Microscopic Asperities Research Articles

Asperity interaction induced vibration excitation of cylindrical roller bearings with localized defect

One of the difficulties in detecting localized faults of cylindrical roller bearings by vibration analysis is the lack of obvious early fault characteristics. To carry out the relationship between internal faults and external vibration signals is meaningful from the perspective of diagnosis. In this paper, a dynamic model of asperity interaction in the joint surface of cylindrical roller bearings is established. A new contact stiffness analysis model based on the microscopic asperity contact model is derived, which enhances the accuracy of the contact stiffness of the cylindrical roller bearing under heavy load condition. Furthermore, the vibration excitation caused by the impact and deformation of the asperity is derived. The effects of surface roughness and radial load on impact energy and vibration excitation are investigated by simulation. Experimental analysis has found that the vibration excitation of bearings is closely related to the impact energy caused by the asperity interaction behavior. The above results underscore the necessity of incorporating asperity contact considerations in the stiffness modeling of cylindrical roller bearings under heavy loads.

Asperity contact analysis: coupling effects of normal interference and nanoscale offset by molecular dynamics simulations

This work investigates the offset contact behaviors of nanosized asperity with spherical and non-spherical shapes, aiming to provide a more realistic analysis of asperity contact for rough surfaces. Molecular dynamics models of nanosized asperity are established, using diamond and copper as exemplary materials. Differences between offset and non-offset contacts, as well as spherical and non-spherical asperities are compared through atomic variations including displacement, shear strain and structure. Coupling effects of normal interference and nanoscale offset on asperity contact behaviors are further analyzed. Comparisons between the established models and the microscopic asperity models in contact force-interference dependence are conducted for verification. Results indicate that atoms in central contact region move vertically with smaller shear strains, while those in peripheral region primarily move diagonally outward with larger shear strains due to directional atomic slip motion. Furthermore, the spherical asperity shows higher atomic deformation and contact force compared to the non-spherical asperity for identical interference and offset conditions. Additionally, the shear resistance of single-crystal copper with {100} crystal orientation can be enhanced along the [1 1 0] and [1 − 1 0] directions.

Contacting Micro Asperity of a Deformable Surface

Abstract Deterministic contact modeling based on half-space theories has satisfied a wide range of applications. However, the half-space theories themselves do not involve shape effects of roughness on Green’s functions/influence coefficients; in deterministic rough-surface contact analyses, the roughness is considered in gap function. This approach can be called the “roughness simplification.” One needs to answer two questions about the validity of the roughness simplification: How appropriate is the roughness simplification in modeling rough-surface contacts? How accurately can the commonly included contact-plasticity behavior be captured under the roughness simplification? This work utilized a double-scale representation of an asperity—a microscopic deformable asperity stacked on a deformable half-space, to obtain their combined contact responses in both elastic and plastic regimes. The deformation and contact behaviors of asperities thus configured were obtained with finite element analysis (FEA) and rough-surface half-space contact solvers. Three stages of asperity contact were discovered: the Hertzian stage, the single-region elastoplastic stage, and the two-region elastoplastic stage where the surrounding base material also takes part in the contact. The comparisons of contact deformation and pressure results from both the finite element analysis and half-space contact solvers support the validity of the half-space theories with the roughness simplification for various ellipsoid-shape asperities with circular-bases in both elastic and elastoplastic rough-surface asperity modeling. The research also reveals that when significant plastic deformations occur, asperities with different aspect ratios can bear different maximum elastoplastic contact pressures.

Gardner-like crossover from variable to persistent force contacts in granular crystals.

We report experimental evidence of a Gardner-like crossover from variable to persistent force contacts in a two-dimensional bidisperse granular crystal by analyzing the variability of both particle positions and force networks formed under uniaxial compression. Starting from densities just above the freezing transition and for variable amounts of additional compression, we compare configurations to both their own initial state and to an ensemble of equivalent reinitialized states. This protocol shows that force contacts are largely undetermined when the density is below a Gardner-like crossover, after which they gradually transition to being persistent, being fully so only above the jamming point. We associate the disorder that underlies this effect with the size of the microscopic asperities of the photoelastic disks used, by analogy to other mechanisms that have been previously predicted theoretically.

Experimental and numerical investigation on multiscale hysteresis friction on artificial printed surfaces

In order to investigate hysteresis friction separately from other friction contributions (mainly adhesive friction), linear friction tests are performed with cut out tread specimens from passenger car tires on 3D-printed sinusoidal surfaces under dry and wet conditions. In this work, a multiscale approach based on the finite element method (FEM) is used to reproduce the hysteresis friction features obtained in the laboratory. On each scale, finite element simulations at block level are evaluated at different contact pressure, sliding velocity and temperature conditions, which are homogenized with respect to time, to build up a friction characterization for the next coarser scale. The height difference correlation function is applied to consider microscopic asperities, which originate from the printing process. Using the Williams-Landel-Ferry equation, the thermal influence on the viscoelastic rubber material is taken into account. By comparing the output of the friction tests against the results of the multiscale simulations, an analysis of contact temperature as well as friction contribution of each length scale is enabled.

Frictional behavior and micro-damage characteristics of rough granite fractures

Natural faults or discontinuities in general are usually rough at different scales, and the roughness of fault surfaces plays a dominant role in determining the mode of sliding (stable or unstable). Understanding the frictional behavior of rough fractures is important for investigating the mechanics and nucleation of earthquakes, and other dynamic geo-hazards such as induced seismicity and fault slip rockburst. We perform direct shear tests on rough granite fractures to investigate the shear behavior, microscopic asperity damage and surface wear characteristics under different normal stresses (10 to 40 MPa). We then compare our test results with those obtained using ground saw-cut fault surfaces. Results indicate that stick-slip occurs on all the rough fractures during the sliding stage, and the amount of stress drops and fault gouges tends to increase with normal stress. The peak friction coefficient is much higher than that of ground fault surfaces, while the steady-state friction coefficient is comparable with that of the ground surfaces. We identify three different scales of brittle fractures, which are considered to be closely related to the stick-slip sliding, namely shear-off of asperities, fracturing of the survived asperities and off-fault tensile fractures propagating into the host rock. The micro-damage of the post-shear surface occurs in the form of powder-sized gouge, micro-cracks and grain edge wear; and the micro-crack number, maximum length and aperture tend to increase with increasing normal stress. Influence of different fault types (rough fractures, ground saw-cut surfaces and gouges) on the stress drop is not very discernable, which is partly because that stick-slip is highly susceptible to different loading conditions. Friction drops for different fault types are found to be clustered around 0.1 but lower than 0.3 below 50 MPa normal stress, which also depend on the loading conditons (e.g., stiffness). Our study contributes to a better understanding of the mechanics underlying the dynamic geohazards (earthquakes, induced seismicity, fault slip rockburst) which are associated with the unstable shear failure on rough faults or fracture surfaces.

Exploring 3D elastic-wave scattering at interfaces using high-resolution phased-array system

The elastic-wave scattering at interfaces, such as cracks, is essential for nondestructive inspections, and hence, understanding the phenomenon is crucial. However, the elastic-wave scattering at cracks is very complex in three dimensions since microscopic asperities of crack faces can be multiple scattering sources. We propose a method for exploring 3D elastic-wave scattering based on our previously developed high-resolution 3D phased-array system, the piezoelectric and laser ultrasonic system (PLUS). We describe the principle of PLUS, which combines a piezoelectric transmitter and a 2D mechanical scan of a laser Doppler vibrometer, enabling us to resolve a crack into a collection of scattring sources. Subsequently, we show how the 3D elastic-wave scattering in the vicinity of each response can be extracted. Here, we experimentally applied PLUS to a fatigue-crack specimen. We found that diverse 3D elastic-wave scattering occurred in a manner depending on the responses within the fatigue crack. This is significant because access to such information will be useful for optimizing inspection conditions, designing ultrasonic measurement systems, and characterizing cracks. More importantly, the described methodology is very general and can be applied to not only metals but also other materials such as composites, concrete, and rocks, leading to progress in many fields.

A multiscale soft-contact modelling method for rough surfaces in contact with coupled slipping/sliding and rolling

This paper presents a multiscale soft-contact modelling (SCM) method for characterising the rough surfaces in contact with coupled slipping/sliding and rolling. The microscale flattening of the random surface asperities subjected to a plastically deformed substrate was accommodated by a statistical analysis with a dynamic explicit finite element analysis (FEA). A soft-contact algorithm was developed to effectively treat the cross-scale deformation of the microscopic asperities and that of the macroscopic body. An efficient modularised strategy including an input module and a hybrid SCM-FEA module was established to analyse the interaction at the interface of the two contacting bodies associated with coupled slipping/sliding and rolling. The modelling method was validated by a typical metal forming process, cold rolling of metal strips.

An improved meshing stiffness calculation algorithm for gear pair involving fractal contact stiffness based on dynamic contact force

The contact stiffness, which is under effects of topography at tooth meshing surface, is determined by the dynamic contact force from the holistic meshing behavior of gears. However, in previous literature, the comprehensive effect of contact force on contact stiffness is often replaced by meshing force, which results in the mismatch between contact stiffness and meshing force. To precisely model contact stiffness and meshing stiffness of gear pair, in this paper, fractal contact stiffness model is first established based on the length scale of microscopic asperities, by which the inherent nonlinear relationship between contact force and contact stiffness is revealed. Subsequently, the improved convergence algorithm based on static and dynamic deformation is applied to calculate the actual contact force. Then the contact stiffness and dynamic response of the system are solved. Based on the proposed method involving fractal contact stiffness, the influences of surface roughness are investigated. The calculation results are also compared with those of traditional method, which reveal that dynamic contact force is irreplaceable in the stiffness modeling.

Nonlinear characteristics of gear pair considering fractal surface dynamic contact as internal excitation

A novel gear fractal backlash model is established to better consider joint action of all microscopic asperities on gear dynamics, which has the dynamic form from gear center motion simultaneously in this paper. Based on the fractal surface dynamic contact as internal excitation, a gear dynamic model involving surface morphology and gear center motion is established, and a corresponding close-loop algorithm is proposed to solve system dynamics by combining mesh stiffness and time-varying pressure angle. Accordingly, the chaos of the gear system is analyzed by the bifurcation diagram, phase portraits, and Poincare mapping. The comprehensive and strong influence of dynamic fractal backlash on the nonlinear characteristics of the gear system is also demonstrated. The intense effects of tooth surface topography through fractal backlash are explained in detail. Comparison with experimental data is conducted to verify the superiority of the proposed model.

The effect of humidity on friction behavior of hydrogenated HIPIMS W-C:H coatings

The study of the effect of humidity on friction behavior in HiPIMS (High Power Impulse Magnetron Sputtering) W-C:H coating/steel ball system confirmed the formation of transfer layer adhered to the ball and consisting of hydrogenated carbon, ferritungstate and small amounts of tungsten (sub)oxides and iron oxides. The corresponding mechano(tribo)chemical reactions driven by flash temperature in the sliding asperities involved oxidation, water vapor dissociation and carbon hydrogenation. Such composition agreed with the results of modelling based on the minimization of free Gibbs energy in the mutually interacting thermochemical reactions. The applicability of modelling to friction was attributed to fast reactions among microscopic asperities eliminating kinetical factors. In inert atmospheres, oxidation and hydrogenation were suppressed and friction seemed to be controlled by the amount of additional carbon.

Transfer layer evolution during friction in HIPIMS W–C coatings

The investigation of the evolution of transfer layer in the contact area in W–C coating/steel ball system during interrupted friction tests in humid air revealed that fast oxidation occurs simultaneously with water vapor dissociation and carbon hydrogenation. These mechanochemical reactions produced a mixture of hydrogenated carbon, ferritungstate with minor amounts of single tungsten (sub-)oxides and iron oxides forming transfer layer preferentially adhered to steel ball. The steady coefficient of friction (COF) was attributed to the development of a thin continuous tribo-film with higher hardness and stiffness in the central zone of the wear track facing the thickest zone of transfer layer on the ball. Despite direct experimental confirmation is necessary, tribo-film was proposed to consist of ordered rehydrogenated carbon film. Friction would be controlled by shear interactions between transfer layer and thin tribo-film on top surface of W–C coating. The effect of humidity on COF can be explained by a control of oxidation, dissociation and hydrogenation reactions and subsequent influence on the composition of transfer layer and tribo-film in the wear track. The existence of phases identified in transfer layers during friction were confirmed via modelling of parallel and mutually interacting mechanochemical reactions based on the minimization of free Gibbs energy. The ability to apply equilibrium thermodynamical calculations to dynamical processes in friction contact was related to very fast reactions among microscopic asperities eliminating kinetical factors necessary for thermodynamical equilibrium at macroscale. Based on the experimental observations and modelling, three stage model of transfer layer evolution was proposed to explain run-in and steady periods on friction curves.

A novel multiscale model for contact behavior analysis of rough surfaces with the statistical approach

This paper presents a contact model to describe the normal and tangential contact behaviors of the rough interface with multiscale topographies. The coupling effect of macroscopic profile deviation and mesoscopic roughness on the contact behaviors is taken into account. First, an improved force expression of single contacting asperity is proposed to characterize the continuous and monotonic feature in the whole deformation process. By adopting the statistical summation, the contact model for the surface with microscopic topography is established. Second, the mesoscopic waviness deformation is characterized with microscopic asperities through multiscale modeling. The macroscopic geometric deviation is defined as a linear shape function. By combining the macro-, meso-- and microscopic topographies, the multiscale normal contact model is established. Then, in terms of tangential model, the Jenkins element is applied to express the slip-stick behavior of asperity. Based on the normal contact model and Coulomb law, the distribution function of the yield slip force is obtained to establish the tangential contact model. Furthermore, the predicted contact responses are compared with experiments and finite element model to validate the effectiveness of the proposed model. Finally, the influences of the multiscale topographies are analyzed, presenting the variation tendencies of contact responses. This work provides a feasible and effective way for studying the contact response of rough interface with multiscale surface topographies.

Laser ultrasonic monitoring of reversible/irreversible modification of a real crack under photothermal loading

We study the responses of laser-generated acoustic waves to localized reversible/irreversible modifications of microscopic asperities on crack surfaces during crack closure, which is an essential process in nonlinear photoacoustic/photothermal crack detection techniques. Our laser ultrasonics technique involves optical measurement of the transmission and mode conversion of the laser-generated surface acoustic waves caused by the crack. Reversible/irreversible modifications of asperities can be achieved via non-contact photothermal loading of the crack. Three photothermal loading cycles were realized in individual succession and were monitored using the laser ultrasonics technique at various experimental locations along the crack. In our experiments, each photothermal loading cycle includes multiple successive subcycles, in which the material is first heated and then cooled to its equilibrium temperature, thereby initiating local closing, followed by opening of the crack. Each subcycle is monitored twice using the laser ultrasonics technique, once each at the end of heating and cooling. Furthermore, each successive subcycle is accomplished at a higher power of heating laser than that of the previous subcycle. Significant differences in the peak-to-peak amplitude of the surface skimming longitudinal acoustic wave, which is excited by mode conversion of the Rayleigh wave by the crack, are revealed during the first cycle of photothermal loading. These differences clearly indicate a partial irreversibility of the mechanical processes occurring in the crack surfaces during subcycles of the first cycle. In a larger temporal scale, irreversible modification of crack surfaces is observed from the significant difference between the experimental results of the first photothermal loading cycle and the subsequent two cycles, whereas a reversible response of the crack surface to thermoelastic loading is observed from the similarity of the measurements accumulated during the two subsequent cycles.

Pressure and sliding velocity dependent surface asperity based friction model: Application to springback simulation

For accurate predictions of the formability and springback in the finite element sheet forming simulations, significant advances have been achieved in constitutive modeling that captures complex deformations occurring in the sheet forming process. However, friction behavior has been still modeled and implemented as a simple way though it is one of critical factors for accurate numerical simulations. For instance, sheet metal forming simulations often employ the Coulomb friction law with a constant friction coefficient, but it is known to be a function of other factors such as contact pressure, sliding rate and other surface quality. In this study, the microscopic surface asperity based friction model originally proposed by Westeneng [1], as one of microscale level friction models, was revisited by using additional contact assumptions. In particular, the model added the strain rate effect to the existing contact pressure dependent model in order to include the effect of both contact pressure and sliding velocity during sheet forming process. The calculated contact friction coefficient was compared to that by the measured for dual-phase 780 steel sheet under different pressure and sliding velocity. Moreover, the predicted friction model was employed in the simulation of U-draw bending springback by using a user friction subroutine, which was evaluated by the comparison of experimentally measured springback profile

Formation of Carbon-Nanotube Layers on Bulk Nickel: The Effect of Surface Topography Defects

The initial stages of the chemical vapor deposition of carbon onto nickel substrates subjected to different mechanical abrasion treatments are studied by optical, scanning electron, and atomic force microscopies, as well as X-ray diffraction and profilometry. We find that the local accelerated deposition of carbon layers with a highly disordered structure, which occurs at the initial stages of the process, is the underlying cause of nonuniformity in carbon nanotube layers synthesized directly onto a metal surface. Enhanced deposition rates are observed at surface topography defects, including microcavities with a large surface-to-volume ratio. Open microvoids in the subsurface layer left behind by mechanical abrasion treatment also act as foci of the accelerated formation of carbon layers. The cause of this enhanced growth lies in the specific chemistry of the extended pyrolysis of hydrocarbons trapped in microvoids. A high carbon deposition rate results in the formation of carbon-containing materials with a highly disordered structure within such defects. Microscopic asperities and microprotrusions formed as a result of mechanical abrasion are sites of intensive hydrocarbon-induced dusting of the metal. Numerous microslits originate from these defects due to corrosion scattering, which in turn facilitate the emergence of sites for accelerated formation in limited volumes of carbon with a disordered structure.

Passive Spine Gripper for Free-Climbing Robot in Extreme Terrain

In this letter, we present a passive spine gripper that can hold rough rocky surfaces of boulders on cliff walls, and we discuss its application to a four-limbed robot for free-climbing in extreme terrain. The limbed robot has four degrees of freedom in each limb, where three are to drive joints of the limb and one for releasing the gripper. The fine spine of the proposed gripper also enables it to passively and adaptively latch on to microscopic asperities of the rough surface, and it is thus an efficient mechanism. In this letter, we present the fundamental design and mechanism of the proposed gripper, after which we introduce its static gripping model. We verify the gripping model by the experimental gripping performance of the prototype gripper. We show a lightweight four-limbed robot that is equipped with the grippers mounted on each limb. To evaluate the climbing capabilities of the robot, we use it to perform climbing experiments on a rugged and steep slope. The results show that the prototype can safely climb over such challenging terrain that is similar to a gravity offload system.

Static and sliding contact of rough surfaces: Effect of asperity-scale properties and long-range elastic interactions

Friction in static and sliding contact of rough surfaces is important in numerous physical phenomena. We seek to understand macroscopically observed static and sliding contact behavior as the collective response of a large number of microscopic asperities. To that end, we build on Hulikal et al. (2015) and develop an efficient numerical framework that can be used to investigate how the macroscopic response of multiple frictional contacts depends on long-range elastic interactions, different constitutive assumptions about the deforming contacts and their local shear resistance, and surface roughness. We approximate the contact between two rough surfaces as that between a regular array of discrete deformable elements attached to a elastic block and a rigid rough surface. The deformable elements are viscoelastic or elasto/viscoplastic with a range of relaxation times, and the elastic interaction between contacts is long-range. We find that the model reproduces the main macroscopic features of evolution of contact and friction for a range of constitutive models of the elements, suggesting that macroscopic frictional response is robust with respect to the microscopic behavior. Viscoelasticity/viscoplasticity contributes to the increase of friction with contact time and leads to a subtle history dependence. Interestingly, long-range elastic interactions only change the results quantitatively compared to the meanfield response. The developed numerical framework can be used to study how specific observed macroscopic behavior depends on the microscale assumptions. For example, we find that sustained increase in the static friction coefficient during long hold times suggests viscoelastic response of the underlying material with multiple relaxation time scales. We also find that the experimentally observed proportionality of the direct effect in velocity jump experiments to the logarithm of the velocity jump points to a complex material-dependent shear resistance at the microscale.

Effect of micropillar surface texturing on friction under elastic dry reciprocating contact

Surface texturing is considered to be a promising method to improve the tribological properties. Depending upon the experimental conditions, the effect of texturing varies from favourable to unnoticeable to detrimental. In this work, surfaces with micropillars are studied under elastic dry reciprocating contact. An array of micropillars with different pillar heights are generated on stainless steel using wire-cut electrical discharge machining. The effect of stiffness of the micropillars on friction is investigated, keeping the number of micropillars in contact with a flat aluminium alloy (Al6061) slider and contact geometry constant. Reciprocating experiments are carried out against a flat surface such that about 81 micropillars are in contact. From the experimental results, it is found that the coefficient of friction is independent of the stiffness of the texture elements. However, work done per cycle significantly varied with the stiffness of texture element and applied normal load. A lumped system model with Coulomb friction shows that the work done per cycle varies quadratically with the normal load. The experimental results agree with this simplified model except in the incipient sliding regime. These results show how the work done per cycle varies, for different contact stiffness under elastic contact even though the coefficient of friction remains constant. The implication of this study for a macroscopic measured coefficient of friction as a function of microscopic asperity level friction is discussed.

Investigation of the pad-conditioning performance deterioration in the chemical mechanical polishing process

The surface roughness of a chemical mechanical polishing (CMP) pad provides microscopic asperities where the actual polishing takes place. Thus, the roughness should be maintained in a proper regimen to obtain stable CMP process quality. A conditioner is a key consumable in the CMP process because it maintains the roughness of the CMP pad surface against surface glazing or flattening during polishing. Because the conditioner mechanically breaks the pad surface to maintain the roughness, previous research has expressed its ability index in terms of the pad wear rate (PWR). In the present study, we investigate the reason for PWR change of CMP pads having vigorously changing moduli with respect to temperature changes during the mass production. The pad property changed steeply in the process temperature range from 60 to 80°C. The tensile strength is a key property change that leads to sudden PWR drop. We found that the shape and configuration of the diamond tip are the major parameters affecting the ability to maintain surface roughness of the pads. Thus, we suggested a proper conditioner design for tungsten (W) CMP based on the experimental data to sustain the process quality and increase the consumable lifetime. As a result, the peak surface roughness has improved from 29.4 μm to 32.6µm and the consumable lifetime was also increased.