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The diagram
shows the form of an S-N curve (a), and the form of the distribution of
cycles to failure at a given load. In designing for fatigue resistance, the
lowest life at a given stress state is the safest guide. The fatigue cracks
in this type of experiment are observed to start at the sample surface, the
region of maximum tensile stress in each cycle.
Sample surface condition
is very important in determining the fatigue life, and surface flaws
such as corrosion pits or mechanical damage reduce the fatigue life by increasing
the local stress in that region of the surface. When selecting a material
for spoke use, it may be better to chose one that has a lower S-N curve but
is more corrosion resistant. In this way the fatigue life will be more predictable
as failures due to local corrosion cracks will be removed. Stainless
steel spokes are an example of this type of choice. |
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