Strong Friction Research Articles

How to build an extensional basin in a contractional setting? Numerical and physical modelling applied to the Mejerda Basin at the front of the eastern Tell of Tunisia

In Tunisia, Africa-Eurasia convergence resulted in the Tell fold-and-thrust belt (FTB), part of the Tell-Rif (or Maghrebides), associated, to the south, to the Atlas intra-continental belt. The Mejerda rift-basin is developed over the compressional boundary between the Tell and the Atlas. The main objective of this study is to identify the mechanism responsible for such a basin development. The abundance of Triassic salt structures in the northern Atlas, compared to the Tell, suggests a friction change along the basal décollement formed by the salt level. This frictional contrast is expected to produce a heterogeneous deformational response and complex structures. On this basis, numerical and analogue modelling were used to investigate the impact of a friction change on the deformation style. The kinematic approach of the limit analysis theory was applied here, via the software Optum G2. The onset of faulting in all numerical tests reveals extensional faults appear at the friction boundary. Beyond the onset of faulting, sandbox experiments show extensional features such as collapsing blocks and extensional faults. Overall, the adopted numerical and physical approaches showed evidences of extension. In particular, the experiment using sandpaper and Silicon putty (PDMS) for the décollement transition shows viscous deformation, mimicking diapiric movement. These observations of numerical and physical model behaviors highlight that compression can produce an extensional basin at the top of a thrust wedge, as expression of a strong basal friction change. Hence, the Mejerda basin is interpreted as a wedge-top basin. It developed, during the growth of the Tell and Atlas FTBs. The same configuration likely exists in other mountain belts: the case of the Molasse Basin between the Jura and the Western Alps is advocated.

Finite element modeling of cable sliding and its effect on dynamic response of cable-supported truss

The cable system of cable-supported structures usually bears high tension forces, and clip joints may fail to resist cable sliding in cases of earthquake excitations or sudden cable breaks. A analytical method that considers the dynamic cable sliding effect is proposed in this paper. Cable sliding behaviors and the resultant dynamic responses are solved by combining the vector form intrinsic finite element framework with cable force redistribution calculations that consider joint frictions. The cable sliding effect and the frictional tension loss are solved with the original length method that uses cable length and the original length relations. Then, the balanced tension distributions are calculated after frictional sliding. The proposed analytical method is achieved within MATLAB and applied to simulate the dynamic response of a cable-supported plane truss under seismic excitation and sudden cable break. During seismic excitations, the cable sliding behavior in the cable-supported truss have an averaging effect on the oscillation magnitudes, but it also magnifies the internal force response in the upper truss structure. The slidable cable joints can greatly reduce the ability of a cable system to resist sudden cable breaks, while strong friction resistances at the cable-strut joints can help retain internal forces.

Cosmology from multimeasure multifield model

We consider the cosmological application of a (variant of) relatively newly proposed model1 unifying inflation, dark energy, dark matter, and the Higgs mechanism. The model was originally defined using additional non-Riemannian measures, but it can be reformulated into effective quintessential model unifying inflation, dark energy and dark matter. Here, we demonstrate numerically that it is capable of describing the entire evolution of the Universe in a seamless way, but this requires some revision of the model setup. The main reason is that there is a strong effective friction in the model, a feature which has been neglected in the pioneer work. This improves the model potential for proper description of the evolution of the Universe, because the friction ensures a finite time inflation with dynamically maintained low-value slow-roll parameters in the realistic scenarios. In addition, the model predicts the existence of a constant scalar field in late Universe.

Precision Numerical Modeling of the Decay of a Metastable State at High Temperatures

The quasistationary decay rate of a metastable state, RD, often is interpreted as the inverse of the average lifetime of this state, τa. There are two ways for finding the rate: to evaluate it using one of the approximate Kramers formulas or to extract the rate RD from numerical modeling. We study quantitatively to what extent the inverse decay rate can be identified with the mean lifetime and to what extent the approximate and numerical quasistationary rates agree with each other. This is done for the values of dissipation strength covering 4 orders of magnitude. Both the numerical rate and the average lifetime are obtained from computer modeling of the decay process using the stochastic differential equations for the phase space diffusion. It is shown that at lower temperature, the numerical rate is in agreement with the inverse lifetime whereas at weak friction and high temperature, τa exceeds $$ {R}_D^{-1} $$ by up to 50%. The Kramers formula for the decay rate at strong friction is found to be in a good agreement with the dynamical rate deviating typically by 5%. For weak friction, the agreement is substantially worse: the difference varies from 20% to a factor of 2.

Characterization and Industrial Performance Evaluation of Duplex-Treated AISI H21 Die Steel during Hot Forging Process

Abstract Die/tooling failure during the hot forging process is mainly due to repeated varying mechanical loads, rigorous thermal shocks, as well as strong friction and erosion. These cause different failure mechanisms, such as wear, thermal fatigue, and plastic deformation. The enhancement of die service life for a hot forging industry is necessary now in order to be competitive globally. Four nitride coatings—TiN, TiAlN, TiCrN, and AlCrN—were deposited onto plasma-nitrided AISI H21 hot work die steel using the cathodic arc evaporation physical vapor deposition technique. The coatings were characterized by scanning electron microscope (SEM), SEM–energy dispersive x-ray spectroscopy (EDS), x-ray power diffraction (XRD), line scan EDS, and x-ray mapping tests and microhardness tester. The wear characteristics were examined using a pin-on-disk method at high temperature and speed against EN31 bearing steel disk. It was observed that the AlCrN and CrTiN coatings have lower wear rates than other coatings at high temperature (550°C). Also, these coatings performed best during actual industrial practice. The enhancement in die life was reported to be 55 and 76 % in terms of parts being hot forged. The low value of the coefficient of friction for all coatings at 550°C can partly be attributed to lower hardness difference between the pins and disks, decreased plastic deformation in the contact, and, consequently, less ploughing (abrasive wear action). Predominately, wear was the main die failure mode there was; when used, dies were observed microscopically.

Microstructure, composition and properties of Cr–GLC graded coating deposited by unbalanced magnetron sputtering

Cr doped graphite-like carbon (Cr–GLC) graded coating with Cr bottom layer, Cr/C composition gradient layer and Cr–GLC top layer was deposited by unbalanced magnetron co-sputtering of pure chromium and graphite targets. The microstructure, composition and properties including adhesion strength, wear resistance of the as-fabricated Cr–GLC graded coating were investigated by TEM, XPS, scratch test and sliding friction test. Three kinds of phases are detected in the graded layer: pure Cr, a Cr/C nanocrystalline/amorphous phase and amorphous carbon. The HRTEM observation for Cr–GLC top layer with 3 at% Cr indicates that Cr-enriched clusters are embedded in an amorphous carbon matrix. Results of the investigation demonstrate that strong adhesion and stable friction coefficient of the Cr–GLC graded coating are attributed to the consecutive interface microstructure in the coating caused by the interlayer design.

Coseismic ultramylonites: An investigation of nanoscale viscous flow and fault weakening during seismic slip

Faults weaken during the propagation of earthquakes due to the onset of thermally-activated mechanisms, which vary depending on the rock type. Recent experimental work suggests that carbonate-hosted faults are lubricated by viscous flow in nano-granular aggregates having ultramylonitic textures. However, their frail nature has often hindered unbiased characterisation of the textures and deformation mechanisms operating at such extreme conditions (strain rates as high as 104), which remain so far poorly investigated and understood.We explore the formation, evolution and deformation mechanisms of coseismic ultramylonites in carbonate-hosted faults generated during high velocity (1.4 ms−1), displacement-controlled shear experiments in a rotary apparatus. Microstructures were analysed using integrated SEM and TEM imaging while detailed crystallographic fabrics were investigated using the electron back-scattered diffraction (EBSD) technique.Mechanical data show that the strength of the experimental fault decays dynamically with slip, according to a characteristic four stage evolution; each stage is associated with characteristic textures. Microstructural observations show that brittle processes dominate when the fault is strong (friction coefficients >0.6). Cataclasis, aided by twinning and crystal plasticity, operates forming an extremely comminuted shear band (mean grain size ∼200 nm). As the fault starts weakening, shear localises within a well-defined principal slip zone. Here, thermally-activated grain size sensitive (GSS) and insensitive (GSI) creep mechanisms compete with brittle processes in controlling fault strength. GSI mechanisms produce strong monoclinic crystallographic preferred orientations in the slip zone, while textures and crystallographic orientations in adjacent locations do not evolve from the previous deformation stage. By the end of the transient weakening stage, the slip zone has reached a steady state thickness (30 μm) and shows a nanogranular ultramylonitic texture. The intensity of the crystallographic preferred orientation in the coseismic ultramylonite is reduced compared to the previous stage, due to grainsize sensitive creep mechanisms becoming gradually more dominant. As the experimental fault re-strengthens, upon deceleration to arrest, the ultramylonite may be partially reworked by brittle deformation.Our findings show that the crystallographic orientations of transient microstructures are preserved in the slip zone of coseismic ultramylonites and in narrow, adjacent deactivated layers, where mirror-like surfaces are located. This shows that EBSD techniques can usefully be employed to determine the deformation mechanisms of coseismic ultramylonites and their evolution during earthquake slip in both experimental and, potentially, natural faults.

Position control of a rodless cylinder in pneumatic servo with actuator saturation

In this paper, positioning control of a rodless cylinder in pneumatic servo systems with actuator saturation is investigated via an active disturbance rejection control. A linear extended state observer is designed to estimate and compensate strong friction force and other nonlinearities in the pneumatic rodless cylinder system. An actuator saturation linear feedback control law is developed to further improve the control performance. Furthermore, a linear matrix inequality-based optimization algorithm is employed to estimate a strictly invariance set for the closed-loop system. Experiment results with response time 0.5 s and accuracy 0.005 mm for a 200 mm step signal demonstrate the effectiveness of the proposed control strategy.

Autonomous Climbing Robot for Tank Inspection

A detailed design of mechanisms is presented, along with the required electromagnetic system to attach the robot to the tank’s wall. This is a novel mechanical design in which the robotic wheels rotate changing the instantaneous center of rotation. The robot mechanics is designed and simulated using Solidworks. The proposed electromagnetic system has to provide the necessary currents to control magnetic forces to be strong enough to attach the robot to the wall. However, because of the electromagnetic forces the control positioning method has to consider the strong Coulomb friction that appears against wheels movement. The sensor and acquisition system for navigation and control are also detailed. An inspection system has been proposed based on client/server architecture. Autonomous tracking and a mapping system powered by IMU sensor fusion is proposed. This design is suitable to be used along with the robot motion positioning strategy and, especially, the system must be robust to the disturbances and noise.The robot runs a server application and a remote PC is the client. The robot configuration has been developed with the capability of climbing up steel walls and navigating welding lines. The remote inspection system has been illustrated. A sensorial system and data fusion strategy to estimate the absolute robot position have been proposed. In order to track the desired trajectories, the robot’s kinematics is described. Some simulations of the position control of the wheels and the complete control for the tracking performance were simulated using Matlab/Simulink. This robotic system has demonstrated the capability to track an accurate trajectory given by the defined kinematic equations.

Fabricating PVDF hollow fiber microfiltration membrane with a tenon-connection structure via the thermally induced phase separation process to enhance strength and permeability

In this work, thermally induced phase separation (TIPS) method was adopted to manufacture polyvinylidene fluoride (PVDF) blend hollow fiber microfiltration membrane with tenon-connection structure, which is used to fasten in Chinese traditional building by the stronger friction between rabbet and mortise, for water treatment by blending polyvinyl butyral (PVB). In this microstructure formation process, the distribution of PVB within the matrix and its regulatory role were investigated. Especially, the comprehensive effect of rabbeting and PVB adhesion on strength (The maximum breaking strength reached 17.08 MPa) was discussed. The results showed that a suitable PVB will markedly reduce the thickness of skin-layer close to the outside surface even for the solid-liquid (S-L) phase separation while the excessive PVB result in a thick skin-layer. Besides the strength, PVB markedly improved the hydrophilicity and permeability of the membrane. The breaking strength is more than 8 MPa, the maximum pure water flux is as high as 929.23 L m−2 h−1 and the rejection rate of carbon particles is nearly 100% even the polymer content is only 22%. In addition, the membrane showed a β-phase and an improved anti-fouling capacity. The investigation presented a simple way to regulate microstructure, enhance mechanical strength, flux and anti-fouling capacity of the membrane via the TIPS process for water treatment.

Comparing Dynamics Initiated by an Attached Oscillating Particle for the Nonholonomic Model of a Chaplygin Sleigh and for a Model with Strong Transverse and Weak Longitudinal Viscous Friction Applied at a Fixed Point on the Body

This paper addresses the problem of a rigid body moving on a plane (a platform) whose motion is initiated by oscillations of a point mass relative to the body in the presence of the viscous friction force applied at a fixed point of the platform and having in one direction a small (or even zero) value and a large value in the transverse direction. This problem is analogous to that of a Chaplygin sleigh when the nonholonomic constraint prohibiting motions of the fixed point on the platform across the direction prescribed on it is replaced by viscous friction. We present numerical results which confirm correspondence between the phenomenology of complex dynamics of the model with a nonholonomic constraint and a system with viscous friction — phase portraits of attractors, bifurcation diagram, and Lyapunov exponents. In particular, we show the possibility of the platform’s motion being accelerated by oscillations of the internal mass, although, in contrast to the nonholonomic model, the effect of acceleration tends to saturation. We also show the possibility of chaotic dynamics related to strange attractors of equations for generalized velocities, which is accompanied by a two-dimensional random walk of the platform in a laboratory reference system. The results obtained may be of interest to applications in the context of the problem of developing robotic mechanisms for motion in a fluid under the action of the motions of internal masses.

Entrepreneurship as a Factor of the Migrant Capacity of Producing Social and Cultural Equality: The Case of Sikh Indian Entrepreneurs in Rome

Currently strong frictions, uncertainties and contrasts characterize society, defined by many as being very complex. The global economic crisis that began in 2006 and especially in Italy became a social and cultural crisis, which affected all societies and nations increasing the distance between the so-called West (Latouche, 2006) and the rest of the world. In this context, migrations have progressively distinguished themselves as a phenomena with global characteristics that have taken the form of real diasporas (Cohen, 2008; Clifford, 1994; Said, 1993;) and which definitively represent the need for a moment of study, analysis and economic and social interpretation that can give vital feedback, not only to scholars and professionals, but also and above all to policy makers.

Bioinspired 3D Printed Locomotion Devices Based on Anisotropic Friction.

Anisotropic friction plays a key role in natural systems, particularly for realizing the purpose of locomotion and strong attachment for the survival of organisms. Of particular interest, here, is the observation that friction anisotropy is promoted numerous times by nature, for example, by wild wheat awn for its targeted and successful seed anchorage and dispersal. Such feature is, however, not fully exploited in man-made systems, such as microbots, due to technical limitations and lack of full understanding of the mechanisms. To unravel the complex dynamics occurring in the sliding interaction between anisotropic microstructured surfaces, the friction induced by asymmetric plant microstructures is first systematically investigated. Inspired by this, anisotropic polymer microactuators with three-dimensional (3D) printed microrelieves are then prepared. By varying geometric parameters, the capability of microactuators to generate strong friction anisotropy and controllable motion in remotely stretched cylindrical tubes is investigated. Advanced theoretical models are proposed to understand and predict the dynamic behavior of these synthetic systems and to shed light on the parameters and mechanisms governing their behavior. Finally, a microbot prototype is developed and cargo transportation functions are successfully realized. This research provides both in-depth understanding of anisotropic friction in nature and new avenues for developing intelligent actuators and microbots.

An Assessment of Experimental Evidence on Agricultural Technology Adoption in Developing Countries

This article reviews recent results on technology adoption in developing countries, primarily from field experiments. It focuses on studies that highlight three constraints to adoption: credit, insurance, and information. Interventions supplying credit are consistently effective in spurring technology adoption for a minority of farmers, while interventions supplying insurance have had more mixed results. This review suggests that one mitigating factor on demand for both of these products is incomplete information, which adds additional uninsurable risk to the technology adoption decision. A broad group of studies identify the presence of strong informational frictions. The review concludes with some potential directions for future research.

Strong negative nonlinear friction from induced two-phonon processes in vibrational systems

Self-sustained vibrations in systems ranging from lasers to clocks to biological systems are often associated with the coefficient of linear friction, which relates the friction force to the velocity, becoming negative. The runaway of the vibration amplitude is prevented by positive nonlinear friction that increases rapidly with the amplitude. Here we use a modulated electromechanical resonator to show that nonlinear friction can be made negative and sufficiently strong to overcome positive linear friction at large vibration amplitudes. The experiment involves applying a drive that simultaneously excites two phonons of the studied mode and a phonon of a faster decaying high-frequency mode. We study generic features of the oscillator dynamics with negative nonlinear friction. Remarkably, self-sustained vibrations of the oscillator require activation in this case. When, in addition, a resonant force is applied, a branch of large-amplitude forced vibrations can emerge, isolated from the branch of the ordinary small-amplitude response.

Strategic human capital management in the context of cross‐industry and within‐industry mobility frictions

Research Summary: We develop and test a theory examining how frictions that restrict mobility across industries and frictions constraining mobility within an industry can co‐occur to effectively isolate individual human capital, ultimately changing the firm's make‐versus‐buy decision for human capital. Empirically, we demonstrate that when cross‐industry frictions in the form of limited skill transferability and within‐industry frictions in the form of noncompete enforceability are both present, employees exhibit longer tenures, firms hire workers with less initial experience, firms change the amount and nature of training provided, and wages marginally increase. These findings suggest that sufficiently strong and complementary mobility frictions shift the emphasis of firms’ human capital management practices toward internal development of human capital relative to acquisition on the external market.Managerial Summary: In the face of frictions to employee mobility both within and across industries, which we capture empirically using measures of noncompete enforceability and limited skill transferability across industries, firms tend to hire less experienced workers, such workers exhibit longer tenures, and firms invest more in their training, particularly in the development of new skills. Our findings imply that for firms operating under such complementary frictions, better hiring and internal development capabilities are particularly important for performance, while those firms without such capabilities may benefit from considering ways to circumvent the mobility frictions, including moving out of the focal state or lobbying for different noncompete laws.

Position Control for Magnetic Rodless Cylinders With Strong Static Friction

An adaptive extended state observer (AESO) based robust controller is proposed for precise position tracking control of a magnetic rodless cylinder with strong static friction. The AESO with dynamic gains is utilized to estimate the static friction and nonlinearities. Precise and robust control is achieved by the proposed controller that integrates the advantages of a sliding-mode controller and a linear active disturbance rejection controller. Experimental results show that both response rate and position accuracy are significantly improved by the proposed method in this paper.

Friction force microscopy of tribochemistry and interfacial ageing for the SiO x /Si/Au system.

Friction force microscopy was performed with oxidized or gold-coated silicon tips sliding on Au(111) or oxidized Si(100) surfaces in ultrahigh vacuum. We measured very low friction forces compared to adhesion forces and found a modulation of lateral forces reflecting the atomic structure of the surfaces. Holding the force-microscopy tip stationary for some time did not lead to an increase in static friction, i.e., no contact ageing was observed for these pairs of tip and surface. Passivating layers from tip or surface were removed in order to allow for contact ageing through the development of chemical bonds in the static contact. After removal of the passivating layers, tribochemical reactions resulted in strong friction forces and tip wear. Friction, wear, and the re-passivation by oxides are discussed based on results for the temporal development of friction forces, on images of the scanned area after friction force microscopy experiments, and on electron microscopy of the tips.

Adhesion and friction in hunting spiders: The effect of contact splitting on their attachment ability

Adhesive foot pads (claw tufts) of spiders consist of hair-like setae representing a two-leveled hierarchical structure, which provides strong attachment to a wide variety of surfaces. Several previous studies on adhesive foot pads concentrated on the function of the terminal contact elements (spatulae) for adhesion and friction, whereas the role of the higher hierarchical level (setae) remained largely unknown. In the present paper, we examined the influence of both setal width and the density of the setal array on attachment and on the ability to adapt to rough substrate surfaces in three hunting spider species that differ in the configuration of the adhesive foot pads: Zoropsis spinimana (Zoropsidae), Cupiennius salei (Ctenidae) and Anyphaena accentuata (Anyphaenidae). We found that spiders with denser and smaller setal tips gained higher vertical pull-off strength (adhesion per total contacting pad area) and traction force (friction) on smooth surfaces. Traction force experiments on surfaces with different roughness revealed a strong friction reduction on substrates with a mean asperity size of 1μm. This supports previous studies that demonstrated the existence of a critical roughness that is equivalent to or smaller than the dimension of spatulae. There was no significant difference in traction forces between smooth surfaces and surfaces with asperities comparable to the dimensions of setal tips (9 and 12μm). This suggests that the setal level is less decisive for the adaptability to the substrate geometry than the spatula level, especially at small length scales of surface roughness.

The Eddy Diffusivity in Barotropic β-Plane Turbulence

Geostrophic turbulent eddies play a crucial role in the oceans, mixing properties such as heat, salt, and geochemical tracers. A useful reduced model for geostrophic turbulence is barotropic (2D) turbulence. The focus of this study is on 2D β -plane turbulence with quadratic drag, which, although arguably the most realistic barotropic model of ocean turbulence, has remained unexplored thus far. We first review and test classical scaling arguments for the eddy diffusivity in three regimes: the strong friction limit, the weak friction/strong β limit, and a transition regime. We then develop a generalized theory by parameterizing the nonlinear eddy–eddy interactions as a stochastic process, which leads to an analytical solution for the eddy diffusivity spectrum, whose integral yields a “bulk” diffusivity. The theory successfully predicts the smooth transition of diffusivity across the three regimes, and echoes with the recent argument that eddy phase propagation relative to the mean flow suppresses the eddy diffusivity. Moreover, the generalized theory reduces to the classical scaling arguments in both the strong friction and strong β limits, which has not been clear from the previous work on diffusivity suppression by flow-relative phase propagation.