Specific Friction Coefficient Research Articles

ANALYSIS OF UNCERTAINTY IN THE DETERMINATION OF THE CRITICAL YAW RADIUS USING MONTE CARLO SIMULATION

Background: The speed of a vehicle in a curve can be estimated by analysing yaw marks, which indicate its orientation around the vertical axis. When it exceeds the "critical speed," the vehicle slides laterally, evidenced by the oscillation of the rear tires relative to the front ones. Aim: To present a methodology for reconstructing yaw accidents, with an analysis of uncertainties. Methods: This article uses Monte Carlo simulation software to address uncertainties in determining the critical yaw speed of vehicles. The radius of curvature is calculated considering the chord, average arrow, and vehicle gauge, while the road is characterized by negative superelevation and specific friction coefficient. The simulation models input variables with probabilistic distributions, analysing uncertainties related to road characteristics, vehicle properties, environmental conditions, and measurement errors. Results: The simulation results provide a detailed assessment of the uncertainties involved in determining the critical yaw speed, highlighting the importance of a probabilistic approach to ensure a more realistic and reliable assessment of vehicular safety. Discussion: When evaluating measurement uncertainties, it is essential to consider the sensitivity coefficient to ensure a precise and comprehensive analysis of uncertainty sources. Conclusions: This study highlights the importance of analysing and controlling critical variables, such as the radius of curvature and friction coefficient, to accurately determine the critical yaw speed and ensure reliable vehicular safety measures..

Modeling and Experimental Investigation of U-R Relationship of AA6061-T6 Tubes Manufactured via Free Bending Forming Process

Forming tubes with various bending radii without changing the bending dies is much easier for the 3D free bending forming (FBF) process. In the 3D-FBF process, different bending radii were realized by adapting the eccentricities of the bending dies. The accuracy of the U-R curve is crucial for the precision forming of complex bending components. In this study, the U-R relation curve of the Al alloy tube with a specific friction coefficient, fixed geometry size, clearance between tubes, and bending die was fitted first based on the forming results of AA6061-T6 tubes under different eccentricities. Second, the U-R relationship curve based on the experiment is used to propose the U-R relationship’s mathematical formula based on many hypotheses. Finally, the modified U-R mathematical formula was applied in the finite element (FE) simulation and the actual FBF experiments for the AA6061-T6 Al alloy complex shape space bending members. The U-R relationship curve’s reliability was verified by comparing the simulation and experimental results. The results obtained from the modified U-R relationship align well with the FE modeling results and can be directly applied to the bending process for the intended components.

Coupled Thermo-Structural Analysis Model of Solid Rocket Motor Nozzle considering the Variation of Friction Coefficient under Operating Conditions

Multi-interface is a typical feature of solid rocket motor nozzles, and the interface evolution law is an important aspect to analyze the thermo-structural response of solid rocket motor nozzles. In this paper, based on the friction coefficient test at different temperatures, a coupled thermo-structural analysis model of solid rocket motor nozzle considering the variation of friction coefficient under operating conditions was established. Firstly, the friction coefficient was tested to model the variation at different temperatures. Then, by adopting structure gap, variable friction coefficient, thermal contact resistance, and friction heat production, a strongly coupled non-linear model was established. Simulations using the non-linear model and traditional model were performed, in which the stress of the throat insert was within the required stress range of the material. The ground firing test result demonstrated the validity of the analysis model, and the non-linear model was in a better agreement with the firing test than the traditional model. Therefore, it can be concluded that with a more specific friction coefficient to represent the friction behaviour of the different parts of the nozzle, the strongly coupled non-linear model established in this paper can reflect the essence of thermo-structural response of solid rocket motor nozzle.

Experimental study of food waste powder FWP influence on compressive and wear behaviour of polyester composite

Composites reinforced by natural fillers approved to have good tribological properties which essentially required in industrial applications demand specific friction coefficient and wear resistance such as flooring materials and brake pads materials…etc. Food wastes powder (FWP) such as crusts are abundant and possess good mechanical properties, these materials evaluated to assess the possibility of using (Polyester/FWP) as a new engineering material with enhanced mechanical properties. In this research, (UPE/FWP) composites prepared by Hand lay-up molding. Compression and wear behavior of UPE composites reinforced with different types of food waste powder FWP have been investigated. Matrix polyester reinforced with four types of FWP (Coconuts Shells, Chestnuts shells, Egg shells, and Pistachio shells) proposed as new engineering materials with enhanced mechanical and frictional properties. The utilize particles size vary between (713.9, 1094.7, 1060.4 and 543.7) nm respectively. Mixing concentration ratio utilize were (2 and 4 wt %) for all. The results showed that all four types of FWP have improved the mechanical and tribological behavior of UPE composites with mild variation. Both, coconuts and chestnuts particles have the significant effect on the compression strength (C.S.), 4wt% ratio present the higher (C.S) value measured for all (FWP) used and the performance increase gradually as filler contain increased. Also, the value of Young’s Modulus observed to be increased as the (FWP) increases and the best value observe at 4wt% (UPE/coconuts shell and UPE/chestnuts shell) composites. Wear rate (W.R.) results present a noticeable reduction during the increase of additive concentration with mild variation. FWP acts as a good reinforcement with polyester composite so the wear resistances consequently increase.

State-of-the-art for Magnetorheological Elastomers Fabrication and Hydro-tribological Characterisation

Magnetorheological effect on mechanical properties of its specific applications has been studied. Mechanical properties such as hardness, tribology characteristic and salt water and water absorption with hygrothermal effect will be further studied on wear performance of MRE on marine equipment applications. Isotropic, anisotropic and anisotropic with added N330 carbon black filler of Ni Zn Ferrite Magnetorheological Elastomers (MREs) were prepared with Ni Zn Ferrite filler, Ni1 - xZnxFe2O4 particles weight fraction of 70%. Images of Scanning Microscopy Electron (SEM) and Energy Dispersive X-Ray Analysis (EDX) are identified with the presence of the structure of Ni Zn Ferrite in the natural rubber matrix. Anisotropic Ni Zn Ferrite MREs with added carbon black filler given highest hardness value of 90.78; least effected on salt water and water absorption and hygrothermal effect; better wear performance with a low average specific coefficient of friction(COF), specific wear rate and least loss of wear volume. Therefore, it is most suitable use for future application in a wet environment for a long period.

The CFD study of three-dimensional model of porous medium regenerator with oscillating flow

The regenerator is the core heat exchange component of the Stirling engine. The performance of it is determined by the configurations and physical properties of the filling material. In this paper, a three-dimensional model of the regenerator filled with powder sintered material is established and simulated with the oscillating flow using ANSYS. After specific post-processing, the relationship between Reynolds number and specific friction coefficient as well as Nusselt number is numerically solved which provides a method to acquire characteristics of flow resistance and heat transfer for the specific configuration. Finally, the parameter of is chosen as an indicator to evaluate the performance of the four fillers, and the potential of being a future regenerator filler is discussed.

The shear stiffness criterion for rock joints considering rock wear behaviour

Rock is a material that is affected by wear, and the curvature of the asperities on a rock joint surface increases with the degree of wear after shearing. Based on the Greenwood and Williamson (GW) model, a new model considering the change of asperity curvature is proposed to explain the wear behaviour of rock joints. First, the shear stiffness formula for a joint surface is derived when the asperity curvature is constant, which shows that the shear stiffness increases with increase of asperity curvature. According to the Mohr–Coulomb criterion, the yield position of a single asperity under normal force and tangential friction force is discussed. Then, the critical normal force for a single asperity at a specific friction coefficient is obtained, which shows that the normal force corresponds to the curvature radius of the asperity. A rough surface model with multi-level curvature radius is proposed. With increase of normal force, the higher-order asperities gradually fail and the curvature radius become larger. A specific pressure value excites a specific radius of curvature, and the larger the pressure, the larger the radius of curvature. The relation between the normal force and the curvature radius is proposed and a shear stiffness formula considering the change of curvature radius of the asperity is derived. The proposed model is verified on the basis of the published experimental results. The calculation results of the proposed model can reflect the test results well: for a given joint surface, with increase in normal force the joint surface gradually becomes smooth; for different joint surfaces, with increase in roughness, the joint surface is more easily smoothed.

Friction and wear performance of a NiAl-8 wt% serpentine-2 wt%TiC composite at high temperatures

A NiAl matrix composite with addition of 8 wt% serpentine and 2 wt% TiC (NAST) is investigated to identify the improvement of friction and wear performance at high temperatures. The results indicate that compared with pure NiAl alloy, NAST exhibits lower friction coefficients (0.30–0.34) and wear rates (2.25–3.12 × 10–5 mm3 N−1 m−1) over a wide range of temperatures. This is attributed to the formation of self-lubricating films with different percentages of coverage, which are mainly composed of SiO2 and MgO. Besides, raising the test temperature increases the forming ability and completeness of the films to some extent, further ameliorating the friction-reducing and anti-wear properties accordingly. Especially at 500 °C, the specific friction coefficient and wear rate of NAST are about 39.8% and 62.6% less than those of pure NiAl alloy.

On the Role of Material Transfer in Friction Between Metals: Initial Phenomena and Effects of Roughness and Boundary Lubrication in Sliding Between Aluminium and Tool Steels

In the absence of a lubricant, the friction we measure in sliding contact between metals is typically high and quite erratic, with rapid fluctuations. If we filter out these rapid fluctuations, we can typically also notice slower trends, which can lead to quite dramatic friction changes. Unless careful studies are performed, the cause to this behaviour cannot be understood. How come a material couple cannot be characterised with a specific coefficient of friction? The present paper sets out to add understanding to this area, by conduction and analysing an experimental series involving sliding between a needle-like aluminium tip against tool steel flats. The load is high enough to cause substantial plastic deformation of the aluminium needle; its tip becomes formed by the contact against the tool steel. These small-scale, low sliding distance tests facilitate detailed studies of the initial stages of various friction trends, and the effects of initial surface roughness and shifts of this roughness caused by material transfer between the sliding surfaces. Specifically, the effects on the transfer and friction behaviour from presence or absence of a boundary lubricant film and atmospheric oxygen were studied. It was found that very smooth sliding surfaces can offer low-friction conditions for these metal types. However, the smooth sliding interface is very fragile. In all unlubricated cases tested, it very rapidly (in less than a few mm sliding) became ruined due to transfer, and the friction level correspondingly increased. The boundary lubricant could only offer low friction in cases where the flat steel surface was very smooth. The lubricant also facilitated smoothening of transferred aluminium. As long has been well known, boundary lubrication films typically do not totally hinder direct metallic contact in solid to solid contact. The present results strengthen this view and further suggests that in these direct contacts one of the major friction reducing effects of the lubricant is to efficiently limit transfer, which otherwise acts to make the sliding surface rough.

Preparation and tribological properties of MoS2/graphene oxide composites

A hydrothermal route is developed for the synthesis of MoS2/graphene oxide (GO) composites based on the hydrothermal reduction of Na2MoO4 and GO sheets with l-cysteine. The MoS2/GO composites in improving friction and wear of the sunshine oil on sliding steel surfaces under low or high applied load were demonstrated. In tests with sliding steel surfaces, the sunshine oil that contains small amounts of MoS2/GO composites exhibited the lowest specific friction coefficient and wear rate under all of the sliding conditions. Scanning electron microscopy and energy dispersive spectrometer performed to analyze the wear scar surfaces after friction confirmed that the outstanding lubrication performance of MoS2/GO composites could be attributed to their good dispersion stability and extremely thin laminated structure, which allow the MoS2/GO composites to easily enter the contact area, thereby preventing the rough surfaces from coming into direct contact.

Well-posed continuum equations for granular flow with compressibility and μ(I)-rheology.

Continuum modelling of granular flow has been plagued with the issue of ill-posed dynamic equations for a long time. Equations for incompressible, two-dimensional flow based on the Coulomb friction law are ill-posed regardless of the deformation, whereas the rate-dependent μ(I)-rheology is ill-posed when the non-dimensional inertial number I is too high or too low. Here, incorporating ideas from critical-state soil mechanics, we derive conditions for well-posedness of partial differential equations that combine compressibility with I-dependent rheology. When the I-dependence comes from a specific friction coefficient μ(I), our results show that, with compressibility, the equations are well-posed for all deformation rates provided that μ(I) satisfies certain minimal, physically natural, inequalities.

Inverse finite element modeling of the barreling effect on experimental stress-strain curve for high temperature steel compression test

Thermomechanical properties used in the modeling of steel forming processes that are determined using high temperature cylindrical coupon compression testing are subject to errors due to barreling of the test specimen. Barreling caused by the friction between specimen and platens reduces the accuracy of the mechanical property determination. In this study, Gleeble hot compression testing was conducted to investigate material behavior for a low carbon structural steel over a range of temperatures (from 900°C to 1200°C) and strain rates (from 1s−1 to 30s−1). An inverse method combined with finite element analysis was developed to correct the experimental stress-strain curves for the observed barreling effect to obtain the actual stress-strain curves for the material. In deformation simulations, the revised stress-strain curves produced barreling shape predictions that agreed well with the barrel shapes observed in experiments. A comprehensive parametric study based on the revised stress-strain curves was performed to study barreling for a range of friction coefficients, temperatures, and strain rates. Results showed that the magnitude of barreling increases with increasing friction coefficient. For a specific friction coefficient, the magnitude of the barreling decreases with increasing temperature and varies non-linearly with strain rate.

Friction Coefficient Variation with Sliding Velocity in Copper with Copper Contact

In this paper, by applying analytical method and using experimental method data, the effect of sliding velocity on friction coefficient for contact of copper with copper is investigated. Some equations for obtaining friction coefficient as a function of sliding velocity for contact of copper with copper by applying second analytical method and using experimental method data are achieved. The results have shown, for each duration of rubbing, the existence of an exclusive equation for friction coefficient. The friction coefficient functions that are achieved by second analytical method are shown as a curve that indicates specific friction coefficient for each sliding velocity. Friction coefficient can be used to obtain friction force and stress in contact region of materials. In order to have a suitable design, the amounts of stress and friction force in contact region are required.

Synergistic effect of hybrid carbon nanotube-graphene oxide as nanoadditive enhancing the frictional properties of ionic liquids in high vacuum.

A remarkable synergetic effect between the graphene oxide (GO) layers and multiwalled carbon nanotubes (MWCNTs) in improving friction and wear on sliding diamond-like carbon (DLC) surfaces under high vacuum condition (10(-5) Pa) and low or high applied load is demonstrated. In tests with sliding DLC surfaces, ionic liquid solution that contains small amounts of GO and MWCNTs exhibited the lowest specific friction coefficient and wear rate under all of the sliding conditions. Optical microscope images of the wear scar of a steel ball showed that GO/MWCNT composites exhibited higher antiwear capability than individual MWCNTs and GO did. Transmission electron microscopy images of nanoadditives after friction testing showed that MWCNTs support the GO layers like pillars and prevent assembly between the GO layers. Their synergistic effect considerably enhances IL-GO/MWCNT composites.

Solving the Savage–Hutter equations for granular avalanche flows with a second‐order Godunov type method on GPU

SummaryIn this article, we apply Davis's second‐order predictor‐corrector Godunov type method to numerical solution of the Savage–Hutter equations for modeling granular avalanche flows. The method uses monotone upstream‐centered schemes for conservation laws (MUSCL) reconstruction for conservative variables and Harten–Lax–van Leer contact (HLLC) scheme for numerical fluxes. Static resistance conditions and stopping criteria are incorporated into the algorithm. The computation is implemented on graphics processing unit (GPU) by using compute unified device architecture programming model. A practice of allocating memory for two‐dimensional array in GPU is given and computational efficiency of two‐dimensional memory allocation is compared with one‐dimensional memory allocation. The effectiveness of the present simulation model is verified through several typical numerical examples. Numerical tests show that significant speedups of the GPU program over the CPU serial version can be obtained, and Davis's method in conjunction with MUSCL and HLLC schemes is accurate and robust for simulating granular avalanche flows with shock waves. As an application example, a case with a teardrop‐shaped hydraulic jump in Johnson and Gray's granular jet experiment is reproduced by using specific friction coefficients given in the literature. Copyright © 2014 John Wiley & Sons, Ltd.

Determination of Friction Coefficients of Interstice of a Shaft-Hub-Connection Manufactured by Lateral Extrusion

A combination of several manufacturing process steps in a simultaneous manner allows savings in energy costs, reduced investment and economizing logistical efforts. In particular, this investigation deals with manufacturing and joining of a frictionally engaged shaft-hub-connection by lateral extrusion. One key challenge of such new process management is the prior layout of both optimal friction conditions during manufacturing process on the one hand and highly expected static friction interaction of joined components in order to transmit tangential and axial forces on the other hand. Therefore, cylindrical, thin-walled hubs have been manufactured and joined to shafts. Several parameters describing the tribological system between shaft and hub such as contact surface topographies and lubricant, have been varied within this study. By measuring the radial deformation of the hub, the contact pressure is determined and a normal force applied to the contact surface of shaft and hub is calculated. When separating hub and shaft in a destructive manner, an extensive axial tensile force is applied and measured. According to Coulomb's friction law, specific friction coefficients are calculated depending on manufacturing process parameters as mentioned above.

Modeling of a horizontal circulation open loop in two-phase helium

In the process of the cryogenic cooling system design of the superconducting magnet of the R3B spectrometer, heat and mass transfer in a two-phase He I natural circulation loop with a horizontal heated section has been investigated experimentally. The experiments were conducted on a 2m high experimental loop with a copper tube of 10mm inner diameter uniformly heated over a length of 4m. All data were obtained near atmospheric pressure. Evolution of the mass flow rates as a function of heat flux in steady state condition are presented and compared to a numerical model that have been developed to assist the design of such a cooling scheme. The model is based on a one-dimensional equations system, which includes mass, momentum and energy balances. It is based on the homogeneous model with a specific friction coefficient for the horizontal heated section. The model reproduces with an acceptable accuracy the experimental results and now serves as a tool for the design.

A Comparative Study of Biologically Inspired Walking Gaits through Waypoint Navigation

This paper investigates the locomotion of a walking robot by delivering a comparative study of three different biologically inspired walking gaits, namely: tripod, ripple, and wave, in terms of ground slippage they experience while walking. The objective of this study is to identify the gait model which experiences the minimum slippage while walking on a ground with a specific coefficient of friction. To accomplish this feat, the robot is steered over a reference path using a waypoint navigation algorithm, and the divergence of the robot from the reference path is investigated in terms of slip errors. Experiments are conducted through closed-loop simulations using an open dynamics engine which emphasizes the fact that due to uneven and unsymmetrical distribution of payload in tripod and ripple gait models, the robot experiences comparatively larger drift in these gaits than when using the wave gait model in which the distribution of payload is even and symmetrical on both sides of the robot body. The paper investigates this phenomenon on the basis of force distribution of supporting legs in each gait model.

Transport and dynamical properties of inertial ratchets

In this paper we discuss the dynamics and transport properties of a massive particle in a ratchet type potential immersed in a dissipative environment. The directional currents and characteristics of the motion are studied as the specific frictional coefficient varies, finding that the stationary regime is strongly dependent on this parameter. The maximal Lyapunov exponent and the current show large fluctuations and inversions, therefore for some range of the control parameter, this inertial ratchet could originate a mass separation device. Also an exploration of the effect of a random force on the system is performed.

Friction laws for lubricated nanocontacts

We have used friction force microscopy to probe friction laws for nanoasperities sliding on atomically flat substrates under controlled atmosphere and liquid environment, respectively. A power law relates friction force and normal load in dry air, whereas a linear relationship, i.e., Amontons' law, is observed for junctions fully immersed in model lubricants, namely, octamethylciclotetrasiloxane and squalane. Lubricated contacts display a remarkable friction reduction, with liquid and substrate specific friction coefficients. Comparison with molecular dynamics simulations suggests that load-bearing boundary layers at junction entrance cause the appearance of Amontons' law and impart atomic-scale character to the sliding process; continuum friction models are on the contrary of limited predictive power when applied to lubrication effects. An attempt is done to define general working conditions leading to the manifestation of nanoscale lubricity due to adsorbed boundary layers.