Constant Friction Research Articles

Mechanical and frictional properties of aesthetic orthodontic wires obtained by hard chrome carbide plating

Background/purposeAlthough aesthetic wire coating has been increasing in demand, it has problems that changes in mechanical properties and increase in frictional force. The aim of this study was to evaluate the coating of the wire, as characterized by aesthetics, in terms of low and constant friction and mechanical properties. Materials and methodsHard chrome carbide-plated (HCCP) wires (HCCP group), commercially available polymer-coated wires (P group), rhodium-coated wires (R group), and uncoated wires (control group) were used. For all wire types, a stainless steel wire of dimensions 0.017 inch × 0.025 inch was used. They were evaluated by three-point bending, friction testing, surface observation, and colorimetric testing. ResultsThe HCCP group was not significantly different from the control group in terms of flexural strength (σ) and flexural modulus (E) (σ: p = 0.90, E: p = 0.35). However, it was significantly inferior compared to the three other groups in terms of the maximum static and kinetic frictional forces under both dry and wet conditions (p < 0.05). In the surface observation, scratches were observed on the wire after the friction test. In the colorimetric test, no significant difference was observed between the HCCP group and the R group (p > 0.05). ConclusionThe mechanical properties of the HCCP wire were not significantly different compared to the control group. The frictional force of the HCCP wire was significantly lower than the other group. Therefore, the HCCP wire was suggested to increase the efficiency of tooth movement in clinics.

Study on Deterioration of Strength Parameters of Sandstone Under the Action of Dry–Wet Cycles in Acid and Alkaline Environment

In order to study the deterioration of strength parameters of sandstone under the action of dry–wet cycles in acid and alkaline environment, laboratory tests were conducted on the sandstone specimens for 1, 3, 6 and 10 dry–wet cycles in the solutions with a pH value of 4, 7 and 9, respectively. With the strength data recorded after each cycle, the Mohr–Coulomb strength criterion parameters and the Hoek–Brown strength criterion parameters were obtained. First, the rock deterioration was briefly analyzed. Then, the failure envelopes and the failure surface on the $$\pi $$ plane at different pH values after different cycles were graphed. The results show that the parameters decrease with the increase in dry–wet cycles. The deterioration is most serious when the number of cycles is small, and then becomes moderate. In addition, the deterioration coefficient of the sandstone is the largest in acidic environment ( $$\hbox {pH}=4$$ ), followed by that in alkaline environment ( $$\hbox {pH}=9$$ ) and that in neutral environment ( $$\hbox {pH}=7$$ ). The deterioration effects of drying and wetting on the following parameters are as follows: fitting compressive strength > cohesion > material constant > internal friction angle. When the pH value of the soaking solution changes from 7 to 4, the deterioration coefficients of the fitting compressive strength and cohesion rise by 30%, while those of the material constant and internal friction angle rise by 10%.

ANALYTICAL APPROACH TO ROTATING COMPRESSION OF A POROUS CYLINDER CONSIDERING CONSTANT SHEAR FRICTION

The volume of the porous material in forming cannot be constant. However, the mass of the porous material remains constant; thereby making the derivations more complicated. A plastic mechanics analytical model with constant shear friction based on the slab method in this study is to explore the characteristics of the rotating compression of a porous cylinder. The effects of the frictional factor and frictional angle on the vertical stress, radial stress, average pressure, compression force, and rotating torque are systematically studied. The analytical results could be advantageous to the industries related to the forging of porous materials.

Cyclic behavior of Chinese ancient wooden frame with mortise–tenon joints: friction constitutive model and finite element modelling

The beams and columns in Chinese ancient wooden buildings were connected with mortise–tenon joints, which are semi-rigid due to the friction and squeezing deformation between mortise and tenon. In this paper, a friction constitutive model for the friction behavior between mortise and tenon was proposed based on the modern frictional theory. A series of surface topography and hardness tests were conducted to obtain some parameters used in the friction constitutive model. A finite element (FE) model of a Chinese ancient wooden frame with mortise–tenon joints under reversed cyclic loading was performed based on the proposed friction constitutive model, and a FE model using a constant friction coefficient was also carried out. Experimental results were used to validate the results modeled by the two models, and better agreement of the proposed model was observed.

Scale- and load-dependent friction in commensurate sphere-on-flat contacts

Contact of a spherical tip with a flat elastic substrate is simulated with a Green's-function method that includes atomic structure at the interface while capturing elastic deformation in a semi-infinite substrate. The tip and substrate have identical crystal structures with nearest-neighbor spacing $d$ and are aligned in registry. Purely repulsive interactions between surface atoms lead to a local shear strength that is the local pressure times a constant local friction coefficient $\ensuremath{\alpha}$. The total friction between tip and substrate is calculated as a function of contact radius $a$ and sphere radius $R$, with $a$ up to ${10}^{3}d$ and $R$ up to $4\ifmmode\times\else\texttimes\fi{}{10}^{4}d$. Three regimes are identified depending on the ratio of $a$ to the core width of edge dislocations in the center of the contact. This ratio is proportional to $\ensuremath{\alpha}{a}^{2}/Rd$. In small contacts, all atoms move coherently and the total friction coefficient $\ensuremath{\mu}=\ensuremath{\alpha}$. When the contact radius exceeds the core width, a dislocation nucleates at the edge of the contact and rapidly advances to the center where it annihilates. The friction coefficient falls as $\ensuremath{\mu}\ensuremath{\sim}\ensuremath{\alpha}{(\ensuremath{\alpha}{a}^{2}/Rd)}^{\ensuremath{-}2/3}$. An array of dislocations forms in very large contacts and the friction is determined by the Peierls stress for dislocation motion. The Peierls stress rises with pressure, and $\ensuremath{\mu}$ rises with increasing load.

A numerical study of equilibrium states in tidal network morphodynamics

The long-term morphodynamic evolution of tidal networks on tidal flats is investigated using a two-dimensional numerical model. We explore the physical processes related to the development of the morphology and the presence of equilibrium configurations. Tidal networks are simulated over a rectangular domain representing a tidal platform, a different setting compared to estuaries (subject to riverine influence) and lagoons (offshore bars constricting the flow). In the early and middle phases of the tidal network evolution, large sediment patches with rhombus-like shape form and gradually migrate in the flood direction, even though the overall sediment flux is ebb-directed. A cross-section-averaged “equilibrium” state is asymptotically approached after about 500 years. The area and peak discharge of the lower flat cross-sections at year 500 approximately show a 1:1 relationship, which is in agreement with field observations. We also show that model results are consistent with the Q-A relationship (peak discharge Q versus cross-sectional area A), which is obtained under the assumption of a constant Chezy friction.

Irreversible Brownian Heat Engine

We model a Brownian heat engine as a Brownian particle that hops in a periodic ratchet potential where the ratchet potential is coupled with a linearly decreasing background temperature. We show that the efficiency of such Brownian heat engine approaches the efficiency of endoreversible engine $$\eta =1-\sqrt{{T_{c}/T_{h}}}$$ [23]. On the other hand, the maximum power efficiency of the engine approaches $$\eta ^{MAX}=1-({T_{c}/T_{h}})^{1\over 4}$$ . It is shown that the optimized efficiency always lies between the efficiency at quasistatic limit and the efficiency at maximum power while the efficiency at maximum power is always less than the optimized efficiency since the fast motion of the particle comes at the expense of the energy cost. If the heat exchange at the boundary of the heat baths is included, we show that such a Brownian heat engine has a higher performance when acting as a refrigerator than when operating as a device subjected to a piecewise constant temperature. The role of time on the performance of the motor is also explored via numerical simulations. Our numerical results depict that the time t and the external load dictate the direction of the particle velocity. Moreover, the performance of the heat engine improves with time. At large t (steady state), the velocity, the efficiency and the coefficient of performance of the refrigerator attain their maximum value. Furthermore, we study the effect of temperature by considering a viscous friction that decreases exponentially as the background temperature increases. Our result depicts that the Brownian particle exhibits a fast unidirectional motion when the viscous friction is temperature dependent than that of constant viscous friction. Moreover, the efficiency of this motor is considerably enhanced when the viscous friction is temperature dependent. On the hand, the motor exhibits a higher performance of the refrigerator when the viscous friction is taken to be constant.

Friction and wear properties of an automobile brake lining reinforced by lignin fiber and glass fiber

PurposeThis paper aims to thrash out friction and wear properties of automobile brake lining reinforced by lignin fiber and glass fiber in braking process.Design/methodology/approachABAQUS finite element software was used to analyze thermo-mechanical coupled field of friction materials. XD-MSM constant speed friction testing machine was used to test friction and wear properties of friction material. Worn surface morphology and mechanism of friction materials were observed by using scanning electron microscope.FindingsThe results show that when the temperature was below 350°C, worn mechanism of MFBL was mainly fatigue wear and abrasive wear, and worn mechanism of GFBL was mainly fatigue wear because MFBL contained lignin fiber. Therefore, it exhibits better mechanical properties and friction and wear properties than those of GFBL.Originality/valueLignin fiber can improve mechanical properties and friction and wear properties of the automobile brake lining.

Journalism Education in India: Quest for Professionalism or Incremental Responses

Journalism education in India is framed in the higher education system, comprising of programs in the universities, both government-supported and media-backed private institutions, as well as in-service and short-term courses offered by press associations and other organizations. They are offered at different levels from certificate to diploma to both undergraduates and postgraduates. Due to requirements of the media industry, there is a constant friction about the need to balance the academic and professional aspects in the curriculum. This has led to skepticism in the past about the relevance of formal journalism education. However, with globalization and growth of the media sector, there is an enhanced need for professionals. Many big media groups have launched journalism programs. Institutional and professional aspects of the programs in India and issues pertaining to curriculum, responses, and critique have been factored in this article.

Squeal analysis of a modal-parameter-based rotating disc brake model

Many squeal studies of brake system finite element (FE) model neglected the disc rotation effect. This paper, based on modal theory and FE modal result, presents a novel method to construct a disc brake model that concerns disc rotation and analyzes its dynamic instability. The rotation effect is embodied in velocity-dependent rotating disc equivalent modal parameters, which are derived through time-space and coordinate transformation and proved to be orthogonal with respect to system matrices in state-space. Through modal synthesis with stationary components, system eigenfunction is formulated in modal space. A comprehensive squeal analysis concerning both rotation and friction negative slope effect is then conducted through complex eigenvalue analysis and modal composition analysis. The result shows the dependency of system instability on rotation speed. Under constant friction, disc rotation is a destabilizing factor to the system as it not only raises the squeal propensity and strengthens the split of eigenvalue loci of mode-merging instability, but also gives rise to additional squeal modes that cannot be generated by mode-merging. Velocity-dependent friction model with negative slope character is proved to generate negative damping and destabilize the system at low speed range. Over high speed range, however, the system becomes more stable due to the decline of friction magnitude. Mechanisms responsible for the squeal are discussed, it is demonstrated that mode-merging induced by the friction coupling is the major mechanism for brake squeal. The destabilizing effects on the system which stems from the split of repeated-root modes caused by rotation, as well as the negative damping effect caused by velocity-dependent negative slope friction, are relatively limited.

Assessment of load dependent friction coefficients and their influence on spur gears efficiency

Traditional procedures to calculate efficiency on gear transmissions generally consider sliding friction as the only dissipative effect, and what is more, they are based on the usage of constant friction coefficients. Although this approach gives acceptable efficiency values depending on the transmission application, the utilisation of a variable friction coefficient provides more reliable results of the friction behavior. Within this framework, the influence of the choice of the friction coefficient on the efficiency of shifted spur gears is assessed in this study. The Niemann’s friction coefficient formulation, which is constant and commonly applied to traditional approaches, was implemented in this proposal, in order to compare it with two hybrid formulations, which are based on elastohydrodynamic lubrication fundamentals and capable to reproduce the friction coefficient in dry contact, boundary, mixed and fluid film conditions of lubrication. These friction coefficient formulations are dependent on the load applied in the conjunction, therefore an enhanced load sharing allows for a better modelling of sliding friction, not only because it depends directly on the normal forces, but due to the friction coefficient load dependence. In this regard, the Load Contact Model previously developed by the authors, which considers the deflections of the adjacent teeth and shifting profile to calculate the load sharing and the friction coefficient, is used, allowing for efficiency values with a high level of accuracy. The efficiency results obtained when hybrid formulations are implemented provides lower values than those determined including Niemann’s formulation. Furthermore, there is a shifting profile which makes optimal the efficiency. This shift factor depends on the implemented friction coefficient formulation, concluding the remarkable importance of the friction coefficient choice.

Seismic isolation of buildings using devices based on sliding between surfaces with variable friction coefficient

Base isolators are effective tools that favor a high level of building performance under lateral load, providing protection to both structural and non-structural elements. In this context, this paper discusses the possibility of employing materials with different frictional properties to enhance the response of flat- and curved-surface base isolators. Two innovative devices, referred to as “BowTie” and “BowC”, are introduced and discussed in some detail. A series of non-linear time history analyses is then conducted using a customized computer program and considering a number of case study structures, designed applying a displacement-based approach. The results of the analyses are used to discuss the key differences between variable friction and constant friction sliding isolation devices. It is shown that the newly proposed isolators may represent an improvement on classic base isolation solutions, in light of their higher energy absorption capacity, which contributes to significantly enhancing their performance.

Thermal Model of Rotary Friction Welding for Similar and Dissimilar Metals

Friction welding is one of the foremost welding processes for similar and dissimilar metals. Previously, the process has been modeled utilizing the rudimentary techniques of constant friction and slip-stick friction. The motivation behind this article is to present a new characteristic for temperature profile estimation in modeling of the rotary friction welding process. For the first time, a unified model has been exhibited, with an implementation of the phase transformation of similar and dissimilar materials. The model was generated on COMSOL Multiphysics® and thermal and structural modules were used to plot the temperature curve. The curve for the welding of dissimilar metals using the model was generated, compared and analyzed with that of practical curves already acquired through experimentation available in the literature, and then the effect of varying the parameters on the welding of similar metals was also studied.

Sensitivity to modelling and design of curved surface sliding bearings in the nonlinear seismic analysis of base-isolated r.c. framed buildings

The main object of this study is to investigate the influence that different modelling assumptions of the curved surface sliding (CSS) bearings may have on the lateral-torsional response of irregular base-isolated structures located in near-fault area, characterized by fling-step and directivity effects with large amplitudes and long period horizontal velocity pulses. The second aim is to evaluate the effects of different design assumptions of the CSS system. To this end, a six-storey base-isolated reinforced concrete (r.c.) framed building, with an L-shaped plan and setbacks in elevation, is designed assuming low- and medium-type friction properties, both with two in-plan distributions of the dynamic-fast friction coefficient, corresponding to: (i) the same value for all isolators; (ii) a different value for each isolator. Four additional cases are compared reducing the friction coefficient in accordance with a temperature increase up to 250°C during ground motions. A computer code for the nonlinear dynamic analysis is developed, in order to compare five models of the CSS bearings that consider constant and variable axial loads combined with constant and variable friction coefficients as function of sliding velocity, axial pressure and stick-slip effect. A lumped plasticity model is used to describe the inelastic behaviour of the superstructure, including a 26-flat surface modelling of the axial load-biaxial bending moment elastic domain at the end sections of r.c. frame members. Near-fault ground motions with significant horizontal components are selected and scaled in line with the design hypotheses adopted for the test structure.

Pin-on-disc study of a friction material dry sliding against HVOF coated discs at room temperature and 300 °C

The tribological behavior of a commercial semi-metallic friction material dry sliding against an uncoated cast iron disc and HVOF coated discs at room temperature and at 300 °C has been investigated. Two types of coatings were investigated, based on Cr3C2-NiCr and WC-CoCr systems. The tests were carried out using a pin-on-disc apparatus. The characteristics of the friction layer that forms on the worn surfaces of the friction material (the pin) and on the counterface disc were analyzed and correlated with the friction and wear behavior of the couplings. At room temperature, quite a long initial stage was observed, required to form a compact friction layer and to achieve a constant average friction coefficient. At 300 °C, steady-state conditions are attained almost from the beginning of the test. The wear rates of the friction materials are mild at room temperature and close to severe at 300 °C because of the thermal softening of the friction material. The wear rates of the coated discs are always negligible, since the contact temperature is not sufficiently high to induce a softening or an oxidative damage of the coatings. Although referring to rather simplified testing conditions, the results obtained in this study provide useful indications on the possibility of using HVOF coatings in braking systems to reduce not only their wear but also, most importantly, the release of particulate matter in the environment.

Nonlinear Frictional Energy Dissipation between Silica-Adsorbed Surfactant Micelles.

The origin of energy dissipation underlies all friction processes, which is crucial in the design of extremely low friction and wear systems. Amontons's friction law shows that the frictional energy dissipation should be linear with load because of the constant friction coefficient. However, in this Letter, we present the nonlinear behavior of frictional energy dissipation in boundary lubrication with silica-adsorbed surfactant micelles. There exist two completely different friction regimes: a near-zero friction regime with very little energy dissipation and a nonlinear friction regime with a great deal of extra energy dissipation. The additional energy dissipation presents a square (nonlinear) relation with applied load, originating from the elastic deformation of the adsorbed micelle layer on the two friction surfaces, which is tightly linked to the stiffness of the micelle layer and the diameter of the contact area.

Numerical investigation of the pressure-time method

The pressure-time method is a discharge measurement technique commonly used to estimate the flow rate in hydro-power plants to assess the turbine hydraulic efficiency. In this paper two-dimensional Reynolds-averaged Navier-Stokes equations with the low-Reynolds k-ω SST turbulence model are used to model and evaluate the pressure-time method. The flow rate decrease is modeled by valve closure and use of the dynamic mesh technique. The results are compared with experimental data showing the close trend. Moreover, the affecting parameters in the pressure-time method including compressibility, calculation of the losses and finding the upper time limit of integration of the pressure-time diagram are studied in detail. Furthermore, the flow characteristics with both compressible and incompressible assumptions are investigated. The obtained results demonstrated the compressibility effect even before the complete valve closure. The physics of the flow, in the closed conduit in the pressure-time method, before the complete valve closure is studied based on the time variation of the wall shear stress and the γ parameter. The γ parameter represents the difference between the turbulence structure in a transient accelerating or decelerating flow and the one in the quasi-steady condition. It is demonstrated that for the pressure-time method, some part of the flow decrease excursion can be characterized as quasi-steady and the rest is unsteady. The dominance of inertia and turbulence dynamics is investigated to evaluate the wall shear stress in the part of the excursion with the unsteady assumption. It is found that the inertia has a dominant effect during the excursion. The flow rate is calculated by evaluating all the terms of losses, e.g., wall shear stress, normal stresses, Reynolds stresses and density variation. Based on the relative magnitude of each term the results indicate that the wall shear stress is a good approximation of the losses for the flow rate calculation and the other terms can be neglected. Due to the primary role of losses in the flow rate calculation other methods in losses calculation are also considered for comparison (e.g., quasi-steady friction or constant friction factor). The available methods to find the upper time limit of the flow rate approximation integral still have significant errors. The flow rate reduction is calculated at each time step before complete valve closure using CFD methods.

Frictional behaviour and friction mechanisms of rolling-sliding contact in mixed EHL

Frictional behaviour of mixed Elasto-hydrodynamic lubrication (EHL) arises from the coupling of the lubricant fluid and asperity interaction frictions. Due to the difficulties in modelling asperity interaction friction, existing approaches often use a constant friction coefficient obtained in boundary lubrication to approach the asperity interaction friction coefficient in mixed EHL. In this study, the asperity interaction friction is, for the first time, considered to result from either the boundary film friction or solid-to-solid ploughing and adhesion friction depending on the local contact and deformation conditions. The friction of mixed EHL was predicted by the combination result of lubricants fluid, boundary film, and solid-to-solid frictions. This development provides an alternative and cost effective method to estimate friction coefficient in mixed EHL.

A numerical and experimental study on the interface friction of ball-on-disc test under high temperature

This paper investigates both experimentally and theoretically the effect of lubricant on the interface stresses under high temperature. A statistical formula was developed to predict the Stribeck curves by considering asperity-lubricant interactions at the contact interface. This enables us to decouple the interface friction stresses contributed by lubricant and asperity solid-solid contact sliding. It was found that the temperature increase at the interface can considerably alter the lubricant performance because of the effect of lubricant additive and that the conventional assumption of constant friction coefficient for asperity contact is unreasonable. As the temperature increased, the boundary layer formed by lubricant molecules dissolved while a soft solid layer was formed by lubricant additives at the contact interface. The finding was confirmed by the effect of lubricant performance on the experimentally measured Stribeck curves. It was also found that in the boundary lubrication regime, friction is sliding-speed-dependent due to the formation-destruction cycle of the soft solid layer formed by lubricant additives when the lubricant temperature increases.

Rough-wall turbulent boundary layers with constant skin friction

A semi-empirical model is presented that describes the development of a fully developed turbulent boundary layer in the presence of surface roughness with length scale$k_{s}$that varies with streamwise distance$x$. Interest is centred on flows for which all terms of the von Kármán integral relation, including the ratio of outer velocity to friction velocity$U_{\infty }^{+}\equiv U_{\infty }/u_{\unicode[STIX]{x1D70F}}$, are streamwise constant. For$Re_{x}$assumed large, use is made of a simple log-wake model of the local turbulent mean-velocity profile that contains a standard mean-velocity correction for the asymptotic fully rough regime and with assumed constant parameter values. It is then shown that, for a general power-law external velocity variation$U_{\infty }\sim x^{m}$, all measures of the boundary-layer thickness must be proportional to$x$and that the surface sand-grain roughness scale variation must be the linear form$k_{s}(x)=\unicode[STIX]{x1D6FC}x$, where$x$is the distance from the boundary layer of zero thickness and$\unicode[STIX]{x1D6FC}$is a dimensionless constant. This is shown to give a two-parameter$(m,\unicode[STIX]{x1D6FC})$family of solutions, for which$U_{\infty }^{+}$(or equivalently$C_{f}$) and boundary-layer thicknesses can be simply calculated. These correspond to perfectly self-similar boundary-layer growth in the streamwise direction with similarity variable$z/(\unicode[STIX]{x1D6FC}x)$, where$z$is the wall-normal coordinate. Results from this model over a range of$\unicode[STIX]{x1D6FC}$are discussed for several cases, including the zero-pressure-gradient ($m=0$) and sink-flow ($m=-1$) boundary layers. Trends observed in the model are supported by wall-modelled large-eddy simulation of the zero-pressure-gradient case for$Re_{x}$in the range$10^{8}{-}10^{10}$and for four values of$\unicode[STIX]{x1D6FC}$. Linear streamwise growth of the displacement, momentum and nominal boundary-layer thicknesses is confirmed, while, for each$\unicode[STIX]{x1D6FC}$, the mean-velocity profiles and streamwise turbulent variances are found to collapse reasonably well onto$z/(\unicode[STIX]{x1D6FC}x)$. For given$\unicode[STIX]{x1D6FC}$, calculations of$U_{\infty }^{+}$obtained from large-eddy simulations are streamwise constant and independent of$Re_{x}$when this is large. The present results suggest that, in the sense that$U_{\infty }^{+}(\unicode[STIX]{x1D6FC},m)$is constant, these flows can be interpreted as the fully rough limit for boundary layers in the presence of small-scale linear roughness.