The Basics Of Metal Fatigue - Flash FLASHV1 User Manual

of the common bicycle frame building materials steel is the
most ductile, titanium less ductile, followed by aluminum.
Metals vary in density. Density is weight per unit of material.
Steel weighs 7.8 grams/cm3 (grams per cubic centimeter),
titanium 4.5 grams/cm3, aluminum 2.75 grams/cm3. Con
trast these numbers with carbon fiber composite at 1.45
grams/Cm3. Metals are subject to fatigue. With enough
cycles of use, at high enough loads, metals will eventually
develop cracks that lead to failure. It is very important that
you read The basics of metal fatigue below.
Let's say you hit a curb, ditch, rock, car, another cyclist or
other object. At any speed above a fast walk, your body will
continue to move forward, momentum carrying you over
the front of the bike. You cannot and will not stay on the bike,
and what happens to the frame, fork and other components
is irrelevant to what happens to your body.
What should you expect from your metal frame? It depends
on many complex factors, which is why we tell you that
crashworthiness cannot be a design criteria. With that im
portant note, we can tell you that if the impact is hard
enough the fork or frame may be bent or buckled. On a steel
bike, the steel fork may be severely bent and the frame
undamaged. Aluminum is less ductile than steel, but you can
expect the fork and frame to be bent or buckled. Hit harder
and the top tube may be broken in tension and the down
tube buckled. Hit harder and the top tube may be broken,
the down tube buckled and broken, leaving the head tub
and fork separated from the main triangle.
When a metal bike crashes, you will usually see some ev
dence of this ductility in bent, buckled or folded metal. It is
now common for the main frame to be made of metal and
the fork of carbon fiber. See Section B, Understanding com
posites below. The relative ductility of metals and the lack of
ductility of carbon fiber means that in a crash scenario you
can expect some bending or bucking in the metal but none
in the carbon. Below some load the carbon fork may be
intact even though the frame is damaged. Above some load
the carbon fork will be completely broken.

The basics of metal fatigue

Common sense tells us that nothing that is used lasts forever.
The more you use something, and the harder you use it, and
the worse the conditions you use it in, the shorter its life.
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