Frictional Shear Stress Research Articles

The effect of local atomic configuration in high-entropy alloys on the dislocation behaviors and mechanical properties

High-entropy alloys (HEAs) are promising as advanced structural materials for various applications owning to their comprehensive properties combining high strength and ductility, good thermal stability and high resistance to wear and corrosion. Although such outstanding mechanical properties have been attributed to the four core effects, it is still unclear how the inherent local chemical variations in HEAs affect dislocation dynamics and plastic deformation behaviors at the nanoscale. Herein, we first introduced an accurate in-house potential for Al x CoCrFeNi HEA, and then employed molecular dynamics simulation methods to investigate the effect of local atomic configuration (LAC) on the dislocation motion, lattice friction and critical shear stress. The results show that compared with pure metals, there is a huge fluctuation in the critical shear stress of dislocation motion in the HEA, indicating that LAC strongly influences the dislocation dynamics and plastic deformation behaviors. It is suggested from the results that introducing large-size atoms in HEA would effectively enhance the dislocation dynamics, which is beneficial to both strengthening and toughening of HEAs.

Rubber Adhesion and Friction: Role of Surface Energy and Contamination Films

We study the influence of the surface energy and contamination films on rubber adhesion and sliding friction. We find that there is a transfer of molecules from the rubber to the substrate which reduces the work of adhesion and makes the rubber friction insensitive to the substrate surface energy. We show that there is no simple relation between adhesion and friction: adhesion is due to (vertical) detachment processes at the edge of the contact regions (opening crack propagation), while friction in many cases is determined mainly by (tangential) stick-slip instabilities of nanosized regions, within the whole sliding contact. Thus while the pull-off force in fluids may be strongly reduced (due to a reduction of the work of adhesion), the sliding friction may be only slightly affected as the area of real contact may be dry, and the frictional shear stress in the contact area nearly unaffected by the fluid.

Fiber Formation of Printed Carbon Fiber/Poly (Ether Ether Ketone) with Different Nozzle Shapes

AbstractThe additive manufacturing industry shows annual growth of more than 10 %. Therefore, the requirements for produced components are increasing, especially for individual medical technology applications printed from poly(ether ether ketone) (PEEK). This paper focuses on an investigation of an uninterrupted nozzle system for printing the high‐performance thermoplastic PEEK with short fiber reinforcements. A custom‐made nozzle design with variable outlet angle is presented and the achievable fiber length distribution and fiber orientation in the printed material are investigated. The applied nozzle angles are 60 °, 90 ° and 120 °. The geometric shape of the tip of the custom‐made nozzles is comparable to that of a reference nozzle. With regard to the height profile, the inner surface is up to three times smoother. With a decreasing nozzle angle, a lower degree of fiber damage caused by deposition can be demonstrated. Thus, the process‐induced shortening of the fibers decreases. The enhanced flow profile for small angles outweighs the simultaneous disadvantageous due to friction loss, shear stress and pressure drop. A clear result for the fiber orientation cannot be deduced. © 2021 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry.

Couette flow of viscoelastic dusty fluid in a rotating frame along with the heat transfer

Viscoelastic fluid is an advanced fluid which exhibits both elastic and viscous properties. Whereas rotation of viscoelastic fluid is a very complex phenomenon and has an immense amount of applications in engineering and product making industries. Due to various applications in real life researchers are working to understand the rheology of viscoelastic fluids. Viscoelastic dusty fluids are used in gas cooling systems. In nuclear reactors, dusty fluids are used to lower the temperature of the system. Such fluids are also used in centrifugal separators, which separate solid particles from the liquid state, etc. Therefore, in the present study, viscoelastic dusty fluid is analyzed. More precisely free convective Couette flow under the influence of the transversely applied uniform magnetic field in a rotating frame is considered. The subject fluid is driven by the sine oscillations of the upper plate along with the effect of free convection. Due to rotation, the fluid and dust particles have complex velocities which is the sum of primary velocity and secondary velocity. The flow regime is modeled in terms of partial differential equations. To non-dimensionalize the system of governing equations, dimensionless variables have been derived through Buckingham-Pi theorem. The system of partial differential equations is solved through assumed periodic solutions (Poincare-Light Hill Technique). The expressions for skin friction (shear stresses at y = 0) and Nusselt number (the rate of heat transfer) are also calculated. Moreover, parametric influence on Nusselt number, and velocity profile of the fluid and the dust particles, is discussed. It is observed that increase in rotation parameter η causes retardation in the velocity of dust particles and the fluid. This is due to the fact that the increase in η enhances the Coriolis forces that are in fact inertial forces, causing a retardation in the velocity of the fluid phase and the dust phase.

Interfacial bonding mechanism of Cu/Al composite plate produced by corrugated cold roll bonding

Corrugated cold roll bonding (CCRB) produces metal composite plate with improved mechanical properties compared with conventional methods, but the interfacial mechanism is not fully understood. Here, Cu/Al composite plate with good plate shape was produced by CCRB, and the bonding mechanism and strength along the corrugated interface were studied by experiments and finite element simulations. The results showed that the average bonding strength of Cu/Al composite plate produced by CCRB was nearly twice that of conventional composite plate at an average reduction of 40% during rolling. Strong friction shear stresses occurred at the interface of the corrugated composite plate, which promoted the plastic deformation of the metals and accelerated the rupture of the brittle interfacial layer. Electron backscattered diffraction analysis showed that higher degrees of grain elongation and refinement occurred in the matrices at the front waist and trough due to the stronger normal and shear stresses. Energy-dispersive spectroscopy line scans showed that the thickest atomic diffusion layer occurred at the front waist. The present combination of experimental and computational analyses provides insights into the underlying mechanism of mechanically improved metal composites prepared by CCRB.

Measurement of Friction Shear Stress in Taper Cup Test

Friction shear stress on the tapered surface of the punch has been directly detected using friction sensors during the taper cup test. An aluminum alloy circular cylinder block was used for the test piece with various lubricants. Punch load increased with the frictional stress on the tapered punch surface. The increase in frictional stress with the stroke was estimated to be due to the increase in surface expansion ratio on the tapered punch surface during punch insertion. The relationship between the frictional stress on the punch and the punch load was compared by FEM analysis using friction shear and friction factors. By this study, it was confirmed that the friction sensor can detect frictional stress directly under lubrication, and it will be applicable to the detection of the local friction change during forging under high pressure and under an increasing surface expansion ratio.

Effect of shear stress on adhesive contact with a generalized Maugis-Dugdale cohesive zone model

The interplay between interfacial shear stress and adhesion has been an active but controversial subject of adhesive contact mechanics, which is currently plagued by diverse, sometimes contradicting, predictions. Recently, McMeeking et al. showed that a reversible interface slip parameter plays an essential role in determining how interfacial shear stress affects adhesion for a Johnson-Kendall-Roberts (JKR) contact interface. In this paper, adhesive contact between a rigid spherical indenter and an elastic half-space is studied with a generalized Maugis-Dugdale (M-D) model, where a constant frictional shear stress presents in the intimate contact area while a constant adhesive stress exists in a cohesive zone near the contact edge. The model solution predicts that the contact behavior is governed by a non-dimensional reversible shear index ατ¯2 as well as the Maugis parameter λ. More specifically, it is found that the impact of interfacial shear stress on adhesion is most significant when the model approaches the JKR limit, and it gets less pronounced in the transitional regime and eventually becomes negligible in the Derjaguin-Mulller-Toporov (DMT) limit. Such behavior is in distinct contrast to Johnson's phenomenological solution. Finally, the proposed model is experimentally validated by adhesion tests on contact interfaces with varying Maugis parameters, where the reversible slip factor is experimentally extracted for the first time.

Empirical relationship between interfacial shear stress and contact pressure in micro- and macro-scale friction

This study examines the empirical relationship between frictional shear stress and pressure in macro- and micro-scale contact and sliding. Two types of friction tests are reported; the macro-scale tests deal with kinetic friction between stainless steel surfaces in a vapor phase lubrication condition, and the micro-scale tests measure kinetic friction at interfaces formed between MoS2 basal planes and surfaces of alumina or stainless steel specimens, using a quartz crystal microbalance microtribometer. A numerical model is used to calculate the contact areas in the macro-scale tests. The results from both friction tests confirmed that the interfacial shear stress in the contact area due to kinetic friction is proportional to the average contact pressure, and the constant of proportionality is close to the coefficient of friction (COF). These observations add to the validation of the Amontons’ law.

Cylinder–flat-surface contact mechanics during sliding

Using molecular dynamics we study the dependency of the contact mechanics on the sliding speed when an elastic block (cylinder) with a cos(q_{0}x) surface height profile is sliding in adhesive contact on a rigid flat substrate. The atoms on the block interact with the substrate atoms by Lennard-Jones potentials, and we consider both commensurate and (nearly) incommensurate contacts. For the incommensurate system the friction force fluctuates between positive and negative values, with an amplitude proportional to the sliding speed, but with the average close to zero. For the commensurate system the (time-averaged) friction force is much larger and nearly velocity independent. For both types of systems the width of the contact region is velocity independent even when, for the commensurate case, the frictional shear stress increases from zero (before sliding) to ≈0.1MPa during sliding. This frictional shear stress, and the elastic modulus used, are typical for polydimethylsiloxane rubber sliding on a glass surface, and we conclude that the reduction in the contact area observed in some experiments when increasing the tangential force must be due to effects not included in our model study, such as viscoelasticity or elastic nonlinearity.

Acoustic induced antifriction and its effect on thermo-mechanical behavior in ultrasonic assisted friction stir welding

The developed ultrasonic assisted friction stir welding (UaFSW) process can reduce the welding load and improve the joint quality, but the interaction mechanism between the exerted ultrasonic vibration and the thermo-mechanical behavior induced by friction stir welding is not yet elucidated. In this study, the classical Norton friction model is modified with the acoustic stress work, and the effect of ultrasonic vibration on the contact state at the tool/workpiece interface is analyzed. The relative velocity between the tool and its adjacent contacting material is taken as one of the main influencing factors to calculate the interfacial friction shear stress. A non-uniform interfacial friction model is combined with the modified constitution equation. And a fully coupled model of UaFSW process is developed to analyze the ultrasonic field, interfacial contact state, heat generation, plastic material flow and heat transfer phenomena. It is found that ultrasound can effectively reduce the interfacial friction stress, and this antifriction effect is more obvious in the welding direction which coincides with ultrasonic vibration. The ultrasound exerted along the radial direction of tool in UaFSW process results in acoustic softening of base material and friction reduction at the tool/material interface. Both effects change the heat generation, material flow and temperature field in UaFSW. Thus, the torque and traverse force are lowered with the ultrasonic induced reduction of interfacial stress, and the high quality of welds are obtained with the improvement of material flow. The model is experimentally validated by comparison of predicted and measured tool torque, heat input and thermal cycles.

Large-Size Jointless Concrete Slab-on-Grade with Combined Prestressing

In the international and national practice of design, a different type of slab on the various types of grade are becoming increasingly common. For such structural elements, shrinkage and temperature influences in combination with low tensile stress, mainly in early age, leads to the risk of cracking in reinforced concrete structures, and as result, a reduction of its durability. The present article describes some of the possible ways of usage of the post-tensioned flat slabs and the rational design procedures to provide their structural reliability. The theoretical background of the punching resistance checking, in the case when the piles support the foundation post-tensioned slabs, presented. For ground floor slabs, an iterative method is given for determining design compression pre-stresses distribution in slab sections, taking into account the restrained effect created by the friction shear stresses in contact between the slab and the base. Besides, the article presents some practical implementations of the post-tensioned slabs as an artificial base in the presence of weak soils and as a large-size ground floor (slab-on-grade) without any joints.

The bioengineering theory of the key modes of action of a cyanoacrylate liquid skin protectant.

The objective of this article is to formulate a new bioengineering theoretical framework for modelling the biomechanical efficacy of cyanoacrylate skin protectants, with specific focus on the Marathon technology (Medline Industries, Inc., Northfield, Illinois) and its modes of action. This work details the bioengineering and mathematical formulations of the theory, which is based on the classic engineering theories of flexural stiffness of coated elements and deformation friction. Based on the relevant skin anatomy and physiology, this paper demonstrates: (a) the contribution of the polymerised cyanoacrylate coating to flexural skin stiffness, which facilitates protection from non-axial (eg, compressive) localised mechanical forces; and (b) the contribution of the aforementioned coating to reduction in frictional forces and surface shear stresses applied by contacting objects such as medical devices. The present theoretical framework establishes that application of the cyanoacrylate coating provides considerable biomechanical protection to skin and subdermally, by shielding skin from both compressive and frictional (shearing) forces. Moreover, these analyses indicate that the prophylactic effects of the studied cyanoacrylate coating become particularly strong where the skin is thin or fragile (typically less than ~0.7 mm thick), which is characteristic to old age, post-neural injuries, neuromuscular diseases, and in disuse-induced tissue atrophy conditions.

Achieving slip-hardening behavior of sanded straight steel fibers in ultra-high-performance concrete

This study develops a slip-hardening straight steel fiber in ultra-high-performance concrete (UHPC). For this, a surface treatment using sandpaper with various grits was adopted in both parallel and perpendicular directions to the fiber axis. Surface roughness was assessed by atomic force microscopy (AFM), and its correlation with the pullout resistance was evaluated. Test results indicate that although the surface treatment using sandpapers in both parallel and perpendicular directions is effective for enhancing the pullout resistance of straight fiber in UHPC, the latter is recommended since it exhibits better slip-hardening characteristics. The benefits resulting from inclination were more significant in the sanded fiber case and increased with increasing the grit. The higher surface roughness resulted in better pullout resistance, and the effectiveness was higher when the fibers were aligned. The sanded fiber achieved superior resistance and excellent slip-hardening response but exhibited smaller magnitude of frictional shear stress than the twisted fiber.

Tunable macroscale structural superlubricity in two-layer graphene via strain engineering

Achieving structural superlubricity in graphitic samples of macroscale size is particularly challenging due to difficulties in sliding large contact areas of commensurate stacking domains. Here, we show the presence of macroscale structural superlubricity between two randomly stacked graphene layers produced by both mechanical exfoliation and chemical vapour deposition. By measuring the shifts of Raman peaks under strain we estimate the values of frictional interlayer shear stress (ILSS) in the superlubricity regime (mm scale) under ambient conditions. The random incommensurate stacking, the presence of wrinkles and the mismatch in the lattice constant between two graphene layers induced by the tensile strain differential are considered responsible for the facile shearing at the macroscale. Furthermore, molecular dynamic simulations show that the stick-slip behaviour does not hold for incommensurate chiral shearing directions for which the ILSS decreases substantially, supporting the experimental observations. Our results pave the way for overcoming several limitations in achieving macroscale superlubricity using graphene.

Heavy Oil Laminar Flow in Corrugated Ducts: A Numerical Study Using the Galerkin-Based Integral Method

Fluid flow in pipes plays an important role in different areas of academia and industry. Due to the importance of this kind of flow, several studies have involved circular cylindrical pipes. This paper aims to study fully developed internal laminar flow through a corrugated cylindrical duct, using the Galerkin-based integral method. As an application, we present a study using heavy oil with a relative density of 0.9648 (14.6 °API) and temperature-dependent viscosities ranging from 1715 to 13000 cP. Results for different fluid dynamics parameters, such as the Fanning friction factor, Reynolds number, shear stress, and pressure gradient, are presented and analyzed based on the corrugation number established for each section and aspect ratio of the pipe.

Effects of sediment load on the abrasion of soil aggregate and hydraulic parameters in experimental overland flow

The breakdown of soil aggregates under rainfall and their abrasion in overland flow are important processes in water erosion due to the production of more fine and transportable particles and, the subsequent significant effect on the erosion intensity. Currently, little is known about the effects of sediment load on the soil aggregate abrasion and the relationship of this abrasion with some related hydraulic parameters. Here, the potential effects of sediment load on soil aggregate abrasion and hydraulic parameters in overland flow were investigated through a series of experiments in a 3.8-m-long hydraulic flume at the slope gradients of 8.7 and 26.8%, unit flow discharges from 2×10−3 to 6×10−3 m2 s−1, and the sediment concentration from 0 to 110 kg m−3. All the aggregates from Ultisols developed Quaternary red clay, Central China. The results indicated that discharge had the most significant (P>0.01) effect on the aggregates abrasion with the contributions of 58.76 and 60.34%, followed by sediment feed rate, with contributions of 39.66 and 34.12% at the slope gradients of 8.7 and 26.8%, respectively. The abrasion degree of aggregates was found to increase as a power function of the sediment concentration. Meanwhile, the flow depth, friction factor, and shear stress increased as a power function along with the increase of sediment concentration at different slope gradients and discharges. Reynolds number was obviously affected by sediment concentration and it decreased as sediment concentration increased. The ratio of the residual weight to the initial weight of soil aggregates (Wr/Wi) was found to increase as the linear function with an increasing flow depth (P=0.008) or Reynolds number (P=0.002) in the sediment-laden flow. The Wr/Wi values followed a power function decrease with increasing friction factor or shear stress in the sediment-laden flow, indicating that friction factor is the best hydraulic parameter for prediction of soil aggregate abrasion under different sediment load conditions. The information regarding the soil aggregate abrasion under various sediment load conditions can facilitate soil process-based erosion modeling.

Lubricated sliding friction: Role of interfacial fluid slip and surface roughness

.We derive approximate mean field equations for the fluid flow between elastic solids with randomly rough surfaces including interfacial fluid slip and shear thinning. We present numerical results for the fluid flow and friction factors for realistic systems, in particular, we consider the case of an elastic cylinder with random surface roughness in relative sliding contact with a flat rigid (low-energy) counter-surface. We present experimental data for the sliding friction between rubber stoppers and glass barrels lubricated with baked-on silicone oil. We find that the frictional shear stress acting in the rubber asperity contact regions is nearly velocity independent for velocities in the 10-1000μm/s range, and very small tau_{{rm f}} approx 0.04 MPa, while for bare glass in silicone oil tau_{{rm f}} is much larger and velocity dependent.Graphical abstract

Damage evolution and lifetime of fiber-reinforced ceramic-matrix composites under stress-rupture with stochastic loading at intermediate temperatures

In this paper, the stress-rupture of fiber-reinforced ceramic-matrix composites (CMCs) with stochastic loading at intermediate temperature is investigated. There are four different stochastic loading sequences under stress-rupture are considered with different loading time and time spacing. The micro stress filed of the damaged CMCs, the damage models of the matrix cracking, fiber/matrix interface debonding, and fiber failure consider the stochastic loading and time-dependent interface and fiber oxidation. The damage evolution of the stress-rupture strain, fiber/matrix interface debonding length and interface oxidation length, and the broken fibers fraction versus the time curves of SiC/SiC composite subjected to four different stochastic loading sequences are analyzed. The effects of the fiber volume, matrix crack spacing, fiber/matrix interface debonding energy, fiber/matrix interface shear stress, and environmental temperature on the damage evolution and lifetime of SiC/SiC composite are discussed. The experimental damage evolution and lifetime of SiC/SiC composite under stress-rupture with stochastic loading are predicted. Under stress-rupture with stochastic loading, the stress-rupture lifetime decreases with the stress level, and the frequency of the stochastic loading; the stress-rupture lifetime increases with the fiber volume, and decreases with environmental temperature; and the time for the interface complete debonding increases with the fiber volume, matrix crack spacing, interface debonding energy and interface frictional shear stress in the slip and oxidation region, and decreases with the temperature.

An Inverse Approach Towards Determination of Friction in Friction Stir Spot Welding

Friction stir spot welding (FSSW) is a solid-state joining process in which a rotating tool is pressed against two materials to be joined for creating a spot weld. The rotating tool comprises a pin with a shoulder. The pin gets inserted inside the material, whereas the shoulder rubs on the surfaces of the two materials. The process encounters friction at the pin-workpiece and shoulder-workpiece interfaces. In order to carry out a realistic simulation of the process, the knowledge of friction is a must. In most of the published work, scant attention has been devoted to the quantitative determination of friction force. This article proposes an inverse approach to the determination of friction force in FSSW. It is assumed that the frictional behaviour at the pin-workpieces interface is sticking in nature and at times, there is a possibility that frictional shear stress is more than the shear strength of the materials due to welding of asperities. This aspect is analysed by carrying out a number of finite element simulations using DEFORM-3D on a shoulder-less pin and validating it through in-house experiments. Later on, a hybrid friction model (combination of Coulomb’s and constant shear models) is used on the shoulder-workpieces interface. The friction parameters are adjusted through an optimization approach by minimizing the error between experiments and simulation. In a typical case, the friction factor (m) on the pin-workpiece interface was found to be 1.05. Friction at the shoulder-work interface was best represented by a hybrid model, with m = 0.77 for the high-pressure region and µ = 0.29 for the low-pressure region, where µ is the Coulomb’s coefficient of friction. Computations with estimated friction predicted temperatures with a reasonable accuracy. It also predicted fairly accurate torque for a tool of different size.

Aqueous surface gels as low friction interfaces to mitigate implant-associated inflammation.

Aqueous surface gels are fragile yet resilient biopolymer-based networks capable of sustaining extremely low friction coefficients despite tribologically-challenging environments. These superficial networks are ubiquitous in natural sliding interfaces and protect mechanosensitive cells from excessive contact pressures and frictional shear stresses from cell-fluid, cell-cell, or cell-solid interactions. Understanding these complex lubrication mechanisms may aid in the development of materials-based strategies for increasing biocompatibility in medical devices and implants. Equally as important is characterizing the interplay between soft and passive yet mobile implant materials and cellular reactions in response to direct contact and frictional shear stresses. Physically interrogating living biological systems without rupturing them in the process is nontrivial. To this end, custom biotribometers have been designed to precisely modulate contact pressures against living human telomerase-immortalized corneal epithelial (hTCEpi) cell layers using soft polyacrylamide membrane probes. Reverse-transcription quantitative polymerase chain-reaction (RT-qPCR) indicated that increased duration and, to a much greater extent, the magnitude of frictional shear stress lead to increased production of pro-inflammatory (IL-1β, IL-6, MMP9) and pro-apoptotic (DDIT3, FAS) genes, which in clinical studies are linked to pathological pain. The hierarchical structure often found in biological systems has also been investigated through the fabrication of high-water content (polyacrylamide) hydrogels through free-radical polymerization inhibition. Nanoindentation experiments and friction coefficient measurements indicate that these "gradient surface gels" reduce contact pressures and frictional shear stresses at the surface of the material while still maintaining stiffness within the bulk. Reducing frictional shear stresses through informed materials and surface design may concomitantly increase lubricity and quiet the immune response, and thus provide bio-inspired routes to improve patient outcomes and quality of life.