YIELDING PROCESS - Fractal Mechanics of Polymers - page 66

cess is realized in densely packed regions of polymer. It is obvious, that
the clusters will be such regions in amorphous glassy polymer and in semi-
crystalline one - the clusters and crystallites [18]. The Eq. (4.9) allows to
estimate relative fraction c of polymer, which remains in the elastic state.
In Fig. 4.12, the temperature dependence of c for HDPE is shown, from
which one can see its decrease at T growth. The absolute values c change
within the limits of 0.516 ÷ 0.364 and the determined by polymer density
crystallinity degree K for the considered HDPE is equal to 0.687 [51]. In
other words, the value c in all cases exceeds amorphous regions fraction
and this means the necessity of some part crystallites melting for yielding
process realization. Thus, the conclusion should be made, that a semicrystal-
line HDPE yielding process includes its crystallites partial mechanical melt-
ing (disordering). For the first time Kargin and Sogolova [52] made such
conclusion and it remains up to now prevalent in polymers mechanics [53].
The concept [35] allows to obtain quantitative estimation of crystallites frac-
tion c cr , subjecting to partial melting − recrystallization process, subtracting
amorphous phase fraction in HDPE from c. The temperature dependence
of c cr was also shown in Fig. 4.12, from which one can see, that c cr value is
changed within the limits of 0.203 ÷ 0.051, decreasing at T growth.
FIGURE 4.12 The dependences of elastically deformed regions fraction c (1, 2) and
crystallites fraction, subjected to partial melting, c cr (3, 4) on testing temperature T in impact
(1, 3) and quasistatic (2, 4) tests for HDPE [51].