Nonconservative Components Research Articles

The model of grain-boundary sliding stimulated by intragranular slip

Abstract The theoretical model of grain-boundary sliding (GBS) stimulated by intragranular dislocation slip has been suggested on the basic of experiments on Zn and Cd bicrystals. Lattice dislocations of two grains dissociate in boundary during the spreading time t s and form two families of mobile gain-boundary dislocations (GBDs) with the opposite sign. The motion of GBDs containing a non-conservative component is the main mechanism of GBS. The GBS rate is determined by the GBD density and by the rate of their annihilation or sink to the surface. The increase in GBD density forms additional elastic fields in the vicinity of boundary, which inhibits the entering of lattice dislocations into the boundary and weaken the source efficiency. The GBD density balance determined by the ratio of influx of lattice dislocations and annihilation of the GBDs is established. Thus the boundary demonstrates the behaviour of system with negative feedback. The model allows one to describe the GBS-time dependence in Cd and Zn bicrystals in the whole volume and to extend the understanding of the interaction of the above-mentioned mechanisms on deformation of the polycrystal.

Robotic stiffness control and calibration as applied to human grasping tasks

In this paper, we study stiffness analysis as applied to human grasping. Grasp stiffness has been demonstrated to be useful for modeling and controlling robotic manipulators. The computation of general linear R/sup 3/spl times/3/ stiffness matrices for grasping, which can be decomposed into symmetric (conservative) and asymmetric (nonconservative) components, offers physical insights for stiffness control in robotics as well as human grasping. Methods of stiffness calibration, using least-squares best fits with and without symmetry constraints, are presented and applied to the force and displacement data obtained from grasping tasks to study human grasping behaviors. The results of this study show that a linear relationship between force and displacement is capable of capturing the characteristics of the experimental data of human grasps for which displacements are small (on the order of one to seven mm). Different measures, proposed and developed in the robotics literature, are employed to predict the behavior of human grasps in reacting to externally applied loads.

Local electrical dissipation imaged by scanning force microscopy

The low internal damping of micromachined monolithic silicon levers with integrated probe tip has been exploited to detect nonconservative components in the interaction forces between the probe tip and sample. This is accomplished by monitoring the mechanical Q while scanning the surface. The gap width was controlled by keeping the lever’s resonance frequency detuning, caused by the gradient of the force between tip and surface, fixed to a preset value. Nonconservative components are present even in Coulomb attraction since, whenever a voltage is applied between tip and substrate, currents are induced by the lever’s oscillation leading to Joule dissipation of energy at a rate that depends on the local conductivity. Strong damping contrast was observed in layered GaAs/AlGaAs semiconductor heterostructures. It depended on the type of material, dopant concentration, illumination, and the applied voltage. Damping variations were resolved over distances of less than 20 nm.

Nonconservative Components of Follower Forces in the Classical Shell Theory

An analysis of follower forces acting on shell structures is presented. Attention is focussed on the expressions of such forces as functions of the generalized displacements. Specific expressions for the follower forces are obtained, according to the order of magnitude of the strains and angles of rotation. For small strains the follower forces allow a decomposition into conservative and non-conservative components. This leads to the equations of dynamic stability of shell problems subjected to follower loads. The dynamic counterparts of Donnell-Mushtari-Vlasov stability equations are presented, by either retaining or omitting the prebuckling rotations.

Neural, mechanical, and geometric factors subserving arm posture in humans

When the hand is displaced from an equilibrium posture by an external disturbance, a force is generated to restore the original position. We developed a new experimental method to measure and represent the field of elastic forces associated with posture of the hand in the horizontal plane. While subjects maintained a given posture, small displacements of the hand along different directions were delivered by torque motors. The hand was held in the displaced positions and, at that time, we measured the corresponding restoring forces before the onset of any voluntary reaction. The stiffness in the vicinity of the hand equilibrium position was estimated by analyzing the force and displacement vectors. We chose to represent the stiffness both numerically, as a matrix, and graphically, as an ellipse characterized by three parameters: magnitude (the area), shape (the ratio of axis) and orientation (direction of the major axis). The latter representation captures the main geometrical features of the elastic force field associated with posture. We also evaluated the conservative and nonconservative components of this elastic force field. We found that the former were much larger than the latter and concluded that the behavior of the neuromuscular system of the multiarticular arm is predominantly spring-like. Our data indicated that the shape and orientation of the stiffness were invariant over subjects and over time. We also investigated the ability of our subjects to produce voluntary and adaptive changes in the stiffness. Our findings indicated that, when a disturbance acting along a fixed and predictable direction was imposed, the magnitude of the stiffness was increased but only minor changes in shape and orientation occurred. Taken together, all of these experiments represent a step toward the understanding of the interactions between geometrical and neural factors involved in maintaining hand posture and its interactions with the environment.

Strain behavior of Fe?Ni?C martensite

This article examines the effect of carbon on the structure of slip traces and characteristics of the deformation resistance of quenched iron-nickel-carbon alloys deformed at 77°K. It is established that an increase in the carbon concentration in the martensite within the range 0.03–0.30 wt. % is accompanied by the formation of “channels” of easy slip during low-temperature deformation, which explains the simultaneously observed increase in the plastic strain of the alloys. Localization of slip in these channels leads to a reduction in the nonconservative component of resistance to deformation.

Variation in Chemical Composition of Oil Field Brines with Depth in Northern Louisiana and Southern Arkansas: Implications for Mechanisms and Rates of Mass Transport and Diagenetic Reaction: ABSTRACT

ABSTRACT Dickey (1966, 1968) has pointed out that there is a nearly linear increase in total dissolved solids (TDS) content with depth in oil-field waters in areas of northern Louisiana and southern Arkansas. Similar relations exist in portions of east-central Louisiana and central Mississippi. A re-examination of brine analyses in the region shows that these linear increases in TDS are primarily the result of linear increases in Na and Cl concentrations with depth. Other dissolved components, such as Mg, Ca, Sr, Ba, HCO3, SO4, and trace constituents can show more complex variations in concentration with depth. It is proposed here that the linear gradients for Na and Cl reflect the on-going, steady-state mass transport of these constituents upward from thousands of feet of depth to the near surface. Na and Cl concentrations at depths exceeding 8,000 to 10,000 ft (2438 to 3048 m) are maintained at constant high levels by the subsurface dissolution of halite. Low dissolved salt concentrations are maintained at the near surface by recharge of meteoric waters. The types of mass transport processes which could produce such nearly linear profiles are limited to molecular diffusion, thermal diffusion (Soret effect), and eddy diffusion. Preliminary estimates of fluxes and mass transport velocities which could result from these processes suggest sodium and chloride ions could be migrating upward at velocities approaching a centimeter per year. Non-linear but systematic variations in the concentrations of other components with depth may reflect the presence of local stratigraphic sources and sinks of material. Zones of depletion of dissolved calcium, for example, may represent areas of active precipitation of calcite. If the mass transport coefficients for these non-conservative components can be estimated, then limits can be put on the absolute rates of on-going diagenetic processes. Of considerable interest is the possibility that dissolved or entrained hydrocarbons are also involved in active vertical migration. The systematic study of brine chemistry promises to provide important clues to the understanding of processes and rates of fluid migration and hydrocarbon migration in the region.

On the Stability of Elastic Systems Subjected to Nonconservative Forces

Free motions of a linear elastic, nondissipative, two-degree-of-freedom system, subjected to a static nonconservative loading, are analyzed with the aim of studying the connection between the two instability mechanisms (termed divergence and flutter by analogy to aeroelastic phenomena) known to be possible for such systems. An independent parameter is introduced to reflect the ratio of the conservative and nonconservative components of the loading. Depending on the value of this parameter, instability is found to occur for compressive loadings by divergence (static buckling), flutter, or by both (at different loads) with multiple stable and unstable ranges of the load. In the latter case either type of instability may be the first to occur with increasing load. For a range of the parameter, divergence (only) is found to occur for tensile loads. Regardless of the non-conservativeness of the system, the critical loads for divergence can always be determined by the (static) Euler method. The critical loads for flutter (occurring only in nonconservative systems) can be determined, of course, by the kinetic method alone.