Biomedical Engineering Reference
In-Depth Information
Materials can have many microscopic defects, such as microporosity, and
macroscopic defects, such as cracks. At the edges of these defects or crack
tips, any applied stress is concentrated, meaning the stress needed to break
the material is lowered. if a material is tough, then it can deform plastically
as the crack propagates, slowing or stopping the crack's motion, or it may
have a second phase which hinders the crack's motion. however, if a material
is brittle, then the concentrated stress causes the crack to propagate very
quickly and catastrophically.
there are several ways to measure fracture toughness, one of the most
common being the single-edge notched beam (Munz and Fett, 2001). This
consists of a setup similar to the three-point bend test, wherein a notch as
thin as possible is placed on the bottom face of the rod, where the specimen
is in tension. if done properly, this notch will serve as a crack initiator and
its propagation will cause the failure of the material. Fracture toughness,
K
ic
, is calculated by:
[5.8]
KY a
ic
KY
ic
KKY
K
ssp
K
sp
KY
where
Y
is a geometrical factor,
s
is the stress at which catastrophic failure
occurs and
a
is the crack length. (in the case of an internal crack,
a
represents
the radius of the crack or one-half the crack length.) toughness can also be
measured indirectly, by indenting the sample with a hard enough load that
cracks emanate from the corners of the indent. Measuring the length of the
cracks enables an estimate of the toughness to be calculated.
fatigue
When a material is placed under continuous cyclic stresses, it may fail by
a mechanism called fatigue. Repetitive loading and unloading can create
microscopic cracks that propagate little by little with each subsequent cycle.
Because cracks and scratches concentrate stresses locally, it will take a
much lower stress than the normal failure stress to propagate the crack and
eventually cause failure. thus the fatigue strength may be as little as one-
quarter to one-third the material's tensile strength.
Fatigue is tested by a cyclic tension or bending test at different maximum
stresses (Dowling, 1998a). the endurance limit is determined by the stress
that causes little to no failure after 10
6
to 10
8
cycles. the fracture surface
is normally perpendicular to the direction of the applied tensile stress.
Because fatigue is due to the gradual formation and propagation of tiny
cracks within the material, the failure mode is brittle, even with a normally
ductile material.
the fatigue behaviour of a material is also sensitive to environment.
Corrosion or other degradation of the material and the cycle rate will affect
