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Alloy does 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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    Everything ages. I have never seen a crank arm "fail", however -- the reasons for replacement are loose fittings or stripped threads. – Daniel R Hicks Oct 3 '20 at 17:49
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    Depends how light those cranks are. Thin hollow pieces or a solid piece of good alloy? Climbing carabiners and similar PPE do not have any lifetime specified. They are safe for decades unless some problem is spotted - mostly due to abrasive wear which can then lead to cracking of the thinned material. – Vladimir F Oct 3 '20 at 19:12
  • @VladimirF 30+ years cranks are most likely solid. Correct me if I'm wrong but as far as I understood it Shimanos hollow crank arms are a bonded two piece design, so the bond itself is more of a concern when it comes to aging. – user430 Oct 3 '20 at 19:20
  • @user430 Only Ultegra, Dura-Ace, and the XC variant of XTR use the glued construction. The rest are basically still a pipe with each end folded shut, no glue involved. – MaplePanda Oct 3 '20 at 23:06
  • @MaplePanda Interesting, didn't know that. Does that mean the lower grade hollow cranks have the more durable design, bc there is no epoxy involved? – user430 Oct 4 '20 at 14:51
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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.

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    Thank you for your answer. Appenrently in my question I mixed up the fact that alloy has no fatigue limit but steel has, thanks for clearing this up. – user430 Oct 4 '20 at 14:55
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    It's heavy, strong, low-cadence riders that have the greatest chance of breaking cranks. The max. achieveable load grows with the weight of the rider. – cmaster - reinstate monica Oct 4 '20 at 20:23
  • The double-negative in the first sentence overly complicates the meaning. – user33335 Oct 5 '20 at 12:56
  • With respect, if there's a double negative there I'm just not seeing it. Doesn't mean you're wrong. – Nathan Knutson Oct 5 '20 at 17:29
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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.

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  • great additional information, thank you. – user430 Oct 4 '20 at 14:53

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