Stick-slip mechanism

The ubiquity of this power-law behaviour in SCG tests on PE has been the subject of considerable discussion, usually based on the assumption of a fibril creep failure mechanism [43, 45, 46, 47, 76, 79]. At high and intermediate K, after a certain induction period, steady-state crack advance is generally observed to occur by a stick-slip mechanism all or part of the fibrillar zone breaks down rapidly after an incubation time during which fibril creep takes place. The crack-tip then advances rapidly over a short distance and a new fibrillar zone stabilises, as sketched in Fig. 12. 

A key factor in the success of the adaptive mechanism is the ability to recreate a tmiform division of air space every time the coat s functionality switches from waterproof to high insulation. The mechanism that enables this is found on the surface of the barbules. Dawson et al. (1999) noticed that tiny hairs, known as cilia, covered the barbules that function as a stick-slip mechanism to keep the barbules entangled and maintains the movement in directions relative to one another to ensure uniformity in creation of air pockets during the coat s function change. 

Li Wen-ping (2000) Stick-Slip Mechanism of Repeated Rupture of Shaft Linings in Yanzhou-Tengzhou-Huaibei-Huainan Mining Area. Journal of China University of Mining Technology 29(5) 459-462. 

A number of other mechanisms [53-65] have been suggested for melt fracture. Based on a stick-slip mechanism, it is purported [53] that, above a critical shear stress, die polymer experiences intermittent slipping due to a lack of adhesion between itself and die wall, in order to relieve the excessive deformation energy adsorbed during the flow. The stick-slip mechanism has attracted a lot of attention [53-63], both theoretically and experimentally. The other school of drought [64,65] is based on thermodynamic argument, according to which, melt fracture can initiate anywhere in the flow field when reduction in the fluid entropy due to molecular orientation reaches a critical value beyond which the second law of thermodynamics is violated and flow instability is induced [64]. 

In a typical force-displacement plot (Fig. 4) describing the pull-out process, the first peak is attributed to debonding and frictional resistance to slipping, whereas subsequent lower peaks are attributed to friction and stick-slip mechanism, giving rise to a serrated portion of the curve [15,37,38,41]. Because of relaxation at... 

Yoshizawa FI and Israelaohvili J N 1993 Fundamental mechanisms of interfacial friction. 2. Stick-slip friction of spherical and chain molecules J. Phys. Chem. 97 11 300-13... 

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],... 

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... 

From the point of view of system d5mamics, the transition from rest to sliding observed in static friction originates from the same mechanism as the stick-slip transition in kinetic friction, which is schematically shown in Fig. 31. The surfaces at rest are in stable equilibrium where interfacial atoms sit in energy minima. As lateral force on one of the surfaces increases (loading), the system experiences a similar process as to what happens in the stick phase that the surface... 

The mechanisms of static friction and stick-slip motion, as discussed in the last section, are supposed to be a good description of dry friction. Another case, perhaps more general in engineering practices, to be addressed in this section is lubricated sliding where liquid lubricant, consisting of a few molecule layers, is confined between two solid walls. Both experimental and theoretical studies indicate, as we have discussed in Chapter 5, that there are substantial changes in rheology of the confined lubricant, and the liquid may transit practically to a solid-like state when film thickness becomes molecularly thin [32,33]. 

In static friction, the change of state from rest to motion is caused by the same mechanism as the stick-slip transition. The creation of static friction is in fact a matter of choice of system state for a more stable and favorable energy condition, and thus does not have to be interpreted in terms of plastic deformation and shear of materials at adhesive junctions. 

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

In impure metals, dislocation motion ocures in a stick-slip mode. Between impurities (or other point defects) slip occurs, that is, fast motion limited only by viscous drag. At impurities, which are usually bound internally and to the surrounding matrix by covalent bonds, dislocations get stuck. At low temperatures, they can only become freed by a quantum mechanical tunneling process driven by stress. Thus this part of the process is mechanically, not thermally, driven. The description of the tunneling rate has the form of Equation (4.3). Overall, the motion has two parts the viscous part and the tunneling part. 

The changes in crack propagation types (from stable to stick-slip) are associated with the crack blunting mechanism, which is favored by high temperatures and low strain rates, conditions that decrease general trends cannot be extended to very high strain rates because a transition from isothermal to adiabatic conditions may... 

It was thought in the past that the only mechanism for wall slip would be polymer desorption, i.e., an adhesive breakdown [25, 53]. However, lack of a strong temperature dependence would be inconsistent with an activation process of chain desorption. Since the onset of the flow discontinuity (i.e., stick-slip) transition was found to occur at about the same stress over a range of experimental temperatures, it was concluded from the outset [9] that the phenomena could not possibly have an interfacial origin. Thus, the idea of regarding the flow discontinuity as interfacial did not receive sufficient and convincing theoretical and experimental support in the past, not only because the transition was often accompanied by severe extrudate distortion and hysteresis, but also because the molecular mechanism for such an interfacial transition involving wall slip was elusive. 

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. 

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... 

To explore the behaviour of ion-implanted polymers microfriction studies were conducted on 1 MeV Ar" " implanted PEEK, and PS implanted to fluences of 5, 10 and 50 x 10 ions m (Rao et al, 1995). The results were compared with macrofriction values obtained using standard pin-on-disc-type tests. The polymers were also characterized for surface mechanical properties using the nanoindentation technique. The most striking aspect of the microfriction tests on the ion-implanted polymers is a marked stick-slip behaviour, which was not observed for the non-irradiated polymers. 

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