State Friction Research Articles

Numerical Investigation of Gas Dynamic Foil Bearings Conjugated with Contact Friction by Elastic Multi-Leaf Foils

With increasing demand for load capacity and dynamic responses, the new generation of Gas Dynamic Foil Bearings (GDFB), assembled from preloaded elastic multi-leaf foils, has been put forward. It exhibits steady and dynamic characteristics that are more sensitive to the contact friction produced by foil–foil bearings and the housings of foil bearings, distinguishing its response to rigidity from those of classic bearings. This study aims to employ numerical simulations to explore GDFBs conjugated with contact friction using elastic multi-leaf foils. A numerical method with Bidirectional Fluid–Structure Interactions (BFSIs), which strengthens the steady and dynamic characteristics of GDFBs, has been developed. The results showed that the distribution of steady stiffness was closely associated with the configuration of the foil assembly and the state of contact friction. The maximum steady stiffness occurred near the key blocks and was accompanied by the support of bump foils. The contact friction increased the stiffness of the foil assembly, which was unfavorable to its operational stability, and the GDFB was prone to rigidity. The contact friction hindered the pull-back of the top foils near the negative pressure area. An increase in the rotational speed and eccentricity increased the load capacity while decreasing the operational stability. The maximum pressure of the gas film was inconsistent with the maximum deformation along the circumferential direction because of the configuration of the foil assembly. The contact friction induced energy dissipation, whereas increases in the integrated stiffness inhibited loss. The fixed constraint of the key block increased the dynamic cross-stiffness more than the contact constraint did, which was unfavorable to operational stability. The results obtained in this study are significant because they provide a physical basis for GDFBs and can be used to guide their operational optimization.

A Detecting Method for “Weak” Friction-Induced Vibration Based on Cross-Correlation Analysis between Vibration and Sound Signals

The “weak” friction-induced vibration can help to monitor the change in friction and wear state of friction pairs and detect the failure of surface damage. However, they are typically submerged in complex interference components during friction. Therefore, detecting accurate “weak” friction-induced vibration is key to using it entirely. A method based on the correlation between friction-induced vibration and sound signals was proposed to achieve this goal. The ball–disk wear experiments were conducted under oil lubrication using a wear tester. Vibration and sound pressure signals generated during the experiments were recorded. By the spectrum analysis of the cross-correlation function calculated from the two types of signals, the “weak” FIV components submerged in the original signals were detected. The experimental results showed that the root mean square change of the “weak” friction-induced vibration detected in the vibration and sound pressure signals was highly consistent with the friction coefficient change. It could effectively characterize the transition of the wear stage from running-in to stable wear of the friction pairs. Therefore, the cross-correlation analysis of vibration and sound signals could be a reliable tool for detecting the “weak” friction-induced vibration.

Influence of joint inclination on mechanical behaviors of shales during unloading-induced slip processes

To investigate the effect of joint inclination on the mechanical behaviors of unloading-induced slip processes, shale samples with different joint inclinations (θ) were tested under constant axial stress and graded unloading of confining pressure to induce slip. The results show that the slip tests resulted in scratches and damage of the rock matrix, thus increasing joint roughness coefficient (JRC) of saw-cut surface samples by values of 4.69, 2.75 and 2.67 for θ = 30°, 40° and 50°, respectively. With the unloading of confining pressure, the samples experienced creep slip, quasi-static slip, and dynamic slip, respectively. The velocity at creep slip, quasi-static slip and dynamic slip decreased with the increase in θ. The average velocities of quasi-static slip are 2.9 × 10−2 mm/s, 1.1 × 10−2 mm/s and 2.1 × 10−3 mm/s for θ = 30°, 40° and 50°, respectively. As the slip progresses, the micro-convexities on the joint are sheared off, resulting in a deterioration of the interlocking effect, which causes the occurrence time of quasi-static slip to advance in an unloading stage and the duration to increase. The friction coefficient (μ) decreased by 2.22%, which changes from 0.45 to 0.44 for the sample with θ = 30°, indicating that the friction strength was slightly weakened during the unloading-induced slip. As θ increases, the friction strength of the sample was enhanced. The μ increased by 19.23% from 0.52 to 0.62 and increased by 28.81% from 0.59 to 0.76 for the sample with θ=40° and 50°, respectively. Based on the experimental results, the unloading-induced slip behaviors were fitted with the rate and state friction law. However, the mechanical behavior of the sample has changed from “shear slip” to “slip failure” as the joint inclination increase, which resulted in the dynamic slip of the sample with θ = 40° and 50° being opposite to the rate weakening described in the rate state friction law.

Influence of temperature on residual strength of clayey soils

This paper presents the influence of temperature on the residual strength by analyzing shearing tests carried out in a temperature- and velocity-controlled ring shear apparatus. Temperature was applied in steps of cooling and heating ranging between 6 °C and 50 °C at a constant shearing velocity of 0.442 mm/min. Additional tests include the evaluation of the strength at 0.890 mm/min before and after the imposed temperature change. Six basic soil types characterized by different mineralogy, grain size and plasticity, as well as, mixture soils having different proportions of sand and clay, are evaluated and the results are interpreted in terms of their properties. The experimental results reveal that the temperature effect becomes relevant when fine particles and plasticity increase being negligible in the case of the sandy soil. Shear strength reacts immediately to temperature changes and a new residual strength establishes soon. The temperature effect is symmetric. An increase in temperature leads to a decrease in strength, and an equal and opposite trend is observed when temperature decreases. The magnitude of the strength variation is significant in clayey soils. For the case of high plasticity soils, the residual friction angle variation observed during cooling and heating (±50 °C) is around 2°. The strain rate effect evaluated in a smaller number of tests indicated a strain rate strengthening in all the materials tested except for the case of the sandy soil, which also was not reactive to temperature changes. The experiments are calibrated with a velocity, state, and temperature-dependent friction model.

Dissipation Mechanisms and Superlubricity in Solid Lubrication by Wet-Transferred Solution-Processed Graphene Flakes: Implications for Micro Electromechanical Devices.

Solution-processed few-layer graphene flakes, dispensed to rotating and sliding contacts via liquid dispersions, are gaining increasing attention as friction modifiers to achieve low friction and wear at technologically relevant interfaces. Vanishing friction states, i.e., superlubricity, have been documented for nearly-ideal nanoscale contacts lubricated by individual graphene flakes. However, there is no clear understanding if superlubricity might persist for larger and morphologically disordered contacts, as those typically obtained by incorporating wet-transferred solution-processed flakes into realistic microscale contact junctions. In this study, we address the friction performance of solution-processed graphene flakes by means of colloidal probe atomic force microscopy. We use a state-of-the-art additive-free aqueous dispersion to coat micrometric silica beads, which are then sled under ambient conditions against prototypical material substrates, namely, graphite and the transition metal dichalcogenides (TMDs) MoS2 and WS2. High resolution microscopy proves that the random assembly of the wet-transferred flakes over the silica probes results into an inhomogeneous coating, formed by graphene patches that control contact mechanics through tens-of-nanometers tall protrusions. Atomic-scale friction force spectroscopy reveals that dissipation proceeds via stick-slip instabilities. Load-controlled transitions from dissipative stick-slip to superlubric continuous sliding may occur for the graphene-graphite homojunctions, whereas single- and multiple-slips dissipative dynamics characterizes the graphene-TMD heterojunctions. Systematic numerical simulations demonstrate that the thermally activated single-asperity Prandtl-Tomlinson model comprehensively describes friction experiments involving different graphene-coated colloidal probes, material substrates, and sliding regimes. Our work establishes experimental procedures and key concepts that enable mesoscale superlubricity by wet-transferred liquid-processed graphene flakes. Together with the rise of scalable material printing techniques, our findings support the use of such nanomaterials to approach superlubricity in micro electromechanical systems.

Nonlinear control with friction compensation to swing-up a Furuta pendulum

Different works in literature have reported that nonlinear controllers based on the energy approach are not effective to completely swing-up an inverted pendulum subjected to friction. Most studies trying to solve this issue consider static friction models in the design of controllers. This consideration is mainly because the stability proof of the system with dynamic friction in closed-loop is difficult. Hence, this paper presents a nonlinear controller with friction compensation to swing-up a Furuta pendulum with dynamic friction. With this aim, we consider that only the active joint of the system is subjected to friction, which is represented via a dynamic model, namely, the Dahl model. We first present Furuta Pendulum dynamic model with dynamic friction. Then, by slightly modifying an energy-based controller that has been previously reported in literature and by including friction compensation, we propose a nonlinear controller that allows to swing-up completely a Furuta pendulum subjected to friction. The unmeasurable friction state is estimated through a nonlinear observer and a stability analysis of the closed-loop system is accomplished with the direct Lyapunov method. Finally, successful experimental results are presented for a Furuta pendulum prototype built by authors. This shows the effectiveness of the proposed controller in achieving a complete swing-up of the Furuta pendulum, in a time feasible for experimental implementation, and ensuring closed-loop stability.

Adaptive friction compensation control of robotic pneumatic end-effector based on LuGre model

PurposeConsidering the response lag and viscous slip oscillation of the system caused by cylinder piston friction during automatic polishing of aero-engine blades by a robotic pneumatic end-effector, the purpose of this study is to propose a constant force control method with adaptive friction compensation.Design/methodology/approachFirst, the mathematical model of the pneumatic end-effector is established based on the continuous LuGre model, and the static parameters of the LuGre model are identified to verify the necessity of friction compensation. Second, aiming at the problems of difficult identification of dynamic parameters and unmeasurable internal states in the LuGre model, the parameter adaptive law and friction state observer are designed to estimate these parameters online. Finally, an adaptive friction compensation backstepping controller is designed to improve the response speed and polishing force control accuracy of the system.FindingsSimulation and experimental results show that, compared with proportion integration differentiation, extended state observer-based active disturbance rejection controller and integral sliding mode controller, the proposed method can quickly and effectively suppress the polishing force fluctuation caused by nonlinear friction and significantly improve the blade quality.Originality/valueThe pneumatic force control method combining backstepping control with the friction adaptive compensation based on LuGre friction model is studied, which effectively suppresses the fluctuation of normal polishing force.

Research on friction state monitoring method of liquid film seal during start-up based on acoustic emission

This paper proposes a method based on acoustic emission to achieve nondestructive testing of the friction state of a liquid film seal. However, the acoustic emission signal is susceptible to noise, and extracting the feature signal is challenging. To address these issues, we propose a signal processing method based on singular value decomposition and adaptive variational mode decomposition (SVD-AVMD). Additionally, we investigate the acoustic emission mechanism of the liquid film seal using the strength theory of shear strain energy and CEB contact model. We design a liquid film seal friction experiment based on acoustic emission technology to extract the characterization signal of the friction state of the liquid film seal using SVD-AVMD. Finally, we apply a convolutional neural network to identify the friction state of the liquid film seal. The results demonstrate that SVD-AVMD has a significantly better ability to capture the center frequency of each mode component and to recover each mode component compared to simple variational mode decomposition. SVD-AVMD can filter out background noise while retaining the most useful information to the maximum extent possible. Furthermore, the root mean square of the extracted AE signal is consistent with the description of the AE mechanism, achieving the goal of studying and judging the friction state of the liquid film seal end face. Finally, we show that combining the convolutional neural network and acoustic emission time-frequency characteristics can improve the recognition accuracy of the friction state of the liquid film seal.

Study on the effect of counter ball materials on fretting wear behavior of solid lubricating films

Solid self-lubricating film can undoubtedly play a superior role in improving the micromotion behavior. The friction pairs of different materials have different fretting behavior and wear mechanism due to their different mechanical properties and surface adhesion properties. Investigating the influence of mechanical properties and surface adhesion on the fretting behavior can make the self-lubricating solid film play a better effect in the practical fretting working conditions. Here, DLC film and MoS2 film are deposited on AISI 304 stainless steel using a TeerUDP-650 unbalanced magnetron sputtering system. The fretting performance of DLC and MoS2 films resistance to GCr15, Al2O3, SiC and Si3N4 balls are studied. The results demonstrate that the fretting characteristics of DLC film are highly dependent on the mechanical properties and adhesion behavior of counter balls. Thereinto, Al2O3 balls show excellent fretting behavior, while SiC and Si3N4 balls make DLC film ineffective due to their high hardness and high adhesion. The MoS2 film maintains low friction and low wear state against any counter ball. DLC film achieves anti-fretting wear via graphitization, while MoS2 film primarily relies on slip and plastic flow. This work provides a theoretical reference for proper selection of solid lubricating films and counter balls under practical fretting conditions and reveals the different tribological mechanisms of friction pairs.

Theoretical and Numerical Investigation of Reduction of Viscous Friction in Circular and Non-Circular Journal Bearings Using Active Lubrication

Reducing friction losses is one of the most common ways to improve fluid film bearings, whose adjustable design provides additional opportunities to improve their dynamic and tribological properties. Previous studies have shown the possibility of reducing viscous friction in actively lubricated bearings by adjusting the rotor position. This work provides a theoretical justification for the mechanism of this effect for the cases of purely laminar lubricant flows in journal bearings. The operating modes connected with the transition to turbulent phenomena and the occurrence of Taylor vortices are beyond the scope of this paper. Conditions that ensure the minimization of friction losses in hydrodynamic and hybrid bearings with hydrostatic parts are determined based on the equations describing viscous friction in a fluid film. In non-adjustable plain hydrodynamic bearings, the minimum of friction is achieved with the centered shaft position that is actually unstable due to the resulting forces configuration. In actively lubricated hybrid bearings, a further reduction in viscous friction is possible by combining film thickness and pressure distributions. Recombining them, along with adjustment of the shaft position, allows the optimization of the distribution of shear stresses in the fluid film. As a result, the shear stresses caused by the rotation of the shaft can be partially compensated by the stresses caused by the pressure gradient, reducing the torque-resisting rotation. In addition, additional benefits can be obtained in the minimum friction state by the reduced lubricant flow and power losses to its pumping. A series of numerical calculations for elliptical, 3-, and 4-lobe bearings show that non-circular bores provide additional variability in film thickness distribution and a premise for optimizing the bearing tribological parameters. Four-lobe bearing demonstrated the best ability for reducing viscous friction among the considered designs. The results obtained can be used as a basis for further optimization of the geometry of fluid film bearings of both active and passive designs by reducing power losses due to viscous friction.

Rate and State Simulation of Two Experiments With Pore Fluid Injection Under Creep Conditions

AbstractThis study uses a spring‐block model and rate and state friction to simulate experiments conducted in a double direct‐shear apparatus on carbonate fault gouge (Scuderi et al., 2017, https://doi.org/10.1016/j.epsl.2017.08.009) and on shale bearing rock (Scuderi & Collettini, 2018, https://doi.org/10.1029/2018jb016084). Both sets of experiments used the same loading protocol and injected pore fluid under creep conditions. When velocity strengthening rate and state friction is used to simulate the experiments on the carbonate fault gouge the results agree well with the observed onset of tertiary creep in the experiment. Thus, the simulation reinforces the observation that pore fluid injection can induce rapid slip even when the friction relation is velocity strengthening. The rate and state framework provides an interpretation alternative to the standard one of the Mohr's circle moving to the left as pressure increases. In the rate and state framework, the friction coefficient must increase with pore pressure increase. The shale has a low nominal friction coefficient (0.28) and is much more velocity strengthening than the carbonate. The simulation agrees with the observations that increases in pore pressure induce an increase in slip velocity but the magnitudes reach only about 100 microns/s by the end of the experiment. The simulation for the shale also agrees well with the magnitude of the observed displacement at the end of the experiment but observed displacement is increasing much more rapidly the calculated. Although the calculations agree well with features of the observations near failure, the overall curves of displacement and velocity are significantly different.

Investigation of the effect of stainless-steel grain size on the continuity of graphene frictional behavior using molecular dynamics (MD) simulation

Nanograin refinement can improve the strength of stainless steel, but the substrate grain boundaries will cause graphene bulging, so it will affect the lubrication effect of graphene. In this study, the effect of stainless-steel grain size on the frictional strengthening behavior of graphene is investigated, and the mechanism of grain boundary distribution and grain boundary crossing on graphene friction strengthening behavior was revealed. A normal load constraint of 0.8 nN is set for the nano-friction process. Stainless steel grain size greater than or equal to 10 nm, the maximum friction force of graphene does not vary with grain size is about 6 nN. However, the friction strengthening behavior is discontinuous, and the friction force fluctuates significantly, which is not conducive to micromechanical structure lubrication. When the grain size of stainless steel is less than 10 nm, the frictional strengthening behavior is continuous and the friction force after strengthening will maintain small fluctuations. Since the small fluctuations of this stick–slip behavior are material properties of graphene and cannot be avoided, this state is already the ideal result for graphene lubrication. However, as the stainless-steel grain size continues to become smaller, the maximum friction of graphene will increase, which is not good for graphene lubrication. This means that after the grain size is less than 10 nm, the continuation of the grain refinement process for stainless steel will improve the strength of stainless steel, but will weaken the graphene lubrication efficiency. Moreover, the difficulty of the nanograin refinement process will be significantly increased. Therefore, the stainless-steel grain size of 7 nm by grain refinement is the best solution to balance the stainless-steel strength and graphene lubrication efficiency. With a grain size of 7 nm, the stainless-steel grain boundaries are densely distributed, and the graphene bulge area will occupy 65% of the total area, allowing for continuous frictional strengthening behavior. The maximum height of graphene bumps is 2.8 Å and the maximum friction force is about 7 nN. Meanwhile, the relationship between the grain size of stainless steel and the continuity of graphene frictional strengthening behavior was explored, and the mechanism of action of grain boundary crossing by modulating the height of graphene bulge and thus affecting the frictional maximum was revealed. The nano-friction simulation of stainless steel/graphene can clearly capture the details of the nano-friction process and the variation of friction values, providing a basis for nano-friction studies of graphene on polycrystalline material surfaces. The simulation can determine the range of stainless-steel grains that exhibit the best friction state with graphene. The simulation results also provide guidance for the refinement of stainless steel nanograins and insights for the application of nanocrystalline stainless steel/graphene in MEMS.

Identification Method for Instantaneous Friction and Wear Energy Density Variation of High-Feed Milling Tool Flank

Instantaneous friction and wear energy density are important indicators of the friction and wear state of the tooth flank of the milling cutter. Existing methods for identifying the friction and wear on the tooth flank of milling cutters mainly focus on the overall level of friction energy consumption and the maximum width of the accumulated wear on the teeth flanks, ignoring the unevenness and variability of the friction energy consumption and wear volume distribution of the tool flank, and the dynamic relationship between the instantaneous friction and wear on each tooth flank of the milling cutter is to be revealed. Transient cutting position of a high feed milling cutter tooth under milling vibration, and the use of the micro-element method to construct a model for solving the transient wear volume on the tool flank. A method of transient friction energy consumption on the cutter flank was proposed. The variation property of the instantaneous friction and wear energy density on the flank was exposed. The identification method of the variation property of the energy density was proposed according to different analysis results of the distribution of friction and wear energy density and verified by experiments. The outcomes indicated the maximum and mean of the energy density at different positions on the flank had a high similarity with the wear depth distribution on the flank. Using this method, the influence property of the friction and wear state and the cutting parameters under the milling vibration of the milling cutter tooth can be identified.

The effect of the structural parameters on the friction and wear properties of some FCC metals

Friction and wear are very complex thermo-mechano-chemical processes of contact interaction. For a better understanding of the mechanisms of plastic deformation affecting friction and wear, four face-centered cubic (FCC) metals (Ag, Cu, Ni, and Al) were studied using a pin-on-disk rig. The topmost surface layer presents nanocrystalline (NC) and ultrafine grain (UFG) structures with the transition to a sublayer with a lamellar structure parallel to the direction of friction. The thickness of the topmost layers is mainly determined by stacking fault energy (SFE), hydrostatic pressure, and shear stress. The thermo-mechanical control of plastic flow during the steady friction state was considered. The effective shear stress as the activation barrier, the strain rate sensitivity, and the contact temperature were calculated. The values of the strain rate sensitivity, m, indicate that NC and UFG surface layers for Ag, Cu, and Ni, exhibit low capacities for strain hardening. The friction of FCC metals in lubricated conditions is characterized by a switch from dislocation-mediated plasticity to grain boundary (GB) plasticity in the topmost surface layers. The similarity between friction with low and high coefficients of friction during dry and lubricated conditions was considered. The wear resistance of studied materials can be compared to strength improvements and easy shearing of the topmost surface layer under dry and lubricated conditions. Dislocation glide and grain growth play a key role in the hardening/softening of surface layers both in dry and lubricated conditions.

Critical State-Based Mohr–Coulomb Bounding Surface Model for Sand under Monotonic Shearing

This study proposes a new constitutive model to describe the smooth transition from an elastic to a plastic response in sands under monotonic shearing. The model modifies the conventional Mohr–Coulomb model by considering the concept of a bounding surface and critical state soil mechanics. The friction angle consists of the critical state friction angle and a portion of the dilatancy angle, which is determined from the distance to the critical state line. Incorporating the bounding surface and the dilatancy angle into constitutive relationships for Toyoura sand produces numerical simulation results that have good agreement with the experimental results.

Friction and wear behavior of multiple steel wires with different corrosion extents under different lubrication conditions

Corrosion and wear of the steel wires are two major factors in the failure of wire ropes and can greatly reduce their service life as well as the reliability of mine hoists. In this paper, to study the fretting wear behavior of multiple wires with different extents of corrosion under different lubrication conditions, fretting wear tests on corroded wires were conducted on a homemade testing apparatus. The results show that corrosion and lubrication had great influence on coefficient of friction (COF) and fretting state. In addition, the wear resistance of steel wire decreased with increasing corrosion extent. The application of lubricant is beneficial. It could significantly reduce the overall specific wear rate and reduce the difference in degree of wear due to corrosion. In two lubricants specially used for mining wire ropes, lubricating grease (IRIS-400 M) had better anti-wear effect than lubricating oil (IRIS-550A). Furthermore, lubrication condition and corrosion degree affected the wear mechanism between steel wires. This study can provide basic data for the safe use and maintenance of wire rope.

Effect of gas-oil contamination on the mechanical behavior of sand-woven geotextile interface: Experimental investigation and constitutive modeling

The accurate estimation of the frictional resistance of interfaces between soils and geosynthetics plays a central role in stability and serviceability of geosynthetic reinforced earth structures. Contamination with hydrocarbons generally impairs soil geotechnical properties; however, its effect on the behavior of soil-geosynthetic interfaces has seldom been examined precisely. For this reason, an extensive series of direct shear tests was performed to investigate the consequence of gas-oil contamination on the mobilization of shear strength and volume change response of gas-oil contaminated angular sand in contact with woven geotextile (WGTX). Complementary tests on the interfaces between glass beads as a replicate for sands with high degree of sphericity and roundness in contact with WGTX were also performed to explore the effect of particle shape. Gas-oil contamination is observed to causes decrease of the peak and critical state friction and dilation angles in both the sand-WGTX and glass bead-WGTX interfaces. However, gas-oil contamination-induced decrease in the frictional efficiency in the glass bead-WGTX interfaces was greater than that in the angular sand-WGTX interfaces. Calibration of a state-dependent sand-structure interface model against the laboratory data of gas-oil contaminated soils-WGTX interfaces results in a reasonable agreement between the model simulations and the laboratory data.

Study on the Matching of Surface Texture Parameters and Processing Parameters of Coated Cemented Carbide Tools

Placing micro-textures on a tool surface can play an anti-wear and friction-reducing role and capture impurities and improve the tool-chip friction state, thus improving the cutting performance of the tool and the quality of the workpiece. To ensure the processing quality in the micro-texture-coated tool-cutting process, the process parameters and micro-texture parameters are limited to smaller parameters, which reduces the processing efficiency and increases the cost. Aiming at this problem, this paper designs orthogonal experiments of the cutting process and micro-texture parameters, builds an experimental platform for milling titanium alloy with a micro-texture-coated ball-end milling cutter, analyzes the influence of cutting parameters on tool milling performance and workpiece quality, establishes a high fitting prediction model, and optimizes parameters. The results show that the cutting parameters significantly affect the milling force, tool wear, and workpiece surface roughness, which are in the first response level, and the micro-texture parameters, which are in the second response level. It is proven that micro-texture has anti-wear and anti-friction effects, and it is found that micro-texture parameters affect the evaluation index by changing the distribution state of the micro-texture. It is found that the multiple linear regression model fits better. Parameter optimization results are: v = 159.4232 (m/min), ap = 0.211 (mm), f = 0.06 (mm/r), micro-pit diameter D = 62.3429 (μm), distance from blade L = 121.5184 (μm), and micro-pit spacing L1 = 235.6443 (μm). It provides some guidance for the selection of micro-texture parameters and cutting parameters on a micro-texture-coated tool.

Ionogel Fiber‐Based Flexible Sensor for Friction Sensing

AbstractThe ability of flexible sensors to detect and recognize the perception of different mechanical stimuli is a prerequisite for meeting the challenges of flexible sensing in many applications. Although monitoring of frictional force and state is important and necessary, there is currently a shortage of highly sensitive sensors that can be used for frictional state sensing. This study demonstrates a high‐performance flexible sensor combining pyramidal micropatterns with ionogel nanofibers, which is capable of detecting both normal and tangential forces. The normal force is detected with a sensitivity of 1.145 kPa−1, a response time of less than 30 ms, and a hysteresis of 3.8%. More importantly, the sensor enables sensitive recognition of sliding speed, normal force load, tangential force, and oil lubrication environments with different viscosities in a reciprocating friction condition. It also enables sensitive recognition of the entire relative sliding process from zero slip to stick‐slip. The excellent frictional force and state recognition performances and long durability of 600 cycles at high‐frequency reciprocating friction indicate that the ionogel fiber‐based flexible sensor has a great potential application in friction sensing.

The Interaction Between Frictional Slip and Viscous Fault Root Produces Slow Slip Events

AbstractWe consider a model where an unstable frictional region, governed by rate and state friction, interacts with a viscous zone with Newtonian rheology. The system is loaded at distance with a constant velocity. Pore pressure variations are considered and we show that the model of Segall and Rice (1995, https://doi.org/10.1029/95jb02403) relating porosity changes to variations of the state variable could be derived considering viscoplastic deformation of a population of identical asperities. We perform a linear stability analysis in the case of a constant pore pressure in agreement with the full numerical results. For a given value of the viscosity of the viscous region, stable slip is promoted at low normal stress and unstable slip at high normal stress. Near the transition from stable to unstable slip, modest acceleration of slip, resembling slow slip events (SSE) are observed. We show that our model can reproduce real SSE sequences in the Guerrero subduction zone which are the largest worldwide. The best fit parameters suggest that SSEs happen in areas of low effective normal stress (for the frictional region) and low viscosity (for the viscous region). In our model, SSEs happen in a regime where the viscous region is able to counteract the instability of the frictional one. We show that considering a rate strengthening rheology or a non newtonian one leads to the same linear stability results. Our work shows that a simple model with homogeneous spatial properties can lead to complex dynamics, covering a wide range of observed sliding modes, from steady‐state creep to seismic slip and SSEs.