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The diagram
shown opposite compares S-N curves for two spoke materials, carbon steel
and stainless steel. The surface condition of each material used in the test
was smooth and corrosion free. In extended use on a bike the carbon steel
is subject to corrosion and this makes it less desirable for long term use,
the corrosion pits acting as stress concentrators that decrease the fatigue
resistance of the alloy.
When selecting a metal alloy for use in a bike
frame or other light structure aluminum alloys look interesting candidates.
The lower S-N curves show the difference in the behavior of this type of
alloy and a carbon steel. The carbon steel shows a "fatigue limit"
below which its fatigue life tends to infinity. An aluminum alloy shows no fatigue
limit and exhibits a steadily decreasing fatigue life as S increases. |
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