Measurements Of Real Contact Area Research Articles

How frictional slip evolves

Earthquake-like ruptures break the contacts that form the frictional interface separating contacting bodies and mediate the onset of frictional motion (stick-slip). The slip (motion) of the interface immediately resulting from the rupture that initiates each stick-slip event is generally much smaller than the total slip logged over the duration of the event. Slip after the onset of friction is generally attributed to continuous motion globally attributed to ‘dynamic friction’. Here we show, by means of direct measurements of real contact area and slip at the frictional interface, that sequences of myriad hitherto invisible, secondary ruptures are triggered immediately in the wake of each initial rupture. Each secondary rupture generates incremental slip that, when not resolved, may appear as steady sliding of the interface. Each slip increment is linked, via fracture mechanics, to corresponding variations of contact area and local strain. Only by accounting for the contributions of these secondary ruptures can the accumulated interface slip be described. These results have important ramifications both to our fundamental understanding of frictional motion as well as to the essential role of aftershocks within natural faults in generating earthquake-mediated slip.

On the use of in situ X-ray computed tomography for soft contact mechanics

The real contact area AR between two solids in contact is an outstanding quantity that controls the frictional and adhesive behavior of a contact interface. Most of the experimental methods to measure AR are based on the contrast in local optical properties of the interface, between in- and out-of-contact regions. Although those methods recently enabled various new insights into contact mechanics and tribology, they suffer from important limitations: they require that at least one of the two solids is optically transparent; they only provide information about the real interface, including AR, but not to the bulk deformation that is at the origin of AR. Here, we propose in situ X-ray Computed Tomography (XRCT) as an appealing alternative method to overcome those limitations. Indeed, it enables three-dimensional access to interfaces within potentially non-transparent contact pairs. We test the advantages and disadvantages of the method on the smooth contact between a smooth elastomer sphere in contact against a smooth rigid plate. Such a tribological system is chosen because the real contact area measurement can be benchmarked against standard optical results. We show that XRCT can, in addition, give unique access to the full surface deformation of the solids in contact, opening the way to deeper comparisons with existing models of adhesive spherical contacts.

The Initiation of Frictional Motion—The Nucleation Dynamics of Frictional Ruptures

AbstractFrictional interfaces lose stability via earthquake‐like ruptures, which are close analogues of shear cracks that are well‐described by fracture mechanics. Interface ruptures, however, need to be first formed—or nucleated. Rupture nucleation therefore determines the onset of friction, replacing the concept of a characteristic “static friction coefficient”. Utilizing rupture arrest at an imposed barrier, we experimentally determine nucleation locations, times and stresses at the origin of each subsequent rupture event. This enables us study the nucleation process via real‐time measurements of real contact area and local strain. Nucleation events initiate as 2D patches that expand at nearly constant velocities, vnuc, that are orders of magnitude lower than the dynamic rupture velocities described by conventional fracture mechanics. We find that: (a) Nucleation has location‐dependent stress thresholds, (b) vnuc is roughly proportional to the local stress level, (c) the nucleation process continues until the patch size reaches Ltran ∼ LG, the Griffith length for the onset of dynamic fracture (d) scaling time by τ = Ltran/vnuc, nucleation patches exhibit self‐similar dynamics (e) dynamic ruptures' cohesive zones are not fully established until significantly beyond Ltran. Many details of nucleation are governed by the local contact area topography, which is roughly invariant under successive rupture events in mature interfaces. Topography‐dependent details of the nucleation process include: precise nucleation site location, patch geometry, critical stress thresholds and the proportionality constant of vnuc with stress. We believe that these results shed considerable light on both how frictional motion is triggered and earthquake initiation.

Elucidation of frictional properties between rubber and flooring using a laser microscope with wide field of view

We investigate the measurement of real contact area, denoted by Ar, between various floor materials and rubber. Observation of Ar plays an important role for better understanding of frictional force. The Ar between them, however, has yet to be observed over regions wider than what an optical microscope can cover. Current optical microscopy techniques are limited by the measurement time required to observe the whole apparent contact area. Further, it is difficult to reproduce actual contact situations between the rubber and the floor materials because at least one of the contacting specimens must be transparent. Usually, glass plates much harder than rubber are used as counter specimens. In this study, we observed the Ar between a transparent silicone rubber and the various floor materials by using a laser microscope with a wide field of view, allowing us to observe the whole apparent contact area of 9 mm × 7 mm for a very short time. By using transparent rubber, we were able to reproduce the actual contact situations of the rubber-floor specimens, which is impossible with glass plates. The measurement of Ar when the apparent contact area was too small suffered from large fluctuations in measurement values due to waviness of the floor specimen. We investigated the apparent contact area required to accurately understand the relationship between frictional force and Ar and clarified the influence of waviness of the floor materials on the distribution of Ar.

Realization of a Novel Morphing Surface Using Additive Manufacturing and Its Active Control in Friction

Abstract As the need for higher efficiency of engineering components increases, so does the demand for functional surfaces. While various tribosurfaces (e.g., texturing and coatings) have been developed, many researches are aimed at static functionality. On the other hand, due to a wide range of environmental adaptability and active control, active-morphing surfaces can be highly efficient and robust. In this paper, we demonstrate a novel morphing surface and its realization using additive manufacturing (AM). By using a diaphragm structure, morphing performance is achieved even if a hard resin material is used. When air pressure is applied to the backside of the diaphragm, it changes to a convex shape and vice versa. The concept requires a complex structure for arranging airflow and a solid morphing system. The AM is one great technique to create such complex structures. As a result of actual manufacturing, the created morphing structure realizes a large morphing of 600 μm or more. In addition, the shape changes reversibly depending on the air pressure. The surface also exhibits very interesting tribological characteristics. The surface shows a friction coefficient of about 0.3 with a concavity, and then increases to 0.5–1.7 with a convexity. A real-contact area measurement reveals that the novel property occurs due to change in the real-contact area depending on surface morphology. In conclusion, the present paper provides a new concept of a novel morphing tribosurface, which selectively performs as a low-friction or break-like surface, created using AM.

Measurement of Real Contact Area for Rough Metal Surfaces and the Distinction of Contribution From Elasticity and Plasticity

Abstract The measurement of the real contact area between rough surfaces is one of the most challenging problems in contact mechanics and is of importance to understand some physical mechanisms in tribology. Based on the frustrated total internal reflection, a new apparatus is designed to measure the real contact area. For metallic samples with various surface topographies, the relation between normal load and the real contact area is measured. The unloading process is first considered to distinguish the contribution of elasticity and plasticity in contact with rough surfaces. It is found that both elasticity and plasticity are involved throughout the continuous loading process, different from some present understanding and assumptions that they play at different loading stages. A quantitative parameter is proposed to indicate the contribution of plasticity. The present work not only provides an experimental method to measure the real contact area but figures out how elastic and plastic deformation works in contact with rough surfaces.

Consideration of Applying Ellipsometry to Real Contact Area Measurement

Consideration of Applying Ellipsometry to Real Contact Area Measurement

Real-Contact Area Between an Elastomer and a Flat Plane Observed by Surface Plasmon Resonance: Optical Model Calculations

This study presents a novel method for high-sensitivity measurement of the area of the real contact between an elastomer surface and a rigid flat plane, using the surface plasmon resonance (SPR) technique in the Kretschmann configuration. Through numerical calculations, the sensitivity of the SPR method for determining the real-contact area is discussed and compared to other typical optical techniques, such as total internal reflection and multiple beam optical interference. The wavelength dependence of the optical reflectance is simulated, as well as its change with the elastomer-plane gap thickness and the optical properties of the gap material. Non-transparent elastomers are also studied. Consequently, it was found that the SPR method has the highest sensitivity for the measurement of real-contact area; it can detect, for example, an ultra-thin air gap (or water gap) formed between the contacting surfaces from the measurement of the intensity of reflected light from the contact region.

Contact pressures and the impact of farm equipment on Latosol with the presence and absence of sugarcane straw

ABSTRACT High contact pressures applied to soil result in a greater degree of compaction, in addition to promoting other negative effects. The objective of this study was to quantify contact areas by using different methodologies, and pressures of farm equipment employed in production activity and evaluate structural changes caused in a Red Latosol with the presence and absence of straw cover. The design was completely randomized in a factorial scheme of type 4 (tire on front axle, tire on rear axle, tire on a sugarcane wagon and metallic track of sugar cane Harvester) x 2 (presence and absence of straw). The contact area (CA) of the run was obtained by three procedures: analytical measure of the area of an ellipse (CA1); digital measurement of area of an ellipse (CA2); and measurement of real contact area (RCA), with digital resources. The contact pressure was calculated from the ratio of mass of each machine's axle and the contact area of the run. The contact area obtained according to the procedure of the ellipse (CA1 and CA2) is overrated when compared to actual area obtained digitally (RCA). The straw reduces the contact pressure in the soil, due to the deep tire treads and consequently, increased contact area. Areas where the traffic of the machines occurred with soil covered with the presence of straw showed reduced soil resistance to penetration, cone index and pre-consolidation pressure, confirming that the increased contact area produced by straw reduced the pressure applied and the compression power dissipated in the soil.

Measurement of Real Contact Area Using Light Interferometry and Its Application

Measurement of Real Contact Area Using Light Interferometry and Its Application

Optical measurements of real contact area and tangential contact stiffness in rough contact interface between an adhesive soft elastomer and a glass plate

Optical measurements of real contact area and tangential contact stiffness in rough contact interface between an adhesive soft elastomer and a glass plate

Measurement of real contact area on thermal print head using a laser microscope with a wide field of view

In a dye-sublimation printer, a thermal print head presses and heats a dye-based ribbon to diffuse ink dyes to the desired positions into the receiving layer of paper. The print quality will deteriorate if the contact condition between the thermal print head and the paper is insufficient. Thus, it is necessary to analyze the contact condition for finer printing. Direct observation of the contact area with an optical microscope is effective to clarify the contact conditions of the thermal print head. However, conventional optical microscopes take too much time to observe the whole apparent contact area, because their fields of view are not sufficiently wide. Previously, we developed a laser microscope with a wide field of view and applied this laser microscope to observe the whole apparent contact area of rubber elements. In this study, we examined the contact conditions between the thermal print head and the paper. In general, when we observe the contact area of two solid surfaces, one is restricted to a transparent material such as a glass plate. Instead of the thermal print head, a glass head of the same shape as an existing thermal print head was pressed on the paper and observed through the glass head. This allowed us to clearly determine the contact situations of the interface and measure the contact widths and distributions of real contact area. In this method, it was confirmed that the contact width and the real contact area changed in accordance with the type of paper used, steps of the printing process, and density of color printing.

画像輝度ヒストグラムに基づく統計的真実接触面積測定（第1報） ―光干渉画像におけるガウス分布の利用―

画像輝度ヒストグラムに基づく統計的真実接触面積測定（第1報） ―光干渉画像におけるガウス分布の利用―

Statistical Measurement of Real Contact Area on the Basis of Image Intensity Histograms (Part 2)

Statistical Measurement of Real Contact Area on the Basis of Image Intensity Histograms (Part 2)

Experimental Study of the Performance of Static Seals Based on Measurements of Real Contact Area Using Thin Polycarbonate Films

To clarify the sealing characteristics of metal gasket seals, leakage rates of gas and the real contact area of the seal surfaces were measured under several closing loads. The static seal consisted of a ring-shaped copper gasket and the two steel flanges that held the gasket in place. Gasket widths in the radial direction were 2 mm, 3 mm, and 5 mm. The contact surfaces of the flanges were finished by lathe turning. To determine the leakage flow paths between the gasket and the flanges, the real contact situation between them was observed using a thin polymer film 1 μm in thickness. The results indicated the leakage flow paths on the gasket surface were the radial direction perpendicular to the lathe-turned groove and the circumferential direction along the groove. As the closing loads increased, the leakage flow in the radial direction ceased and only that in the circumferential direction remained. Therefore, the cross-section of the aperture for the leakage flow in the circumferential direction was evaluated from the measured real contact area, and the leakage rates were estimated by assumption of laminar flow. The results agreed well with the measured leakage rates.

Measurement of real contact area and analysis of stick/slip region

The real contact area obtained between a rough rubber hemisphere and a glass plate, under unlubricated conditions, was visualized by white light interferometry. The contacting friction condition of the real contact area in the starting friction process was analyzed using interference intensity. The real contact area was extracted by fitting the intensity histogram of interference images to Gaussian distribution. This was seen to decrease with the increase of the applied tangential force, and it became approximately constant after initiation of macroscopic sliding. Real shear stress increased linearly in the micro-sliding regime and then became constant in the macro-sliding regime with the increase of the glass plate's displacement. The statistical ratio of stick region to contact area was estimated with the use of a differential histogram of the interference image intensity based on a tangential force coefficient of zero. This ratio showed a tendency to be more than a theoretical solution offered by Mindlin. In addition, we performed particle image velocimetry analysis, which assumed real contact spots as chase markers, and succeeded in the mapping of the stick/slip region within the real contact area.

Shear characteristics of a boundary film for a paper-based wet friction material: friction and real contact area measurement

A paper-based wet friction material lubricated with automatic transmission fluids (ATFs) produces plateau and positive slope regions in the friction–sliding velocity diagram. This feature observed at less than 1 mm/s of sliding velocity is discussed in this paper. The relationship between the friction and contact pressures of less than 5 MPa was determined by a reciprocating friction test rig lubricated with mineral oil including phosphoric acid ester, one of the additives for ATFs. The real contact area for dry condition was also determined by optical interferometry. The consistency of the real contact pressure dependency of the shear strength of the boundary film was indicated at pressures of less than 1 GPa obtained by steel-on-steel boundary friction (oiliness) test. This suggests that the generation of the positive slope region is due to thin film lubrication. To the plateau region, possibility of application of the phase transition model is considered.

Friction and thermal phenomena in chemical mechanical polishing

The friction and thermal phenomenon was investigated to verify the effect of temperature on the material removal mechanism in chemical mechanical polishing (CMP). To this end, the polishing of various materials, temperature measurement by way of infrared ray camera, frictional force measurement and real contact area measurement experiment were conducted. From the results of these experiments, we concluded that the material removal mechanism in CMP under the influence of increasing temperature, is dominated by the increase in chemical reactions rather than mechanical ones. Furthermore, the removal rate did not remain constant during the polishing process and this has an effect on the within wafer non-uniformity (WIWNU).

101 PC 薄膜を用いた真実接触面積の測定 : 圧痕形状の影響

101 PC 薄膜を用いた真実接触面積の測定 : 圧痕形状の影響

Novel Stage for Ultrasonic Measurement of Real Contact Area Between Rough and Flat Parts Under Quasi-Static Load

Abstract The paper contains a description of an ultrasonic method and stage for real contact area (RCA) measurement on steel flat parts with technological surface layer. The stage allows measurement of RCA of test surface in contact with a rigid flat counterface. The measurement can be taken during the process of surface deformation as well as for the surfaces already deformed.