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The photograph
shows a fatigue crack in the frame tube of a bicycle. The crack started
at a tube joint region and then propagated, under the fluctuating stresses
from road shocks, into the tube. In this picture the crack has been made
visible by filling it with chalk dust. An alternative method is to use a
liquid fluorescent dye that will soak into the crack and can be seen (after the
tube surface is wiped clean) when the bike is illuminated with ultra-violet
light. The mechanism of crack propagation in a metal frame tube is the same
as that for the spokes considered above. The crack will grow until a critical
crack length is reached at which point the tube will fail by fast fracture.
For
steel frame tubes there is a 'fatigue limit' - - a stress value
below which the fatigue life of the component is essentially infinite. If the
tubes are of aluminum alloy, the fatigue lifetime increases as the maximum
applied fatigue stress is reduced. The frame tubes are also susceptible
to stress concentration due to corrosion or local surface damage. The tube shape
and wall thickness should be selected so as to reduce the maximum tensile
stresses and thereby increase fatigue life and decrease tube elastic deformation
under load. |
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