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Corrosion
& Environmental Degradation |
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·
The
comparative fatigue behavior of chrome-vanadium steel is shown for various
environments . A joint probably has three million loading-unloading cycles
per year, and for a system life of 10 years, the number of cycles to failure
must be more than 3 x 107.
·
The
condition of the material and its ambient determine the stress level that
the structure can be expected to support for a desired number of cycles.
Maximum normal knee loads may be 5 X body weight. |
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From:
J. Black, "Biological Performance of Materials,"
Marcel
Dekker (1981) |
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·
The
allowed stress level for a given fatigue life is reduced by a factor of
8 in the sea water (~ body fluid) by comparison with the same material
tested in air.
·
A
pre-corroded sample tested in air shows the effects of corrosion induced,
near surface damage. Pit corrosion and local residual stress are important.
· Corrosion
in an electrolyte will increase the fatigue rate through crack growth rate
enhancement due to stress corrosion, crevice corrosion in the propagating
crack, and the effect of corrosion deposits in the newly formed crack. |