Stick-slip Characteristics Research Articles

Analysis of corrugation development on a Tianjin metro straight line focusing on wheel-rail contact mechanics

The development features of corrugation on the straight track metro line are studied to find the root cause of corrugation. Firstly, the measured corrugation characteristics on the line are investigated and a three-dimensional wheel-rail (WR) rolling contact finite element (FE) model is constructed. Then, the WR stick-slip (SS) characteristics, WR contact and rail wear are analyzed to understand the evolution process of corrugation from the micro transient point of view. Finally, combined with a system stability analysis, the development trend of corrugation is characterized from the macro perspective. The results show that the WR contact has a significant SS variation under the corrugation condition, which promotes the initial corrugation to develop further. For the fixed node in the contact area on the rail surface, the maximum stress and strain of the section where it is located will gradually transfer from the subsurface to the surface with the wheel running, so the damage prone position will also form first on the subsurface and gradually transfer to the surface. This shows from the micro viewpoint that the formation of wave crest/trough in the corrugation section is a bottom-up process of damage accumulation. The corresponding frequency of 648 Hz of the unstable vibration mode in the WR system is close to the passing frequency of 656 Hz of measured corrugation, demonstrating that the initial corrugation will tend to intensify with the wheel running.

Experimental Investigation of the Effect of Fault Reactivation Induced by Water Injection

An understanding of fault reactivation induced by water injection is of great significance for geothermal energy development and utilization. We conducted a series of water injection shear tests on low-permeability granite samples that each contained a single saw-cut fault under locally undrained conditions. Slip characteristics were analyzed by varying the fluid pressurization rate, confining pressure, and stress state of the fault to understand fault reactivation. The experimental results demonstrated that at a high pressurization rate, a higher local fluid pressure was needed to reactivate the fault than had been estimated theoretically, and the required fluid pressure increased with an increase in pressurization rate. The fluid pressurization rate and confining pressure both controlled the slip mode of the fault. The slip mode changed from dynamic slip to quasi-static slip at a high pressurization rate, and the peak slip rate of dynamic slip increased with an increasing pressurization rate. The fault showed significant stick-slip characteristics under a high confining pressure, as fault locking and reactivation phenomena occurred repeatedly. Faults with different initial stress states had little influence on the slip mode after the onset of slip.

Unsteady wetting of soft solids

HypothesisSpreading of liquids on soft solids often occurs intermittently, i.e., the liquid's wetting front switches between sticking and slipping. Studies of this so-called stick-slip wetting on soft solids mostly are confined within quasi-static or forced spreading conditions. In these situations, because the sticking duration is set much larger than the viscoelastic relaxation time of the solid, a ridge is persistently and fully developed at the wetting front as the soft solid yields to the liquid's surface tension. The sticking duration and spreading velocity, therefore, were shown to have little impact to the contact angle change required for stick-to-slip transitions. For unsteady wetting of soft solids, a commonly encountered but largely unexplored situation, we hypothesize that the stick-to-slip transition is controlled not only by a combination of sticking duration and the spreading velocity, but also by an increasing depinning threshold caused by the growing ridge at the wetting front. ExperimentWe performed unsteady wetting experiment on soft solids by letting water droplets spread freely on soft solid surfaces of various stiffness. We capture both the stick-slip spreading behavior and growing wetting ridges using synchronous high-speed imaging and high-speed interferometry. Recorded data of liquid spreading and solid deforming at the wetting front were analyzed to shed light on the relation between stick-slip characteristics and the growing wetting ridge. FindingsWe find that intermittent wetting on a soft solid surface results from a competition between three key factors: liquid inertia, capillary force change during sticking, and growing pinning force caused by the solid's viscoelastic response. We theoretically formulate their quantitative contributions to predict how stick-to-slip transitions occur, i.e., how the contact angle change and sticking duration depend on the liquid's spreading velocity and the solid's viscoelastic characteristics. This provides a mechanistic understanding and methods to control unsteady wetting phenomena in diverse applications, from tissue engineering and fabrication of flexible electronics to biomedicine.

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.

Wheel-Rail Stick-Slip Characteristics under Coupling Damages of Rail Corrugation and Wheel Flat in Metro

To investigate wheel-rail contact stick-slip characteristics (SSCs) under coupling damages of rail corrugation and wheel flat and explain the co-action mechanism of the two damages, a vehicle-track system model was built, and the SSCs were analyzed by employing the index of the overall dispersion rate of adhesion coefficient (ODRAC). The results show that both inner and outer wheel-rail systems (WRSs) on the curve have a propensity for stick-slip motion. Since the values of SSCs of the inner WRS are 1.204 to 2.963 times higher than those of the outer one, the inner WRS is easier to stick-slip vibration (SSV). When the rail corrugation is fixed, the overall dispersion rates of adhesion coefficients (ODRACs) progressively decrease with the increase of flat depth and length, and the decrease range is 2.766–41.775%. When the wheel flat is fixed, the ODRACs also progressively decrease as the corrugation wavelength increases, and the decrease ranges from 7.129% to 42.474%; the ODRACs incrementally increase together with the increase of the corrugation wave depth, and the increase ranges from 10.030% to 30.380%. The presence of flat will attenuate the level of SSV of the original system with rail corrugation, whilst the presence of corrugation will intensify the level of SSV of the original system with wheel flat.

Nucleation Mechanism and Rupture Dynamics of Laboratory Earthquakes at Different Loading Rates

The loading rate of tectonic stress is not constant during long-term geotectonic activity and significantly affects the earthquake nucleation and fault rupture process. However, the mechanism underlying the loading rate effect is still unclear. In this study, we conducted a series of experiments to explore the effect of the loading rate on earthquake nucleation and stick–slip characteristics. Through lab experiments, faults were biaxially loaded at varying rates to produce a series of earthquakes (stick–slip events). Both shear strain and fault displacement were monitored during these events. The findings indicate a substantial effect of the loading rate on the recurrence interval and the shear stress drop of these stick–slip events, with the recurrence interval inversely proportional to the loading rate. The peak friction of the fault also decreases with the increasing loading rate. Notably, prior to the dynamic rupture of earthquakes, there exists a stable nucleation phase where slip occurs in a quasi-static manner. The critical nucleation length, or the distance required before the dynamic rupture, diminishes with both the loading rate and normal stress. A theoretical model is introduced to rationalize these observations. However, the rupture velocity of these lab-simulated earthquakes showed no significant correlation with the loading rate. Overall, this study enhanced our comprehension of earthquake nucleation and rupture dynamics in diverse tectonic settings.

Influence of rail side lubrication and slip rate dependent effects upon the corrugation on the metro small curve track

For investigating the rail corrugation characteristics of metro small curve track under the combined effect of rail side lubrication and slip rate dependence, by developing a three-dimensional vehicle-track coupling numerical model, the wheel-rail (WR) contact stick-slip (SS) characteristics under different transition areas of rail side lubrication in the presence of corrugation were analyzed. Meantime, on the basis of considering the friction slip rate dependence, the SS vibration characteristics under different rail side lubrication conditions were further analyzed to understand the effect mechanism of rail surface friction characteristics on corrugation. The results show that, when there is rail side lubrication, both inner and outer WR systems have the possibility for SS vibration, and the longitudinal SS vibration intensity is greater than the transverse SS vibration intensity; the SS vibration intensity of the inner WR system is greater than that of the outer WR system, indicating that the inner rail is more susceptible to corrugation and the corrugation is more acute. With the expansion of the rail side lubrication range, the standard deviations of longitudinal and transverse adhesion coefficients decrease, illustrating that the expansion of the lubrication range can reduce the SS vibration intensity of WR system. When the co-effects of rail side lubrication and slip rate dependence are considered, the increase of the rail side lubrication range leads to the increase of standard deviations of adhesion coefficients, which may be related to the enhancement of slip effect induced by the increase of the lubrication range. The introduction of slip rate dependent effect can reduce the standard deviation of the WR adhesion coefficient, which demonstrates that the slip rate dependent effect can ease the degree of SS vibration of WR system, thus reducing the probability of corrugation occurrence caused by the SS vibration.

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.

Stick–Slip Characteristics of Drill Strings and the Related Drilling Parameters Optimization

To eliminate or reduce stick–slip vibration in torsional vibration of the drilling string and improve the rate of penetration (ROP), a stick–slip vibration model of the drilling string considering the ROP was established based on the multidimensional torsional vibration model of the drilling string. The model was verified by simulation analysis. The characteristics of the drilling string stick–slip vibration in the three stages of stationary, slip, and stick were analyzed. This paper investigated the influence of rotary torque, rotary speed, and weight on bit (WOB) on stick–slip vibrations in the drill string. Based on this, the relationship between the drilling parameters and ROP was established. Drilling parameter optimization was completed for soft, medium-hard, and hard formations. Results showed that appropriately increasing torque and decreasing WOB can reduce or even eliminate stick–slip vibrations in the drill string and increase the ROP. The parameter optimization increased the ROP by 11.5% for the soft formation, 13.7% for the medium-hard formation, and 14.3% for the hard formation. The established drill string stick–slip vibration model provides theoretical guidance for optimizing drilling parameters in different formations.

Analysis of rail corrugation characteristics in the vehicle braking section of metro

To study the evolution characteristics of corrugation in the vehicle braking section of the metro, a three-dimensional wheel–rail transient contact finite element model is established, and the contact stick–slip characteristics and rail wear distribution features are analyzed. The results show that, under the braking condition, the interface has stick–slip motion, indicating that the system is in a state of instability; the longitudinal relative slip is unevenly distributed; the peak position of the initial corrugation has a large longitudinal relative slip; and the transverse relative slip in the initial corrugation area is significantly larger than that in other areas, indicating that the initial corrugation will continue to develop and have a remarkable transverse wear trend. Under the nonbraking condition, there is no stick–slip motion at the interface, and the longitudinal and transverse relative slip values are less than the corresponding results under the braking condition. The cause of corrugation in the vehicle braking section is the wheel–rail unstable stick–slip motion triggered by the vehicle braking, and the fixed operating mode of the train provides the wavelength-fixing condition for the development of the initial corrugation, which leads to its continuous deterioration.

Modelling and stability analysis of a high-speed train braking system

The stability of a braking system is crucial for the steady and safe operation of trains. To investigate the dynamic behaviors and stability of a train braking system as influenced by key parameters, a numerical model integrating mode-coupling, negative friction–velocity slope (NFVS), and stick–slip theories was established. Additionally, the wheelset connected with the braking system was contained in the model considering that mode-coupling and stick–slip characteristics were affected by system structures. Based on this model, the influence and mechanism of stiffness, damping, and NFVS characteristics on mode-coupling of the braking system were revealed. The results showed that when the tangential and normal contact stiffness values of the pad were close, the system experienced mode-coupling instability and vibrations in these two directions were coupled. When the tangential and normal damping values of the pad were unequal, the energy dissipation in the two directions became different and the system instability was enhanced. Additionally, the system complex eigenvalues always had positive real parts and the dynamic behaviors were more complicated by considering NFVS at relatively low speeds. This is because NFVS characteristics can be equivalent to negative damping values, which cause the difference in tangential and normal damping values and continuously input energy into the braking system, then weakening the system stability.

Evaluation of the dynamic behaviors of a train braking system considering disc–block interface characteristics

Disc–block interface properties have prominent influence on the dynamic behaviors of train braking systems. In this study, the tribological characteristics of small-scale train friction blocks with different perforation diameters at the centre of the block surface were investigated using a disc–block test rig. And the Stribeck model parameters of the disc–block friction interface were identified to intuitively characterize the evolution law of stick–slip characteristics with the variation of block perforation diameters. Then, a trailer bogie braking system model of a high-speed train considering the coupling effects between the essential brake unit, wheelset and bogie frame was established. And the identified Stribeck parameters were introduced into the proposed braking system dynamics model to establish a relationship between the friction characteristics of the disc–block interface and the dynamic behaviors of the train braking system. Based on this, the cross-scale analysis was performed to evaluate the dynamic performance of a train braking system affected by the perforated friction block via classical stability theory and numerical simulation. The results show that the friction block with a perforation diameter of 3 mm can effectively suppress the stick–slip vibration of the disc–block friction interface, then reducing the oscillation of system components and enhancing the stability of the entire trailer bogie system.

Trend analysis of rail corrugation on the metro based on wheel–rail stick-slip characteristics

According to the actual metro situation, the vehicle-track rigid-flexible coupling numerical model was established, and the wheel–rail stick-slip characteristics on straight and curve tracks were analysed. Meantime, the stick-slip characteristics under the negative friction condition and its influence on corrugation were studied. The results show that for the straight track without irregularity or with long wavelength irregularity, the reason for the occurrence or further development of corrugation may be related to the saturation of transverse adhesion coefficient; for the straight track with short wavelength irregularity, the further development of corrugation may be related to the saturation of longitudinal adhesion coefficient. Both the inside and outside wheel–rail interfaces on the curve track have the trend of stick-slip vibration, and the existence of short wavelength irregularity will aggravate the intensity of stick-slip vibration. The fixed defect on rail surface will lead to the same phase rail wear at the same position, give the same phase periodic stick-slip vibration. The overall dispersion rates of longitudinal and transverse adhesion coefficients at the inside wheel–rail interface are greater than the corresponding values at the outside, which indicates that the inner rail is more prone to stick-slip vibration with large intensity.

Cause Analysis of Rail Corrugation Based on Stick-slip Characteristics

By analyzing the measured corrugation data, the cause of rail corrugation was discussed theoretically. Then, the vehicle-track coupling model was used to study the stick-slip characteristics of wheel-rail system. Finally, the cause of rail corrugation was explained by combining modal analysis and wheel-rail stick-slip relation. The transverse stick-slip curve on the inner rail-inner wheel of the leading wheelset presents a negative slope characteristic, which makes the inner wheel easy to slide on the inner rail surface and aggravates the wear of inner rail; there are also negative slope sections of the stick-slip curve on the outer rail-outer wheel of the trailing wheelset, but the adhesion coefficient is relatively large, indicating that the outer wheel will not slide obviously and the wear is relatively small. Meantime, with the increase of line curve radius, the negative slope phenomenon of stick-slip curves on the inner and outer rail sides shows a decreasing trend, which illustrates that the sliding wear of wheel is gradually reduced, and thus the occurrence probability of rail corrugation decreases. Combined with modal analysis, it can be seen that the coupling action of wheel discs bending vibration and inner rail bending-torsional vibration is the main cause of rail corrugation under the condition of wheel-rail system with stick-slip negative slope.

Stick–Slip Characteristic Analysis of High-Speed Train Brake Systems: A Disc–Block Friction System with Different Friction Radii

Inspired by the difference in the friction radii of the pads from the high-speed train brake system, stick–slip experiments for a disc–block friction system with different friction radii were carried out via a test device. Based on the test results, the stick–slip vibration characteristics of the disc–block friction system with variation in the friction radius were analyzed, and the corresponding Stribeck model parameters in exponential and fractional forms were identified. The experimental results show that with an increase in the friction radius the vibration amplitude first increased and then decreased and the frequency of stick–slip vibration increased. The identified Stribeck model parameters show that the decay factors increased, the static friction coefficient decreased, and the dynamic friction coefficient decreased first and then increased as the friction radius increased. Moreover, the identified Stribeck model in an exponential form can more accurately reflect the stick–slip characteristics of a disc–block friction system than the model in a fractional form. It can be further applied in the investigation of the dynamic behaviors of high-speed train brake systems.

Stick-Slip Phenomena and Acoustic Emission in the Hertzian Linear Contact

AE detection and analysis usually requires a specific, costly platform due to its particular burst nature and high-frequency content. This experimental study investigates the relationship between low-demand acoustic emission parameters (AE) and the occurrence of stick–slip (SS) at the Hertzian linear contact. Hence, the correlation of basic AE characteristics (amplitude, energy, and evolution in time) with stick–slip characteristics (static and kinetic friction coefficients, amplitude, energy, and evolution in time) is pursued. Tribological tests were conducted on cylinder–plane specimens under dry friction conditions with different loads at different low driving speeds and Hertzian contact pressures at a constant stiffness. The AE, normal, and friction forces were recorded simultaneously on the experimental stand. At the cylinder–plane interface, the jumps specific to the stick–slip phenomenon (friction coefficient—COF) were followed after a few milliseconds by AE jump peaks. The results of the experiments show that the amplitude and energy generated by AE were sensitive to the occurrence of the stick–slip phenomenon, while the AE and COF energies in the stick and slip phases had the same law of variation based on the driving velocities. The results show that the amplitude and energy of the sampled low-frequency AE signals were enough to detect the friction in SS and demonstrate the potential of AE as a tool for detecting and monitoring the tribological behaviour of SS at the linear Hertzian contact.

Investigation of Shear Mechanical Behavior and Slip Weakening Characteristics of Rough Joints in Rock Mass

The surface morphology of a structural plane is an important factor affecting the shear mechanical behavior of a structural plane. A direct shear test of a rough structural plane is carried out, and the shear mechanical behavior and slip weakening characteristics of a structural plane under different levels of roughness and normal stress conditions are studied; the normal stress conditions ranged from 2 MPa to 14 MPa. The results show that the shear strength and shear stress drop of a rough structure increase as the normal stress and roughness levels also increase. The higher the roughness level, or the greater the normal stress level, the more elastic energy the structural plane accumulates before shear failure. Once the shear stress is great enough and shear failure occurs, the shear slip of the rough structural plane shows obvious stick slip characteristics, and it releases more energy. Under high normal stress conditions, the convex body of the structural plane is damaged earlier in the process of direct shear, and the duration of convex body damage and failure is longer. After direct shear, the roughness of the structural plane decreases exponentially as normal stress levels increase. The shear slip of the structural plane, which has a significant degree of roughness under high normal stress conditions, shows a significant number of slip weakening characteristics, which is the main reason that the stick slip of the structural plane releases a lot of energy.

Investigation on axial-lateral-torsion nonlinear coupling vibration model and stick-slip characteristics of drilling string in ultra-HPHT curved wells

In view of the vibration failure of drilling string system in ultra-high temperature and high pressure (ultra-HPHT) curved wells, an axial-lateral-torsion coupling (ALTC) nonlinear vibration model of drilling string system was established using energy method and Hamiltonian principle, in which, the influence of wellbore trajectory change, wellbore constraint, interaction between bit and rock and ultra-HPHT of wellbore on elastic modulus and viscosity of drilling fluid were taken into account. The finite element method (FEM) is used to realize the numerical solution of the nonlinear vibration model. The correctness and validity of the ALTC nonlinear vibration model was verified by comparing the measured data of four ultra-HPHT wells with the theoretical calculation results of the proposed model.Based on this, according to the parameters of M directional well in Ledong ultra-HPHT block, South China Sea, the influences of ground rotation speed, drilling string length, weight on bit (WOB), torsional impact tools and drill collar length on stick slip vibration characteristics were investigated, and the method of increasing drilling speed in ultra-HPHT curved wells was put forward. The results obtained demonstrate that, firstly, when other requirements were met, the rotary speed should be increased as much as possible, which can effectively improve the drilling efficiency. Secondly, with the increase of drilling depth on-site, the drilling efficiency will be reduced, the whirl phenomenon of drill string system will be intensified, and the service life of bottom hole assembly (BHA) will be reduced. Thirdly, the optimized parameters of WOB and the torsional impact tool depended on the well structure, downhole tool size, etc. and can be determined using the proposed analysis method. The research results provide a theoretically sound guidance for designing and practically sound approach for effectively improving rate of penetration (ROP) and the service life of drilling string in ultra-HPHT curved wells.

Influence of Microroughness on Stick–Slip Characteristics of Fault Under Constant Normal Stiffness

Influence of Microroughness on Stick–Slip Characteristics of Fault Under Constant Normal Stiffness

Study on Axial-lateral-torsion Coupling Vibration Model and Stick-slip Characteristics of Drilling String in Ultra-HPHT Curved Wells

Study on Axial-lateral-torsion Coupling Vibration Model and Stick-slip Characteristics of Drilling String in Ultra-HPHT Curved Wells