bluuebunny/crossref_metadata_2025 huggingface dataset:
Computational Models for High-Temperature Multilayered Composite Plates and Shells
Authors: Noor Ahmed K. , Burton W. Scott
Published: April 2009

The focus of this review is on the hierarchy of composite models, predictor-corrector procedures, the effect of temperature-dependence of material properties on the response, and the sensitivity of the thermomechanical response to variations in material parameters. The literature reviewed is devoted to the following eight application areas: Heat transfer; thermal stresses; curing, processing and residual stresses; bifurcation buckling; vibrations of heated plates and shells; large deflection and postbuckling problems; and sandwich plates and shells. Extensive numerical results are presented showing the effects of variation in the lamination and geometric parameters of temperature-sensitive angle-ply composite plates on the accuracy of thermal buckling response, and the sensitivity derivatives predicted by nine different modeling approaches (based on two-dimensional theories). The standard of comparison is taken to be the exact three-dimensional thermoelasticity solutions. Some future directions for research on the modeling of high-temperature multilayered composites are outlined.
Stress Minimum Forms for Elastic Solids
Authors: Wheeler Lewis T.
Published: April 2009

This article concerns the problem of determining shapes of minimum stress concentration. Special emphasis is placed upon the mathematically precise formulation of these problems and upon the study of cases in which the optimum shape can be found in a highly explicit form. For all the cases discussed, a mathematical proof of the optimality is given.
Stability of Laminated Composites
Authors: Guz’ A. N. , Chekhov Vik N.
Published: April 2009

The characteristic special feature of deformation behavior of modern laminated composite materials and structural elements fabricated from these materials, at current levels of loading and operating conditions is the occurrence of the purely three-dimensional stress-deformed state. In this process some specific mechanical phenomena and effects may occur, which is impossible to describe within the framework of applied or approximate approaches existing currently in deformable solid body mechanics. The structure of massive laminated materials may be included in this class of phenomena when the critical parameters of the problem depend only on the ratio between mechanical and geometrical characteristics of single layers and are independent of the dimensions and the form of the total laminated body as a whole. Since this phenomenon may be the beginning of the process of fracture of these materials, and the loss of the load-carrying capacity of structure elements fabricated from them, we consider below, in three-dimensional formulation, the problem of the surface and internal instability in laminated composite materials under compressive surface loads. The classification of the existing types of stability problems is presented for laminated materials and approaches for their solution presented in the literature. On the basis of three-dimensional linearized stability theory, within the framework of the piecewise-homogeneous media model, the general formulation of the most characteristic classes of stability problems of laminated materials is given in Lagrangian coordinates at small and finite, homogeneous and inhomogeneous precritical deformation. Analytic and variational methods of investigation of formulated problems are given with application to various models of laminated bodies models, in accordance with accepted stability criteria. The accuracy of these models is evaluated, based on the example of the solution of certain model problems; the range of application is given for existing applied and approximate approaches to the analysis of formulated problems. Results of solution of specific problems are given; specific mechanical effects, characteristic for the phenomenon considered, are found. Recommendations are formulated for engineering design and computational methods. New promising areas of research are pointed out for the problem of laminated composite materials stability.
Microbubble Drag Reduction in Liquid Turbulent Boundary Layers
Authors: Merkle Charles L. , Deutsch Steven
Published: April 2009

The interactions between a dense cloud of small bubbles and a liquid turbulent boundary layer are reviewed on the basis of available experimental observations to understand and quantify their capability for reducing skin friction. Gas bubbles are generally introduced into the boundary layer by injection through a porous surface or by electrolysis. After injection, the bubbles stay near the wall in boundary-layer-like fashion giving rise to strong gradients in both velocity and gas concentration. In general, the magnitude of the skin friction reduction increases as the volume of bubbles in the boundary layer is increased until a maximum skin friction reduction of typically 80–90% of the undisturbed skin friction level is reached. The volumetric gas flow required for this maximum is nominally equal to the volume flow of the liquid in the boundary layer. Bubble size estimates indicate that in most microbubble experiments the bubbles have been intermediate in size between the inner and outer scales of the undisturbed boundary layer. Additional studies with other nondimensional bubble sizes would be useful. However, the bubble size is most likely controlled by the injection process, and considerably different conditions would be required to change this ratio appreciably. The trajectories of the bubble clouds are primarily determined by the random effects of turbulence and bubble-bubble interactions. The effects of buoyancy represent a weaker effect. The trajectories are unlike the deterministic trajectory of an individual bubble in a time-averaged boundary layer. Bubbles are most effective in high speed boundary layers and, for the bubble sizes tested to date, produce an effect that persists for some one hundred boundary layer thicknesses. Modeling suggests that microbubbles reduce skin friction by increasing the turbulence Reynolds number in the buffer layer in a manner similar to polymers. Although the effects of microbubbles are consistent and reproducible, their primary practical limitation is the volume of gas needed. Studies aimed at reducing the volumetric gas flow requirements are recommended. Potential applications would favor high speed vehicles operating near the surface where pumping work is minimized.
Heat-Transfer Control: Principal Focus on Japanese Research
Authors: Aihara Toshio
Published: April 2009

Rapid transient heat transfer of milli- or micro-second order has come to play an important role in advanced engineering and technology, and the demand for active and direct control of heat transfer is growing. In this article, first, the recent needs for heat-transfer control are considered in detail. Then, the difference in concept between heat-transfer control and conventional temperature or thermal control is discussed. In the following sections, the methods of heat-transfer control are divided into passive and active methods, and the active control methods are classified into eight categories: transient heat-transfer method, molecular dynamics method, variable physical property method, external-force control method, boundary-layer control method, phase change method, multiphase media method, and porous media method. With respect to each of them, the controllability of heat transfer and its feasibility are discussed, quoting principally the recent Japanese literature. In the concluding remarks, the future trends of heat-transfer control are described.
Optimum Dimensions of Extended Surfaces Operating in a Convective Environment
Authors: Aziz A.
Published: April 2009

This article is devoted to the review of the literature on optimum dimensions of extended surfaces losing heat by pure convection to the surroundings. The review covers straight (longitudinal) fins, annular (radial) fins, and spines of different profile shapes. The optimum dimensions for each shape are given both in terms of the volume of the material as well as the heat dissipation. The effects of tip heat loss, variable heat transfer coefficient, internal heat generation, temperature dependent thermal conductivity, base convection, and primary surface thickness on the optimum dimensions are discussed. The optimization procedure is illustrated with several numerical examples. Areas of extended surface technology where further optimization studies are needed are identified. It is hoped that the article would serve the dual purpose of the state-of-the-art as well as a pedagogical review.
Overview of Micro Heat Pipe Research and Development
Authors: Peterson G. P.
Published: April 2009

The concept of a micro heat pipe was first proposed in 1984. Since that time, numerous analytical and experimental investigations have been conducted to determine the fundamental parameters that govern the operation of these devices. Micro heat pipes ranging in size from 1 mm in diameter and 60 mm in length to 30 μm in diameter and 10 mm in length have been analyzed, modelled, and fabricated. The following review describes the historical development of these devices, along with the analytical and numerical techniques used to model and predict their performance and the results of several recent experimental investigations. Because of recent advances in the development of micro heat pipes fabricated as an integral part of semiconductor wafers, particular emphasis has been placed on various construction and charging methods currently under investigation.
Fluorescence Thermometry
Authors: Dowell L. Jonathan
Published: April 2009

The fluorescence of rare-earth-doped ceramic phosphors depends on temperature. Particularly, the fluorescence lifetime is decreased by temperature. This dependence makes the fluorescence suitable for measuring temperature. This paper describes the physics of the fluorescence of these ceramics, noting the works of Forster, Dexter, Inokuti, Hirayama, and others. Next, it outlines the several advantages of fluorescence thermometry. These advantages include (a) measurement of temperature by transfer to measurement standards for time, (b) remote sensing for surface thermometry, (c) high speed of measurement relative to many physical and mechanical phenomena, (d) narrow-spectrum optical sensing suitable for hostile electrical and luminous environments, and (e) the transfer of calibration standards for precise thermometry. The paper presents engineering considerations for realizing these advantages. It presents parameter-estimation techniques that allow measurement of the temperature-dependent fluorescence parameters. It describes instrumentation techniques that transfer the measurement of temperature to measurement of units of time, with instrument calibration by atomic standards. It also discusses other measurement and instrumentation details.
Extensile Cracking in Porous Rock Under Differential Compressive Stress
Authors: Myer L. R. , Kemeny J. M. , Zheng Z. , Suarez R. , Ewy R. T. , Cook N. G. W.
Published: April 2009

Under differential compressive stress rocks exhibit nonlinear deformation that includes initial compaction, near-linear elastic behavior, and strain-hardening followed by strain-softening and dilation (or compaction in clastic rocks) and localization. This behavior derives largely from changes in the microstructure of the rocks. Much of it has been attributed to the growth of extensile microcracks. The stress-induced microstructural changes brought about by successively more complicated states of stress produced by uniaxial and triaxial compression of circular cylinders, axisymmetric stresses in hollow cylinders, and indentation by hemispheres in Indiana limestone and Berea sandstone have been preserved using Wood’s metal porosimetry. In this technique molten Wood’s metal at about 100°C is used as a pore fluid at a pressure of about 10 MPa, and the experiments are conducted using the concepts of effective stress. At the deformation state of interest, the temperature is lowered to solidify the metal, thereby preserving the microstructure as it exists under load and facilitating subsequent preparation of the specimen for microscopic study. Mode I microcrack growth is observed to occur by a variety of mechanisms such as bending, point loading and sliding cracks. The effects of this are analyzed using an elastic continuum within which Mode II displacement across microcracks and Mode I microcrack growth results from heterogeneous stress concentrations that produce local tensile stresses. While the continuum model replicates many of the observations, it fails to account for localization by en echelon arrays of extensile microcracks that precede macroscopic shear faulting. Using a “zero order” continuum approximation, the spatially stochastic distribution of grains in clastic rocks is shown to be important in the formation of the en echelon arrays of microcracks that form shear bands.
Effect of Loading Path and Porosity on the Failure Mode of Porous Rocks
Authors: Wong Teng-fong , Szeto Hiram , Zhang Jiaxiang
Published: April 2009

Grain crushing and pore collapse are the dominant compaction mechanisms in high porosity clastic rocks. These micromechanical processes control the evolution of strain hardening during cataclastic flow, and they can also result in embrittlement of the rock. The mechanics of the transition from brittle fracture to homogeneous cataclastic flow for the Berea and Kayenta sandstones were investigated in the laboratory. The mechanical data show that the transition is sensitively dependent on the stress state as well as the porosity. In the stress space, the complete locus for brittle failure by shear localization can be determined by tests on normally consolidated and overconsolidated samples along different loading paths. Using porosity as the hardening parameter, the evolution of the inelastic yield locus with strain hardening can be mapped out in the stress space. This yield locus expands with decreasing porosity. Scanning electron microscope and acoustic emission measurements were used to elucidate the micromechanics. The onset of grain crushing and pore collapse was marked by a surge in acoustic emission activity. A Hertzian fracture mechanics model was formulated to analyze the roles of porosity, grain size and fracture toughness in controlling the onset of hydrostatic and shear-enhanced compaction. Stereological measurements of the microcrack density show that significant stress-induced anisotropy was induced by shear-enhanced compaction, with preferred orientations of the stress-induced microcracks subparallel to the maximum compression direction.
A Micromechanics-Based Model for Creep Behavior of Rock
Authors: Yoshida H. , Horii H.
Published: April 2009

Recently, various ideas on underground development have been proposed and associated technical problems have been studied. One of the issues of concern is the prediction of the long-term behavior of rock, such as creep phenomena and fatigue. The mechanical behavior of rock is known to be greatly affected by temperature, confining pressure, pore fluid pressure and pH. It is necessary to establish a prediction method for creep deformation and creep failure of rock in order to ensure the long-term safety of the underground structures such as vaults for nuclear waste and power stations. Studies with the scanning electron microscope (SEM) revealed that the mechanisms of creep deformation and creep failure is the growth of microcrack nucleated at a pre-existing defect. Under compression below the failure strength, the microcrack gradually grows and the rock specimen fails after a certain time. The mechanism of time-dependent crack growth is understood as the stress corrosion at the crack tips. The objective of this study is to establish a prediction method of creep behavior. It is necessary to understand the governing mechanism of phenomena and to build a model for the reproduction of creep behavior. In the present study, an analytical model of microcrack growth under compression on the basis of micromechanics is proposed. The analytical results of the proposed model are compared with the experimental results. It appears that the experimental data are reproduced by the model. Moreover, a constitutive equation is derived from the proposed micromechanical model and is implemented into a finite element program to analyze the creep behavior of underground structures. As an example, a problem of elliptical excavation under hydrothermal conditions is analyzed and a crack length field is predicted as a function of time at different temperatures. It is concluded that the results of the finite element analysis indicate the possibility that rock may fail due to the effect of high temperature.
Effective Elastic Properties of Cracked Solids: Critical Review of Some Basic Concepts
Authors: Kachanov Mark
Published: April 2009

The problem of effective moduli of cracked solids is critically reviewed. Various approaches to the problem are discussed; they are further assessed by comparing their predictions to results for sample deterministic arrays. These computer experiments indicate that the approximation of non-interacting cracks has a wider than expected range of applicability. Some of the deficiencies of various approximate schemes seem to be related to inadequacy of the conventionally used crack density parameter (insensitive to mutual positions of cracks). An alternative parameter that has this sensitivity, is suggested. Finally, the problem of effective moduli is discussed in the context of “damage mechanics”. It is argued that, contrary to the spirit of many damage models, there is no direct quantitative correlation between progression of a microcracking solid towards fracture and deterioration of its stiffness; thus, the effective moduli may not always serve as a reliable indicator of damage.
Remarks on Crack-Bridging Concepts
Authors: Bao G. , Suo Z.
Published: April 2009

The article draws upon recent work by us and our colleagues on metal and ceramic matrix composites for high temperature engines. The central theme here is to deduce mechanical properties, such as toughness, strength and notch-ductility, from bridging laws that characterize inelastic processes associated with fracture. A particular set of normalization is introduced to present the design charts, segregating the roles played by the shape, and the scale, of a bridging law. A single material length, δ0E/σ0, emerges, where δ0 is the limiting-separation, σ0 the bridging-strength, and E the Young’s modulus of the solid. It is the huge variation of this length—from a few nanometers for atomic bond, to a meter for cross-over fibers—that underlies the richness in material behaviors. Under small-scale bridging conditions, δ0E/σ0 is the only basic length scale in the mechanics problem and represents, with a pre-factor about 0.4, the bridging zone size. A catalog of small-scale bridging solutions is compiled for idealized bridging laws. Large-scale bridging introduces a dimensionless group, a/(δ0E/σ0), where a is a length characterizing the component (e.g., hole radius). The group plays a major role in all phenomena associated with bridging, and provides a focus of discussion in this article. For example, it quantifies the bridging scale when a is the unbridged crack length, and notch-sensitivity when a is hole radius. The difference and the connection between Irwin’s fracture mechanics and crack bridging concepts are discussed. It is demonstrated that fracture toughness and resistance curve are meaningful only when small-scale bridging conditions prevail, and therefore of limited use in design with composites. Many other mechanical properties of composites, such as strength and notch-sensitivity, can be simulated by invoking large-scale bridging concepts.
Does the Process Zone Control Crack Growth?
Authors: Abe´ H. , Saka M. , Ohba S. , Hashida T.
Published: April 2009

Fracture toughness tests were carried out by using small compact tension specimens of Westerly granite. Relation between the load-line displacement and the initial crack-tip separation changed from linear to nonlinear and then to linear again with increasing loading. The onset of the main crack growth was found to occur in a loading level at which the relation was nonlinear. This result disclosed that the onset of main crack growth in granite occurred without experience of the complete area under the tension-softening curve. The critical value of the initial crack-tip separation for onset of the main crack growth was obtained independently of the initial notch length. Also monitoring of the growth of the fracture process zone by ultrasonic technique showed that its length related with the crack-tip separation independently of the initial notch length. By combining the results just mentioned, it was concluded that the fracture process zone controlled the onset of the main crack growth.
Effect of Fiber Extensibility on the Fracture Toughness of Short Fiber or Brittle Matrix Composites
Authors: Jain L. K. , Wetherhold R. C.
Published: April 2009

A micromechanical model based on probabilistic principles is proposed to determine the effective fracture toughness increment and the bridging stress-crack opening displacement relationship for brittle matrix composites reinforced with short, poorly bonded fibers. Emphasis is placed on studying the effect of fiber extensibility on the bridging stress and the bridging fracture energy, and to determine its importance in cementitious matrix composites. Since the fibers may not be in an ideal aligned or random state, the analysis is placed in sufficiently general terms to consider any prescribable fiber orientation distribution. The model incorporates the snubbing effect observed during pull-out of fibers inclined at an angle to the crack face normal. In addition, the model allows the fibers to break; any fiber whose load meets or exceeds a single-valued failure stress will fracture rather than pull out. The crack bridging results may be expressed as the sum of results for inextensible fibers and an additional term due to fiber extensibility. An exact analysis is given which gives the steady-state bridging toughness G directly, but presents a non-linear problem for the bridging stress-crack opening (σb – δ) relationship. An approximate analysis is then presented which gives both G and σb – δ directly. To illustrate the effect of extensibility on bridging stress and fracture energy increment due to bridging fibers, a comparison with the inextensible fiber case is provided. It is found that effect of extensibility on fracture energy is negligible for common materials systems. However, extensibility may have a significant effect on the bridging stress-crack opening relationship. The effect of other physical and material parameters such as fiber length, fiber orientation and snubbing friction coefficient is also studied.
Recent Developments in the Force Method of Structural Analysis
Authors: Kaveh A.
Published: April 2009

The development of the force method of structural analysis has been hindered by the difficulty of generating a suitable maximal set of independent self-equilibrating stress systems, known as statical or null basis. Recent developments to overcome this problem can be classified as topological, combinatorial, algebraic, mixed and integrated force methods. Combinatorial methods are efficient and provide insight to the problems involved, however, further research is needed before a unified formulation can be made. Algebraic methods are simple and general in nature, however, their automation requires considerable amount of operations and storage. The insight to the problems are not present in these methods. Mixed methods incorporate the efficiency of the existing combinatorial methods and generality of the algebraic approaches. In this paper, the progress made in the analysis of structures by means of the force method is reviewed, and directions required for further advances are specified. It is hoped that such advances will make the force method competitive to the displacement approach in fields such as non-linear analysis and optimization of structures. This will hopefully put the force method as an efficient counterpart to the displacement approach in computational mechanics.
Finite Element Analysis of Aeroelasticity of Plates and Shells
Authors: Bismarck-Nasr Maher N.
Published: April 2009

A review of the finite element method applied to the problem of supersonic aeroelastic stability of plates and shells is presented. The review is limited to linear models. Some new contributions in the field are presented and future trends are discussed.
Nondestructive Testing and Structural Condition Monitoring of Mechanical Cables
Authors: Laura Patricio A. A.
Published: April 2009

This article concerns the problem of evaluating the `structural health’ of cables or ropes by means of non-destructive testing methods. Special emphasis is placed upon electromagnetic techniques and the acoustic emission method.
Aging of Distribution and Other Lifeline Systems Due to Corrosion
Authors: Isenberg J.
Published: April 2009

Statistics derived from public works maintenance records for buried steel and cast iron pipelines indicate that aging may be seen in increasing rates of repairs. Maintenance increases with age due to cumulative traffic loads, ground settlement and, among the most important causes, corrosion. The tendency for repair rates gradually to increase and the opposing effects of corrosion control and planned replacement are punctuated by the rapid rise in leakage and required maintenance in the aftermath of an earthquake. These data were uncovered as a byproduct of studying five western US earthquakes in which performance of steel pipelines under seismic conditions and under normal operating conditions appear to be correlated. Evidence also points to temporary and, sometimes, to permanent increase in the rate of leakage and failure in the aftermath of an earthquake. The underlying cause of this correlation is thinning of pipe walls due to corrosion, which is facilitated by stray current and conductive soil.
Significance and Role of Deformation Micromechanisms in Life-Predictive Modeling of Aging Structures
Authors: Murty K. L.
Published: April 2009

Transitional creep mechanisms are exhibited by materials used in power systems and a thorough knowledge of these is a prerequisite in reliable extrapolations of short-term data to service conditions mainly due to the dominance of viscous creep mechanisms such as Nabarro-Herring, Coble and more importantly Harper-Dorn creep at low stresses. The paper summarizes the deformation mechanisms pertinent to various classes of materials. It is shown that blind extrapolation of the short-term laboratory test results to long-term service conditions leads to nonconservative predictions of the creep-rates and life times of the structural materials. Application of such transitional creep mechanisms to the prediction of dimensional changes of Zircaloy cladding in light water reactors is summarized.
Effect of Aging on the Seismic Performance of Petrochemical Facilities
Authors: Johnson Gayle S.
Published: April 2009

This paper discusses the effects of aging of petrochemical facilities with regard to the impact on seismic performance. Examples of typical construction and serious concerns are presented. Requirements in evaluation and retrofits are discussed. A comparison is made with the current state-of-practice in offshore platforms and nuclear power plants.
Life Extension of High Temperature Components
Authors: Dyson B. F. , Leckie F. A.
Published: April 2009

Power producing plant operating at high temperature are designed for finite life. This is necessary because the operating conditions are sufficiently high to cause continuous growth of material damage. Failure occurs when damage reaches a critical value. Design is based on stress levels with an appropriate factor of safety. Since life is dependent on a strongly non-linear function of stress the actual life can be many times greater than the design life. In these circumstances it is then natural to explore the possibility of extending the working life. To increase the working life it is necessary to decrease the factor of safety, but it may be possible to follow this course of action provided the growth of physical damage is carefully monitored. In this paper creep damage mechanisms are reported and growth laws are proposed.
Robust Inspection and Interpretation Techniques
Authors: Marriott Douglas L.
Published: April 2009

This paper explores the value of approximate methods of a specific type, as an efficient aid to performing residual life assessments, particularly when input data are incomplete. These methods, referred to as “robust” have the desirable attributes of providing satisfactory answers using less-than-perfect input, and being very economical in time of execution. Some examples of robust methods, which have been used successfully in high temperature design and fracture evaluations, are presented.
Some Methods for Assessing the Safety of Dams Which May Have Applicability to Energy Facilities
Authors: Schaefer M. G.
Published: April 2009

Two of the key tasks in providing for the maintenance and upgrade of aging industrial facilities are the establishment of criteria for use in evaluating the condition/safety of various elements of a facility, and the development of a prioritization scheme for accomplishing repairs and improvements. This paper briefly presents some methods seeing increased usage in the dam safety community for accomplishing these tasks. An overview is presented of the decision framework used by the Washington State dam safety program for selecting probabilistic design/performance goals for critical project elements. An overview is also given of schemes used by some Federal and State dam safety programs for setting priorities in allocating resources for repairs and improvements.
Current Progress Toward Natural Phenomena Hazards Mitigation for Aging DOE Structures, Systems, and Components
Authors: Murray Robert C.
Published: April 2009

This paper outlines a program under development to establish requirements, develop criteria and guidance for systematically evaluating the behavior of structures, systems, and components (SSC) in existing Department of Energy (DOE) facilities located across the United States when subjected to natural phenomena hazards (NPH). The requirements have been established in DOE Order 5480.28. Criteria are summarized in the implementing standards which accompany the order. An approach to implement the requirements for existing systems and components is currently being developed.
Mechanics of Materials in Lunar Base Design
Authors: Smith A.
Published: April 2009

The construction of a lunar base has been thought about for almost five decades. The earliest concepts dealt with size and shape concepts rather than actual design of structures and equipment. As the exploration of space has received emphasis in more recent times, the need to consider the factors of design and design approaches have received more attention. It still is not possible to completely rationalize the design of a habitat, for example, but we are now very aware of the need to include the fracture and fatigue properties of the materials of which the hardware of a lunar infrastructure is constructed. It is reasonably certain that the same techniques used for terrestrial designs can be used for lunar bases provided the space environment is taken into account. The extensive amount of research on fracture and fatigue that has been and is being conducted, along with the ease and speed of computation of their effects on alternative design features will be invaluable in providing a highly reliable, safe lunar base.
Tensile-Integrity Structures for the Moon
Authors: Benaroya Haym
Published: April 2009

A brief review of lunar base structural concepts is presented followed by a discussion of tensile structural concepts. Specifically, tensile integrity (tensegrity) structures are suggested for study for possible use on the lunar surface due to their efficiencies and advantages. Such application could conceivably be for second generation structures. These structures, like pneumatic and truss structures, are self-sustaining and can be designed not to require any significant foundation structure. One exciting advantage of the class of structure discussed here is the possibility of designing deployable versions.
Structural Concepts for Lunar-Based Astronomy
Authors: Chua Koon Meng , Johnson Stewart W. , Nein Max E.
Published: April 2009

Lunar-based astronomy requires structures that provide the required stiffness, strength, and dimensional stability in the challenging lunar vacuum, thermal, radiation, micrometeorite, secondary ejecta, and dust environments. Materials used in the structures must be durable to provide extended service life. They must not outgas and thereby interfere with optics. Structures must function as part of an integrated system involving detectors, metrology, and optics. The thermal environment of the lunar surface is particularly taxing because the long-duration exposures to extremes of temperature. Designers must cope with transient thermal gradients associated with passing from sunlight to darkness and back to sunlight. Also, the shape of mirrors and other components at night-time low temperatures may differ from shape at day-time high temperatures because of material properties varying significantly over the range from about 70 K to nearly 400 K. Materials for structural components include several types of glass, metals and their alloys (e.g., aluminum, beryllium, and titanium), graphite epoxy and the associated class of materials, and metal matrix composites (e.g., graphite magnesium and graphite aluminum). Examples of material properties are presented. Structural design concepts for several possible observatories are presented. Structural configurations will be partially determined by the need to reduce direct human involvement in the construction process. Concepts discussed include the 16-meter optical/ultraviolet telescope and a small telescope with 0.8-m aperture intended to be landed on the Moon by a robotic spacecraft. For lunar-based astronomy, structures must rest on the lunar regolith with loads transferred to the regolith through a foundation. The overall response of the structure is dependent on the interaction of the foundation and the regolith.
Homoclinics in the Reconstruction of Dynamic Systems From Experimental Data
Authors: Anishchenko V. S. , Safonova M. A.
Published: April 2009

The role of homoclinic effects in solution of a reconstruction problem of system attractors and model equations from experimental observable in the presence of external noise is investigated numerically. It is shown that the possibility of reconstruction essentially depends on character of origin system homoclinic trajectories and noise intensity. If the homoclinic structure belongs to the attractor, then the reconstruction results in restoration origin system attractors. A small noise influence causes in this case a small perturbation of attractors probability measure and practically disappears due to filtering properties of the reconstruction algorithm. The homoclinic structure does not belong to the attractor, then in the absence of noise the probability measure concentrates at the attractor, the structure of which is not defined by the homoclinics. The noise perturbation induces new regimes. Then the attractor structure essentially depends on the homoclinics structure and noise level. In this case the model system attractor of which reproduces “invisible” homoclinic structure, is obtained as a result of reconstruction.
Destruction of Quasiperiodic Oscillations in Weakly Nonlinear Systems
Authors: Belogortsev Andrey B. , Vavriv Dmitry M. , Tretyakov Oleg A.
Published: April 2009

We consider the main regularities of the arising of the dynamical chaos in weakly nonlinear oscillatory systems. We show that the chaotic oscillations in such systems can occur due to the destruction of quasiperiodic oscillations. Various analytical approaches are applied to study the properties of the quasiperiodically forced passive and active single-mode oscillators as well as the conditions for the appearance of chaos. The results of numerical and experimental investigations are also discussed.
Controlling Chaos in Mechanical Systems
Authors: Błaz˙ejczyk Barbara , Kapitaniak Tomasz , Wojewoda Jerzy , Brindley John
Published: April 2009

The problem of controlling chaos, that is to convert the chaotic behaviour to a periodic time dependence is discussed. We described a number of effective controlling methods in the context of mechanical systems.
Hopf Bifurcation in Gas Journal Bearings
Authors: Czołczyn´ski K.
Published: April 2009

This paper reviews a numerical investigation of the problem of small self-excited vibrations in gas journal bearings. The method of analysis is based on the Hopf bifurcation theory, in which the approximate periodic solutions of nonlinear equations of motion are computed using the Fredholm alternative. This theory enables us to construct the bifurcating periodic solutions and to determine their stability. The equations of motion of the investigated gas journal bearing have been formulated after estimating the damping and stiffness coefficients of a gas film. For this purpose, a new method of identification has been proposed.
Bifurcations and Chaos in Voice Signals
Authors: Herzel Hanspeter
Published: April 2009

The basic physical mechanisms of speech production is described. A rich variety of bifurcations and episodes of irregular behaviour are observed. Poincare´ sections and the analysis of the underlying attractor suggest that these noise-like episodes are low-dimensional deterministic chaos. Possible implications for the very early diagnosis of brain disorder are discussed.
Sectional Curvature and Chaos in Dynamical Problems: Toward the Invariant Measure of Chaos in Hamiltonian Systems
Authors: Szydłowski Marek , Krawiec Adam
Published: April 2009

Chaotic phenomena in general relativity are investigated. In relativistic astrophysical problems no space-time coordinate system is privileged in any way as far as the physical description of phenomena is concerned. Effects which depend on the choice of the particular coordinate system should be treated as an artifact of the incorrect methods. To avoid such difficulties the gauge invariant theory of chaos is proposed.
Dynamics of a Rolling Wheelset
Authors: True Hans
Published: April 2009

We discuss the kinematics and dynamics of a wheelset rolling on a railway track. The mathematical model of a suspended wheelset rolling with constant speed on a straight track is set up and its dynamics is investigated numerically. The results are presented mainly on bifurcation diagrams. Several kinds of dynamical behavior is identified within the investigated speed range. We find a stationary equilibrium point at low speeds and at higher speeds symmetric and asymmetric oscillations are found and ranges with chaotic motion are identified. The bifurcations are described.
Boundary Integral Equations in Aerodynamics
Authors: Morino Luigi
Published: April 2009

A review of the use of boundary integral equations in aerodynamics is presented, with the objective of addressing what has been accomplished and, even more, what remains to be done. The paper is limited to aerodynamics of aeronautical type, with emphasis on unsteady flows (incompressible and compressible, potential and viscous). For potential flows, both incompressible and compressible flows are considered; the issue of the boundary conditions on the wake and on the trailing edge are addressed in some detail (in particular, some unresolved issues related to the impulsive start are pointed out). For incompressible viscous flows, the use of boundary integral equations in the non-primitive variable formulation are addressed: the Helmholtz decomposition and a decomposition recently introduced (and here referred to as the Poincare´ decomposition) are presented, along with their relationship. The latter is used to examine the relationship between potential and attached viscous flows (in particular, it is shown how the Poincare´ representation, for vortex layers of infinitesimal thickness, reduces to the potential-flow representation). The extension to compressible flows is also briefly outlined and the relative advantages of the two decompositions are discussed. Throughout the paper the emphasis is on the derivation and the interpretation of the boundary integral equations; issues related to the discretization (ie, panel methods, boundary element methods) are barely addressed. For numerical results, which are not included here, the reader is referred to the original references.
Arterial Fluid Mechanics and Biological Response
Authors: Friedman M. H.
Published: April 2009

To better understand the response of the arterial wall to the adjacent blood flow, corresponding hemodynamic and histomorphometric data are obtained at multiple sites in human arteries. The hemodynamic data are obtained by perfusing realistically compliant flow-through casts of vascular segments with physiologically realistic pulsatile flows and measuring near-wall velocities by laser Doppler velocimetry. The hemodynamic and histologic data in combination suggest that the thickening response of the innermost layer of the vessel wall, which may precede atherosclerosis at the site, varies with time and wall shear: at early times, sites exposed to relatively high and unidirectional shears are thicker, while at later times, their thickness is exceeded by that at sites exposed to relatively low or oscillatory shear forces. A biologically plausible mathematical model of the thickening process supports the hypothesis that this behavior can be the consequence of multiple shear-dependent processes in the vessel wall.
Interfacial Transport in Large and Small Blood Vessels
Authors: Weinbaum S.
Published: April 2009

In this paper we shall review some recent mathematical models which have led to new conceptual views of the ultrastructural pathways by which water, solutes and large molecules cross the endothelial interface between tissue and blood. In particular, we shall show how a sequence of models for the endothelium and underlying tissue in large arteries have finally led to the experimental discovery of the large pore via which LDL and other large molecules enter the artery wall and how a new three-dimensional model for the interendothelial cleft in capillaries might reconcile the several long standing paradoxes relating to the measured filtration and solute permeability coefficients in the transcapillary exchange of water and hydrophilic solutes in the microcirculation.
Triphasic Theory for Swelling Properties of Hydrated Charged Soft Biological Tissues
Authors: Mow V. C. , Lai W. M. , Hou J. S.
Published: April 2009

Swelling phenomenon of biological soft tissues, such as articular cartilage, depends on their fixed charge densities, the stiffness of their collagen-proteoglycan solid matrix and the ion concentration in the interstitium. Based on the thermodynamic continuum mixture theory, a multiphasic mixture model is developed to describe the equilibrium and transient swelling properties. For articular cartilages in a single salt environment (e.g. NaCl), a three phase model (triphasic theory) suffices to describe its swelling behavior. The three phases are: solid matrix, interstitial water and the mobile salt. The equations of motion in this theory shows that the driving forces for interstitial water and salt are the gradients of their chemical potentials. Constitutive equations for the chemical potentials of the phases and for the total stress under infinitesimal strain but large variation of salt concentration are presented based on the physico-chemical theory for polyelectrolytic solutions and continuum theory. Application of this theory to equilibrium problems yields the well known Donnan equilibrium ion distribution and osmotic pressure equations. The theory indicates that at equilibrium the applied load on the tissue is shared by 1) the solid matrix elastic stress due to deformation; 2) the Donnan osmotic pressure; and 3) the chemical expansion stress due to the charge-to-charge repulsive forces between the charged groups in the solid matrix. For the transient isometric swelling problem, the theory is shown to describe the experimentally observed responses very well.
Mechanics of the Anterior Cruciate Ligament and Its Contribution to Knee Kinematics
Authors: Woo S. L.-Y. , Wayne J. S.
Published: April 2009

The anterior cruciate ligament (ACL), as one of the major stabilizers of knee motion, is anatomically complex. Its inability to heal when torn often requires surgical reconstruction using biological tissues or artificial ligament replacements to achieve knee stability and appropriate kinematics for young and active patients. Yet, there is much debate as to which graft and reconstruction technique best emulate the natural ligament. To increase our knowledge of the function of the ACL in the knee joint, it is crucial to understand the tensile properties of the ligament as well as the its role in maintaining knee kinematics. This information should aid in the selection, design, and evaluation of ligament replacements and reconstruction techniques.
Boundary Layer Receptivity Theory
Authors: Kerschen E. J.
Published: April 2009

The receptivity mechanisms by which free-stream disturbances generate instability waves in laminar boundary layers are discussed. Free-stream disturbances have wavelengths which are generally much longer than those of instability waves. Hence, the transfer of energy from the free-stream disturbance to the instability wave requires a wavelength conversion mechanism. Recent analyses using asymptotic methods have shown that the wavelength conversion takes place in regions of the boundary layer where the mean flow adjusts on a short streamwise length scale. This paper reviews recent progress in the theoretical understanding of these phenomena.
Boundary Layer Receptivity to Sound: Navier-Stokes Computations
Authors: Reed H. L. , Lin N. , Saric W. S.
Published: April 2009

Here we discuss the computational modeling of the receptivity of the laminar boundary layer on a semi-infinite flat plate with an elliptic leading edge. The incompressible flow is computed in a spatial simulation using the full Navier-Stokes equations in general curvilinear coordinates. Finite differences are used in both space directions and in time. First, the steady basic state is obtained in a transient approach using spatially varying time steps. Then, small-amplitude acoustic disturbances of the freestream velocity are applied as unsteady boundary conditions, and the governing equations are solved time-accurately to evaluate the spatial and temporal developments of instability waves (Tollmien-Schlichting waves) in the boundary layer. A sharper leading edge is found to be less receptive to freestream disturbances.
Dynamic Phase Transitions in a One-Dimensional Elastic Continuum
Authors: Abeyarartne Rohan
Published: April 2009

This paper summarizes the results of some recent investigations on the continuum mechanical modeling of thermo-elastic phase transitions in solids carried out jointly with J. K. Knowles. Our most recent efforts have been focussed on the role of a kinetic law and an initiation criterion in modeling dynamic phase transitions, and their relationship to theories that include viscosity and strain-gradient effects.
Possibilities of Incorporating Coherent Structure Models in Turbulent Shear Flow Calculations
Authors: Liu J. T. C.
Published: April 2009

We discuss possibilities of using coherent structure models in turbulent shear flow description. The nonuniversality of different classes of such flows is directly attributed to the nonuniversality of hydrodynamic instability mechanisms. This is fully explored in discussions of free shear flows, where dynamical instabilities are important and in wall-bounded flows where longitudinal vorticity system and its nonlinear consequence are at play.