Friction Stress Research Articles

Friction through molecular adsorption at the sliding interface of hydrogels: theory and experiments.

We report on the frictional properties of thin (≈μm) poly(dimethylacrylamide) hydrogel films within contacts with spherical silica probes. In order to focus on the contribution to friction of interfacial dissipation, a dedicated rotational setup is designed which allows to suppress poroelastic flows while ensuring a uniform velocity field at the sliding interface. The physical-chemistry of the interface is varied from the grafting of various silanes on the silica probes. Remarkably, we identify a velocity range in which the average frictional stress systematically varies with the logarithm of the sliding velocity. This dependency is found to be sensitive to the physical-chemistry of the silica surfaces. Experimental observations are discussed in the light of a molecular model where friction arises from thermally activated adsorption of polymer chains at the sliding interface, their elastic stretching and subsequent desorption. From this theoretical description, our experimental data provide us with adhesion energies and characteristic times for molecular adsorption that are found consistent with the physico-chemistry of the chemically-modified silica surfaces.

Natural state parameter for sand

Based on the frictional state concept, the natural state parameter for sands is defined as an extension of the state parameter defined by Been and Jefferies. The proposed definition of the natural state parameter is the sum of the distance between the normalised difference of the current stress ratio and the critical frictional stress ratio and the difference between the current and critical frictional state void ratio for the same mean principal stress. Therefore, it combines the difference between the current and critical frictional state in the q ‒ p’ and e ‒ p’ planes and can be treated as an extension of Been and Jefferies’ definition of the state parameter in the e ‒ p’ plane. The results of drained triaxial compression tests for Toyoura sand and Dog’s Bay sand, presented in the literature, are analysed. The values of the natural state parameter at failure for these sands are equal to zero. Therefore, the critical state in the q ‒ p’ and e ‒ p’ planes can be determined by analysing the conditions at failure. At failure, the deformations of the samples are almost homogeneous, and the stresses and deformations (void ratios) can be correctly determined. Additionally, the critical frictional state and critical state are very similar for these sands. The relationship between the dilatancy and the state parameter at failure, similar to that given by Been and Jefferies, was obtained directly by using the frictional state concept and the proposed definition of the natural state parameter. The natural state parameter, like the state parameter, can be used for modelling of sands in the future.

Friction in roller technological machines

The study is devoted to solving the problem of determining the patterns of friction stress distribution in the symmetric roll module contact zone. A mathematical model was developed that describes the patterns of friction stress distribution in the contact zone of roll modules. It was established that the ratio of friction stress to normal stress in the contact zone of roll modules is not constant and changes at each point of the contact zone. It was revealed that the ratio of friction stress to normal stress depends on external forces acting on the roll supports, geometric and kinematic parameters of the roll module, and deformation properties of the strip and roller coating.

Study on modeling of contact interaction in roll modules

The main problem of the mechanics of contact interaction in roll modules is to determine the stress-strain state in the technological zone. The article presents mathematical models of the laws of distribution of contact stresses and analytical dependencies of their main indices, which make it possible to calculate the power parameters of roll modules of technological machines. When developing mathematical models, empirical formulas for the laws of distribution of normal stresses along roll contact curves and a friction stress model were used, considering the effect of external forces, roll drive, and compliance of the roll coating on the distribution of shear stresses. It is shown that normal stresses in the compression zone increase from zero to the maximum value by the harmonic law, and in the recovery zone, they decrease from the maximum value to zero by the parabolic law. It was revealed that in the driven roll, the neutral point is located on the left side of the point of maximum normal stress, and the points that determine the maximum normal stress and compression of the interacting bodies coincide and are shifted from the line of centers towards the supply of the processed material into the contact zone.

Excellent cryogenic mechanical properties of a novel medium-entropy alloy via vanadium-doping and hetero-grain/precipitation engineering

A novel vanadium-doped f.c.c. medium-entropy alloy (MEA) Ni42.4Co24.3Cr24.3Al3Ti3V3 was designed to achieve excellent cryogenic mechanical properties. The vanadium was added in order to increase the temperature-dependent friction stress, and aging of the MEA, which had been given a homogenization anneal at 1150 °C followed by cold-rolling to 80% reduction, at 700–900 °C was used both to produce partial recrystallization and precipitate L12 nanoparticles. Aging produced a heterogeneous microstructure composed of fine, soft recrystallized grains surrounded by harder, relatively-large non-recrystallized grains, containing L12-precipitates. Increasing the aging temperature from 700 °C to 900 °C promoted V partitioning into the f.c.c. matrix with the partitioning coefficient increasing from 0.8 to 1.9; the grain size increasing from 2.3 μm to 4.6 μm; an increase in the precipitate size from 10.5 nm to 80.0 nm; a reduction in the residual geometrically-necessary dislocation (GND) density from 9.0 × 1014 m−2 to 2.8 × 1014 m−2; and a decrease in the volume fraction of precipitates fraction from 28% to 14%. All three aged MEAs showed extraordinary cryogenic mechanical properties with an increase in the aging temperature resulting in an increase in strength but a decrease in ductility, viz, a yield strength (YS) of 1929 MPa, an ultimate tensile strength (UTS) of 2147 MPa, and an elongation to failure (ε) of 6.6% at 700 °C, YS ∼ 1735 MPa, UTS ∼ 1976 MPa, ε ∼ 9.0% at 800 °C, and YS ∼ 1274 MPa, UTS ∼ 1694 MPa, ε ∼ 24.8% at 900 °C. The dislocation back stress rose progressively with increasing strain and was invariably higher than the effective stress, accounting for 51–59% of the flow stress i.e. the back stress played a dominant role in the ultrahigh flow stress. Increasing the aging temperature also produced a greater abundance of nanoscale deformation twins, stacking faults, and dislocation networks, which are responsible for excellent strain hardening capacity and greater ductility. After deformation, the density of low-angle grain boundaries increased by 3–6 times to 0.6–1.3 μm/μm2, and the GND density increased by 4–6 times to 1.2–2.0 × 1015 m−2.

Grain boundary strengthening in nanocrystalline Mo0.2CoNi medium-entropy alloys produced via high-pressure torsion

In this study, we produced nanocrystalline (NC) MoCoNi-based medium-entropy alloys (MEAs) with a single face-centered cubic (FCC) phase, which exhibit exceptional hardness through combining compositional and microstructural features of alloys. The alloys were prepared via powder metallurgy and high-pressure torsion, and the processing condition was optimized to achieve full densification and prevent grain growth in order to maximize the effect of grain boundary strengthening. Consequently, NC Mo0.2CoNi MEAs revealed the refined microstructures with a grain size of approximately 50 nm and outstanding hardness of 857 HV (corresponding to approximately 2.86 GPa in yield strength). This remarkable property, even compared to the single FCC phase alloys with similar grain size, is attributed to the existence of alloying elemental atoms of Mo with large atomic radius, which leads to severe lattice distortion allows to improve friction stress as well as the Hall-Petch coefficient. This study aims to discuss the potential of developing hard metallic materials by using grain boundary strengthening from alloys with large atomic size difference.

INFLUENCE OF ROLLING SPEED ON THE TEMPERATURE FIELD DURING COLD ROLLING OF ALUMINIUM SHEETS

One of the effects of plastic deformation is an increase in the temperature of the formed metal. The rolling pressure, friction stress, and contact state show different characteristics in different zones along the deformation zone, which causes the heat generation and transfer states to be different, so the strip temperature is subject to a complex change process. The paper presents the results of experimental temperature measurements at several points on the width of the rolled strip and on the surface of the working rolls depending on the rolling speed in the range of 0.5-4.0 m/s. The results obtained showed a clear dependence of both the strip temperature and the working roll temperature on the rolling speed, with the increase in strip temperature being more intense. At very high rolling speeds, the temperature of the rolled sheet may even exceed 1000C, which is unfavourable due to the possibility of the formation of deposits on the surface of the rollers.

Closed‐form solution for axially loaded end‐bearing piles in two‐layered soil

AbstractThis paper presents a mathematical model for the analysis of axially‐loaded end‐bearing piles embedded in a two‐layer elastic soil deposit. The governing equations of the soil layers surrounding the pile are cast by means of a Tajimi‐type continuum formulation, and this allows deriving a closed‐form expression that provides the frictional stresses that develop at the soil‐pile interface. The pile is treated as two virtual one‐dimensional elastic rods, and pile displacements are obtained by solving a system of linear equations that results from considering the boundary and continuity conditions. This results in improved efficiency compared to existing Tajimi‐based solutions for piles in layered soil, at no expense of accuracy. The model is used to study the effect of the relative stiffness of the soil layers on the serviceability behavior of axially‐loaded piles, and drawn conclusions of practical importance.

A life off the beaten track in biomechanics: Imperfect elasticity, cytoskeletal glassiness, and epithelial unjamming.

Textbook descriptions of elasticity, viscosity, and viscoelasticity fail to account for certain mechanical behaviors that typify soft living matter. Here, we consider three examples. First, strong empirical evidence suggests that within lung parenchymal tissues, the frictional stresses expressed at the microscale are fundamentally not of viscous origin. Second, the cytoskeleton (CSK) of the airway smooth muscle cell, as well as that of all eukaryotic cells, is more solid-like than fluid-like, yet its elastic modulus is softer than the softest of soft rubbers by a factor of 104-105. Moreover, the eukaryotic CSK expresses power law rheology, innate malleability, and fluidization when sheared. For these reasons, taken together, the CSK of the living eukaryotic cell is reminiscent of the class of materials called soft glasses, thus likening it to inert materials such as clays, pastes slurries, emulsions, and foams. Third, the cellular collective comprising a confluent epithelial layer can become solid-like and jammed, fluid-like and unjammed, or something in between. Esoteric though each may seem, these discoveries are consequential insofar as they impact our understanding of bronchospasm and wound healing as well as cancer cell invasion and embryonic development. Moreover, there are reasons to suspect that certain of these phenomena first arose in the early protist as a result of evolutionary pressures exerted by the primordial microenvironment. We have hypothesized, further, that each then became passed down virtually unchanged to the present day as a conserved core process. These topics are addressed here not only because they are interesting but also because they track the journey of one laboratory along a path less traveled by.

Physics-based model to predict yield strength of single-phase FCC high-entropy alloys over wide temperature range

A physics-based temperature-dependent yield strength model without fitting parameters was developed for single-phase FCC high-entropy alloys. The model considered the temperature dependence of lattice friction stress, solid solution strengthening, grain boundary strengthening, dislocation strengthening, and their evolution with temperature to the overall yield strength. The results show that a quantitative relationship between temperature, material parameters, and yield strength was successfully captured by the model. This model can predict the yield strength at different temperatures only by using the easily available material parameters at room temperature. The accuracy of model was well verified by 17 sets of available experimental data over a wide temperature range (4.2–1273 K). Moreover, the contribution of different strengthening mechanisms to the yield strength was quantitatively analyzed and discussed from 4.2 to 1273 K, and some suggestions for improving the temperature-dependent yield strength were put forward.

Experimental and theoretical study of the shear strength of ice-rock interface

Frost heave failure of fractured rock is an important and challenging problem faced by geotechnical engineering structures in cold regions. The shear strength of the ice-rock interface is a critical parameter in understanding this type of failure. In this study, we conducted shear tests on ice-rock interfaces to examine effects of freezing temperature, normal stress, and interface roughness on the shear strength of the interface. Furthermore, we derived a theoretical model for the shear strength of the ice-rock interface considering these three influencing factors. Comparisons between the theoretical and experimental results confirmed that the shear stress at the ice-rock interface was composed of two components, i.e., the ice-rock particle bonding stress and the frictional stress caused by the rough interface. The freezing temperature mainly affected the bonding stress of ice-rock particles, while the normal stress and the roughness affected the frictional stress. The ice melting happened during the shearing process, leading to the appearance of a second or third peak in the stress-strain curve.

Phenomenon-Based Model for Virtual Hot Strip Rolling

Every year digitalization is taking a bigger role in the steel industry. Models for predicting metallurgical phenomena, roll forces and microstructure have been commonly used in development of novel steel grades. These individual models may predict certain phenomena thoroughly, but input values are usually based on an assumption or on a “good guess”. To produce reliable boundary conditions for these models of individual phenomena, a virtual rolling model is developed. This model computes the whole process of the hot strip mill from roughing to accelerated water cooling on a run-out table. Strip location and temperature evolution is calculated continuously. Thermal and thermo-mechanical (rolling stands) boundary conditions are according to process layout. Input data for the model is automatically read from raw process data. Rolling parameters are calculated using a coupled ARCPRESS model, which is developed by authors, and calculates normal and frictional shear stress distributions in the roll gap to predict roll forces and displacements of the work roll surface. Recrystallization is considered when calculating the flow stress of the rolled strip. Phase fractions during water cooling are calculated as well. The virtual rolling model minimizes the need for parameter speculation as all parameters are calculated throughout the process. All the input values are read from actual process data and the metallurgical and mechanical state of the strip are computed throughout the whole process. As required by the state-of-art virtual rolling model, this model is based on generally accepted theories and experimentally studied metallurgical and physical phenomena along with the thermo-mechanical response of the actual rolling process.

Effect of different friction coefficient models on numerical simulation of inertia friction welding of 2219 Al alloy to 304 stainless steel

In inertia friction welding (IFW) of Al alloy to steel, the friction coefficient of interface influences the friction heat production and determines the accuracy of the numerical simulation of welding process. A fully coupled three-dimensional model was established to simulate the IFW process of 2219 Al alloy to 304 stainless steel. By taking different simplifications in the influencing factors (i.e., temperature, sliding velocity, and pressure) of friction coefficient, three types of friction coefficient models were established and utilized in the simulation of IFW process. The strain compensated Arrhenius and Johnson-Cook models were employed to describe the deformation behavior of Al alloy and steel, respectively. Both Al alloy and steel were assumed to be the elastic-plastic body, and the friction stress was calculated by the Coulomb friction model. The friction interface temperature, angular velocity attenuation, axial shortening distance, and joint appearance were measured to verify and compare the accuracy of the established models. The temperature and stress evolutions during the IFW process were also analyzed. With the smallest error in validation, the friction coefficient model, which was a function of temperature, sliding rate, and pressure, showed the best accuracy in describing the friction behavior. By considering the effect of pressure, a more accurate shear stress at the center region of the interface was calculated with a lower value. It means that the friction resistance at the region was lower, promoting the flow of internal Al alloy and the formation of curly-shape flash.

Modeling and analysis of the cutting temperature of titanium alloys for quasi-intermittent vibration assisted swing cutting

EVC has proven to be an effective means of machining difficult-to-process materials. QVASC inherits EVC's intermittent cutting and friction reversal advantages, eliminating EVC cutting residues and improving machining quality. In order to deeply understand the cutting mechanism of QVASC, it is necessary to understand the cutting heat generation and tool surface temperature distribution in the process of QVASC. In this paper, the kinematic and frictional characteristics of the QVASC machining process are analyzed, the heat generation and heat transfer model of QVASC machining process is established, and the cutting temperature prediction model considering the characteristics of QVASC machining is proposed. According to whether there is heat convection and frictional shear stress distribution between the tool and the environment, the heat flow distribution of the front surface of the tool is divided into four regions, and the heat flux of the tool is calculated respectively. On this basis, the tool temperature of QVASC is solved by using the heat balance equation in the form of the partial differential equation. Finally, a systematic comparison experiment was carried out, and the effects of cutting depth, cutting speed, tool swing frequency, and amplitude on cutting temperature were studied by single factor method. The results show that the average cutting temperature of QVASC is 10 % lower than that of OC, which verifies the advantage of QVASC in reducing cutting temperature. Under different cutting depth, cutting speed, tool swing frequency and amplitude, the change trend of theoretical and experimental cutting temperature is the same, and the average error between them is 9 %, 8 %, 10 % and 9 %, respectively, which is within the acceptable range, proving the validity of the theoretical model. This research contributes to a deep understanding of the cutting process of QVASC and effectively determines the optimal cutting parameters.

Ultrahigh grain boundary strengthening ability of VCoNi medium entropy alloy

Tensile tests of VCoNi medium-entropy alloy with different grain sizes were conducted at 77 K, 194 K and 293 K. The results show that the grain boundary strengthening ability characterized by the Hall-Petch slope of the alloy increases with decreasing temperature and is evidently higher than that of FCC pure metals, CuAl alloys, and some common FCC high-entropy alloys/medium-entropy alloys at room temperature. A new model correlated to intrinsic stacking fault energy and lattice friction stress is derived to describe the high grain boundary strengthening ability. The high grain boundary strengthening ability and its temperature dependence of the VCoNi medium-entropy alloy are mainly attributed to the high lattice friction stress of the VCoNi alloy. The findings provide a possibility for designing the single-phase FCC medium-entropy alloy with ultra-high yield strength and considerable plasticity.

Anisotropic behaviors of calcareous sand dependent on loading direction and initial shear stress

Calcareous sand has been a suitable unbound granular filler in island reef engineering. Given its unique microstructure and the complex loading situation in practical engineering, anisotropy is a crucial factor that cannot be ignored. In light of this, we conducted a set of hollow cylindrical shear tests to explore how the loading direction and initial shear affect the anisotropic behaviors of calcareous sand, with Silica sand as the control material. A remarkable finding is that calcareous sand is more anisotropic than silica sand due to its irregular particles, as evidenced by a greater variation in internal friction angle(φf) and stress ratio at failure(Mf) with influencing factors. To accurately represent these variations caused by anisotropy, a parameter for quantifying anisotropy was found. The dependence of the two influencing factors on Mf was formulated based on the parameter. The formulated Mf was then applied to the UH model, and the predicted and experimental results of the shear response were compared to evaluate the model performance.

Imaging of Accumulated Mechanical Stresses Using Self-Assembled Layered Conjugated Polymer.

When mechanical stresses, such as tensile, compressive, and frictional stresses, are applied to objects by various motions, they are accumulated in materials. Conventional mechanoresponsive materials and sensors detect one-time applied stress. However, the accumulated stresses are not visualized or measured in previous works. The present study demonstrated imaging and sensing of not only one-time but also accumulated tensile, compressive, and frictional stresses. Polyurethane (PU) film was combined with 2D layered polydiacetylene (PDA), a stimuli-responsive color-changing polymer. PDA generally exhibits no color changes with the application of tensile and compression stresses because the molecular motion leading to the color change is not induced by such mechanical stresses. Here the versatile mechanoresponsiveness was achieved using a block copolymer guest partially intercalated in the layered PDA. As the interlayer and outerlayer segments interact with PDA and PU, respectively, the applied stresses to the film are transferred from PU to PDA via the block copolymer guest. The color changes of the film imaged and quantified the accumulated work depending on the number and strength of the applied multiple stresses such as tensile, compressive, and frictional stresses. The design strategy of materials and methodology of sensing can be applied to the development of new sensors for accumulated mechanical stresses in a wide range of length and strength scales.

Analytical solution for axial load transfer of floating piles

This paper presents an analytical solution for the analysis of axially-loaded compressible floating piles embedded in a homogeneous elastic soil layer of finite thickness. The mathematical model is based on a Tajimi-type elastic continuum formulation, that allows deriving an accurate closed-form expression for the frictional stresses developing at the soil-pile interface. The pile and its underlying soil column are considered as one-dimensional axially deforming rods, and solution of their governing equations leads to closed-form expressions for pile and soil displacements and stresses. The proposed model provides results of similar accuracy to boundary integral solutions for the special case of floating piles in elastic half-space, and to numerical analyses for the general problem of floating piles embedded in a layer of finite thickness. As such, the model is used to prepare charts for the settlement analysis of floating piles in practice, based on a parametric analysis that unveils the sensitivity of the axial response of floating piles to the different problem parameters.

Wall friction and heat transfer in turbulent pulsating flow in a square duct

For the first time, a DNS study of pulsating turbulent flows and heat transfer in a square duct is performed. Calculations were carried out at Re=2200 in the current-dominance mode for eight values of the oscillation period and three values of the Prandtl number. It was found that the time-mean friction coefficient in the studied pulsating flows has a lower value than in the stationary flow. The difference reaches −14.7%. The values of the Nusselt numbers are also decreasing, albeit by a smaller amount. The features of secondary flows of Prandtl’s 2nd kind at different phases of the flow oscillations are studied. The profiles of the wall friction stress and heat transfer coefficients and their evolution over time are determined. The time-oscillations of the wall friction are entirely determined by the oscillations of the main flow, being ahead of it in phase by π/4, which coincides with the behavior of laminar pulsating flows at high frequencies. The oscillations in heat transfer on the wall are determined by oscillations in the intensity of turbulent heat exchange, and the change in the latter occurs almost in phase with a change in the volume-average intensity of the transverse velocity fluctuations.

Modelling gas fracturing experiments in saturated clay using zero-thickness interface elements

The injection of gas in a liquid-saturated clay-rich material may lead to the formation of narrow channels or fractures created by the mechanical action of gas pressure. However, the complexity of the mechanical and transport properties of clays, combined with the high compressibility of gases, makes it challenging to understand and address this phenomenon. Gas fracturing can occur in a range of engineered processes, for instance the stimulation of sensitive hydro-carbon reservoirs [1], carbon dioxide injection and storage in subsurface reservoirs [2], pneumatic fracturing for enhanced remediation of contaminated soils [3], or gas transport through natural and engineered clay barriers in geological disposal facilities for radioactive waste [4]. Despite the multiple environmental and economic implications associated with gas fracturing it remains difficult to predict and control due to the lack of fundamental scientific insight.
 One of the most challenging aspects of understanding gas-driven cracking in clays is the difficulty in visualising the crack formation in real time. In recent years, a new experimental setup has been proposed by the British Geological Survey, which makes it possible to induce and observe the formation of “two dimensional” cracks in clay-rich low-permeability materials by the injection of gas or water [5,6]. In this setup, a thin layer (~1mm) of clay paste is compressed between a 110 mm diameter sight glass and a steel plate, while it is held laterally in place by a ring filter (Figure 1). Then, gas is injected at the centre of the lower platen at a controlled volumetric rate while the cracks induced are registered by a camera through the sight glass.
 In order to aid the interpretation of the results obtained with this new device, the authors have carried out a series of numerical simulations performed with a finite element approach recently published [7], and compared them to experimental data. The model uses a fully coupled Pneumo-Hydro-Mechanical (PHM) formulation in 2D, and is implemented in the code LAGAMINE [8]. Two different types of finite elements are used: continuum and interface elements. Continuum elements are used to represent the mechanical and flow processes in the bulk clay material and interface elements are used to represent cracks. Interface elements are introduced a priori in between the continuum elements in order to provide potential cracking paths. The model parameters are such that, as long as the interface elements remain closed, they do not have any significant effect on the response of the model.
 These simulations required two additional numerical developments. First, since the model is 2D (plane strain), the tangential, frictional stresses developed at top (clay-glass interface) and bottom (clay-steel interface) were introduced as a body force, using an elastoplastic formulation in terms of the in-plane displacements and the out-of-plane stress. Second, a mixed boundary condition was implemented to make it possible to impose a given volumetric gas injection rate as in the experiments.
 The simulations show that the model proposed can reproduce the experimental results, providing valuable insights into the factors controlling the onset and propagation of the gas fractures (Figure 1). These insights have the potential to enhance the performance and safety of projects involving gas fracturing processes