Slipping mechanism

The. Homogeneous Equilibrium Model (HEM) assumes uniform mixing of the phases across the. pipe diameter, no phase slip (mechanical equilibrium), thermal equilibrium between, the..phases and complete vapour/ liquid, equilibrium. "Homogenous" in the context of the HEM refers to the flow in the vent line. 

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. 

Evidence for wall slip was suggested over thirty years ago [9,32,63]. One of the first attempts at a slip mechanism was the performance of a Mooney analysis by Blyler and Hart [32]. Working in the condition of constant pressure, they explicitly pointed out melt slip at or near the wall of the capillary as the cause of flow discontinuity. On the other hand, they continued to insist that bulk elastic properties of the polymer melt are responsible for the flow breakdown on the basis that the critical stress for the flow discontinuity transition was found to be quite insensitive to molecular weight. Lack of an explicit interfacial mechanism for slip prevented Blyler and Hart from generating a satisfactory explanation for the flow oscillation observed under a constant piston speed. 

Extrudate distortion has been viewed as explicit evidence for melt flow instabilities or melt fracture. This is another calamitously misleading assertion in the massive literature of over 3000 papers on the subject. Because extrudate distortion was also observed even without any signature of slip, it was concluded by Blyler and Hart [32] that the slip process is not an essential part of the flow instability. This dilemma, that the flow anomalies including flow oscillation cannot be accounted for in terms of either a constitutive instability or interfacial slip mechanism, has persisted until very recently. Denn coined this plight the paradox [10b]. 

Figure 10.14. The single-slip mechanism, in which crack growth occurs in the direction of the primary slip systems, results in a zig-zag crack path.

In brittle materials, like glass, calculating the theoretical strength on the basis of breaking the bonds separating two layers in a material and forming two new surfaces makes sense in teams of the types of specific bonds in these systems. The metallic bond is non-specific, however, and failure of a theoretically perfect structure involves slip mechanisms along certain planes in the crys-... 

Regioselectivity is quite predictable, and consistent in a simple way with typical electrophilic activation of an alkene (Markovnikov s rule). Just as in bromination of an nnsymmetrical alkene, initial coordination of an electrophile (M+, Br+) activates the alkene toward nncleophilic addition of a nncleophile, the addition is preferred at the end of the alkene that best stabilizes a cation. Electronic effects dominate over steric effects. An molecnlar orbital (MO) analysis has been pnt forward ( the slip mechanism ) to rationalize the activating effect of the metal and, in a secondary way, the regioselectivity. It focnses on the reactants and prodncts, and notes that the metal moves dnring the reaction from the approximate midpoint of the alkene to one end. As that slip occurs toward one end of the alkene, the lowest unoccupied molecular orbital (LUMO) for the complex changes and a large coefficient develops at the other end. 

McLaren, A. C., Etheridge, M. A. (1976). A transmission electron microscope study of naturally deformed orthopyroxene. I. Slip mechanisms. Contrib. Mineral. Petrol., 57, 163-77. 

Preliminary Dislocation Dynamics (DD) simulations using the model developed by Verdier et al. provide a plausible scenario for the dislocation patterning occuring during the deformation of ice single crystals based on cross-slip mechanism. The simulated dislocation multiplication mechanism is consistent with the scale invariant pattemings observed experimentally. 

Figure 5 represents a typical evolution of the dislocation pattern during the deformation. The simulation was performed in a 20 mm diameter crystal, with 2 initial basal planes activated (one system in each plane) at the beginning of the deformation. It clearly appears that the double cross-slip mechanism propagates the plasticity in many other basal planes. One can also notice the asymmetry in the plane expansion due to the dislocation interactions. 

The double cross-slip mechanism can then be considered as the most probable deformation process, complementary to the basal slip. Indeed, dislocation climb can hardly be invoked in this torsion loading conditions since most of the dislocations are of screw type. 

Usually, creep deformation of ice single crystals is associated to a steady-state creep regime, with a stress exponent equal to 2 when basal glide is activated . In the torsion experiments performed, the steady-state creep was not reached, but one would expect it to be achieved for larger strain when the immobilisation of the basal dislocations in the pile-ups is balanced by the dislocation multiplication induced by the double cross-slip mechanism. 

Silicon will serve as the paradigmatic example of slip in covalent materials. Recall that Si adopts the diamond cubic crystal structure, and like in the case of fee materials, the relevant slip system in Si is associated with 111 planes and 110> slip directions. However, because of the fact that the diamond cubic structure is an fee lattice with a basis (or it may be thought of as two interpenetrating fee lattices), the geometric character of such slip is more complex just as we found that, in the case of intermetallics, the presence of more than one atom per unit cell enriches the sequence of possible slip mechanisms. 

Fig. 8.37. Schematic of the Friedel-Escaig cross slip mechanism (adapted from Bonneville et al. (1988)).

The author expresses a personal opinion that the slipping mechanism, first conceived by Houwink44, taken up by Dannenberg19 and given a mathematical treatment as a saltation process by Rigbi77, is the only theory actually dealing with the problem. 

The dynamic mechanical analysis of virgin PP, untreated sisal/PP composites and MAPP treated sisal PP composites revealed an increase in the storage modulus (E ) in the PP matrix with the addition of fibers and MAPP (Figure 9.5a) [71]. The loss modulus displayed three relaxation peaks at -80°C (y), 8°C (P), and 100°C (a), respectively. The temperature of P relaxation maximum corresponds to the Tg of the matrix, while the a relaxation peak is related to the slip mechanism in the crystallites. The y relaxation peak is due to the... 

Reed-Hill (1964) had shown previously that the two conditions of Cahn were not necessary to obtain a twin intersection. He had shown that slip mechanisms could allow the displacement to be the same on each side of the preexisting twin layer even if the shear vector was not the same. The mechanism in this case is quite different from the one that we observed in the B structure. Reed-Hill took into account the fact (which is obvious) that the displacement must be the same on each side of the preexisting twin layer but did not suggest any condition for epitaxial continuity. 

Although polymer crystal structures are known, and some slip mechanisms (slip plane and slip direction) determined, these are less important than for metals. Firstly, the amorphous phase plays an important part in the mechanical properties. Secondly, polymer yield strengths are not determined by obstacles to dislocation movement. However, it is possible to fabricate highly anisotropic forms of semi-crystalline polymers, so crystal characterization and orientation are important. 

Carkner CJ, Mosey NJ (2010) Slip mechanisms of hydroxylated a-Al203 (0001)/(0001) interfaces a first-principles molecular dynamics study. J Phys Chem C 114 17709-17719... 

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. 

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