Macroscopic shear process

Howe (26) has reviewed the evidence from earlier studies which have indicated that a non-shock process may be controlling the initiation of detonation. Howe concludes that a macroscopic shear process is probably the mechanism in many of these cases. The new results we have presented here are certainly consistent with such a mechanism but considerably more work is needed before any firm conclusions can be drawn. 

Inconsistency of and values for metals results to a search for another mechanism of yielding realization. At present, it is commonly accepted that this mechanism is the motion of dislocations by sliding planes of the crystal [3]. This implies that interatomic interaction forces, directed transversely to the crystal sliding plane, can be overcome in case of the presence of local displacements number, determined by stresses periodic field in the lattice. This is strictly different from macroscopic shear process, during which all bonds are broken simultaneously (the Frenkel model). It seems obvious that with the help of dislocations total shear strain will be realized at the... 

SCORIM results in substantial enhancement of the physical properties of iPR The enhanced properties are related to the formation of shish-kebab morphology, and hence the high levels of molecular orientation that result from the application of macroscopic shears to a solidifying melt in a mold cavity. In this chapter, SCORIM technology is first described, and then relationships between processing conditions and mechanical properties are discussed, including a discussion on shish-kebab morphology, orientation and the occurrence of 7 phase in processed iPP. 

HDPE, where no p relaxation is observed [29], In these sheets, the crystal lamellae make an acute angle of about 40° with the initial draw direction [30], Applying the stress along the initial draw direction then gives the maximum resolved shear stress parallel to lamellar planes. We see from Figure 10.10 that the maximum loss is tan 3o, confirming that the a relaxation in HDPE is primarily an interlamellar shear process from a macroscopic mechanical viewpoint. 

In the case K > fi, the usual diffusion determines the kinetics for any gel shapes. Here the deviation of the stress tensor is nearly equal to — K(V u)8ij since the shear stress is small, so that V u should be held at a constant at the boundary from the zero osmotic pressure condition. Because -u obeys the diffusion equation (4.18), the problem is trivially reduced to that of heat conduction under a constant boundary temperature. The slowest relaxation rate fi0 is hence n2D/R2 for spheres with radius R, 6D/R2 for cylinders with radius R (see the sentences below Eq. (6.49)), and n2D/L2 for disks with thickness L. However, in the case K < [i, the process is more intriguing, where the macroscopic critical mode slows down as exp(- Q0t) with Q0 oc K. 

The connection between processing conditions and crystalline perfection is incomplete, because the link is missing between microscopic variations in the structure of the crystal and macroscopic processing variables. For example, studies that attempt to link the temperature field with dislocation generation in the crystal assume that defects are created when the stresses due to linear thermoelastic expansion exceed the critically resolved shear stress for a perfect crystal. The status of these analyses and the unanswered questions that must be resolved for the precise coupling of processing and crystal properties are described in a later subsection on the connection between transport processes and defect formation in the crystal.. 

In terms of tonnage, polyolefins are by far the most important polymeric materials for structural applications, and there is consequently enormous interest in optimising their fracture properties. A rational approach to this requires detailed understanding of the relationships between macroscopic fracture and molecular parameters such as the molar mass, M, and external variables such as temperature, T, and test speed, v. Considerable effort is therefore also devoted to characterising the irreversible processes (crazing and shear deformation) that accompany crack initiation and propagation in these polymers, some examples of which will given. 

---