Chemistry Reference
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some
D
cr
reduction at sharp notch length growth, observed experimentally.
Secondly, strictly speaking, at calculation one should not use sharp notch
length
a
, but values
a
and stable crack length sum should be used, since it is
a matter of fracture process, which is realized by instable crack propagation
the adduced in Fig. 8.10 comparison shows, that the fractal Griffith crack
model gives exhaustive explanation of crack self-braking effect.
FIGURE 8.10
The dependences of fracture stress sf
f
on notch length
a
for PAASO (1, 1')
and blends PAASO/PC with PC contents 20 (2, 2') and 80 (3, 3') mas. %. 1, 2, 3 - calculation
according to the Eq. (8.4); 1', 2', 3' - the experimental data [14].
Let us note in conclusion, that the adduced above results have shown that
the fractal Griffith crack model application can not only improve quantita-
tive conformity of theoretical and experimental data, but also obtain qualita-
tively new picture of fracture processes.
The term “cracking” at simultaneous action of stress and environment”
was introduced for polymers (mainly polyethylenes) being in stressed state
at mobile polar liquids presence. It has been shown [17], that in the end
the most weak amorphous part of semicrystalline polymer is responsible for
materials strength at this fracture kind. This allows to connect occurring at
cracking phenomena to polar liquid diffusion in amorphous regions.
In Ref. [18], it has been shown that in case of HDPE, modified by high-
disperse mixture Fe/FeO (Z), strong extreme growth of stability to cracking,
expressed by time up to fracture t
50
, is observed. So, if for the initial HDPE
the normative value t
50
makes up 10 h, then for composition HDPE + Z with
