Friction shear slip

According to Coulomb s law of friction, the relationship among shear stress, normal stress, coefficient of friction and pore-pressure at the moment of shear slip (critical condition) due to increasing of pore-pressure are described as follow. 

We attribute the low friction of the lamellar liquid crystalline phases to the repulsion between the surfactant layers in the structure, culminating in a load-carrying capacity. This repulsion will naturally cause a low friction if the planes are sheared. We do, however, expect an increase in the friction if the surfaces are sheared at a faster rate, due to a disruption of the lamellar order, resulting in a higher friction. This slipping between the layers is the reason for the low friction. Hence, a surfactant system should be designed such that a lamellar liquid crystalline layer is formed at the interface between the sliding layers in order to obtain a low friction. 

Frictional shear bond occurs when the bond strength is exceeded and certain relative displacement (slip) occurs. 

Foam rheology has been a challenging area of research of interest for the yield behavior and stick-slip flow behavior (see the review by Kraynik [229]). Recent studies by Durian and co-workers combine simulations [230] and a dynamic light scattering technique suited to turbid systems [231], diffusing wave spectroscopy (DWS), to characterize coarsening and shear-induced rearrangements in foams. The dynamics follow stick-slip behavior similar to that found in earthquake faults and friction (see Section XU-2D). 

For a monolayer film, the stress-strain curve from Eqs. (103) and (106) is plotted in Fig. 15. For small shear strains (or stress) the stress-strain curve is linear (Hookean limit). At larger strains the stress-strain curve is increasingly nonlinear, eventually reaching a maximum stress at the yield point defined by = dT Id oLx x) = 0 or equivalently by c (q x4) = 0- The stress = where is the (experimentally accessible) static friction force [138]. By plotting T /Tlx versus o-x/o x shear-stress curves for various loads T x can be mapped onto a universal master curve irrespective of the number of strata [148]. Thus, for stresses (or strains) lower than those at the yield point the substrate sticks to the confined film while it can slip across the surface of the film otherwise so that the yield point separates the sticking from the slipping regime. By comparison with Eq. (106) it is also clear that at the yield point oo. 

The solidihed layer yields and returns to the liquid phase if the shear stress excesses the critical value, which initiates the sliding. When the stress is relaxed as a result of slip, the solid phase resumes again. The periodic transition between the solid and liquid states has been interpreted in the literature as a major cause of the stick-slip motion in lubricated sliding. Understanding the stick-slip and static friction in terms of solid-liquid transitions in thin films makes a re-... 

Fig. 19—Shear stress and chain angle as a function of sliding distance, from simulations of alkanethiolates on Au(111) at temperature 1 K (a) results from commensurate sliding show a stick-slip motion with a period of 2.5 A, (b) in incommensurate case both shear stress and chain angle exhibit random fluctuations with a much smaller average friction [45],...

In both models, the magnitude of friction can be determined in terms of the shear strength of boundary films, i.e., the critical shear stress when slip occurs, regardless of how the films are formed, by adsorption, reaction, or solidification. The existence of a critical shear stress, independent of ap-... 

The model proposed by Bowden and Tabor has been regarded as the most successful one for presenting a simple and logical theory capable of explaining the Amontons friction law. However, suspicions concerning the two fundamental assumptions in the model were gradually aroused over past years. Friction has been attributed, in Bowden and Tabor s model, to the adhesion between asperities in contact and torn-off of the adhesive junctions when the shear stress exceeds a critical value. This implies that plastic flow and surface destruction may occur at the moment of slip, and that friction is dominated by the shear strength of the adhesive conjunctions, which is material dependent. 

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. 

During the friction and wear tests of PTFE Him, two zones can be classified according to the load. One is the load below 70 nN, the friction force which was created in friction and wear tests is too small to make the PTFE film to shear. Within this zone the friction force increases linearly with the load, and there are no transfer of atoms and no worn marks. The second zone is when the load is above 70 nN, and the friction force created in the friction and wear tests is large enough to force the PTFE molecular atom to slip. So there were obvious worn mark and projection in the film, and the friction force stayed almost constant with load. 

In their study, Park et al.100 investigated the frictional properties of fluorine-terminated alkanethiol SAMs grafted to gold surfaces. The frictional properties of the system were investigated by sliding two SAMs past one another at velocities in the stick-slip regime under various external loads. The simulations yield the shear stress as and the kinetic friction coefficient pk can be estimated from the slope of a plot of as versus load, using the relationships contained in Eqs. [4] and [5]. 

Broadly speaking, the mechanical properties can be divided into two classes bulk and interfacial . Within the bulk properties are included the shear and extensional viscosities, moduli and yield stresses (material constants that relate stress to strain or strain rate), and within interfacial rheology are included the wall-slip and friction effects. The interfacial properties are independent of bulk mechanical properties and governed by the frictional or surface forces which are thought to operate at relatively... 

Differences in the frictional properties of most plastics can be explained in terms of the ratio of shear strenghth to hardness. Shooter and Tabor observed that the coefficients of friction for polytetrafluoroethylene are 2—3 times lower than anticipated by this calculation. It is believed that this discrepancy is caused by the inherently low cohesive forces between adjacent polymer chains and is responsible for the absence of stick-slip. The large fluorine atoms effectively screen the large carbon-fluorine dipole, reducing molecular cohesion so that the shear force at the interface is low. The shear strength of the bulk material is higher because of interlocking molecular chains. 

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