Change In Friction Coefficient Research Articles

Study on particle plugging in propagating fractures based on CFD-DEM

In the drilling and completion process of fractured formations, wellbore stability is a key factor affecting the safety of drilling and completing engineering. Previous studies have demonstrated that propping moderately and plugging fractures with soluble particles can improve formation fracture pressure. When it comes to particle transport in 3D rough propagation fractures, the interactions between particle-fracture-fluid need to be considered. Meanwhile, size-exclusion, particle bridging/strain effects all influence particle transport behavior and ultimately particle plugging effectiveness. However, adequate literature review shows that fracture plugging, and fracture propagation have not been considered together. In this study, a coupled CFD-DEM method was put forward to simulate the particle plugging process of propagating fracture, and the effects of positive pressure difference, fracture roughness, particle concentration, and particle shape on the plugging mechanism were examined. It is concluded through the study that: 1) Positive pressure difference too large will lead to excessive fracture aperture, making the particles unable to form effective plugging in the middle of the fracture; positive pressure difference too small will lead to fracture aperture too small, making particles unable to enter into and plug the fracture. 2) No matter how the concentration, particle size and friction coefficient change, they mainly affect the thickness of the plugging layer, while the front end of the particle is still dominated by single-particle bridging, and double-particles bridging and multiple-particles bridging are hardly ever seen. For the wellbore strengthening approaches, such as stress cages, fracture tip sealing, etc., specific analysis should be carried out according to the occurrence of extended fractures. For example, for fractures with low roughness, the particles rarely form effective tight plugging in the middle of the fracture, so it is more suitable for fracture tip sealing; For the fracture with high roughness, if the positive pressure difference is controlled properly to ensure reasonable fracture extension, the particle plugging effect will be good, and the stress cage method is recommended for borehole strengthening.

Role of polysaccharides in tribological and sensory properties of model dairy beverages

In this study, we investigated the contribution of different polysaccharides that varied in conformation and molecular weight, i.e. maltodextrin, xanthan gum, guar gum, carboxymethyl cellulose, and maize starch, to the frictional and sensorial properties of model dairy-based beverage systems, containing 5% emulsion droplets (stabilized either by 0.3% soy lecithin (SL) or 0.1% whey protein isolate (WPI), and 4.5% protein (either WPI or whey protein aggregates (WPA). In the presence of polysaccharide, systems containing SL-stabilized emulsion droplets (SE) and WPI showed a high degree of aggregation and coalescence, whereas systems containing WPI-stabilized emulsion droplets (WE) and WPA were less sensitive to such phenomena. Aggregation or micro-phase separation was especially observed for mixtures containing xanthan, which led to a heterogenous structure, and thus higher friction coefficients. A more homogeneous structure without aggregation was mostly observed for mixtures with guar gum, and provided lower friction coefficients, i.e. better lubrication. The rheological and tribological properties in different sliding regimes were correlated with sensory perception. Systems with a relatively low degree of shear-thinning (i.e. viscosity showed low dependency of shear rate) and low friction coefficients (0.1–0.25, at 10–100 mm/s) gave higher values for the attributes creamy, thick, and fatty. This was mostly observed for homogeneous structures. The attributes slippery, dry and mouthcoating showed strong correlations with friction coefficients (at 10–50 mm/s) and the slope of frictional curves (at 0.1–10 and 30–470 mm/s). These results can be used to optimize the physical and sensory properties of such mixed systems. • Adding polysaccharides in dairy beverages affected microstructure of the mixture. • Aggregation and micro-phase separation increase friction coefficients. • Among the used polysaccharides guar gum promoted lubrication most efficiently. • Sensory perception could be linked to frictional behavior at various sliding speeds. • Changes in friction coefficients with speed were linked to perception.

Material Investigation of Omni-Wheel’s Rollers based on Changes of Friction Coefficient w.r.t Pressing Force

Material Investigation of Omni-Wheel’s Rollers based on Changes of Friction Coefficient w.r.t Pressing Force

Preparation and Tribological Properties of a Superhydrophobic TB6 Titanium Alloy

Superhydrophobic surfaces of conventional Ti-6Al-4V (TC4) titanium alloys facilitate drag reduction and anti-wear. However, the economical fabrication of a wear-resistant superhydrophobic Ti-10V-2Fe-3Al (TB6) titanium alloy for application in modern aerospace equipment remains to be achieved. Here, an efficient wear-resistant superhydrophobic TB6 titanium alloy surface using commercially available materials and processing equipment was fabricated through nanosecond laser texturing and facile chemical modification. The wettability of textured surfaces, including line and grid patterns, ablated at different laser scanning intervals were investigated and analyzed in terms of surface morphology and chemical composition. Moreover, the mechanisms of coefficient of friction changes on different surfaces under dry and water sliding conditions were studied based on surface morphology and wettability. Under dry sliding and water lubrication conditions, the average coefficient of friction of the prepared superhydrophobic surface can be reduced by 29% and 74%, respectively, compared with that of the original hydrophilic surface.

Numeric simulation of steel twin disc system under rolling-sliding contact

Various mechanical parts come into high-load rolling and sliding contact at the contact surface. Even with technological advancements, mechanical failures still occur. Rolling-sliding mechanical contact issues are the primary cause of over 90 % of surface and subsurface metallic failures and they are only becoming worse. Using discretised continuum 2D finite element methods (FEM), this research investigates the parametric contact effect of a steel twin disc system subjected to rolling-sliding contact at varied surface friction and comprehensive load condition. The equivalent von Mises stress distribution, contact pressure distribution and shift position of maximum subsurface stress on the contour region are all influenced by changes in compressive load and coefficient of friction, according to a numerical surface-to-surface contact simulation performed with Abaqus at mean Hertzian pressure. The maximal equivalent stress, for a given load, reaches a peak in the subsurface and moves farther away from the surface when the coefficient of friction decreases and comes close to the contact surface when the coefficient of friction increases. Consistency is shown by the analytical and numerical results.

Clarifying the effects of comonomer distribution between short and long chains on physical and mechanical properties of ethylenic copolymers

AbstractIn this paper, two Zigler‐Natta catalysts (ZNCs) were used to synthesize a commercially available linear low‐density polyethylene (LLDPE), widely used in the packaging industry, on an industrial scale. The catalysts differ only in their ability to distribute comonomers between short and long chains. Both catalysts were fully characterized in the first section, and two similar ethylene/1‐butene copolymers were made using them. Afterward, the produced copolymers were fully characterized using different techniques; namely, differential scanning calorimetry (DSC), successive self‐nucleation and annealing (SSA), oxygen induction time (OIT), melt flow index (MFI), rheometric mechanical spectroscopy (RMS), and a wide range of mechanical experiments. It was revealed that while the presence of comonomers in short chains can reduce their resistance against oxidation (by more than 30%) and can cause a dramatic change in friction coefficients (by more than 20%), some of the other main mechanical properties of the made copolymers were independent of comonomer distribution between long and short chains. In addition, it was shown that ethylenic copolymers' strain hardening modulus (SHM) takes advantage of the homogenous distribution.

Study on friction and wear properties of cast iron material in line contact condition based on free abrasive grinding experiment

In order to study the friction and wear properties of cast iron in free grinding environment under the condition of line contact, this paper presented a new method to calculate the friction coefficient by measuring the friction torque. Through the self-developed free abrasive grinding experimental device in line contact condition, this work carried out friction and wear experiment with relative speed, load, abrasive particle diameter, and abrasive mass fraction as process parameters, and studied the evolution rules of friction coefficient and wear amount of cast iron materials under different process parameters. Based on the response surface method Box-Behnken experimental design, the significance of the experimental results was analyzed, and the prediction model of the friction coefficient and wear amount of cast iron was established. According to the significance of the prediction model, the influence degree of each process parameter on the friction coefficient and wear amount was revealed. The results showed that under the action of free abrasive, the friction coefficient of cast iron material in line contact with GCr15 was between 0.18 and 0.35. When the experimental time was 1.5 h, the removal amount of cast iron material was between 20 and 65 mg, which is significantly increased compared with that without abrasive and the wearing capacity of cast iron increased with the increase of relative speed, load, abrasive particle diameter, and abrasive mass fraction. The friction coefficient had a negative correlation with relative speed, and a positive correlation with abrasive particle diameter and abrasive mass fraction. With the increase of load, the change of friction coefficient decreased first and then increased. The influence degree of each process parameter on the friction coefficient and wear amount of cast iron material: the particle diameter and relative velocity of abrasive were relatively significant, the abrasive mass fraction was the second, and the load was the smallest. Through random experiments, the prediction model of friction coefficient and wearing capacity were verified to be highly accuracy, and could be used to predict the friction and wear performance parameters of cast iron in the process of free grinding under the condition of line contact. It provided a certain theoretical basis for the friction and wear behavior of cast iron as a grinding tool material in the grinding process.

Thermal and Tribological Properties Enhancement of PVE Lubricant Modified with SiO2 and TiO2 Nanoparticles Additive

The addition of nanoparticles may have a positive or negative impact on the thermal and tribological properties of base lubricant. The objective of this paper is to investigate the effect of nanoparticle dispersion in lubricant base in relation to its application in refrigeration system compressors. An investigation of tribological and thermal properties of nanolubricants for rolling piston rotary systems was carried out through four-ball tribology tests and thermal conductivity measurements. Nanolubricants dispersed with SiO2 and TiO2 nanoparticles were tested at various concentrations and temperatures. The changes in thermal conductivity and coefficient of friction (COF) were analyzed while wear weight loss was also calculated from wear scar size. A regression model of thermal conductivity enhancement was proposed for both types of nanoparticles. Zeta potential results show that nanolubricants have excellent stability. The thermal conductivity increases by the increment of nanoparticle concentration but decreases by temperature. The R-square for the regression model is more than 0.9952 with an average deviation not more than 0.29%. The COF for SiO2/PVE nanolubricant at 0.003 vol.% reduced 15% from the baseline. The COF for nanolubricants exceeds the result for base lubricants when the concentration is more than the threshold value. The optimum concentration of SiO2 and TiO2 nanoparticles improved the thermal and tribological properties of PVE lubricant and may offer an advantage when applied to refrigeration systems.

The relationship between adhesion morphology and cutting force in orthogonal cutting of 6061-T6 aluminum alloy

Due to the sticky and soft characteristics of aluminum alloy, it is easy to form adhesion on the tool rake face during cutting process. In this work, the orthogonal cutting tests for 6061-T6 aluminum were carried out at cutting speeds of 20–300 m/min and feed rates of 0.05–0.3 mm, and the relationship between adhesion and cutting force has been explored. A slip-line model was used to separate the edge force of cutting edge to obtain the pure friction force of tool rake face, thereby obtaining the average friction coefficient. Adhesion morphologies were observed under the scanning electron microscope (SEM) and compared with the average friction coefficients. It was found that different adhesion morphologies correspond to different friction coefficient ranges. Simultaneously, the relationship between the temperature and normal stress on the tool rake face and the adhesion morphology and friction coefficient was investigated. The results showed that the temperature is the major factor of adhesion evolution and friction coefficient change. Combining the tool rake face temperature and the adhesion morphology, it was noted that the reduction of the friction coefficient is not only owing to the self-lubrication caused by thermal softening, but also due to the decrease of the total shear force caused by the incomplete contact. Finally, the relationship between the friction coefficient of tool rake face and the cutting force and specific cutting energy was discussed, with the conclusion that the friction coefficient has an impact on the cutting force and specific cutting energy by changing the shear angle.

The effect of the coefficient of friction between the roller and the powder on the rolling process

The friction coefficient between the roller and the powder is one of the important factors affecting the rolling process. Since the friction coefficient is difficult to measure during rolling, it is usually regarded as a constant value in previous studies. However, the friction coefficient changes with the changes in process parameters such as the gap size and the rolling speed. In order to study the effect of the friction coefficient between the roller and the powder on powder rolling, the Drucker Prager Cap model for nickel powder rolling was established by ABAQUS. Firstly, the changes of contact stress (CPRESS) between the roller and the powder, the width (PE33), and relative density (SDV1) of the strip during rolling were analyzed. Then, the effect of the friction coefficient between the roller and the powder on the contact stress, the width, and the relative density of the strip was studied especially. The results show that during the rolling process, the contact stress remains basically unchanged. However, a large transverse deformation occurs on both sides of the strip, and the deformation in the middle is basically small. The relative density is greatest at the smallest seam between the rollers, and it decreases slightly after moving away from the roller, and then it remains stable. The coefficient of friction between the roller and the powder has an impact on the rolling result, in which the contact stress between the roller and the powder, the width, and the relative density of the strip will increase as the friction coefficient increases.

Experimental Method Research on Non-linear Characteristics of Static Friction Coefficient to Temperature

The temperature difference in the working range of precision instruments is large, and the change in temperature causes the static friction coefficient between the internal assembly contact surfaces to change. The change of the static friction coefficient will cause asymmetric displacement of the contact surface, which will affect the accuracy and stability of the instrument during operation. In order to study the non-linear variation law of the static friction coefficient under different temperature conditions, this paper proposes a method to measure the static friction coefficient at different temperatures accurately and designs and develops the corresponding static friction coefficient measuring device. The static friction coefficient of a typical steel-steel contact surface was measured from 20°C to 80°C using the static friction coefficient measuring device. It was found to be non-linear with increasing temperature, with the average static friction coefficient changing by about 5% at a temperature difference of 60°C. Based on the experimental data, a simulation study on the effect of non-linear variation of static friction coefficient at different temperatures was conducted using a simulated sample of a gyroscope assembly structure, and it was found that the asymmetric deformation of the assembly structure caused by the variation of static friction coefficient will be one of the factors causing the change of gyroscope accuracy stability. This research has important implications for maintaining and optimizing precision instrumentation accuracy stability.

Heat transfer analysis in Darcy Forchheimer flow of hybrid nanofluid for multiple shape effects over a curved stretching surface

Nanoparticles and their shapes have practical implications in engineering and industrial processes. For this purpose, we will present a comprehensive study of nanoparticles shape effects on the flow over a curved stretch surface. The Runge Kutta (RK) method is used to numerically solve the non-linear differential equations resulting from the transformation of the flux process equations. First, the equations are transformed into a collection of first-order problems using the shooting method; next, the RK method is applied to solve them. Graphical analysis is carried out to study the implications in flow and heat transfer. Further, changes in skin friction coefficient and local heat transfer rate are graphically displayed and comprehensive discussions on the results are provided.

Friction dynamics of human hair treated with water or cationic surfactant aqueous solution

AbstractCationic surfactants are adsorbed on the surface of human hair and exhibit a lubricating effect. Here, we evaluated the friction when the hair treated with water or 1 wt% cationic surfactant (cetyltrimethylammonium bromide) aqueous solution was rubbed by a contact probe equipped in a sinusoidal motion friction evaluation system. Because of the rough structure of the cuticle on the human hair surface, an oscillatory phenomenon with a frequency of 50–70 Hz was observed when untreated hair was rubbed with a contact probe. On the other hand, the oscillatory phenomenon was not observed when the hair contained a large amount of water because stick–slip phenomena were inhibited on the soft, swollen hair surface. Furthermore, the change in kinetic friction coefficient and delay time, which is a normalized value of the time difference between the reaction of the friction force to accelerated motion, from untreated hair, Δμk and Δδ, was almost zero. However, we found a large difference in Δμk and Δδ for the hair treated with cationic surfactant aqueous solution. The treatment with cationic surfactant reduced both friction parameters, Δμk and Δδ, indicating that the treatment induced not only the frictional force but also the profile at the beginning of frictional sliding. The significant lubrication is due to cationic surfactant adsorption on the hair surface. These data imply that the smoothness of hair treated with a cationic surfactant is related to a reduction in friction coefficient and delay time δ.

Study on friction properties of different abrasive materials and polylactic acid materials

In recent years, 3D printing technology is often used in various fields such as industry, medical care, and academia, because it can produce components with complex shapes at a lower cost than traditional processing. However, the size of 3D products is often limited by the printable range of printing equipment, so large objects often need to be disassembled and printed before joining. Friction stir welding is used to join different thermoplastics and has considerable potential for industrial applications, and the influence of the design of the welding tool on the formation of the welding zone is an important key. In this study, different abrasive materials and polylactic acid materials were used for friction, and the changes of friction coefficient and friction temperature during the friction process were discussed. The experimental results show the surface roughness of the abrasive tool has a significant effect on the coefficient of friction and temperature rise. As increasing the roughness of the abrasive tool, the wear scars, the cracks below the wear scar are both significant. It can be inferred from the crack shape that the PLA softens at high temperatures and then solidifies again, so the cracks are seen with a streamlined appearance, which means that the softened PLA can be carried by the abrasives tool with highly rough surfaces.

Regulation Mechanism for Friction Coefficient of Poly(vinylphosphoric acid) (PVPA) Superlubricity System Based on Ionic Properties.

Adjustable lubrication aims to achieve active control of the relative motion of the friction interface, providing a new idea for intelligent operation. A new phenomenon of sudden changes of friction coefficient (COF) in the poly(vinylphosphoric acid) (PVPA) superlubricity system by mixing different lubricants, was found in this study. It was found that anions were the critical factor for the COF change. The change degrees of the COF were investigated by a universal micro tribometer (UMT). A quartz crystal microbalance (QCM)-D was used to analyze the adsorption quantity of anions on the PVPA surface. The hydratability of the PVPA interface was controlled by changing the anionic properties (the amount of charge and structure), thus regulating the COF. The adsorption difference of anions is an important reasoning of how anionic properties can regulate the hydratability. It was analyzed by molecular dynamics simulation. For anions carrying different numbers of charges or double bonds, the adsorption quantity of anions was mainly affected by the adsorption degree on the PVPA surface, while the adsorption quantity of anions with different molecular configuration was synergistically regulated by the adsorption degree and adsorption area of anions on the PVPA surface. This work can be used to develop smart surfaces for applications.

Оцінка впливу стабільності характеристик гальмівних механізмів на процес гальмування транспортних засобів

The purpose of the article is to evaluate the influence of the stability of the characteristics of brake mechanisms on the process of braking of passenger vehicles. In the article, using the example of a mixed braking system of a passenger vehicle of segment C, an assessment was made of the influence of the stability of the characteristics of its braking mechanisms on the braking process. The results obtained allow us to conclude that with "cold" and "cooled" brake mechanisms of the vehicle, the values of the mathematical expectation of the friction coefficient and the dispersion of its values are approximately the same. The performed calculation shows that when the friction coefficient changes by 8-20%, the braking torque changes by 20-30%. In this case, the difference between the largest and smallest braking torque can be 50%. Given that the left and right brake mechanisms (due to various reasons) may have different coefficients of friction of the friction surfaces, then the unevenness of the braking forces on the rear axle of the vehicle under consideration can be up to 50%. In order to prevent advanced blocking of the rear wheels during vehicle braking in the event that the actual coefficient of distribution of braking forces between the axles deviates from the calculated value due to the instability of the brake mechanisms, it is necessary to ensure the fulfillment of the condition according to the dependence given in the work. The results obtained can be used in the design of braking systems and the assessment of the braking properties of vehicles, as well as in the examination of road accidents.The instability of the braking torques on the wheels does not have a significant effect on the reduction of the total braking force (braking efficiency) in the presence of amplifiers in the drive, but it significantly affects the stability of the vehicle during braking, since it causes a significant change in the coefficient of distribution of braking forces between the axles and the appearance of their onboard unevenness. The statistical analysis of the coefficients of friction of friction pairs (on the example of a passenger vehicle of segment C) showed their wide spread, which significantly affects the onboard and axial unevenness of the braking forces; for example, for friction linings DTM, the average difference between the friction coefficients on the left and right wheels is 0.17, and the standard deviation is ± 0.08. The difference in the coefficients of friction of friction pairs of different types of brake mechanisms, due to the use of friction linings from different manufacturers, can lead to a change in the coefficient of distribution of brake forces over a wide range.

On the Impact of Surface Morphology and Transfer Film on Brake System Performance of High-Capacity Metro Train

The brake system of Melbourne’s High-Capacity Metro Train (HCMT) suffers from consistently extended braking distances after repeating a set of high-speed tests and the commission process. The degradation of brake system performance affects the safety of rolling stock and its conformance to the design standard. In this paper, the root cause leading to the degraded brake performance was analyzed. The brake discs and brake pads of the affected train and another train with normal working conditions were removed and a series of examinations was to determine the reason for the change of friction coefficient between friction surfaces. The results revealed that brake disc samples from the affected TS02 trainset suffered from changed transfer film and surface morphology after multiple consecutive high-speed braking applications. The factors that may affect the brake system performance were analyzed in the laboratory. It was found the brake disc surface had a lower hardness level, coefficient of friction, and smaller contacting area with the brake pad when compared to the brake disc and pad samples from another trainset. These factors harmed the performance of the braking system, and the decrease in the braking effort led to a longer braking distance than expected and failed braking tests.

Design of Ball-Ramp dual clutch transmission to reduce uncertainties in clutch actuator and tie-up effect

Dual-clutch transmissions (DCT) have high efficiency, low drop in tractive force during shifting, and fast shifting is possible. However, it is infeasible to measure clutch torque due to cost and durability problems, and difficult to control clutch torque due to change in friction coefficient. Also, the energy consumed by the actuator for maintaining engagement is large. To address these limitations, a Ball-Ramp DCT (BR-DCT) was developed based on the self-energizing principle. The BR-DCT can reduce energy consumption of the clutch actuator. In addition, it is possible to reduce the uncertainties in actuator by using only measurable values to calculate the actuator input; and, when a tie-up occurs in which two clutches are engaged at the same time, the magnitude of negative torque can be reduced. However, design constraints for implementing last two features were not addressed in previous studies. In this paper, we propose the two design constraints to reduce the uncertainties in actuator and reduce the tie-up effect. For verification, a BR-DCT prototype and powertrain test bench were manufactured with the design constraints. As a result of the shift experiment, it was verified that accuracy of the actuator model can be improved, and the tie-up effect can be reduced.

Synthetic Kilogram Carbon Dots for Superior Friction Reduction and Antiwear Additives.

Because of the high synthesis cost, strong chemical inertness, complex process, and easy to endanger environment of traditional carbon-based nanolubricant additives, the development of its application in lubrication is limited. Therefore, a new type of lubricant additive with low cost, high yield, high performance, and environmental protection is urgently needed. Herein, a kilogram-scale carbon dots (CDs) lubricant additive was prepared by a simple and green one-step reaction of aldol condensation, which showed excellent lubricating properties in water and sunflower oil. The tribological properties of the CDs lubricant additive at different concentrations, loads, and speeds were systematically studied. The results show that the average friction coefficient of water is significantly reduced by 75% by a CDs lubricant additive. In particular, CDs not only exhibited excellent service life and lubrication stability during friction but also kept the friction coefficient change rate of sunflower seed oil close to 0 within 500 min. According to the tribological evaluation and wear surface analysis, the lubrication mechanism of CDs was attributed to their own morphological characteristics and abundant oxygen-containing functional groups on the surface. In the friction process, the charge adsorption effect, the adsorption protective film, and the hydrogen bonding layer are generated, which play an essential role in obvious antiwear and friction reduction. Therefore, this work provides a reference for the preparation of high-performance and high-yield lubricant additives.

Brownian motion with time-dependent friction and single-particle dynamics in liquids.

A microscopic theory of molecular motion in classical monatomic liquids, proposed by Glass and Rice [Phys. Rev. 176, 239 (1968)10.1103/PhysRev.176.239], is revisited and extended to incorporate the dynamic friction in the Brownian description of the atomic diffusion in a mean-time-dependent harmonic force field. A modified, non-Markovian Langevin equation is utilized to derive an equation of motion for the velocity autocorrelation function with time-dependent friction coefficient. Numerical solution of the equation gives an excellent account of the velocity autocorrelation function in Lennard-Jones liquids, liquid alkali, and transition metals over a broad range of density and temperature. Derivation of the equation of motion leads to a self-consistent expression for the time dependence of friction coefficient. Our results demonstrate that the nature of time dependence of the friction coefficient changes dramatically with the liquid density. At low and moderate densities, the dynamic friction decays exponentially whereas it increases exponentially at high liquid densities. Our findings provide an opportunity for a different outlook of the Brownian description of atomic dynamics in liquids.