Fracture, Fatigue, and Notch Behavior of PEEK - PEEK Biomaterials - page 61

loading); K II (sliding mode, due to in-plane shear); and
K III (tearing mode, due to out-of-plane shear)
[12,15,16] . Mixed-mode conditions can also occur. In
the majority of failures, however, failure occurs under
mode I loading conditions. Hence, the fracture
toughness of a material is typically evaluated under
mode I loading conditions. A minimum, geometry-
independent, value of the fracture toughness can often
be determined for a material, given the proper testing
conditions [12,15,16] . Most important, a precracked
specimen must be tested under plane strain loading
conditions such that the extent of plastic deformation
at the crack tip is small relative to the remainder of the
structure (i.e., the structure must undergo primarily
elastic deformation). The minimum value for K C is
designated as K IC and is an intrinsic material property
because it does not depend on specimen geometry.
One of the most common ways K C is experimen-
tally determined is with a compact tension (CT)
specimen [12,15,16] ( Fig. 5.1 ). ASTM D5045-99
gives guidance for CT specimen dimensions and for
K C testing of polymers [19] . Currently, ASTM
F2026-10, which defines the mechanical properties
necessary for medical PEEK, does not include frac-
ture toughness test specifications [20] .

specimen [15,16] . The results from these tests are
highly dependent on specimen size, notch geometry,
the amount and rate of loading, and the method of
support of the specimen. Thus, they do not provide
intrinsic material behavior properties, but the results
can be used for relative comparisons if all test
conditions are held constant. Currently, the only
fracture property requirement for implantable PEEK
materials is a notched Izod fracture toughness of at
least 4 kJ/m 2 according to ASTM F2026-10 [20] .

5.2.3 Stress Life Testing
One of the classic methods of quantifying the
fatigue behavior of a material is with a W ¨ hler S e N
(stress life) curve [14,15] . In this approach, a test
specimen (typically having a uniform gage region) is
subjected to a constant cyclic stress, waveform, and
frequency until failure occurs. This is repeated for
a range of cyclic stresses. The stress range (

Ds ¼

s /2) is then

plotted on the y -axis, and the associated lifetime in
cycles ( N f ) for each cyclic stress is plotted on the
x -axis on a logarithmic scale ( Fig. 5.2 ). It was shown
by Basquin that for many engineering materials, the
S e N behavior can be modeled with a power law
relationship [14,15] :

s min ) or the stress amplitude ( Ds max

5.2.2 Impact Testing
Both the Izod test and the Charpy test are high-
strain-rate tests that measure the energy absorbed by
a material during high-speed impact. Both tests rely
on a pendulum being dropped from a specific height,
impacting and fracturing a notched or cracked

Ds ¼ AN f

where A and d are dependent on both intrinsic
(material) and extrinsic (test conditions and specimen
geometry) factors. The benefit of this type of fatigue

Figure 5.1 CT specimen geometry and stress intensity
factor,

K , determination. P is the applied load, F p is the

geometry factor, and a ¼ a / b . All other terms are Figure 5.2 Typical curve. Here failure was

defined as a strain of 12%. Courtesy of Ref. [42] .

S e N

defined in the image.