Indirect shear

At a superficial, qualitative level, the effects are clear. More detailed, quantitative explanations raise more difficulties, and there is little detailed theory available. The direct shear stresses, due to friction or bending, should, at least in principle, be calculable from the overall applied mechanics. The indirect shear stresses depend on the stress distribution around a discontinuity as shown in Fig. 12. 

Figure 1.35 Difference between direct and indirect shear testers. (Jenike direct shear tester inside square and biaxial tester solid rectangle.)...

Direct and indirect shear stress equipment is usually limited to research projects because of the expense, the skill and time evolved to determine the yield loci and failure functions of particulate materials from bi- and ffi-axial shear testers. The designation of which type of shear tester has been used may be defined by the location of the shear failure zone. With direct shear testers - Jenike type - the major principal stress rotates during shear (Figure 1.35), whilst with indirect shear testers - tri- and bi-axial - the directions of the principal stresses are fixed and orientated in either three or two dimensions, respectively, and remain constant during the test. 

Hypothermia—Indirect cryodestruction Metabolic uncoupling Energy deprivation Ionic imbalance Disruption of acid-base balance Waste accumulation Membrane phase transitions Cytoskeletal disassembly Frozen State—Direct cryodestruction Water solidification Hyperosmolality Cell-volume disruption Protein denaturation Tissue shearing Intracellular-ice propagation Membrane disruption Microvascular Thawed State Direct effects... 

To avoid this damage great efforts were made to improve the cultivation systems. Feder and Tolbert [5] developed special stirrers with low shear damage and Lehmann et al. [6] replaced gas bubbling through indirect aeration by diffusion of oxygen through porous polypropylene membranes. 

Thus, only the normal Reynolds stresses (i = j) are directly dissipated in a high-Reynolds-number turbulent flow. The shear stresses (i / j), on the other hand, are dissipated indirectly, i.e., the pressure-rate-of-strain tensor first transfers their energy to the normal stresses, where it can be dissipated directly. Without this redistribution of energy, the shear stresses would grow unbounded in a simple shear flow due to the unbalanced production term Vu given by (2.108). This fact is just one illustration of the key role played by 7 ., -in the Reynolds stress balance equation. 

The contribution of this component is shown in Figures 4.16 and 4.17. One of the important features to recognise about the retardation spectrum is that it only has an indirect relationship to both the zero shear rate viscosity and the high frequency shear modulus. Both these properties are contained in the relaxation spectra. We shall see in Section 4.5.7 that, whilst a relationship exists between H and L it is somewhat complex. 

Endothelial cells produce vasoactive substances, which modulate the permeability of vessel walls accordingly, shear stresses may indirectly affect this wall parameter by influencing the secretory activity of endothelial cells. 

The interaction of two substrates, the bond strength of adhesives are frequently measured by the peel test [76]. The results can often be related to the reversible work of adhesion. Due to its physical nature such a measurement is impossible to carry out for particulate filled polymers. Even interfacial shear strength widely applied for the characterization of matrix/fiber adhesion cannot be used in particulate filled polymers. Interfacial adhesion of the components is usually deduced indirectly from the mechanical properties of composites with the help of models describing composition dependence. Such models must also take into account interfacial interactions. 

The preceding explanation suggests that all suspensions of solids in liquids should exhibit dilatant behavior at high solids contents. Few data are available for evaluation of this conclusion, as the usual examples of dilatant behavior (starch, potassium silicate, and gum arabic in water) (A3, G3) are not true suspensions. The excellent studies of Daniel (Dl) and Verway and De Boer (V3) have indicated under what conditions more dilute suspensions may also exhibit dilatancy. Some of these factors have been summarized by Pryce-Jones (P6). If Reynolds explanation is a valid one, it should be possible to measure the expansion or dilation of the fluid with increases in shear rate. This has been done indirectly Andrade and Fox (A5) measured the dilation of sand suspensions and arrays of cylinders upon the imposition of localized stresses. 

Although the indirect method is more difficult to implement, it renders better results because the rate of deformation and temperature fields are smoother and better bounded than the viscosity field. For example, when using the power law shear thinning model, the viscosity goes to infinity when the rate of deformation goes to zero. 

Cilostazol is a PDE3 inhibitor and also is a drug with multiple mechanisms of action (23,24), some of which are directly platelet-related, and others of which exert their effects indirectly via endothelial cells (Figs, 4 and 5), As a PDE inhibitor, it increases the concentration of cyclic nucleotides within platelets and inhibits platelet aggregation in response to shear forces (via ADP) and a variety of agonists. Like other inhibitors of platelet activation, it blocks the expression and release of P-selectin, a key adhesion molecule on the membrane of platelet alpha granules that externalizes with activation, and plays an important role in the interactions between platelets, endothelial cells, and leukocytes, The effect of cilostazol on P-selectin expression appears to be additive to those of aspirin and clopidogrel (25). 

Indirect determination of the yield stress simply involves extrapolation of the experimental shear stress-shear rate data to obtain the yield value as the shear stress limit at zero rate of shear. The extrapolation is performed numerically on the available data, or the latter can be fitted to a suitable rheologic model representing the fluid, and the yield stress parameter in the model is determined. 

The indirect method can be employed by extrapolating the rheologic models or the shear stress-shear rate data to zero shear rate. The computer software Table Curve 1.12 was used to fit the shear stress-shear rate data to the different rheologic models. This software uses the Simplex method for a nonlinear regression curve fit. 

The techniques that have been used to characterise the mechanical properties of microparticles may be classified as indirect and direct. The former includes measurement of breakage in a "shear" device, for example, a stirred vessel (Poncelet and Neufeld, 1989) or bubble column (Lu et ah, 1992). However, the results from these indirect techniques are rather difficult to use since the mechanical breakage depends not only on the mechanical properties but also the hydrodynamics of the processing equipment, and the latter are still not well understood. To overcome this problem, a cone and plate viscometer that can apply well-defined shear stresses has been used to study breakage of hybridomas (Born et ah, 1992), but this is not a widely applied or applicable technique because the forces are too small to break most cells. 

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