Cannondale Bicycle Owner's Manual page 70

PaRt ii
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
101 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. "
fiGURe a
this is a
BENT
metal foRk.
68
this is a
COMPLETELY BROKEN
CaRBon foRk.