Chemistry Reference
In-Depth Information
FIGURE 10.14
The dependence of impact toughness
A
p
on fracture dimension
d
fr
for
HDPE samples in tests with varied sharp notch length
a
(1) and testing temperature
T
(2) [47].
In Fig. 10.14, the dependences
A
p
(
d
fr
) for both indicated HDPE samples
series are adduced. As one can see, for the same polymer and at tests identi-
cal conditions the correlations
A
p
(
d
fr
) with opposite tendencies are obtained
- the samples with varied notch length series shows
A
p
increase at
d
fr
growth
and at
T
variation
A
p
reduction at
d
fr
enhancement is observed. Such discrep-
ancy has several key reasons, the part of which is noted above. Firstly, the
impact toughness
A
p
is the integrated characteristics, which is defined by
polymer plasticity (
G
Ic
) and critical defect stiffness (for a sharp notch - its
length
a
) [22]. As it is known [48], a polymers, as and all real (physical)
fractals possess statistical self-similarity and are fractals only in a definite
linear scales range, which was determined experimentally and equal to ~3
¢ 50 Ả [49]. The processes, defining plasticity (both local and macroscopic)
for polymers, are controlled by exactly this structure level [50], whereas a
sharp notch has the size on about 5 order more and in virtue of this is consid-
ered as Euclidean object within the frameworks of mechanics of continua.
Secondly, as it was noted above, the polymers fracture process by instable
crack (exactly such mechanism is realized in the considered tests) does not
practically require energy expenditures and that is why the fracture energy
U
value is defined not by fracture process (main crack propagation, dividing
the initial sample into two parts), but by local plastic deformation processes,
