Elongation shear

Later products employed double extrusion. The viscous mass consisting of protein continuous and carbohydrate inclusions was extruded, with some air present, in long dies. The elongational shear produced layered structures with flake-like substructure (Figure 18.8). 

Recently, the simultaneous application of pressure and elongational shear by extrusion, allows the production of elongated particles, lightly adhered to their neighbours. This produces a remarkably meatlike aligned flake structure (Cheftel et al. 1992). For fine comminutes, the adhesion occurs in the batter itself, and casings are used to form the final product shape. 

Tensile strength (GPa) Initial tensile modulus (GPa) ultimate elongation (%) Shear modulus (GPa) Transverse comp, modulus (GPa) Reference... 

From the above equations it is possible to calculate the size of the largest drop that exists in a fluid undergoing distortion at any shear rate. In these equations, the governing parameters for droplet breakup are the viscosity ratio p (viscosity of the dispersed phase to that of the matrix) the type of flow (elongational, shear, combined, etc.) the capillary number Cfl, which is the ratio between the deforming stress (matrix viscosity x shear rate) imposed by the flow on the droplet and the interfacial forces a/R, where ais the interfacial... 

This class of fibre is melt-spun from a nematic liquid-crystalline phase, which leads to the production of highly-oriented filaments at quite low elongational shear rates. The fibres have high modulus and, if thermally post-treated over an extended period to raise the molecular weight, they also have very high tensile strength. However, so far they are limited as additives by their fusibility and poor adhesive properties. 

Figure 2.6 Model experiments illustrating laminar and elongational shear fields.

With no elongational shear there could be limits related to viscosity ratio, particularly as there can be difficulties in laminar shear mixing of viscous fluids with widely differing viscosities. However, results by Yu and co-workers [25] for trials using a viscosity ratio of 34 at 180 °C and 45 at 160 °C indicates that this may not be a limitation for many applications. 

Heat treatment Tensile yield strenaUi Ultimate tensile strenath Elongation, % Shear strensrth ... 

As discussed Sect. 10.3.2, in the molten state, and with no effect of elongational shear, the morphology of blends of LCP and thermoplastics consist of near-spherical droplets of PC in the matrix polymer (see Fig. 10.1). 

The significance of fhe second assumption is that the deformation field in the film is essentially elongational. Shear stresses are not present, if the film thickness is very small. The first assumption makes the final equations simpler without losing any significant information. Finally, overall these assumptions resemble those used in Section 9.1.2 in the thin-filament theory for the fiber-spinning process. 

Here is tire tensile stress and = lL-/L-, where is tire initial lengtli of tire sample and AL is tire sample elongation. In shear experiments, tire shear relaxation modulus G(t) is defined as where... 

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

The elastic and viscoelastic properties of materials are less familiar in chemistry than many other physical properties hence it is necessary to spend a fair amount of time describing the experiments and the observed response of the polymer. There are a large number of possible modes of deformation that might be considered We shall consider only elongation and shear. For each of these we consider the stress associated with a unit strain and the strain associated with a unit stress the former is called the modulus, the latter the compliance. Experiments can be time independent (equilibrium), time dependent (transient), or periodic (dynamic). Just to define and describe these basic combinations takes us into a fair amount of detail and affords some possibilities for confusion. Pay close attention to the definitions of terms and symbols. 

In packed beds of particles possessing small pores, dilute aqueous solutions of hydroly2ed polyacrylamide will sometimes exhibit dilatant behavior iastead of the usual shear thinning behavior seen ia simple shear or Couette flow. In elongational flow, such as flow through porous sandstone, flow resistance can iacrease with flow rate due to iacreases ia elongational viscosity and normal stress differences. The iacrease ia normal stress differences with shear rate is typical of isotropic polymer solutions. Normal stress differences of anisotropic polymers, such as xanthan ia water, are shear rate iadependent (25,26). 

Magnesium alloys have a Young s modulus of elasticity of approximately 45 GPa (6.5 x 10 psi). The modulus of rigidity or modulus of shear is 17 GPa (2.4 X 10 psi) and Poisson s ratio is 0.35. Poisson s ratio is the ratio of transverse contracting strain to the elongation strain when a rod is stretched by forces at its ends parallel to the rod s axis. 

Drop breakage occurs when surrounding fluid stresses exceed the surface resistance of drops. Drops are first elongated as a result of pressure fluctuations and then spHt into small drops with a possibiUty of additional smaller fragments (Fig. 19). Two types of fluid stresses cause dispersions, viscous shear and turbulence. In considering viscous shear effects, it is assumed that the drop size is smaller than the Kohnogoroff microscale, Tj. 

Material Tensile strength, MPa Shear strength, MPa Elongation, % Modulus of elasticity, GPa Specific gravity Hardness... 

Extensional Viscosity. In addition to the shear viscosity Tj, two other rheological constants can be defined for fluids the bulk viscosity, iC, and the extensional or elongational viscosity, Tj (34,49,100—107). The bulk viscosity relates the hydrostatic pressure to the rate of deformation of volume, whereas the extensional viscosity relates the tensile stress to the rate of extensional deformation of the fluid. Extensional viscosity is important in a number of industrial processes and problems (34,100,108—110). Shear properties alone are insufficient for the characterization of many fluids, particularly polymer melts (101,107,111,112). 

Duetility (ASTMD113). The ductihty of an asphalt is expressed as the distance in cm which a standard briquet can be elongated before breaking. Ductihty is a combination of dow properties and redects both cohesion and shear susceptibiUty. 

As an aside, when a large liquid droplet is broken up by shear stress, it tends initially to elongate into a dumbbell shape, which determines the particle size of the two large droplets formed. Then, the neck in the center between the ends of the dumbbell may explode or shatter. This would give a debris of particle sizes which can be quite different than the two major particles produced. 

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