Jeffrey L. Streator

Meeting the Contact-Mechanics Challenge

This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one.

Dynamic Contact of a Rigid Sphere With an Elastic Half-Space: A Numerical Simulation

A numerical simulation is performed to investigate the development of forces between a rigid sphere and an elastic half-space during normal, dynamic contact in the absence of friction. Of interest is to quantify the magnitude of forces that arise and to identify any sources of hysteresis between approach and separation, the latter being associated with energy dissipation. In the simulation a rigid sphere approaches and separates from an isotropic, linearly elastic half-space at a prescribed, constant speed. Surface forces are incorporated in the model by ascribing a surface interaction potential derived from the Lennard-Jones 6-12 intermolecular potential. Dynamical equations of motion for the interface are integrated numerically during the approach-separation event. During the approach phase, it is found that the magnitude of adhesive force is generally consistent with well-known static-equilibrium based analytical models (e.g., DMT and JKR), depending upon the strength of the interaction potential. However, during separation, the attractive force computed in this dynamic simulation may be several times higher than the predictions of the analytical models. Additionally, the maximum compressive forces attained during the contact process far exceed the predictions of Hertzian contact theory. The discrepancy between results of this simulation and those of the static-equilibrium analytical and numerical models indicate that dynamic interactions play a significant role in determining the development of contact forces. Moreover, dynamic effects persist even when the approach-separation speed of the sphere is small compared to the dilatation and shear wave speeds of the half-space.

A Generalized Formulation for the Contact Between Elastic Spheres: Applicability to Both Wet and Dry Conditions

The interaction between two elastic spheres with an intervening liquid film of given volume is studied theoretically. Using an energy minimization approach, equilibrium contact configurations are determined through numerical computation. Several dimensionless groups are identified that govern the character of the solution. Curve fits are performed to reveal analytical relationships among the dimensionless groups. At extreme values of particular parameters, the curve fits are found to recover the analytical results of the well-known Hertzian and Johnson-Kendall-Roberts elastic (dry contact) models, as well as the force of a liquid bridge between rigid spheres. Qualitative agreement is found between the current model and some published experiments.

Velocity-Dependent Adhesion with Lubricants on Thin-Film Disks

The well-known problem of stiction in a magnetic disk drive largely depends on the forces induced by the presence of a thin liquid film. It is commonly recognized that both adhesive and viscous effects contribute to the magnitude of the stiction force, but is is not known what relative roles the two effects have in a lubricated contact. In the present work, the nature of adhesive and viscous effects is investigated for the slider/disk interface under conditions of constant-speed sliding. Friction measurements are conducted over a range of sliding speeds, 0.25-250 mm/s, with eight perfluoropolyether (PFPE) lubricants applied in various thicknesses, 0-6.6 nm, to carbon-coated magnetic thin-film disks. The lubricants were selected to cover a broad range of viscosities. For several sliding speeds and lubricant film thicknesses, the friction force is found to decrease significantly with increasing sliding speed for all lubricants. In several instances, large friction forces are observed at the lowest sliding s...

A Computational Leakage Model for Solid Oxide Fuel Cell Compressive Seals

One of the key obstacles precluding the maturation and commercialization of planar solid oxide fuel cells has been the absence of a robust sealant. A computational model has been developed in conjunction with leakage experiments at Oak Ridge National Gaboratory. The aforementioned model consists of three components: a macroscopic model, a microscopic model, and a mixed lubrication model. The macroscopic model is a finite element representation of a preloaded metal-metal seal interface, which is used to ascertain macroscopic stresses and deformations. The microscale contact mechanics model accounts for the role of surface roughness in determining the mean interfacial gap at the sealing interface. In particular, a new multiscale fast Fourier transform-based model is used to determine the gap. An averaged Reynolds equation derived from mixed lubrication theory is then applied to approximate the leakage flow across the rough annular interface. The composite model is applied as a predictive tool for assessing how certain physical parameters (i.e., seal material composition, compressive applied stress, surface finish, and elastic thermophysical properties) affect seal leakage rates. The leakage results predicted by the aforementioned computational leakage model are then compared with experimental results.

Voltage Saturation in Electrical Contacts

Electrical contact resistance is important to the performance of electrical switches and other current-carrying interfaces. This study investigates the behavior of electrical contact resistance for an aluminum sphere-on-flat contact as s function of current through the interface. It is observed that the contact resistance may either increase or decrease with increasing current, depending on the current level as well as the current history. At low current levels the voltage drop across the interface increases initially with increasing current until it saturates, after which the voltage level remains constant. If the current is increased beyond the value corresponding to saturation, a subsequent decrease in current yields a corresponding decrease in voltage, so that the associated current cycle shows substantial hysteresis. However, subsequent cycles of current are reversible so long as the voltage remains below the saturation point. Such behavior suggests that irreversible morphological changes occur at the interface when the current exceeds the level associated with the attainment of voltage saturation.Copyright

Effect of Current Polarity on Tribological Behavior of Copper-Aluminum Electrical Interface

Wear and friction in sliding electrical contacts is affected by a multitude of factors such as contact load, current density, sliding speed, sliding distance and materials in contact [1], [2]. Very limited studies have been done to investigate the effect of current polarity on wear and coefficient of friction. In the present study, the authors investigate the effect of current polarity on the frictional response and wear of copper-aluminum interface. Copper wire mounted on a fixture is slid on aluminum flat under different current levels. Large scale melting at the interface was observed when slider (copper) was maintained as anode. Observations of the slider and flat surface made under microscope reveal material transfer of aluminum on copper even at low current levels of 60 Amperes. The results of the study can be used in the light of applying different coatings or surface design for anode and cathode in order to minimize their wear or degradation.Copyright

A Numerical Simulation of Interfacial Slip and its Role in Friction

The transition from static friction to kinetic friction results from the attainment of a point of instability, whereby interfacial slip becomes more energetically favorable than sticking. Such an instability is explored in this work via a plane-strain elastostatic analysis. A rigid pin of prescribed geometry is placed in contact with an elastic slab and translated horizontally under conditions of constant load. An intrinsic static coefficient of friction is prescribed, which limits the ratio of shear stress to contact pressure at each location within the interface. Additionally, the surface of the elastic slab is given a desired undulation to simulate the effects of surface roughness. As the pin is translated horizontally, a lateral reaction force (i.e., friction force) is developed and is observed to grow nearly linearly with increasing lateral displacement. At a critical point, a substantial portion of the interface experiences slip, leading to a large decrease in the friction force and thereby revealing a stick-slip behavior. It is found that the overall (macroscopic) static friction coefficient can be significantly less than the intrinsic friction coefficient and that the presence of even a small amount of roughness can have a large effect on the friction force.Copyright

Investigation of a Spectral-Based Surface Contact Model via Pointwise Computations

One of the outstanding issues in the analysis of contacting surfaces regards properly accounting for the multi-scale nature of the surface topography. Many treatments of surface contact, particularly those that derive from the well known Greenwood Williamson (GW) model, proceed from the basis that one can determine a representative curvature of the surface peaks (or summits) by means of computing the appropriate numerical derivative of the sampled profile data. However, as has been demonstrated over the last 20 years or so, numerical derivatives of surface data are sensitive to the lateral spacing between sampled surface points. In view of the potential ambiguity associated with such a determination of peak curvature, a number of models have been put forth that account for the fact that topographical features exist at many scales of observation. In the present work, predictions of a recently developed spectral-based model of elastic (rough) surface contact are compared to those of a deterministic numerical computation in which the equations of elasticity are solved at every nodal point of a 3D surface.Copyright

Simulation of Thermal Effects in Stationary and Sliding Electrical Contacts

Sliding electrical contacts are subject to surface damage and wear, which can be enhanced by the heating at the interface arising from electrical contact resistance. For example, in electromagnetic launcher (EML) technology, thermally assisted wear processes can result in unacceptable levels of material loss at the armature-rail interface. The control of the interface tribology in sliding electrical contacts requires an understanding of the Joule heating in the vicinity of the interface. In the current study, a multiphysics numerical simulation is conducted of transient heat conduction in both a stationary and a sliding electrical contact. The interface under investigation consists of a flat-ended aluminum cylindrical pin sliding against an aluminum rail. Electrical contact resistance is modeled by introducing a thin layer of high resistivity between the pin and the rail. Results show that shortly after sliding has commenced, (1) the maximum temperature rise occurs in the bulk of the pin rather than at the interface, (2) the bulk of the Joule heat goes into the rail, and (3) that sliding can have a significant effect on the temperature field, even when the speed is quite low.Copyright