YIELDING PROCESS - Fractal Mechanics of Polymers - page 50

FIGURE 4.1 Schematic representation of deformation of two sequences of atoms according
to the Frenkel model. Positions before (a) and after (b) deformation [3].
As it has been shown in Ref. [21], dislocation analogies are also true for
amorphous metals. In essence, the authors of Ref. [21] consider the atoms
construction distortion (which induces appearance of elastic stress fields)
as a linear defect (dislocation) being practically immovable. It is clear that
such approach correlates completely with the offered above structural defect
concept. Within the frameworks of this concept, Fig. 4.1a may be considered
as a cross-section of a cluster (crystallite) and, hence, the shear of segments
in the latter according to the Frenkel mechanism - as a mechanism limiting
yielding process in polymers. This is proved by the experimental data [32],
which shown that glassy polymers yielding process is realized namely in
densely packed regions. Other data [23] indicate that these densely packed
regions are clusters. In other words, one can state that yielding process is
associated with clusters (crystallites) stability loss in the shear stresses field
[24].
In Ref. [25], the asymmetrical periodic function is adduced, showing the
dependence of shear stress t on shear strain g sh ( Fig. 4.2 ). As it has been
shown before [19], asymmetry of this function and corresponding decrease
of the energetic barrier height overcome by macromolecules segments in the
elementary yielding act are due to the formation of fluctuation free volume
voids during deformation (that is the specific feature of polymers [26]). The
data in Fig. 4.2 indicate that in the initial part of periodic curve from zero up
to the maximum dependence of t on displacement x can be simulated by a