This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: John Unger <firstname.lastname@example.org>
I think that some of the confusion (and heat...) on this subject
arises because people misunderstand the term fatigue and equate it
with some sort of "work hardening" phenomena.
By definition, metal fatigue and subsequent fatique failure are
well-studied phenomena that occur when metal (steel, aluminum,
etc.) is subjected to repeated stresses within the _elastic_ range
of its deformation. Elastic deformation is defined as deformation
that results in no permanent change in shape after the stess is
removed. Example: your forks "flexing" as the bike rolls over a
(an aside... The big difference between steel and aluminum
as a material for bicycles or anything similar is that you
can design the tubes in a steel frame so that they will
NEVER fail in fatigue. On the other hand, no matter how
over-designed an aluminum frame is, it always has some
threshold in fatigue cycles beyond which it will fail.)
This constant flexing of a steel frame that occurs within the
elastic range of deformation must not be confused with the
permanent deformation that happens when the steel is stressed beyond
its elastic limit, (e. g., a bent fork). Repeated permanent
deformation to steel or to any other metal changes its strength
characteristics markedly (try the old "bend a paper clip back and
forth until it breaks" trick).
Because non-destructive bicycle riding almost always limits the
stresses on a frame to the elastic range of deformation, you don't
have to worry about a steel frame "wearing out" over time.
I'm sorry if all of this is old stuff to the majority of this
newsgroup's readers; I just joined a few months ago.
I can understand why Jobst might be weary about discussing this
subject; I can remember talking about it on rides with him 20 years