Do different frame materials "wear out"?
From: John Unger
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 cobblestone street. (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.
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