Shear locking

Many early simulations for plate structures were based on the shell element formulations introduced by Ahmed et al. (1970). While such elements are capable of dealing satisfactorily with thick plate and shell problems, their structural response becomes too stiff as the plate or shell becomes thin. This phenomenon is called shear locking and is known to be related to the inability of the element to approach the limiting condition of zero transverse shear strain at the appropriate quadratic rate. 

The Ferranti-Shidey viscometer was the first commercial general-purpose cone—plate viscometer many of the instmments stiU remain in use in the 1990s. Viscosities of 20 to 3 x 10 mPa-s can be measured over a shear rate range of 1.8-18, 000 and at up to 200°C with special ceramic cones. Its features include accurate temperature measurement and good temperature control (thermocouples are embedded in the water-jacketed plate), electrical sensing of cone—plate contact, and a means of adjusting and locking the position of the cone and the plate in such a way that these two just touch. Many of the instmments have been interfaced with computers or microprocessors. 

A common theme with natural products is the variability of the fiber with the particular plant or animal and the particular conditions under which they were grown. The proportion of kemp to wool varies from breed to breed as does the number of crimps per inch. Thus, wool from different breeds raised in different parts of the world will have different properties and contributions to the fiber. Generally, the wool and kemp are physically separated. In fact, after shearing, the wool is divided into five groups fleece (which is the largest), pieces, bellies, crutchings, and locks. The last four are packaged and sold separately. The desired part, the fleece, is further classified. 

The expl components of each of the above fuzes consist of a delay element, a detonator, a booster lead, and a booster chge (1.9-oz of Tetryl). Its safety features consist of a safety (cotter) pin, which passes thru holes in the flange of the vane-cup and in the flange of the vane-cup support and locks the reduction mechanism to prevent the fuze from arming. The arming wire keeps the fuze unarmed until it is withdrawn when the bomb is released. These fuzes are "detonator-safe and "shear-safe"... 

A more serious deficiency resides in reliance on MFI to characterize different polymers. No single rheological property can be expected to provide a complete prediction of the properties of a complex material like a thermoplastic polymer. Figure 11-27 shows log — log flow curves for polymers having the same melt index, at the intersection of the curves, but very differeni viscosities at higher shear stress where the materials are extruded or molded. This is the main reason why MFI is repeatedly condemned by purer practitioners of our profession. The parameter is locked into industrial practice, however, and is unlikely to be displaced. 

The threshold is even more visible on this representation, as when the strong slip starts, the shear rate experienced by the polymer is no longer proportional to Vt. In fact, at the onset of strong slip, the shear stress remains locked, while the velocity at the wall strongly increases. 

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