have not have a fatigue limit. Does that mean that forged alloy crank arms have a theoretical end of life? Should I worry about 40 year old crank arms?
Could someone with a little knowledge in metallurgy shine some light on this issue?
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Aluminum has no fatigue limit and thus it is impossible to make an aluminum bike part that won't fail with enough use.
Fatigue accumulates with load, not time.
Good forged cranks with designs that avoid stress risers in the spider area tend to be pretty good at resisting fatigue failures more or less indefinitely in practice. Weight weenie designs and bad spider transitions do cause exceptions to this. Some riders are also just plain good at breaking cranks, i.e. by being strong and favoring a low cadence.
Heavily used 40 year old aluminum do cause you to make a decision about the risk. If they're lightly used, there is no cause for concern.
The usual way that old aluminum machine elements are "proven safe" is by nondestructive testing at specified intervals, to detect fatigue cracking before it causes the part to fail. The simplest method for looking for cracks in aluminum parts is the dye penetrant test; dye penetrant test kits are commercially available. One kit is good for a large number of tests.