Structure shear strain parameter

Fig. 13. Schematic drawing (top) of the VPI-5 structure and the plot (bottom) of the quadru-pole coupling constants, of Al as a fimction of the shear strain parameter of AIO4 tetra-hedra in aluminate sodalites (A, V), feldspars ( ), and VPI-5 ( ) [112,113]...

As wc pointed out in S tion 4.3.1, the confiiKHl fluid < an be cxj)osed to a nonvanisliiug shear strain by misaligning the two chemically striped surfaces. Misaligmnent is specified quantitatively in terms of the parameter a in Eq. (4.48a). On account of the discrete nature of our model, a can only be varied discretely iu increments of Aa = l/n. This section is devoted to a discussion of both structure and phase behavior of a confined lattice fluid exposed to a shear strain. 

There are no convenient data from low-temperature shear or uniaxial tensile or compressive large-strain deformation experiments available because of the onset of a brittle-like response as discussed in Section 7.5.5 for metallic glasses. Therefore, we evaluate the model above in the context of the simulations by Demkowicz and Argon of the plasticity of amorphous Si. This provides some direct comparisons of the model with the results presented in Fig. 7.20(a) and formally separates the brittle-like response from the plastic response. For the detailed comparison we note that, for Si, v = 0.42, giving By = 0.282 and = 0.544, and that // = 39.7 GPa at 300 K. Since the simulations presented in Fig. 7.20(a) pertain to the most slowly quenched structure, we take = 0.22 and complete the parameter selection by choosing A in eq. (7.31) as 1.38 for 0 K and 1.667 for 300 K, resulting in corresponding steady-state concentrations of of 0.45 for 0 K and 0.375 for 300 K. The best choice for the characteristic relaxation shear strain y for both cases is taken as 0.1. 

Then, using eq. (7.42) with the material parameters chosen above, we obtain three shear-stress-shear-strain curves for the three chosen for deformations at 300 K. These, given in Fig. 7.21, represent only a rigid plastic response in which elastic response is entirely neglected. We note that for the two slowest quenched structures with

structures develop large initial overshoots of plastic resistance because of the very low pj, followed by prominent strain softening, whereas in the most rapidly quenched structure with (p = 0.6, which is substantially larger than the eventual steady-state level of 0.375, plastic flow... 

Another set of physical properties, very important to structural adhesives, are those that are determined when the polymer is subjected to a force that is tangential to the surfaces of the material. This is shown schematically in Figure 4. A force applied in this manner is known as a shearing force. Parameters analogous to tensile stress and strain can be defined for samples subjected to shearing forces thus a shear stress can be defined by Eq. (5) ... 

In a recent attempt to bring an engineering approach to multiaxial failure in solid propellants, Siron and Duerr (92) tested two composite double-base formulations under nine distinct states of stress. The tests included triaxial poker chip, biaxial strip, uniaxial extension, shear, diametral compression, uniaxial compression, and pressurized uniaxial extension at several temperatures and strain rates. The data were reduced in terms of an empirically defined constraint parameter which ranged from —1.0 (hydrostatic compression) to +1.0 (hydrostatic tension). The parameter () is defined in terms of principal stresses and indicates the tensile or compressive nature of the stress field at any point in a structure —i.e.,... 

The structure of Immiscible blends Is seldom at equilibrium. In principle, the coarser the dispersion the less stable It Is. There are two aspects of stability Involved the coalescence In a static system and deformability due to flow. As discussed above the critical parameter for blend deformability Is the total strain In shear y = ty, or In extension, e = te. Provided t Is large enough In steady state the strains and deformations can be quite substantial one starts a test with one material and ends with another. This means that neither the steady state shearing nor elongatlonal flow can be used for characterization of materials with deformable structure. For these systems the only suitable method Is a low strain dynamic oscillatory test. The test Is simple and rapid, and a method of data evaluation leading to unambiguous determination of the state of miscibility is discussed in a later chapter. 

---