bluuebunny/crossref_metadata_2025 huggingface dataset:
Integrity Assurance of Natural Gas Transmission Pipelines
Authors: Posakony G. J.
Published: April 2009

Natural gas transmission pipelines have proven to be a safe and efficient means for transporting the trillions of cubic feet of natural gas used annually in the United States. Since the peak of construction of these pipelines occurred between 1950 and the mid-1960s, their average age is now over thirty years. However, replacement of these pipelines because of age would be prohibitively expensive and unnecessary. Preventive maintenance and rehabilitation programs put into practice by the pipeline industry provides the key to ensuring the continued integrity of the transmission pipeline system. This article reviews the preventive maintenance practices commonly used by the gas industry. These practices include right-of-way patrols, corrosion control procedures, in-line inspection with intelligent or smart pigs that inspect the pipe while traveling through the inside of the pipe, direct access inspection of the pipe from bellhole excavations, and hydrostatic retesting of pipelines. When pipelines are properly maintained, these practices can ensure the integrity and long-term serviceability of transmission pipelines well into the 21st Century.
Generic Steam Generator Life Cycle Management From a Utility Perspective
Authors: Baker Ronald L.
Published: April 2009

Steam generator repairs and replacements, which have occurred over the last 10 years, have lead many utilities to evaluate the economics of continued maintenance on existing steam generators against the economics of steam generator replacement. Such an endeavor requires an assessment of the expected rate and types of degradation. In addition, an identification of possible preventative or remedial measures and their potential effectiveness must be made. To arrive at an assessment of the economic impact of various combinations of potential courses of action many utilities have employed in-house developed or customized commercial programs to convert technical assessments into economic impact evaluations. This paper intends to give the reader an introduction to the technical issues and insight into a method of addressing the economic impact of possible management strategies.
Radiation Embrittlement Mechanistic Modelling
Authors: Pavinich W. A. , Server W. L. , Griesbach T. J.
Published: April 2009

The mechanistic models of radiation embrittlement for reactor vessel steels are reviewed and direction for improving these models is provided. Improvement in these mechanistic models will lead to predictive expressions for parameters of engineering interest. This paper provides the initial direction of modelling efforts that will improve existing or develop new predictive equations for damage attenuation, temperature effects, thermal annealing, the effect of post weld heat treatment, transition behavior and upper shelf behavior of reactor vessel steels.
Memphis Light, Gas and Water Division Experience With Aging of Electric, Gas, and Water Systems
Authors: Pickel Charles
Published: April 2009

Electric, gas and water distribution systems can have an extremely long life when properly designed, installed and maintained. MLGW is proof positive that aging distribution systems can be managed in an effective manner. Customer satisfaction is a high priority with Division management. According to a recent survey, Memphians enjoy the lowest average monthly utility bills among the 25 largest cities in the United States.
Materials Considerations in Service Life Prediction
Authors: Wei Robert P. , Harlow D. Gary
Published: April 2009

To address the issues of aging of energy production and distribution systems, and those associated with durability and reliability of new systems, improvements in the methods for service life prediction are needed to adequately account for the contributions of material and environmental variables. A mechanistically based probability approach is proposed. The efficacy of this approach and the importance of materials considerations are discussed. Support of research, utilizing this approach to address specific failure mechanisms, is recommended.
Thermal Cycling Damage of Metal Matrix Composites: Analytical Study on Dimensional Change
Authors: Taya Minoru , Armstrong William D. , Dunn Marty
Published: April 2009

An analytical model to predict dimensional changes in thermal cycled metal matrix composites (MMCs) with and without constant stress is proposed. The present model assumes that the temperature-time relation to simulate thermal cycling is of a step-function type and the matrix metal deforms as elastic/plastic/creep material while the fiber remains elastic throughout thermal cycling. The model can explain well the experimental results of W-ThO2/FeCrAlY composite during the early stage of cycling and those of the previous works. The present model is then compared with the existing ones including our previous model. Another model is also constructed to simulate the later stage of thermal cycling damage where the debonding of the matrix-fiber interface would presumably take place progressively with number of cycles. This model with a progressive debonded interface was found to explain reasonably well the later stage of dimensional changes observed in W-ThO2/FeCrAlY composite. Then, the dimensional change in thermal cycled MMC under constant stress loading is studied by use of the above model for early (first) stage of thermal cycling that is modified to account for applied stress. This model can explain well the experimental results of W/Cu composite.
Structures and Geriatrics From a Failure Analysis Experience Viewpoint
Authors: Hopper David M.
Published: April 2009

In a failure analysis consulting engineering practice one sees a variety of structural failures from which observations may be made concerning geriatric structures. Representative experience with power plants, refineries, offshore structures, and forensic investigations is summarized and generic observations are made regarding the maintenance of fitness for purpose of structures. Although it is important to optimize the engineering design for a range of operational and environmental variables, it is essential that fabrication and inspection controls exist along with common sense based ongoing monitoring and operations procedures.
Dynamic Characteristics of Deteriorating Engineering Structures
Authors: Virgin Lawrence N.
Published: April 2009

The monitoring of energy production and distribution systems with a view to predicting their future performance and service life is a difficult but increasingly important exercise. Specific interest here concerns the behavior of aging offshore oil and gas structures. The maintenance and safety of these systems is a major challenge considering their generally deteriorating nature and harsh operating environment. The dynamic response of a structural component or structure can be used to assess properties such as mass, stiffness and damping. If periodic assessment is conducted then any changes, specifically deterioration, during the lifetime of a structure can be monitored using vibration measurement methods. This information can then be used to predict the useful life of the structure given a constant rate of deterioration or provide the basis for remedial action to be taken to strengthen the structure. The prevention of catastrophic failure is included in this scenario. Suggestions for possible research directions are made with regard to nonlinear behavior.
Risk Management for Existing Energy Facilities: A Global Approach to Numerical Safety Goals
Authors: Pate´-Cornell M. Elisabeth
Published: April 2009

This paper presents a structured set of numerical safety goals for risk management of existing energy facilities. The rationale behind these safety goals is based on principles of equity and economic efficiency. Some of the issues involved when using probabilistic risk analyses results for safety decisions are discussed. A brief review of existing safety targets and “floating numbers” is presented, and a set of safety goals for industrial risk management is proposed. Relaxation of these standards for existing facilities, the relevance of the lifetime of the plant, the treatment of uncertainties, and problems of failure dependencies are discussed briefly.
Session VIII Summary: Assessment
Authors: Cornell C. Allin
Published: April 2009

The working group took its objective to be the formulation of a set of research and development recommendations for assessment of aging energy systems. The material discussed in every session at this workshop has relevance to the assessment of characteristics such as the current state of these facilities or their components. Indeed many of the items we cite will appear elsewhere as well. The focus we have maintained, however, is assessment for the direct purpose of decision making. The decisions may have to do with increasing inspection, monitoring or analysis, or with repair, or even with closure and replacement. The assessment may be the risk assessment of an individual structure or the problem may be that of designing a cost-efficient inventory of the status of a large “fleet” of facilities.
Applied Mechanics of Lunar Exploration and Development
Authors: Duke Michael B. , Benaroya Haym
Published: April 2009

An overview of current concepts for lunar outposts is provided, with emphasis on identifying design issues that are also prospective research problems in applied mechanics. The authors believe that the conjugation of new applications and the unique features of the lunar environment will provide many interesting problems whose solution will provide fundamental insights into problems in applied mechanics.
Lunar Environment
Authors: Utreja Lajpat R.
Published: April 2009

As one of the key elements of the Space Exploration Initiative, the Moon provides a waypoint for scientific exploration and travel to Mars. The Moon’s stable ground in the vacuum of space is an ideal platform for astronomical observatories. Conditions on the Moon are similar to what human beings will face on other planets, so it is a natural test bed to prepare for a manned mission to Mars. A knowledge of the lunar environment is therefore important before undertaking any missions of construction, operations, and habitation on the lunar surface. The purpose of this paper is to review and assemble information on the lunar environment so that engineers and scientists can refer to this as they begin lunar-based engineering studies. The lunar environment is categorized into three major elements: lunar physical constants, lunar atmosphere, and lunar surface. The description of lunar size, orientation, period of rotation, and lunar month are all treated as part of lunar physical constants. Lunar atmosphere includes gas composition, pressure and density, solar flux and radiation, micrometeorite flux, and lunar dust. The geophysical and geochemical properties are provided as lunar surface characteristics. The geophysical properties include terrain characteristics, topography and surface tremors; soil and rock characteristics; mechanical, thermal, electrical, magnetic, and optical properties. The chemical composition of the regolith and rocks are described in geochemical properties.
Properties and Mechanics of the Lunar Regolith
Authors: Johnson Stewart W. , Chua Koon Meng
Published: April 2009

Knowledge of the lunar regolith is essential to success in lunar missions whether crewed or robotic. The regolith is the loose material overlying more intact strata on the Moon. It varies in thickness from several meters on the maria or lunar seas to many meters on the highlands of the Moon. The regolith is the material humans walked and drove on from 1969 to 1972. In the future, people will use it for radiation protection and as a resource for recovery of oxygen, silicon, iron, aluminum, and titanium. Implanted in the regolith by the solar wind are recoverable amounts of volatiles such as hydrogen and helium. Increasing our knowledge of the mechanical properties of the regolith will enable constructors of the 21st Century to build habitats, do mining, establish manufacturing, and erect telescopes on the Moon. We already know much of the regolith from robotic and astronaut missions to the Moon. There is much more to be learned.
Engineering Elements for Transportation on the Lunar Surface
Authors: Wallace Brian E. , Rao Niranjan S.
Published: April 2009

Major aspects of engineering planetary transportation vehicles are discussed. Emphasis is placed on the the mechanics of mobility and mobility system design, including considerations for designing the wheels, drive, steering, and suspension systems. Because mobility performance of any vehicle is extremely sensitive to environmental (e.g. soil properties) and design (e.g. vehicle weight) parameters, the mechanics of mobility are addressed by parametric application as well as empirical (derived relationships and equations) form. Finally, and more philosophical in nature, discussions of the key considerations for designing and analyzing each of the components of mobility (locomotion, steering, suspension, and stability) are also included. These discussions include some general guidelines to assist in determining the values of key design parameters.
Indigenous Materials for Lunar Construction
Authors: Happel John A.
Published: April 2009

An important step in the exploration and colonization of the solar system is to build a permanently inhabited base on the Moon. The lunar environment is stark and hostile to unprotected humans. Structures are needed that protect the inhabitants from vacuum, radiation, extreme temperatures, dust, and meteoroids. Transporting the necessary construction materials from Earth is extremely expensive. Fortunately, lunar structures can be built utilizing indigenous materials. The locally available materials include lunar regolith, cast regolith, glass and glass composites, metals and concrete. Their mechanical properties are summarized and their suitability for lunar construction is evaluated. The most promising materials are cast regolith and lunar glass. Several lunar bases concepts utilizing indigenous materials are described and evaluated. Precast modules and large cast in place structures can be fabricated from lunar concrete. Large cylindrical modules, curved and flat panels and arches cast from lunar regolith are also feasible. A tied arch system is considered very promising because of its structural efficiency.
Synchronization and Chaotization of Oscillations in Coupled Self-Oscillating Systems
Authors: Landa P. S. , Rosenblum M. G.
Published: April 2009

The effects of synchronization of chaotic and periodic self-oscillation systems are discussed. Two mechanisms of synchronization of periodic systems which manifest themselves at synchronization of chaotic systems are determinated. Examples of synchronization of chaotic systems by harmonic external force, mutual synchronization of periodic and chaotic systems, as well as of mutual synchronization of two and more chaotic systems are discussed.
Vibrations of Planar Curved Beams, Rings, and Arches
Authors: Chidamparam P. , Leissa A. W.
Published: April 2009

This work attempts to organize and summarize the extensive published literature on the vibrations of curved bars, beams, rings and arches of arbitrary shape which lie in a plane. In-plane, out-of-plane and coupled vibrations are considered. Various theories that have been developed to model curved beam vibration problems are examined. An overview is presented of the types of problems which are addressed in the literature. Particular attention is given to the effects of initial static loading, nonlinear vibrations and the application of finite element techniques. The significantly different frequencies arising from curved beam theories which either allow or prevent extension of the centerline during vibratory motion are shown. An extensive bibliography of 407 relevant references is included.
Structural Adaptation of Bones
Authors: Cowin S. C.
Published: April 2009

Living bone tissue, like many other connective tissues, is a structural material that adapts its form and microstructure to changing environmental loading conditions. Bone tissue adapts not only its shape, but also its density and the details of its microstructure including its anisotropy. The anisotropy of bone is adapted in both its degree or strength and in the orientation of its principal axes of symmetry. These adaptive features of bone tissue are often referred to as aspects of Wolff’s law, although, strictly speaking, the term “Wolff’s law” applies only to the structural adaptation of spongy or trabecular bone. In this paper the composition, microstructure, mechanical properties and structurally adaptive features of bone are briefly reviewed. An algebraic formulation of Wolff’s law at remodeling equilibrium is described, and the nature of an evolutionary Wolff’s law is sketched.
Nonlinear Triple-Deck Studies in Boundary-Layer Receptivity
Authors: Bodonyi R. J.
Published: April 2009

A numerical study of the generation of Tollmien-Schlichting waves due to the interaction between a small free-stream disturbance and a small localized variation of the surface geometry such as a hump or suction strip has been carried out using both finite difference and spectral methods. The nonlinear steady flow is of the viscous-inviscid interactive type while the unsteady disturbed flow is assumed to be governed by linearized disturbance equations. The numerical solutions illustrate the growth or decay of the TS waves generated by the interaction, depending on the value of a scaled Strouhal number.
Measurement of Acoustic Receptivity at Leading Edges and Porous Strips
Authors: Wlezien R. W. , Parekh D. E. , Island T. C.
Published: April 2009

The receptivity of a laminar boundary layer to acoustic disturbances in the vicinity of a leading edge and a narrow porous surface is investigated experimentally. The relative importance of the receptivity mechanisms is explored for leading edge and porous surface configurations. Several methods to decouple the acoustic and instability wave velocity perturbations are discussed, and consistent estimates of the Tollmien-Schlichting modes are achieved when the acoustic field is directly estimated from profiles of the total fluctuating velocity. A 24:1 leading edge produces negligible Tollmien-Schlichting response when interacting with symmetric acoustic disturbances and is used to confirm the porous-surface receptivity mechanism.
Mean Flow Validation for Numerical Study of Leading Edge Boundary Layer Receptivity
Authors: Gatski Thomas B.
Published: April 2009

Numerical calculations are presented for the incompressible flow over a parabolic cylinder. Cylinder radii, smaller than the Tollmien-Schlichting wavelength of the boundary-layer flow, and smaller than those examined in previous studies, are considered. The calculation includes the flow upstream of the body, as well as the leading-edge region itself, and extends downstream into the region where the Blasius boundary-layer solution holds. A steady mean flow solution is computed and the results for the scaled surface vorticity and the displacement thickness are compared to previous studies. The unsteady problem is then formulated as a perturbation solution starting with and evolving from the mean flow. Comparisons are made with the Stokes wave solutions at various periods and locations within the boundary-layer. In the initial phase of this study reported here, these mean flow results are used to analyze some of the features observed in previous forced flow numerical studies.
Microstructure of Shape-Memory and Magnetostrictive Materials
Authors: James R. D.
Published: April 2009

Recent advances in the analysis of microstructure is providing models and methods for treating the kinds of optimization problems that arise in the study of microstructure. The main advance has been the development of theory and methods for treating the case in which arbitrary microstructures compete for the minimum (or maximum). This contrasts for example with micromechanics in which the geometry of the microstructure is assumed, or assumed up to the choice of a few parameters, and then the optimization or stress analysis is carried out under severe geometric restrictions. Micromechanics is effective in dealing with a particular experimentally observed microstructure, but not for understanding microstructures that might be optimal in a certain sense. Much of this recent research has been fueled by critical discussions among engineering scientists, mathematicians and electron microscopists. The intent of this paper is first to summarize, in terms accessible to a broad audience, the nature of this research and then to describe applications to the improvement of shape-memory and magnetostrictive materials. The general part of the lecture will focus on three areas, effective properties of materials, optimal design of materials and phase transformation and active materials. A central role is played by the question “How does one meaningfully average a quantity whose values vary rapidly on a microstructural scale?” A second recurring theme is that the optimal microstructure is predicted to have fine structure. The latter is closely related to the failure of conditions of material stability.
Localization in Granular Materials
Authors: Jenkins James T.
Published: April 2009

We sketch an analysis of the onset of shear bands in dry granular materials and provide some direction to recent work.
Adiabatic Shear Bands
Authors: Wright T. W.
Published: April 2009

Some recent results on the analysis of adiabatic shear bands are reviewed.
Coherent Structures in Turbulent Shear Flows
Authors: Wallace J. M. , Hussain F.
Published: April 2009

What is firmly known about the kinematic properties and dynamic importance of coherent structures in bounded and unbounded turbulent shear flows is briefly summarized. The nature of instabilities giving rise to these structures is discussed. Unanswered questions requiring further research are posed.
Dynamical Significance of Turbulent Wall Layer Streaks
Authors: Bernard P. S. , Handler R. A.
Published: April 2009

The role of low speed streaks in the dynamical processes leading to the generation of Reynolds stress is investigated using ensembles of computed particle paths obtained from a direct numerical simulation of turbulent channel flow. Simultaneous visualization of appropriate Eulerian fields and trajectories of fluid particles which are most indicative of Reynolds stress production are given. These graphically illustrate the occurrence of ejection events at a series of discrete locations along low speed streaks. A strong association between streamwise vortices and the ejecting fluid is found. In particular, visualization of the ejecting fluid shows the presence of vortices which drive fluid from the sides up and over the low speed regions. As part of this process fluid from within the streaks appears to be entrained outward from the wall. Some of the implications of these results for turbulence modeling will be described.
Coherent Structures and the Generalized Lagrangian Mean Equation
Authors: Phillips W. R. C.
Published: April 2009

The Generalized Lagrangian mean equations are used to derive evolution equations for the perturbation flow about a turbulent mean base flow which is homogeneous in the streamwise and spanwise directions. The equations expose the mechanism which leads to the formation of streamwise vortices in the wall region of turbulent bounded flows and may be solved numerically. The advantage of this formulation is that the form of the coupling terms in the equations is known precisely; moreover, they can be expressed in terms of space time correlations, most of which have been measured. The result is a set of partial differential equations for the rectified flow field with the fluctuations filtered out. The rectified flow field then provides a way to represent the underlying coherent structures in the flow. It also permits the determination of their dynamics, which may be chaotic.
Application of Slender Body Theory to Describe Wall Turbulence
Authors: Hanratty Thomas J. , Kontamaris K.
Published: April 2009

Observations of turbulent flow close to a wall reveal turbulent eddies which are elongated in the flow direction. This has motivated the use of a slender body assumption to simplify the Navier Stokes equations. Derivatives in the flow-direction are neglected so that three velocity components are calculated in a plane. The application of this 2 1/2D model to the viscous wall region (y+ < 40) shows that the turbulent velocity field can be represented by interaction of two eddies with spanwise wavelengths of 100 and 400 wall units. This model has been used to investigate the effect of favorable pressure gradients on a turbulent boundary-layer and to explore what determines the size of the stress producing eddies close to the wall. The accuracy of the basic physical assumptions are explored by examining resulte from a computer simulation of the three-dimensional time dependent turbulent flow in a channel. Some possible improvements are discussed, which make use of the observation that spatial derivatives in the flow direction can be related to time derivatives by using a convection velocity.
Steady-State Mechanics of a Growing Crack Paralleling an Elastically Constrained Thin Ductile Layer
Authors: Charalambides P. G. , Mataga P. A. , McMeeking R. M. , Evans A. G.
Published: April 2009

The steady-state near tip mechanics of brittle cracks growing under mixed mode conditions parallel and at close proximity to a thin ductile layer in a sandwich specimen morphology are examined. The plastic dissipation in the elastically constrained ductile layer is determined analytically through an approximate method and numerically via rigorous finite element calculations. Sensitivity studies regarding the effects of plasticity, layer thickness and crack location are presented. Thus, crack shielding characteristics are addressed and relationships between the remote (Gapp) and near tip (Gtip) energy release rates are established. The associated crack tip phase angle ψtip is extracted numerically via the method of finite elements. The above analysis is used to interpret experimental data for a sapphire/gold/sapphire system obtained using the plane strain four-point flexure model specimen geometry.
Analyses of Interfacial Failure
Authors: Needleman A.
Published: April 2009

A cohesive zone type model, taking full account of finite geometry changes, is used to provide a unified framework for describing the process of interfacial decohesion from initial debonding through complete separation. Constitutive relations are specified independently for material phases and for the interface. This model permits the prediction of interfacial decohesion without the necessity of introducing some additional failure criterion. Also, since the mechanical response of the interface is specified in terms of both a critical interfacial strength and the work of separation per unit area, dimensional considerations introduce a characteristic length. Various issues associated with the analysis of interfacial failure phenomena within this framework will be discussed.
Single Crystal Hardening
Authors: Bassani J. L.
Published: April 2009

The hardening behavior of individual slip systems under multislip deformations is reinterpreted, which leads to a new analytical description of hardening. Most significantly, the transition from stage I (single slip easy glide) to stage II (multiple slip) hardening for “single slip” orientations of the tensile axis and, generally, the orientation dependence of hardening is well predicted. Even within a time-independent framework the hardening description leads to a unique determination of the active set of slips. Numerical calculations are based on an extremum principle and utilize a modified quadratic programming algorithm.
Search for Exceptional Waves: Tellurium Dioxide, an Example
Authors: Abbudi M. , Barnett D. M.
Published: April 2009

The authors review the basic theory governing the existence of exceptional bulk waves which propagate in an infinite anisotropic elastic medium while leaving a family of parallel planes traction-free. A solution scheme proposed by Al’shits and Lothe is implemented numerically and solution lines of exceptional waves for tetragonal tellurium dioxide are computed as an example. It is found that tellurium dioxide possesses nine acoustic axes, five great circles on the unit sphere as solution lines of transverse exceptional waves, and a number of closed curves as solution lines of non-transverse exceptional waves.
Wave Propagation in Fluid-Loaded Laminated Cylindrical Shells
Authors: Braga Arthur M. B. , Barbone Paul E. , Herrmann George
Published: April 2009

Harmonic waves propagating in a fluid-loaded, composite circular cylinder are investigated. The solid in this case is composed of perfectly bonded, isotropic, linearly elastic, cylindrical layers stacked radially to form a laminated cylinder. The surface impedance of the laminated shell is calculated using a recursive invariant-imbedding algorithm. This algorithm is numerically stable for a wide range of frequencies and circumferential mode numbers. Particular attention is given to the study of subsonic interface waves which propagate unattenuated along a fluid-solid interface. Results are presented which show the effects of layering and curvature on the dispersion spectrum of the subsonic interface waves.
Finite Element Solution and Dispersion Analysis for the Transient Structural Acoustics Problem
Authors: Abboud N. N. , Pinsky P. M.
Published: April 2009

In this paper a finite element formulation is proposed for solution of the time-dependent coupled wave equation over an infinite fluid domain. The formulation is based on a finite computational fluid domain surrounding the structure and incorporates a sequence of boundary operators on the fluid truncation boundary. These operators are designed to minimize reflection of outgoing waves and are based on an asymptotic expansion of the exact solution for the time-dependent problem. The variational statement of the governing equations is developed from a Hamiltonian approach that is modified for nonconservative systems. The dispersive properties of finite element semidiscretizations of the three dimensional wave equation are examined. This analysis throws light on the performance of the finite element approximation over the entire range of wavenumbers and the effects of the order of interpolation, mass lumping, and direction of wave propagation are considered.
Elasto-Plastic Analysis of Reissner-Mindlin Plates
Authors: Papadopoulos Panayiotis , Taylor Robert L.
Published: April 2009

A finite element analysis of elasto-plastic Reissner-Mindlin plates is presented. The discrete field equations are derived from a nonlinear version of the Hu-Washizu variational principle. Associative plasticity, including linear hardening, is employed by means of a generalized von Mises-type yield function. A predictor/corrector scheme is used to integrate the plastic constitutive rate equations. Numerical simulations are conducted for a series of test problems to illustrate performance of the formulation.
Effects of Elastic Stress on the Stability of a Solid-Liquid Interface
Authors: Spencer B. J. , Davis S. H. , McFadden G. B. , Voorhees P. W.
Published: April 2009

The effects of elastic stress on the stability of solid-liquid interfaces under a variety of conditions are discussed. In the cases discussed, the nonuniform composition field in the solid, which accompanies either the melting process or the development of a perturbation on the solid-liquid interface during solidification, generates nonhydrostatic stresses in the solid. Such compositionally generated elastic stresses have been shown experimentally to induce a solidifying solid-liquid interface to become unstable. We are in the process of analyzing the effects of these stresses on the conditions for morphological stability of a directionally solidified binary alloy.
Float Zone Modelling: Transport Phenomena and Morphological Stability
Authors: Young G. W.
Published: April 2009

A model is presented for a float zone established in a vertical sheet. Heat transfer between the system (melt, feed and crystal) and the surrounding environment (including the heating source) is assumed to take place via radiation. Asymptotic solutions for the temperature, concentration, and melt flow profiles and the melting, solidifying and melt/gas interfacial shapes are developed in the limit of small aspect ratio (zone half-width/length) and weak surface tension driven flows. We find that convective heat transport leads to melt back of the solidification front near the edges. Further, lateral solute segregation is due to both convective effects and curved solidification fronts. Increasing the flow or increasing the velocity of solidification leads to increased lateral solute segregation in melts that are not well mixed. Hence, flat solidification fronts may not yield flat concentration profiles. To reduce the level of convection, hot wall, low temperature gradient float zone techniques are also investigated. Morphological stability of the solidification front is of concern for such heating configurations. We present the results of a linear stability analysis in the limits of small segregation coefficient, slow velocities of solidification, and large surface heat transfer. It is found that shorter melt zones are less susceptible to long wavelength morphological instabilities.
Effects of Anisotropy Properties on Interface Pattern Formation
Authors: Trivedi R.
Published: April 2009

The anisotropy in interfacial energy and interface kinetics significantly influence the interface patterns that develop when an alloy is directionally solidified beyond the threshold of the planar interface stability condition. These anisotropy effects not only influence the planar to cellular and cellular to dendritic transitions, but they also cause tilting of cells and dendrites with respect to the heat flow direction. The extent of tilt depends upon the velocity as well the crystal orientation at the interface. Experimental studies on pattern formation in anisotropic materials are presented, and these results are compared with the linear and weakly nonlinear analyses of the planar interface stability.
Long–Wave Morphologies in Directional Solidification
Authors: Riley D. S.
Published: April 2009

Long–wave instabilities in a directionally–solidified binary mixture may occur in several limits. Sivashinsky identified a small–segregation–coefficient limit and obtained a weakly–nonlinear evolution equation governing subcritical two–dimensional bifurcation. Brattkus and Davis identified a near–absolute–stability limit and obtained a strongly–nonlinear evolution equation governing supercritical two–dimensional bifurcation. In this presentation these previous analyses are set into a logical framework, and a third distinguished (small–segregation–coefficient, large–surface–energy) limit identified. The corresponding strongly–nonlinear, evolution equation equation links both of the previous and describes the change from sub– to super–critical bifurcations.
Use of Fractal Dimension in the Characterization of Chaotic Structural Dynamic Sytems
Authors: Hall E. , Kessler S. , Hanagud S.
Published: April 2009

The purpose of this paper is to investigate the use of fractal dimensions in the characterization of chaotic systems in structural dynamics. The investigation focuses on the example of a simply-supported, Euler-Bernoulli beam which when subjected to a transverse forcing function of a particular amplitude responds chaotically. Three different nonlinear models of the system are studied: a complex partial differential equation (PDE) model, a simplified PDE model, and a Galerkin approximation to the simpler PDE model. The responses of each model are examined through zero velocity Poincare´ sections. To characterize and compare the chaotic trajectories, the box counting fractal dimension of the Poincare´ sections are computed. The results demonstrate that the fractal dimension is a spatial invariant along the length of the beam for the specific class of forcing function studied, and thus it can be used to characterize chaotic motions. In addition, the three models yield different fractal dimensions for the same forcing which indicates that fractal dimensions can also be used to quantify whether a simplification of a chaotic model accurately predicts the chaotic behavior of the full-blown model. Thus the conclusion of the paper is that fractal dimensions may play an important role in the characterization of chaotic structural dynamic systems.
Structural Reliability of Reactor Fuel Rods After Emergency Cooling
Authors: Heller R. A. , Thangjitham S. , Wang X.
Published: April 2009

The structural reliability of layered cylinders such as clad nuclear reactor fuel rods, subjected to a thermal shock is investigated. Emergency cooling may reduce the temperature of rods from operating conditions to lower levels in seconds. The high rate of temperature change requires the solution of a coupled thermo-elasticity problem that takes into consideration the coupling effects of thermal and mechanical parameters but neglects thermal-dynamic interactions. The Fourier transform-inverse transform relationship between frequency response and impulse response is used to calculate stresses as well as the reliability of each layer. The “weakest link” principle which takes into consideration both the applied stress and the effected volume of material is employed in the reliability analysis.
Integration of Information From Design and Manufacture Through Decision Support Problems
Authors: Karandikar Harshavardhan , Mistree Farrokh
Published: April 2009

Effectively addressing life-cycle considerations early in the design process and a reduction in design cycle time are key issues in concurrent engineering. They require making good design decisions in the face of uncertainty, ambiguity and incomplete information. In this paper an overview of our research concerning the integration of material selection and manufacturing considerations with dimensional synthesis of a component is presented. The integration is achieved by formulating appropriate Decision Support Problems and using multiobjective optimization for solution in order to provide decision support. Our approach facilitates concurrent handling of a number of issues associated with designing for manufacture and enables a reduction in the number of iterations in the process of designing. This has been shown through case studies dealing with the design of structures made of composite materials.
Experiments on the Excitation of Resonances of Elastic Objects by Acoustic Pulses and Theoretical Interpretation
Authors: Bao Xiao-Ling , U¨berall Herbert , Niemiec Jan
Published: April 2009

We have carried out tank experiments on the excitation of vibrational resonances of submerged elastic objects by incident short acoustic pulses. We have studied steel spheres, cylinders with hemispherical endcaps, and prolate as well as oblate spheroids. Theoretical interpretations of the resonance positions are provided by the matching of the phases of surface waves excited by the incident pulses, that encircle the object along closed paths. Some results of a French experimental study (G Maze and J Ripoche, University of Le Havre) are interpreted using the same approach.
Investigation of Finite Deformations and Shear Banding: Theory and Experiment
Authors: Bayoumi A. E. , Joshi R. B. , Zbib H. M.
Published: April 2009

An experimental method using a digital image processing technique is developed for the purpose of characterizing material behavior at large elastoplastic deformations and the associated phenomenon of localization of plastic flow into shear bands. This allows for a detailed description of the evolution of the nonuniform deformation pattern in the post-localization regime. The experimental results are utilized to calibrate a recently developed gradient-dependent constitutive equation which takes into account the effect of heterogeneous plastic flow, anisotropy and large deformations. The measured values of the gradient coefficients are of small magnitude suggesting that higher order gradients are important only in the highly inhomogeneous region as expected. Moreover, it is found that anisotropic effects become significant in the post-localization regime where the anisotropy ratio changes considerably.
Harmonious and Transient Behavior of Rotors
Authors: Paz Mario
Published: April 2009

The paper presents an analytical solution to predict the behavior of eccentric rotors, both during the transient motion as well as the conditions for self-synchronization. Hamilton’s principle is applied to predict the condition of self-synchronization of two rotors arranged in arbitrary configuration. The transient behavior of two electric motors with eccentric rotors is presented. It is shown that the motors may not attain the final operating speed and be locked at a resonant frequency of the system unless they possess sufficient power.
Multiple Integral Constitutive Equations in Unsteady Motions and Rheometry
Authors: Siginer D. A.
Published: April 2009

The use of the integral fluid of order three to predict some simple nearly viscometric unsteady flows of viscoelastic liquids, driven by periodic forcing, is discussed. Flow enhancement effects, due to the parallel and orthogonal superposition of oscillatory and simple shear fields, are predicted. It is shown that it is feasible to determine the constitutive constants involved from a series of experiments of rheometry.
Dynamic Analysis of a Stone Column Excited by a Sine Wave Ground Motion
Authors: Sinopoli A.
Published: April 2009

The aim of this work is the identification of an analytical model to study the dynamic behaviour of a stone column, either monolithic or multiblock, excited by a sine wave ground motion. It is demonstrated that the motion can be described by a family of forced damped Hill’s equations. The operative limits of the model in relation to the problem are shown and discussed; in fact, the system exibihits some kind of unpredictability. In any case, criteria for a stability analysis of the motions are given.
Efficient Mode Superposition Method for Seismic Analysis of Nonlinear Systems
Authors: Villaverde Roberto , Hanna Melad M.
Published: April 2009

A step-by-step integration method is proposed to compute within the framework of the conventional mode superposition technique the response of bilinear hysteretic structures subjected to earthquake ground motions. The method is computationally efficient because only a few modes need to be considered to obtain an accurate estimate of such a response, and because it does not require the use of excessively small time steps to avoid problems of accuracy or stability. It is developed on the basis that the nonlinear terms in the equations of motion for nonlinear systems may be considered as additional external forces, and on the fact that by doing so such equations of motion can be interpreted as the equations of motion of an equivalent linear system, excited by a modified ground motion. These linear equations are then subjected to a conventional modal decomposition and transformed, as with linear systems, into a set of independent differential equations, each representing the system’s response in one of its modes of vibration. To increase the efficiency of the method and properly account for the participation of higher modes, these independent equations are solved using Nigam-Jennings technique in conjunction with the so-called mode-acceleration method. In addition, an iterative scheme is introduced to avoid an inefficient recalculation of the system’s eigenvectors and eigenvalues every time there is a change in the stiffness of one of its elements. The accuracy and efficiency of the method is verified by means of a comparative study with solutions obtained with a conventional direct integration method. In this comparative study, with only a few modes considered, the proposed method accurately predicts the seismic response of three two-dimensional frame structures, but requiring only, on the average, about 43 per cent of the computer time spent when using the direct integration method.
Response and Discretization Methods for Axially Moving Materials
Authors: Wickert J. A. , Mote C. D.
Published: April 2009

Through a convective acceleration component, the equations of motion for axially-moving materials are skew-symmetric in the state space formulation, so that the response problem is best analyzed within the broader context of continuous gyroscopic systems. With particular application to the prototypical traveling string and beam models, a modal analysis that associates degrees of freedom with the complex state eigenfunctions and their conjugates is presented. This procedure is well-suited for harmonic excitation sources, and in some instances, it is more convenient than previous methods which decompose the modal coordinates, eigenfunctions, and generalized forces into real and imaginary components. Also from the state space perspective, Rayleigh’s quotient for gyroscopic systems provides a variational method for determining the eigensolutions of axially-moving materials. Ritz discretization of the quotient can make effective use of the speed-adapting modes of the traveling string and beam models as they are rich in phase, as well as amplitude, content.
Mixed Volume Boundary Element Approach for Aerodynamics
Authors: Branda˜o Mauri´cio P.
Published: April 2009

A review is presented of theoretical methods in aerodynamics and aeroacoustics which lead to the present approach. A formulation is developed for the analysis of three-dimensional, unsteady, and viscous flows. The integral solution to the Ffowcs Williams and Hawkings equation mixes surface and volume contributions. The surface terms are treated following the traditional boundary element technique. Special care is taken in revealing the hidden singularities of curved surfaces. The volume terms are treated following a cell-based approach. The concept of finite-part of singular integrals is used to interpret these terms. A computational procedure is proposed to determine simultaneously the pressure and velocity fields around bodies moving in incompressible and compressible fluids. Finally, current and future directions of research regarding the present method are pointed out and discussed.
Nonlinear Deformation of Composite Beams: Unification of Cross-Sectional and Elastic Analyses
Authors: Atilgan Ali R. , Hodges Dewey H. , Fulton Mark V.
Published: April 2009

A unified analysis is presented for predicting the deformation of anisotropic beams. Outlines of the derivations of both a linear, two-dimensional, cross-sectional analysis and a nonlinear, one-dimensional analysis are given from application of a general kinematical foundation, in which only small strain and small local rotation are assumed. The deformation may be arbitrary except that restrained warping effects are not treated. Predictions from the unified analysis agree quite well with published experimental results for both nonlinear static and linearized dynamic behavior about equilibrium.
Implementation of the Nonlocal Microplane Concrete Model Within an Explicit Dynamic Finite Element Program
Authors: Cofer William F.
Published: April 2009

The microplane concrete material model is based upon assumptions regarding the behavior of the material components. At any point, the response to the strain tensor on arbitrarily oriented surfaces is considered. Simple, softening stress-strain relationships are assumed in directions perpendicular and parallel to the surfaces. The macroscopic material behavior is then composed of the sum of the effects. The implementation of this model into the explicit, nonlinear, dynamic finite element program, DYNA3D, is described. To avoid the spurious mesh sensitivity that accompanies material failure, a weighted integral strain averaging approach is used to ensure that softening is nonlocal. This method is shown to be effective for limiting the failure zone in a concrete rod subjected to an impulse loading.