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The izod impact fracture behaviour of phenolphthalein poly(ether ketone).

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Die Angewandte Makromolekulare Chemie 221 (1995) 131-138 (Nr. 3973)
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, P. R. China
The Izod impact fracture behaviour of
phenolphthalein poly(ether ketone) a
Yanchun Han*, Binyao Li, Yuming Yang, Zhiliu Feng, Xuhui Wang
(Received 8 September 1994)
SUMMARY:
The Izod impact fracture behaviour of notched specimens of phenolphthalein poly(ether ketone) (PEK-C) has been studied over a temperature range from room temperature to 240 "C by using an instrumented impact tester. The temperature dependence of
the maximum load, total impact energy, initiation energy, propagation energy, ductility
index (DI) and the relationships between these parameters and the relaxation processes
have been investigated.
ZUSAMMENFASSUNG:
Das Izod-Schlagbruchverhaltenvon gekerbten Probekljrpern aus Phenolphthalein
Polyetherketon (PEK-C) wurde iiber einen Temperaturbereich von Raumtemperatur bis
240 "C mit einem instrumentierten Schlagversuchstester gemessen. Die Temperaturabhangigkeit der Maximalbelastung, der Gesamtschlagenergie, der Energie zur Brucheinleitung und -fortpflanzung, des Duktilitatsindexes (DI) und die Zusammenhange zwischen diesen Parametern und den Relaxationsprozessen wurden untersucht.
Introduction
The impact resistance of polymeric materials often determines their usefulness in many applications. Substitution of plastics for wood, metal, or ceramic
parts is determined, to a large extent, by the mechanical durability offered by
the replacement materials. Accurate evaluation of a polymer's impact strength
is, therefore, essential if optimum part performance is to be obtained.
The impact strength of polymers is one of the most difficult properties to
measure. Besides being dependent on the polymer structure itself, the fabrication and environmental conditions, as well as the type and frequency of the
impact, affect the impact resistance. Numerous types of testing machines have
*
a
Correspondence author.
Key Project of the National Natural Science Foundation of China.
0 1995 Huthig & Wepf Verlag, Zug
CCC 0003-3146/95/$07.00
131
Y. Han, B. Li, Y. Yang, Z. Feng, X. Wang
been developed for evaluating the impact strength of materials. Instrumented
impact testers'-3 have achieved widespread popularity within the past decade
for characterizing the response of polymers to short-term loads, and several
types are now commercially available. The instrumented impact test provides
much more information than can be obtained from the conventional impact
test. Unlike conventional impact test equipment which yields only the energy
required to break a specimen, the instrumented devices provide detailed forcevs.-time curves of the impact process. So, the load and energy absorbed by the
specimen can be determined during the entire duration of the impact. Thus,
it is possible to determine the dynamic strength as well as to separate the total
impact energy into the initiation energy and propagation energy. Such devices
require fewer test specimens than conventional testing apparatus and allow the
various failure types to be detected readily, especially in cases where mixed
failure modes occur4.
Phenolphthalein poly(ether ketone) (PEK-C) is an amorphous material with
a high T, of about 215 "C and an average molecular weight of 326000.
r
1
PEK-C can be used as an engineering thermoplastic and as matrix of
composites. As PEK-C is being increasingly used in engineering applications,
there is a need to understand the effect of temperature on the impact
properties. The aim of this paper is to use an instrumented impact tester to
investigate the temperature effect on Izod impact properties of PEK-C and
discuss the relationships between impact properties and relaxation processes.
Experimental
Phenolphthalein poly(ether ketone) was supplied by Xu Zhou Engineering Plastics
Co., China. The specimens were injection-moulded bars of PEK-C of size 63.5 mm x
6 mm X 12.7 mm. The notches were machined to the required depth of 2.54 mm.
Izod impact tests were carried out on JJ-20 Model Izod instrumented impact pendulum tester over a range of temperature from room temperature to 240°C. Tests at
elevated temperature involved heating the sample to the required temperature, and then
impacting it within 5 s of removal from the oven.
132
Izod impact fracture behaviour of PEK-C
Results and discussion
Load-time and energy-time curves
With the recent introduction of an instrumented impact test, as was
employed in the present study, a load-time history for a standard Izod impact
test may be obtained. The amount of energy dissipated at the various stages
of fracture may then be observed. The maximum load which the specimen can
sustain is also available.
Fig. 1 shows typical load-time and energy-time curves which were obtained
from the instrumented Izod impact test of PEK-C at room temperature. The
monotonically increasing curve represents the energy input to the test specimen up to a given time. It is obtained by integrating the load-time trace after
the load-time trace has been converted to a load-deflection trace. The time axis
(the horizontal axis) may be easily converted to represent the midpoint
deflection of the test specimen by assuming that the velocity of the striking
pendulum is essentially constant during the fracture of the specimen. That is,
the midpoint deflection is found by multiplying the time representing a
particular point in the fracture process by the velocity of the pendulum at
impact. The portion of the load-time trace up to the maximum or peak load
is assumed to be the elastic portion of the impact event. The portion of the
curve following the first peak load represents the post fracture process.
60 I
10.4
Time(ms)
Fig. 1.
Load-time and energy-time curves of PEK-C at room temperature.
133
Y. Han, B. Li, Y. Yang, Z . Feng, X. Wang
If the assumption is made that the area under the load-deflection curve to
the point of the maximum load is the energy required to initiate the fracture
of a polymer sample, and if it is further assumed that fracture is not initiated
until this point of the maximum load is reached, then the total strain energy
stored within the specimen should equal the energy integrated beneath the
load-deflection curve to the point of maximum load. It must also be pointed
out that even for a "ductile" material, in which substantial amounts of energy
are dissipated after maximum load has been reached, the initial fracture can
be analysed in a similar manner.
The failure of a material when subjected to an impact load involves a
complex fracture process. When the applied load exceeds a certain level, shear
or slip bands are initiated at the notch root. Being developed by further
loading, they become curved and the form of a logarithmic spiral becomes
evident. The extension of the plastic zone including slip bands increases the
stress at the elastic-plastic boundary due to the plastic constraint to initiate a
crack-like flaw, i.e., a fracture nucleus propagation in a brittle manner to lead
to a final fracture. In other words, the fracture process passes through three
stages as follows: (a) initiation and extension of shear bands from the notch,
(b) nucleation and slow growth of the fracture nucleus at the elastic-plastic
interface and (c) rapid propagation of the nucleus.
Temperature dependence of the maximum load
The effect of temperature on the maximum load is shown in Fig. 2. We
consider the characteristic regions for PEK-C in the light of Fig. 3, where the
fractographs near the notch root are shown.
"i
n
0
Fig. 2.
134
I
50
_
I
_
150
200
Temperature("C)
100
250
Temperature dependence of the maximum load of PEK-C.
Izod impact fracture behaviour of PEK-C
Fig. 3.
Fractographs at characteristic temperature regions in PEK-C; (a) 40"C,
(b) 140 "C, (c) 160 "C, (d) 200 "C. The length of the bars corresponds to 1 mm.
(i) Region I: The stress required for craze initiation is probably smaller than
the shear yield stress. Consequently, crazes are initiated at the notch root
before shearing yielding occurs, and grow radially around the notch root. One
of the crazes initiated opens to form a crack and the crack propagates slowly
through the crazes to lead to a subsequent fast fracture. The fracture origin is
adjacent to the notch root.
(ii) Region 11: Fracture stress decreases with decreasing temperature. As
reflected in Fig. 3, shear bands are initiated at the notch root before crazing
occurs. Before the stress at the elastic-plastic interface becomes high enough
to satisfy the condition for internal-craze nucleation, some crazes form at the
135
Y. Han, B. Li, Y. Yang, Z. Feng, X. Wang
notch root. Once the fracture nucleus forms among these crazes, it suddenly
propagates in a brittle manner.
(iii) Region 111: The final fracture starts at an elastic-plastic boundary, where
the stress reaches a critical level required for internal-craze nucleation.
(iv) Region IV: General yielding occurs before fracture.
As is evident from the above results, how crazes form around the notch
seems to determine the characteristic fracture regions of PEK-C. Slip-induced
fracture becomes dominant at relatively high temperatures, while crazeinduced fracture is valid at low temperatures.
Temperature dependence of initiation energy, propagation energy, total
fracture energy and ductility index
As shown in Fig. 1, the load-time history can be divided into two distinct
regions, a region of fracture initiation and a region of fracture propagation.
In the initiation region, as the load increases, elastic strain energy is
accumulated in the specimen on contact with the striking head of the
pendulum. In this region, no gross failure takes place, but failure mechanisms
on a microscale. When a critical load is reached at the end of the initiation
phase, the specimen may fail, either by a tensile or a shear failure depending
on the relative values of the tensile and interlaminar shear strengths. At this
point, the fracture propagates either in a catastrophic “brittle” manner or in
a progressive manner continuing to absorb energy at smaller loads. The total
impact energy E,, as recorded on the energy-time curve on the oscilloscope, is,
therefore, the sum of the initiation energy, Ei, and propagation energy, E,.
Since a high-strength brittle material, which has a large initiation energy but
a small propagation energy, and a low-strength ductile material, which has a
small initiation energy but a large propagation energy, may have the same total
impact energy, knowing the value of E, alone is not sufficient to properly
interpret the fracture behaviour of the material. The values of total impact
energy, E,, and initiation energy, Ei, can be divided by the cross-sectional area
of each specimen in order to obtain their normalized values. Thus:
E,/B(D - a)
(1)
ei = E,/B(D - a)
(2)
e,
=
and
136
Izod impact fracture behaviour of PEK-C
The propagation energy per unit area, ep, is given by:
ep = E,/B(D - a)
=
E,/B(D - a) - Ei/B(D - a)
(3)
or
e = e, - ei
(4)
Another proposed characteristic of the material that can be obtained from
these relationships is the ductility index (DI)5, which is a dimensionless
parameter and is defined as the ratio of propagation energy to the initiation
energy. Thus
DI = E,/Ei
(5)
DI = e,/ei
(6)
and
For PEK-C, the temperature dependence of E,, Ei and E, is shown in
Fig. 4. We can gain some insight from their changes with temperature where
the different regimes of behaviour are related to the maximum load. The
temperature dependence of DI is shown in Fig. 5 . From Fig. 5 it can be seen
P -401
0
*
32
8 ‘
10
120
175
Temperature(”C)
65
230
Fig. 4. Temperature dependence of the total impact energy, E, (o),the initiation
energy, Ei (O),
and propagation energy, E, (A), of PEK-C.
137
Y. Han, B. Li, Y. Yang, Z. Feng, X. Wang
I
E; 1.0
0.8
1.2
0.6
Fig. 5.
:&
'
10
65
120
175
Temperature("C)
I
230
Temperature dependence of the ductility index of PEK-C.
that the temperature dependence of DI shows two maxima, similar to the tan6
loss. This suggests that the tan6 loss has some effect on the ductility.
Conclusions
Within the past decade, instrumented impact testers have become important
tools for characterizing the response of polymers to short-term loads. Unlike
conventional test equipment which yields only the fracture energy, the instrumented devices are capable of providing detailed force-vs.-time plots of the
impact process. The Izod impact fracture behaviour of notched specimens of
phenolphthalein poly(ether ketone) (PEK-C) has been studied over a range of
temperature by using an instrumented impact tester. The temperature
dependence of the maximum load, total impact energy, initiation energy,
propagation energy, ductility index (DI) and the relationships between these
parameters and the relaxation processes have been investigated.
I
A. J. Hemingway, A. D. Channel], E. Q. Clutton, Plast. Rubber Process. Appl. 17
(1992) 147
Y. Nakamura, M. Yamaguchi, M. Okubo, Polym. Eng. Sci. 33 (1993) 279
A. J. Kinloch, G. A. Kodokian, M. B. Jamarani, J. Mater. Sci. 22 (1987) 41 1 1
H. Gonzalez, W. J. Stowell, J. Appl. Polym. Sci. 20 (1976) 1389
P. Beaumont, P. Riewald, C. Zweben, Methods for Improving the Impact
Resistance of Composites Materials, ASTM STP 568 (1 974) 134
138
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phenolphthaleins, behaviour, izod, ethers, ketone, poly, fractured, impact
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