Biomedical Engineering Reference
In-Depth Information
6.10, it can be seen that the fatigue strength of fine- and coarse-grained
Ti-6Al-4V significantly decreases when a notch is present.
34
This decrease
is even greater than expected, considering the magnitude (1.95) of the stress
concentration factor. The fatigue strength under the notch of the fine grained
material is only slightly higher than that of the coarse grained material, which
suggests (taking into account the difference in the fatigue limit of unnotched
specimens of both microstructures) that the effect of the as-machined notch
is more severe in the fine grained material. Viceconti
et al
., studying a group
of hip prostheses with a small systematic machining defect that produced a
stress concentration factor of 1.13, which is equivalent to that produced by
the notch (1.95), reported a fatigue limit reduction factor of 1.4.
35
a further consideration concerns the effect of surface treatments on
the mechanical properties of the bulk. For instance, efforts to improve the
osteointegration, fixation and stability of Ti implants have been addressed
by creating a rough surface that increases the surface area available for
bone/implant apposition. Particularly important has been the activity for
the production of randomly rough surfaces by sandblasting, which is
mechanical abrasion of the surface by using oxide particles (mostly sio
2
,
zro
2
, or al
2
o
3
) with angular shapes that are shot against the implant. The
treatment yields a severe plastic deformation of the surface which causes a
roughness increase, whose magnitude depends on size, shape and the kinetic
energy of the particles reaching the surface. In addition, the treatment leaves
compressive residual stresses with a maximum value close to the surface.
36
This compressive stress state is known to delay crack initiation and/or slow
900
Ti-6Al-4V
Unnotched
notched
800
700
600
500
400
300
200
100
0
Fine grain size
Coarse grain size
6.10
Fatigue strength of unnotched and notched specimens as a
function of microstructure (adapted from Chao and López
34
).
