Cannondale Chase Series Owner's Manual page 66

When all metal bikes are crashed you will usually
see some evidence 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
the composites 0 section 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.
metal fatigue 101
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.
Fatigue is the term used to describe accumulated
damage to a part caused by repeated loading. To
cause fatigue damage, the load the part receives
must be great enough. A crude, often-used
example is bending a paper clip back and forth
(repeated loading) until it breaks. This simple
definition will help you understand that fatigue
has nothing to do with time or age. A bicycle in
a garage does not fatigue. Fatigue happens only
through use.
So what kind of "damage" are we talking about?
On a microscopic level, a crack forms in a highly
stressed area. As the load is repeatedly applied,
the crack grows. At some point the crack becomes
visible to the naked eye. Eventually it becomes so
large that the part is too weak to carry the same
load that, without the crack, it could carry. At that
point there can be a complete and immediate
failure of the part.
One can design a part that is so strong that
fatigue life is nearly infinite. This requires a lot
of material and a lot of weight. Any structure that
must be light and strong will have a finite fatigue
life. Aircraft, race cars, motorcycles: all have parts
with finite fatigue lives. If you wanted a bicycle
with an infinite fatigue life, it would weigh far more
than any bicycle sold today. So we all make a
trade-off: the wonderful, lightweight performance
we want requires that we inspect the structure.
In most cases a fatigue crack is not a defect. It is
a sign that the part has been worn out, a sign the
part has reached the end of its useful life. When
your car tires wear down to the point that the tread
bars are contacting the road, those tires are not
defective. Those tires are worn out and the tread
bar says "time for replacement." When a metal
part shows a fatigue crack, it is worn out. The
crack says "time for replacement."
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