Fully tensioned bolts

[polevesque]

Is it possible for a fully tensioned 1" A325 bolt to loosen under repetitive loading? It has always been my (and others) impression that it can not, but I am interested in what you guys know about this.

Also, are there any ill effects of tack welding the nut/bolt to eliminate the danger of loosening? My suspicion is that you may cause embrittlement in the bolt that could have a negative effect on fatigue resistance or strength.

 

[boo1]

Welding is not allowed on high strength, medium carbon or alloy material which undergoes a heat treating process to develop the strength properties necessary to meet the requirements of a given specification.

 

[connectegr]

I recently attended a Geoff Kulak lecture on bolting issues. Based on his presentation and slides, I would say that the pretension can be released if the shear forces exceed the slip critical value of the connection. He stated that the shear strength of the bolts is not effected by pre-tension. He showed lab results of snug-tight and slip critical connections that created deformation of the bolts under extreme loading. But, Dr. Kulak also stated that many safety factors are in place to prevent overloading of the bolts.

Do not welded quenched and tempered bolts (A325 or A490). These heat can create brittle fractures of the bolts.

Under fatigue loading additional safety factors are applied to avoid slip of the faying surfaces.

http://www.FerrellEngineering.com

 

[HgTX]

There are a few other threads around here about welding structural fasteners (or, rather, not welding them).

The same force holding your slip-critical connection together (the tension on the bolt and thus the friction between the faying surfaces) is also holding the nut and bolt head against the outer surfaces of the plates. It's not going to just up and turn under normal repetitive loading.

If you exceed the slip capacity, you are now in bearing. If you have a thick filler between the main loadbearing plates, you'll get bending of the bolts. But there's still nothing there that would cause the plies of your connection to squish together and away from the nut or the bolt head.

Hg

Eng-Tips policies: faq731-376

 

[rb1957]

i would expect preload to relax over time (for the bolt to creep) no matter what you do to retain it. welding nuts just sounds bad, jam nuts would be much better IMHO

 

[paddingtongreen]

A325 and A490 bolts don't have enough thread for jam nuts. They do not unwind unless it is done with a wrench.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[boo1]

Fastener loosing can occure from side sliding of the nut or bolt head relative to the joint, resulting in relative motion occurring in the threads, thermal effects or high vibrational loading.

To prevent loosing provide a sufficient clamp force at the joint interface, to prevent movement between the bolt head or nut and the joint. Thread locking compounds (Loctite) have been shown effective in preventing loosening. If thread locking compound is used reduce lubricated torque loading (I have seen a common 20% reduction).

 

[connectegr]

Release of bolt pretension is due to yielding of the material. Not reversal of the nut. If repetitive loading or load reversal is a concern, then specify slip critical connections and adjust design loads to account for fatigue. Yielding of the bolt is due to bearing or applied tension on the bolts. Adhesive nut locking or damaging threads will not prevent yielding. Installation of structural bolts (A325 or A490) must conform to RCSC provisions.

http://www.FerrellEngineering.com

 

[Ron]

Yes. It can happen. Connectegr has it right. It is a yielding phenomenon, mostly due to creep from repetive loading.

A study was done of the high strength bolts in one of the rides at a "major theme park" near Orlando. Ultrasonic length measurements were taken before and after....elongation occurred with repetive loading.

Repeating what others have said...don't weld.

 

[boo1]

Permanent deformation will occur when the elastic limit (yield strength) is exceeded. It is the lowest stress at which permanent deformation can be measured. Most metals have a linear stress-strain relationship up to the yield point.

http://www.eduresourcecollection.com/civil_sm_Elasticity.php

Creep deformation can be from both the clamping force of the bolt and the service-load and coatings.

http://appliedbolting.com/pdf/DTIs_mean_less_creep.pdf

http://www.boltcouncil.org/files/2004RCSCSpecification.pdf