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Graphical Abstract Static Friction at Fractal Interfaces Tribology International 2016, vol. 93, 229-238
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... G. Huang et al. meanwhile, greatly influences the natural frequency and frictional response of the whole machine [7]. It is therefore essential to precisely model the features of rough surface and accurately calculate the tangential contact stiffness for better characterization of mechanical systems [8][9][10]. ...
The tangential contact stiffness of joint interface is investigated in this work, using both theoretical and experimental methods. Firstly, the contact characteristics of a single asperity are analyzed on the basis of the fractal theory. Second, the total normal load and tangential stiffness of multiple asperities on the contact area are obtained by the integral over the entire surfaces. Besides, the effects of influential factors, including fractal dimension, fractal roughness, material property, friction coefficient, and load ratio, are comprehensively evaluated and discussed. Furthermore, in order to verify the effectiveness of the proposed model, two groups of test specimens are machined from different material types (45#steel and 20CrMnTi), and their actual surface profiles are measured. The structure function method is adopted to determine each profile’s fractal parameters. Under different normal load conditions, the experimental results of tangential contact stiffness on the rough surface between the upper and lower specimen are processed and obtained. By comparing the numerical results with experimental data, it is concluded that the proposed model has higher prediction accuracy on tangential contact stiffness than the comparison model. The establishment of the model not only provides a new theoretical supplement to the contact mechanics research, but also lays the foundation for the subsequent dynamic analysis of interfacial behavior on the combined structures.
... The coefficient of static friction depends on the combined effects of material deformation characteristics and surface roughness, both of which originate from the chemical bonding between the atoms of the materials between their surfaces and any adsorbed material. The fractality of surfaces, a parameter that describes the integration of structural and molecular interactions across a range of scales of surface roughness, plays an important role in determining the magnitude of static friction [5]. The amount of friction generated depends primarily on the materials which are in sliding contact, the coefficient of friction (μ) is a dimensionless quantity that describes the ratio of the force of the friction between two bodies and the force of them pressing together. ...
Establishing a standard measurement for drilling and screwing bone implants in different amounts and qualities of bone tissue, in a simple and adequate way to control and predict results, is the gold standard for successful primary stability and better results on long-term osseointegration. So far, the maximum insertion torque (MIT) has been used as the main parameter to achieve success in primary stability and osseointegration, although it has shown conflicting results in the literature for over four decades when predicting standard or minimum values. Basically, the surgeon's experience guides the planning and execution of the surgical procedure, adapted in each case according to his tactile experience, guided by X-ray analysis and the bone and general conditions of the patient. In this work, using a new biomechanical simple machine as a dental implant, a new method will be described mathematically and experimentally, which standardizes the compression and torque in the implant-bone contact, in five different bone densities, during the achievement of mechanical primary stability. The results described the relationship between the MIT, maximum removal torque, and maximum force of static friction between implant-bone and bone-to-bone, achieving a controlled and predictable standard steady-state torque that maintains equilibrium in elastic stress for the primary stability of bone implants, hereby established for an innovative simple machine Bioactive Kinetic Screw.
... Wang et al. [20] established a three-dimensional sliding friction pair contact model based on fractal theory and studied the effects of load, rotational speed, and roughness on tribological properties. Dorian et al. [21] studied the interaction between friction surface structure and fractal parameters, and found that there is more excellent wear on the surface of the higher fractals. Agile et al. [22] constructed a fractal prediction model of the static friction coefficient of flange joint surface based on fractal theory, and verified the accuracy of the prediction model by friction test. ...
The hydrostatic bearing slipper pair of the hydraulic pump has a unique adaptive friction characteristic, which has a better friction reduction and anti-wear ability than the general sliding friction pair, and also has a certain recovery effect on the performance degradation caused by the early wear of the slipper. This paper attempts to reveal the friction adaptive mechanism. Based on the fractal theory, two fractal parameters of fractal dimension and scale coefficient are used to characterize the surface morphology of the slipper mathematically, and the adaptive friction mechanism model is established by combining the friction coefficient equation. The effects of different fractal parameters on the friction coefficient and other performance parameters of slipper pairs are obtained by means of the numerical analysis method. The wear test was carried out by replacing specimens at different intervals to observe the worn surface morphology and the degradation process of the slipper to verify the correctness of the theoretical results. The results show that the friction performance and load-bearing capabilities of the slipper can be recovered to a certain extent within a short period when early wear occurs, and its surface performance shows the variation characteristics of deterioration-repair-re-deterioration-re-repair.
... Friction and wear processes occupy a special place among the factors afecting the reliability of friction pairs. Te evaluation of these processes is complicated since the kinetics of their fow depend on the specifc pressure between the interacting surface layers, microstructural changes of the surface layer, and chemical reactions between the materials of the friction pairs [1][2][3]. ...
Self-organization mechanisms of metastable dissipative structures during friction depending on base and oil functional additives for hypoid gears are considered. Research was conducted on a software-hardware complex with simulation of gears’ operation in rolling with slipping conditions in start-stop mode. Indicators of formation of wear-resistant dissipative structures include the following: improvement of antifriction characteristics, lubricant boundary layers’ formation, contact surfaces’ strengthening, and formation of heterogeneous deformation microrelief with a fine-grained structure. The formation of chemically modified boundary layers on 90% of the contact area of tribo-coupling elements ensures an increase in the wear resistance of leading and lagging surfaces by 2 and 1.4 times, respectively. The sclerometry method was used to establish that the formation of dissipative structures when lubricating tribo-coupling elements with various transmission oils can reduce deformation processes in metal near-surface layers by 23%. Highly viscous flavored lubricant with distillate oil and additive composition ensures wear-resistant dissipative structures with active components, including oxygen, sulfur, and phosphorus.
... A fractal dimension is a term used in the mathematics field of geometry to provide a logical statistical index of a pattern's level of complexity ( [5]). In order to identify relationships between surface structure and scaling behavior and performance, such as frictional behavior, fractal dimensions are being used more and more in recent years ( [6]). Although the mechanisms of gas adsorption, desorption, diffusion and other physical and chemical processes in coal seams are different, they have one thing in common, that is, the surface irregularity of coal samples has an important impact on these mechanisms ( [7,8]). ...
... Compared with the modified coal samples of the six different surfactants, the coal samples modified by SDBS and rhamnolipid had the strongest gas desorption abilities. (6) In the context of sustainable considerations, the optimal surfactant order is: rhamnolipid, CDEA, tea saponin, and sucrose ester, from the perspective of fractal dimension D S . Meanwhile, from the perspective of NMR, the optimal surfactant order is: tea saponin, rhamnolipid, CDEA, and sucrose ester. ...
In this paper, the fractal dimension of coal surfaces was calculated using the fractal theory, and the influence of different surfactants on the fractal dimension of coal surface was studied. Six kinds of sustainable and traditional surfactants used in coal gas desorption processes were compared and analyzed. We used mineral gas fertilizer coal from Huainan Liuzhuang, China, as the research object and studied sodium dodecyl benzene sulfonate (SDBS), cohol diethanolamide (CDEA), nonylphenol polyoxyethylene ether (NP-10), tea saponin, sucrose ester and rhamnolipid surfactants used to modify coal. The methane adsorption capacity of coal before and after surfactant modification was measured by low field nuclear magnetic resonance methane adsorption, and Langmuir volume and Langmuir pressure were obtained according to the Langmuir fitting equation. The results showed that from the perspective of fractal dimensions, the best surfactant in the context of sustainability (in order) is rhamnolipid, CDEA, tea saponin, sucrose ester. According to the two important parameters VL and PL in the Langmuir equation, the coal sample modified by sucrose ester had the strongest gas adsorption capacity. According to the numerical analysis of the surface fractal dimension DS of the coal modified by surfactants, the soluble organic matter in the raw coal samples dissolved, and the micropore morphology evolved to become mesoporous or macroporous, which is more favorable for desorption. The coal samples modified by rhamnolipid and SDBS had the strongest gas desorption ability.
... In theory, it may be possible to tailor the friction of rough surfaces by adjusting appropriate surface parameters. For example, numerical modelling in Hanaor et al. [20] revealed greater friction for surfaces exhibiting higher fractality. However, controlling and adjusting rough surface parameters (such as the fractal dimension) is extremely challenging in practice. ...
This paper investigates why the static friction peak is mostly absent in reciprocating sliding and gross-slip fretting literature. Here, reciprocating sliding tests were conducted on ultra-smooth silicon surfaces. A prominent static friction peak was present in the initial cycles. However, a rapid wear-induced decay in the static friction peak occurred after the first cycle with the peak being mostly absent by about 30 cycles. Two possible explanations are proposed for the wear-induced decay: (1) that increasing surface roughness (with cycles) reduces the ‘fully stuck’ contact area and (2) that wear reduces the bonding strength of the stuck interface by removing ‘third body’ contaminant molecules. Predictions from a multi-asperity friction model are used to support these arguments.
... As higher surface slopes result greater normal forces at contact points, surfaces of greater fractality unsurprisingly exhibit a general tendency towards higher resistance to shear and a larger macroscopically observed coefficient of friction. [10] According to an article published by The Hebrew University of Jerusalem, "while frictional motion is often thought of as the motion of two bodies against each other, separated by a perfectly smooth plane, in fact, due to the microscopic roughness of sliding surfaces, all of the contact between sliding bodies takes place in only a tiny area". Thus, only a sparsely spaced microscopic "bumps" are responsible for maintaining the contact between two sliding bodies. ...
... For many applications, it is sufficient to describe static friction and the crossover to sliding according the linear Amonton's law, where the coefficient of friction is proportional to the applied load, However, for small loads, for complex couplings between surfaces, for physiochemical interactions, or for soft contacts, the shear resistance may not follow a linear dependence with load, and time effects may become relevant [24,25]. In addition to the dependence on the normal load, there may be a dependence on the contact area due to a shear resistance arising from atomic or molecular friction due to contact bonding or intermolecular adhesion at the contacts [26]. ...
Oscillatory tribometry is presented as a new method for investigations on the transition from static friction to kinetic friction. The method distinguishes between purely elastic deformations and plastic frictional motions. Friction moduli are defined which are directly proportional to the stored and dissipated energy during an oscillation cycle. The elastic deformations are represented through the elastic friction modulus, which is directly proportional to the average elastic energy over an oscillation cycle, while the frictional motions yield the dissipation friction modulus, which is directly proportional to the average dissipated energy over an oscillation cycle. Energy dissipation in the static and boundary regime can be attributed to local dissolution of contact bonds or intermolecular adhesion bonds at the contact points, and thus to local slip. The ability to measure at very small sliding distances in oscillatory tribometer tests is a crucial prerequisite for such investigations. The methodology provides access to local friction phenomena at the intermolecular level while simultaneously measuring macroscopic samples. The application examples presented show that oscillatory tribometry on a highly sensitive rheometer is particularly valuable for applications such as dry contacts between polymers, elastomers or soft materials.
... It is important to understand that the COF is a system property. Several mechanisms occur simultaneously on certain scales, including the surface asperity interaction, molecular forces, and the shear properties of the solids and of the substances between the surfaces [2,3]. Previous studies have shown that three mechanisms of friction can be isolated to describe the effects within a tribological system: adhesion, tribochemistry, and deformation [4][5][6][7]. ...
The coefficient of friction (COF) is an important parameter for mechanical engineers to consider when designing frictional connections. Previous work has shown that a surface microstructuring of the harder friction partner leads to a significant increase in the COF. However, the impact of the changes in the tribological system on the COF are not known in detail. In this study, the tribological influence factors such as the nominal surface pressure, the material pairing, lubrication, and the surface properties of the counterbody are investigated. Microstructuring is applied by turn-milling of an annular contact surface of cylindrical specimens. A torsional test bench is used to measure the torque depending on the displacement of the two specimens, thus enabling the determination of the COF. All tests with the microstructured specimens result in higher COF than the reference test with unstructured samples. The manufacturing process of the counterbody surface, the nominal surface pressure, and the materials in contact have a significant influence on the COF. While lubrication reduces friction in the case of unstructured specimens, the COF does not change significantly for microstructured samples. This proves that the deformative friction component dominates over the adhesive. Microstructuring the harder friction partner increases the transmittable torque in frictional connections and reduces the sensitivity towards possible contamination with lubricants.
... They found that the number of asperities in contact plays a significant role in the development of surface structure and resultant frictional force. In their subsequent contribution, Hanaor et al. (2016) proposed a roughness modelling approach for the prediction of static friction between rough-to-rough surfaces. Their model facilitates an understanding of how molecular scale friction and adhesive interactions contribute to the development of frictional forces. ...
This paper describes a systematic study of the effect of fractal rough surfaces on the contact area with the aim of advancing contact constitutive laws used in the Discrete Element Method. An in-house Boundary Element code is adopted to investigate the mechanical behaviour of computer-generated surface roughness. Surfaces are generated to have methodically controlled root mean square height (Sq), root mean square gradient (Sdq), short wavevector (q0), large wavevector (q1), Hurst exponent (H) and fractal dimension (Df). The effect of each parameter on the contact area is investigated. Two recently proposed analytical solutions in tribology (i.e. Persson-Tosatti and Pastewka-Robbins) are applied to predict the real contact area. A parameter based on Sq, q0, and H is proposed and its sensitivity for real contact area prediction is demonstrated. Surfaces of natural sand are simulated and their mechanical response shows similar trend as the computer-generated surfaces, albeit more complex.
