Stick-slip Amplitude Research Articles

Size effects of brake pads on stick-slip phenomena

Stick-slip phenomena at a low-velocity range are investigated to study the effect of brake pad size on friction instability, focusing on the contact plateaus on the pad. The friction test is performed using commercial brake pads in three different sizes. The results show that smaller pads produce higher stick-slip amplitudes within narrower velocity ranges. Friction levels are influenced by the total contact plateau area, whereas the stick-slip amplitude was affected by the size of high-pressure plateaus. The pronounced stick-slip with small pads is attributed to the large high-pressure plateaus that increase the static friction further with stick time, indicating that a smaller brake assembly could lead to a high propensity of friction-induced noise and vibrations.

Triaxial test on glass beads simulating coarse-grained soil

It is feasible to study the mechanical characteristics of coarse-grained soil by simulated granular materials such as glass beads. In this paper, 3 mm diameter glass beads are used to conduct drained and undrained triaxial tests under different confining pressures to explore their strength, deformation and critical state characteristics. Specifically, the influence of drainage and confining pressure on the stick-slip phenomenon of glass beads is reported. The experimental findings from triaxial tests show that the stress-strain relationship of glass beads softens when the confining pressure is high. Under the undrained condition, the initial modulus increases with the increase in the confining pressure. In contrast, it is not significantly affected by the confining pressure in the drained condition. It is quite evident that the glass beads hardly contract during the shearing process, and their stress path is approximately a segmented straight line. The slope of the critical state line under the undrained condition is greater than that under the drained condition, and the friction angle of the glass beads under the undrained and drained conditions is calculated to be 28.1° and 29.5°, respectively. The phenomenon of stick-slip has been depicted for the different test conditions, and the stick-slip amplitude linearly increases with the confining pressure, especially in the undrained condition. It is also found that the maximum energy released from the phenomenon of stick-slip increases linearly with the confining pressure.

Linking macroscale frictional properties to nanoscale water capillary bridges

Friction and unloading experiments of an ox-Si3N4 ball against a PDMS film were conducted using an in-situ optical microtribometer. The interface was first wetted with 0.1 mL water and allowed to dry out during the sliding. A transient friction coefficient 60% greater than in dry condition always occurred in accordance with discretely distributed nanoscaled water bridges, increased pull-off force, stick-slip amplitude, and contact deformation. In summary, this study provides experimental support for the hypothesis that multi-asperity capillary bridges could significantly impact the macroscale frictional behavior of solid-solid contacts.

Friction at nanopillared polymer surfaces beyond Amontons’ laws: Stick-slip amplitude coefficient (SSAC) and multiparametric nanotribological properties

Frictional and nanomechanical properties of nanostructured polymer surfaces are important to their technological and biomedical applications. In this work, poly(ethylene terephthalate) (PET) surfaces with a periodic distribution of well-defined nanopillars were fabricated through an anodization/embossing process. The apparent surface energy of the nanopillared surfaces was evaluated using the Fowkes acid-base approach, and the surface morphology was characterized using scanning electron microscope (SEM) and atomic force microscope (AFM). The normal and lateral forces between a silica microparticle and these surfaces were quantified using colloidal probe atomic force microscopy (CP-AFM). The friction-load relationship followed Amonton’s first law, and the friction coefficient appeared to scale linearly with the nanopillar height. Furthermore, all the nanopillared surfaces showed pronounced frictional instabilities compared to the smooth sliding friction loop on the flat control. Performing the stick–slip amplitude coefficient (SSAC) analysis, we found a correlation between the frictional instabilities and the nanopillars density, pull-off force and work of adhesion. We have summarised the dependence of the nanotribological properties on such nanopillared surfaces on five relevant parameters, i.e. pull-off force fp, Amontons’ friction coefficient μ, RMS roughness Rq, stick–slip amplitude friction coefficient SSAC, and work of adhesion between the substrate and water Wadh in a radar chart. Whilst demonstrating the complexity of the frictional behaviour of nanopillared polymer surfaces, our results show that analyses of multiparametric nanotribological properties of nanostructured surfaces should go beyond classic Amontons’ laws, with the SSAC more representative of the frictional properties compared to the friction coefficient.

The significance of displacement control mode in direct shear tests of rock joints

Servo-controlled direct shear tests of rough (RMS = 500 μm) and smooth (RMS = 7 μm) surfaces of Gabbro are performed under a constant normal stress of 5 MPa and a constant shear displacement rate of 0.01 mm/s. The tested interfaces are 10 cm long and 8 cm wide. Two servo-controlled shear displacement modes are studied: 1) Piston Displacement Control [PDC], and 2) Block Displacement Control [BDC]. In PDC mode the displacement output from the shear piston is used as the feedback signal for servo control whereas in BDC mode the displacement output from two horizontally oriented LVDT's positioned directly on the sheared interface are used for control. We find that when the surfaces are tested under BDC mode they exhibit a distinct peak shear stress followed by a stress drop to residual shear strength, whereas a continuous transition to “steady state” sliding is exhibited by the same surfaces when tested under PDC mode. This leads to the important observation that while peak shear strength is similar in both control modes the residual shear strength in PDC mode is distinctly higher. All tested interfaces exhibit stick-slip oscillations, the magnitude and frequency of which are strongly related to the control mode. We show that stick slip amplitude and frequency are artificially higher in BDC mode because of servo-control effects and that the correct stick slip behaviour can only be obtained in PDC mode. We find that the displacement control mode in direct shear tests drastically affects the obtained results and therefore investigators must consider the consequences before specifying which servo-control mode to use in direct shear tests.

Comparative study on dynamic shear behavior and failure mechanism of two types of granite joint

Frictional instability such as fault slip rockburst, injection-induced earthquake and landslide is frequently associated with the unstable sliding of pre-existing discontinuities. This study aims to investigate the influence factors and mechanism of the unstable sliding of granite joints. Two types of granite (granite A and granite B) which are mainly composed of microcline, albite, quartz and biotite are used to prepare a number of splitting joints, with the average JRC values of 12.7 and 15.7 respectively. Direct shear tests are then conducted under various normal stresses, during which acoustic emissions are monitored and recorded. Test results reveal that the peak shear strength is generally greater for joint B than joint A, while the post-peak failure intensity with respect to post-peak stress drop, amplitude of stick-slip, peak energy rate, the sound emitted during stress drops for joint B are smaller than that of joint A, especially under the high normal stress condition. The underlying contributing factors are studied. According to the petrographic observation and the two dimensional joint surface roughness analysis, the higher peak shear strength for joint B is related to the rougher joint surface which is associated with a larger mineral grain size. The more violent post-peak failure intensity in joint A is related to its smaller mineral grain size, compact structure and fewer inherent cracks, which favor a greater energy build-up in the asperities. The present study suggests that not only the roughness of discontinuity plane, but also the micro-texture of the host rock should be considered in the assessment of the fault slip rockburst hazards caused by the unstable sliding. Results of this research contribute to the better understanding of mechanism and influencing factors of fault slip rockburst, as well as the mitigation of geohazards induced by the dynamic shear failure of rock discontinuities.

Friction instability induced by iron and iron oxides on friction material surface

This study analyzed the effect of ferrous wear particles, which can be transferred from gray iron brake discs to the friction material surface during brake applications, on friction instability. To simulate the chemical change of the sliding surfaces, ferrous particles, such as pure iron, Fe2O3, and Fe3O4 particles, were embedded on the sliding surface of a friction material, and their friction characteristics were investigated using a scale dynamometer. The results showed that the ferrous particles aggravated friction instability, showing larger stick-slip amplitudes and wider velocity ranges for friction instability than the bare specimen. The embedded ferrous particles increased the static coefficient of friction and produced larger stick-slip amplitudes in the order of Fe2O3, iron, and Fe3O4, while the friction material without ferrous particles showed the least instability. The surface energy and amount of high-pressure contact plateaus on the sliding surface were attributed to the high friction instability caused by the interfacial adhesion increased by ferrous particles at the sliding interface. This explained the frequent friction-induced noise and vibrations found with a corroded or relatively soft gray iron disc.

Effect of plateau distribution on friction instability of brake friction materials

The effect of contact plateau distribution on stick-slip of brake friction materials was studied based on experimental results and computer simulation of the sliding interface. Experimental results showed that plateau distribution determined the amplitude and frequency of friction oscillation, which were governed by the tangential stiffness and failure strength of each plateau. The numerical analysis showed that the surface with few large plateaus produced larger stick-slip amplitudes than one with several small contact plateaus. This study suggested that the friction material should be designed to produce small contact plateaus on the sliding surface to diminish its noise propensity during brake applications.

Analytical Approximations for Stick–Slip Amplitudes and Frequency of Duffing Oscillator

Linear spring mass systems placed on a moving belt have been subjected to numerous investigations. Dynamical characteristics like amplitude and frequency of oscillations and bifurcations have been well studied along with different control mechanisms for this model. But the corresponding nonlinear system has not received comparable attention. This paper presents an analytical investigation of the behavior of a Duffing oscillator placed on a belt moving with constant velocity and excited by dry friction. A negative gradient friction model is considered to account for the initial decrease and the subsequent increase in the frictional forces with increasing relative velocity. Approximate analytical expressions are obtained for the amplitudes and base frequencies of friction-induced stick–slip and pure-slip phases of oscillations. For the pure-slip phase, an expression for the equilibrium point is obtained, and averaging procedure is used to arrive at approximate analytical expressions of the periodic amplitude of oscillations around this fixed-point. For stick–slip oscillations, analytical expressions for amplitude are arrived at by using perturbation analysis for the finite time interval of the stick phase, which is linked to the subsequent slip phase through conditions of continuity and periodicity. These analytical results are validated by numerical studies and are shown to be in good agreement with them. It is shown that the pure-slip oscillation phase and the critical velocity of the belt remain unaffected by the nonlinear term. It is also shown that the amplitude of the stick–slip phase varies inversely with nonlinearity. The effect of different system parameters on the vibration amplitude is also studied.

Effects of humidity and substrate hydrophilicity on nanoscale friction

The effect of water menisci at the asperity junction on the friction level and oscillation was investigated by using atomic force microscopy. The menisci formed through capillary action were controlled by changing the humidity of the experimental environment and the ZnO substrate's hydrophilicity. The hydrophilic ZnO substrate showed higher friction levels than the hydrophobic substrate, while this difference was indistinguishable at humidity levels above 40% relative humidity. Indentation experiments demonstrated that the friction level was determined by the stiffness of the water menisci that was changed by hydrophilicity of the substrate and humidity level. The stick-slip amplitude was also increased in the case of the hydrophilic substrate, supporting the hypothesis of repeated formation and deformation of the solid-like menisci during sliding.

Stick-slip instabilities in sheared granular flow: The role of friction and acoustic vibrations.

We propose a theory of shear flow in dense granular materials. A key ingredient of the theory is an effective temperature that determines how the material responds to external driving forces such as shear stresses and vibrations. We show that, within our model, friction between grains produces stick-slip behavior at intermediate shear rates, even if the material is rate strengthening at larger rates. In addition, externally generated acoustic vibrations alter the stick-slip amplitude, or suppress stick-slip altogether, depending on the pressure and shear rate. We construct a phase diagram that indicates the parameter regimes for which stick-slip occurs in the presence and absence of acoustic vibrations of a fixed amplitude and frequency. These results connect the microscopic physics to macroscopic dynamics and thus produce useful information about a variety of granular phenomena, including rupture and slip along earthquake faults, the remote triggering of instabilities, and the control of friction in material processing.

Sustained Frictional Instabilities on Nanodomed Surfaces: Stick–Slip Amplitude Coefficient

Understanding the frictional properties of nanostructured surfaces is important because of their increasing application in modern miniaturized devices. In this work, lateral force microscopy was used to study the frictional properties between an AFM nanotip and surfaces bearing well-defined nanodomes comprising densely packed prolate spheroids, of diameters ranging from tens to hundreds of nanometers. Our results show that the average lateral force varied linearly with applied load, as described by Amontons' first law of friction, although no direct correlation between the sample topographic properties and their measured friction coefficients was identified. Furthermore, all the nanodomed textures exhibited pronounced oscillations in the shear traces, similar to the classic stick-slip behavior, under all the shear velocities and load regimes studied. That is, the nanotextured topography led to sustained frictional instabilities, effectively with no contact frictional sliding. The amplitude of the stick-slip oscillations, σf, was found to correlate with the topographic properties of the surfaces and scale linearly with the applied load. In line with the friction coefficient, we define the slope of this linear plot as the stick-slip amplitude coefficient (SSAC). We suggest that such stick-slip behaviors are characteristics of surfaces with nanotextures and that such local frictional instabilities have important implications to surface damage and wear. We thus propose that the shear characteristics of the nanodomed surfaces cannot be fully described by the framework of Amontons' laws of friction and that additional parameters (e.g., σf and SSAC) are required, when their friction, lubrication, and wear properties are important considerations in related nanodevices.

MODULATION WAVE EQUATION FOR SPRING-BLOCK ORBITS WITH NONLINEAR RATE AND STATE FRICTION

A four-dimensional parameter model that captures the essence of spring-block propagation with rate- and state- friction laws was stated. In order to establish the existence of waves phenomena for all velocity-dependent critical value of stiffness and stick-slip amplitude for stiffness variations, we derived the modulation equation for our model. Conditions under which the amplitude of the modulation wave equation produces many small solutions near the instability threshold for finitely large enough time scales were established.

Visualization of contact line motion on hydrophobic textures

Contact angle hysteresis (CAH) has important implications in a variety of natural and industrial applications. The contact line motion of Cassie drops on microtextured surfaces is directly visualized using high-magnification imaging and fluorescence microscopy. The advancing contact line is observed to glide smoothly over the micro-posts, whereas the receding contact line exhibits significant stick-slip. Using simultaneous measurements of drop width and contact angle, the authors quantify stick-slip amplitude and define contact width hysteresis. A new scaling is proposed for drop roll-off angle α wherein sinα is linearly proportional to the practical work of adhesion (1 + cosθr). This study provides insight into the dynamic nature of CAH and the design of superhydrophobic surfaces with optimal drop shedding.

Intermittent stick-slip dynamics during the peeling of an adhesive tape from a roller

We study experimentally the fracture dynamics during the peeling at a constant velocity of a roller adhesive tape mounted on a freely rotating pulley. Thanks to a high speed camera, we measure, in an intermediate range of peeling velocities, high frequency oscillations between phases of slow and rapid propagation of the peeling fracture. This so-called stick-slip regime is well known as the consequence of a decreasing fracture energy of the adhesive in a certain range of peeling velocity coupled to the elasticity of the peeled tape. Simultaneously with stick slip, we observe low frequency oscillations of the adhesive roller angular velocity which are the consequence of a pendular instability of the roller submitted to the peeling force. The stick-slip dynamics is shown to become intermittent due to these slow pendular oscillations which produce a quasistatic oscillation of the peeling angle while keeping constant the peeling fracture velocity (averaged over each stick-slip cycle). The observed correlation between the mean peeling angle and the stick-slip amplitude questions the validity of the usually admitted independence with the peeling angle of the fracture energy of adhesives.

The influence of humidity on the sliding friction of brake friction material

Effects of humidity on the friction coefficients of brake friction materials at slow sliding speeds were studied. By measuring the static and dynamic coefficients of friction (COF) at high and low humidity levels, the mechanism of humidity-induced friction instability was investigated as functions of exposure time, applied pressure, and sliding speed. Tribological properties were examined using a pad-on-disk type tribometer and a 1/5 scale brake dynamometer. Results showed that the kinetic and static COFs were significantly affected by the exposure time in the humid air, showing saturation of the static COF within 2s. The static COF increased in humid air due to the increased adhesion by water menisci produced at the contact junctions. On the other hand, the static and kinetic COFs increased as a function of pressure in dry air due to the increased real contact area at high pressure, while they were not affected by applied pressure at high humidity levels. The test results also showed that the humidity increased the amplitude of stick-slip and its critical speed, which explains high propensity of friction instability at slow speeds such as creep groan at humid weather conditions.

The correlation between contact stiffness and stick–slip of brake friction materials

Friction-induced stick–slip of brake friction materials was investigated by examining the correlation between contact stiffness (kn) and surface topography changed by material wear at the sliding interface. By changing the surface roughness and thickness of the friction material, friction instability during sliding at low speeds was analyzed using a scale brake dynamometer. The results showed that the intensity and frequency of stick–slip were determined by the relative magnitudes of tangential contact stiffness (kt), bulk stiffness (kb), sliding velocity, and the difference (Δμ) between the static (μs) and kinetic coefficients (μk) of friction and these results were supported by the contact theories proposed by Greenwood and Williamson. While Friction materials with rough surfaces showed small stick–slip amplitudes due to small tangential contact stiffness (kt), friction materials with high compressibility exhibited pronounced stick–slip because the amplitude inversely varied with the bulk stiffness (kb). The high stick–slip propensity observed from heat affected friction materials also supported the correlation of the surface condition and contact stiffness (kn).

A Novel Pin-on-Disk Machine for Stick-Slip Measurements

A novel pin-on-disk machine is designed and constructed to provide the facility of vibrating the test samples at different amplitudes and frequencies of vibration in order to examine their effects on stick-slip amplitudes. The machine is equipped with control features and sensors with an integrated data acquisition system to regulate normal applied load and relative spindle speed. This machine is used as pin-on-disk type with the option of changing the sample jig and fixture. Friction stability of different types of soft and hard materials is investigated by means of variation of stick-slip amplitude with amplitude of normal vibration when mild steel pin slides on mild steel, glass fiber reinforced plastic (GFRP), polytetrafluoroethylene (PTFE), and rubber. Proper construction of the machine was validated by comparing the friction coefficients for tests performed on the machine with published results using similar machine. The results revealed that within the observed range the stick-slip amplitude depends on the material type and stick-slip amplitude decreases with the increase of amplitude of vibration. The rate of stick-slip amplitude reduction has a particular relationship with material, amplitude and frequency of vibration.

Effect of surface contact conditions on the stick–slip behavior of brake friction material

Brake-induced stick–slip, which determines the propensity of a brake system to generate noise, was investigated by analyzing the friction oscillation at different loads, speeds, and surface topographies. The friction characteristics at the sliding interface were obtained using a pad-on-disk type tribometer and a 1/5-scale brake dynamometer. Results showed that the size and stiffness of the surface contact plateaus significantly affect the propensity of stick–slip and wear. The stick–slip amplitude increased with normal load and decreased with sliding speed. Contact stiffness of the friction material, which was modified by producing shallow slots on the rubbing surface, also strongly affected the stick–slip propensity, whereas Δμ(=μs−μk) was not changed by the surface modification. The strong effects from the contact stiffness suggest that the height distribution of the surface plateaus can change the stick–slip propensity of a brake friction material.

금속계와 유기계 마찰재의 분포에 따른 하이브리드 마찰재의 마찰 특성

Tribological properties of hybrid type friction materials were studied. Hybrid friction materials were produced by combining non-steel(NS) and low-steel(LS) type friction materials. The emphasis of the inves- tigation was given to possible synergistic effects from the two different friction materials, in terms of friction sta- bility at high temperatures and the amplitude of friction oscillation, also known as stick-slip at low sliding speeds. The high temperature friction test results showed that the friction effectiveness of the hybrid friction material was well sustained compared to LS and NS friction materials. Wear resistance of the hybrid type was similar to LS friction materials. Examination of the rubbing surfaces after tests revealed that the friction characteristics of the hybrid friction material were attributed to the wear debris produced from low-steel friction materials, which were migrated to the surface of the non-steel friction material, forming new contact plateaus. The stick-slip amplitude and its frequency were pronounced when non-steel friction material was tested, while hybrid and low-steel types showed relatively small stick-slip amplitudes. These results suggest possible improvement of tribological prop- erties by designing a hybrid composite of low-steel and non-steel friction materials. Keywordsstick-slip(스틱-슬립), friction( 마찰), wear( 마모), temperature( 온도), low-steel( 금속성 마찰재), hybrid(하이브리드), non-steel(유기계 마찰재)