Homogeneous crazes

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.

If the particle volume content is in average above 10%, a remarkable superposition of the individual particle stress fields appears. The result is a higher stress concentration and the more intense initiation of local matrix yielding in the form of fibrillated crazes, homogeneous crazes, or shear yielding. 

Deformation zones (also called homogeneous crazes) possess the same orientation as fibrillated crazes, but they do not contain voids, do not show volume increase, and are the result of homogeneous deformation processes with gliding of macro-molecular segments (see Figs. 1.49 and 1.50 in PC in Part 11). 

A very broad and several narrow homogeneous deformation bands (homogeneous crazes) in front of a crack tip in SAN copolymer ... 

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