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.