bluuebunny/crossref_metadata_2025 huggingface dataset:
Site‐directed spin labeling of a bacterial chemoreceptor reveals a dynamic, loosely packed transmembrane domain
Authors: Barnakov Alexander , Altenbach Christian , Barnakova Ludmila , Hubbell Wayne L. , Hazelbauer Gerald L.
Published: July 2002

We used site‐directed spin labeling and electron paramagnetic resonance spectroscopy to investigate dynamics and helical packing in the four‐helix transmembrane domain of the homodimeric bacterial chemoreceptor Trg. We focused on the first transmembrane helix, TM1, particularly on the nine‐residue sequence nearest the periplasm, because patterns of disulfide formation between introduced cysteines had identified that segment as the region of closest approach among neighboring transmembrane helices. Along this sequence, mobility and accessibility of the introduced spin label were characteristic of loosely packed or solvent‐exposed side chains. This was also the case for eight additional positions around the circumference and along the length of TM1. For the continuous nine‐residue sequence near the periplasm, mobility and accessibility varied only modestly as a function of position. We conclude that side chains of TM1 that face the interior of the four‐helix domain interact with neighboring helices but dynamic movement results in loose packing. Compared to transmembrane segments of other membrane proteins reconstituted into lipid bilayers and characterized by site‐directed spin labeling, TM1 of chemoreceptor Trg is the most dynamic and loosely packed. A dynamic, loosely packed chemoreceptor domain can account for many experimental observations about the transmembrane domains of chemoreceptors.
Solubilization and disaggregation of polyglutamine peptides
Authors: Chen Songming , Wetzel Ronald
Published: July 2002

A method is described for dissolving and disaggregating chemically synthesized polyglutamine peptides. Polyglutamine peptides longer than about Q20 have been reported to be insoluble in water, but dissolution in – and evaporation from ‐ a mixture of trifluoroacetic acid and hexafluoroisopropanol converts polyglutamine peptides up to at least Q44 to a form readily soluble in aqueous buffers. This procedure also has a dramatic effect on peptides which appear to be completely soluble in water, by removing traces of aggregate that seed aggregation. The protocol makes possible solution studies—including in vitro aggregation experiments—on polyglutamine peptides with repeat lengths associated with increased risk of Huntington's Disease and other expanded CAG repeat diseases. It may also be useful in conducting reproducible, quantitative aggregation studies on other polypeptides.
Effect of the N2 residue on the stability of the α‐helix for all 20 amino acids
Authors: Cochran Duncan A.E. , Doig Andrew J.
Published: July 2002

N2 is the second position in the α‐helix. All 20 amino acids were placed in the N2 position of a synthetic helical peptide (CH3CO‐[AXAAAAKAAAAKAAGY]‐NH2) and the helix content was measured by circular dichroism spectroscopy at 273K. The dependence of peptide helicity on N2 residue identity has been used to determine a free‐energy scale by analysis with a modified Lifson‐Roig helix‐coil theory that includes a parameter for the N2 energy (n2). The rank order of ΔΔG(relative to Ala) is Glu−, Asp− > Ala > Glu0, Leu, Val, Gln, Thr, Ile, Ser, Met, Asp0, His0, Arg, Cys, Lys, Phe > Asn, > Gly, His+, Pro, Tyr. The results correlate very well with N2 propensities in proteins, moderately well with N1 and helix interior preferences, and not at all with N‐cap preferences. The strongest energetic effects result from interactions with the helix dipole, which favors negative charges at the helix N terminus. Hydrogen bonds to side chains at N2, such as Gln, Ser, and Thr, are weak, despite occurring frequently in protein crystal structures, in contrast to the N‐cap position. This is because N‐cap hydrogen bonds are close to linear, whereas N2 hydrogen bonds have poor geometry. These results can be used to modify protein stability rationally, help design helices, and improve prediction of helix location and stability.
A partially folded intermediate species of the β‐sheet protein apo‐pseudoazurin is trapped during proline‐limited folding
Authors: Reader John S. , Van Nuland Nico A. J. , Thompson Gary S. , Ferguson Stuart J. , Dobson Christopher M. , Radford Sheena E.
Published: July 2002

The folding of apo‐pseudoazurin, a 123‐residue, predominantly β‐sheet protein with a complex Greek key topology, has been investigated using several biophysical techniques. Kinetic analysis of refolding using far‐ and near‐ultraviolet circular dichroism (UV CD) shows that the protein folds slowly to the native state with rate constants of 0.04 and 0.03 min−1, respectively, at pH 7.0 and at 15°C. This process has an activation enthalpy of ∼90 kJ/mole and is catalyzed by cyclophilin A, indicating that folding is limited by trans‐cis proline isomerization, presumably around the Xaa‐Pro 20 bond that is in the cis isomer in the native state. Before proline isomerization, an intermediate accumulates during folding. This species has a substantial signal in the far‐UV CD, a nonnative signal in the near‐UV CD, exposed hydrophobic surfaces (judged by 1‐anilino naphthalenesulphonate binding), a noncooperative denaturation transition, and a dynamic structure (revealed by line broadening on the nuclear magnetic resonance time scale). We compare the properties of this intermediate with partially folded states of other proteins and discuss its role in folding of this complex Greek key protein.
Human muscle sympathetic neural and haemodynamic responses to tilt following spaceflight
Authors: Levine Benjamin D. , Pawelczyk James A. , Ertl Andrew C. , Cox James F. , Zuckerman Julie H. , Diedrich André , Biaggioni Italo , Ray Chester A. , Smith Michael L. , Iwase Satoshi , Saito Mitsuru , Sugiyama Yoshiki , Mano Tadaaki , Zhang Rong , Iwasaki Kenichi , Lane Lynda D. , Buckey Jay C. , Cooke William H. , Baisch Friedhelm J. , Robertson David , Eckberg Dwain L. , Blomqvist C. Gunnar
Published: July 2002

Orthostatic intolerance is common when astronauts return to Earth: after brief spaceflight, up to two‐thirds are unable to remain standing for 10 min. Previous research suggests that susceptible individuals are unable to increase their systemic vascular resistance and plasma noradrenaline concentrations above pre‐flight upright levels. In this study, we tested the hypothesis that adaptation to the microgravity of space impairs sympathetic neural responses to upright posture on Earth. We studied six astronauts ∼72 and 23 days before and on landing day after the 16 day Neurolab space shuttle mission. We measured heart rate, arterial pressure and cardiac output, and calculated stroke volume and total peripheral resistance, during supine rest and 10 min of 60 deg upright tilt. Muscle sympathetic nerve activity was recorded in five subjects, as a direct measure of sympathetic nervous system responses. As in previous studies, mean (±s.e.m.) stroke volume was lower (46 ± 5 vs. 76 ± 3 ml, P= 0.017) and heart rate was higher (93 ± 1 vs. 74 ± 4 beats min−1, P= 0.002) during tilt after spaceflight than before spaceflight. Total peripheral resistance during tilt post flight was higher in some, but not all astronauts (1674 ± 256 vs. 1372 ± 62 dynes s cm−5, P= 0.32). No crew member exhibited orthostatic hypotension or presyncopal symptoms during the 10 min of postflight tilting. Muscle sympathetic nerve activity was higher post flight in all subjects, in supine (27 ± 4 vs. 17 ± 2 bursts min−1, P= 0.04) and tilted (46 ± 4 vs. 38 ± 3 bursts min−1, P= 0.01) positions. A strong (r2= 0.91–1.00) linear correlation between left ventricular stroke volume and muscle sympathetic nerve activity suggested that sympathetic responses were appropriate for the haemodynamic challenge of upright tilt and were unaffected by spaceflight. We conclude that after 16 days of spaceflight, muscle sympathetic nerve responses to upright tilt are normal.
Voltage and concentration dependence of Ca2+ permeability in recombinant glutamate receptor subtypes
Authors: Jatzke Claudia , Junryo Watanabe  , Wollmuth Lonnie P.
Published: July 2002

The channels associated with glutamate receptor (GluR) subtypes, namely N‐methyl‐d‐aspartate receptors (NMDARs), and Ca2+‐permeable α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionate receptors (AMPARs) and kainate receptors (KARs), are to varying degrees permeable to Ca2+. To compare the mechanism of Ca2+ influx, we measured Ca2+ permeability relative to that of Na+ (PCa/PNa) using fractional Ca2+ currents (Pf) and reversal potential measurements over a wide voltage and Ca2+ concentration range in recombinant NMDAR NR1‐NR2A, AMPAR GluR‐A(Q) and KAR GluR‐6(Q) channels. For NR1‐NR2A channels, PCa/PNa derived from Pf measurements was voltage independent but showed a weak concentration dependence. A stronger concentration dependence was found when PCa/PNa was derived from changes in reversal potentials on going from a Na+ reference solution to a solution with Ca2+ as the only permeant ion (‘biionic’ condition). In contrast, PCa/PNa was concentration independent when derived from changes in reversal potentials on going from a Na+ reference solution to the same solution with added Ca2+ (‘high monovalent’ condition). For GluR‐A(Q) channels, PCa/PNa derived from all three approaches was concentration independent, and for the reversal potential‐based approaches were of comparable magnitude. Their most distinctive property was that PCa/PNa derived from Pf measurements was strongly voltage dependent. For GluR‐6(Q) channels, PCa/PNa derived from Pf measurements was weakly voltage dependent. On the other hand, PCa/PNa derived from all three approaches was the most strongly concentration dependent of any GluR subtype and, except for low Ca2+ concentrations, the values were of comparable magnitude. Thus, the three Ca2+‐permeable GluR subtypes showed unique patterns of Ca2+ permeability, indicating that distinct biophysical and molecular events underlie Ca2+ influx in each subtype.
Direct and indirect modulation of rat cardiac sarcoplasmic reticulum function by n–3 polyunsaturated fatty acids
Authors: O'Neill S. C. , Perez M. R. , Hammond K. E. , Sheader E. A. , Negretti N.
Published: July 2002

Measurements were made of trans‐sarcolemmal Ca2+ fluxes and intracellular [Ca2+]i in rat ventricular myocytes loaded with Indo‐1 to determine how the n‐3 polyunsaturated fatty acid eicosapentaenoic acid (EPA) suppresses spontaneous waves of Ca2+ release. We report that in 10 μm EPA, the Ca2+ efflux generated by individual waves increased by 11.3 ± 4.9 % over control levels. However, wave‐generated efflux per unit time fell overall by 19 ± 5.3 %. On removal of EPA, wave frequency increased transiently such that Ca2+ efflux was greater than normal and the cell lost 28.0 ± 10.6 μmol l−1 Ca2+. This probably represents the loss of extra Ca2+ accumulated by the sarcoplasmic reticulum (SR), while Ca2+ release was inhibited. These results are evidence of inhibition of the SR Ca2+‐release mechanism and reduced availability of Ca2+ to the SR From the relationship between average intracellular Ca2+ and the frequency of spontaneous waves, we have calculated the relative contributions of these different mechanisms to the lower frequency of waves. In EPA, the frequency of spontaneous waves fell by 37.5 ± 8.1 %, the majority of this (29.2 ± 8.8 %) is due to inhibition of the Ca2+‐release mechanism. In EPA, the rate of fall of Ca2+ in the caffeine response (an indicator of surface membrane Ca2+ efflux pathway activity) was not altered. We conclude, therefore, that the lower resting level of Ca2+ observed in EPA is due to a lower influx of Ca2+ across the surface membrane rather than increased activation of efflux pathways. How these effects might contribute to the anti‐arrhythmic actions of EPA is discussed.
Tachycardia evoked by disinhibition of the dorsomedial hypothalamus in rats is mediated through medullary raphe
Authors: Samuels B. C. , Zaretsky D. V. , DiMicco J. A.
Published: July 2002

Activation of neurons in the region of the dorsomedial hypothalamus (DMH) appears to generate the sympathetically mediated tachycardia seen in experimental stress in rats. The purpose of this study was to assess the role of neurons in the area of the medullary raphe pallidus (RP) in the tachycardia caused by stimulation of the DMH. The cardiovascular response to microinjection of the GABAA receptor antagonist bicuculline methiodide (BMI) 10 pmol (100 nl)−1 into the DMH was assessed before, and after, injection of the GABAA receptor agonist muscimol 80 pmol (100 nl)−1 or saline vehicle 100 nl into the RP in urethane‐anaesthetized rats. Tachycardia evoked by microinjection of BMI into the DMH was mimicked by microinjection of BMI 30 pmol (75 nl)−1 into the RP. This DMH‐induced tachycardia was markedly suppressed after injection of muscimol into the RP, but the response was unaffected by injection of saline into the same region. Thus, DMH‐induced tachycardia is mediated through activation of neurons in the area of the RP, suggesting that these neurons may play a previously unrecognized role in stress‐induced cardiac stimulation.
Modeling of Conduction Mode Laser Welding Process For Feedback Control
Authors: Tsai Fuu-Ren , Kannatey-Asibu, Elijah
Published: July 2002

The response of conduction mode laser weld pool dimensions, specifically weld width, to a step change in power input has been modeled using two-dimensional heat flow analysis. The goal is to develop a simplified model suitable for feedback control. The weld pool geometry was approximated by a tear-drop shape. The workpiece thermal properties were assumed to be lumped and temperature-independent. The result was a first-order weld pool thermal model. A series of experiments was performed using different welding conditions (plate thickness, step power changes, and welding speeds) to validate the model. The weld pool image was recorded using a vision system and digitized. The process time constant as calculated by the model was of the order of 10−4 seconds. The response of the laser machine, estimated by the least squares method, was found to be about 10−2 seconds, which is much slower than that of the weld pool. Thus, within the constraints of the assumptions on which the model is based, the entire laser welding process is considered to be dominated by the laser machine dynamics. [S1087-1357(00)00502-5]
Effect of Relative Impeller-to-Volute Position on Hydraulic Efficiency and Static Radial Force Distribution in a Circular Volute Centrifugal Pump
Authors: Baun Daniel O. , Ko¨stner Lutz , Flack Ronald D.
Published: July 2002

The hydraulic performance and radial hydraulic force characteristics of a circular volute centrifugal pump are strongly affected by the impeller to volute relative position. For a typical design configuration the geometric center of the impeller will be coincident with the volute geometric center. However, assembling a circular volute pump with the impeller center eccentric from the volute center can radically alter both the hydraulic performance and the radial hydraulic force characteristics. In particular, at the design flow coefficient an optimum impeller to volute relative position exists where the efficiency is maximized and the resultant radial force is minimized. At the optimal relative position a 5 percent and a 3.5 percent increase in the efficiency was realized compared to the centered positions for the circular and spiral volutes, respectively. In addition the nondimensional resultant radial force at the design flow coefficient was reduced from 0.045 at the centered position to 0.005 at the optimal position for the circular casing. This value of radial thrust is similar in magnitude to the radial thrust for the spiral volute operating at the design flow coefficient. By assembling a circular volute pump with the appropriate relative impeller to volute position the design simplicity of a circular volute can be utilized without compromising pump hydraulic performance or radial force characteristics as compared to a typical spiral volute. [S0098-2202(00)02303-8]
Mechanical Properties of Flat Braided Composites With a Circular Hole
Authors: Ohki Takeru , Ikegaki Shinya , Kurasiki Ken , Hamada Hiroyuki , Iwamoto Masaharu
Published: July 2002

In this study, fracture behavior and strength in the flat braided bar with a circular hole were investigated by static and fatigue test. Two types of specimen were prepared. They are a braided flat bar with an integrally formed braided hole and a braided flat bar with a machined hole. Moreover, we also examined a specimen that had a metal pin inserted at the circular hole. This specimen was subjected to a static tensile test. The results of the tensile tests indicate that the strength of the flat bar with a braided hole was larger than that of the one with the machined hole. Furthermore, from the results of the fatigue tests, the flat bar with the braided hole showed higher fatigue property than that of the one with the machined hole. [S0094-4289(00)02604-9]
Wavenumber-Harmonic Analysis of a Periodically Supported Beam Under the Action of a Convected Loading
Authors: Cheng Chih-Chun , Kuo Chun-Pao , Yang Jer-Weir
Published: July 2002

The vibro-acoustic response of an infinite, fluid-loaded and periodically supported Timoshenko beam under a convected harmonic loading has been obtained in a mathematical form. Instead of considering the dynamics of a single substructure alone, an essentially different technique named as wavenumber-harmonic method for the analysis of periodic structures is presented. The method involves the use of Fourier transform, then the associated wavenumber response is expressed in a series form that represents wave components of the flexural motion. This approach differs from the space-harmonic analysis which describes the beam motion in spatial domain. With this approach the periodic boundary conditions and the phase relation between two substructures are not required. Furthermore, incorporating the heavy fluid loading effect is easy. This method can be used conveniently to calculate the sound power radiated from a fluid-loaded infinite beam, which is subject to either a moving point or distributed loading. The influence of periodic supports on the sound radiation has also been examined for different speeds of convected loading. [S0739-3717(00)01002-3]
Heat Transfer and Pressure Distributions on a Gas Turbine Blade Tip
Authors: Azad Gm. S. , Han Je-Chin , Teng Shuye , Boyle Robert J.
Published: July 2002

Heat transfer coefficient and static pressure distributions are experimentally investigated on a gas turbine blade tip in a five-bladed stationary linear cascade. The blade is a two-dimensional model of a first-stage gas turbine rotor blade with a blade tip profile of a GE-E3 aircraft gas turbine engine rotor blade. The flow condition in the test cascade corresponds to an overall pressure ratio of 1.32 and exit Reynolds number based on axial chord of 1.1×106. The middle 3-blade has a variable tip gap clearance. All measurements are made at three different tip gap clearances of about 1, 1.5, and 2.5 percent of the blade span. Heat transfer measurements are also made at two different turbulence intensity levels of 6.1 and 9.7 percent at the cascade inlet. Static pressure measurements are made in the midspan and the near-tip regions as well as on the shroud surface, opposite the blade tip surface. Detailed heat transfer coefficient distributions on the plane tip surface are measured using a transient liquid crystal technique. Results show various regions of high and low heat transfer coefficient on the tip surface. Tip clearance has a significant influence on local tip heat transfer coefficient distribution. Heat transfer coefficient also increases about 15–20 percent along the leakage flow path at higher turbulence intensity level of 9.7 over 6.1 percent. [S0889-504X(00)00404-9]
Optical Observation of the Supercavitation Induced by High-Speed Water Entry
Authors: Shi Hong-Hui , Itoh Motoyuki , Takami Takuya
Published: July 2002

When a high-speed projectile penetrates into water, a cavity is formed behind the projectile. The gas enclosed in the cavity experiences a nonequilibrium process, i.e., the gas pressure decreases as the projectile moves more deeply into water. As a result, the cavity is sealed near the free surface (surface closure) and subsequently the cavity breaks up in water (deep closure). Accompanying the break-up of the cavity, secondary shock waves appear. This is the so-called supercavitation in water entry. This paper describes an experimental investigation into the water entry phenomenon. Projectiles of 342 m/s were generated from a small-bore rifle that was fixed vertically in the experimental facility. The projectiles were fired into a windowed water tank. A shadowgraph optical observation was performed to observe the entry process of the projectile and the formation and collapse of the cavity behind the projectile. A number of interesting observations relating to the motion of the free surface, the splash, the underwater bubbly flow and so on were found. [S0098-2202(00)00204-2]
Secondary Flow Measurements in a Turbine Passage With Endwall Flow Modification
Authors: Aunapu Nicole V. , Volino Ralph J. , Flack Karen A. , Stoddard Ryan M.
Published: July 2002

A flow modification technique is introduced in an attempt to allow increased turbine inlet temperatures. A large-scale two half-blade cascade simulator is used to model the secondary flow between two adjacent turbine blades. Various flow visualization techniques and measurements are used to verify that the test section replicates the flow of an actual turbine engine. Two techniques are employed to modify the endwall secondary flow, specifically the path of the passage vortex. Six endwall jets are installed at a location downstream of the saddle point near the leading edge of the pressure side blade. These wall jets are found to be ineffective in diverting the path of the passage vortex. The second technique utilizes a row of 12 endwall jets whose positions along the centerline of the passage are based on results from an optimized boundary layer fence. The row of jets successfully diverts the path of the passage vortex and decreases its effect on the suction side blade. This can be expected to increase the effectiveness of film cooling in that area. The row of jets increases the aerodynamic losses in the passage, however. Secondary flow measurements are presented showing the development of the endwall flow, both with and without modification. [S0889-504X(00)01004-7]
Wave Group Statistics of Numerically Simulated Mixed Sea States
Authors: Rodriguez German R. , Guedes Soares C. , Ferrer Luis
Published: July 2002

Run length and total run length probabilities are examined for mixed sea states, resulting from the combination of a remotely generated wave field and a locally generated wave system, based on numerically simulated wave records. Two peaked sea states are classified in nine representative groups, in terms of two dimensionless parameters. Results are compared with those corresponding to a mean JONSWAP target spectrum and those predicted by the Kimura’s model. [S0892-7219(00)00404-0]
A Poisson-Based Formulation for Frictional Impact Analysis of Multibody Mechanical Systems With Open or Closed Kinematic Chains
Authors: Lankarani Hamid M.
Published: July 2002

Analysis of frictional impact in a multibody mechanical system requires a friction model capable of correct detection of all possible impact modes such as sliding, sticking, and reverse sliding. Conventional methods for frictional impact analysis have either shown energy gain or not developed for jointed mechanical system, and especially not for closed-chain multibody systems. This paper presents a general formulation for the analysis of impact problems with friction in both open- and closed-loop multibody mechanical systems. Poisson’s hypothesis is used for the definition of the coefficient of restitution, and thus the energy gains inherent with the use of Newton’s hypothesis are avoided. A canonical form of the system equations of motion using Cartesian coordinates and Cartesian momenta is utilized. The canonical momentum-balance equations are formulated and solved for the change in the system Cartesian momenta using an extension of Routh’s graphical method for the normal and tangential impulses. The velocity jumps are calculated by balancing the accumulated system momenta during the contact period. The formulation is shown to recognize all modes of impact; i.e., sliding, sticking, and reverse sliding. The impact problems are classified into seven types, and based on the pre-impact system configuration and velocities, expressions for the normal and tangential impulses are derived for each impact type. Examples including the tip of a double pendulum impacting the ground with some experimental verification, and the impact of the rear wheel and suspension system of an automobile executing a very stiff bump are analyzed with the developed formulation. [S1050-0472(00)02304-7]
Discharge Coefficients of Critical Venturi Nozzles for CO2 and SF6
Authors: Nakao Shin-ichi , Takamoto Masaki
Published: July 2002

The discharge coefficients of critical Venturi nozzles were measured for CO2 and SF6 on the Reynolds number range from 3×103 to 2×105. The results showed that the measured discharge coefficients for both gases were about 2 percent larger than the theoretical estimation based on the assumption of isentropic flow of a perfect gas and this large deviation could not be reduced even by introducing real gas effects. The experimental results also showed that the large deviation for CO2 could be explained through the assumption of a nonequilibrium flow at the throat. On the other hand, the reason of the deviation observed for SF6 has not been clear yet, but one possible explanation would be the inadequate estimation of the boundary layer at the throat because the theory is based on the laminar boundary layer of a perfect gas. [S0098-2202(00)02004-6]
Suppression of Rotating Stall by Wall Roughness Control in Vaneless Diffusers of Centrifugal Blowers
Authors: Ishida Masahiro , Sakaguchi Daisaku , Ueki Hironobu
Published: July 2002

By positioning the completely rough wall locally on the hub side diffuser wall alone in the vaneless diffuser, the flow rate of rotating stall inception was decreased by 42 percent at a small pressure drop of less than 1 percent. This is based on the fact that the local reverse flow occurs first in the hub side in most centrifugal blowers with a backswept blade impeller. The three-dimensional boundary layer calculation shows that the increase in wall shear component normal to the main-flow direction markedly decreases the skewed angle of the three-dimensional boundary layer, and results in suppression of the three-dimensional separation. It is also clarified theoretically that the diffuser pressure recovery is hardly deteriorated by the rough wall positioned downstream of R = l.2 because the increase in the radial momentum change, resulting from reduction in the skewed angle of the three-dimensional boundary layer, supports the adverse pressure gradient.
Nanomechanical and Nanotribological Properties of Carbon, Chromium, and Titanium Carbide Ultrathin Films
Authors: Lu W. , Komvopoulos K.
Published: July 2002

The nanomechanical and nanotribological properties of 10-nm-thick amorphous carbon (a-C) films and 100-nm-thick polycrystalline chromium (Cr) and titanium carbide (TiC) films were investigated using a surface force microscope (SFM). The films were deposited on Si(100) substrates by radio frequency (RF) sputtering and pulsed laser deposition (PLD) techniques. The experiments were performed with diamond tips of nominal radius of curvature equal to 20 nm, 100 nm, and 20 μm, and contact forces in the range of 3–400 μN. Nanoindentation experiments performed with the 20-nm-radius pyramidal diamond tip revealed that, for a 20 μN maximum contact force, the deformation of the a-C films was purely elastic, whereas that of the Cr film and Si(100) substrate was predominantly plastic. Although the RF sputtered a-C films and the PLD films of TiC exhibited similar nanohardness (∼40 GPa), the a-C films showed a superior nanowear resistance. Despite the identical hardness-to-elastic modulus ratio values of the RF sputtered polycrystalline Cr films and the single-crystal Si(100) substrate, the Cr films demonstrated a greater nanowear resistance. The wear behavior of the films is interpreted in terms of the relative specific energy dissipated during the nanowear process. Results from nanowear tests show that, in addition to the nanohardness and hardness-to-elastic modulus ratio, the microstructure, type of atomic bonding, and deposition process affecting the composition and residual stress in the films influence the nanowear resistance of the films.
Analysis of EHL Circular Contact Start Up: Part I—Mixed Contact Model With Pressure and Film Thickness Results
Authors: Zhao Jiaxin , Sadeghi Farshid , Hoeprich Michael H.
Published: July 2002

In this paper a model is presented to investigate the start up condition in elastohydrodynamic lubrication. During start up the lubrication condition falls into the mixed lubrication regime. The transition from solid contact to lubricated contact is of importance when investigating the start up process and its effects on bearing performance. The model presented uses the multigrid multilevel method to solve the lubricated region of the contact and a minimization of complementary energy approach to solve the solid contact region. The FFT method is incorporated to speed up the film thickness calculation. An iteration scheme between the lubrication and the solid contact problems is used to achieve the solution of the mixed lubrication contact problem. The results of start up with smooth surfaces are provided for the case when speed increases from zero to desired speed in one step and the case when speed is linearly increased to desired speed. The details of the transition from full solid contact to full lubricated contact in EHL start up are presented. The change of pressure and film thickness as well as contact forces and contact areas are discussed.
The Optimal Size of a Resonant Vibrating Beam Gyroscope
Authors: Katz Amnon , Highsmith Alton
Published: July 2002

The level of thermal noise in a resonant vibrating beam gyroscope is lower, the longer the beam. A limitation on the length of the beam arises from the consideration that the beam acts as a mechanical bandpass filter transmitting the signal being measured (the instantaneous rate of rotation) from the drive axis to the sense axis. A longer beam vibrates at a lower frequency and has a narrower resonance. The length that is acceptable depends on the bandwidth of the time variation of angular rate, which in turn, depends on the nature and mission of the vehicle that uses the gyroscope for inertial navigation. High rates of maneuvering indicate millimeter scale gyroscopes that are realized as micro-electro-mechanical devices. In the case of General Aviation, considerations of noise, as well as cost, favor a centimeter scale device.
A Complementary Sensor Approach to Reverse Engineering
Authors: Bradley C. , Chan V.
Published: July 2002

A complementary sensor technique for reverse engineering objects that are represented by a three-dimensional (3D) cloud data set is reported. The research focuses on objects whose surface form is manifest as a set of distinct free-form surface patches, each of which is enclosed by a boundary. The method incorporates three stages: (1) laser scanner-based digitization of all the free-form surface patches, (2) touch probe-based digitization of the surface patch boundaries, and (3) modeling of both data sets to create a complete B-spline curve and surface representation of the object. The patch boundary data, defined by the touch probe, is employed to segment the free-form surface data into the constituent patches. Furthermore, the boundary data is incorporated within a B-spline surface fitting process to constrain the boundaries. The two sensors functionally complement each other; the range sensor provides the required dense resolution of 3D points on the free-form surfaces whereas the touch probe accurately defines the patch boundaries. The method is ideal for objects comprised of both functional engineering features, e.g. bearing holes or precise mounting locators, and aesthetic features, such as hand grips or part covers. The touch probe is also ideal for digitizing boundaries where occlusion prevents the use of an optical digitizer. The laser-based sensor has an accuracy specification of 50 microns (over a 40-mm depth of field) whereas the touch probe is accurate to 4 microns over a 25-mm measurement length. An example part is modeled that has multiple free-form patches (defining the part’s outer cover) that require a large cloud data set for complete coverage. The corresponding patch boundaries accurately define the location of critical part mounting locations that require the touch probe’s precision.
Three-Dimensional Tube Geometry Control for Rotary Draw Tube Bending, Part 2: Statistical Tube Tolerance Analysis and Adaptive Bend Correction
Authors: Lou Huazhou , Stelson Kim A.
Published: July 2002

The control of each individual bend and overall process is presented in Part 1 of the paper. In Part 2 of the paper, statistical methods are used to analyze and improve 3-D tube bending accuracy. The relationship between bending process error and tube geometry error is obtained with Monte Carlo simulation. For the same tube tolerance requirement, the required process tolerance varies in a large range based on tube geometry. Among the three bending errors: bend angle, bend plane and distance between bends, bend angle error has the largest influence on tube error. For a tube with multiple bends, the overall tube geometry error can be minimized by intentionally modifying the nominal values of the bends to be made based on the errors in the existing bends. The required modification of the bending commands is calculated with an adaptive bend correction algorithm.
Biphasic Poroviscoelastic Simulation of the Unconfined Compression of Articular Cartilage: I—Simultaneous Prediction of Reaction Force and Lateral Displacement
Authors: DiSilvestro Mark R. , Zhu Qiliang , Wong Marcy , Jurvelin Jukka S. , Suh Jun-Kyo Francis
Published: July 2002

This study investigated the ability of the linear biphasic poroelastic (BPE) model and the linear biphasic poroviscoelastic (BPVE) model to simultaneously predict the reaction force and lateral displacement exhibited by articular cartilage during stress relaxation in unconfined compression. Both models consider articular cartilage as a binary mixture of a porous incompressible solid phase and an incompressible inviscid fluid phase. The BPE model assumes the solid phase is elastic, while the BPVE model assumes the solid phase is viscoelastic. In addition, the efficacy of two additional models was also examined, i.e., the transversely isotropic BPE (TIBPE) model, which considers transverse isotropy of the solid matrix within the framework of the linear BPE model assumptions, and a linear viscoelastic solid (LVE) model, which assumes that the viscoelastic behavior of articular cartilage is solely governed by the intrinsic viscoelastic nature of the solid matrix, independent of the interstitial fluid flow. It was found that the BPE model was able to accurately account for the lateral displacement, but unable to fit the short-term reaction force data of all specimens tested. The TIBPE model was able to account for either the lateral displacement or the reaction force, but not both simultaneously. The LVE model was able to account for the complete reaction force, but unable to fit the lateral displacement measured experimentally. The BPVE model was able to completely account for both lateral displacement and reaction force for all specimens tested. These results suggest that both the fluid flow-dependent and fluid flow-independent viscoelastic mechanisms are essential for a complete simulation of the viscoelastic phenomena of articular cartilage.
The Impact of Surface Errors on the Location and Orientation of a Cylindrical Workpiece in a Fixture
Authors: Sangnui Supapan , Peters Frank
Published: July 2002

Surface error at the contact region is one factor that contributes to the displacement of the workpiece in a fixture. This work employs the Newton-Raphson technique to develop a mathematical model that predicts the impact of surface errors on the location and orientation of a cylindrical workpiece. Model validation and measurement analysis were conducted to verify the proposed mathematical model. An experimental fixture and workpiece were developed to simulate the effect of surface errors on the placement of a workpiece. The study showed that the numerical model is reliable in predicting the behavior of the workpiece.
Random Flow Generation Technique for Large Eddy Simulations and Particle-Dynamics Modeling
Authors: Smirnov A. , Shi S. , Celik I.
Published: July 2002

A random flow generation (RFG) technique is presented, which can be used for initial/inlet boundary generation in LES (Large-Eddy-Simulations) or particle tracking in LES/RANS (Reynolds-Averaged Navier-Stokes) computations of turbulent flows. The technique is based on previous methods of synthesizing divergence-free vector fields from a sample of Fourier harmonics and allows to generate non-homogeneous anisotropic flow field representing turbulent velocity fluctuations. It was validated on the cases of boundary layer and flat plate flows. Applications of the technique to LES and particle tracking are considered.
A Simple Model of Bipedal Walking Predicts the Preferred Speed–Step Length Relationship
Authors: Kuo Arthur D.
Published: July 2002

We used a simple model of passive dynamic walking, with the addition of active powering on level ground, to study the preferred relationship between speed and step length in humans. We tested several hypothetical metabolic costs, with one component proportional to the mechanical work associated with pushing off with the stance leg at toe-off, and another component associated with several possible costs of forcing oscillations of the swing leg. For this second component, a cost based on the amount of force needed to oscillate the leg divided by the time duration of that force predicts the preferred speed–step length relationship much better than other costs, such as the amount of mechanical work done in swinging the leg. The cost of force/time models the need to recruit fast muscle fibers for large forces at short durations. The actual mechanical work performed by muscles on the swing leg appears to be of relatively less importance, although it appears to be minimized by the use of short bursts of muscle activity in near-isometric conditions. The combined minimization of toe-off mechanical work and force divided by time predicts the preferred speed–step length relationship.
An Elastodynamic Model of Frictional Contact and Its Influence on the Dynamics of a Workpiece-Fixture System
Authors: Fang B. , DeVor R. E. , Kapoor S. G.
Published: July 2002

A model to predict the dynamics of a workpiece-fixture system has been developed based on the equivalency of the dynamic frictional contact problem with a nonlinear friction law to that of a variation inequality problem. Nonlinear finite element method has been employed to solve the problem. A series of experiments under various fixture configurations were conducted on a modular fixture test-bed to determine the dynamic characteristics of the workpiece-fixture system. Model predictions of the dynamic characteristics have been compared to those obtained from the experiments and are found to be in good agreement. It was found that frictional sliding contact at the workpiece-fixture interfaces provides damping to the system and that the nature of this contact and level of damping are highly dependent on contact forces developed through clamping. Clamping sequence was also found to be important in determining dynamic characteristics.
Theory of Torsional Chatter in Twist Drills: Model, Stability Analysis and Composition to Test
Authors: Bayly Philip V. , Metzler Sandra A. , Schaut Adam J. , Young Keith A.
Published: July 2002

The mechanism of torsional chatter in drilling differs qualitatively and quantitatively from other types of chatter. In this paper we show that torsional chatter can be explained by the torsional-axial coupling inherent in a twisted beam; the beam “untwists” and extends in response to an increase in cutting torque. Based on a model of this mechanism, predictions of stability boundaries and chatter frequencies are derived by frequency domain analysis, and confirmed by numerical simulation and experimental tests. The effect of the torsional-axial coupling is opposite that of traditional cutting in that an increase in cutting forces leads to axial extension and greater chip load. Because of this sign difference, the limiting depth of cut is governed by the positive real part of the frequency response function, which explains the unexpected fact that torsional chatter occurs below the natural frequency of the tool.
Size Effects in a Slowly Sheared Granular Media
Authors: Antony S. J. , Ghadiri M.
Published: July 2002

In this paper, we analyze the nature of stress distribution experienced by large particles in a dense granular media subjected to slow shearing, using the distinct element method. The particles were generated in a three-dimensional cuboidal periodic cell in which a large solid spherical particle was submerged (“submerged particle”) at the center of a bed of monodispersed spherical particles. The granular systems with different size ratio (i.e., the ratio of the diameter of submerged particle to that of the surrounding monodispersed particles) were subjected to quasi-static shearing under constant mean stress condition. The evolution of stress distribution in the submerged particle during shearing was carefully tracked down and presented here. The nature of stress distribution is bifurcated into two components, viz., (i) hydrostatic and (ii) deviatoric components. It has been shown that, for size ratio greater than c.a. 10, the nature of stress distribution in the submerged particle is hydrostatically dominant (increases the ‘fluidity’). For smaller size ratios, the nature of stress distribution in the submerged particle is dominantly deviatoric.
Bifurcation Analysis of Self-Acting Gas Journal Bearings
Authors: Wang Cheng-Chi , Chen Cha’o-Ku`ang
Published: July 2002

This paper studies the bifurcation of a rigid rotor supported by a gas film bearing. A time-dependent mathematical model for gas journal bearings is presented. The finite differences method and the Successive Over Relation (S.O.R) method are employed to solve the Reynolds’ equation. The system state trajectory, Poincare´ maps, power spectra, and bifurcation diagrams are used to analyze the dynamic behavior of the rotor center in the horizontal and vertical directions under different operating conditions. The analysis shows how the existence of a complex dynamic behavior comprising periodic and subharmonic response of the rotor center. This paper shows how the dynamic behavior of this type of system varies with changes in rotor mass and rotational velocity. The results of this study contribute to a further understanding of the nonlinear dynamics of gas film rotor-bearing systems.
Dynamic Modeling and Vibration Analysis of the Atomic Force Microscope
Authors: Fung Rong-Fong , Huang Shih-Chien
Published: July 2002

The objective of this paper is to formulate the equations of motion and to investigate the vibrations of the atomic force microscope (AFM), which is divided into the contact and noncontact types. First, the governing equations of the AFM including both base oscillator and piezoelectric actuator are obtained using Hamilton’s principle. In the dynamic analysis, the piezoelectric layer is treated as a sensor to measure the deflection and as an actuator to excite the AFM via an external voltage. The repulsive force and van der Waals (vdW) force are considered in the contact and noncontact types of the AFM, respectively. Some important observations are made from the governing equations and boundary conditions. Finally, numerical results using a finite element method are provided to illustrate the excitation effects of base oscillator and piezoelectric actuator on the dynamic responses.
The Role of Flow-Independent Viscoelasticity in the Biphasic Tensile and Compressive Responses of Articular Cartilage
Authors: Huang Chun-Yuh , Mow Van C. , Ateshian Gerard A.
Published: July 2002

A long-standing challenge in the biomechanics of connective tissues (e.g., articular cartilage, ligament, tendon) has been the reported disparities between their tensile and compressive properties. In general, the intrinsic tensile properties of the solid matrices of these tissues are dictated by the collagen content and microstructural architecture, and the intrinsic compressive properties are dictated by their proteoglycan content and molecular organization as well as water content. These distinct materials give rise to a pronounced and experimentally well-documented nonlinear tension–compression stress–strain responses, as well as biphasic or intrinsic extracellular matrix viscoelastic responses. While many constitutive models of articular cartilage have captured one or more of these experimental responses, no single constitutive law has successfully described the uniaxial tensile and compressive responses of cartilage within the same framework. The objective of this study was to combine two previously proposed extensions of the biphasic theory of Mow et al. [1980, ASME J. Biomech. Eng., 102, pp. 73–84] to incorporate tension–compression nonlinearity as well as intrinsic viscoelasticity of the solid matrix of cartilage. The biphasic-conewise linear elastic model proposed by Soltz and Ateshian [2000, ASME J. Biomech. Eng., 122, pp. 576–586] and based on the bimodular stress-strain constitutive law introduced by Curnier et al. [1995, J. Elasticity, 37, pp. 1–38], as well as the biphasic poroviscoelastic model of Mak [1986, ASME J. Biomech. Eng., 108, pp. 123–130], which employs the quasi-linear viscoelastic model of Fung [1981, Biomechanics: Mechanical Properties of Living Tissues, Springer-Verlag, New York], were combined in a single model to analyze the response of cartilage to standard testing configurations. Results were compared to experimental data from the literature and it was found that a simultaneous prediction of compression and tension experiments of articular cartilage, under stress-relaxation and dynamic loading, can be achieved when properly taking into account both flow-dependent and flow-independent viscoelasticity effects, as well as tension–compression nonlinearity.
Wrinkling Limit in Tube Bending
Authors: Wang Xi , Cao Jian
Published: July 2002

Thin-walled tube bending has found many of its applications in the automobile and aerospace industries. This paper presents an energy approach to provide the minimum bending radius, which does not yield wrinkling in the bending process, as a function of tube and tooling geometry and material properties. A doubly-curved sheet model is established following the deformation theory. This approach provides a predictive tool in designing/optimizing the tooling parameters in tube bending.