IMPACT TOUGHNESS - Fractal Mechanics of Polymers - page 217

PMMA 7.0 — 0 . 420 2.840 1.10 0.10 2.05

HDPE, sharp notch with length
of 1.5 mm

6.8 — 0.280 2.560 1.40 0.40 2.27

Plastic deformation

A p ,

Polymer sample

T
f

d

kJ/m 2

D ch d fr

n h d

HDPE, without notch 0.410 2.820 1.85 0.85 2 .99 >167

PC 0.400 2.800 1.58 0.58 2.99 >167

PVC 0.428 2.856 1.45 0.45 2 .31 14.5

PMMA — — — — — 6.2

HDPE, sharp notch with
length of 1.5 mm

— — — — — 4.7

In the general case d f value is connected with Poisson's ratio n by the
Eq. (1.9). Hence, using the values n, obtained according to the Eq. (2.20),
the values

cl

f d can be calculated according to the Eq. (1.9) and then, using
the adduced in Table 10.1 v alues D f g , the values

Y

f d according to the Eq.
(10.30). Further recalculation of the parameters D ch (or h d ) for the indicated
samples plastic deformation region was conducted, excluding HDPE sam-
ples with a notch and PMMA, which break down by quasibrittle mode, that
is, earlier yield stress reaching.
The calculation according to the Eq. (10.30) is correct only for those
samples, for which yielding zone in impact tests occupies the entire cross-
section of sample [55]. As it follows from the data of Table 10.1, a values
d fr , close to 3 and characterizing superplasticity state (determined according
to the Eq. (10.28)), were obtained for PC and HDPE. As before, this effect
reason for PC is C ∞ small value and for HDPE - high values h d (or D ch ). For
PVC, having molecular mobility level comparable with such one for PC, the
higher C ∞ value defines smaller d fr value, without allowing superplasticity
effect realization. In the Table 10.1, last column the impact toughness A p val-
ues for the considered samples are cited, in addition the A p values for PC and
HDPE were estimated by the division of device the greatest impact energy (4
J) by sample cross-section area. These data demonstrate clearly fracture type
influence to A p value, which is the most important operating characteristic of
engineering polymers [54].
The transition from superplasticity to intermediate behavior (even at duc-
tile fracture possibility) reduces A p more than an order and some even less
the value A p in brittle fracture case.