity of the PD-LC and NPD-LC algorithms.
Single Degree-of-Freedom Morphing Wing (Design and Synthesis)
Authors: Stubbs Matthew D. , Whittier William B. , Reinholtz Charles F.
Published: April 2009

Recent research in morphing wing technology has focused on complex multiple-degree-of-freedom (MDOF) mechanisms and smart structures to provide a specified shape change; single degree-of-freedom actuation concepts have generally been ignored or overlooked. In this research, the authors propose a novel design for a single degree-of-freedom (SDOF) mechanism for mission morphing of aircraft wings. A general design methodology has been developed, and this has been applied to a Hyper-Elliptic Cambered Span (HECS) wing developed by engineers at NASA Langley Research Center. The design tools developed include a synthesis procedure for determining the dimensions of the single-degree-of-freedom morphing mechanism, and a sensitivity analysis to determine the effects of manufacturing errors.
Direct Singular Positions of 3RPS Parallel Manipulators
Authors: Liu C. H. , Cheng Shengchia
Published: April 2009

In this study a procedure to obtain direct singular positions of a 3RPS parallel manipulator is presented. If the heights of 3 spherical joints, denoted by d1n, d2n, and d3n respectively, are used as coordinate axes, then the workspace of the moving platform may be represented as an inclined solid cylinder in this coordinate system. The location of a point on the solid circular cylinder determines a configuration of the manipulator’s moving platform. The procedure to locate direct singular positions consists of two steps, the orientation of the moving platform is assumed first, from which the horizontal position of the moving platform may be obtained; then in the second step the heights that make determinant of Jacobian matrix vanish may always be determined. Results show that unless the moving platform is normal to the base, in which case there exist only one or two singular configurations, otherwise there are always three singular configurations corresponding to a moving platform’s orientation.
Dynamic Characteristics of a Six-Bar Hinge Mechanism as Used in Cabinets
Authors: Chen Fu-Chen
Published: April 2009

The dynamic characteristics of a six bar hinge mechanism as used in home cabinets were investigated using the method of equation of motion. The derived equations of motion were numerically solved and the motion of the hinge mechanism was simulated. The influence of mass and width of the cabinet door on the dynamic characteristics of the hinge mechanism as well as the effect of the hinge number on the force applied on the handle were also investigated. The experimental and simulation results showed good agreement with an error of under 2%, which validated the simulation results. The proposed approach can be used by hinge manufacturers for the design and analysis of similar hinge mechanisms.
Kinematic Performance of 5-UPS/PRPU 5-Axis Parallel Machine Tool
Authors: Zhao Yong Sheng , Zheng Kui Jing , Li Qin Chuan , Tian Xiao Jing , Qin Yan Ming
Published: April 2009

A novel 5-UPS/PRPU 5-axis PMT (Parallel Machine Tool) is studied. The stationary platform is connected with the moving platform by the same five UPS actuated limbs and a PRPU passive constraining limb. Three translational DOF (degrees of freedom) and two rotational DOF can be achieved. by using kinematic screw theory and D-H parameter method, the kinematic forward and inverse solutions of the PRPU constraining limb are deduced. The rotational constraint along the vertical axis of the moving platform acted by the PRPU limb is confirmed. Moreover, the configuration of the moving platform can be acquired online by installing sensors on the joints of the constraining limb. The kinematic inverse solution equation and jacobian matrix for this 5-axis PMT are presented. Finally the workspace and the dexterity of the PMT are analyzed. Theoretical analysis is proved by the actual operation of the prototype of the 5-UPS/PRPU 5-axis PMT in our laboratory.
Kinematics of Special-Type Manipulators by Displacement-Groups
Authors: Galletti Carlo , Fanghella Pietro
Published: April 2009

After a brief summary of the basic aspects of displacement-group theory, several applications of this theory to the kinematics of robot mechanisms are described. Then, we show in greater detail how to obtain, in a simple way, the characteristic polynomials of manipulators with special structural parameters. The goal is to find the polynomials of degree two for these special-geometry manipulators. In our paper, we apply displacement-group theory to provide an elegant and compact presentation and discussion of this subject, and we demonstrate that some geometrical conditions on the shapes of the manipulator links, as stated in other scientific papers, can be eliminated without changing the degree of the characteristic polynomials.
Visually Guided Object Insertion by Cooperative Robots in Unstructured Environments
Authors: Sujan Vivek A.
Published: April 2009

In field environments it is not usually possible to provide robots in advance with valid geometric models of its environment and task element locations. The robot or robot teams need to create and use these models to locate critical task elements by performing appropriate sensor based actions. Here, an information-based iterative algorithm to intelligently plan the robot’s visual exploration strategy is proposed to enable it to efficiently build 3D models of its environment and task elements. The method assumes mobile robot or vehicle with cameras carried by articulated mounts. The algorithm uses the measured scene information to find the next camera position based on expected new information content of that pose. This is achieved by utilizing a metric derived from Shannon’s information theory to determine optimal sensing poses for the agent(s) mapping a highly unstructured environment. Once an appropriate environment model has been built, the quality of the information content in the model is used to determine the constraint-based optimum view for task execution. Experimental demonstrations on a cooperative robot platform performing an assembly task in the field show the effectiveness of this algorithm for single and multiple cooperating robotic systems.
Direct Position Analysis in Analytical Form of Parallel Manipulators Generating Structures With Topology SR-2PS
Authors: Di Gregorio Raffaele
Published: April 2009

A wide family of parallel manipulators (PMs) is the one that groups all the PMs with three legs where the legs become kinematic chains constituted of a passive spherical pair (S) in series with either a passive prismatic pair (P) or a passive revolute pair (R) when the actuators are locked. The topologies of the structures generated by these manipulators, when the actuators are locked, are ten. One out of these topologies is the SR-2PS topology (one SR leg and two PS legs). This paper presents an algorithm that determines all the assembly modes of the structures with topology SR-2PS in analytical form. The presented algorithm can be applied without changes to solve, in analytical form, the direct position analysis of any parallel manipulator which generates a SR-2PS structure when the actuators are locked. In particular, the closure equations of a generic structure with topology SR-2PS are written. The eliminant of this system of equations is determined and the solution procedure is presented. Finally, the proposed procedure is applied to a real case. This work demonstrates that the solutions of the direct position analysis of any parallel manipulator which generates a SR-2PS structure when the actuators are locked are at most eight.
Robotic System to Evaluate Force Feedback in Minimally Invasive Computer Aided Surgery
Authors: Mayer Hermann , Nagy Istva´n , Knoll Alois , Schirmbeck Eva , Bauernschmitt Robert
Published: April 2009

We present an experimental system for robot assisted, minimally invasive surgery that is capable of force measurement and haptic feedback. While minimally invasive surgery with robots provides several advantages like reduced tissue trauma and shorter recovery times, there are also some inherent short-comings. Referring to surgeons, the most significant issue is the lack of haptic feedback. This often causes collateral trauma and leads to prolonged operation time. Therefore we have developed an open platform, which combines the advantages of present systems with the possibility of force reflection. We have included features known from commercial available systems, like full Cartesian control of the end effector, stereo vision and ergonomic input devices. We used the system to perform basic surgical tasks (like knot-tying) and to evaluate force feedback.
Exact-Gradient Optimization Method for Rigid-Body Guidance Synthesis of Planar Mechanisms
Authors: Sancibrian Ramon , Garcia Pablo , Viadero Fernando , Fernandez Alfonso
Published: April 2009

In this paper an approximate kinematic synthesis method is presented with application to rigid-body guidance in planar multibody systems. The problem of finding the optimal dimensions in linkages with rigid-body guidance constraints has been widely studied. Many techniques have been developed and applied to numerous kinematic chains. However, some problems remain without appropriate solution, such as a large number of required poses or low computational cost. The proposed method uses exact-gradient determination to search for an optimal solution. The modelling of the mechanism uses fully Cartesian coordinates and is formulated by means of algebraic constraint equations. Furthermore, the formulation allows the use of a large number of prescribed poses giving high accuracy in the definition of synthesis conditions. Examples are included to illustrate the new approach to some synthesis specifications.
A More General Mobility Criterion for Parallel Platforms
Authors: Rico J. M. , Aguilera L. D. , Gallardo J. , Rodriguez R. , Orozco H. , Barrera J. M.
Published: April 2009

This contribution presents a more general mobility criterion applicable to parallel platforms, unlike previously employed mobility criteria based on the well-known Kutzbach-Gru¨bler criterion that often fails to provide the correct number of degrees of freedom of parallel manipulators, the mobility criterion introduced in this contribution provides the correct number of degrees of freedom for a wider class of parallel manipulators. Furthermore, the analysis provides insight into why the criteria based on the Kutzbach-Gru¨bler criterion often fails. Moreover, the newly developed criterion computes the passive degrees of freedom in parallel platforms and the mobility of some classes of kinematically defficient parallel platforms. Finally, it is important to note that this criterion is based on an analysis of the subalgebras of the Lie algebra, se(3), also known as screw algebra, of the Euclidean group, SE(3). As is common in many areas of kinematics, the criterion presented here can be also obtained by an analysis of the subgroups of the Euclidean group.
Properties of the SX-YS-ZS Structures and Singularity Determination in Parallel Manipulators Which Generate Those Structures
Authors: Di Gregorio Raffaele
Published: April 2009

The instantaneous forward problem (IFP) singularities of a parallel manipulator (PM) must be determined during the manipulator design and avoided during the manipulator operation, because they are configurations where the end-effector pose (position and orientation) cannot be controlled by acting on the actuators any longer, and the internal loads of some links become infinite. When the actuators are locked, PMs become structures consisting of one rigid body (platform) connected to another rigid body (base) by means of a number of kinematic chains (legs). The geometries (singular geometries) of these structures where the platform can perform infinitesimal motion correspond to the IFP singularities of the PMs the structures derive from. In this paper, the singular geometries of the structures with topology SX-YS-ZS (S stands for spherical pair, whereas X, Y and Z stand for three generic one-dof pair which may be or may not be of the same type) are studied with a unified approach. The presented approach leads to obtain an analytic condition which allows all the singular geometries of these structures to be determined. Moreover, the geometric interpretation of the found singularity condition and the exhaustive enumeration of the types of singular geometries is provided. Finally, the use of the presented results in the design of the manipulators which become one structure with topology SX-YS-ZS when the actuators are locked is discussed.
Spread-Band: A Smart Machine Element Enables Compact Parallel Robot Design
Authors: Soetebier Sven , Raatz Annika , Maris Ciprian , Krefft Mathias , Hesselbach Ju¨rgen
Published: April 2009

This paper presents a planar parallel robot, built of spread-band elements. The robot has a workspace that exceeds its original size by far. The mass of the mechanical structure is extremely low, allowing accelerations up to 120 m/s2. A spread-band element consists of a roll with flanges and a cambered metal tape. The arch form of the cambered band allows the transmission of tensile and compressive forces and of one moment. The flange of the roll changes the orientation of the bending radius axis by 90°. Nevertheless the combination of roll and cambered band retains the bands flexural rigidity against a moment around an axis perpendicular to the longitudinal axis of the cambered band and the axis of the roll. This leads to the possibility of transforming rotation into translation and vice versa using spread-band elements. The combination of several spread-band elements performed in the presented robot results in a rigid mechanical system.