Stick-slip frictional behavior

Foam rheology has been a challenging area of research of interest for the yield behavior and stick-slip flow behavior (see the review by Kraynik [229]). Recent studies by Durian and co-workers combine simulations [230] and a dynamic light scattering technique suited to turbid systems [231], diffusing wave spectroscopy (DWS), to characterize coarsening and shear-induced rearrangements in foams. The dynamics follow stick-slip behavior similar to that found in earthquake faults and friction (see Section XU-2D). 

In technical applications, stick-slip friction is detrimental in terms of wear, vibrations, and precision of movement. The main factors determining stick-slip behavior are as follows ... 

It has been recognized that the behavior of atomic friction, such as stick-slip, creep, and velocity dependence, can be understood in terms of the energy structure of multistable states and noise activated motion. Noises like thermal activities may cause the atom to jump even before AUq becomes zero, but the time when the atom is activated depends on sliding velocity in such a way that for a given energy barrier, AI/q the probability of activation increases with decreasing velocity. It has been demonstrated [14] that the mechanism of noise activation leads to "the velocity... 

The frictional properties of PTFE arc unique Its unusually low static coefficient of friction decreases with increasing load and is lower than the dynamic coefficient of friction. This precludes stick-slip behavior. The low surface energy also prevents wetting by liquids other than low-surface-tension fluids like fluorocarbons. 

Figures 17b and 17c show the response in the lateral and normal directions to a lateral constant velocity drive for the stick slip regime that occurs at low driving velocities. This behavior is similar for the presently discussed model. The separation between the plates, which is initially Zq at equilibrium, starts growing before slippage occurs and stabilizes at a larger interplate distance as long as the motion continues. Since the static friction is determined by the amplitude of the potential corrugation exp(l — Z/A), it is obvious that the dilatancy leads to a decrease of the static friction compared to the case of a constant distance between plates.

Detailed direct experimental evidence for the frictional behavior during the slip part of a stick-slip cycle is found in the work reported by Ko and Brockley [17]. The rider and its elastic coupling were fitted with the following transducer devices (a) a strain gage to measure the displacement of the rider from its null position (b) an electromagnetic transducer to measure the velocity of the rider during vibration or transport and (c) an accelerometer to measure the acceleration of the rider. The data obtained with this instrumentation were used in the equation for stick-slip (c(S. Eqn 8-18) ... 

Consider the behavior illustrated in Fig. 9-4, which is typical of the slow-speed sliding of many metals in ambient air. When the lubricant is a highly refined mineral oil, the friction trace indicates stick-slip sliding. Addition of a small amount of long-chain fatty acid (e.g. lauric acid) to the lubricant almost immediately results in a change to smooth sliding with a coefficient of friction less than 0.1. Since... 

In the chain-parallel direction, only the HOPE showed a periodic stick slip behavior with a repeat distance of ca. 2.5 A. This distance is equal to the repeat unit of polyethylene (16, 20). For PTFE the LFM friction loops in our experiments did not reveal any stick slip behavior. Thus, in this case, we cannot determine the value of do in the chain-parallel direction. In this case we can, however, assume that the value of d is close to 0. Based on equation 1, the friction anisotropy is therefore expected to be larger for PTFE, than that for HDPE. For a semi-quantitative comparison of friction forces predicted by equation (1) on one hand and experimental results obtained on highly oriented polymer surfaces on the other hand, one should be... 

Phenomenologically, the behavior of polycarbonate can be represented by a mechanical model containing a friction or stick-slip element to represent yielding. Such a model was first introduced by Bingham (See Bingham, (1922) and Reiner, (1971)) to explain the behavior of certain fluids such as paint and later adapted to explain yielding in various materials including polymers with various modifications as shown in Fig, 11.15. 

Figure 1.10 Force vs. time data for a ball dragged through a pile of rods of aspect ratio 12 (a), 20 (h), and 40 (c), all in a 5 cm tube. The low aspect ratio particles show the stick-slip behavior common in ordinary granular materials. The large aspect ratio particles, however, act as a single solid body, with small fluctuations characteristic of dry friction. Intermediate aspect ratios show both behaviors in a single experiment on both large and small (inset) time scales. Continued)...

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