Interfacial shear stresses, effect

The basic Nusselt analysis ignores inertial effects in the condensed film and subcooling effects. Approximate methods of accounting for subcooling were discussed above. A method of accounting for both effects is discussed in this section. To illustrate the method, condensation on an isothermal vertical plate is again considered [52] to [54]. Interfacial shear stress effects will be neglected. 

In the macrocomposite model it is assumed that the load transfer between the rod and the matrix is brought about by shear stresses in the matrix-fibre interface [35]. When the interfacial shear stress exceeds a critical value r0, the rod debonds from the matrix and the composite fails under tension. The important parameters in this model are the aspect ratio of the rod, the ratio between the shear modulus of the matrix and the tensile modulus of the rod, the volume fraction of rods, and the critical shear stress. As the chains are assumed to have an infinite tensile strength, the tensile fracture of the fibres is not caused by the breaking of the chains, but only by exceeding a critical shear stress. Furthermore, it should be realised that the theory is approximate, because the stress transfer across the chain ends and the stress concentrations are neglected. These effects will be unimportant for an aspect ratio of the rod Lld> 10 [35]. 

Barsoum, M. and Tung, F.C. (1991). Effect of oxidation on single fiber interfacial shear stresses in a SiC--borosilicate glass system. J. Am. Ceram. Soc. 74, 2693-2696. 

For practical values of H and Prf, Eq. 14.33 was found to be near unity, indicating that acceleration and convection effects are negligible. Chen [34] included the effect of vapor drag on the condensate motion by using an approximate expression for the interfacial shear stress. He was able to neglect the vapor boundary layer in the process and obtained the results shown in Fig. 14.8. The influence of interfacial shear stress is negligible at Prandtl numbers of ordinary liquids (nonliquid metals, Pr< > 1). Chen [34] was able to represent his numerical results by the approximate (within 1 percent) expression ... 

It is assumed that the interfacial shear stress t, is the dominant force on the liquid film and that gravitational effects can be ignored. In reality (and particularly close to the onset of annular flow), the shear stress may vary considerably across the liquid film. 

Reynaud. P., Rouby, D. and Fantozzi, G., Effects of temperature and of oxidation on the interfacial shear stress between fibres and matrix in ceramic-matrix composites , Acta mater., vol 46, No. 7, pp 2461-2469, 1998... 

To investigate the effect of viscosity ratio between the two liquids, different values of 2 are chosen. Figure 4 shows the dimensionless velocity profiles at the symmetric line when 2 has different values. The velocity profile of the nonconducting fluid is very much like the Couette flow as the nonconducting liquid is dragged by the conducting liquid fluid through the interfacial shear stress. As 2 = smaller value of P2... 

Recently, Hanratty presented a comprehensive review of the attempts to account for the interfacial waviness in modelling the interfacial shear stress for the stability analysis of gas-liquid two-phase flows [53]. Basically, the approach taken was to implement the models obtained for the surface stresses in air flow over a solid wavy boundary as a boundary condition for the momentum equation of the liquid layer over its it mobile wavy interface. Craik [98] adopted the interfacial stresses components which evolve from the quasi-laminar model by Benjamin [84]. Jurman and McCready [99], Jurman et al. [100], and Asali and Hanratty [101] used correlated experimental values of shear stress components (phase and amplitude) based on turbulent models which consider relaxation effects in the Van Driest mixing length. Since the characteristics of the predicted surface stresses are dependent on the wave number, Asali and Hanratty picked the phase and amplitude values which correspond to the wave lengths of the capillary ripples observed in their experiments of thin liquid layers sheared by high gas velocities [101]. It was shown that the growth of these ripples is controlled by the interfacial shear stress component in phase with the wave slope. 

Eiber lengths between 5 and 10 mm are conveniently selected for the microindentation test (111). Eor a carbon fiber/epoxy system, as the fiber volume fraction increases from 10 to 50 vol%, the indentation displacement distance decreases from 44 to 36 pm but the interfacial shear strength increases from 33 to 46 MPa. When the interphase-to-matrix modulus ratio increases from 1.0 to 7.5, the interfacial shear stress increases by only 10%. Likewise, the interphase thickness and fiber diameter have marginal effects on the interfacial shear stress. Three types of thermoplastic polymers (polyester, polyamide, and polypropylene) were tested for their interfacial shear strength to the glass fiber by Desaeger and... 

At the substrate-adhesive interface both the shear and tensile stresses reach a maximum at the free edge of the adhesive bond. Harrison and Harrison used a finite element analysis to determine the effect of varying Poisson s ratios on interfacial shear strengths.Rubbery materials can distribute the stress over larger areas while materials with lower Poisson s ratios produce greater interfacial shear stresses. 

After failure of adhesion, the interfacial shear stress r-, = pp, increases with increasing pull-out distance [46], but the normal stress from the matrix acting on the fiber will produce a reduction in cross-sectional area due to Poisson s effect resulting in a reduction of interfacial normal stresses. 

The distribution of stress around discontinuous fibers in composites has been studied by a number of researchers. Theoretical analyses have been performed by Cox [82] and Rosen [83]. In these models only fiber axial stress distribution and the fiber-matrix interfacial shear stress distribution are determined. Amirbayat and Hearle [84] studied the effect of different levels of adhesion on the stress distribution, that is, no bond, no adhesion, perfect bond, and the intermediate case of limited friction. They also considered the inhibition of slippage by frictional forces resulting from interfacial pressure due to Poisson s lateral contractions of the matrix but did not consider the shrinkage of the matrix during curing. 

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