Real Contact Area Research Articles

A discussion of model asperities as a method to study friction in metal forming

The present work discusses the use of model asperities as a method for studying asperity flattening in metal forming. Asperity flattening and the resulting real contact area largely influence friction in metal forming. In order to gain a fundamental understanding of asperity flattening, a method of choice often is to use model asperities. Here, macroscopic model surfaces are emulating real surface topographies. The work presents the method in a historic context, explains the method, lists advantages and disadvantages, and discusses the method in terms of whether the insights gained on model asperities are of qualitative or quantitative nature. In conclusion, the method is a valid tool to fundamentally study asperity flattening. It is, however, not always possible to transfer obtained conclusions quantitatively from model surfaces to real surfaces.

Structural and technological support of tightness of interference fits

The conditions for contact interaction of mating parts in interference fit joints are considered. It is shown that the main factor determining the strength and tightness of joints is the state of the surface layer. Ways are established to control the performance of joints with an interference fit by using surfactants, both in the technological preparation of parts before assembly, and in the assembly of joints. Keywords: contact interaction of surfaces, real contact area, joint tightness, influence of surfactants. kurnosov-ne@mail.ru

A novel fractal contact model based on size distribution law

This study develops a new fractal contact model to evaluate contact status on rough surfaces. Specifically, the three-dimensional Weierstrass-Mandelbrot function is used to characterize fractal surfaces, in which the asperity level range is determined by the power spectral density of surface. A novel size distribution law of truncation regions has been proposed based on rough surface truncation in our previous study to overcome the limitation of the law proposed by Mandelbrot. By using the size distribution law, a multi-asperity contact model is established to calculate the contact force and real contact area. The evolution of the asperity level range is considered during the compression process. Furthermore, the elastic or plastic deformation status of each asperity in the level range is determined. In the implementation of the proposed theoretical model, the numerical truncation of fractal surfaces is conducted to obtain the effective asperity level range. Finite element simulation of fractal surface contact and compression experiments of copper specimens are conducted to validate the fractal contact model. The theoretical results of the contact force and total contact area agree well with the simulation and experimental results. The effects of the fractal parameters on the contact behavior are further investigated. The results indicate that the proposed fractal contact model can provide an accurate prediction of rough contact behaviors.

Evolution of the real contact area of self-affine non-Gaussian surfaces

The impact of non-Gaussian height distribution on the real contact area evolution of elastic, frictionless and non-adhesive contact is studied. The Weibull probability function is used to model the height distribution, as it can capture surfaces of practical relevance. The set of variables needed to parametrise the problem is discussed, including the shape parameter of the height distribution, the topography’s wavelength ratio and the Hurst exponent. As the topographies become more non-Gaussian, a significant deviation from the Gaussian case is observed. Moreover, the spectral properties show a distinct effect on different non-Gaussian surfaces and the dependency on Nayak’s parameter is not inherited. A power-law evolution of the real contact area is found to fit the numerical results obtained with the boundary element method precisely. Finally, two semi-analytical asperity-based models are compared with numerical results, using the statistics of artificially generated topographies. The qualitative behaviour predicted with the numerical simulations is captured.

Molecular Probing of the Stress Activation Volume in Vapor Phase Lubricated Friction.

When two solid objects slide over each other, friction results from the interactions between the asperities of the (invariably rough) surfaces. Lubrication happens when viscous lubricants separate the two surfaces and carry the load such that solid-on-solid contacts are avoided. Yet, even small amounts of low-viscosity lubricants can still significantly lower friction through a process called boundary lubrication. Understanding the origin of the boundary lubricating effect is hampered by challenges in measuring the interfacial properties of lubricants directly between the two surfaces. Here, we use rigidochromic fluorescent probe molecules to measure precisely what happens on a molecular scale during vapor-phase boundary lubrication of a polymer bead-on-glass interface. The probe molecules have a longer fluorescence lifetime in a confined environment, which allows one to measure the area of real contact between rough surfaces and infer the shear stress at the lubricated interfaces. The latter is shown to be proportional to the inverse of the local interfacial free volume determined using the measured fluorescence lifetime. The free volume can then be used in an Eyring-type model as the stress activation volume, allowing to collapse the data of stress as a function of sliding velocity and partial pressure of the vapor phase lubricant. This shows directly that as more boundary lubricant is applied, larger clusters of lubricant molecules become involved in the shear process thereby lowering the friction.

Charge Density Evolution Governing Interfacial Friction.

It is well-known that the electron nature of a solid in contact plays a predominant role in determining the many properties of the contact systems, but the general rules of electron coupling that govern interfacial friction remain an open issue for the surface/interface community. Here, density functional theory calculations were used to investigate the physical origins of friction of solid interfaces. It was found that interfacial friction can be inherently traced back to the electronic barrier to the change in the contact configuration of the joints in slip due to the resistance of energy level rearrangement leading to electron transfer, which applies for various interface types ranging from van der Waals, metallic, and ionic to covalent joints. The variation of the electron density accompanying contact conformation changes along the sliding pathways is defined to track the frictional energy dissipation process occurring in slip. The results demonstrate that the frictional energy landscapes evolve synchronously with responding charge density evolution along sliding pathways, yielding an explicitly linear dependence of frictional dissipation on electronic evolution. The correlation coefficient enables us to interpret the fundamental concept of shear strength. The present charge evolution model thereby provides insights into the classic hypothesis that the friction force scales with the real contact area. This may shed light on the intrinsic origin of friction at the electronic level, opening the way to the rational design of nanomechanical devices as well as the understanding of the natural faults.

A new algorithm for uncertainty quantification for thermal conductivity measurement on polymers with the Haakvoort method using differential scanning calorimetry considering specimen height and real contact area

A new algorithm for the quantification of uncertainty in thermal conductivity measurements on polymers according to the Haakvort method is presented. This fast and convenient method using differential scanning calorimetry has been established as DIN EN ISO Standard 11357–8 with an error margin of 5–10%, which is a rather large value when considering that this is an important material parameter for many applications and is often used in combined quantities, such as thermal diffusivity or thermal effusivity. Unfortunately, the DIN EN ISO standard does not provide useful information on the dependence of the error range on the number of specimens or important parameters, such as the height of the specimens or their real contact area. Applying a rigorous statistical approach, based on the law of large numbers (LLN) and different techniques which are also used in well-known methods, such as Monte-Carlo- or Markov chain Monte Carlo (MCMC) algorithms, we establish and investigate a method to optimize the experimental effort to a specific target, especially the number of specimens, the aspect ratio and the real contact surface of the specimen.

Enhanced adhesion friction behaviors of nature rubber composites by applications of carbon nanotube: Experiment and molecular insight

The tribological performance of the rubber composite and tire products is affected by adhesion behaviors. The sliding friction tests of the CNT/NR composites and related simulations were performed to reveal the influenced factors of the adhesion behavior. The results indicated that the sliding friction process of the CNT/NR composite can be divided into three stages. The COF of the composite was significantly affected by the real contact area which was mainly determined by the CNT-influenced tensile properties, interfacial properties, and the load condition. This work expands the multiscale understanding of adhesion behavior and its influenced mechanism.

Experimental study on the load-area relation of rough surfaces and comparison with theoretical model

It is of critical significance to accurately measure or predict the real contact areas between rough solids in engineering. In this study, based on the frustrated total internal reflection technique and an Otsu method, an optical apparatus is designed to capture the real contact areas between rough metallic samples and flat quartz glass. Moreover, an incremental equivalent contact model developed recently by our group is adopted to predict the load-area relationship. For samples of various materials and morphologies, it is found that the proposed contact model agrees well with the experimentally measured results, which verifies the efficiency of the incremental equivalent contact model.

Heat transfer in granular media consisting of particles in humid air at low confining pressure

I study the heat transfer in systems of weakly interacting particles at low confining pressure in the normal atmosphere. The particles have surface roughness with self-affine fractal properties, as expected for mineral particles produced by fracture, e.g., by crunching brittle materials in a mortar. I show that for small particles (say <10 mathrm{mu m}) with hydrophilic surfaces, at large humidity water capillary bridges dominate the heat transfer, while for big particles heat transfer via the air dominates. This differs from vacuum conditions where the propagating electromagnetic (EM) waves give the dominant heat transfer for large particles, while for small particles both the evanescent EM-waves and the phononic contribution from the area of real contact are most important.

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.

Time-Varying Wear Calculation Method for Fractal Rough Surfaces of Friction Pairs

For the wear problem of the real rough surface during sliding friction, based on fractal theory and Hertz contact theory, a 3-D fractal rough surface with random characteristics is constructed, and the relationship between the wear deformation depth of the rough peak and its real contact area during the wear process is derived. Furthermore, considering the peak wear and pit scratch phenomena of rough surfaces in different contact states, the time-varying wear calculation model of the worn surface and the compensation wear calculation model of the unworn surface are established, respectively, and the relationship between the instantaneous wear amount and the dynamic change in the rough surface topography is comprehensively characterized. Combined with image digitization technology, the 3-D rough surface is converted into a 2-D discrete plane with 3-D information. According to the dynamic real-time update of the graph data, the iterative calculation of the wear cycle is completed, the time-varying wear calculation method for fractal rough surfaces of friction pairs is proposed, and the dynamic change in the wear amount and surface topography of the rough surface is simulated. The simulation results are experimentally verified and the influence of friction parameters on the surface topography is analyzed. The results show that after the wear simulation, the profile height of the rough surface is reduced, and the average wear depth is 0.02 mm. Increases in rotational speeds and external loads can exacerbate surface wear, surface topography tends to be flattened, and surface carrying capacity increases. This provides theoretical guidance for the development and manufacture of friction pairs.

Estimating the real contact area in lubricated hot rolling of aluminium

Predictions of the real contact area in metal forming processes are essential for the calculation of friction and, consequently, the optimisation of energy consumption and quality of rolled products. This study develops of a model for estimating the real contact area in hot rolling of a 6061 aluminium alloy based on: thermo-viscoplastic material properties, measured roll topography, and hydrodynamic and hydrostatic roles of lubricant in the contact. Results show the interplay between load and contact area, as regions of solid contact, fluid channels and pockets are formed. Quantification in a rolling pass shows both solid contact and lubricant entrapment play the major roles, which is highly determined by the topography and the flow stress of the workpiece.

Friction Behavior between Carbon Fiber Plain Weave and Metal Semi-Cylinder Tool

The deformations that occur during composite forming processes are governed by the friction between the fabrics and tooling material on the mesoscopic level. The effect of normal load and multi-plies on the frictional behavior of the carbon plain weave is investigated by simulating the friction between the fabric and metal semi-cylinder tool by using the experimental method. The periodic wavy friction-displacement curve between the metal tool and fabric is caused by the interwoven structure of the fabric. Both the increase in the normal load and the number of layers cause an increase in the real contact area during friction, leading to an increase in the friction force. The real contact area is calculated based on the Hertzian contact model and the self-designed testing method. The friction force values obtained from multiplying the real contact area with shear strength are closely aligned with the measured results.

Investigation of the real contact area of tensile fractures with different normal stresses and sizes by using pressure-sensitive films

More than 240 experimental tests were conducted and the characteristics of real contact area of the mated tensile fractures were investigated by using pressure-sensitive films. The effects of normal stress, fracture size as well as rock type on the real contact area of mated tensile fractures were evaluated and discussed. The results show that the real contact area of the mated rock fractures has a parabolic increasing relationship with the normal stress ratio (the ratio of the applied normal stress on the fracture surface to the uniaxial compressive strength of the joint wall). Also, the size of the tensile fracture can affect its real contact area, and the larger the applied normal stress is, the more significant the size effect of the rock fracture on the real contact area is. Furthermore, lithology has a significant influence on the real contact area of the structural plane, which is also related to the applied normal stress. Under the same stress condition, the real contact areas of shale and granite fractures are the smallest, while that of green sandstone fractures is the largest. By analyzing the nominal normal stress and the real normal stress on rock fractures, it was found that even if the nominal normal stress applied on the rock fractures is only 1.5 MPa, the average real normal stresses can reach 20 MPa ∼ 45 MPa, which is about 15–30 times the nominal applied normal stress. The contact characteristics of tensile fractures were analyzed by adopting the cutting methods. It was revealed that within a mated tensile fracture, the smaller the JRC of an area is, the larger the real contact area will be. The results of this study are of great significance for revealing the characteristics and laws of the contact areas of tensile fractures, and the 248 sets of data specimens obtained from the tests can provide important support or reference for establishing mechanical models for rock mass contact.

A new test for sheet metal asperity flattening under varying subsurface strain conditions

Friction in sheet metal forming is largely determined by asperity flattening and the real contact area between sheet asperities and tool surface. This paper presents a new experimental test, which enables the investigation of asperity flattening under varying sub-surface strain ratios with or without the application of lubrication. The test is based on the Marciniak-Kuczynski test for inducing in-plane strains, while an asperity region in the centre is subjected to normal loading. The test was deployed to investigate model asperity flattening due to normal loading combined with controlled linear in-plane strain paths with and without the presence of liquid lubrication.

Rubber-ice friction

We study the friction when a rectangular tire tread rubber block is sliding on an ice surface at different temperatures ranging from −38 to −2 °C, and sliding speeds ranging from 3 µm/s to 1 cm/s. At low temperatures and low sliding speeds we propose that an important contribution to the friction force is due to slip between the ice surface and ice fragments attached to the rubber surface. At temperatures above −10 °C or for high enough sliding speeds, a thin premelted water film occurs on the ice surface and the contribution to the friction from shearing the area of real contact is small. In this case the dominant contribution to the friction force comes from viscoelastic deformations of the rubber by the ice asperities. We comment on the role of waxing on the friction between skis and snow (ice particles).

The generation mechanism of demolding force based on themold‐partinterface contact mode in micro‐injection molding

AbstractDemolding is the key step of micro‐injection molding (μIM), which influences the molding quality of polymer parts to a great extent. The essence of demolding is to overcome interface interactions between the mold cavity surface and the part surface, which is closely related to factors such as the cavity surface topography and process parameters. However, a theoretical model predicting the demolding force of μIM is still lacking, resulting in the difficulty of improving the quality of demolding in μIM. Therefore, a comprehensive demolding force model is proposed to clarify the demolding mechanism of μIM. The model focuses on the adhesion behavior and friction behavior in the special contact mode formed by the replication effect of μIM. The theory proof and computation results indicate that the cavity surface topography determines the contact mode of the mold‐part interface (MPI), which in turn changes the real interface contact area, the composition mode of adhesion stress, and the friction forms, thereby affecting the adhesion and friction force during demolding. In addition, the process parameters can also change the contact mode of MPI as the process parameters have a great effect on the filling capacity and the degree of replication, thereby influencing the demolding process.

Friction hysteretic behavior of supported atomically thin nanofilms

Hysteretic friction behavior has been observed on varied 2D nanofilms. However, no unanimous conclusion has yet been drawn on to the exact mechanism or relative contribution of each mechanism to the observed behavior. Here we report on hysteretic friction behavior of supported atomically thin nanofilms studied using atomic force microscopy (AFM) experiments and molecular dynamics (MD) simulations. Load dependent friction measurements were conducted on unheated and heated samples of graphene, h-BN, and MoS2 supported by silica substrates. Two diverging friction trends are reported: the unheated samples showed higher friction during unloading than during loading, and the heated samples showed a reversed hysteresis. Further, the friction force increased sub-linearly with normal force for heated samples, compared with unheated samples. Tapping mode AFM suggested that the interaction strength of the substrate was increased with heating. Roughened substrates in the MD simulations that mimicked strong/weak interaction forces reproduced the experimental observations and revealed that the evolution of real contact area in different interface interaction situation caused the diverging behaviors. Surface roughness and interaction strength were found to be the key parameters for controlling the out-of-plane deformation of atomically thin nanofilms.

Application of the Finite Element Method to Simulate the Friction Phenomenon in a Strip Drawing Test

Friction is an undesirable phenomenon in the flange area of the drawpiece in sheet metal forming processes, causing a deterioration in surface finish and a decrease in the formability limits of the sheet metal. The aim of this work is numerical analysis using the finite element method of the strip drawing test with two rounded countersamples. This test simulates friction conditions in the flange area of the drawpiece. The results of the experimental research on the influence of surface roughness on the value of the friction coefficient of S235 steel samples were used to verify the numerical results. The relation between the real contact area and the mean roughness Ra of the countersamples was determined. The real contact area increases with the increase of the mean roughness Ra. In sheet metal forming processes, the coefficient of friction depends on the real contact area, and its value increases with the increase of the real contact area.