Biomedical Engineering Reference
In-Depth Information
Chapter 5
Fracture, Fatigue, and Notch Behavior
of PEEK
Michael C. Sobieraj M.D., Ph.D., and Clare M. Rimnac Ph.D.
5.1 Introduction
overview of fracture and fatigue behavior in the
presence of stress concentrations (notches) and
cracks. The effect of stress concentrations is dis-
cussed because static fracture/fatigue failures most
often initiate at such design features. The next
several sections are meant to provide a brief intro-
duction of the subject matter. The reader is referred
to several works
[12
e
18]
for a more in-depth
treatment. These methods, combined with analysis
of
The assessment of the failure behavior of a mate-
rial is one of the fundamental needs of engineering.
Components can fail for a variety of reasons, and
failure by fracture, often emanating from a stress
concentration, be it in cyclic or static loading, can
be one of the most catastrophic. By design, compo-
nents that must undergo repetitive loading often
incorporate stress concentrations. Long-term reliable
performance under repetitive loading conditions is
one of the fundamental challenges designers face,
and the field of orthopedic implant engineering is no
exception. For example, there are reports in the
orthopedic literature of cracks and/or gross fracture
in ultrahigh-molecular-weight polyethylene compo-
nents along the rim of acetabular components
[1
e
4]
,
the stabilizing posts in noncruciate-sparing tibial
components
[5
e
9]
, and along the rims of total disk
replacements
[10]
; these areas of stress concentration
are all necessitated by functional requirements.
With polyaryletheretherketone (PEEK) and related
formulations being incorporated into orthopedic
components, including in spinal applications
[11]
,it
is important to understand how this material behaves
in terms of static and cyclic fracture, particularly in
the presence of stress concentrations.
fracture surfaces, provide the basis for
the
majority of
research on fracture properties of
materials.
5.2.1 Stress Intensity Factor, K
If there is a crack present in a structure, failure can
occur at stresses well below the yield stress of the
material. The field of fracture mechanics provides
a framework to understand the propensity for fracture
in the presence of cracks and crack-like defects. In
this regard, the stress intensity factor,
K
, can be used
to characterize the severity of the stress at the crack
tip and takes into account component geometry,
crack size, and the applied stress:
ðpaÞ
p
K
¼ Fs
MPa
p
Þ
where
K
is the stress intensity factor
ð
,
F
is
a dimensionless geometry factor,
is the applied stress
(MPa), and
a
is the crack length (m). If
K
is greater than
or equal to a critical value for the material,
K
C
,the
crack will grow and fracture will occur
[12,15,16]
.
There are three defined fracture modes for which there
are three different
K
:
K
I
(opening mode, due to tensile
s
5.2 Fracture and Fatigue of
Materials
To understand the possible failure mechanisms of
PEEK components, we will first begin with an
