Three-dimensional molecular dynamics (MD) simulation is used to study the atomic-scale indentation process of a spherical diamond tip in contact with a copper substrate. In the indentation simulations, the force-displacement curve is obtained and compared with a modified elastic solution of Hertz. The contact area under different indentation depths is also investigated. The force-displacement curve under different maximum indentation depths is obtained to investigate elastic-plastic deformation during the loading and unloading processes.

When solving the problems involving inhomogeneous materials, the influence of the inhomogeneity upon contact behavior should be properly considered. This research proposes a fast and novel method, based on the equivalent inclusion method where inhomogeneity is replaced by an inclusion with properly chosen eigenstrains, to simulate contact partial slip of the interface involving inhomogeneous materials. The total stress and displacement fields represent the superposition of homogeneous solutions and perturbed solutions due to the chosen eigenstrains. In the present numerical simulation, the half space is meshed into a number of cuboids of the same size, where each cuboid is has a uniform eigenstrain. The stress and displacement fields due to eigenstrains are formulated by employing the recent half-space inclusion solutions derived by the authors and solved using a three-dimensional fast Fourier transform algorithm. The partial slip contact between an elastic ball and an elastic half space containing a cuboidal inhomogeneity was investigated.

In this work, by utilizing the shear strength criterion for the sliding inception, a finite element model for obliquely loaded spherical contact has been developed, which realized a friction transition from perfect slip case to full stick case with increasing normal approach. Both tangential force and normal force during tangential loading were investigated using different models. It was found that with elastic-plastic normal displacement preload, there is an obvious normal force release during tangential loading. Furthermore, both Coulomb friction model and the proposed model predict a lower tangential force at the same tangential displacement compared to the full stick model. However, the Coulomb friction is more empirically determined with some arbitrary friction coefficient whereas the proposed model is based on physics parameters.

This paper presents a finite element method approach for modeling a thermal-electrical-mechanical coupled-field contact comprised of an elastic hemisphere and an elastic half space. The goal of this model is to develop a fundamental understanding of the behavior of a multi-physics contact with a particular interest in the contact area. The results from the model illustrate a clear difference in contact behavior between force control and displacement control in the presence of an applied electrical potential. It is shown that, while Hertzian contact theory can be used to accurately predict the behavior of the contact under force control, a new relationship must be established to accurately predict the behavior of the contact under displacement control. This relationship can be applied to RF MEMS switch contact bumps as well as macro scale electrical contacts.

Over the past decade there has been a significant increase the demand of hard disk drives (HDD) for the mobile platforms. In such applications, a mobile HDD is often subjected to harsh working environment. For example, components like DVD drive and built-in speakers present in a laptop can excite its HDD and hence can degrade its read/write (R/W) performance. In this study, we develope a complete model for a mobile HDD to analyze and characterize its vibration performance due to external disturbances. The current model includes a spinning disk, a fluid dynamic bearing (FDB) based spindle motor, a base plate, an actuator assembly (E-block and suspension) and a detailed model for the air bearing slider. Prediction of slider dynamics during an excitation essentially requires the solution of coupled fluid-structure problem in which head gimbal assembly (HGA) is coupled with the disk-support system through the air bearing. Off-track displacement of the slider is used to quantify the vibration performance of the HDD for excitation of different frequencies and orientations. It is observed that during an excitation, the sway modes of the actuator assembly provide the most significant contribution to the off-track vibration. This knowledge about the vibration characteristics of mobile drive can be used to design better a vibration isolation system for mounting a HDD in a laptop.

Great flexibility in prosthetic sockets, with long range of spring like action is needed while designing a socket. A finite element analysis is conducted to investigate the stress and displacement distribution for different socket cases where relief is needed. The relieved socket after optimization of the pressure distribution areas showed less wear and more comfort for amputees than the non-relieved socket. A Design of Experiment using 2 level factorial design is used to optimize the relief areas with respect to pressure distribution. Stresses and displacements were included in the model desirability. Higher deformation of the pressure relief areas is sought while preserving the socket structural integrity. A compromise with the structural integrity of the socket and the optimized values with high desirability of the Design of Experiment Model imply that Duraform socket A with 1.69mm relief area could be the best design scenario between several cases. Wear and skin analysis indicated that the relieved socket is significantly better than the unrelieved one.

This paper presents an investigation of the fluid properties that affect starting efficiency in hydraulic motors. Starting efficiency is important because it often determines the minimum displacement of motors in mobile hydraulic systems. The boundary friction, thermophysical and pressure-viscosity coefficients of prototype fluids have been characterized. These hydraulic fluids have been evaluated in geroler, axial piston, and radial piston motors under constant torque starting conditions in accordance with the ISO 4392-2 standard test method. Correlations between starting efficiency and fluid boundary friction, thermophysical and pressure-viscosity coefficients have been identified. The relative impact of these fluid properties varied with motor design.

Nonlinear behavior of force and displacements in rolling contacts with the presence of surface defects are studied. Model-based fault assessments in rolling element bearings and gears require detailed modeling and dynamics of faults. A detailed model of rolling element bearings with direct correspondence between parameters of the model and physical components is developed. The model incorporates dynamics of faults, nonlinear contacts, slips and surface separations. Mechanics of contacts with inner race faults (IRF), ball faults (BF), and outer race faults (ORF) are studied using the developed model. Contacts force, displacement and impulse signals are studied for different size and types of surface defects. It is shown that impulse signals contain useful information about the severity of surface defects in rolling element bearing. Results provide model-based diagnostics a deep knowledge of rolling contact mechanics with surface defects to be used for fault assessments.

The mechanical behavior of eukaryotic cell components is critical to cell mobility, division, signaling, and proliferation. Cell components are in such harmony with each other that often mechanical testing of individual components produces less meaningful results than testing performed within living cells. Moreover, cell components are constantly undergoing growth and remodeling, normally through polymerization and depolymerization. An appropriate experimental and modeling framework is therefore essential for cell mechanics. In this study, a special atomic force microscope (AFM) with controlled adhesion area of the cell to a substrate was used to probe the mechanical response of cells to an external force. This experimental set up enables direct control of the applied force and precise measurement of the cell penetration by the AFM tip. A modeling framework that combines the displacement field with the applied force/stress obtained with the AFM is developed to estimate the mechanical properties of each cell component. The tribological relevance of this research is the viscoelastic deformation of cell components under different contact loads.

A steam generator tube of a nuclear power plant is damaged by a fretting phenomenon caused by flow induced vibrations (FIV). In this work, the surface of the tube was coated with CrN or TiN as a measure to improve performance of the fretting wear resistance. Fretting wear regime was classified by determining a phase difference between friction and relative displacement signals and contact characteristics were analyzed. As a result, coating increased the friction coefficient. At a lower load, contact condition shifted from gross slip to stick slip.

A pin joint simulation machine has been built to test a real landing gear pin joint under realistic loading and reciprocation conditions. The pin is loaded hydraulically using a hydraulic actuator to apply a fixed displacement cycle whilst measuring the reactive torque. The machine was used to measure the torque cycle (and hence friction coefficient) required to operate the joint. In this work a method of evaluating different formulation greases has been proven. This involved measuring their frictional torque and also evaluating performance using a Sommerfeld type approach that displays the different lubrication regimes in the joint for different conditions. Measured friction coefficients were in the region of 0.02 to 0.12 depending on the joint load and articulating speed. In actual gear the surface sliding speed is low and so the joint operates in the boundary regime. The required torque and coefficient of friction have been related to the lubrication mechanisms occurring as a function of articulation angle, reciprocal frequency and applied axial load for lubrication starvation in a reciprocating journal bearing.

Due to the friction moment, there is an angular displacement when two flat rough surfaces come to sliding contact or quasi sliding contact. A 2-DOF inclined rough surface contact model is presented in this work, and the effects of the angular displacement on the friction coefficient, interfacial forces and moments are investigated. The numerical simulations show that both interfacial forces and moments increase with the increasing inclined angle, while the friction coefficient decreases instead. In addition, for a given sliding mass block system, the effects of the friction coefficient and the base sliding speed on the stability of the sliding contact are also discussed. The simulations indicate that a larger friction coefficient and a higher base sliding speed tend to turn over the mass block during the sliding.

When two surfaces in static contacts are subjected to combined loads applied in the normal and tangential directions, or just a normal load for dissimilar materials, microscopic slip would take place at certain areas of the contact region even though the contacting bodies remain still without macroscopic movement. The micro-slip is considered a major cause of fretting wear for the materials in contacts under alternating dynamic load or vibration, referred as the fretting contacts in this study. The fretting contact problem was solved using a semi-analytical method (SAM), in which analytical relations between a unite stress and corresponding surface displacement were obtained on the basis of Green functions. The contact pressure and shear tractions were then calculated by minimizing the complementary energy, and by a numerical procedure based on Conjugate Gradient Method (CGM) and Fast Fourier Transform (FFT) technique. The algorithm is very effective since the meshes are applied to the positions just in the contact areas of interest, which saves the computing time. The fretting contacts of dissimilar materials were studied and the effects of surface roughness were analyzed. Results show that the coupled effects of shear traction and material dissimilarity make the traction distributions quite different with the solutions from similar materials. The solutions under dynamic load depend on the path or history of the loading process, but the stress distributions and load-displacement curves will quickly converge to a periodic stability after several load cycles.

This paper designs a composite microtribology experiment system (CMES) to observe the sidewall friction of silicon micro-component pairs. The CMES is composed of a microtribological chip, an out-chip driving system, a video acquiring system, a laser triangulation displacement measure system (LTDMS), and a data processing system. Each microtribological chip has four cantilever friction pairs arranged anti-symmetrically. The size of the cantilever is 200 u m×50 u m×5 u m. The out-chip driving system loads the normal press force on the microtribological chip and produces reciprocating vibration meanwhile to achieve the friction. The video acquisition system is made up of a zoom microscope, a CCD, and a monitor. The LTDMS includes a diode laser, two optical focusing system, and a PSD (Position Sensitive Detectors). Both the video acquiring system and the LTDMS measures the friction characteristics from static to dynamic. The preliminary experiments shows: 1) The CMES has sufficient robustness and reliability. 2) The phenomenon of stick-slip is observed at static state by the LTDMS; 3) and the friction coefficient decreases with the increasing of the reciprocating frequency.

Tilting pad thrust bearings are designed to transfer high axial loads from rotating shafts to the static support structure. In the present paper, formulation of Reynolds’ equation for the bearing is done in two dimensions (planar). A finite difference method is used to convert the terms of the Reynolds’ equation in to a set of simultaneous linear algebraic equations. A solution procedure for finding value of the pressure in the oil film is described. Numerical integration of the pressure values gives the load distribution. Subsequently, the study of angular stiffness of the film is done by varying the value of the oil film thickness and calculating torques at three different film shapes 1, 2 & 3 of the pad. The angular stiffness, differential displacement and load pertaining to the 3-2 pair are calculated for 0.5%–20% variation of h o . The results obtained show that the values of Kt* and K* converge asymptotically.

A continuum model of adhesive interaction between elastic surfaces is presented. Surface interaction between two elastic spheres is modeled by the Lennard-Jones (L-J) potential. The analysis is based on the equivalent system of a rigid sphere of reduced radius in close proximity with an elastic half-space of effective elastic modulus. The critical gap at the instant of the “jump-in” and “jump-out” contact instabilities is determined by an elastic solution of the half-space surface displacement. A finite element model, in which surface interaction is modeled by nonlinear springs of a prescribed force-displacement governed by the L-J potential, is also used to analyze adhesive surface interaction. The analytical model is validated by finite element results of the critical central gap at the instant of jumpin and jump-out instabilities for different values of the Tabor parameter.

The contact area, friction force and relative displacement evolution at the very early stage of fretting are investigated experimentally. Copper and steel spheres of various diameters are loaded against a hard sapphire flat by a range of normal loads deep into the elastic-plastic regime of deformation. A reciprocating tangential loading is then applied with a maximum loading below the static friction to avoid gross slip. Real-time and in situ direct measurements of the contact area, along with accurate measurements of the friction force and relative displacement, reveal substantial junction growth and energy dissipation mainly in the first loading cycle. The so called “slip amplitude” is found to be attributed to residual tangential plastic deformation rather than to interfacial slip. Elastic shake-down is observed for the 2.5% hardening steel spheres while plastic shake-down is observed in the case of the elastic perfectly plastic copper spheres.

The effect of angular misalignment of plain journal bearings on the oil whirl and oil whip phenomena was investigated using a test rig comprising a flexible one inch diameter shaft supported on two cylindrical fluid film bearings. A matrix of experiments was designed where a precisely controlled angular misalignment between the shaft and the drive end, the non-drive end, or both bearings was induced. In each case we measure the onset of oil whirl and/or the onset of oil whip and the maximum speed that the test rig can reach before reaching a maximum vibration displacement of 1 mm at the location of the displacement sensor. The results show clearly that the angular misalignment of the bearings with respect to the shaft help delay the onset of oil whip from 3200 rpm to 4500 rpm in some cases. An interesting and new observation was the effect of the configuration of the misalignment on the overall stability characteristics. In particular, when the two bearings were misaligned in the same direction such that the two misaligned bearing planes were parallel to each other the onset of instability was more delayed and the overall operation was smoother.

Performances of point contact EHL on multilayered or coated substrates have been analyzed in this paper via computer simulations, with emphasis on comparing the effects of Newtonian and non-Newtonian lubricants. The lubrication system consists of a rigid ball in contact with a smooth coated flat. The coating is perfectly bonded to an elastic substrate and it has a uniform thickness. The rigid ball has surface velocity U relative to the contact point. The hydrodynamic pressure p is governed by a generalized Reynolds equation in which the non-Newtonian effects of lubricants are characterized by two factors whose values are determined based on lubricant rheology. The Papkovich-Neuber potentials were employed to get the response functions in frequency domain for layered contact problems, and the influence coefficients relating pressure to surface displacements and stresses can be obtained via invert Fourier transform. The surface deformation was then calculated in terms of the pressure-displacement influence coefficients and the DC-FFT method was used to speed up the computation. The distributions of pressure, film thickness and subsurface stress have been analyzed for lubricants with different rheological behaviors, from which pressure and film thickness profiles along the rolling direction are calculated for Newtonian and Non-Newtonian lubricants. The central film thickness become thicker for stiffer coatings in the case of Newtonian lubricants, but the trend is reversed for Non-Newtonian lubricants. The surface stresses along the rolling direction show a spike corresponding to the pressure, which is more significant with stiffer coatings in the Newtonian case, but the spike is less visible for Non-Newtonian lubricants.

The behavior of an elastic-plastic contact between a deformable sphere and a rigid flat under combined constant normal and reciprocating tangential loading is investigated in the present work. The theoretical model is based on the assumptions of full stick contact condition and two kinds of the sphere material hardening. Hysteretic change of friction force versus tangential displacement during reciprocating tangential loading is investigated along with the study of the change of the contact area and stress state in the elastic-plastic sphere. Good agreement between theoretical and experimental results is obtained.