Stick-slip oscillation

INTERMITTENT MOTION IN FRICTIONAL SLIDING STICK-SLIP OSCILLATION... 

Figure 14.13 shows the time domain waveform and spectrogram of a creaky door hinge. The door sound starts off with some relatively slow pops as initial sticking and slipping takes place (0.0-0.22 s), followed by a silent period, then more rapid pops (0.4-0.6 s) followed by an audio-rate oscillation that sweeps upward in frequency (0.6-0.68 s), settling in a relatively stable oscillation (0.7-1.2 s). A little silence is followed by more quasi-periodic creaky oscillation. This system and vibration is much more complex than the relatively simple oscillation of the bowed prayer bowl, but it is still produced by a stick/slip physical mechanism. The prayer bowl, the door, and many other stick/slip oscillation systems can be simulated using the relatively simple bow table model. 

There is another class of frictional interaction that we might call semi-deterministic. These actually behave like a hybrid between stick/slip oscillations and random peak/valley fiiction interactions. Most of these semi-deterministic interactions are caused by human-made objects, created through intentional engineering of the surface features of these objects. Essentially any surface that has a deterministic (especially periodic) structure has the potential to slide with a quasi-periodic motion. Some obvious examples of these come from our driving experience, like the ka-dunk, ka-dunk... sound produced by the seams between freeway concrete tiles, or the scored pavement patterns (or Botts dots) placed by the road edges to tell us we re straying from the allowed driving surface. 

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. 

A torque feedback system has been developed to dampen the surface torque oscillations and consequently the stick-slip motion at the bit. The system consists of (see Figure 4-309)... 

Fig. 1. Typical flow curve of commercial LPE. There are five characteristic flow regimes (i) Newtonian (ii) shear thinning (iii) sharkskin (iv) flow discontinuity or stick-slip transition in controlled stress, and oscillating flow in controlled rate (v) slip flow. There are three leading types of extrudate distortion (a) sharkskin like, (b) alternating bamboo like in the shaded region, and (c) spiral like on the slip branch. Industrial extrusion of polyethylenes is most concerned with flow instabilities occurring in regimes (iii) to (v) where the three kinds of extrudate distortion must be dealt with. The unit shows the approximate levels of stress where the sharkskin and flow discontinuity occur respectively. There is appreciable molecular weight and temperature dependence of the critical stress for the discontinuity. Other highly entangled melts such as 1,4 polybutadienes also exhibit most of the features illustrated herein...

Many polymers exhibit neither a measurable stick-slip transition nor flow oscillation. For example, commercial polystyrene (PS), polypropylene (PP), and low density polyethylene (LDPE) usually do not undergo a flow discontinuity transition nor oscillating flow. This does not mean that their extrudate would remain smooth. The often observed spiral-like extrudate distortion of PS, LDPE and PP, among other polymer melts, normally arises from a secondary (vortex) flow in the barrel due to a sharp die entry and is unrelated to interfacial slip. Section 11 discusses this type of extrudate distortion in some detail. Here we focus on the question of why polymers such as PS often do not exhibit interfacial flow instabilities and flow discontinuity. The answer is contained in the celebrated formula Eqs. (3) or (5). For a polymer to show an observable wall slip on a length scale of 1 mm requires a viscosity ratio q/q equal to 105 or larger. In other words, there should be a sufficient level of bulk chain entanglement at the critical stress for an interfacial breakdown (i.e., disentanglement transition between adsorbed and unbound chains). The above-mentioned commercial polymers do not meet this criterion. 

Having unraveled the specific characteristics of the stick-slip transition, it is rather straightforward to describe the physical origin of the oscillating flow ob-... 

In summary, the origin of the oscillating flow (sometimes termed slip-stick regime) observed in the constant piston speed mode is the oscillation of the HBC between the no-slip and slip states due to a reversible coil-stretch transition of either adsorbed chains or the first layer of unbound chains entrapped with the adsorbed chains. The experimental demonstration of an abrupt large stick-slip... 

We have surveyed the most recent progress and presented a new molecular level understanding of melt flow instabilities and wall slip. This article can at best be regarded as a partial review because it advocates the molecular pictures emerging from our own work over the past few years [27-29,57,62,69]. Several results from many previous and current workers have been discussed to help illustrate, formulate and verify our own viewpoints. In our opinion, the emerging explicit molecular mechanisms have for the first time provided a unified and satisfactory understanding of the two major classes of interfacial melt flow instability phenomena (a) sharkskin-like extrudate distortion and (b) stick-slip (flow discontinuity) transition and oscillating flow. 

Excessive noise in gears and transmissions is often related to frictional problems, especially friction-induced oscillations or stick-slip. Addition of molybdenum disulphide can sometimes reduce the friction problems. An example was... 

Other complex fluids, such as polymer melts, contain no solvent that can serve as a lubricant, and mechanisms for shp at or near a solid surface—and even the existence of wall slip-—are less obvious (Denn 1990). Suspicion that slip may be occurring is aroused by observations of jumps, or abrupt slope changes, in curves of shear stress versus shear rate, or by oscillations in stress or pressure at fixed apparent flow rate, suggesting stick-slip — that is, alternating periods of stick and slip (Benbow and Lamb 1963 Blyler and Hart 1970 Vinogradov et al. 1972 Kalika and Denn 1987 Lim and Schowalter 1989 Piau et al. 1990 Hatzikiriakos and Dealy 1992). But molecular theories of slip for complex fluids such as... 

Other frictional phenomena of interest in explosives sensitivity include the temperature rise during sliding, stick-slip (friction-induced oscillations), friction of rolling bodies, fretting (a severe form of wear), and internal friction [74-80]. Fracturing can also occur in sliding and grinding operations, and friction is interrelated with electrostatics. Those topics are discussed in Chapter 9, Volume 1. 

An important element in melt fracture is also wall slip phenomenon [5, 49]. It is related to the so-called sharkskin, or sharkskin melt fracture, which is also called surface melt fracture. It is a low amplitude surface distortion of extruded polymer. Sharkskin is generally observed in case of linear polymers with narrow MWD, below the oscillating stick-slip transition. Sometimes (but not always), there is a change... 

To casual inspection traces of stick-slip motion such as Fig. 8-8a seem to be composed of linear segments, but Eqns 8-21 and 8-28 show that in actuality they are functions of the cosine or the sine of the angular displacement. Quasiharmonic oscillation gives the distinctly different type of trace shown in Fig. 8-8b. Brockley and Ko [16] showed that a linear relation between kinetic friction and tractive velocity of the kind illustrated in Fig. 8-9a leads only to either stick-slip or smooth sliding for quasiharmonic oscillation a humped curve such as is seen in Fig. 8-9b is required. 

After the [Tst junction rapture, the pulled yam slips in the weave. Therefore, the pullout force and consequently the fabric displacements decrease to some extent. But it increases later due to new adhesion formation. This happens in the second step, called the stick-shp motion, and recurs until the free end of the pulled yam leaves the [Ist crossover of the weave. Thereafter the stick-slip motion adopts a reductive trend, creating the third step. Figure 7 presents the yam pullout force displacement pro (He typically. It shows an oscillation behavior in the second and the third steps, due to the mentioned shps and adhesions. 

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). 

Figure 17 shows RE and phE measurements on a faster time scale (0.01 s/channel). We find oscillations in both the RE and phE rates, which rise and fall simultaneously. The irregular pattern in Figure 17 corresponds to the patch wise failure of the adhesive in an oscillating or low-peel-speed stick-slip mode,(8 > an effect which is accentuated by the... 

Adhesion may be much concerned with friction and abrasion of rubber [65]. There is a transition developing the so-called Schallamach oscillation in friction [66]. It may be a transition of adhesion-cohesion or stick-slip [57,67]. Abrasion also indicates a similar velocity-dependent spectrum [27]. 

---