Sliding deformation

Yeoh, S.L., Papadakis, G., and Yianneskis, M. (2004) Numerical simulation of turbulent flow characteristics in a stirred vessel using the les and rans approaches with the sliding/deforming mesh methodology. Chem. Eng. Res. Des., 82 (A7), 834-848. 

Phase angle describing mode-mixity of crack driving force describes the relative amounts of mode I (opening) and mode II (sliding) deformation at a crack tip. 

After the reservoir was impounded, the highest water level raised to 565 m (Figure 6). 66% portion of the landslide was submerged under the water when the water level changed from 494 m to 565 m. Distinctive sliding deformation evidence was observed during the initial impoundment experiment. 

When structures of the starting compound and the product of MA are identical, the interaction of components can occur via the plastic deformation mechanism without destruction of the initial matrix by means of a plastic strain, particularly, the slide deformation that detaches structural planes of the initial matrix favouring substitution and combination reactions to form a target product. This mechanism is characterized by the disruption of only the long-range atomic order of the starting compoimd, while the structural matrix and its local atomic order remain during the mechanical activation. 

Sliding deformation is assumed to occur whenever the acceleration of a sliding block exceeds the yield acceleration, which is the horizontal acceleration that results in a factor of safety of 1 in a pseudo-static slope stability analysis, and the sliding stops when the acceleration falls below the yield acceleration. Mathematically, the Newmark sliding block displacements are computed by double integration of acceleration time history on the sliding block for the portion of accelerations that exceeds the yield acceleration (Fig. 7). 

The fracture behaviour and mechanical properties of the composites are generally influenced by the interfacial structure or the fibre/matrix interactions. The nonlinear behaviour in the L-D curve and the pull-out of the fibre from the matrix without plastic deformation have demonstrated experimentally that the weak bonding between the fibres and by the van der Waals force allows the shear sliding deformation at tiie interface. The weak interaction between the... 

A number of friction studies have been carried out on organic polymers in recent years. Coefficients of friction are for the most part in the normal range, with values about as expected from Eq. XII-5. The detailed results show some serious complications, however. First, n is very dependent on load, as illustrated in Fig. XlI-5, for a copolymer of hexafluoroethylene and hexafluoropropylene [31], and evidently the area of contact is determined more by elastic than by plastic deformation. The difference between static and kinetic coefficients of friction was attributed to transfer of an oriented film of polymer to the steel rider during sliding and to low adhesion between this film and the polymer surface. Tetrafluoroethylene (Telfon) has a low coefficient of friction, around 0.1, and in a detailed study, this lower coefficient and other differences were attributed to the rather smooth molecular profile of the Teflon molecule [32]. 

Polymers will be elastic at temperatures that are above the glass-transition temperature and below the liquiflcation temperature. Elasticity is generally improved by the light cross linking of chains. This increases the liquiflcation temperature. It also keeps the material from being permanently deformed when stretched, which is due to chains sliding past one another. Computational techniques can be used to predict the glass-transition and liquiflcation temperatures as described below. 

A series of events can take place in response to the thermal stresses (/) plastic deformation of the ductile metal matrix (sHp, twinning, cavitation, grain boundary sliding, and/or migration) (2) cracking and failure of the brittle fiber (5) an adverse reaction at the interface and (4) failure of the fiber—matrix interface (17—20). 

A Hquid is a material that continues to deform as long as it is subjected to a tensile and/or shear stress. The latter is a force appHed tangentially to the material. In a Hquid, shear stress produces a sliding of one infinitesimal layer over another, resulting in a stack-of-cards type of flow (Fig. 1). 

A sliding plate rheometer (simple shear) can be used to study the response of polymeric Hquids to extension-like deformations involving larger strains and strain rates than can be employed in most uniaxial extensional measurements (56,200—204). The technique requires knowledge of both shear stress and the first normal stress difference, N- (7), but has considerable potential for characteri2ing extensional behavior under conditions closely related to those in industrial processes. 

The exponential term appears for the same reason as it does in diffusion it describes the rate at which molecules can slide past each other, permitting flow. The molecules have a lumpy shape (see Fig. 5.9) and the lumps key the molecules together. The activation energy, Q, is the energy it takes to push one lump of a molecule past that of a neighbouring molecule. If we compare the last equation with that defining the viscosity (for the tensile deformation of a viscous material)... 

Let US now look at how this contact geometry influences friction. If you attempt to slide one of the surfaces over the other, a shear stress fj/a appears at the asperities. The shear stress is greatest where the cross-sectional area of asperities is least, that is, at or very near the contact plane. Now, the intense plastic deformation in the regions of contact presses the asperity tips together so well that there is atom-to-atom contact across the junction. The junction, therefore, can withstand a shear stress as large as k approximately, where k is the shear-yield strength of the material (Chapter 11). 

In the lightly cross-linked polymers (e.g. the vulcanised rubbers) the main purpose of cross-linking is to prevent the material deforming indefinitely under load. The chains can no longer slide past each other, and flow, in the usual sense of the word, is not possible without rupture of covalent bonds. Between the crosslinks, however, the molecular segments remain flexible. Thus under appropriate conditions of temperature the polymer mass may be rubbery or it may be rigid. It may also be capable of ciystallisation in both the unstressed and the stressed state. 

When polymer melts are deformed, polymer molecules not only slide past each other, but they also tend to uncoil—or at least they are deformed from their random coiled-up configuration. On release of the deforming stresses these molecules tend to revert to random coiled-up forms. Since molecular entanglements cause the molecules to act in a co-operative manner some recovery of shape corresponding to the re-coiling occurs. In phenomenological terms we say that the melt shows elasticity. 

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