Stick-slip Process Research Articles

Investigation on acoustic emission characteristics of fault stick-slip under different lateral pressures

To explore the effect of different stress environments on fault-slip rockbursts. Bidirectional shear friction experiments with different lateral pressures were conducted on precracked syenogranites buried at 800 m. The macroscopic statistical parameters (cumulative number of AE events, magnitude and b value) and local characteristic parameters (amplitude and dominant frequency) of acoustic emission during the stick-slip process under different lateral pressures were investigated. In addition, based on fractal theory, the nonlinear characteristics of AE spectrum were analyzed. On this basis, the microscopic mechanism of fault stick-slip was discussed. The results show that the lateral pressure influences the friction strength of the fault and stick-slip motion characteristics. With increasing lateral pressure, the proportion of transgranular shear fractures increases, which leads to an increase of cumulative number of AE events and magnitude. The periodic decrease in the b value is more significant at high lateral pressure. There is a good correlation between a high-magnitude AE event and a stress drop. The AE frequency with phased response characteristics can be used to effectively identify the evolution of fault stick-slip instability at the laboratory scale. A sharp increase in the amplitude of the dominant frequency can be regarded as one of the precursory features of fault stick-slip instability. The AE frequency spectra have multifractal characteristics, that differ among the different stages. The maximum multifractal dimension and spectral width can reflect the difference in energy released during fault stick-slip motion.

Numerical simulations of fault stick–slip characteristics in different temperature fields at laboratory scale

With the increasing demand for deep engineering development, there is an urgent need to study the evolution and formation mechanisms of the stick–slip process in faults at high temperatures. Although research on fault stick–slip behaviors at room temperature and laboratory scales has yielded some findings, exploring them at high temperatures is challenging, especially when acoustic emission (AE) monitoring is involved. To address this, the research conducted numerical simulations of fault stick–slip characteristics under thermo-mechanical coupling using the discrete element method to solve the problem. This approach aimed to provide insights into fault stick–slip behaviors and AE characteristics across different temperature fields. Subsequently, the evolution of the stress–strain characteristics (the number of stick–slip cycles, slip-initiation stress, slip-initiation stress drop, and maximum stress drop), energy dissipation, and AE characteristics (energy, magnitude, failure mechanism, and b-value) in the stick–slip process were systematically analyzed. The findings of this study aim to address existing shortcomings in AE tests and simulation methods related to fault stick–slip behaviors at high temperatures.

Identification of Stribeck Model Parameters to Accurately Reveal Stick–Slip Characteristics of a Disc–Block Friction System

Stick–slip vibration is simultaneously influenced by several factors that share complicated coupling relationships. In this research, an experimental investigation was performed to analyze the evolution law of stick–slip characteristics at different working conditions using a tribometer. The results showed that the disc rotational speed and normal force had noticeable impact on both stick–slip vibration and coefficient of friction (COF). Then the Stribeck model, which can characterize COF characteristics in the stick–slip process, was employed and the parameters were identified to construct a dynamic relationship between working conditions and COF. Further, numerical simulations respectively considering and neglecting the dynamic relationship between working conditions and Stribeck model parameters were conducted via an established tribometer analytical model. It was found that the simulation results considering the influence of working conditions on Stribeck model parameters were in better agreement with the experimental results, as the stick–slip vibration depends on the combined effect of working conditions and Stribeck model parameters. Therefore, the dynamic relationship between working conditions and Stribeck model parameters is crucial to accurately reveal stick–slip characteristics.

Sliding modes of fault activation under constant normal stiffness conditions

Fault activation has been the focus of research community for years. However, the studies of fault activation remain immature, such as the fault activation mode and its major factors under constant normal stiffness (CNS) conditions associated with large thickness of fault surrounding rock mass. In this study, the rock friction experiments were conducted to understand the fault activation modes under the CNS conditions. Two major parameters, i.e. the initial normal stress and loading rate, were considered and calibrated in the tests. To reveal the response mechanism of fault activation, the local strains near the fault plane were recorded, and the macroscopic stresses and displacements were analyzed. The testing results show that the effect of displacement-controlled loading rate is more pronounced under the CNS conditions than that under constant normal load (CNL) conditions. Both the normal and shear stresses drop suddenly when the stick-slip occurs. The decrease and increase of the normal stress are synchronous with the shear stress in the regular stick-slip scenario, but mismatch with the shear stress during the chaotic stick-slip process. The results are helpful for understanding the fault sliding mode and the prediction and prevention of fault slip.

A large-step stick-slip rotary piezoelectric actuator with high velocity under low frequency and small backward motion

Stick-slip rotary piezoelectric actuators (SRPAs) are commonly used nowadays. However, most of them achieve high velocity by high exciting frequency, which causes the problems of wide power-source passband requirement and the wear of actuators. Moreover, their further applications are limited, due to the poor motion stability caused by the backward motion. To solve the problems, the stick-slip process is analyzed with kinematics, indicating that the large step contributes greatly to SRPAs for achieving high velocity under low operating frequency and backward motion elimination. Then a large-step SRPA is proposed, fabricated, and tested. The experiments show that under the sawtooth signal with 100 V and 400 Hz, the prototype can reach a maximum velocity of 1.854 rad s−1, benefiting from the large step (above 4.636 mrad). While other works require the exciting frequency of several kilohertz to reach the same level of velocity. Additionally, by increasing the step, the backward ratio decreases from 14.43% to 8.89% at the frequency of 1 Hz, and the minimum no-backward frequency decreases from 120 Hz to 60 Hz. The results indicate the effectiveness of the large step for solving the problems, which is significant for the design of SRPA.

Temperature dependent model for the quasi-static stick-slip process on a soft substrate.

The classical Prandtl-Tomlinson model is the most famous and efficient method to describe the stick-slip phenomenon and the resulting friction between a slider and a corrugated substrate. It is widely used in all studies of frictional physics and notably in nanotribology. However, it considers a rigid or undeformable substrate and therefore is hardly applicable for investigating the physics of soft matter and in particular biophysics. For this reason, we introduce here a modified model that is capable of taking into consideration a soft or deformable substrate. It is realized by a sequence of elastically bound quadratic energy wells, which represent the corrugated substrate. We study the quasi-static behavior of the system through the equilibrium statistical mechanics. We thus determine the static friction and the deformation of the substrate as a function of temperature and substrate stiffness. The results are of interest for the study of cell motion in biophysics and for haptic and tactile systems in microtechnology.

Этапы подготовки к прерывистому скольжению на предварительно вырезанных разломах в лабораторных моделях и проверка этапов в природе

This study is focused on analog modeling of the stick-slip process along an existing large fault in an elastic-viscoplastic model subjected to loading at a constant strain rate. Based on our model results and data from (Ma et al., 2012, 2014), we distinguish stable, meta-stable, and meta-instable stages of the stick-slip process (the latter includes the early and late sub-stages). Our experiments show that the fault is periodically reactivated by segmentation. We analysed this mechanism from one fault reactivation to another, and identified regressive and progressive trends of segmentation. The regressive segmentation takes place during the stable and meta-stable stages of the stick-slip process. Under regressive segmentation the number of active segments and their lengths are reduced. The progressive segmentation is initiated at the early meta-instable sub-stage of the stick-slip process. Its activity is desplayed by an increase in the number of active segments to a certain critical density, while their pattern becomes more chaotic. In the late sub-stage, number of segments decreases as they rapidly grow and join with each other to form larger active segments, up to full reactivation of the entire fault. For comparison with the model results, we interpret rock deformation monitoring records before the Bystroe earthquake. Our analysis confirms specific features of the anomalous rock deformation that are similar to the strain features observed along the model fault during the meta-instable stage. There are evidence to suggest that meta-instability of a fault is a potential candidate to short-term precursor of earthquakes.

An alternate state-space algorithm for dynamic solution, sensitivity analysis and parameter identification of dry friction systems

Dry friction is one of the most complicated nonlinear phenomena in mechanical systems, due to its discontinuous stick–slip property. This property certainly brings difficulty to the solution and identification of dry friction systems. To overcome the difficulty, an alternate state-space algorithm is proposed in this paper to transform the dry friction system into a piecewise linear system. In doing so, the stick–slip process is firstly modeled by a zero–one representation, with which the stick–slip transition conditions are deduced. Then, the transition points are detected by the transition conditions to break the system into several pure stick or slip phases; each phase corresponds to a linear system. Dynamic solution of the piecewise linear system is quickly acquired by precise time step integration over an arbitrary pure stick or slip phase and the sensitivity analysis, pertaining also to a piecewise linear equation, can proceed exactly in a similar way. As is noteworthy, the sensitivity may admit jumps at the transition points due to the discontinuity of the dry friction system and therefore, the affine relation between the sensitivity before and after a transition point is elaborately derived. Eventually, based on the acquired sensitivity and the Tikhonov regularization, a sensitivity approach is developed to inversely identify the parameters of the dry friction system. Numerical examples and an experimental case are studied to demonstrate the accuracy and efficiency of the proposed alternate state-space algorithm.

Friction coefficient between glass surfaces and soft materials

The dry sliding friction between glass surface and soft materials like paper, towel and cloth for wiping glasses was explored experimentally. The UMT- 2 Tribometer has been fitted with a unidirectional oscillator including a mass-spring system. With a linear speed ranging from 0.05 to 8mm/s, a mass weighing 242 grams was sliding across planar surfaces covered with soft materials. The values of static and dynamic friction coefficients have significant variation as a function of linear speed and soft materials. The transition between the stick-slip process and continuum sliding has also been demonstrated experimentally. A theoretical model for variation of the dynamic friction coefficient as function of linear speed has been included in the dynamic equations of mass displacement. The dynamic equations have been solved by Runge Kuta method and compared with experimental results.

Visualization of stick-slip shear failure process of granite by 3D reconstruction technique and DEM

Understanding the mechanism of stick-slip instability of faults is of great significance for earthquake forecast and seismic fortification. In this paper, combined with the 3D reconstruction technology and discrete element method (DEM), the stick-slip shear failure process is visualized and the evolution mechanism of each stage of stick-slip process is studied. First, the 3D morphology scanning tests are carried out to define the 3D topography of the sample interface, which is imported into PFC3D to generate particle assembly, and micro-parameters of the particle assembly are calibrated by the experimental results. Second, the loading conditions are applied to the particle assembly, the stick-slip instability of the simulated fault occurs spontaneously, and the evolution of fault displacement and velocity distribution during the whole stick-slip period are studied. Third, the effects of normal stress, shear loading rate and interface roughness on stick-slip instability are investigated. Finally, nine parameters related to the steady-state friction coefficient, namely normal stress, natural density, Young's modulus, Poisson's ratio, maximum static friction coefficient, reference slip velocity, reference friction coefficient, a-b and slip velocity, are selected to establish the steady-state friction coefficient forecast model based on back propagation neural network (BPNN), and the relative importance analysis of relevant parameters is also implemented. Our research is helpful to reveal the seismogenic mechanism.

Friction behavior of 2D hydrogenated diamond-like films and bilayer graphene

It is important to distinguish the friction behavior between 2D hydrogenated diamond-like film (HDLF) and bilayer graphene (BG), which makes it difficult to determine the friction mechanisms due to the complexity of the sliding interface. Nano-scratch simulation is carried out to investigate the change of the average friction force and the degree of friction of the epitaxial 2D carbon film covering the surface of 4H-SiC under different normal loads. The results show that along with the increase of normal load, the average friction force on the interface of HDLF increases sharply compared with that in BG. During the stick-slip process, the huge damping produced by HDLF differs from the friction behavior of BG films due to the fast transport of phonons at the interface.

Anti-Poiseuille flow: Increased vortex velocity at superconductor edges

Using the time-dependent Ginzburg Landau equations we study vortex motion driven by an applied current in two dimensional superconductors in the presence of a physical boundary. At smaller sourced currents the vortex lattice moves as a whole, with each vortex moving at the same velocity. At larger sourced current, vortex motion is organized into channels, with vortices in channels nearer to the sample edges moving faster than those farther away from sample edges, opposite to the Poiseuille flow of basic hydrodynamics where the velocity is lowest at the boundaries. At intermediate currents, a stick-slip motion of the vortex lattice occurs in which vortices in the channel at the boundary break free from the Abrikosov lattice, accelerate, move past their neighbors and then slow down and reattach to the vortex lattice at which point the stick-slip process starts over. These effects could be observed experimentally, e.g. using fast scanning microscopy techniques.

Dislocation avalanches are like earthquakes on the micron scale

Compression experiments on micron-scale specimens and acoustic emission (AE) measurements on bulk samples revealed that the dislocation motion resembles a stick-slip process – a series of unpredictable local strain bursts with a scale-free size distribution. Here we present a unique experimental set-up, which detects weak AE waves of dislocation slip during the compression of Zn micropillars. Profound correlation is observed between the energies of deformation events and the emitted AE signals that, as we conclude, are induced by the collective dissipative motion of dislocations. The AE data also reveal a two-level structure of plastic events, which otherwise appear as a single stress drop. Hence, our experiments and simulations unravel the missing relationship between the properties of acoustic signals and the corresponding local deformation events. We further show by statistical analyses that despite fundamental differences in deformation mechanism and involved length- and time-scales, dislocation avalanches and earthquakes are essentially alike.

Mechanism analysis and improved model for stick-slip friction behavior considering stress distribution variation of interface

Studying the evolution of interface contact state, revealing the “black box” behavior in interface friction and establishing a more accurate friction model are of great significance to improve the prediction accuracy of mechanical system performance. Based on the principle of total reflection, a visual analysis technology of interface contact behavior is proposed. Considering the dynamic variation of stress distribution in interface contact, we analyze the nonlinear characteristics of contact parameters in different stages of stick-slip process using the above-mentioned experimental technology. Then, we find that the tangential stiffness of the interface is not a fixed value during the stick-slip process and the stress distribution variation is one of the important factors affecting the tangential stiffness of interface. Based on the previous experimental results, we present an improved stick-slip friction model, considering the change of tangential stiffness and friction coefficient caused by the stress distribution variation. This improved model can characterize the variation characteristics of contact parameters in different stages of stick-slip process, whose simulation results are in good agreement with the experimental data. This research may be valuable for improving the prediction accuracy of mechanical system performance.

Laboratory earthquake prediction of granite

Earthquake prediction refers to predicting the magnitude, location, and time of earthquakes and is challenging. Four attempts to predict the magnitude, location, and time of the laboratory earthquake of granite were made. First, the stick-slip failure process of granite samples was studied by a homemade test setup, and the whole process was monitored in real-time by strip strain gauges, rosette strain gauges, and acoustic emission (AE) transducers. Second, six parameters related to the stress drop were selected: dry density, Young's modulus, loading rate, static friction coefficient of the interface, normal stress, and shear stress. The prediction model of stress drop was established based on a backpropagation neural network (BPNN) considering the above six parameters. The stress drop in this paper is positively correlated with the released energy (magnitude) when the simulated fault slips suddenly. Third, the simulated fault interface was scanned by a high-precision 3D topography scanner before the stick-slip tests, and then the 3D topography scanning image was quantitatively measured based on digital image processing (DIP). According to the measurement results, the relationship between the nucleation position and the microstructure of the simulated fault interface was discussed. Fourth, the characteristics of AE parameters (absolute energy, ringing count rate, amplitude, b1 value) in the stick-slip process were studied. This paper aims to determine the precursory factors of sudden slip. This study helps explain the mechanism of shallow-focus earthquakes.

Influence of the stiffness and the speed on the stick-slip process

In this paper the authors experimentally investigated the influence of the linear speed and the stiffness of a single degree-of-freedom mass-spring system on the evolution of the stick-slip process. Using the UMT-2 Tribometer a steel mass was sliding on glass surface with a linear speed varied between 0.02 mm/s and 1 mm/s and spring stiffness varied between 40 N/m and 442 N/m. The experimental results evidenced that increasing both the linear speed and the spring stiffness can reduce the stick-slip process. Thus, the authors proposed an original friction model to simulate the stick-slip process by considering the dependence of the dynamic coefficient of friction with the sliding speed in the “slip” phase. The theoretical model is based on dynamic equilibrium of all the forces acting on the mass in transient phases. The proposed theoretical model was validated with experiments by simulating the stick-slip process for given linear speed and spring stiffness.

Synthesis and characterization of end-functionalized solution polymerized styrene-butadiene rubber and study the impact of silica dispersion improvement on the wear behavior of the composite

Two end-functionalized solution polymerized styrene-butadiene rubber was synthesized through anionic polymerization for using in formulation of well-dispersed silica compounds with study the wear behavior of the vulcanizates. Polymerization and functionalization were analyzed and confirmed via FT-IR, GPC, and NMR analyses. Novel functionalization, using an amine-containing agent, improved the distribution of silica. With the fine dispersion of silica, the wear rate dropped; the correlations between vulcanizate abrasion resistance, hardness, tear strength, glass transition temperature, and the SEM micrographs of the worn surface were studied. The abrasive stick-slip process was the primary wear mechanism and pattern spacing was the least for amino-modified vulcanizates. Lower reinforcement caused deep grooves, scratches, and pits in the non-modified sample. The amino-functionalized samples had desirable SEM characteristics, low hardness, more dispersed silica with fewer agglomerates and higher tear strength, confirming the higher interaction of silica particles and rubber chains.

In-Plane Potential Gradient Induces Low Frictional Energy Dissipation during the Stick-Slip Sliding on the Surfaces of 2D Materials.

Understanding the nanoscale friction properties of 2D materials and further manipulating their friction behaviors is of great significance for the development of various micro/nanodevices. Recent studies, taking advantage of the close relationship between friction and surface charges, use an external out-of-plane electric field to control the interfacial friction. Nevertheless, friction increases with the application of the out-of-plane electric field in most cases. Here, an in-plane potential gradient is applied for the investigation of the contribution of electric charges to friction on the surfaces of 2D materials. Experimental results show that the friction between an atomic force microscope tip and the flakes of 2D materials decreases with the application of the in-plane potential gradient, and the higher the potential gradient, the greater the friction decrease. By comparing the in situ atomic-level stick-slip maps before and after the application of the in-plane potential gradient, it is proposed that the promotion of low friction dissipative motion during the stick-slip process owing to the presence of the potential gradient gives rise to the friction reduction. These results not only help to reveal the origin of friction, but also provide a novel way to manipulate friction through an electrically-controlled sliding process.

Study of the formation mechanism of bundle structures using AFM tip-based nanoscratching approach

Abstract To investigate the formation mechanism of bundle structures on polycarbonate (PC) surfaces, atomic force microscopy (AFM) was utilized to scratch a PC sheet. A theoretical model for the AFM tip, which accounted for the friction force exerted on the probe, was established to illustrate the torsion angle of the tip. The formation of bundle structures was determined by a critical friction force, and a stick-slip process was used to explain the formation mechanism. The influence of the scratching parameters, including the scratching angles, normal loads, velocities, and feed values, on the amplitude and wavelength of the bundle structures was studied. These results can provide guidance for the fabrication of higher quality bundle structures on a polymer sample surface.

Improving load capacity of stick-slip actuators in both driving directions via a shared driving foot

Stick-slip piezoelectric actautors are promising actautors with prinicipally unlimited stroke and positioning resolution. However, it is challenged with a low load capacity, which limits the field of application. Various types of compliant driving foot were proposed in literature to improve the load capacity but the improved load capacity is direction-dependent. To improve the load capacity in both forward and backward directions, this paper proposes an approach via a shared driving foot. The basic idea is to employ two piezoelectric actuators and a shared driving foot to work in a way that either forward or backward driving is a stick-slip process and both involve a clamping action during the ‘stick’ phase and a releasing action during the ‘slip’ phase, so that a large driving force/load capacity can be achieved in both driving directions. Following this approach, a shared driving foot was proposed and designed. Finite element simulations were carried out and have validated that the designed driving foot can realize the proposed approach as desired. A prototype was built and tested and the effectiveness of the proposed approach has been validated by experiments. Under the sawtooth waveform voltage of 100 V at 1 kHz, the prototype achieved a free-load forward and backward driving speed as large as 18.6 mm s−1 and 16 mm s−1 respectively and a load capacity larger than 2 kg for both driving directions. Under a driving load of 2 kg, it can still move stably with forward and backward driving speeds of 1.8 mm s−1 and 0.6 mm s−1 respectively.