Stick-slip phenomenon

Leonov A. I., Rheol. Acta, "A linear model of the stick slip phenomena in... 

In the SFA experiments there is no way to determine whether shear occurs primarily within the film or is localized at the interface. The assumption, made by experimentalists, of a no-slip flow boundary condition is invalid when shear localizes at the interface. It has also not been possible to examine structural changes in shearing films directly. MD simulations offer a way to study these properties. Simulations allow one to study viscosity profiles of fluids across the slab [21], local effective viscosity inside the solid-fluid interface and in the middle part of the film [28], and actual viscosity of confined fluids [29]. Manias et al. [28] found that nearly all the shear thinning takes place inside the adsorbed layer, whereas the response of the whole film is the weighted average of the viscosity in the middle and inside the interface. Furthermore, MD simulations also allow one to examine the structures of thin films during a shear process, resulting in an atomic-scale explanation [12] of the stick-slip phenomena observed in SFA experiments of boundary lubrication [7]. 

The main objective of this paper is to present a detailed view of various aspects of adhesion and deformation friction based mainly upon the research findings of this laboratory. An attempt is also made to analyse the large number of results reported elsewhere in order to develop a coherent presentation. Certain fundamental aspects of friction are discussed at first from a phenomenological point of view. A discussion of the law of friction relating to motion follows with a study of the stick-slip phenomena. The next part deals with the isothermal adhesive friction of elastomers on nominally flat and smooth surfaces. 

Control Valve Dynamics. Control valve dynamics tend to be relatively fast compared to the dynamics of the process itself. However, the overall behavior of pneumatic control valves can include nonlinear phenomena such as dead band, stick-slip phenomena, backlash and hysteresis (Blevins et al., 2003 Edgar et al., 2008). Dead band and hysteresis are illustrated in Fig. 9.9. Fortunately, their effects can be reduced significantly by employing valve positioners. 

The local friction image (Figure 4.a), shows the initial situation turns to high friction with some stick-slip phenomena, inducing contact vibration, due to the low longitudinal stiffness of the test machine. The low fnction effect of the tribofilm is found to last about 25 cycles. 

Feeny B, Guran A, Hinrichs N, Popp K (1998) Historical review on dry friction and stick-slip phenomena. Appl Mech Rev 51(5) 321-341... 

Slow moving slides and tables in machine tools are subject to jerky motion due to alternating slipping and sticking of the sliding surfaces. Consequently, specially developed lubricants are required to prevent and/or reduce the stick-slip phenomenon. Slideway oils usually contain polar surface-active compounds, mostly fatty acid derivatives, together with anti-wear additives and oxidation inhibitors. The polar surface-active agents form oriented boundary layers which prevent adhesive friction. 

The fault thickness depends on the ratio of solid friction resistance (to slippage) and the strength of the intact rock material. At the transition to the middle crust they begin to have the same order, the breaking of faults edges takes place. The stick-slip phenomenon. Brace (1972), is an indicator of competition of fault asperity crushing and slippage. This happens in the PT zone (III) where the faults are widened due to dilatancy. Then low velocity zones (LVZ) as porous/fractured layers appear. 

Sliding of the Steel Sphere on Flat PET Surfaces. Figure 2 shows typical friction traces in the sliding of a steel sphere at a speed 0,25 mm/s under a load 8 N. It is seen that the static friction is considerably higher than the kinetic friction and there is no stick-slip phenomenon. Comparing the friction of PET with that of other polymers obtained in the sliding experiment ( ) similar to that in the present work, it was found that PET exhibited relatively lower... 

An example for self-excited vibrations is the stick-slip phenomenon. In machining applications, stick-slip typically arises at the glides. The mechanical model can be seen in Fig. 12. The block of mass m is fixed to the moving wall by a spring of stiffness k and a dashpot of damping c. The wall is moving with velocity vq-The friction force acting on the block is... 

Dynamics, Fig. 12 Mechanical model for the stick-slip phenomenon... 

The AFM probing of polymeric surfaces can, besides imaging the surface, also produce a number of anomalies. Surface contaminants, such as those caused by adsorbed polar molecules, were found to cause significant perturbations on the images produced. From the calculations, it appears that when the AFM tip encounters a polar defect, it is initially attracted (phenomenon) and becomes trapped for a short period of time. This type of stick-slip phenomenon leads to an enhanced frictional energy dissipation which, in turn, causes an increase in the surface temperature of both the AFM tip and the polymer surface. The increase in temperature can subsequently induce rotational defects in a polymer chain and ultimately cause deformations on a long time-scale. 

Fig. 13 shows the dynamic friction coefficient during the reciprocating operation for each PFPE. The amplitude of the saw tooth pattern in the friction curve is significantly high for the Z-DOL tapes these fluctuations in sliding resulted from the stick-slip process and are associated with squeal and chatter. However, the dynamic friction coefficient was relatively constant and the stick-slip phenomenon is only slightly observed for the ammonium salt tapes (Lubricant 11). 

Stick-Slip Phenomenon and the Schallamach Waves for Elastomers... 

Imposition of no-slip velocity conditions at solid walls is based on the assumption that the shear stress at these surfaces always remains below a critical value to allow a complete welting of the wall by the fluid. This iraplie.s that the fluid is constantly sticking to the wall and is moving with a velocity exactly equal to the wall velocity. It is well known that in polymer flow processes the shear stress at the domain walls frequently surpasses the critical threshold and fluid slippage at the solid surfaces occurs. Wall-slip phenomenon is described by Navier s slip condition, which is a relationship between the tangential component of the momentum flux at the wall and the local slip velocity (Sillrman and Scriven, 1980). In a two-dimensional domain this relationship is expressed as... 

Stick-slip motion is another issue that has been explored using SFA. It is found that the occurrence of stick-slip depends on the sliding velocity and the stiffness of the system, and the mechanism of the phenomenon can be interpreted in terms of periodic transition between liquid and solid states of the conhned lubricant [40],... 

At the beginning of sliding, the system is accelerated because the driven force must excess the resistance from lubricating film. For this reason, the system actually jumps from A to the point B, instead of B, to gain a shear stress lower than the critical value This phenomenon, so called velocity-weakening has been regarded widely in the literatures as the cause for instability and stick-slip motion in lubricated systems. 

Virtually in all dry sliding contacts we observe that the frictional force required to initiate motion is more than that needed to maintain the surfaces in the subsequent relative sliding thus there are two values reported for the coefficient of friction. The static coefficient of friction is used in reference to the initial movement of the object from the rest position. In this case, the F ml. The kinetic coefficient of friction is used for two surfaces in relative motion. This feature, together with the inevitable natural elasticity of any mechanical system, can often lead to the troublesome phenomenon of stick-slip motion (the displacement of surface materials with time). Displacement increases linearly with time during periods of sticking when slipping occurs, the deformed surface materials are released. Representative of dry static and kinetic coefficients of friction for various material pairs are found in tribology and physics references 11 see Table 3.1. 

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