Variable Friction Research Articles

Variable-Friction-Based In-Hand Manipulation of Fabrics Applied to Unfolding Operations

The automatic state recognition and handling of fabrics, which are typical deformable objects, is difficult to realize. The development of related technologies for fabric manipulation remains impractical. This research was inspired by the process by which humans handle fabrics using their fingers. We proposed an in-hand manipulation method for fabrics based on variable friction and verified its effectiveness by sliding a single sheet of fabric and unfolding using in-hand manipulation functions. Experimental results demonstrate that the in-hand manipulation functions proposed in this study have the potential to solve complex problems in fabric handling.

Variable Friction Dampers (VFD) for a modulated mitigation of the seismic response of framed structures: Characteristics and design criteria

In this paper a new approach for the energy dissipation is discussed based on the use of Variable Friction Dampers (VFDs). The VFD device is borrowed from braking systems mechanical engineering, able to modulate its capability of dissipation, providing a constant pure friction force coupled with an additional (variable) damping force with the increasing of the displacement.Resorting to a previous study, the characteristics and the efficiency of the “braking” system in terms of reduction of the displacements and the restoring forces is described for SDOF systems under seismic excitations and compared with the performances achievable by classic constant friction dampers (CFDs). Then, a design criterium based on structure stiffness, viscous damping and seismic demand is proposed evaluating its effectiveness through a statistical assessment of the dynamic response of SDOF models subjected to natural accelerograms.The results obtained by the proposed design approach have proven that is a robust approach and indicate that the device can be really considered suitable for the improvement of the energy dissipation and the reduction of the drift in framed structures in an extended range of the seismic demands (from low to high) differently from the classic friction devices whose characteristics make them effective in a smaller range of the seismic intensities.

Dry Coupled Ultrasonic Non-Destructive Evaluation Using an Over-Actuated Unmanned Aerial Vehicle

Unmanned aerial vehicles (UAVs) are seeing increasing adoption to automated remote and in situ inspection of industrial assets, removing the need for hazardous manned access. Aerial manipulator architectures supporting pose-decoupled exertion of force and torque would further enable UAV deployment of contact-based transducers for sub-surface structural health assessment. Herein, for the first time, we introduce an over-actuated multirotor deploying a dry-coupled ultrasonic wheel probe as a novel means of wall thickness mapping. Using bi-axial tilting propellers in a unique tricopter layout, this system performs direct thrust vectoring for efficient omnidirectional flight and application of interaction forces. In laboratory testing, we demonstrate stable and repeatable probe deployment in a variety of representative asset inspection operations. We obtain a mean absolute error (MAE) in measured thickness of under 0.10 mm when measuring an aluminum sample with varying wall thickness. This is maintained over repeated exit and reentry of surface contact and when the sample is mounted vertically or on the underside of a 45° overhang. Furthermore, when rolling the probe dynamically across the sample surface in an area scanning modality, an MAE in wall thickness below 0.28 mm is recorded. Multi-modal operational confidence bounds of the system are thereby quantitatively defined. Note to Practitioners—Motivation for this article stems from the desire to enhance the speed and level of insight into structural health currently offered through remote aerial inspection processes. We approach this by integration of a thrust vectoring multirotor platform with a dry-coupling wheel probe for aerial ultrasonic thickness measurement. This system reliably presses the probe into the target surface and obtains point measurements across various surface orientations without a stabilizing frame. This broadens applicability and permits novel inspections where couplant gel would otherwise contaminate the surface and require manual cleaning. We profile thickness along a scanned linear section, a mode suited to corrosion mapping of large surface areas such as petrochemical storage tanks, pipework, or similar assets. We also make detailed consideration toward measurement accuracy, repeatability, and localization, an aspect commonly overlooked in literature. Future work to characterize variable friction effects currently limiting rolling scan speed and measurement coverage density may be beneficial. Quantitative study of the effectiveness of dry-couplant in the presence of any uncommon surface contaminants specific to a desired use-case is also advised.

Theoretical and experimental study on a dual-stage variable stiffness friction damper for satellite flywheel

Micro-vibration reduction devices are widely utilized to mitigate disturbances from on-board flywheel in order to enhance the performance for high precision payloads in satellite. However, the conventional micro-vibration suppression devices cannot endure the high-level loadings and effectively control the dynamic responses of the flywheel in launching stage. In this paper, firstly, a new type of dual-stage variable stiffness friction damper (DS-VSFD) is proposed and investigated under the on-orbit and launch loads. In order to demonstrate the effectiveness of the developed damper, the mechanism and hysteresis characteristics of the device are introduced in detail and the variable sliding force and displacement of the DS-VSFD are theoretically analyzed. Based on the relationship between output force and displacement of the device, a double-tail-shaped hysteresis curve is illustrated and corresponding stiffness and energy dissipation capacity at different stages are provided. Secondly, a single-degree-of-freedom dynamic model of the flywheel system with the DS-VSFD is constructed. The Harmonic Balance Method (HBM) is adopted to derive the force and displacement transmissibility curves in the frequency domain based on the dynamic model under different excitation conditions. In on-orbit stage, it can be observed that the resonance frequency is shifted to higher frequency and the amplitude is lower than that of the linear case in higher frequencies. In launching stage, the peak of displacement transmissibility of the flywheel system has been decreased in the resonance region comparing with the conventional linear isolator. Besides, the resonant frequency of the system has shifted to higher frequency region. Effects of the external excitation amplitude on the force transmissibility are also examined. Finally, comparison between the theoretical and experimental results has been carried out through static and dynamic tests. According to the static mechanical experimental results, it can be demonstrated that the DS-VSFD with wedge-block configuration exhibits stable and reliable hysteresis characteristics. The actual experimental results agree with the calculation results of the system dynamics model with the DS-VSFD in terms of force and displacement transmissibility. The results discussed in this research indicate that the DS-VSFD guarantees both the structural safety of the flywheel system under the launch loads and the micro-vibration reduction of the flywheel under the on-orbit condition.

Mode locking in gaseous detonation propagation in a channel with periodically varying friction

We consider a one-dimensional gaseous detonation wave propagating in a channel with a spatially periodic friction factor with the purpose of understanding how non-uniform momentum losses affect the detonation dynamics. The problem is investigated by means of the shock-fitting numerical integration of reactive Euler equations. We examine in detail the detonation velocity oscillations for a range of wavelengths of the variation of the friction factor. It is found that such periodic momentum losses lead to mode locking whereby the oscillations of the detonation velocity are locked into particular simple modes imposed by the variable friction. Outside of the mode locking regions, the dynamics is found to be highly irregular. For such regimes, we estimate the largest Lyapunov exponents of the detonation-velocity signal that points to the presence of friction-driven chaotic solutions.

The strange case of shear-thinning in non-Brownian suspensions

<h2>Abstract</h2> Shear thinning is generally reported in Brownian suspensions where it is understood to arise from a decrease in the relative contribution of entropic forces. However it has also been often reported in experiments with non-colloidal systems and nominally-Newtonian matrices at high volume fractions. In spite of the generality of the observations, its origin is still a matter of debate and the behaviour is difficult to reproduce in numerical simulations where shear thickening is typically observed instead. Several explanations have been proposed in recent years ranging from hidden non-Newtonian effects in the small interparticle gaps (Vázquez-Quesada et al., Phys. Rev. Lett. 117 (10), 108001 (2016), J. Non- Newt. Fluid Mech. 248, 1-7(2017)), excluded volume effects (Chatté et al. Soft Matter,14, 879-89 (2018)) and variable friction coefficients (Chatté et al.Soft Matter,14, 879-89 (2018); Tanner et al.Rheol. Acta 57:635–643 (2018); Lobry et al. J. Fluid Mech., 860, 682–710 (2019)). Another possibility that has emerged recently is related to slip occurring in the interparticle gaps at the liquid-solid interface (Kroupa et al. Phys. Chem. Chem. Phys., 19, 5979-5984 (2017); Vázquez-Quesada et al., Phys. Rev. Fluids 3 (12), 123302 (2018); Kumar et al.J. Non-Newt. Fluid Mech., 281, 104312 (2020)). In this talk I will review some of these issues and recent theories developed in this context.

An equivalent approach for modelling butterfly-hysteresis passive variable friction damper

This research work proposes an equivalent approach modelling the peculiar hysteretic behavior of passive variable friction damper, for engineers who call for a simplified way to simulate such damper in anti-earthquake engineering practice with acceptable accuracy. This approach models the special nonlinear behavior of the damper via paralleling 3 basic link elements that commonly built-in commercially available structural analysis software, instead of developing direct hysteretic model that demands for both post development functionality on software side and programming skill on engineer side. The mathematical derivation and analytical geometric equating work are comprehensively conducted. The numerical accuracy of analysis output yielded using equivalent model is digitally verified against the one yielded using generic FEM simulation of the same damper subject to several loading cases. Code-automated optimization workflow for yielding product-wise modelling parameters is developed that is convenient and straightforward for application by engineers without requirement of high expertise, which is demonstrated by generating optimized parameters to equivalently model tested specimen.Article highlightsPassive variable friction damper can be modelled by assembling friction spring, elastic spring and wen model into parallel.Code-automated optimization process can locate the most ideal modelling parameters combination in predefined numeral fields.Hysteretic behaviors yielded from equivalent modelling approach match well corresponding ones from generic FEA simulation.

Multiscale Shear Properties and Flow Performance of Milled Woody Biomass

One dominant challenge facing the development of biorefineries is achieving consistent system throughput with highly variant biomass feedstock quality and handling performance. Current handling unit operations are adapted from other sectors (primarily agriculture), where some simplifying assumptions about granular mechanics and flow performance do not translate well to a highly compressible and anisotropic material with nonlinear time- and stress-dependent properties. This work explores the shear and frictional properties of loblolly pine at multiple experimental test apparatus and particle scales to elucidate a property window that defines the shear behavior over a range of material attributes (particle size, size distribution, moisture content, etc.). In general, it was observed that the bulk internal friction and apparent cohesion depend strongly on both the stress state of the sample in granular shear testers and the overall particle size and distribution span. For equipment designed to characterize the quasi-static shear stress failure of bulk materials ranging from 50 to 1,000 ml in test volume, similar test results were observed for finely milled particles (50% passing size of 1.4 mm) with a narrow size distribution (span between 10 and 90% passing size of 0.9 mm), while stress chaining and over-torque issues persisted for the bench-scale test apparatus for larger particle sizes or widely dispersed sample sizes. Measurement of the anisotropic particle–particle friction ranged from coefficients of approximately 0.20 to 0.45 and resulted in significantly higher and more variable friction measurements for larger particle sizes and in perpendicular alignment orientations. To supplement these laboratory-scale properties, this work explores the flow of loblolly pine and Douglas fir through a pilot-scale wedge-shaped hopper and a screw feeder. For the gravity-driven hopper flow, the critical arching distance and mass discharge rate ranged from approximately 10 to 30 mm and 2 to 16 tons/hour, respectively, for both materials, where the arching distance depends strongly on the overall particle size and depends less on the hopper inclination angle. Comparatively, the auger feeder was found to be much more impacted by the size of the particles, where smaller particles had a more consistent and stable flow while consuming less power.

Deformational in curved fold-and-thrust belts: As a function of backstop shape and basal friction: Insights from analogue modeling and application to the Pamir salient, Hindu Kush region

Curved fold-and-thrust belt (CFTB) is a common structural phenomenon in orogenic belts and accretionary wedges, which has attracted the attention of several geologists. However, the formation mechanism of CFTB, particularly the cause of Pamir salient, has been greatly controversial. Therefore, based on previous studies, CFTB was studied by 12 groups of analogue modeling using the different shapes of the backstop and variable basal friction properties, and different compression velocities. The results show that the formation of CFTB is closely associated with the initial geometric characteristics of CFTB, the strength of the basal ductile layer, and the influence of lateral friction. Although the variation in backstop geometries is dominated by the pure Coulomb wedge, CFTB with completely different structural styles can form because of different boundary shapes. The CFTB formed by lower basement friction strength has a larger deformation range, and the CFTB formed by high strength of basal friction, which is narrow, has a high uplift amplitude, forming a tensile structure around its uplift. A high compressional rate forms the higher arc curvature, and a lower compressional rate forms the smaller arc curvature. Models of a brittle basement with triangular backstop and models with ductile basement and boundary parallel to the compression direction may provide a more reliable explanation for the formation of the forward thrust, backward thrust, and symmetrical pop-up structures of the Pamir salient and its structural vergence from south to north. This study further reveals that initial boundary geometry, stratigraphic structure, and rheology with a difference in brittle and ductile layers will yield more complex structural styles and different kinematic characteristics.

Seismic optimization of variable friction pendulum tuned mass damper with hysteretic damping characteristic

A variable friction pendulum-tuned mass damper (VFP-TMD) with hysteretic damping characteristics was proposed. The mechanism of the proposed VFP-TMD demonstrated linear behavior and nonlinear properties. Further, the fully nonlinear (FN) and partial linear (PL) equations of motion of the structure with the VFP-TMD system were derived. The VFP-TMD demonstrated stable and excellent seismic mitigation capacities and stroke limitation functionality. Based on the fixed-point theory, the closed-form exact solutions to design parameters of an undamped structure with a VFP-TMD system subjected to the ground motion were presented for different performance indices. The parametric studies showed that optimal designed VFP-TMD can be perfectly tuned in the frequency domain, which proved the applicability of closed-form solutions. To better understand VFP-TMD with hysteretic damping, constant friction pattern and multi-stage variable friction pattern were proposed for comparison. In contrast with constant friction and multi-stage variable friction, VFP-TMD with hysteretic damping characteristic was significantly optimized by the closed-form solutions. To verify the effectiveness of the closed-form solutions on VFP-TMD, twenty sets of real earthquake records were conducted which exhibited the superiority of linear variable friction patterns on seismic mitigation simultaneously. Results showed that the optimized VFP-TMD had an “infinitely large bandwidth”, which enabled it to reach its full potential according to the excitation amplitude.

Experimental validation and numerical simulation of a dual-self-centering variable friction braced frame under strong ground motions

The dual self-centering variable friction brace (DSC-VFB) is a novel resilient system with a loading stiffness higher than the unloading stiffness and an enhanced energy dissipation capability. This study presents experimental and numerical analyses of a 1/3-scaled dual-self-centering variable friction braced frame (DVBF). The pinned beam–column, column–base connections in the DVBF ensured that the DSC-VFBs were a standalone lateral seismic resisting system. Three ground motions scaled to six shaking intensities were conducted to evaluate the seismic responses of the DVBF. The ground motions were subjected to far- and near-field earthquakes. Owing to the high seismic resistance of the DVBF, the stiffness degradation could be omitted after several strong earthquakes. The frame returned to its initial position with nearly zero residual drift, although the maximum drift exceeded 2.5%. The grooved friction plates and self-centering system, which could be disassembled and replaced conveniently, had no observable damage after the tests. A 3D simulation model was developed based on practical constructions. The nonlinear analysis results were compared with the experimental results. The two exhibited good agreement. The results showed that the DVBF under near-field pulse earthquakes resulted in larger displacement and acceleration responses than those under far-field and near-field no-pulse earthquakes. The present study verified the effective working mechanism and good resilience of the DSC-VFB and DVBF even under strong seismic shaking intensities.

Surface undulation parameters of continuously variable transmission friction during turning

City traffic conditions contain frequent starts and stops. Thus, Single-Stage Continuously Variable Transmissions are subject to high loads when operating at high gear ratios, leading to excessive friction that cause slipping and heating of friction elements. This study proposes a microprofile with undulation on the surface of the pulley. A mathematical dependence on the technological quality assurance of the surface layer of the V-belt transmission parts formed during turning is proposed, enabling the prediction of height parameters of undulation depending on the change in the spindle speed, feed, and cutting speed. Estimating the error in the field experiment data and calculating values of the proposed methodology showed that the height parameters of the undulation of pulleys have a relative error of 6.3%.

Adaptive finite element simulation and experimental verification for fretting wear of PVDF piezoelectric thin films

An adaptive finite element method linked with user-defined subroutine UMESHMOTION is developed for simulating the tangential fretting wear of PVDF piezoelectric thin films. The three-dimensional (3D) sphere-on-flat contact model is established for the fretting wear simulation by using commercial software ABAQUS, where the Si3N4 ceramic ball is used as the punch in contact with PVDF piezoelectric thin films. The variable friction coefficient obtained from experiments is taking into consideration. The calculation of wear is based on the Archard's wear model. The tangential fretting motion with different loads and variable friction coefficients are applied in the finite element simulation. The fretting wear test are carried out to verify the numerical results under the same conditions. The results indicate that the present adaptive finite element model has a certain accuracy comparable to the experiment results for the fretting wear of PVDF piezoelectric thin films.

Seismic behavior of variable friction timber dampers: Experimental and numerical investigation

This paper proposed variable friction timber dampers (VFTDs) made of replaceable grooved timber plates, seismic behavior of such dampers and its influence on the lateral performance and dynamic response of heavy timber frames were investigated. Firstly, in total four specimens with different geometry characteristics and pre-stressing forces were conducted with reversed cyclic loading test. Typical damage and failure modes were identified and hysteretic curves were analyzed. Secondly, a modelling method on the hysteretic behavior of VFTDs was proposed by using multiple materials in OpenSees. Combined an existing test of a simple timber frame, the lateral performance and dynamic response of timber frames with VFTDs were numerically investigated and compared to those of a simple frame and a frame with a conventional friction timber damper. The experimental results indicated that timber plates of VFTDs cracked perpendicular to wood grain under combined compression and friction. The cracking weakened the integrity of timber plates, resulting to asymmetry of hysteretic curves of VFTDs. VFTDs were able to dissipate energy meanwhile provide higher damping force when timber plates slid on slope surfaces. Increasing pre-stressing force in a certain range considerably improved damping force and energy dissipation capacity of VFTDs. The numerical simulation demonstrated that VFTDs increased the lateral load-resisting capacity and energy dissipation capacity of timber frames more than 55% and 710%, respectively. Owing to superior lateral load-resisting capacity and extraordinary energy dissipation capacity of VFTDs, the peak drift and acceleration of timber frames decreased up to 55%. VFTDs with slope sliding surfaces outperformed VFTDs with consecutive flat and slope sliding surfaces and conventional friction timber dampers in seismic response reduction.

A 4D-Printed Structure With Reversible Deformation for the Soft Crawling Robot

Reversible deformations of the 4D-printed structures are attractive and promising for various application fields. In this study, the principle of reversible deformations for the bilayer structure consisting of SMP and elastic material is illustrated. By exploring the influence of printing parameters on deformation and resistance, a low-cost reversible bilayer structure with rational resistance distribution is designed to realize reversible deformation. Subsequently, the bilayer structure is employed to design a soft crawling robot with asymmetrical variable friction coefficient feet. By revealing the principle of locomotion by force analysis and deformation process analysis, a wave-like strategy is proposed to actuate the robot. Experiments verify the effectiveness of the designed structures.

Novel variable reconstruction and friction term discretisation schemes for hydrodynamic modelling of overland flow and surface water flooding

Due to the recent advances in computing and data acquisition technologies, numerical models solving the 2D shallow water equations (SWEs) are now widely used in predicting overland flow and surface water flooding process in natural catchments. In catchment-scale flood modelling, in addition to the calculation of convective terms that are usually achieved through solving local Riemann problems to capture transient flow dynamics, correct discretisation of source terms is also essential to handle complex domain topography and ensure accurate and stable numerical solutions. The external forces induced by gravity and bed friction create the key source terms that drive the change of momentum in the SWEs and affect the stability and accuracy of the adopted numerical scheme, especially for applications involving very shallow water depth (e.g. wet and dry fronts, overland flows). Herein, new schemes are introduced to reconstruct the face values of flow variables and a fully implicit algorithm is improved to discretise the stiff friction terms, in the context of developing a Godunov-type finite volume hydrodynamic model for accurate and stable simulation of overland flow and surface water flooding involving very shallow water depth. Five analytical test cases are considered to validate the resulting models and the numerical results are compared with those produed by two alternative numerical schemes. The performance of the models are further demonstrated by reproducing a flood event in the 2500 km2 Eden Catchment, UK.

Shaking table test of a three-story frame with resilient variable friction braces

A novel resilient variable friction brace (RVFB) has been proposed to avoid the complex prestressing process of a traditional resilient brace. This study presents a shaking table test of the 1/3 scaled RVFB frame (RVFBF) to evaluate its seismic performance. In particular, the concept and mechanical model of the RVFB were introduced, including the theory of variable friction and stiffness. Then, a three-story steel RVFBF with pinned connections was designed, including one braced frame and two gravity frames. This arrangement guaranteed that the lateral force resistance was only provided by the RVFBs, while other components of the RVFBF were kept in the elastic stage. After the frame was tested under several earthquake conditions with increasing ground motion levels, the accelerations, drifts, and shear forces of the RVFBF were evaluated. The results demonstrated that the RVFB exhibited a novel flag-shaped curve with greater loading stiffness. Good agreement was verified between the mechanical and experimental results. Although the peak floor drift of the RVFBF exceeded 2.6% under the strongest ground motion level, the damage-free frame returned to the initial position with nearly zero residual displacements, and the braces had negligible performance degradation after the entire test. Therefore, a RVFBF that has satisfied seismic requirements can effectively control the structural damage and quickly restore its function after several earthquakes.

Effects of landing gear layout on the safe rollout envelope of equipped–skid aircraft

Rollout is an inevitable phase of realizing horizontal landing for an equipped–skid aircraft; therefore, one should pay attention to the safety of the entire rollout process. The research presented herein analyzes the effects of the landing gear layout on the safe rollout envelope, which is the set of all states where the aircraft can remain in the desired state region during the entire rollout phase. First, the nonlinear reachability analysis and its internal relation with a safe envelope are briefly described. To improve the calculation efficiency of approximating the safe envelope, a mixed implicit explicit (MIE) scheme is introduced. Subsequently, a simple yet accurate model of nonlinear aircraft dynamics is established and the dangerous state region during the rollout phase is defined. Finally, based on the numerical results, the effects of the layout type on the safe envelope are discussed. In addition, a reasonable safety evaluation index is proposed to quantitatively analyze the effects of position parameters on the rollout safety. The results indicate that with the application of the variable friction skid, the safe envelope of the wheel–skid aircraft is larger than that of the all–skid aircraft. Compared with the original position parameters, the optimized values increase the rollout safety of the wheel–skid aircraft by 4.18%.

Coupled Thermo-Structural Analysis Model of Solid Rocket Motor Nozzle considering the Variation of Friction Coefficient under Operating Conditions

Multi-interface is a typical feature of solid rocket motor nozzles, and the interface evolution law is an important aspect to analyze the thermo-structural response of solid rocket motor nozzles. In this paper, based on the friction coefficient test at different temperatures, a coupled thermo-structural analysis model of solid rocket motor nozzle considering the variation of friction coefficient under operating conditions was established. Firstly, the friction coefficient was tested to model the variation at different temperatures. Then, by adopting structure gap, variable friction coefficient, thermal contact resistance, and friction heat production, a strongly coupled non-linear model was established. Simulations using the non-linear model and traditional model were performed, in which the stress of the throat insert was within the required stress range of the material. The ground firing test result demonstrated the validity of the analysis model, and the non-linear model was in a better agreement with the firing test than the traditional model. Therefore, it can be concluded that with a more specific friction coefficient to represent the friction behaviour of the different parts of the nozzle, the strongly coupled non-linear model established in this paper can reflect the essence of thermo-structural response of solid rocket motor nozzle.

Design and Implementation of an Electromechanical Brake System

Nowadays, hydraulic brakes are already being replaced by electromechanical brakes (EMB) to improve quick-response brakes, efficient fuel consumption, environmentally sound, simple maintenance, and enhanced safety design. It is suggested that the electromechanical brake will be one of the most important brake systems in the future. This study focuses on designing and implementing an electromechanical brake based on a brushless DC (BLDC) motor and position controller to generate and control the required braking force at a variable friction coefficient between disc and pad. A feedback controller equipped with a measuring sensor is usually utilized to control this type of brake. Thus, three controllers for current, speed, and position were implemented in successive loops to control the motor movement. This system has current, speed, and force sensors. Due to implementation difficulties and cost issues of braking, the clamping sensor should be replaced with a position sensor with some modification where a position controller has been designed and implemented. The results showed that the clamping force of the brake system can follow the target accurately and it has good performance. Also, it is shown that this system can adjust the brake force more accurately and quickly compared with the traditional.