Plastic deformation friction mechanics

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. 

A traditional explanation of solid friction, which is mainly employed in engineering sciences, is based on plastic deformation.12 Typical surfaces are rough on microscopic length scales, as indicated in Figure 3. As a result, intimate mechanical contact between macroscopic solids occurs only at isolated points, typically at a small fraction of the apparent area of contact. 

Based on the discussion in earlier sections of this chapter, one may expect atomically flat incommensurate surfaces to be superlubric. Indeed the first suggestion that ultra-low friction may be possible was based on simulations of copper surfaces.6,7 Furthermore, the simulations of Ni(100)/(100) interfaces discussed in the previous section showed very low friction when the surfaces were atomically flat and misoriented (see the data for the atomically flat system between 30° and 60° in Figure 21). In general, however, it is reasonable to assume that bare metals are not good candidates for superlubric materials because they are vulnerable to a variety of energy dissipation mechanisms such as dislocation formation, plastic deformation, and wear. 

Impurities, such as grit, shreds of cotton, even in small quantities, sensitize an expl to frictional impact. That is why utmost cleanliness must be exercised in the preparation of expls. There are differences in the sensitivity of azides to mechanical and thermal influences. They have been correlated with the structure of the outer electronic orbits, the electrochemical potential, the ionization energy and the arrangement of atoms within the crystal. Functions of the polarizability of the cation are the plastic deformability of the crystals, and their surface properties. The nature of cation in an azide, such as Pb(Nj)2, has little effect on the energy released by the decomposition, which is vested in the N ion. The high heat of formation of the N2 molecule accounts... 

For all materials, the adhesive mechanism and the plastic deformation should be the main processes. During testing, the variation in friction coefficient values could be influenced by the specific AM behavior. Since AM corresponds to the crosslink density of a composite, for a qualitative assessment it can be concluded that the crosslink density decreases with increasing absorbed dose. PTFE500kGy-EPDM showed much lower AM and f90 values. It can be inferred that the state of cure is strongly dependent on the irradiation dose absorbed by the PTFE powder. 

Under boundary friction conditions, the exposed metal surface is extremely reactive due to mechanical activation. Exoemission occurs when a material surface is disturbed by plastic deformation, abrasion, fatigue cracking, and phase... 

Figure 1. Schematic representation of various possible friction mechanisms (a) Geometric interlocking of asperities with typical angle 0, (b) elastic deformation (stretched dashed bonds) to interlock atoms and/or macroscopic peaks, resulting in multiple metastable states, (c) defect pinning (circles), (d) pinning by an intervening layer of weakly bound material, (e) plastic deformation or plowing, and (f) material mixing or cold welding.

Nieminen et al. [152] observed a different mechanism of plastic deformation in essentially the same geometry, but at higher velocities (100 m/s versus 5 m/s) and with a different model for the potential between Cu atoms. Sliding took place between (100) layers inside the tip. This led to a reduction of the tip by two layers that was described as the climb of two successive edge dislocations under the action of the compressive load. Although wear covered more of the surface with material from the tip, the friction remained constant at constant normal load. The reason was that the portion of the surface where the tip advanced had a constant area. As in Sprensen et al. s work [63], dislocations nucleated at the comers of the contact and then propagated through it. 

One of the commonly described mechanisms for producing friction is plowing (Fig. le) [31]. In this case a hard tip is indented into a softer material and plows a permanent groove into the material as it slides. The work needed to produce this plastic deformation of the substrate has to be provided by the frictional force. This mechanism clearly occurs whenever a substrate is scratched during sliding. This naturally leads to rapid wear, which may be desirable in the context of machining. 

Chip Formation and Chip Flow The different chip formation zones in turning are shown in Fig. 4. The material is separated mechanically by shearing in the primary shear zone 1. In the secondary shear zone 2, the frictional forces on the rake face result in plastic deformation of the material from zone 3. In addition, in the separation zone, high compressive stresses occur and lead to a deformation and friction and shearing of the material in zone 4, whereby both elastic and plastic deformations occur. 

As a result of this process, a joint is produced in solid state. Because of various geometrical features of the tool, the material movement around the pin can be quite complex. During FSW process, the material undergoes intense plastic deformation at elevated temperature, resulting in generation of line and equiaxed recrystallized grains. The fine microstructure in friction-stir welds produces good mechanical properties of the joints, both static and dynamic. 

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