Homogeneous crazes formation

At higher strains, shear zones become broader, become connected with each other and finally cover the whole visible part of the sample, and subsequent deformation proceeds in a homogeneous manner on the level of dimensions of optical microscopy. No craze formation has been recorded. 

The same mechanism can appear in ABS polymers. Besides the formation of the fibrillated crazes, and depending on the matrix and local stress state, a homogeneous plastic deformation between particles, comparable to the appearance of homogeneous crazes in SAN (12, 13), is also possible (Figure 6). The homogeneous deformation in ABS is associated with cavitation inside the rubber particles. In general, this mechanism precedes the formation of the fibrillated crazes. 

Figure 6. Formation of fibrillated and homogeneous crazes at rubber particles in ABS (HVEM image). The deformation direction is shown by the arrow.

Case b stress-induced formation of homogeneous crazes. The stress concentration at the particles causes homogeneous crazes to start at the particle-matrix interfaces. Propagation of these crazes into the matrix is accomplished by an increase of volume, which arises from cavitation inside the particles (the possible mechanism of cavitation inside the originally homogeneous crazes is unlikely). Therefore, these crazes are closely connected to the cavitated rubber particles—they cannot propagate for distances as long as those of the fibrillated crazes—and appear mainly between particles. 

Figure 19. Schematic representation of the three different toughening mechanisms in dispersed systems, where the assumed loading direction is vertical (a) induced formation of fibrillated crazes (i.e., with microvoids in them) at the equatorial zones of rubber particles (b) induced formation of homogeneous crazes at cavitated particles and (c) induced formation of shear deformation between cavitated particles.

In the separation of an adhering system at or near an interface, in terms of a craze mechanism, there will be four differences from separation within a bulk polymer. One is, that interfacial voids (or proto-voids) may exist. These can act as cavitation nuclei and interfacial craze formation, starting from such nuclei, would be orders of magnitude more rapid than crazing by homogeneous nuclea-tion. Such cavitation could also be more rapid than the processes that occur in the Taylor instability mechanism, particularly if it should happen that the voids at the interface formed a two-dimensional continuum. Patches of low-energy matter in the solid surface can also be loci of void initiation, even if no voids are present before loading. 

In contrast to metals, plastics are mostly subject to a purely physical process in which time-dependent diffusion and swelling processes play a considerable role. At first, fine crazes are observed at the surface from which fracture will later develop. Crazes are crack-like damage zones within which no complete material separation takes place as is seen in cracks, but where load transfer is still possible via fibrillated or homogeneously stretched material between the craze walls. Such crazes represent weak points under impact load and can develop into true cracks under long-lasting load that will lead to fracture. Such craze formation is visible to the eye in transparent plastics and sometimes also in unfilled, non-transparent plastics [13]. 

If fibrillated crazes (case a) coexist with homogeneous deformation (cases b or c), the homogeneous mechanisms and rubber-particle cavitation precede the formation of crazes. 

Bulk PS deforms with the formation of fibrillated crazes (a) as well as thick fibrils (b). Fibrils thinner than a critical value (here 225 nm, depending on temperature) deform with necking, cold drawing, and homogeneous stretching. In the necking zones, the fiber diameter decreased from about 225 nm to nearly 60-80 nm [19, 20]. 

---