Fracture, Fatigue, and Notch Behavior of PEEK - PEEK Biomaterials - page 63

significantly longer than an unnotched (smooth)
specimen with the same cyclic notch stress evenly
distributed across the entire cross-section. Because
of this observation, researchers have suggested
that failure is dictated by the stresses in a volume
of material in the vicinity of the notch root and not
just at the root itself [24,25] . Therefore, one cannot
simply adjust estimates of fatigue life by using
k t alone (even if the loading is primarily elastic); it
is necessary to test notched specimens in fatigue
to assess how the material will respond in
the presence of a notch under cyclic loading
conditions.

Figure 5.4 A cylindrical bar with a circumferential
groove. The enlarged element shows the directions of
the principal stresses generated at the notch under
an applied tensile load. Courtesy of Ref. [43] .

5.3 PEEK Fracture Studies

5.3.1 Mechanical Properties
As mentioned earlier, the minimum-notched Izod
fracture toughness for implantable PEEK is 4 kJ/m 2
[20] . According to the material property data sheets
for Victrex PEEK, an increase in molecular weight
results in an increase in impact strength/toughness
[26] . An increase in crystallinity has been shown to
decrease Charpy impact toughness [27] . Increased
aging of the polymer has been shown to decrease
Charpy toughness [27] , though it should be noted that
PEEK is relatively difficult to age in vivo. An increase
in the radius of curvature of the notch, r r , of the impact
specimen increases the Charpy impact toughness [27]
( Tabl e 5 .1 ).

This leads to an elevation in the apparent yield strength
of the bulk specimen, known as notch strengthening
[16] . In a material that is notch sensitive (notch
weakening), the material has little plastic flow capa-
bility; fracture in the vicinity of the notch is initiated,
and the specimen fails primarily in a grossly brittle
manner.
Interestingly, the fatigue behavior of a notched
specimen is not necessarily determined only by the
cyclic stress at the notch (which is higher than
that experienced by the rest of the specimen). In
many materials, a notched specimen will survive

Table 5.1 Summary of Material and Testing Effects on Impact Toughness, Fracture Toughness
( K IC ), and FCG Resistance of PEEK

Effect on
Impact Toughness Material/Testing Parameter K IC FCG

Molecular weight

[ [ [ [

Percent crystallinity

[ Y Y [

Spherulite size

[ Y Y

Aging

[ Y

[ r r [

Loading rate

[ Y

50 to 150 C) Temperature ( ( J IC )

[ [