Showing papers on "Contact area published in 1992"

An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments

Abstract: The indentation load-displacement behavior of six materials tested with a Berkovich indenter has been carefully documented to establish an improved method for determining hardness and elastic modulus from indentation load-displacement data. The materials included fused silica, soda–lime glass, and single crystals of aluminum, tungsten, quartz, and sapphire. It is shown that the load–displacement curves during unloading in these materials are not linear, even in the initial stages, thereby suggesting that the flat punch approximation used so often in the analysis of unloading data is not entirely adequate. An analysis technique is presented that accounts for the curvature in the unloading data and provides a physically justifiable procedure for determining the depth which should be used in conjunction with the indenter shape function to establish the contact area at peak load. The hardnesses and elastic moduli of the six materials are computed using the analysis procedure and compared with values determined by independent means to assess the accuracy of the method. The results show that with good technique, moduli can be measured to within 5%.

On the generality of the relationship among contact stiffness, contact area, and elastic modulus during indentation

Abstract: Results of Sneddon's analysis for the elastic contact between a rigid, axisymmetric punch and an elastic half space are used to show that a simple relationship exists between the contact stiffness, the contact area, and the elastic modulus that is not dependent on the geometry of the punch. The generality of the relationship has important implications for the measurement of mechanical properties using load and depth sensing indentation techniques and in the measurement of small contact areas such as those encountered in atomic force microscopy.

A study of the mechanics of microindentation using finite elements

Abstract: In this paper the finite element method is used to explore the mechanics of the microindentation process. In the simulations discussed, aluminum and silicon are investigated both in their bulk forms and in thin film-substrate combinations. Among the quantities readily computed using this approach and given in this paper are hardness (computed using actual contact area), contact stiffness, effective composite modulus, and surface profile under load. Importantly, this investigation builds on previous work by providing a more critical examination of the amount of pileup (or sink-in) around the indenter in the fully loaded configuration, as well as the variation of the actual contact area during indenter withdrawal. A key conclusion of this study is that finite element simulations do not support the widely used assumption of constancy of area during unloading (for either bulk materials or thin film systems). Furthermore, the amount of pileup or sink-in can be appreciable. The implication of these findings is that for many situations the commonly used straight-line extrapolation of a plastic depth may render an estimate for the contact area that is quite distinct from the actual area. This assertion is demonstrated herein through comparison of hardnesses calculated using actual contact area with values calculated using the straight-line extrapolation of plastic depth.

An investigation of microscopic aspects of contact angle hysteresis: pinning of the contact line on a single defect

Abstract: We examine the microscopic pinning of a contact line on a single heterogeneity. We find that many features of this behavior are predicted by models based on the same energy functionals which describe macroscopic contact angle hysteresis on surfaces with many such imperfections. As the defect emerges from the undistorted contact line, the contact line pins, increasingly distorts, slides across the defect, and then jumps back to an undistorted position. The distortion of a stably pinned contact line is described by a logarithmic dependence. While the force pinning the contact line depends on the defect microstructure, the elastic restoring force follows Hooke's law and is dependent on the properties of the fluid and contact angle on the underlying surface. The sliding motion and unstable jumps of the contact line occur at small capillary numbers.

A review of the processes that control snow friction

Abstract: : There is a long history of interest in snow friction, but it is still necessary to speculate about the details of the processes Roughness elements and contact areas must be characterized before the basic processes can be well understood These parameters change with movement over snow and, in fresh snow, probably change along the length of the slider Friction results from a mixture of processes: dry, lubricated, and possibly capillary Dry rubbing occurs at low speeds, loads, and/or temperatures and is characterized by solid-to-solid interactions requiring solid deformation With small quantities of meltwater present, elastohydrodynamics must be used to account for processes at partially separated surfaces and, when too much water is present, the contact area increases and there may be capillary attachments Static charging probably occurs and may attract dirt that, even in the size range of micrometers, could complicate the processes Slider thermal conductivity and even color are very important Heat is generated by friction and solar radiation absorbtion but some is conducted away by the slider and ice particles The remaining heat is available to generate meltwater, which acts as a lubricant Polyethylene bases offer many advantages including low ice adhesion, high hydrophobicity, high hardness and elasticity, good machinability, and good absorption of waxes While sliders must be designed for use over a narrow range of snow and weather conditions, polyethylene bases can be structured and waxed to broaden that range The important processes operate, not at the air temperature, but at the ski base temperature, which is highly dependent on such things as snow surface temperature, load, and speed Friction, Rubbing, Snow physics, Plastics, Skis, Wax, Polyethylene, Sliding, Polymers, Snow

The influence of fluids on grain contact stiffness and frame moduli in sedimentary rocks

Abstract: The frame moduli of sedimentary rocks are strongly influenced by the properties of the grain contacts. A modified Hertz contact theory is presented for the self consistent calculation of deformation, equilibrium separation distance (film thickness), and contact area of deformation for two spherical asperities in contact and subjected to an external load. We show that surface forces, i.e., electrostatic repulsion, Born, structural, and Van der Waals forces can be incorporated into the contact deformation problem. From the results presented, it is evident that surface forces play an important role in determining seismic wave velocities and attenuations at low confining stresses. The velocities and attenuations computed from the model are compared with measured values for glass beads, Navajo, Berea, Obernkirchner, and Fort Union sandstones. The velocities and attenuations calculated as functions of stress, frequency, fluid type, and saturation are in good agreement with reported experimental data.

Atomic force microscope study of boundary layer lubrication

Abstract: An atomic force microscope is used to investigate the lubrication properties of a simple surfactant (n‐dodecanol) on mica at 24 °C. The liquid becomes strongly layered as it is confined between the tip and the mica and a bilayer structure was often observed. The lubrication properties of the adsorbed molecules are clearly evident in the very low friction forces observed at appreciable applied loads (∼100 MPa). It is only when the last layer of molecules is removed that a significant friction signal is observed. It appears that the tip‐mica interaction is dominated by adhesive forces and hence the measured friction and normal forces are strongly influenced by the effective contact area of the tip.

Dynamic model of stationary contacts based on random variations of surface features

Abstract: A statistical model was developed to characterize surface profiles and calculate the density of contact spots produced when rough surfaces are pressed together. This model incorporates contact force, microhardness, and real and apparent contact area as parameters which can be varied. The effects these variables have on the number of contact spots and asperity deformation were calculated. These results were subsequently used to evaluate the impact on electrical performance of aged contacts. This was accomplished using a previously developed degradation model due to the author (1990) that utilizes the concept of a third level of constriction to simulate aging. The results indicate that both contact force and geometry play important roles in electrical performance of aging contacts. The connection between geometry and apparent pressure was estimated using the Hertz theory and indicates that one can view pressure as an important vehicle as well. >

Three-dimensional finite element analysis of elastic continua for tactile sensing

Abstract: This article presents a method for determining three-dimensional stress and strain tensors within elastic layers covering or embedding tactile sensors. When an object comes in contact with the elastic surface of a sensing array, the distribution of forces on the surface is mechanically filtered before reaching the underlying sensing elements. In order to predict the response of embedded sensors and to understand the nature of the transduced variables, the relationship between surface forces and interior stress and strain tensors must be known. To determine this, one needs to know the shape of the surface contact area and the distribution of forces over that area, both normal and tangential. The algorithm described here discretizes the known surface force distribution into a dense array of independent point loads, approximating the surface force profile. The stress field produced by a single normal and/or a single tangential point load can be expressed analytically, given the elasticity and compressibility of the material. A detailed example is illustrated, analyzing the interaction between a sphere and a planar elastic half space. From the example the relative effects of depth and distance from the center of contact on the response of underlying sensors is discussed. Using results frommore » the example, methods are proposed to exploit sensors at different depths and at different orientations within the medium. An algorithm for determining the direction of surface tangential forces and a method for determining the ration between normal and tangential forces are analyzed, using unidirectionally sensitive sensors at different depths in the elastic layer.« less

Effect of the hydrodynamic bearing on rotor/stator contact in a ring-type ultrasonic motor

Abstract: A hybrid numerical analysis that includes the hydrodynamic bearing effect and elastic contact in a ring-type ultrasonic motor is presented. The two-dimensional time-dependent compressible Reynolds equation is solved numerically by a second-order time accurate, noniterative, factored implicit finite-difference algorithm. The rotor deformation is described by a one-dimensional Green's function. The contact problem is solved by an iteration method so that the contact condition and the hydrodynamic bearing condition are satisfied simultaneously. The results show that the hydrodynamic bearing effect, especially the squeeze effect, is significant for ultrasonic frequency contact of the rotor and stator. Surface roughness, contact area, and normal vibrating speed of the stator are important parameters in the hydrodynamic bearing. A disagreement between the friction coefficient needed in the numerical analysis and the experimentally measured one in a previous study. >

Effect of Contact Area Size on Enamel and Composite Wear

Abstract: The effect of contact area dimensions on the wear of composite specimens and their opposing enamel cusps was evaluated in vitro. Thirty-six standardized cylindrical composite specimens were placed into metal cavities (8 mm x 2 mm) and divided randomly into five groups. The composite used was a fine-particle hybrid and was stressed as follows: storage in 75% aqueous ethanol solution for 24 h, toothbrush/toothpaste-abrasion for 30 min, followed by 300 thermal cycles in water ranging from 5° to 55°C and simultaneous 120,000 occlusal chewing loads at a frequency of 1.7 Hz at 53 N maximum force. In group 1 (n = 12), the occlusal chewing loads were applied by palatal cusps of extracted human maxillary molars with natural morphology. In groups 2 to 5 (n = 6), the cusp tips had standardized contact area dimensions of 0.26, 0.38, 1.18, and 4.10 mm2, respectively. Wear of composite specimens and antagonistic enamel cusps (means ± SD) was assessed in μm by means of a 3-D scanner. Additionally, the contact surfaces o...

Surface Forces and Adhesion

Abstract: This chapter is concerned, not with the controversial topic of the adhesion component of friction (covered later in this book), but with how surface forces may influence a wide range of friction processes. This influence is either direct, or as a result of changes in the real area of contact.

Normal and Angular Motions at Rough Planar Contacts During Sliding With Friction

Abstract: The planar dynamics of a rough block in nominally stationary or sliding contact with a counter-surface is studied in this work. Starting with the Greenwood-Williamson model of a rough surface, the analysis of elastic contact deflections is extended to accommodate angular as well as normal motions. The real area of contact and the normal contact force are obtained in terms of the relative approach and orientation of the surfaces. It is shown that angular and normal motions at frictional contacts are generally coupled. The contact area and normal contact force are shown to be nonlinearly related to the normal and angular motions

Variable capacitor and position indicator using same

Abstract: PURPOSE:To largely change a capacitance value with pressure or vary small displacement by separating between a first electrode attached to the one surface of dielectrics, a flexible second electrode arranged in the other surface and between the second electrode and the other surface of dielectrics for only a little distance, except for the part thereof CONSTITUTION:A housing member 61 is formed almost cylindrical with a bottom surface and the external side of the bottom 723 of a core material 71 is formed as the spherical part In the case where pressure or displacement is not applied to the core material 71, the other surface 1b of dielectrics 1 is separated by an electrode 3 with a spacer 4, allowing formation of the air layer 10 Meanwhile, when pressure or displacement is applied to the core material 71 and thereby a cap body 72 bends an electrode 3 toward the dielectrics 1 through an elastic material 5, capacitance by air layer 10 becomes larger is reverse proportion to thickness of air layer 10 and thereby a capacitance value between terminals 8 and 9 also becomes large Moreover, when pressure or displacement increases and the electrode 3 is placed in contact with the other surface 1b of the dielectrics 1, a capacitance value between the terminals 8 and 9 also increases almost in proportional to the contact area

Zirconium oxide ceramics for surfaces exposed to high temperature water oxidation environments

Abstract: Disclosed is an apparatus and a process for high temperature water oxidation of combustibles in which during at least a part of the oxidation, corrosive material is present and makes contact with at least a portion of the apparatus over a contact area on the apparatus, wherein at least a portion of the contact surface area comprises a zirconia based ceramic, with the temperature of the oxidation process in excess of about 300°C and the pressure of the oxidation process is in excess of about 27.5 bar (400 psi).

The effects of fibular and talar displacement on joint contact areas about the ankle.

Abstract: The purpose of this study was to advance prior techniques and studies regarding the effects of fibular and talar displacement of contact areas of the tibio-talar joint. Type IV supination external rotation injuries were experimentally created on five fresh cadaveric specimens. Tibio-talar contact and peak pressures were measured using Fuji pressure-sensitive film with the talus in a neutral position and then displaced 1, 3, 4, 5, and 8 mm laterally as well as a repeat measurement following reduction and plate fixation. Results demonstrated a 50% reduction in contact area with only 1 mm of talar displacement along with a linear increase in average peak pressures. Reapproximation of the fibular osteotomy with a plate and rereduction of the talus allowed for return to normal pretesting contact areas. This study confirms prior studies demonstrating the significance of 1 mm of talar displacement in regards to marked reduction in contact area of the tibio-talar joints. In addition, restoration of the normal anatomy and fixation of the osteotomy allows for return of normal contact areas.

Nanometer scale mechanical properties of Au(111) thin films

Abstract: The mechanical properties of gold films of (111) orientation were studied as a function of load when contacted by a single asperity Pt-Rh alloy tip. The interaction forces were measured in the direction perpendicular to the surface. The contribution of various types of forces (van der Waals, capillarity from contaminants, and metallic adhesion) in the process of contact was determined. We investigated the elastic and plastic response of the gold film as a function of applied load by examination of the contact area in subsequent imaging with STM and AFM.

Some Comparisons of Average to Peak Soil-Tire Contact Pressures

Abstract: Comparisons are made among several methods of determining the contact pressure between a tire and the soil. Results show that on compacted soils the peak pressures measured at the soil-tire interface are much greater than mean pressures determined from measure-ments and much greater than pressures calculated by dividing the dynamic load by contact area. On uncompacted soil, peak pressures are almost equal to the inflation pressure.

The classification of rough surfce contacts in relation to tribology

Abstract: Height and wavelength parameters of surface roughness are varied to show that three types of surface contact can exist in relation to tribology. These are (i) predominantly plastic asperity contact with gaps in the contact interface, (ii) the same contact geometry but with predominantly elastic asperity contact and (iii) almost total surface contact under an elastic state of stress (this state approaches the theoretical smooth surface conditions). These contact states are confirmed by rough surface contact techniques of a ball-on-flat geometry. Results from simple boundary-lubricated sliding tests are also presented to show that the frictional and tribological behaviour differs significantly for each of these contact states. The fundamental implications for tribology are discussed in relation to this work.

Charge transfer between a polymer particle and a metal plate due to impact

Abstract: To elucidate the mechanism of contact-impact electrification of insulating particles, the authors measured the charge generated by collision of a single polymer particle onto a metal plate. It was not exclusively the normal component of the impact velocity that determined the amount of impact charging. In order to investigate this great contribution of the impact angle to the impact charging, impact stresses, contact times, and contact areas were measured, and the impact-contact process was discussed. Based on the idea that the "apparent contact area" is supported by some "real contact points" and that the number of these real contact points increases with the particle slipping, a new model called "effective contact area model" was proposed. >