Biomedical Engineering Reference
In-Depth Information
The reported fracture toughness,
K
IC
, for PEEK
the conical structures in the fractograph are the nuclei
of the spherulites and the fibrillar material connecting
them to the base plane represent lamellar bundles.
Chu and Schultz also commented that the high-
molecular-weight material (450G) showed several
distinct fracture features. In the stable crack growth
region, there were both large parabolic curves that
originated at inclusions/flaws and very small “hood-
shaped” structures (
Fig. 5.6
). There was also a region
of crack instability, which corresponded to the fast
fracture region (
Figs 5.7 and 5.8
). This fracture
pattern has been reported for high-molecular-weight
PEEK in other studies
[30,31,33]
. One of the more
interesting observations that Chu and Schultz made
was that the small-hooded structures were similar in
size to spherulites; they concluded that these struc-
tures imply that for PEEK 450G, fracture is a purely
interspherulitic process with the fracture propagating
strictly in the amorphous region. This results in the
observed hooded features forming around intact
spherulites.
In an FCP study of PEEK
[34]
, reported fracto-
graphs of Victrex PEEK 150G resemble those seen
in the work by Chu and Schultz in their (mono-
tonic) fracture toughness study (
Fig. 5.9
). Once
again the Victrex 450G did not show these features.
Instead, for the 450G PEEK fatigue fracture
behavior, there appear to be three regions: stria-
tions, followed by parabolic features similar to
those seen in the monotonic fracture toughness
studies, and finally a region of catastrophic fracture.
This fracture pattern for 450G PEEK was also
observed by Brillhart and Botsis
[36,37]
(
Fig. 5.10
);
they concluded that the striations corresponded to
individual cycles of crack growth
[37]
. Inspection
of these fractographs reveals that they, too,
demonstrate the hooded-shaped structures in both
the striated and parabolic regions.
ranges between 2 and 8 MPa
p
[28
e
33]
. Research
has shown that for neat PEEK, several testing and
morphological parameters affect fracture toughness.
First, it has been shown that increasing the molecular
weight of PEEK increases fracture toughness
[28,29]
. Second, several authors have shown that
increasing the percent crystallinity decreases fracture
toughness
[28,30,32,33]
. Chu and Schultz
[29]
have
also reported that increasing the size of the spheru-
lites in PEEK decreases fracture toughness. Finally,
unlike many mechanical properties of PEEK that are
relatively rate insensitive, it has been shown that the
fracture toughness decreases with increasing loading
rate
[30]
. Rae et al.
[31]
also showed that
J
IC
(another
measure of fracture toughness
[12,15,16]
) increased
with increasing temperature between
50 and
150
C.
It has also been shown by several authors that the
FCG behavior of PEEK can be modeled using the
Paris law
[29,33
e
35]
. Several trends have been
reported for the dependence of the FCG resistance of
PEEK on morphological parameters. Increasing
molecular weight has been shown to increase FCG
resistance
[29,34]
. Increasing crystallinity has also
been shown to increase FCG resistance
[34,35]
,
whereas increasing spherulite size has been shown to
decrease FCG resistance
[33]
.
5.3.2 Fractography and Fracture
Micromechanisms
In the work by Kemmish and Hay
[27]
, the authors
present fractographs of 20% crystalline PEEK
Charpy impact specimens with different notch radii.
The authors found that as the specimen notch radius
decreased, resulting in a transition from plane strain
to plane stress loading, the fracture surface appear-
ance changed progressively toward brittle behavior.
They also observed that as the notch radius increased,
the craze site moved progressively further from the
notch, and there was more surface deformation
(
Fig. 5.5
).
In fracture toughness studies using CT specimens,
several observations have been made about the
fracture behavior of PEEK. Chu and Schultz
[29]
observed for low-molecular-weight PEEK (PEEK
150P) that the fracture surfaces showed “nucleus
pull-out” (
Fig. 5.6
). They concluded that this indi-
cated that intraspherulitic fracture occurs in low-
molecular-weight PEEK. The “knobby apexes” of
5.4 PEEK Notch Studies
There is little in the literature regarding the frac-
ture behavior of PEEK in the presence of notches. In
a monotonic tension study of PEEK 450G
[38]
,
several observations were made by our group. First,
PEEK is a notch-weakening material in which almost
all gross specimen plasticity is suppressed in the
presence of several notches of different severity used
in that study. Second, the ultimate fracture stress
decreased as the severity of the notch increased
