Localized shear zones

Figure 11. Growth of a localized shear zone in rubber-toughened epoxy, obtained using the spherical model. The yielded elements at the different steps are shaded. The applied strain (loading direction) is 3.0% (top), 4.0% (middle), and 5.0% (bottom).

Although the Eq. (5.3) gives for polymers good correspondence to experiment in the case of both crazes [6] and local shear zones [9], but it does not connect the parameter q with any structural characteristics of polymers. 

Thus, the offered in Refs. [29, 30] treatment assumes percolation not by a sample cross-section area, but by a line - sample width B. Such treatment reason will be considered below. Besides it is supposed [30], that percolation cluster formation beginning (percolation threshold) corresponds to any small amount of local shear zone (shear bands network) formation in deformation process and therefore, to the first approximation the percolation... 

FIGURE 10.10 The relation between samples fracture energy U and local shear zone relative length for HDPE [29]. 

Deformation behavior of amorphous polymers is well understood by using the concept of strand network density. Whether crazing or shear deformation is observed depends on the total network density, and the extension ratio of each deformation zone is proportional to the theoretical maximum extension ratio. Macroscopic shear deformation, leading to ductile behavior under tension, can be achieved by using thin layers whose thickness is below the critical value, even for normally brittle polymers like PS. The observed extension ratio of macroscopic shear zones is the same as that observed for localized shear zones. 

Kneaders are effective mixers for making color concentrates because they are excellent for both dispersive and distributive mixing. The main attribute of a kneader is uniform control of heat and dwell since there is no localized high-shear zone to overmix or overheat sensitive polymer like polyvinyl chloride (PVC). 

The spatial distribution of plastic deformation adjacent to the fault plane can be explained by a model in which the propagating fault is preceded by a zone of stress concentration at its tip. Calculations by Blenkin-sop and Drury (1988) indicate that, for remote applied stresses of less than 100,MPa, the zone of stress concentration, with stress greater than 145 MPa, will have a lateral width of 1-3 m once the initial fracture is longer than 12 m. The high stresses estimated from the quartz microstructures adjacent to the fault are consistent with a low average stress in the crust because very high transient values of stress may occur locally, particularly at the propagating termination of fractures, shear zones, and folds. 

PC shows either craze or shear behavior, with no mixed behavior, i.e., successively regenerated localized DCG zones. There is a sharp transition between the craze and the shear branches, as seen in Fig. 35. The competition between crazing and shear is temperature and stress sensitive. The mode, once determined, persists as the barrier is high between these two modes. At 75 °C and above, no craze-crack growth is observed, although shear fracture does persist down to —25 °C, albeit only at high stresses. 

Kelemen P. B. and Dick H. J. B. (1995) Focused melt flow and localized deformation in the upper mantle juxtaposition of replacive dunite and ductile shear zones in the Josephine Peridotite, SW Oregon. J. Geophys. Res. B Solid Earth Planets 100, 423-438. 

Roll Mills Roll mills can provide extremely high localized shear while retaining extended surface area for temperature control. A typical roll mill has two parallel rolls mounted in a heavy frame with provisions for accurately regulating the pressure and distance between the rolls. Since one pass between the rolls does only a little blending, the mills are usually used as a series of mixers. Only a small amount of material is in the high-shear zone at a time, thus allowing time and exposure for cooling. 

---