bluuebunny/crossref_metadata_2025 huggingface dataset:
Phenomenological Theories of Elastoplasticity and Strain Localization at High Strain Rates
Authors: Nemat-Nasser Sia
Published: April 2009

In this paper certain fundamental concepts underlying the phenomenological theories of elastic-plastic deformations at finite strains and rotations are presented, and some of the commonly discussed theories are summarized, emphasizing the constitutive parameters which influence strain localization and material instability often observed in finite deformation of ductile materials. Particular attention is paid to the thermodynamic basis of inelastic deformation. Conditions for the existence of inelastic potentials are discussed. The results are presented in terms of a general material strain and its conjugate stress, and then specialized for particular applications, emphasizing quantities and theories which are reference- and strain measure-independent. Rate-independent and rate-dependent elastoplasticity relations are developed, starting from a finite deformation version of the J2-plasticity with isotropic and kinematic hardening, and leading to theories which include dilatancy, pressure sensitivity, frictional effects, and the noncoaxiality of the plastic strain and the stress deviator. A class of commonly used deformation plasticity theories is then examined and its relation to nonlinear elasticity is discussed. The question of plastic spin, and its relation to the decomposition of the deformation gradient into elastic and plastic constituents, is reviewed in some detail, and it is shown that this decomposition yields explicit relations which uniquely define all spins in terms of the velocity gradient and the elastic and plastic deformation rates, hence requiring no additional constitutive relations for the plastic spin. The phenomenon of strain localization at high strain rates is illustrated and discussed, and a series of numerical results are given. Finally, a recent breakthrough in elastoplastic explicit computational algorithms for large-strain, large-strain-rate problems is briefly reviewed.
Surface Effects in Brittle Materials and Internal Length Estimation
Authors: Frantziskonis G.
Published: April 2009

The concept of state of degradation is employed as a tool to study the effect of initial material (structure) inhomogeneity and the implications of surface (skin) effects on brittle material response. The initial heterogeneity pattern and its growth under external load is studied experimentally via ultrasonic scanning measurements on prismatic simulated rock samples subjected to mechanical load. Numerical solutions are compared with the experimental results. The skin effects are also studied experimentally through the spatial variation of dissipated energy within the samples tested. In addition, microstructure considerations highlight the skin effect and its implications. The material length for the simulated rock is estimated (for load levels up to peak load) through the experimental observations which yielded 1≃0.24 inches (0.61 cm). The analytical solution of a simple problem including microstructure considerations for the same material yielded 1≃0.21 inches (0.53 cm). This paper integrates and extends the recent work of the author and co-workers.
A Model for a Propagating Shear Band on the Basis of a Tilt Wall Dislocation Array
Authors: Hirth J. P.
Published: April 2009

Microstructural parameters influencing shear band propagation and, ultimately, fracture in steels are reviewed for two types of test. These are the plane strain tension characteristics of spheroidized steels and the mixed mode I/III J-resistance behavior of a rotor steel. In the former case, shear band propagation is associated with voids forming at carbide particles because of incompatibility effects. In the latter case, the mixed mode I/III toughness is less than that in either pure mode I or pure mode III because local plastic flow in the path of the crack produces damage in the form of voids by a process analogous to the first case. A model for the interaction of a shear band and particles, suggested by these results, is proposed in the form of a discontinuous tilt wall of dislocations that breaks away and propagates past the particles. The model is shown to be consistent with several experimental observations, both of shear band characteristics and of the stress required to propagate the band.
Nonlinear Analysis Techniques for Shear Band Formation at High Strain-Rates
Authors: Tzavaras Athanasios E.
Published: April 2009

One of the most striking manifestations of instability in solid mechanics is the localization of shear strain into narrow bands during high speed, plastic deformations of metals. According to one theory, the formation of shear bands is attributed to effective strain-softening response, which results at high strain rates as the net outcome of the influence of thermal softening on the, normally, strain-hardening response of metals. Our objective is to review some of the insight obtained by applying nonlinear analysis techniques on simple models of nonlinear partial differential equations simulating this scenario for instability. First, we take up a simple system, intended as a paradigm, that describes isothermal shear deformations of a material exhibiting strain softening and strain-rate sensitivity. As it turns out, for moderate amounts of strain softening strain-rate sensitivity exerts a dissipative effect and stabilizes the motion. However, once a threshold is exceeded, the response becomes unstable and shear strain localization occurs. Next, we present extensions of these results to situations where explicit thermal effects are taken into account.
Finite Strain Plasticity and Damage in Constitutive Modeling of Metals With Spin Tensors
Authors: Voyiadjis George Z. , Kattan Peter I.
Published: April 2009

The analysis of damage and plastic deformation in metals is very important towards the full understanding of the various damage mechanisms in these materials. A coupled theory of damage mechanics and finite strain plasticity is proposed. The theory is based on a sound mathematical and mechanical background and is thermodynamically consistent. It is formulated using spatial coordinates utilizing a von Mises type yield criterion with both isotropic and kinematic hardening. The derivation is based on the concept of effective stress that was originally proposed by Kachanov [1] for the case of uniaxial tension. The plasticity model is first formulated in a fictitious undamaged configuration of the body. Then certain transformation equations are derived to transform this model into a damage-plasticity model in the damaged configuration of the body. Certain assumptions are made in order to make this transformation possible. These assumptions include small elastic strains and the hypothesis of elastic energy equivalence of Ref 17. The corotational stress rate equations are also discussed since they are used extensively in the constitutive relations. Therefore, the use of spin tensors is also discussed since they play a major role in the definition of the corotational rates. In addition, a modified spin tensor is proposed to be used in the coupled model. Furthermore, the nature of the fourth-rank damage effect tensor is discussed for a general state of deformation and damage. Also, the explicit matrix representation of this tensor is rigorously derived and can be used in future applications to solve plane stress and plane strain problems involving damage. Finally, the problem of finite simple shear is investigated using the proposed model. The resulting equations are solved using a Runge-Kutta-Verner fifth order and sixth order method. The stress-strain curves are obtained for a certain expression of the modified spin tensor and are compared with other spin tensors. Also, the evolution of the backstress and damage variables is presented. The results obtained compare favorably with previous results.
Machining and Grinding: A Historical Review of the Classical Papers
Authors: Komanduri R.
Published: April 2009

Major contributions to machining and grinding research in the US came in the twentieth century. The seminal work by Frederick Winslow Taylor on the Art of Cutting Metals published in 1907 was the beginning of a series of serious and systematic studies on the various aspects of metal cutting and grinding in this century. This monumental work, which became an American classic, continues to inspire many a researcher in this field even today. It was followed by the works of other pioneers, including Orlan W Boston, Hans Ernst, M E Martellotti, Max Kronenberg, M Eugene Merchant, Milton C Shaw, Michael Field, John Kahles, K J Roubik, K Armitage, Ken Trigger, B T Chao, Alfred Schmidt, William W Gilbert, Fran Boulger, Lester Colwell, Carl Oxford, Erich Thomsen, Robert Hahn, and many others. Many of the associates of the pioneers including Nathan Cook, Iain Finnie, B F von Turkovich, Shiro Kobayaski, Inyong Ham, E Loewen, and others including W B Rice, S M Wu, and J Tlusty have made significant contributions to these fields in their own right. There is no doubt that this century will be heralded by the historians as the golden age of metal cutting and grinding research, particularly the period between 1940 and 1960. It was, however, M Eugene Merchant’s paper on the Basic Mechanics of the Metal Cutting Process in 1945 that took a giant step from the art of metal cutting to the science of metal cutting. This work laid the foundation for much of the work that is practiced today. It can be stated unequivocally that because of the significant contributions by the pioneers and their associates, metal cutting and grinding research today is rich in its heritage and contents, and has contributed towards the improvement of manufacturing productivity. It has thereby facilitated the improvement of living standards around the world. In this review, the following ten topics are addressed briefly: Physics of Machining; Mechanics of Machining; Shear and Friction in Machining; Thermal Aspects of Machining and Grinding; Tool Materials, Tool Wear, and Machinability; Multiple Cutting Points; Grinding; Vibrations in Machining; Surface Integrity; and Economics of Machining.
Reduced Constraint Manifolds
Authors: Boulos Paulo , Achcar Nelson
Published: April 2009

The formalization of the concept of internal constraint is usually done by means of restrictions on the deformation gradients, by imposing that they must stay on a submanifold of Lin+. Our point of view is to impose restrictions on the right Cauchy-Green tensors, by imposing that they must stay on a submanifold of P sym, which we call reduced constraint manifold. We determine: all the constraint manifolds that are independent of the local reference configuration, a new formula for the reaction space, all the constraint manifolds whose reaction spaces are independent of the deformation gradient.
A General FEM Formulation of Nonlinear Dynamics Applied to Accessing the Statical Loading Effect Upon the Dynamic Response of Planar Frames
Authors: Brasil Reyolando M. L. R. F. , Mazzilli Carlos E. N.
Published: April 2009

This paper initially discusses the dynamics of discrete structural systems of geometricaly nonlinear behaviour costituted by linear elastic materials. Two formulations are derived, namely global and incremental. They are both suitable to general FE modelling, as the matrix equations of motion are written in explicit form. Matrices and vectors involved are characterized in terms of constraint equations defined within the continuum discretization. In principle, such formulations are applicable to any structural theory, as the theories of beams, plates and shells. As an example, the Bernoulli-Euler beam element is studied herewith. Both global and incremental formulations capture the effect geometrical nonlinearities have upon inertial and elastic forces alike. The ANDROS FEM program, developed by the authors, which is based upon the global formulation, has been successfully used in several nonlinear analyses. From this general background, the paper proceeds to consider the effect statical loading may have upon the free undamped vibration frequencies of a structure. It is shown that the tangent stiffness matrix of the incremental formulation should be used in the resultant eingenvalue problem. In some cases, axial forces are seen to have a strong influence on the internal resonance tuning. It is shown, in a sample structure thus tuned and subjected to dynamical loading, that a nonlinear regime may appear in the response.
Thermoelastoplastic Analysis of a Stainless Steel Plate Considering Phase Transformations
Authors: Tauchert T. R. , Webster G. A. , Reed R. C.
Published: April 2009

A quasistatic thermoelastoplastic analysis is given for the response of a traction-free plate subject to surface heating. The formulation incorporates temperature-induced phase changes which model experimentally obtained data for X22 CrMoV 12 1 stainless steel. The plate is discretized into layers, with material properties considered homogeneous within each layer. An incremental, iterative approach based upon Kirchhoff bending theory is employed to obtain an approximate solution to the problem. Numerical results illustrate the effects of phase transformations and temperature sensitivity of the material properties upon the transient and residual stress distributions.
Stability of Linear Mechanical Systems With Holonomic Constraints
Authors: Mu¨ller Peter C.
Published: April 2009

Singular systems (descriptor systems, differential-algebraic equations) are a recent topic of research in numerical mathematics, mechanics and control theory as well. But compared with common methods available for investigating regular systems many problems still have to be solved making also available a complete set of tools to analyze, to design and to simulate singular systems. In this contribution the aspect of stability is considered. Some new results for linear singular systems are presented based on a generalized Lyapunov matrix equation. Particularly, for mechanical systems with holonomic constraints the well-known stability theorem of Thomson and Tait is generalized.
Dynamic Modelling and Control Strategies of Space Manipulators
Authors: Ercoli-Finzi A. , Mantegazza P.
Published: April 2009

In this work the dynamics of a manipulator arm is analyzed by Maggi’s formulation and by automatic development of the equations of motion. Two control strategies, that is an adaptive and a standard one, are taken into account in order to satisfactory perform pick and place maneuvers. Numerical simulations emphasize performance qualities and limits of the two choices and suggest a possible implementation of mixed control strategies.
Electroelastic Analysis of Piezoelectric Laminated Structures
Authors: Sosa Horacio , Castro Marco
Published: April 2009

A two-dimensional electroelastic formulation is developed to study the response of piezoelectric laminated structures. The methodology is based on a state space formulation where a particular collection of elastic and electrical variables is used to determine the behavior of layered structures subjected to arbitrary loading conditions. The application of this methodology in the area of multilayer electromechanical devices and of adaptive or smart structures is discussed.
Development of Numerical Simulations of Dynamic/Aerodynamic Interactions
Authors: Luton J. A. , Mook D. T. , Nayfeh A. H. , Mracek C. P.
Published: April 2009

A procedure for simulating aerodynamic/dynamic interactions is described. The equations of motion of the flowing air and the lifting structure are integrated numerically by an iterative technique based on Hamming’s predictor-corrector method. The solution provides the flowfield and the motion of the lifting body simultaneously. The technique is modular and different models of the flow and the structure can be incorporated by simply changing subroutines. For the current examples of wingrock of highly swept delta wings and flutter of very flexible, very high-aspect-ratio wings, the flow-fields are considered incompressible and modeled by vortex-lattice and continuous vorticity-panel methods. These models are inherently nonlinear and provide a force-free wake as part of the solution. It is shown that the instabilities can be easily suppressed by means of actively controlled flaps.
Jump Phenomena, Bifurcations, and Chaos in a Pressure Loaded Spherical Cap Under Harmonic Excitation
Authors: Gonc¸alves Paulo Batista
Published: April 2009

This paper investigates the dynamic non-linear behaviour of a pre-loaded shallow spherical shell under a harmonic excitation. For this, the Marguerre partial differential equations of motion for an imperfect, pre-loaded cap is reduced to a finite degree of freedom system using the Galerkin method. The displacements and stress functions are described by a linear combination of Bessel functions and modified Bessel functions that satisfy all the relevant boundary and continuity conditions. The resulting differential equations of motion are solved by the Galerkin-Urabe procedure, or, alternatively, by numerical integration. To study the response of the shallow cap under harmonic excitation, phase plane portraits, Poincare´ maps, resonance curves, and bifurcation diagrams are plotted for a number of loading conditions. Results indicate that, for static load levels between the upper and lower limit point loads, the shell may display jumps due to the presence of competing potential wells and the presence of non-linear resonance curves within each well. Additionally, different physical situations are identified in which period-doubling phenomena and chaos can be observed.
Transformation of Liposomes: Mechanical Behavior and Stability
Authors: Pamplona D.
Published: April 2009

Liposomes are small artificial vesicles of lipid bilayer, wich enclose and are surrounded by water. Morphological transformations in liposomes, starting from a spherical shape, due to changes in the osmotic pressure, have been described in the literature. The first transformation is into a circular biconcave form, afterwards the biconcave side view is maintained, while the front view reveals transformations into elliptical or regular polygonal forms, usually triangular, square or pentagonal. Finite elasticity and the theory of thin shells were used to analyse the behavior of the liposomes under decreasing volume. The biological membrane was considered as a two dimensional fluid layer, exhibiting solid properties to some extent, e.g., elasticity. The stability of the liposmes was studied by using the method of elastic perturbation to obtain the critical pressure for the biconcave transformation and the long liposome tubes. The transformations to elliptical and regular polygonal forms were studied using the linear stability equations of elasticity.
Coupling of a State-Space Inflow to Nonlinear Blade Equations and Extraction of Generalized Aerodynamic Force Mode Shapes
Authors: de Andrade Donizeti , Peters David A.
Published: April 2009

The aeroelastic stability of helicopter rotors in hovering flight has been investigated by a set of generalized dynamic wake equations and hybrid equations of motion for an elastic blade cantilevered in bending and having a torsional root spring to model pitch-link flexibility. The generalized dynamic wake model employed is based on an induced flow distribution expanded in a set of harmonic and radial shape functions, including undetermined time dependent coefficients as aerodynamic states. The flow is described by a system of first-order, ordinary differential equations in time, for which the pressure distribution at the rotor disk is expressed as a summation of the discrete loadings on each blade, accounting simultaneously for a finite number of blades and overall rotor effects. The present methodology leads to a standard eigenanalysis for the associated dynamics, for which the partitioned coefficient matrices depend on the numerical solution of the blade equilibrium and inflow steady-state equations. Numerical results for a two-bladed, stiff-inplane hingeless rotor with torsionally soft blades show the importance of unsteady, three-dimensional aerodynamics in predicting associated generalized aerodynamic force mode shapes.
Evaluation of the Structural Health of Mechanical Cables
Authors: Laura Patricio A. A.
Published: April 2009

Mechanical cables have long been used for towing, for remote control or for the supplying of communication or other service links between two vehicles. They also constitute the basic structural element in suspended bridges. On the other hand they are used in biomedical engineering applications; to lower men underground as deep as 16,000 ft and they are considered in superconductivity applications. Obviously the problem of evaluating the “structural health” of cables or ropes by means of non-destructive test methods is of the utmost importance. This article is concerned with a survey of the available methods.
Theoretical and Experimental Study on Nonlinear Response of a Rotor Blade to a Gust
Authors: Tang D. M. , Dowell E. H.
Published: April 2009

A rotating slotted cylinder (RSC) gust generator installed in the Duke University low speed wind tunnel was used to generate a gust excitation field and the nonlinear response of a flexible rotor blade to gust excitations was studied theoretically and experimentally. The quantitative agreement between theory and experiment indicates that the assumption of an uniform chordwise gust angle of attack and an approximate solution procedure of the nonlinear equations proposed are allowable for the parameter range considered in this study. For a periodic gust flow field, the effects of geometric structural nonlinearity and aerodynamic nonlinearity on dynamic aeroelastic behavior are significant and lead to aperiodic or chaotic motion when stall occurs.
Implementation of Viscoelastic Behavior in a Time Domain Boundary Element Formulation
Authors: Schanz M. , Gaul L.
Published: April 2009

The boundary element method (BEM) provides a powerful tool for the calculation of elastodynamic response in frequency and time domain. Field equations of motion and boundary conditions are cast into integral equations, which are discretized only at the boundary. The boundary data often are of primary interest because they govern the transfer dynamics of members and the energy radiation into a surrounding medium. Formulations of BEM currently include conventional viscoelastic constitutive equations in the frequency domain. In the present paper viscoelastic behaviour is implemented in a time domain approach as well. The constitutive equations are generalized by taking fractional order time derivatives into account.
Transient and Steady-State Operational Limits for Ship Roll
Authors: Bishop Steven R. , de Souza Jesse Rebello
Published: April 2009

This paper investigates the safe operational limits of roll for small vessels in regular waves through the use of an archetypal model of ship motion. The technique incorporates steady-state analysis with methods of quantifying the engineering integrity of transient motions. Both analytical and numerical methods are used and compared. The resonant response of the vessel is examined to reveal unacceptable solutions and superimposed on parameter space diagrams which include lower bounds for fractal erosion of basins of attraction.
New Theoretical Developments in Aeroacoustics and Aerodynamics
Authors: Branda˜o Mauricio Pazini
Published: April 2009

The Acoustic Analogy is studied regarding its application to Aerodynamics. The concept of generalized derivatives is reviewed and extended. As consequences, new versions of the generalized continuity, momentum, energy, and energy momentum conservation equations are presented. These conservation laws form a basis for theoretical ideas regarding three-dimensional, linear and non-linear, potential and non-potential, steady and unsteady flows of incompressible and compressible, inviscid and viscous fluids. Extensions made in the definition of generalized derivatives relate fluid properties to new surface terms linked to tangential flow. In integral form, some of these relations recall the classical circulation and downwash integrals of Aerodynamics. An iterative solution technique of the resulting singular integral equations is proposed.
Nonlinear Equations of the Theories of Elasticity and Shells in Terms of Anholonomic Components
Authors: Altman Wolf , Oliveira Antonio Marmo de
Published: April 2009

This paper is concerned with some new aspects of the theory of anholonomic components of tensors of single and double fields. Consistent definitions of these components are given. As applications, the geometrically nonlinear equations of elasticity and shells referred to general curvilinear coordinates are written in terms of anholonomic components of vectors and tensors involved.
Effective Behavior of Piezoelectric Composites
Authors: Wang Biao
Published: April 2009

In this paper, general relations between the overall properties of piezoelectric composites and the properties of their constituents are derived. Based on the solution for an ellipsoidal inclusion in a piezoelectric material developed by Wang (Int. J. Solids and Structures, 29, 293, 1992), it is found that the coupled elastic and electric field inside a spheroidal inclusion in a transversely isotropic, piezoelectric matrix can be expressed in terms of a system of the linear algebraic equations which contains only some simple integrals. These internal fields are then used to obtain the effective constants of a piezoelectric composite.
Fracture Statistics Models and Crack Propagation in Random Media
Authors: Jeulin D.
Published: April 2009

Crack propagation in heterogeneous media is of primary interest for engineering purposes, in order to predict the overall toughness and the probability of fracture from data on the microstructure. Probabilistic models for mode I crack propagation in two dimensions are presented. They are developed for brittle elastic materials with a random distribution of fracture energy. These models enable us to calculate in a closed form the probability of fracture involving crack nucleation and propagation that differ from the usual fracture statistics models based on the weakest link model. The use of the Griffith’s crack arrest criterion is applied to random function models for the distribution of the fracture energy and for various loading conditions resulting in stable or unstable crack propagation. From the models are deduced some statistical size effects.
A Stochastic Model for Slow Crack Growth in Brittle Materials
Authors: Kunin Boris I.
Published: April 2009

In the last decade, progress in brittle material technology (CC-composites, ceramics, ceramic composites, etc) made these so attractive for structural applications that one has to put up with their tendency to fail unpredictably, provided that one can predict with confidence probabilities of failure. The present paper addresses prediction of life-time scatter for brittle materials. In the paper, we model a commonly observed mode of slow crack growth in brittle materials, namely a Markovian stochastic pattern of a microscopic random jump, followed by a random waiting time, followed by a random jump, and so on. The waiting times are related, on physical grounds, to random energy barriers at the arrest points, whereas random magnitudes of the jumps are treated within the existing framework of Crack Diffusion Theory. This leads to a description of crack growth as a random process whose transition probability density satisfies a hyperbolic PDE. Relation to probabilistic life-time prediction is discussed and a simple illustrative example considered.
Micromechanics as a Basis of Continuum Random Fields
Authors: Ostoja-Starzewski M.
Published: April 2009

Many problems in solid and geomechanics require the concept of a meso-continuum, which allows a resolution of stress and other dependent fields over scales not infinitely larger than the typical microscale. Passage from the microstructure to such a meso-continuum is based on a scale dependent window playing the role of a Representative Volume Element (RVE). It turns out that the material properties at the mesoscale cannot be uniquely approximated by a random field of stiffness with continuous realizations, but, rather, two random continuum fields, corresponding to essential and natural boundary conditions on RVE, need to be introduced to bound the material response from above and from below. In this paper Monte Carlo simulations are used to obtain the first- and second-order one- and two-point characteristics of these two random fields for random chessboards and matrix-inclusion composites. Special focus is on the correlation functions describing the autocovariances and crosscovariances of effective random meso-scale conductivity tensor Cij and its dual Sij. Following issues are investigated: i) scale-dependence of noise-to-signal ratios of various components of Cij and Sij, ii) spatial structure of the correlation function, iii) uniform strain versus exact calculations in determination of the correlation function, iv) correlation structure of composites with inclusions without and with overlap.
Macroscopic Behavior of Random Media From the Microstructure
Authors: Torquato S.
Published: April 2009

Recent progress that we have made on the problem of determining the macroscopic properties of random heterogeneous media from the microstructure is reviewed. A variety of different effective properties are considered, including the effective thermal (or electrical) conductivity and elastic moduli of composites, fluid permeability of porous media, and effective time scales associated with diffusion and reaction among traps. A unified approach to treat these seemingly disparate problems is emphasized.
Self-Consistent Approaches and Strain Heterogeneities in Two-Phase Elastoplastic Materials
Authors: Bornert Michel , Herve´ Eveline , Stolz Claude , Zaoui Andre´
Published: April 2009

The Generalized Self Consistent Scheme [GSCS] extended to the nonlinear case with help of a deformation theory of elastoplasticity is used to predict the strain heterogeneities that spread out in two phase elastoplastic materials submitted to a monotonic uniaxial load. Materials with different microstructural morphologies are considered. The single composite inclusion of the GSCS is an accurate representation of “matrix/inclusion” microstructures but it does not give a sufficient representation of the considered morphologies. That’s why this model is extended to more general cases by using two or even more different spherical composite inclusions: local concentration fluctuations and local morphological inversions can then be modeled. The nonlinear extension is also modified: the composite inclusions are discretized into several concentric layers in order to take into better account the strain gradient along the radius and a new definition of the work-hardening parameter of each of these layers is proposed. The elastoplastic strain field in the single composite inclusion is also computed numerically by means of finite element methods and compared to the analytical result. Unfortunately, these modifications do not basically modify the strain heterogeneity predictions of the GSCS, which widely underestimate the measured strain heterogeneities in most of the cases. In fact, the inaccuracy of the GSCS in these cases is basically due to the appearance of long range shear bands that cannot be described by a local self-consistent approach.
Kinematics and Thermodynamics Across a Propagating Non-Stoichiometric Oxidation Phase Front in Spent Fuel Grains
Authors: Stout R. B. , Kansa E. J. , Wijesinghe A. M.
Published: April 2009

Spent fuel from power reactors contains mixtures, alloy subsets, and compounds of elements; but the aggregate atomic densities in spent fuel are dominated by uranium and oxygen atoms. With the exception of some UO2 fuels with burnable poisons (primarily gadolinia in BWR rods), the other elements with significant atomic densities in spent fuel evolve during reactor operation from neutron reactions and fission plus fission decay events. Due to nuclear decay processes, the intrinsic chemical composition and activity of spent fuel will continue to evolve after it is removed from reactors as its radioactivity decays over time. During the time interval when the radioactivity levels are significant, which is the time interval relevant for design and for performance assessment of a geological repository, it is important to develop an understanding and to develop models that describe potential chemical responses in spent fuel and its potential degradational impacts on repository design and performance. One such potential impact is the oxidation response of spent fuel. The oxidation of spent fuel results in an initial phase change of the UO2 lattice to a U4O9 lattice, and the next phase change is probably to U3O8 although it has not been observed yet at low temperatures (<200°C). The U4O9 lattice is non-stoichiometric with a oxygen to uranium weight ratio (O/U) at ~ 2.4. Preliminary indications are that the UO2 has a O/U of ~ 2.4 at the time just before it transforms into the U4O9 phase.[1,2] Also, in the oxygen weight gain versus time response, a plateau appears as the O/U approaches ~ 2.4. Part of this plateau response is due to geometrical effects of a U4O9 phase change front propagating into UO2 grain volumes. However, the plateau time response may be indicative of a metastable phase change delay kinetics or a diffusional related delay time until the oxygen density can attain a critical value to satisfy the stoichiometry and energy conditions for phase changes. In a previous paper and as a first step, a kinematic and thermodynamic ayalysis was developed to model spatially homogeneous oxidation phase transitions.[3] However, the experimental data clearly show a front of U4O9 lattice structure propagating into grains of the UO2 lattice structure. To describe this spatially inhomogeneous oxidation phase transition, as well as the expected U3O8 phase transition from the U4O9 lattice, lattice models are developed and spatially discontinuous kinematic and energetic expressions are derived. The approach will use concepts from statistical mechanics, discontinuum mechanics, and non-equilibrium thermodynamics. In addition, analytical techniques from shock wave analysis will be used to derive the surface discontinuity energetic expressions across the propagating oxidation phase front.
Failure of Compressible/Dilatant Geomaterials
Authors: Cristescu N. D.
Published: April 2009

The paper presents a general constitutive equation for geomaterials allowing to describe dilatancy and/or compressibility during transient and stationary creep. The constitutive equation also describes the instantaneous response of the geomaterial, work-hardening during transient creep, instantaneous failure and creep failure. The damage produced by dilatancy is used to formulate a criterion for creep failure. Thus ultimate failure may be involved in various ways, depending on the initial and boundary conditions and certainly on the constitutive equation. Typical mining engineering examples are given. First is discussed the creep closure of a deep vertical cylindrical cavern, various possible instantaneous failures, creep failure, and spreading of damage by dilatancy into the rock mass. Second example discusses the instantaneous failure and creep failure around a horizontal tunnel, and the location where damage by dilatancy is more pronounced. The third example presents the case of a rectangular-like shaped cavern.
Entrainment and Mixing in Stratified Flows
Authors: Breidenthal Robert E.
Published: April 2009

Entrainment, mixing, and their dependence on each other are discussed in a variety of stratified flows. In general, entrainment rates decline or are constant as the stratification is increased. The precise dependence of entrainment rate on Richardson number seems to have a variety of possible relationships. Recently, a new model has been proposed in an attempt to describe them in a unified pattern. This model introduces a new parameter, the persistence, to explain the observations. It also distinguishes between cases when the vorticity impinges on the interface from when the vorticity is already there. Surprisingly, mixing seems to be have no effect on stratified entrainment for many of these regimes. However, under some conditions the molecular mixing plays a central role in the flow. This is clear in the limit of large Richardson number, where molecular diffusion controls the rate at which mass or heat crosses a stratified interface. An even more dramatic case, similar to combustion, is mixing-induced buoyancy reversal. There even the global motions are controlled by molecular-scale mixing.
Sidewall Instability and Eddy Generation in a Rotating Fluid Subject to Periodic Forcing
Authors: Hart John E.
Published: April 2009

Laboratory experiments concerned with the generation of waves and vortices by sidewall instability in a rotating cylinder containing a homogeneous liquid are described. Motions are driven by modulating the basic rotation rate Ω of the cylinder sinusoidally in time at amplitude δ and frequency γ. In addition, a rotating lid, revolving differentially at rate ω provides a steady bias to the motion. The basic state consists of an axisymmetric flow composed of solid body rotation, sloshing periodically in time, with a Stokes-Stewartson boundary layer at the cylinder’s vertical wall. This oscillatory boundary layer is subject to a number of quasi-geostrophic (depth independent) and ageostrophic (depth varying) instabilities. At low values of the rotation modulation δ, the dominant instabilities are ageostrophic small scale propagating-horizontal and stationary-spiral roll vortices that are imbedded in the vertical boundary layer adjacent to the wall, along with propagating vertically-oriented waves that are also trapped in the boundary layer. These instabilities are found only when ω > 0. As the modulation amplitude is increased from zero, columnar vortices form by barotropic instability of the Stokes-Stewartson layer, and for sufficiently supercritical conditions these eddies may penetrate deep into the interior. At low modulation frequency (long period forcing), the interaction of the instabilities with the fluctuating horizontal shear results in a predominance of anticyclonic vortices. On the other hand, at moderate to high frequency modulation dipoles are favored. The formation of both ageostrophic and columnar instabilities is enhanced when the gyre is biased by a cyclonic steady surface stress (ω > 0). Gyres with anticyclonic bias (tending to generate a cyclonic boundary layer) are more stable.
Experimental Investigation of a Turbulent Jet Produced by an Oscillating Surface Actuator
Authors: James R. D. , Jacobs J. W. , Glezer A.
Published: April 2009

A round turbulent water jet produced normal to, and at the center of a resonantly driven piezoceramic actuator is investigated experimentally. The flow is produced without mass injection and is comprised entirely of radially entrained fluid. The jet is created by the formation and disappearance of cavitation bubbles during each oscillation cycle near the actuator surface. It appears that this process produces a series of vortex puffs from radially entrained fluid which coalesce to form the jet. Although the jet results from strong time periodic excitation, its time averaged behavior in the far field is similar to that of a classical turbulent round jet in that the increase its width and decrease in the inverse of its centerline velocity are both linear functions of the distance from the actuator. The time periodic features of the jet are observed throughout the flow field and are superimposed on the mean flow. The transient characteristics of the jet have also been investigated and indicate that it can be manipulated on relatively small time scales suggesting that it may be utilized for control of wall bounded shear flows.
Control of Bursting in Boundary Layer Models
Authors: Coller B. D. , Holmes Philip , Lumley John L.
Published: April 2009

We continue our investigation of using feedback to control low dimensional models of bursting in a turbulent boundary layer. We begin by describing, from the viewpoint of a time-dependent coordinate transformation, our previous control strategy developed for a four dimensional model. Using this new point of view, we develop a similar strategy for the ten-dimensional model of Aubry et al. [1988].
Continuing Airworthiness Challenges
Authors: Goranson Ulf G.
Published: April 2009

Structural safety is an evolutionary accomplishment, and attention to detail design features is key to its achievement. A multitude of design considerations is involved in ensuring the structural integrity of Boeing jet transports that have common design concepts validated by extensive analyses, tests, and three decades of service. As airplanes approach their design service objectives, the incidences of fatigue and corrosion may become widespread. Continuing airworthiness of the aging jet fleet requires diligent performance from the manufacturer, the airlines, and airworthiness authorities.
Some Aspects of Near-Surface Vortices
Authors: Gharib M.
Published: April 2009

An experimental investigation of the dynamics of near surface vortices is presented. Using digital particle image velocimetry, the velocity and vorticity fields of a vortex ring and a pair of undulated vortices were mapped as they rise toward a free surface. Circulation measurements indicate that vortex disconnection and appearance of the surface-normal vortices are simultaneous. Vortex disconnection due to the vorticity flux through the surface is shown to be related to the free-surface deceleration.
Upwellings, Downdrafts, and Whirlpools: Dominant Structures in Free Surface Turbulence
Authors: Banerjee Sanjoy
Published: April 2009

Fluid motion at flat, unsheared interfaces develops primarily due to impingement of coherent turbulent structures from the far field. On the other hand, when shear is imposed, alternating low-speed/high-speed regions are formed with ejection-sweep cycles qualitatively similar to those seen in wall turbulence. The transition to this “active” state depends on a shear rate non-dimensionalized by the Reynolds stress and dissipation rate. Turning back to the unsheared (or free) surface case, the bulk turbulence structures cause “upwellings” when they approach the interface. The regions between upwellings appear as stagnation lines on the surface plane—the surface-normal velocity being downwards. Whirlpool-like attached vortices also form at the edges of the upwellings. These attached vortices are remarkably persistent—the main annihilation mechanism being interaction with a subsequent upwelling. For situations where the surface patterns convect away from a region of turbulence generation, i.e. a decaying pattern, the attached vortices become the dominant structure since new upwellings and downdrafts are not formed. The attached vortices pair and decay in a manner such that the near-surface turbulence structure is essentially two-dimensional. Even in situations where turbulence generation occurs quite close to the free-surface, measures such as energy spectra indicate a quasi two-dimensional near-surface structure.
Dynamics of Free-Surface Flows With Surfactants
Authors: Swean T. F. , Beris A. N.
Published: April 2009

There is ample quantitative evidence (through, for example, surface tension measurements) of the presence of surfactants at the air-sea interface in sufficient quantities to influence the sea surface dynamics and its interactions with ambient flow turbulence. The importance of the role of the surfactants can also be judged from independent observations of phenomena such as suppression of short wavelength capillary waves and the presence of long-lived slick structures at the ship wakes. Although there is consensus on the presence of surfactants as the underlying reason behind these phenomena, the capability of quantitative predictions is still lacking for most of them. The objective of the present work is to introduce to the general engineering mechanics community the governing equations and the relevant issues associated with the study of free surface flows with surfactants. In particular, we focus on the interactions between a high Reynolds number flow, interface deformation and surfactant distribution next to and at the water-air interface. In addition, recent progress is briefly reviewed. Then, the remaining outstanding issues to allow the understanding of the dynamics of nonlinear interactions between turbulent flow and surfactant structure and concentration at the air-water interface are outlined.
Heterogeneous Material Mechanics at Various Scales
Authors: Christensen Richard M.
Published: April 2009

Some contemporary problems involving the mechanics of materials will be examined at widely different length scales. The three scales chosen are i) the near atomic scale, ii) an intermediate range of the order of microns, and iii) large scale macro or structural behavior. An additional theme transcending these scales will be carbon based, or carbon like heterogeneous material behavior. At the near atomic scale, examples are chosen from the various forms that elemental carbon can take including diamond, graphite, newly discovered fullerenes, and ideal carbon fibers. Examples at the intermediate length scale feature low density materials, since typical cell sizes are about a micron or somewhat larger or smaller. The effect of microstructure becomes the dominant and determining characteristic of such materials. A hierarchy of microstructures will be revealed which gives a coherent connection between microstructure and mechanical behavior. Finally, at the macroscale, involving structural behavior, a methodology for the life prediction of fiber dominated, graphite epoxy composites will be given. These three classes of problems are intended to show that virtually equal opportunities for meaningful research contribution occur across the spectrum of length scales.
Numerical Simulation of Elastohydrodynamically Lubricated Contacts With Rough Surfaces
Authors: Ai Alan Xiaolan , Cheng Herbert S.
Published: April 2009

Transient numerical analysis to elastohydrodynamically lubricated point contacts with rough surfaces is described. The numerical simulation is based on the multigrid method. Three types of surface roughness: single dent or bump, oblique waviness and random roughness are reviewed. Under heavily loaded conditions, results reveal a strong domination of Couette flow (convective flow). The presence of sliding greatly increases pressure fluctuation and as a consequence may lead to surface-initiated spallings. For obliquely orientated roughness, lubricant flow intends to deform the ridges and create primarily longitudinal passages.
Compressive Kinking of Fiber Composites: A Topical Review
Authors: Budiansky Bernard , Fleck Norman A.
Published: April 2009

This is a review of the results of several selected theoretical studies concerned with the localized kinking, or microbuckling, of aligned fiber composite materials subjected to compression in the fiber direction. Compressive kinking is of primary concern in polymer matrix composites, for which kinking failure can limit the compressive strength to a value that is usually much lower than the tensile strength. A similar situation can occur in carbon matrix fiber composites. Compressive kinking failure may be understood on the basis of an elementary theoretical approach that ignores the influence of bending resistance of the reinforcing fibers, but takes into account the nonlinearity of composite constitutive relations as well as the effects of initial imperfections in fiber alignment. Kink bands bounded by fiber breaks are produced by deformations that occur after the attainment of peak compressive loads. The theoretical calculation of the widths of such kink bands does require consideration of fiber bending resistance; on the other hand, the results for kink width are not sensitive to the sizes of initial fiber misalignments. Progress in the study of the following additional kinking topics is summarized briefly: correlation of static kinking strength and random fiber misalignment; effects of shear and transverse loads on static kinking; viscoelastic and creep kinking; kinking fatigue; and a phenomenological theory of kinking “toughness”.