Asperity Interaction Research Articles

Predicting resistance of bearing steels to surface initiated fatigue: Application to hybrid contact

The constant demand for improved energy efficiency in lubricated rolling/ sliding contacts, such as in rolling bearings, is generally addressed through increased power density and the use of low-viscosity (i.e., thin-film) lubricants. In these conditions, mixed lubrication regime can dominate with intensive asperity interactions between two contacting surfaces, thus increasing the risk of surface-initiated fatigue (e.g., in the form of micropitting). In the present study, the influence of steel mechanical properties on the micropitting accumulation in a hybrid contact (steel versus Si3N4 ceramics of different roughness) has been experimentally and theoretically investigated. It was found that higher roughness of Si3N4 surface could increase the risk of micropitting on the steel component. However, despite formation of some mild micropitting, the bearing steels tested here show good performance. For better understanding of the relationship between the mechanical properties of bearing steels and micropitting resistance, a new theoretical framework is introduced. The new micropitting model, based on the finding that the nucleation of a micropit is driven by (i) local accumulation of the plastic strain in mixed rough contact, and (ii) reduction of the total energy by debonding of the plastically deformed zone is introduced. The new micropitting model was shown to match with experiments across the range of test conditions. Micropitting performance maps based on the mechanical properties of steels (yield strength, work hardening and fracture toughness) have been obtained. The new theory has implications not only for predicting micropitting but also for predicting adhesive wear resistance of different steels.

Evaluation of surface texturing on chrome-coated cylinder liners via deterministic mixed lubrication simulation

This study investigates the mixed lubrication performance of various surface texture configurations in the piston ring/cylinder liner conjunction of a two-stroke internal combustion engine using a deterministic mixed lubrication model. The numerical model simultaneously solves the Reynolds equation with mass-conserving cavitation to calculate inter-asperity hydrodynamic pressures and an elastic, perfectly plastic, rough contact model to determine contact pressures at each asperity interaction. Gaussian Mixture Model clustering was employed to enhance surface characterization. The deterministic simulation approach considers the full-scale representation of the cylinder liner topography to accurately capture the influence of surface features on the hydrodynamic support and friction under mixed lubrication conditions. The investigated cylinder liners were initially hard-chrome-coated and honed, resulting in a stochastic arrangement of surface pores, and then deterministic patterns of surface pockets were created by micro electrodischarge machining (EDM). Surface measurements were performed using laser interferometry, providing input for the mixed lubrication simulations. The study also explored the virtual removal of ridges formed around the pockets by the EDM technique. Key findings indicate that the stochastic texture outperformed the hybrid texture (stochastic + deterministic) in the boundary and mixed lubrication regimes, showing higher hydrodynamic support at low separations but increased hydrodynamic shear stresses at higher speeds. Conversely, deterministic textures exhibited a significant decrease in average hydrodynamic shear stress at high velocities. These results highlight the critical role of surface texture in tribological behavior and suggest that localized textures on cylinder liners can potentially optimize engine performance. The study recommends further exploration of a broader range of texture geometries, densities, and distribution patterns to enhance engine design strategies.

In-situ measurements of contact evolution for fractal rough surfaces under normal compression

The contact interface plays a key role in the overall functionality and stability of structures. Understanding the evolution of the contact interface over time and its dependency on materials and load is crucial for functional integrity and operational safety assessment. In this study, we employ in-situ three-dimensional X-ray computed tomography (3DXRCT) to examine the creep behavior of 3D-printed surfaces exhibiting various roughness under constant normal compression. We observe that the overall contact area enlargement during the contact creep decreases with roughness amplitude and fractal dimension. The variation of interfacial separation distance is found to increase with roughness amplitude and decrease with fractal dimension. Correlation analysis reveals that the microcontact size played a more important role than the asperity shape in determining the microcontact enlargement. By examining the calculated interfacial strains extracted from XRCT measurements, significant deformations are found to occur at the non-contacting zones, indicating strong asperity interactions. This study offers high-resolution experimental measurements and unravels the asperity micromechanics for contact creep on rough surfaces, providing insights into understanding and optimizing the performance of rough interfaces.

Effects of inclination angle and confining pressure on triaxial unloading-induced slip behaviors of shale fractures

To explore the effects of fracture inclination angle θ and confining pressure σ3t on the slip behaviors and friction properties of fractures, the triaxial unloading-induced slip experiments were performed on the shale fractures. The results show that the σ3t controls the slip modes of fractures, while the θ affects the occurrence of the stick-slip events during the quasi-static slip stage. With the increase in σ3t, the main slip modes of fracture transform from the stable-slip to stick-slip, and eventually to the creep-slip. The increase in θ facilitated the occurrence of stick-slip events. As the θ increased from 30° to 50°, the number of stick-slip events increased from 0 to 3 and from 2 to 4 for σ3t = 10 MPa and 20 MPa, respectively. For σ3t = 40 MPa, no stick-slip event occurred in the slipping process. The θ and σ3t have great effects on interaction modes between asperities, which directly affected the friction properties of fractures. With increasing σ3t, the void spaces between the asperities were further compacted, resulting in the transition of asperity interaction from overriding mode to shear-off mode. The transition of asperity interaction model therefore brought about the weakening of friction coefficient at the activation point and the onset of dynamic slip stage. There is a competitive relationship between the θ and σ3t for the evolution of the friction properties of fractures. As the θ increase from 30° to 50°, for σ3t = 10 MPa, the mean sheared-off thickness decreased from 0.502 mm to 0.433 mm, while for σ3t = 40 MPa, the mean sheared-off thickness decreased from 0.505 mm to 0.319 mm. With the increment of θ, the anisotropy of joint roughness coefficient was weakened. We suggested that by adjusting the fracturing angle of hydro-fracturing, the earthquakes with large seismic moments may be effectively mitigated.

A lubrication contact stiffness model considering asperity interactions

In practical engineering applications, most connection interfaces require lubrication. The contact stiffness of the lubrication interface can affect the static and dynamic characteristics of the interface, thus affecting the performance of the mechanical system, such as vibration, fatigue, and wear characteristics. Therefore, the measurement of lubrication contact stiffness is very important for the analysis and optimization of mechanical system performance. However, measuring the stiffness of the lubrication interface is a significant challenge. In this paper, a mixed lubrication contact stiffness model considering asperity interactions is proposed, which assumes the lubrication rough interface as an equivalent thin layer. The proposed model was compared with the lubrication contact model that does not consider the asperity interactions. Based on the microscopic contact behavior of the contact interface, the relationship between the mechanical parameters of the virtual thin layer and the contact pressure was analyzed. Finally, the predicted results of interface stiffness are verified by published theoretical and experimental results. The results show that the proposed model can more reasonably describe the actual contact behavior of the rough lubrication interface and give the variation law of the mechanical parameters of the virtual thin layer.

Normal Contact Model of Fractal Surfaces Considering Friction and Asperity Interactions

Friction as well as asperity interactions have significant influence on the contact area and force of rough surfaces. In this work, a single-asperity, elastoplastic contact model, considering friction, is established by means of continuum mechanics and power-exponential functions. Then, the deformation of the asperity interactions is modeled by the displacement of the average line for asperity height. Using fractal theory, the actual deformation and contact stiffness of an asperity are derived, which take the friction and asperity interactions into account. Furthermore, the actual contact area and normal stiffness models for fractal surfaces are developed, accounting for friction and asperity interactions. The accuracy of the developed models is confirmed by using published experimental results. The developed models are closer to experimental results than the reported fractal contact models. Finally, the effects of the friction coefficient and surface morphology parameters on contact characteristics are discussed.

Lateral force controlled impact tribometer characterizing transient friction: Effect of contact pressure

A new Lateral Force Impact Tribometer (LFIT) is designed to quantify the dynamic friction of synthetic seat belt and D Ring Polymer (DRP) under transient conditions during the initial sliding period. LFIT simulates the sliding between two bodies subjected to a sudden lateral force applied, resulting in relative motion. Transient friction under suddenly applied initial acceleration conditions is obtained from the displacement response. LFIT overcomes stiffness concerns of most displacement-controlled tribometers in which the sensors and actuators influence the contact stiffness, nature of asperity interaction, and frictional characteristics. The displacement vs. time response showed the variation in displacement during the initial 0.3 mm sliding distance. LFIT is modelled as a spring-mass-Coulomb damper system applied for transient analysis. The effect of contact pressure on the transient Coefficient of Friction (CoF) is studied in the present work. This paper perhaps presents the first report that enables the capture of the friction transition within 10 ms from the start of sliding.

Simplified Formulation for Bush, Gibson, and Thomas Model

Abstract The random process model, also known as the multi-asperity contact model or statistical model, is one of the dominant methodologies for analyzing the contact between nominally flat rough surfaces. In 1975, Bush, Gibson, and Thomas developed a complete random process model (known as the BGT model) assuming that solid–solid contacts occur on the summits of the asperities, which are semi-ellipsoids uniquely described by three random variables (the asperity height and two principal peak curvatures). The Hertzian elliptic contact theory states that the contact area and normal load are implicit functions of the random variables, which results in an original BGT model with a complex formulation. In this study, we used an adapted Hertzian elliptic contact theory to simplify the formulation of the BGT model. The relative contact area to normal load relation predicted by the simplified BGT model perfectly agrees with that of the original formulation. It is anticipated that rough surface contact models with complex asperity interactions can be effectively built under the framework of this simplified BGT model.

Analysis of the discrete contact characteristics based on the Greenwood-Williamson model and the localization principle

The contact of a rigid body with nominally flat rough surface and an elastic half-space is considered. To solve the contact problem, the Greenwood-Williamson statistical model and the localization principle are used. The developed contact model allows us to investigate the surface approach and the real contact area with taking into account the asperities interaction. It is shown that the mutual influence of asperities changes not only contact characteristics at the macroscale, but also the contact pressure distribution at the microscale. As follows from the results, the inclusion in the contact model of the effect of the mutual influence of asperities is especially significant for studying the real contact area, as well as the contact characteristics at high applied loads. The results calculated according to the proposed approach are in a good agreement with the experimentally observed effects, i.e., the real contact area saturation and the additional compliance exhaustion.

A Fractal Prediction Method for Contact Stiffness of Helical Gear Considering Asperity Lateral Contact and Interaction

The normal contact stiffness (NCS) on rough surfaces has a significant impact on the dynamic characteristics of helical gear. Aiming at the problem of inaccurate calculation of the NCS model under the traditional Hertz theory of smooth surfaces, a fractal prediction model of helical gear contact stiffness considering asperity lateral contact and interaction between asperities is proposed in this paper. The variation formula of asperity and the correction coefficient of a tooth contact surface under asperity lateral contact and interaction are derived, and the influence of micro-elements on normal load and NCS is qualitatively analyzed. The results show that the NCS of considering the interaction and lateral contact of asperity is closer to the experimental results; the contact surface correction coefficient increases with the increase of curvature radius and load. The NCS of a tooth surface increases with the increase in fractal dimension D or the decrease in roughness amplitude G. The influence of asperity lateral contact and interaction decreases with the increase in D and the decrease in G. The NCS of the helical gear decreases under the lateral contact and interaction of the asperity, which is critical for exact estimation of the NCS of contact surfaces in gear.

Automatic Measurement of Contact Stress and Connection Force of Interference Components by Ultrasound

Tiny interference components play an important role in the aerospace and instrumentation fields. Contact stress and connection force are important factors in evaluating the reliability and service life of components. In this study, the ultrasonic method is used to measure the stress distribution and the connection force on the mating interface. Based on statistical microcontact theory, the virtual thin-layer model, and the asperity interaction model, the contact acoustic model is optimized. On this basis, the mapping relationship between contact stress and contact stiffness can be further obtained. This relationship was obtained by calibration experiments. Then, the stress distribution on the interface with two different structures, namely the uniform cylinder and the step cylinder, was measured and compared with the theoretical results of the thick-walled cylinder theory. The results show that the two have good consistency. Finally, based on the stress distribution, the connection force was calculated and compared with the experimental results. The results show that the ultrasonic measurement method can accurately measure the connection force, and the relative error is within 16%. Therefore, this study can provide a good method for the quality assessment of interference components.

A rough surface electrical contact model considering the interaction between asperities

A rough surface electrical contact model considering the asperity interaction is developed to study the electrical contact behavior. The rough surface is generated by using a two-dimensional Weierstrass-Mandelbrot function. First, the mechanical contact area of the rough surface is determined by the boundary element method in the absence of the electric potential. The electrical contact area is then calculated by a fitting function based on the mechanical contact area. The fitting data is provided by the single-asperity electrical contact model developed in our previous study. By using the electrical contact area of each asperity, the contact force, electrical contact resistance, electric current, and temperature rise on the asperities are calculated. Finally, the electrical contact behavior on the rough surface is thus determined by summarizing the contributions of each contact spot. The finite element simulation of electrical contact of rough surfaces is also conducted to validate the proposed model. The proposed model is used to investigate the effects of the electric potential, fractal surface parameters, and material parameters on the electrical contact behavior. The results illustrate that the developed model can accurately predict the multi-physics behavior of electrical contact.

A Modified Model for Identifying the Characteristic Parameters of Machine Joint Interfaces

The contact parameters of joint interfaces have a great influence on the dynamic performance of the machine tool, and the dynamic performance determines the machining precision of the machine tool. Therefore, it is important to accurately identify the contact parameters of joint interfaces to improve the machining precision of the machine tool. In this paper, a method of identifying the joint interface stiffness parameters based on a joint simulation in Matlab and Ansys is first proposed, and then stiffness parameters of joint interfaces between the bed and column of the precision grinding machine are identified by applying the method. Finally, the feasibility and high efficiency of the proposed method are proved by comparing the natural frequencies of the simulation and the experiment. In order to further analyze the influence of joint contact parameters on the performance of the joint interfaces, the theoretical model of the normal stiffness of the joint interfaces based on fractal geometry is amended and verified by taking into account the asperity interaction and elastic–plastic deformation. Then, the modified model is extended to a three-dimensional model, which is more in line with the actual situation. Finally, the influencing factors of the joint interface stiffness are analyzed. The conclusions of this paper are of great significance in the dynamic performance analysis and structural design of machine tools.

A meticulous friction torque model for a lubricated ball screw considering the surface roughness

This study proposes a meticulous friction torque model considering both the asperity interaction and fluid traction mechanisms in micro-scale physical analysis. The fractal theory is employed to characterize the surface roughness in the asperity interaction component. The modification factor is derived to describe the actual contact condition precisely. A dedicated friction coefficient is obtained by fitting experimental data, enabling an accurate characterization of the friction exhibited by the ball screw. The fluid traction component involves the formulation of the modified oil film thickness, achieved by subtracting a modification factor from 1. This model is validated through the experimental data and is more suitable for light to moderate working conditions, contributing to a better understanding and prediction of friction torque.

Lubricating characteristics and sealing performance of mixed TEHD analysis of piston-cylinder interface in the piston pump

PurposeIn high-pressure pumps, due to the interaction of asperities on the upper and lower surfaces, the piston–cylinder interface suffers severe lubrication and sealing problems during mixed lubrication. This study aims to establish a mixed thermo-elastohydrodynamic (EHD) model for the lubrication gap to determine how working conditions affect the lubricating characteristics and sealing performance of the interface.Design/methodology/approachA mixed thermo-EHD lubrication model is established to investigate the lubricating characteristics and sealing performance of the interface between the piston and cylinder. The model considers piston tilting, thermal effect, surface roughness and bushing deformation. The interface lubricating characteristics and sealing performance under different working conditions are calculated by the proposed numerical model.FindingsA higher inlet pressure contributes to an increase in the minimum film thickness. Increased shaft speed can significantly reduce the minimum film thickness, resulting in severe wear. Compared to roughness, the impact of the thermal effect on the interface sealing performance is more significant.Originality/valueThe proposed lubrication model in this study offers a theoretical framework to evaluate the lubricating characteristics and sealing performance at the lubrication gap. Furthermore, the results provide references for properly selecting piston-cylinder surface processing parameters.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-03-2023-0072/

An improved elastic–plastic contact model with asperity interactions based on Greenwood–Williamson theory

An improved elastic–plastic contact model with asperity interactions based on Greenwood–Williamson theory

Improved anti-adhesive wear performance of rail/armature pair via interfacial energy modulation for electromagnetic launching applications

The prevention of adhesive wear between rail (made of high-strength Cu alloy) and armature (made of 7075 Al alloy) in the electromagnetic launching system is challenging due to the strong adhesion forces that arise from the interaction of asperities. Here, we propose a novel approach to enhance the anti-adhesive wear performance of rail through interfacial energy modulation between rail/armature pair for electromagnetic launching applications. Our findings indicate that the interfacial energy of the rail/armature pair can be significantly reduced by magnetron sputtering a W thin film onto the Al alloy surface. A lower coefficient of friction and a substantial alleviation in adhesion and mechanical damage of the Cu alloy are thus obtained. Molecular dynamics simulations reveal that the immiscibility and low interfacial energy of the Cu/W pair result in the formation of an amorphous interfacial layer during friction, facilitating countermotion at the contacting surfaces, thus in turn enables the low friction. The concept proposed in this work is further validated by real electromagnetic launching tests.

Numerical Simulation of the Lubricant-Solid Interface Using the Multigrid Method

Solid asperity interactions are common and inevitable under severe loading conditions for any lubricated contact. Heavy-duty machine components (gears, bearings, etc.) generally operate under Mixed Lubrication (ML), where uneven surface features contact each other when the generated fluid pressure is not enough to support the external load. The Reynolds equation is commonly used to simulate smooth lubricated contacts numerically. In rough lubricated interfaces where opposite surface asperities make contact, the Reynolds equation alone cannot accurately predict pressure using the traditional numerical simulation method. In this paper, lubrication–contact interface conditions (LCICs) have been implemented and extended to solve the multiple asperity contact problem using the full-multigrid approach. The developed novel algorithm has significantly accelerated the solution process and improved the accuracy and efficiency of pressure calculation for fluid–solid sub-interactions that can occur in ML regions. The results of the finite difference method (FDM) results have been compared with those of computational fluid dynamics (CFD) simulation to validate the newly developed model. Hence, the proposed optimized solution method will provide valuable insight to researchers and industry engineers interested in simulating the ML problem where the effect of the fluid–solid interface can be captured effectively to improve reliability in the calculation of the life expectancy of the lubricated parts.

Influence of dumbbell base oil blends on micropitting

Micropitting is a type of surface fatigue that occurs in rolling-sliding contacts. It results from stress fluctuations caused by surface asperity interactions. Both the number and severity of asperity interactions increase at low lambda ratios, so it is becoming a significant challenge to develop low viscosity lubricants for gears and rolling bearings that mitigate micropitting effectively. In this paper, the influence on micropitting of dumbbell blends of PAOs and PIBs with an equivalent kinematic viscosity at test temperature is studied. The result shows that appropriate blends of PAOs and PIBs are able to provide thick oil film thicknesses and reduce micropitting while maintaining low viscosity. The insights here can help with the design of lubricants that are effective in controlling micropitting.

Numerical investigation of progressive damage and associated seismicity on a laboratory fault

Understanding rock shear failure behavior is crucial to gain insights into slip-related geohazards such as rock avalanches, landslides, and earthquakes. However, descriptions of the progressive damage on the shear surface are still incomplete or ambiguous. In this study, we use the hybrid finite-discrete element method (FDEM) to simulate a shear experiment and obtain a detailed comprehension of shear induced progressive damage and the associated seismic activity. We built a laboratory fault model from high resolution surface scans and micro-CT imaging. Our results show that under quasi-static shear loading, the fault surface experiences local dynamic seismic activities. We found that the seismic activity is related to the stress concentration on interlocking asperities. This interlocking behavior (i) causes stress concentration at the region of contact that could reach the compressive strength, and (ii) produces tensile stress up to the tensile strength in the region adjacent to the contact area. Thus, different failure mechanisms and damage patterns including crushing and sub-vertical fracturing are observed on the rough surface. Asperity failure creates rapid local slips resulting in significant stress perturbations that alter the overall stress condition and may trigger the slip of adjacent critically stressed asperities. We found that the spatial distribution of the damaged asperities and the seismic activity is highly heterogeneous; regions with intense asperity interactions formed gouge material, while others exhibit minimal to no damage. These results emphasize the important role of surface roughness in controlling the overall shear behavior and the local dynamic seismic activities on faults.