Solid Friction Research Articles

Statistical analysis of stretch shaping process of biconvex skin

The results of shaping process were obtained by the process scheme of serial stretching for biconvex skin of workpieces using the ANSYS/LS-DYNA software. The analysis is based on different flow material characteristics during plastic deformation at solid friction. A statistical analysis of the calculation results for disturbing factors assessment is carried out.

NANOMATERIALS OF SHS TECHNOLOGY FOR TRIBOLOGICAL APPLICATIONS: А REVIEW

The paper reviews the results of using the self-propagating high-temperature synthesis (SHS) powder technology to obtain various nanomaterials, which can be utilized for tribological purposes. Firstly, these are low-cost nanopowders of sulfides, oxides, nitrides, carbides, borides and metals, which can be used as solid lubricants and friction modifiers for liquid and semisolid lubricants. Secondly, these are solid compact nanostructured ceramic and composite materials for the production of tribological structures. This type of nanomaterials can be obtained either ex situ from SHS nanopowders by sintering or introducing into the melt or in situ in a single stage from initial powdered reagents by the methods of gasostatiс SHS technology, force SHS compaction, SHS casting, and SHS in the melt, which significantly simplifies and cheapens production of such materials. Thirdly, these are SHS materials for application of nanostructured coatings of different thickness with high wear resistance and low friction factor, such as nanostructured materials for surfacing and spraying, electro-spark alloying electrodes, multicomponent targets for magnetron sputtering and cathodes for vacuum arc evaporation, nanosized fillers for electrochemical and electroless chemical coatings.

Wave-induced motion of magnetic spheres

We report an experimental study of the motion of magnetized beads driven by a travelling-wave magnetic field. For sufficiently large wave speed, we report the existence of a backward motion, in which the sphere can move in the direction opposite to the driving wave. We show that the transition to this new state is strongly subcritical and can lead to chaotic motion of the bead. For some parameters, this counterpropagation of the sphere can be one order of magnitude faster than the driving-wave speed. These results are understood in the framework of a model based on the interplay among solid friction, air resistance and magnetic torque.

Application of continuous normal–lognormal bivariate density functions in a sensitivity analysis of municipal solid waste landfill

The variability of untreated municipal solid waste (MSW) shear strength parameters, namely cohesion and shear friction angle, with respect to waste stability problems, is of primary concern due to the strong heterogeneity of MSW. A large number of municipal solid waste (MSW) shear strength parameters (friction angle and cohesion) were collected from published literature and analyzed. The basic statistical analysis has shown that the central tendency of both shear strength parameters fits reasonably well within the ranges of recommended values proposed by different authors. In addition, it was established that the correlation between shear friction angle and cohesion is not strong but it still remained significant. Through use of a distribution fitting method it was found that the shear friction angle could be adjusted to a normal probability density function while cohesion follows the log-normal density function. The continuous normal–lognormal bivariate density function was therefore selected as an adequate model to ascertain rational boundary values (“confidence interval”) for MSW shear strength parameters. It was concluded that a curve with a 70% confidence level generates a “confidence interval” within the reasonable limits. With respect to the decomposition stage of the waste material, three different ranges of appropriate shear strength parameters were indicated. Defined parameters were then used as input parameters for an Alternative Point Estimated Method (APEM) stability analysis on a real case scenario of the Jakusevec landfill. The Jakusevec landfill is the disposal site of the capital of Croatia – Zagreb. The analysis shows that in the case of a dry landfill the most significant factor influencing the safety factor was the shear friction angle of old, decomposed waste material, while in the case of a landfill with significant leachate level the most significant factor influencing the safety factor was the cohesion of old, decomposed waste material. The analysis also showed that a satisfactory level of performance with a small probability of failure was produced for the standard practice design of waste landfills as well as an analysis scenario immediately after the landfill closure.

Stability Analysis of On-Board Friction Modifier Systems at the Wheel–Rail Interface

Friction control at the wheel–rail interface, using on-board solid stick friction modifier systems can lead to enhanced track life, reduced wear, and increased fuel economy in railroads. Frictional contact between the solid stick and the railway wheel itself can potentially cause vibrations within the modifier systems, influencing their stability and performance. A frequency domain linearized stability analysis of the state of steady sliding at the frictional contact between the solid stick and the wheel is performed. The proposed approach relies on individual frequency response functions (FRFs) of the wheel and the applicator–bracket subsystems of the on-board friction modifier. Stability characteristics of three representative bracket designs are qualitatively compared, using the FRFs generated by their respective finite element (FE) models. The FE models are validated by comparing the predicted natural frequencies with corresponding experimentally measured values on a full wheel test rig (FWTR) facility. The validated FE models are then used to compute stability maps which delineate stable and unstable regions of operation in the design parameter space, defined by train speed, angle of applicator, friction coefficient, and bracket design. Strong dependence of stability upon the bracket designs is observed. The methodology developed here can be used by design engineers to assess the effectiveness of design changes on the stability of the applicator–bracket assembly in a virtual environment—thus avoiding costly retrofitting and prototyping. Directions for further model refinement and testing are provided.

Granular friction: Triggering large events with small vibrations.

Triggering large-scale motion by imposing vibrations to a system can be encountered in many situations, from daily-life shaking of saltcellar to silo unclogging or dynamic earthquakes triggering. In the well-known situation of solid or granular friction, the acceleration of imposed vibrations has often been proposed as the governing parameter for the transition between stick-slip motion and continuous sliding. The threshold acceleration for the onset of continuous slip motion or system unjamming is usually found of the order of the gravitational acceleration. These conclusions are mostly drawn from numerical studies. Here, we investigate, in the laboratory, granular friction by shearing a layer of grains subjected to horizontal vibrations. We show that, in contrast with previous results, the quantity that controls the frictional properties is the characteristic velocity, and not the acceleration, of the imposed mechanical vibrations. Thus, when the system is statically loaded, the typical acceleration of the vibrations which trigger large slip events is much smaller than the gravitational acceleration. These results may be relevant to understand dynamic earthquake triggering by small ground perturbations.

Euler’s friction of fluids theory and the estimation of fountain jet heights

In 1761, Leonhard Euler (1707–1783) published a treatise with the title “Attempt at a Theory of the Friction of Fluids”, in which he assumed that, as is the case for solid friction, fluid friction is proportional to pressure. Several experiments were proposed by Euler to derive a friction factor, which were intended to experimentally confirm his equations. Detailed developments of five different problems of discharge were presented in his treatise, taking into account the loss of head in the conduits. In the Appendix, an example is given of the calculation of the jet heights of a particular fountain, fed with conduits of different cross-sectional areas. Application of the current method for the calculation of head losses in pipes reveals that Euler grossly overestimated the fountain jet heights.

Microstructure and Tensile Properties of Friction Stir and Gas Tungsten Arc Welded AZ91D Magnesium Alloy Joints

Influence of a fusion welding process namely gas tungsten (GTAW) and a solid state friction stir welding (FSW) on the microstructural changes and tensile strength variation of cast magnesium alloy joints were investigated and the results are compared. Of the two joints fabricated, FSW joint showed superior tensile properties compared to GTAW joint due to the difference in microstructural gradient across the weld cross section. Wider partially melted zone and heat affected zone was observed in GTAW joint due to the high heat input nature process compared to narrow heat affected zone in FSW joint. Microhardness measurement indicates that, the weld metal is overmatched compared to the base metal irrespective of welding process used.

Effect of bulk solid on strength of cylindrical corrugated silos during filling

The paper presents 3D numerical analysis results on the effect of bulk solid on strength and stability of metal cylindrical silos with corrugated walls (without stiffeners) during filling. The behaviour of two bulk solids (dry sand and wheat) was described with a hypoplastic constitutive model. Non-linear FE analyses with both geometric and material non-linearity were performed. The numerical results were compared with the Eurocode formulae. The strengthening effect of the stored solid, wall thickness, solid granular hardness, initial void ratio of solid and wall friction angle on buckling strength was investigated. The major contribution of the paper is the quantitative estimation of the effect of the bulk solid stiffness on the silo stability and wall stresses.

Closure to “Discussion of ‘Specific Heat of Tribological Wear Debris Material’” (2015, ASME J. Tribol., 137(3), p. 031601)

A variety of models are proposed to explore the complex nature of friction and its effects. Therefore, my conclusions from the discussion on specific heat of debris “in statu nascendi” must be treated as open to discussion [1]. I initially assumed in Ref. [2] (in line with your comments) that specific heat of a refined solid is not essentially different than its specific heat before being refined. I took the formula for the first law of thermodynamic for open stationary systems and balanced fluxes of different forms of energy and mass within the system and on its boundaries with the environment. I went on to introduce two systemic quantities, C and D, and provided their physical interpretations. C—specific heat times maximum temperature of the friction zone—was very high for selected experimental examples. Given the limited value of this temperature, the postulate that specific heat of debris is very high seemed the only possibility. That persuaded me to modify the original thermal balance. In the circumstances, absorption of friction heat is not accompanied by a marked rise in the temperature of particles being generated. This interpretation is corroborated by the recently discovered “cooling effect” associated with friction and wear of solids [3]. The high value of specific heat in the process of material dispersion may also be explicated by analogy to the phase transition of melting, where specific heat of a substance tends toward infinity. The unconventional properties of matter accompanying tribological wear have been explained, inter alia, by means of the so-called magma–plasma model [4]. Other interpretations of unconventional effects associated with friction are possible, proof that we are still a long way from a full understanding of the nature of friction.The enthalpy change h° as per Eq. (1) characterizes reversible elastic strain of a certain quantity of a substance (a closed thermodynamic system) and its concomitant thermal effect. Equations (1) and (2) also include specific heat of the substance, which remains continuous. They do not apply to debris detachment from a material involved in friction. In my work, I use opportunities offered by phenomenological thermodynamics for open systems. In open systems, enthalpy is the energy released out of a system into its environment together with debris (the latter's further role is not a subject of my discussion). Density of its fluxes is described by Eq. (12)J[adyss+cp'(Θo-Θ)]Increment of specific enthalpy has two components—growth of internal energy caused by a change in temperature of a unit of debris mass and unit work of mechanical dissipation causing tribological wear. This is in line with the definition of enthalpy as the sum total of internal energy and work performed by a substance. The work includes two components: superficial and volumetric [3]. It is required for a debris particle to detach. This kind of energetic interaction is not taken into consideration by Eq. (1).Application of Fourier's equation to description of thermal processes in the area of direct energy dissipation at the time of solid friction is not reasonable in physical terms as physical processes other than conduction occur and properties of the substance filling this area are not uniquely defined. That is the reason why I did not use this equation in my discussion. Thank you for your contributions and suggestions.

A model of solids friction factor for fluidized dense phase pneumatic conveying

Solids friction factor is a parameter required for predicting the pressure drop in a process of pneumatic conveying. It depends upon a number of non-dimensional parameters. In this paper, experimental data for a 2m long section of a 173m long pipeline has been used to develop a mathematical model for solids friction factor. The model predicts the pressure drop with a low error margin for the 2m long pipeline. Although the model has been developed for a 2m long straight pipeline with fly ash as the conveying material, it has also been scaled-up for a 173m long straight pipeline. By scaling-up, the predicted pressure drop lies within an acceptable error margin. Since the model seems to be having less dependence upon the parameter of particle density, it predicts the pressure drop with less error margin for the experimental data of other conveying materials such as alumina and cement. The model shows high error in predicting pressure drop for the experimental data for a different pipeline configuration.

Biomolecular interactions control the shape of stains from drying droplets of complex fluids

When a sessile droplet of a complex fluid dries, a stain forms on the solid surface. The structure and pattern of the stain can be used to detect the presence of a specific chemical compound in the sessile droplet. In the present work, we investigate what parameters of the stain or its formation can be used to characterize the specific interaction between an aqueous dispersion of beads and its receptor immobilized on the surface. We use the biotin-streptavidin system as an experimental model. Clear dissimilarities were observed in the drying sequences on streptavidin-coated substrates of droplets of aqueous solutions containing biotin-coated or streptavidin-coated beads. Fluorescent beads are used in order to visualize the fluid flow field. We show differences in the distribution of the particles on the surface depending on biomolecular interactions between beads and the solid surface. A mechanistic model is proposed to explain the different patterns obtained during drying. The model describes that the beads are left behind the receding wetting line rather than pulled towards the drop center if the biological binding force is comparable to the surface tension of the receding wetting line. Other forces such as the viscous drag, van der Waals forces, and solid–solid friction forces are found negligible. Simple microfluidics experiments are performed to further illustrate the difference in behavior where is adhesion or friction are present between the bead and substrate due to the biological force. The results of the model are in agreement with the experimental observations which provide insight and design capabilities. A better understanding of the effects of the droplet–surface interaction on the drying mechanism is a crucial first step before the identification of drying patterns can be promisingly applied to areas such as immunology and biomarker detection.

Specific Heat of Tribological Wear Debris Material

Stationary processes of solid friction, heating and wear are analyzed in this paper on the basis of the first principle of thermodynamics. Analytical dependences between physical parameters of a tribological system have been determined. Densities of extensive quantity fluxes are referred to elementary surface and elementary time, which has permitted to include intensive quantities, especially temperature, in the model presented here. Although the discussion is restricted to the phenomenological approach, conclusions regarding some microscopic properties of the matter in the process of fragmentation are drawn directly from the laws of energy and mass conservation. Differences between specific heat of the starting material cp and of the debris produced cp′ are emphasized. The model of the friction process described by Maciąg, M. (2010, “Thermodynamic Model of the Metallic Friction Process,” ASME J. Tribol., 132(3), pp. 1–7) has been modified and a new method of evaluating cp′ is proposed. Results of standard friction and wear testing are used to describe selected tribological systems in quantitative terms based on the thermodynamic model discussed here (Sadowski, J., and Żurowski, W., 1992, “Thermodynamic Aspects of Metals' Wear-Resistance,” Tribol. Lubr. Eng., 3, pp. 152–159). Very high specific heat of tribological wear debris material is found at the moment of the material's production. To conclude, results of theoretical and experimental analysis are discussed and their interpretation is proposed. Applicability of the system magnitudes C and D to modeling of friction and wear is highlighted.

On the Commonality Between Theoretical Models for Fluid and Solid Friction, Wear and Tribochemistry

Tribology is concerned with the influence of mechanically applied forces on interfacial phenomena that accompany and control sliding. A wide range of models has been developed to describe these phenomena, which include frictional dissipation, wear and tribochemical reactions. This paper shows that these apparently disparate models are based on the same fundamental concept that an externally applied force accelerates the rate of thermal transition of atoms or molecules across energy barriers present in solid and liquid materials, thereby promoting flow, slip or bond cleavage. Such “stress-assisted” effects and the associated thermal activation concepts were developed independently and in different forms by Prandtl (Z Angew Math Mech 8:85, 1928) and Eyring (J Chem Phys 4(4):283–291, 1936). These two works have underpinned subsequent theories of dry friction, boundary lubrication, EHD rheology, tribochemistry and nanoscale wear modelling. This paper first reviews the historical development of the concepts, focussing in particular on the models of Prandtl and Eyring and how they have subsequently been used and adapted by others. The two approaches are then compared and contrasted, noting that although superficially similar, they contain quite different assumptions and constraints. First, the Prandtl model assumes that the force is exerted through a compliant spring, while constant force sliding is assumed by Eyring. Second, different approximations are made in the two models to describe the change in energy barrier with external force. Prandtl explores the asymptotic behaviour of the energy barrier as the applied force become sufficiently high to reduce it to zero, while Eyring assumes that the energy barrier is reduced by an amount equal to the external work carried out on the system. The theoretical underpinnings of these differences are discussed along with the implications of compliant coupling and constant force sliding on the velocity and temperature dependence of the friction forces for the two models.

Critical Assessment Of The Issues In The Application Of Hilbert Transform To Compute The Logarithmic Decrement

Abstract The parametric OMI (Optimization in Multiple Intervals), the Yoshida-Magalas (YM) and a novel Hilbert-twin (H-twin) methods are advocated for computing the logarithmic decrement in the field of internal friction and mechanical spectroscopy of solids. It is shown that dispersion in experimental points results mainly from the selection of the computing methods, the number of oscillations, and noise. It is demonstrated that conventional Hilbert transform method suffers from high dispersion in internal friction values. It is unequivocally demonstrated that the Hilbert-twin method, which yields a ‘true envelope’ for exponentially damped harmonic oscillations is superior to conventional Hilbert transform method. The ‘true envelope’ of free decaying strain signals calculated from the Hilbert-twin method yields excellent estimation of the logarithmic decrement in metals, alloys, and solids.

Experimental study of hydraulic transport of coarse basalt

In many industrial applications, as in dredging or deep sea mining, coarse solid particles (typical size in the order of millimeters or tens of millimeters) are transported hydraulically in mixture with water through pipelines. This paper discusses experimental results obtained for flows of water-based slurries of three fractions of coarse basalt pebbles in a 100-mm pipe of the laboratory loop of the Institute of Hydrodynamics in Prague. The collected experimental database includes concentration distributions measured in the horizontal section of the pipe loop. The data are used to evaluate friction mechanisms of slurry flow and their relation to the internal structure of the flow. Measured frictional pressure drops are analysed and compared with existing predictive formulae. A theoretical background is proposed for a modified predictive formula for the frictional pressure drop in horizontal flow of slurry carrying coarse grains transported in saltation or as a sliding bed. The relative excess pressure drop in such flow is found to be sensitive primarily to the slip ratio, the thickness of a sliding bed and the solids friction coefficient. An accurate determination of frictional energy losses is essential to a successful assessment of an economy of hydraulic transport and to an optimisation of operations of slurry pipelines conveying very coarse solids.

Acoustics and frictional sliding in granular materials

We study numerically the propagation of an acoustic pulse through a loaded granular material under the hypothesis that the conventional modeling of solid friction used in soft sphere discrete element modeling remains valid at acoustic time scales. As the pulse crosses the material, it temporarily suppresses sliding contacts, making it difficult to prepare states that correspond to experimental conditions. The pulse speed is strongly affected by the loading in a very anisotropic way, varying by as much as a factor of two depending on the propagation direction. We separate the contribution of the contact network from that of sliding contacts, and show that sliding contacts can reduce the propagation speed as much as changes in coordination number. Sliding contacts have a characteristic acoustic signature: pulse speed depends on sign (compression or rarefaction), even at very small amplitudes.

Relationship between Solid Lubricant Layer and Friction Coefficient of PPS Races-PTFE Retainer Hybrid Thrust Bearings under Dry Condition

Due to the advantage on low coefficient and self-lubricant, PPS races-PTFE retainer hybrid polymer bearings can offer better service performance under certain condition. In our previous study, the relationship between friction coefficient and rotation speed was investigated, and formation of black layer was found. In the present study, EPMA was introduced to investigate the component and thickness of the black layers. Based on the observations, the relationship between friction coefficient, solid lubricant layer and thrust load was also discussed.

On the mathematical basis of solid friction

A piecewise-smooth ordinary differential equation model of a dry-friction oscillator is studied, as a paradigm for the role of nonlinear and hysteretic terms in discontinuities of dynamical systems. The friction discontinuity is a switch in direction of the contact force in the transition between left- and rightward slipping motion. Nonlinear terms introduce dynamics that is novel in the context of piecewise-smooth dynamical systems theory (in particular they are outside the standard Filippov convention), but are shown to account naturally for static friction, and moreover provide a simple route to including hysteresis. The nonlinear terms are understood in terms of dummy dynamics at the discontinuity, given a formal derivation here. The result is a three-parameter model built on the minimal mathematical features necessary to account for the key characteristics of dry friction. The effect of compliance can be distinguished from the contact model, and numerical simulations reveal that all behaviours persist under smoothing and under small random perturbations, but nonlinear effects can be made to disappear abruptly amid sufficient noise.

Transient solid–fluid interactions in rat brain tissue under combined translational shear and fixed compression

An external mechanical insult to the brain may create internal deformation waves, which have shear and longitudinal components that induce combined shear and compression of the brain tissue. To isolate such interactions and to investigate the role of the extracellular fluid (ECF) in the transient mechanical response, translational shear stretch up to 1.25 under either 0 or 33% fixed normal compression is applied without preconditioning to heterogeneous sagittal slices which are nearly the full length of the rat brain cerebrum.The normal stress contribution is estimated by separate unconfined compression stress–stretch curves at 0.0667/s and 1/s engineering strain rates to 33% strain. Unconfined compression deformation causes lateral dimension expansion less than that predicted for an incompressible material under large deformation and often a visible loss of internal fluid from the specimen so that the bulk brain tissue is not incompressible in vitro, as sometimes assumed for mathematical modeling.The response to both slow 0.001/s and moderate 1/s shear translational stretch rates is deformation rate dependent and hardening under no compression but under 33% compression is nearly linear perhaps because of increased solid–solid friction. Both shear and normal stress relaxation are faster after the fast rate deformation possibly because higher deformation rates produce higher ECF hydrostatic pressure that primarily drives stress relaxation.The experimental results on ECF behavior guide the form of our nonlinear viscoelastic mathematical model. Our data are closely fit by non-equilibrium evolution equations that involve at most three specimen-specific empirical parameters and that are based on the idea that stretch of axons and glial processes resists load-induced ECF pressure.