Slip-stick effect

Torsional vibrations are due to the stick-slip effect of the stabilizers in deviated boreholes. They can be seen at surface as large torque oscillations with a period of 3 to 10 s. Figure 4-308 shows a near-bit stabilizer in a deviated borehole. The stick-slip effect increases with WOB and RPM. 

Figure 4-308. Stick-slip effect due to near-bit fraction in a deviated borehole. (Courtesy Anadrill [113].,)...

During extrusion of polymer melts with high throughputs, the elastic melt properties can also lead to elastic instabilities which can result in surface distortions of the extrudate. One example are wavy distortions also described as sharkskin. Depending on the polymer, this can also lead to helical extrudate structures (stick-slip effect) or to very irregular extrudate structures (melt fracture) at even higher throughput rates [10]. 

It is of interest that counter intuitively wet wood, along with wet composite materials, shows higher friction compared to dry materials. Surface of wet deck boards is less slippery than that of dry deck boards. This phenomenon is a result of sticktion or stick-slip effect of wet surfaces, which in turn originates in a capillary nature of wood and extruded composite materials. 

Static coefficient of friction, 369, 370, 377 Polyethylene, 369 Pressure treated lumber, 369 Southern yellow pine, 369 Steady shear viscosity, 639 Steady shear, 640 Steady shearing conditions, 640 Steady-state viscosity, 640 Steam explosion, 77, 84, 96, 522 Corrosion of the equipment, 96 Stearic acid, 81 Stick-slip effect, 380, 658 Sticktion, 380... 

In practice it is essential that the manometer and mercury be thoroughly clean to eliminate. stick-slip effects in the movement of the mercury meniscus. A tipping manometer is frequently used to transfer mercury from the reservoir into the arms of the manometer after evacuation. During the tipping operation it is essential that the mercury not reach the stopcock, because this is always greased and the resulting contamination will spread rapidly through the manometer [27],... 

The above defects are due to different reasons. In the article (41), the stick-slip effect is related to self-excited oscillations initiated by the dependence of static friction-stress between the billet and the die at the time when they are in contact. The latter is connected with the viscoelastic properties of the rough billet surface and by lubricant squeezing out from the region of contact. Ward and co-workers (1) explain the stick-slip effect by the heating of billet dining the deformation. The pulsatory flow is assumed to be due to a competition between the viscosity and high elasticity of polymers (42). 

In accordance with different physical causes of interruption in the normal course of SSE (eg 41,42), criteria have been obtained and the boundaries of the input-variable range have been determined for the normal regime of the process. For example, for the stick-slip effect the lower bound of the velocity of plunger motion has been determined in (41). The upper bound of the velocity region of normal process has been derived in (42) based on the Weissenberg (Debora) criterion. 

In (41,44), different engineering approaches to stick-slip effect elimination have been proposed. The most effective are the application of a tractive force to the front end of the product being extruded. 

The shark skin effect can be avoided by using special additives such as fluoropolymers. These coat the machine and the die surfaces the wall slippage rate is increased, and thus the stick-slip effect is counteracted. 

The inclusion of the all-purpose rigid-body dynamic model in the design and control improves the mechanical system behaviour and performance fidelity. However, friction imposes additional nonlinearity in the dynamic equations of connected moving bodies. The discontinuous nature of all kinds of friction causes vibrations and stick-slip effects which limit the accuracy of end-effector position or path. 

McGuiggan, P. M. (2008) Stick slip contact mechanics between dissimilar materials Effect of charging and large friction. Langmuir, 24, 3970-3976. 

Differences in the frictional properties of most plastics can be explained in terms of the ratio of shear strenghth to hardness. Shooter and Tabor observed that the coefficients of friction for polytetrafluoroethylene are 2—3 times lower than anticipated by this calculation. It is believed that this discrepancy is caused by the inherently low cohesive forces between adjacent polymer chains and is responsible for the absence of stick-slip. The large fluorine atoms effectively screen the large carbon-fluorine dipole, reducing molecular cohesion so that the shear force at the interface is low. The shear strength of the bulk material is higher because of interlocking molecular chains. 

Fig. 6. A rough surface of die wall, where the stagnant (thin) chains allow the unbound chains (the middle thick chain without dot) to entangle and adsorb at the effective interface (dashed line). Adsorbed chains (thick chains with dots) are also present. Such an interface can also produce a stick-slip transition upon a coil-stretch transition involving the thick chains. See Ref. [27]. For clarity, we only draw three tethered chains (two adsorbed and one entangled) besides the stagnant chains in one valley. The chains not drawn here, of course, fill up all the space away from the rough wall...

Linear polyethylenes (PE) are one polymer that possess an important ingredient necessary for a display of interfacial stick-slip transition. In the past, the coincidence that PE is both the most widely used polymer and most prone to suffer from melt flow instabilities has challenged the PE industry. Today we still face the task of how to effectively remove instabilities that result in various types of extrudate distortions. 

The effects of moisture on epoxy fracture are not conclusive. Scott et al. reported that an amine cured epoxy, normally displaying stick-slip fracture at room temperature and low rates, exhibited stable behavior when immersed in distilled water. Also, they found that the rate necessary to promote the unstable to stable crack growth transition at room temperature was increased by two orders of magnitude in the presence of the water. Yamini and Young , on the other hand, found that testing in water tended to suppress stable behavior and promote stick-slip fracture in an amine cured epoxy over a wider range... 

Sol fraction 17, 25, 35, 39, 50 Soluble fractions 123,130 Spanning tree approximation 22 Stable fracture 134 Static light-scattering 7 Statistical links 126 Stick-slip fracture 133, 135 Strain hardening 145 Stress intensity factor 133 —strain measurement 42 Substitution effect 21, 28, 30... 

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]. 

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