Do bolts get weaker if loosened and tighten often?

[Al]

Torque is a very inexact way of determining bolt stretch. Based on material composition; if you know the exact "stretch" (which you can't know.) three mechanisms come into play (at least): "creep", metal fatigue, and location of stresss on the stress/strain curve. (you can get on the internet to learn about these items.)

Readers digest version: replace the fastener with a better grade..you don't have the ability to determine the nature of the fracture.

 

[doitmyself]

Originally Posted By: Al
Torque is a very inexact way of determining bolt stretch.


I'm glad you brought that up. Everyone should study some sites about the science of torque, proper torque wrench use, and the limitations of torque wrenches. Numerous engineering sites agree that using a torque wrench has about a +/- 25% range in the final result due to so many variables (user error, fastener condition, lubrication, etc.).

http://www.surebolt.com/torque_errors.htm

I always liked the Sheldon Brown wrench:

 

[Shannow]

Just want to correct a flaw in some of the statements.
Fatigue can occur inside the elastic limits of a material...just needs cyclic stresses of sufficient magnitude...doesn't have to yield.

 

[Merkava_4]

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?


Is it an Allen head bolt? I would've tightened it with one of my Eklind T-handles. The T-handles will flex telling me the torque is just right.

 

[Linctex]

Originally Posted By: Trav
Try a higher grade bolt to replace it with but don't increase the torque or use grease or never seize.


If it's a steel bolt threaded into aluminum.... ALWAYS use grease or antiseize!!

 

[Linctex]

Originally Posted By: Shannow
Just want to correct a flaw in some of the statements.
Fatigue can occur inside the elastic limits of a material...just needs cyclic stresses of sufficient magnitude...doesn't have to yield.


Very true, of softer steels and aluminum alloys.

Also, Especially aluminum. That's why aircraft structures are rated in "cycles" (number of landings or pressurizations) - not by hours.

Some chinese steel products have an elastic range similar to aluminum.

 

[Shannow]

Yep,
traditional steels have a "fatigue limit", where if stressed low enough, they have an infinite life...Al, and Ti and the like will always break eventually...maybe a long, long time away given low enough stresses, but they get there.

 

[Silk]

I remember about 20 years ago when wheel studs started stripping and breaking...obviously being over tightened. But at that time I was also maintaining a daily driver 1938 Chev...I'm pretty sure it was on the original wheel studs and nuts. I'm sure in the '40's, '50's, '60's and '70's people were over tightening wheel nuts as much as they do these days, probably even more so.

 

[Wheel]

I've got to guess that Bontrager went with a lighter rated bolt to try and save the aluminum threads in the stem should they be overtightened. I don't think I'd use a stronger bolt for that reason. I would also ask Bontrager what their recommendation is for lube on those bolts, mixed metals and all.