Do bolts get weaker if loosened and tighten often?

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So I had a bolt on my bike snap even though I was under-torquing it by about 0.7nm (max is 5.2nm and torque wrench was at 4.5nm). This was one of two stem pinch bolts. I usually tighten gradually since I try to balance the torque across the two opposing bolts. Usually, I have no problems getting them to 5.2nm. But today, one of the bolts snapped at 4.5nm. I often loosen and tighten these bolts to adjust the headset after long mountain bike rides. I'm just wondering if the frequent loosening and tightening might have weakened the bolts to the point that they snapped even at less than max torque.

Thanks
 
This is a metallurgical issue, and I'm going to venture a guess that the bolt was made from bad steel from it's origin.

Fatigue cycle is ZERO on many steels (you can go up to a certain % of its yield strength indefinitely and it will NEVER fatigue)

I don't think it was your fault - - replace with better quality hardware.
 
I suppose there is the rare (VERY rare) chance this was specifically a "torque to yield" style of bolt, then if that is the case it's definitely your fault. But I kind of doubt it.
 
Yes. Torque applied by a fastener is based on stretching of the metal; for critical applications one does not measure bolt torque, but instead bolt stretch.

The materials used in the fastener play a role here ... high strength bolts don't like to re-stretch. Repeatedly re-usimg a fastener will result in bolt failure.
 
Good info. So it sounds like I should just go ahead and replace the bolts every few months if I am going to adjust the headset often.
 
I believe you should look at calibrating your torque wrench first. There is a high possibility that you've been over torquing these bolts without knowing it. Or maybe there is some sort of lubricant or other contamination on the threads that alters the torque reading.

As mentioned before, steel bolts don't fatigue, unless they are torque to yield. Torque to yield takes the material form its elastic range (that's where it always comes back to the original length), to the plastic range (that's where the material stretches beyond its ability to spring back).
Once the bolt goes into the plastic range, the structural changes are irreversible and it is only a matter of time when the bolt fails.
 
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Thanks KrisZ. As for the torque wrench, it's fairly new. I've used it a few times on the handlebar stem bolts and a few other bolts using the correct torque settings. The wrench clicked without tightening the bolts further. That is, most of the other fasteners didn't need additional tightening with this wrench. Do you think this is good enough info to trust the wrench?
 
One more question for you guys. If the bolt fails, is that likely to compromise the threads as well? I'm worried that I may have to replace the stem.
 
Clicker type wrenches will not tell you if a fastener is over torqued. They will click at their setting and that's about it. If there is a little bit of rotation before it clicks, then the fastener is at the set torque, but if the wrench clicks right away, then the fastener could either be at the correct setting or higher.
 
If the new bolt tightens smoothly and solidly into place, then the threads are OK. My brother is a biker and does all his own work. I'll ask him, but I don't think he owns a torque wrench. His road bike is worth more than most of my cars.
 
I wouldn't worry about proactive replacement. If I were in your shoes, I would simply have a spare on hand in case it breaks. This assumes that you use it often and can't wait for replacements to be available.
 
How would you know if the bolt is a "Torque to yield?" The best description I got on my stem bolts is: ``Uses M5 countersunk, corrosion-resistant steel bolts with 5.2Nm torque spec''
 
Originally Posted By: Johnny2Bad
Yes. Torque applied by a fastener is based on stretching of the metal; for critical applications one does not measure bolt torque, but instead bolt stretch.

The materials used in the fastener play a role here ... high strength bolts don't like to re-stretch. Repeatedly re-usimg a fastener will result in bolt failure.
Well the cyclic fatigue of retightening a bolt should be absolutely minimal if the torque applied is a small enough proportion of the bolt's elastic region.

If you're stretching a bolt to the plastic region every time you tighten it down then of course it'll fail quickly after retorquing. But that's only going to be on critical engine components like head bolts.

I have a specific counter point to your last statement.
Many German cars use screw fasteners to attach wheels to their hubs. If these bolts were prone to fail after multiple uses then they'd need to be replaced often. Instead they are of a high enough grade that torquing it repeatedly to remove and reattach wheels does not put them on the path to failure.
 
It's highly unlikely that a header bolt would be a "torque to yield" type.

What I do is install a nut on an over-length bolt, fully seat the bolt, then tighten the nut to secure the header. You could also use two nuts, the outer one secured against the inner one (hold the inner in place with a wrench).

In that way the bolt is never re-installed unless you are removing the header completely.
 
What make is the stem ? Is it possible the bolts are not steel or had some damage on the far side of the threads ?
 
The make is Bontrager (Trek). I think I might have discovered one possibility for over-tightening. I removed the other bolt that did not break and it had grease on it. I suspect that the bike mechanic greased the bolts before tightening them down which could have resulted in over-torquing either by him or by me after the last time he worked on it. Bike mechanics seem to love greasing everything. I don't know how you get accurate torque readings with grease.

In any case, any of the experts in this thread think this might be the explanation?
 
Originally Posted By: KrisZ
As mentioned before, steel bolts don't fatigue, unless they are torque to yield. Torque to yield takes the material form its elastic range (that's where it always comes back to the original length), to the plastic range (that's where the material stretches beyond its ability to spring back).
Once the bolt goes into the plastic range, the structural changes are irreversible and it is only a matter of time when the bolt fails.

That is a very good explanation. If steel is kept in its elastic range it does not fatigue.

There are a couple of other possible contributors. The first is stress corrosion cracking where a corrosive agent (salt water for one) in combination with a repeated stress may lead to a crack which over time grows and extends across the bolt, until it suddenly fails under an otherwise normal load.

The second is that a stress riser, perhaps at a thread or a deep scratch, perhaps at a large flaw in the metal, results in local increased stress sufficient to start a crack which progresses in much the same way.

An expert can tell a lot by looking at the broken end. If it looks like a piece of toffee torn apart it was a plastic failure, if there is a coarse surface it was a brittle failure, if a large flat area with concentric "ripples" a fatigue failure.
 
Originally Posted By: VeeDubb
I think I might have discovered one possibility for over-tightening. I removed the other bolt that did not break and it had grease on it. I suspect that the bike mechanic greased the bolts before tightening them down which could have resulted in over-torquing either by him or by me after the last time he worked on it.

This is a good possibility.
 
Originally Posted By: VeeDubb
The make is Bontrager (Trek). I think I might have discovered one possibility for over-tightening. I removed the other bolt that did not break and it had grease on it. I suspect that the bike mechanic greased the bolts before tightening them down which could have resulted in over-torquing either by him or by me after the last time he worked on it. Bike mechanics seem to love greasing everything. I don't know how you get accurate torque readings with grease.

In any case, any of the experts in this thread think this might be the explanation?


Grease or other thread lubricants require you to reduce torque by up to 40% depending on the compound, 25% is a good number if the compound is unknown. IMO that's probably why the bolt broke. Try a higher grade bolt to replace it with but don't increase the torque or use grease or never seize. The higher grade bolt with tolerate higher torque just in case your TW isn't very accurate at these low numbers.
 
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