Boundary Friction Research Articles

Effect of boundary conditions on tensile bending strength of glass under four-point bending

Tensile bending strength is the most crucial property of structural glass. An evaluation procedure of tensile bending strength is specified in EN 1288-3 using the four-point bending (4PB) tests. However, the effects of boundary conditions of the supporting and bending rollers on tensile bending strength have not been well documented. The purpose of this paper is to provide a modified procedure for evaluating tensile bending strength considering the combined effects of boundary conditions and friction. The 4PB test method was re-evaluated by performing a comprehensive experimental, analytical and numerical study on ultra-white annealed glass (ANG) and fully tempered glass (FTG) subjected to large deflection. The results demonstrate that different boundary conditions can lead to a deviation between the EN 1288-3 standard method and the newly proposed evaluation procedure, which is strongly dependent on boundary conditions and friction effects. If laboratory conditions permit, it is highly recommended to ensure that the supporting and bending rollers can rotate freely during the 4PB test to avoid overestimating the tensile bending strength of structural flat glass. Otherwise, it is necessary to employ the newly proposed evaluation procedure to obtain the tensile bending strength accurately.

Regularized SPH model for soil–structure interaction with generalized frictional boundary

AbstractThis paper proposes a regularized smoothed particle hydrodynamics (SPH) scheme to solve the problems of tensile instability and stress noise in large‐deformation soil–structure interaction. Particle shifting and stress regularization are combined to maintain uniform stresses and particle distributions. A generalized frictional boundary condition is presented to model soil–structure interfaces with abitrary friction coefficients. A modified particle shifting technique is developed to regularize particles near the frictional boundaries. Furthermore, GPU acceleration algorithm is adopted to improve the efficiency. The proposed scheme is compared with the conventional SPH methods using numerical examples, including the collapse of three‐dimensional granular column, the collapse of the cohesive soil, and pipe penetrates into soft clay. The comparison indicates that the proposed regularized SPH model can give highly accurate results for large‐deformation soil–structure interaction problems with arbitrary friction coefficient.

Effect of cross section aspect ratio and bearing surfaces treatment on the compressive strength of solid fired clay brick specimens

This study addresses the evaluation of the confinement effect in the experimental determination of compressive strength in solid fired clay units. The experimental campaign has focused on two different types of solid fired clay bricks, namely mechanically extruded and handmade, with a total amount of 458 specimens. The research considers different standard specimens, such as whole or half brick, and 100 × 100 × 40 mm3 specimen, and nonstandard 40 × 40 × 40 mm3 specimen, subjected to different standard bearing surface treatments, i.e. grinding, capping with cement mortar or gypsum plaster, placing with birch plywood or fibreboard. Additionally, two novel bearing surface treatments are proposed, i.e. covering with gypsum powder, and placing two oiled PTFE leaves. The experimental campaign has focused on four main aspects. First, the evaluation of the compressive strength value in specimens with hardening response. Second, the influence of the cross section’s aspect ratio, defined as the ratio between the specimen’s length and width. Third, the influence of the bearing surface treatment on the determination of the compressive strength. Fourth, the evaluation of the standard compressive strength through the comparison amongst reference standards. The results highlight and quantify the different factors that influence the confinement, while detecting differences depending on the manufacturing process of the unit. In addition, the results reveal the use of oiled PTFE leaves as a promising and fast possibility of low boundary friction to obtain the strength regardless of the specimen shape.

Frictional vibration behaviors of a new piezo-damping composite under water-lubricated friction

Water-lubricated bearings have become an important component of marine propulsion systems due to the requirements of environmental friendliness and concealability. However, water-lubricated bearings are often subjected to bending stress, additional stress, frictional resistance and other multi-factor effects, resulting in boundary friction or even dry friction on the friction interfaces when encountering unique working conditions such as engine start-stop, steering, low-speed, and heavy-load. The severe frictional processes often induce vibration behaviors and generate frictional noise, which seriously weakens the sound concealment performance and service life of marine propulsion systems. Therefore, it is urgent to improve the vibration and sound pressure behaviors of the composite bearings under water-lubricated conditions. In this study, novel barium titanate/carbon black/polyurethane composites (BaTiO3/CB/PU) are fabricated and then explored the damping capacities and tribological properties of the new polymers. The results showed that the BaTiO3 concentrated on transforming the mechanical energy generated by frictional vibration into electrical energy which generates the thermal energy. The water cooled the frictional heat and the piezo-damping heat. This energy dissipation method increased the damping capacity to reduce the vibration and noise behaviors induced by friction. The composite with 3.0 wt% BaTiO3 performed the highest loss factor under the operating temperatures and presented the best frictional vibration reduction properties. The knowledge obtained in this study is useful not only to explore a new piezo-damping composite, but also to provide a theoretical basis for developing higher performance polymer-based water-lubricated bearing.

The island rule with multiple islands and its application to the Indonesian Throughflow

The island rule theory in the case of complex geometry with multiple islands referring to the Indo-Pacific Maritime Continent is investigated on the basis of Godfrey’s island rule theory. The bottom friction and lateral friction of multiple channels are considered by employing the Munk and Stommel boundary layer models. Five idealized cases with various spatial distributions of islands are designed to examine the influence of shape and size of the islands. The analytical solutions of the streamfunctions of the through-flows among the islands are obtained and the volume transport through each channel is estimated. The Indonesian Throughflow (ITF) transport is then calculated using the analytical solutions with wind stress and compared with observations and previous theoretical results. The ITF transport from the multiple island rule is about 14.5 Sv during 2004–2006, which is close to the observed ITF transport (about 15.0 Sv) from the International Nusantara Stratification and Transport. We find that the multiple islands rule reproduces well the mean value and interannual variability of the observed ITF transport, and inclusion of wind stress in the North Pacific Ocean may improve the estimate of ITF transport. Sensitivity experiments indicate that frictional boundary layer thickness and channel size influence the estimated ITF transport under the multiple island framework. These results imply that the multiple island rule shows improvements in estimating the ITF transport relative to previous studies, and the multiple island rule can be used to produce long time series of ITF transport and might have implications for paleo-ITF study.

Dual separation control and drag mitigation in high speed flows using viscoelastic materials

Boundary layer separation and friction drag form key delimiting phenomena that subvert the aerial platforms from achieving greater efficiency and accessing wider operation envelope. Both these phenomena are significantly aggravated in supersonic platforms due to the interactions between shock waves with the boundary layer that develops over the vehicle surface and within the engines. The present work demonstrates a new paradigm that leverages the native or programmable material properties of the aerostructures to engender simultaneous reduction in the separation scales and mitigation of skin friction drag. As a first step toward realizing this paradigm, the present work demonstrates, for the first time, the simultaneous skin friction drag mitigation in a Mach 2.5 boundary layer and control of shock induced boundary layer separation, both using viscoelastic implants placed under the flow. It is experimentally demonstrated that the appropriately chosen viscoelastic materials can simultaneously reduce the skin friction coefficient at the measurement location by 11% and mitigate the size of a large-scale separated flow by up to 28%. The reported performance matches the current generation flow effectors in both separation scale and skin friction mitigation. The present study opens a new application space for soft/programmable materials in high speed aerial vehicles.

Some rheological properties of plastic greases by Carreau-Yasuda model

Various greases are often used for lubrication of various roller bearing and more seldom for slide bearings operating under rather lower speed values, when the mixed or boundary friction occur, for example, during operation in the start-stop regime. Three various commercial greases were tested, differing in the value of the kinematic viscosity of the base oils used in them, measured at temperature 40 °C and 100 °C, and the resulting different dropping point temperature. The rheological properties of such greases have been determined using the rotary rheometer. Based on the resulted dependencies of the apparent viscosity on the shear rate the parameters of the Carreau-Yasuda rheological model chosen, have been estimated. The prediction of rheological parameters for three plastic greases studied obtained with particularly good accuracy. Among the tested greases, the grease with the highest drop point temperature turned out to be a mechanically stable grease. The differences in parameters values could be affected by differences in the compounds of the greases studied.

Properties of Lubricant Additives in Slip with Boundary Friction

Properties of Lubricant Additives in Slip with Boundary Friction

Relationship between salivary lubrication and temporal sensory profiles of wine mouthfeel and astringency sub-qualities

Wine mouthfeel plays an important role in sensory quality. Astringency, as an important aspect of mouthfeel, is composed of distinct sub-qualities associated with tannin-salivary colloidal interactions and loss of oral lubrication. Perception of wine mouthfeel is temporal – the sensory intensity of wine mouthfeel attributes changes over the oral process of evaluation. It remains unclear whether these changes in sensory are relevant to distinct physical processes. This study took sensory-instrumental and multivariate approaches to provide mechanistic insights into the wine mouthfeel during the temporal evaluation process. The sample set was created by doctoring ethanol, tannin, pH and mannoprotein in a Malbec. Wines were sensorily evaluated by a progressive profiling method and instrumentally tested using a soft-tribology protocol involving saliva. Results indicate that the scores of mouthfeel attributes are dependent on the interactive effect of matrix components. The decaying rate of the score for roughness after expectoration correlates with those for drying and pucker. The multivariate analyses indicate that tannin-protein interaction and high boundary friction in tribology are important in explaining the perception of all astringency sub-qualities. At the end of the evaluation, the boundary friction becomes less important in explaining the score of drying/roughness, while pH becomes less important in explaining the score of pucker. It is inferred that, after expectoration, saliva secretion recovers the oral lubrication; astringency sub-qualities are contributed less from the frictional mechanism but more from an alternative perceptive mechanism. The outcome could assist the wine industry in better managing the wine matrix to attain preferable mouthfeel characteristics.

Granular stresses in granular flows subjected to different obstacles

Although the application and development of continuum approaches demonstrate immense promise to model granular flows, the assumption of isotropic stress state in a constitutive law needs to be carefully examined, especially for accurately modeling the kinematics and kinetics of obstructed granular flows. This paper investigates the effect of obstacles on granular stresses and stress ratios in 3D granular silo flows by using discrete element method (DEM). The adopted obstacles include a conical insert, disk insert, BINSERT, hollow cylinder insert and hollow silo insert, as well as the absence of any insert for comparison. A novel (simple but robust) method with ring measurement cells was proposed to determine average stress tensors for granular flows. An innovative slice method with null friction boundaries was also proposed to significantly reduce DEM computational time. The validity of the proposed DEM model was justified by a repose angle experiment and two numerical verification examples. Results reveal that the blocking effect from the obstacles and the hopper wall of the silos led to a significant increase in normal and shear stresses. The normal stresses exhibited slight anisotropy in steady and uniform granular flows, and resulted in a stress ratio of approximately 1.0, indicating an approximately isotropic stress state. Unlike the normal stresses, the shear stresses however showed strong anisotropy even in the steady and uniform regime. The obstructed granular silo flows exhibited strong anisotropy not only in normal stresses, but also in shear stresses. The stress ratio in the obstructed granular flows can appreciably deviate from unity, varying between 0.27 and 2.24. Moreover, the vertical stresses dominated the contact force transmission in the granular silo flows, and showed the formation of dynamic granular arching.

Mechanical Characteristics Regulation of V-Shaped Standing-Wave Ultrasonic Motors With Minimal Quantity Lubrication

Friction drive and wear control is a tough issue for linear standing-wave ultrasonic motors with intermittent contact and impact friction. For the widely recognized reduction of friction/wear, minimum quantity lubrication was adopted to improve the output performances and services for motors through friction regulation. Given different preload and driving voltages, the mechanical characteristics of motors with minimal quantity lubrication were investigated using a homemade test rig. Results show that output performances and driving efficiency under film lubrication differ from those of dry friction. The optimized driving voltage was reduced by 31.5% and the minimum startup voltage was reduced by 41.67% using film lubrication. Additionally, the output speed increased by 26.29%, and thrust decreased by 41.26% under uniform exciting conditions (e.g., 150 V at 39.14 kHz). The existence of film lubrication broadens the effective frictional contact boundary, and effective friction contribution enhances the driving efficiency (the maximum efficiency of motors increased by 14%). Furthermore, the singularity of the effect of preload is significantly improved after lubrication, and the friction interface of the stator/slider becomes more controllable. Suitable lubrication is critical to the improvement of startup performance and prone to realize low-voltage driving, and the combination of lubrication regulation and wear inhibition is conducive to the durability and reliability of motors. This study provides an additional alternative for performance improvement of linear standing-wave motors apart from frictional materials.

On the Finite Element Approximation of a Semicoercive Stokes Variational Inequality Arising in Glaciology

Stokes variational inequalities arise in the formulation of glaciological problems involving contact. We consider the problem of a two-dimensional marine ice sheet with a grounding line, although the analysis presented here is extendable to other contact problems in glaciology, such as that of subglacial cavitation. The analysis of this problem and its discretization is complicated by the nonlinear rheology commonly used for modelling ice, the enforcement of a friction boundary condition given by a power law, and the presence of rigid modes in the velocity space, which render the variational inequality semicoercive. In this work, we consider a mixed formulation of this variational inequality involving a Lagrange multiplier and provide an analysis of its finite element approximation. Error estimates in the presence of rigid modes are obtained by means of a specially-built projection operator onto the subspace of rigid modes and a Korn-type inequality. These proofs rely on the fact that the subspace of rigid modes is at most one-dimensional. Numerical results are reported to validate the error estimates.

Longitudinal velocity profile of flows in open channel with double-layered rigid vegetation

Aquatic vegetation of different heights is widely scattered in natural rivers and is conducive to their environmental function while affecting the flow hydrodynamic conditions. A semi-analytical velocity model is constructed and used to study the longitudinal velocity profile in open channel flow through double-layered rigid vegetation. The double-layered vegetation flow is separated into three zones according to the velocity profile: 1) nearly uniform distributed velocity zone 1A in the lower region of the short vegetation layer, 2) a mixing layer zone B, 3) uniform distributed velocity zone 2A in the upper region of the tall vegetation layer. Two force equilibrium equations about the gravity-driving and vegetation drag are solved to obtain the uniform velocity distribution equations in zone 1A and 2A. The velocity of zone 1A and B is further modeled as a linear superposition of two concepts: the uniform velocity distribution term of zone 1A and a hyperbolic tangent profile. Meanwhile, longitudinal velocity and the lateral vorticity profiles of open channel flow through double-layered rigid vegetation are studied by laboratory flume tests of different vegetation arrangements exposed to two water depths and three slopes. The experimental results show that the longitudinal velocity increases with the slope increase. The verification of the velocity model is based on the instantaneous velocity measured by Acoustic Doppler Velocimetry (ADV), which shows acceptable agreement, indicating that the model can give a reference to the longitudinal velocity of multi-layered vegetation flow in some cases. The effects of wake vortices and boundary friction on the model are further explored in the discussions. The results presented in this study could contribute to the management of aquatic vegetation configurations and the restoration of freshwater ecology.

Investigation of the Wear Resistance of a Journal Bearing with Polymer-Coated Grooved Support Ring

Introduction. In modern heavy-loaded friction units, metallopolymer coated bearings operating in the boundary friction mode are widely used. Their successful application is provided by the viscoelastic deformation of these coatings under load. To pass from boundary friction to liquid friction, it is required to create a bearing hydrodynamic wedge. Currently, the use of journal bearings with polymer-coated grooved support ring is hindered by the lack of a methodology for their calculation. This work analyzes a model of movement of a micropolar lubricant in the operating clearance of a journalbearing with a nonstandard support profile having a PTFE composite coating with a groove on the bearing surface. The study aims at establishing the dependence of the stable hydrodynamic regime on the width of the groove on the surface of the bearing profile. Materials and Methods. Tribological tests of journal bearings with a nonstandard bearing profile having a polymer coating with a groove on the surface were carried out on samples in the form of partial bushes (blocks). Using the equation of movement of a lubricant with micropolar rheological properties, as well as the continuity equation, new mathematical models were obtained that took into account the width of the groove, polymer coating, and nonstandard bearing profile. Results. A significant expansion of the applicability of design models of journal bearings with structural changes has been achieved. Polymer-coated bearings with a groove provided a hydrodynamic lubrication mode. The results obtained allowed us to evaluate the operational characteristics of the bearing: hydrodynamic pressure value, load capacity, and coefficient of friction. Discussion and Conclusions. The design of polymer coated journal bearing and a groove 3 mm wide on the surface of the liner provided a stable ascent of the shaft on the hydrodynamic wedge, which was validated experimentally. The experiments were carried out for journal bearings with a diameter of 40 mm with a groove 1–8 mm wide, at a sliding speed of 0.3–3 m/s and a load of 4.8–24 MPa.

In vitro oral simulation based on soft contact: The importance of viscoelastic response of the upper jaw substitutes.

Real oral processing is the squeezing and shearing between two soft surfaces. The importance of soft palate surface cannot be ignored while focusing on tongue substitutes. Thus the effects of viscoelasticity, roughness of upper jaw substitutes, and fluid rheological properties on lubrication properties were explored by in vitro oral tribology experiments. Different palate substitutes significantly changed the friction curves of pure water, milk, and yogurt. The boundary friction coefficients of pure water and milk are higher under softer or smooth palate substitutes due to stronger viscoelastic responses of friction pairs. Their boundary friction coefficients are lowest at rigid upper jaw substitutes owing to smaller contact angles and deformation. However, the boundary friction coefficient of yogurt is lower owing to its high viscosity, low loss factor, and large particle size under soft friction pairs. In addition, it is highest at rigid palate friction pair because a smaller contact area reduces the entrainment of yogurt, resulting in poor lubricating performance.

Development and application of the multifunctional transmission roll-sliding friction tester

This paper aims to elaborate on the development and the main features of the multifunctional transmission roll-sliding friction tester. The tester adopts the independent drive for controlled relative movement, and the contacting specimen that sustains the radial loads can be replaced, so it can complete the rolling friction, sliding friction, and roll-sliding friction test. The integrated transmission performance test machine not only has solved the inconsistency between the friction and transmission experimental boundary conditions but also can precede the friction drive research. Piecewise calibration data and interpolating correction algorithm are used to avoid the affecting generated by the spindle friction to the specimen. The monitoring system software is developed by the LabVIEW platform, and the parameters are controlled automatically by the computer. Thus it can simulate actual working conditions very well and achieve automatic real-time data acquisition, processing and showing the results, etc.

Determination of the properties of lubricant additives operating under boundary friction conditions

Sodium silicate and sodium hydroorthophosphate dissolved in glycerin are tested on an ИИ-5018 friction machine during sliding under boundary friction conditions. The dependences of the change in the friction force on normal loads and the coefficient of friction on the sliding speed are obtained. The optimal composition of the lubricant with a complex of additives is determined. Keywords: friction machine, laboratory tests, lubricant, coefficient of friction, additive. vkohan@yandex.ru, mia@rgups.ru, glazunovdm@yandex.ru

Finite element simulations for slip flow and heat transfer phenomenon through a cosine-based wavy channel

The objective of current communication is to study heat transfer phenomenon for slip flow of viscous fluid due to wavy channel with general cosine function boundaries and fixed amplitude. The walls along with slip boundary constraints are kept at different temperatures. The flow is incompressible and Newtonian with AIS as a predicting material being used to check the fluids and thermal properties. The Navier–Stokes expressions with 2D flow regime subject to heat transfer due to convection are used to develop the simulations. A parametric theoretical assumptions analysis is performed for specified range of Reynolds number (100–1000) with upper and lower surface vibration periods of 1 to 6. The results are displayed with graphs, surface and contours plots and first, ever a novel work was done to represent the percentage change in velocity magnitude and local Nusselt number as surface plots and contours, respectively. The results are authentic due to mesh independent study and verification with the experimental correlation. A periodic flow at the lower wall was deducted. The maximum and average rotation rates attain a linear relationship with Reynolds number and their correlation was found. The simulations show the strict relationship of Reynolds number and the geometry of the channel with shear rate. The pressure gradient in [Formula: see text]-direction was found minimum in trough and maximum in the crest region. It has been observed that the boundary friction is reduced due to periodic variation of walls surface.

Low-voltage driving linear piezoelectric motors having textured sliders of surface multi-connected vesicle-like microarray

Friction behavior and serious wear of the stator/slider pair are critical to restrict the development and commercialization of standing-wave linear ultrasonic motors. Owing to the effective friction regulation of laser surface microtextures, a multi-connected vesicle-like microarray consisting of the micropits and microgrooves was prepared on the slider surface. Finite element analysis at the microscale showed the proposed micro-textures were prone to cause the stator tip to sink, resulting in an increase of horizontal impact effect of ultrasonic vibration and the alternately regulating of static-dynamic friction boundary, which enhances the tangential force and improves the driving effect. The experimental results approved that the composite microtextures combining the pits of 100 μm and the grooves of 50 μm are positive for friction drive and output performance enhancement. Specifically, the output power of the low-voltage driving linear piezoelectric motor with the complex texturing array is 66% higher than that of non-textured ones. This study provides a reliable approach to enhancing the friction-driving performance and operation stability of piezoelectric motors or actuators.

Synthesis mechanism of Ni–Co alloy uniting ultrahigh strength with tensile ductility via abrasive-assisted electroforming

Persistent efforts are being made to combine strength and ductility in metallic nanomaterials. We successfully prepared a homogeneous nanocrystalline Ni−27%Co alloy exhibiting ultrahigh strength and sufficient ductility using abrasive-assisted electroforming (AAE) with free ceramic beads. The AAE process permits superior control of the microstructure of nanocrystalline Ni–Co alloy. Our results demonstrated that the structure of the Ni−27%Co alloy (AAE) consists of ultrafine equiaxed nanocrystals with an average grain size of 88 nm and a twin boundary fraction of 24%. The alloy structure results from the combined effect of the addition of Co as an alloying element and the electrochemical and physical action of the moving ceramic beads on the nucleation and growth. The Ni−27%Co alloy (AAE) exhibited superior yield and tensile strength of 1103 ± 47 and 1639 ± 19 MPa, respectively, while having excellent ductility with tensile elongation to failure of 8.5 ± 0.5%. The yield strength of the Ni−27%Co alloy (AAE) was approximately 1.6 times that of the Ni−26%Co alloy (Direct current electroforming, DCE). This result can be attributed to the contribution of strengthening mechanisms that involve frictional stress, dislocation, grain boundary, solid solution, and twin boundary strengthening. Among them, the grain boundary (Hall-Petch) strengthening, arising from grain boundary-dislocation interactions, and twin boundary strengthening, arising from the intersection of the twin-dislocation, account for approximately 70% of the total yield strength. The tensile ductility of the Ni−27%Co alloy (AAE) at ultrahigh flow stress can be preserved by simultaneous strain and strain-rate hardening using the AAE method. This study provides a novel strategy for synthesizing ductile ultrahigh strength materials for applications in microelectromechanical systems, surface coatings, and X-ray telescopes.