Interfacial shear

Carpick R W, Agrait N, Ogletree D F and Salmeron M 1996 Measurement of interfacial shear (friction) with an ultrahigh vacuum atomic force microscope J. Vac. Sc/. Technol. B 14 1289... 

Wear. Ceramics generally exhibit excellent wear properties. Wear is deterrnined by a ceramic s friction and adhesion behavior, and occurs by two mechanisms adhesive wear and abrasive wear (43). Adhesive wear occurs when interfacial adhesion produces a localized Kj when the body on one side of the interface is moved relative to the other. If the strength of either of the materials is lower than the interfacial shear strength, fracture occurs. Lubricants (see Lubricants and lubrication) minimize adhesion between adj acent surfaces by providing an interlayer that shears easily. Abrasive wear occurs when one material is softer than the other. Particles originating in the harder material are introduced into the interface between the two materials and plow into and remove material from the softer material (52). Hard particles from extrinsic sources can also cause abrasive wear, and wear may occur in both of the materials depending on the hardness of the particle. 

Compressive interfacial stresses increase the interfacial shear resistance. Although usually detrimental to toughening, these stresses can enhance toughening if bridge pullout is the operative toughening process. 

Fig. 2. Results of interfacial shear strength measurements of the same fiber/matrix systems using four different micro-mechanical tests during a round-robin program involving 12 different laboratories, (a) Results for untreated, unsized carbon fibers, (b) Results for carbon fibers with the standard level of surface treatment. Redrawn from ref. [13].

Assuming the work of adhesion to be measurable, one must next ask if it can be related to practical adhesion. If so, it may be a useful predictor of adhesion. The prospect at first looks bleak. The perfect disjoining of phases contemplated by Eq. 1 almost never occurs, and it takes no account of the existence of an interphase , as discussed earlier. Nonetheless, modeling the complex real interphase as a true mathematical interface has led to quantitative relationships between mechanical quantities and the work of adhesion. For example, Cox [22] suggested a linear relationship between Wa and the interfacial shear strength, r, in a fiber-matrix composite as follows ... 

The interfacial shear strength for carbon/nylon may be taken as 4 MN/m Solution... 

Example 3.18 Calculate the maximum and average fibre stresses for glass fibres which have a diameter of 15 /xm and a length of 2.5 mm. The interfacial shear strength is 4 MN/m and i,/ = 0.3. 

Bonded-bolted joints generally have better performance than either bonded or bolted joints. The bonding results in reduction of the usual tendency of a bolted joint to shear out. The bolting decreases the likelihood of a bonded joint debonding in an interfacial shear mode. The usual mode of failure for a bonded-bolted joint is either a tension failure through a section including a fastener or an interlaminar shear failure in the composite material or a combination of both. 

Sf = tensile stress at the fiber, and T = fiber-matrix interfacial shear strength. 

The analysis depends on whether the interfacial failure occurs by yielding or by crack propagation. The simplest analysis is based on interfacial yielding where the shear stress is assumed to be distributed uniformly over the interface from top to bottom. According to this analysis, the interfacial shear stress increases uniformly until every location in the interface gives way simultaneously. 

For this, failure interfacial shear strength (t) is obtained by dividing the maximum load P, by interfacial area A. 

The Bowyer and Bader [96] methodology can be used to predict stress-strain response of short fiber-rein-forced plastics. The stress on the composite (cT( ) at a given strain can be computed by fitting the response to a form of Eq. (4) with two parameters, the fiber orientation factor (Cfl) and interfacial shear strength (t,). 

The purpose of these experiments was to characterize different flow details under conditions when the superficial gas velocity is constant and the superficial liquid velocity increases. The upward flow regimes are presented in Fig. 5.33. Figure 5.33a shows the stratified flow pattern at [/gs = 20 m/s and J/ls = 0.005 m/s. In the region of pure stratified flow the liquid layer is drawn upward by the gas via the interfacial shear stress. No droplets could be observed at the interface. Such a regime was also observed by Taitel and Dukler (1976), and Spedding et al. (1998). 

Melcher, JR Taylor, GI, Electrohydrodynamics A Review of the Role of Interfacial Shear Stresses, Annual Review of Fluid Mechanics 1, 111, 1969. 

A study on a commonly used demulsifier, namely, a phenol-formaldehyde resin, elucidated how various parameters such as interfacial tension, interfacial shear viscosity, dynamic interfacial-tension gradient, dilatational elasticity, and demulsifier clustering affect the demulsification effectiveness [1275]. 

Dukler and Taitel, 1991b). The simplest configuration of annular flow is a vertical falling film with concurrent downward flow. Given information on interfacial shear and considering the film to be smooth, the film thickness and heat transfer coefficient between the wall and the liquid film can be predicted from the following basic equations (Dukler, 1960) ... 

Due to lack of understanding of the wave structure and motions, modeling of the interfacial shear remains empirical. 

Cheremisinoff and Davis (1979) relaxed these two assumptions by using a correlation developed by Cohen and Hanratty (1968) for the interfacial shear stress, using von Karman s and Deissler s eddy viscosity expressions for solving the liquid-phase momentum equations while still using the hydraulic diameter concept for the gas phase. They assumed, however, that the velocity profile is a function only of the radius, r, or the normal distance from the wall, y, and that the shear stress is constant, t = tw. ... 

The liquid in the film alongside the Taylor bubble flows in the opposite direction, with negligible interfacial shear from the gas on the bubble. The average gradient due to friction and acceleration across a slug unit is... 

Annular flow. Modeling the interfacial shear is central to the problem of modeling hydrodynamics and transport during annular flow. The mechanisms are not clear, and the extent of basic modeling that has appeared is still very limited (Dukler and Taitel, 1991b). Only empirical treatments are currently available (see Sec. 3.5.3.3). 

For the interfacial shear stress with roll waves, the following expression was used ... 

Annular flow. In annular flow, as mentioned in Section 3.4.6.1, modeling of the interfacial shear remains empirical. For adiabatic two-phase flow, Asali et al. (1985) suggested that the friction factor, fjfs, is dependent on a dimensionless group for the film thickness, 8+, as defined in Eq. (3-136), and the gas Reynolds number, Rec ... 

The interfacial shear viscosities are measured by the deep channel viscous traction surface viscometer (5) at the Illinois Institute of Technology. The oil-water equilibrium tensions are measured by either the spinning drop or the du Nouy ring (6) method. 

Our results suggest that the lowering of interfacial shear viscosity, although necessary, is not a sufficient criterion for effective demulsification. In addition, a demulsifier must also rapidly dampen any fluctuations in the oil-water interfacial tension. 

---