Fracture, Fatigue, and Notch Behavior of PEEK - PEEK Biomaterials - page 67

yielding does not occur but cavitation does occur;
whereas in zone III, neither yielding nor cavitation
occurred. Similar to the Charpy test findings of
Kemmish and Hay [27] , our group found that with
increasing notch severity there was a progressive
increase in the surface that was dominated by fast
fracture with a lessening amount of deformation.
In S e N fatigue studies of notched specimens by
our group [39] and by Nisitani et al. [40] , it was
shown that increasing notch severity decreased total
fatigue lifetime. Our work also showed that a razor
notch, which was meant to approximate a crack,
caused both a shift in the S e N curve and a change in
the slope of the curve; that is, both the coefficient A
and the exponent d of the Basquin relationship was
altered by the introduction of a notch ( Fig. 5.15 ).
Both studies concluded that the majority of the
specimen lifetime is spent in initiation of the crack
and not in propagation. Furthermore, our analysis of
the fracture surfaces ( Figs 5.16 and 5.17 ) led our
group to conclude that fracture initiated near the
notch root but not at the surface. The crack proceeded
through parabolic features that were produced by the
nucleation and growth of cavities ahead of the
primary crack front [16] . In addition, there were
striation-like markings that were observed in this
region and were likely fatigue striations representing
individual cycles of crack growth, consistent with the
FCG studies of Brillhart et al. [37] . An observed
patch morphology was similar to that seen in the
monotonic study by our group [38] and likely formed
in a similar manner through crazing of the material.

Figure 5.9 Scanning electron micrograph of an FCP
specimen of PEEK 150G. Note the similarity to the
monotonically loaded fracture surface in Fig. 5.6 . Cour-
tesy of Ref. [34] .

Figure 5.10 Scanning electron micrograph of an FCP
specimen of PEEK 450G. Crack propagated from left
to right. Note the presence of both fatigue striations
and parabolic features similar to those in the monoton-
ically loaded PEEK 450G specimen in Fig. 5.8 .
Courtesy of Ref. [36] .

5.5 Summary

For neat PEEK, it has been shown that intrinsic
factors, such as molecular weight, percent crystal-
linity, spherulite size, and aging, as well as
a variety of

extrinsic (testing and specimen

Figure 5.11 The maximum axial
true stress and strain for an
unnotched and two notched
geometries of PEEK 450G in
monotonic tension. Notice the
significant decrease in both the
stress and strain at

fracture.

Courtesy of Ref. [38] .