Chemistry Reference
In-Depth Information
FIGURE 10.17
The dependences of fracture surface fractal dimension
d
fr
, calculated
according to the Eq. (10.28), on characteristic ratio
C
¢
at fixed dissipated energy fraction h
d
:
0.2 (1), 0.5 (2) and 0.8 (3). The shaded lines indicate
d
fr
level for ductile fracture (1') and super
plasticity (2') [57].
As one can see, both PC and HDPE in the first from the indicated re-
gions have the values
d
fr
, calculated according to the Eq. (10.28) larger than
2.7, that is, they are subjected to ductile fracture according to the criterion
(10.29). For PC this threshold
d
fr
value is reached at the expense of
C
∞
small
values h
d
(or
D
ch
), that is the consequence of devitrificated amorphous phase
availability for the latter at testing temperature
T
= 293 K. For PVC the value
n in principle allows ductile fracture, proceeding from the Eq. (4.51), but h
d
(or
D
ch
) small values result to the fact, that samples are broken down either
by brittle mode or immediately beyond yield stress. For PMMA the small
values h
d
(or
D
ch
) are typical and, as consequence, small values
d
fr
, indicat-
ing quasibrittle fracture, that is, 2.0 <
d
fr
< 2.5 [6].
The results of comparison for HDPE samples with sharp notch and
without it show, that notch (stiff macroscopic defect) introduction results
to n reduction, that is, local order level enhancement, which is expressed in
mechanical vitrification of devitrificated amorphous phase part [59]. This
effect defines molecular mobility considerable reduction, expressed by
D
ch
decrease, and h
d
reduction. The last factor results to
d
fr
decrease up to the
values typical for quasibrittle fracture, that is,
d
fr
< 2.5 [6].
