Does fatigue occur in non-preloaded bolts under load range which is compressive only? 1

[siberiankhatru]

Bolts are seldom under compressive load, but what if we had an situation, where a non-preloaded bolt was under varying compressive load (load ratio R>1, i.e the loading is fully compressive).

For example, the situation could be a column base connection without grout where the anchor bolts and nuts (and baseplate) carry the compressive load from the column.

Details and FAT classes are presented in the design standards Eurocode 1993-1-9 and DNVGL RP C203 but the details are meant for bolts which are mainly or somewhat under tension. How much better FAT class would a non-preloaded bolt have if the load range was fully compressive?

 

[BridgeSmith]

Failure due to fatigue is a fracturing of the metal. No tension = no fracture = no failure due to fatigue = no fatigue limit.

 

[Agent666]

Just be aware with no grout you may have bending in the bolts under shear which may be enough to cause tension in parts of the bolt if you overcome any compressive loads.

 

[Tmoose]

Leveling nuts under the base plate ?

Like this, but without the grout?

https://ascelibrary.org/cms/attachment/cbabef68-9bc7-477e-b5cf-ae3071406886/figure1.jpg

https://laurelleaffarm.com/item-pho...uloid-Laurel-Leaf-Farm-item-no-nt716235-2.jpg

As others suggested, it seems like bending loads are a real possibility.

Even if in this pie-sky example, the column is laterally restrained by other means so bending is not allowed, Unless the top nuts are tightened to provide clamp loads >> than the maximum vertical load, then I think the nut and bolt threads will be subjected to varying bending loads, and fatigue is a possibility.

 

[siberiankhatru]

The mentioned example of the connection is just an example. There are no bending or tension loads, only compression load so the load range is purely on the compressive side.

The main question is (everything else is irrelevant), do bolts experience fatigue under fully compressed varying load and if they do, how much better their fatigue class is? (under varying tension, bolt's FAT class is 50MPa in the Eurocode)

 

[BridgeSmith]

The bottom line is, whether bolts can be said to "experience fatigue" under strictly compression or not, steel cannot fail due to fatigue while in compression. The fatigue failure mechanism is progressive fracture. The range of stress for checking fatigue resistance can include compression (referred to in the AASHTO spec as "reversal"), but the steel must experience tension somewhere in the range in order to fail due to fatigue.

 

[BrianE22]

There's probably tension somewhere in the bolt - poisson if nothing else.

 

[Ron]

If the bolt experiences cyclic loading, even zero tension and high compression, fatigue can be an issue. Check the fatigue limit curves for the material involved.


A Great Place For Engineers to Help Engineers

Follow me there.....

 

[BridgeSmith]

Ron, I don't understand how fatigue can be an issue without tension. As I said in my previous post, as far as I understand it, fatigue is only an issue if it results in fracture. I don't see how it's possible to have fracture without tension.

 

[Agent666]

Hotrod, for what it's worth our local steel standard NZS3404 states "Compressive stress ranges should be considered to be as damaging as tensile stress ranges unless it can be shown to be otherwise".

Keep in mind also that shear also causes fatigue, often you'd need to look at a combination of the axial and shear stresses (NZS3404 uses a rule called miners rule to look at combined stresses).

 

[BridgeSmith]

Agent, I'd be curious as to what that statement means. AASHTO has fatigue stress range limits exclusively for tension and reversal, with no fatigue limits for components strictly in compression. There are fatigue limits for shear also, of course, but that was not the OP's question. I don't believe AASHTO has combined stress limits for fatigue, either. Maybe buried in the sign spec, but I haven't found them.

 

[Agent666]

Interestingly in AISC appendix 3 it explicitly notes in contrast that you also only need to take tensile loads into account.

But if you consult Google there seems to be a lot of research indicating compressive fatigue in steel is a real effect.

 

[chez311]

Even if in this pie-sky example, the column is laterally restrained by other means so bending is not allowed, Unless the top nuts are tightened to provide clamp loads >> than the maximum vertical load, then I think the nut and bolt threads will be subjected to varying bending loads, and fatigue is a possibility.

I'm thinking along the same lines - I would imagine that it would be extremely difficult to ensure all loads were perfectly along the axis of the bolt, there would almost certainly be some out of plane loading as some scale.

If we follow your example though, in addition to the bending that will almost certainly occur in the threads, I would think the nut and head itself would experience some as well. Also depending on the length of the bolt buckling will come into play.

 

[BridgeSmith]

There are a myriad of ways bolts can see tension, and a few situations where they won't. The OP asked about whether fatigue is an issue in the situation where the axial compression is large enough that the bolts never experience net tension. I contend fatigue not an issue in that situation, because in order for there to be failure (fracture) due to fatigue, there must be net tension in the bolt, either due to bending or shear. My reading of the AASHTO bridge design spec would tend to support my contention.

I think we would all be in agreement that if a bolt (or any other steel part) experiences cyclic stresses that include net tension, fracture due to fatigue is something that must be considered.

 

[chez311]

There are a myriad of ways bolts can see tension, and a few situations where they won't

Besides a theoretical hydrostatic compression (or maybe a bolt sitting at the bottom of the ocean with no other forces acting on it) what situations can you imagine where all the parts of a nut/bolt would be under *pure compression?

I'm also not sure how the forces being "large enough" plays into it.

*Edited

 

[BridgeSmith]

I didn't say "pure compression", I said no net tension. I'm not sure why you think the bolt needs to be at the bottom of the ocean to accomplish that. The compressive force has to be large enough to produce a net compressive stress on all areas of the bolt cross section. If bending produces 10 ksi tension at the edge of the bolt (and 10 ksi compression at the opposite face), but the axial load is large enough to produce 20 ksi compression stress across the entire cross section of the bolt, then anywhere in the cross section there is at least 10 ksi compression stress, hence no net tension in the bolt.

Edit: I guess I should clarify that I'm referring to axial tension and compression in the bolt. Of course there's always tension perpendicular to any compression stress, and compression perpendicular to any tension stress (yes, I took Mechanics of Materials in college, too). In the practical sense of failure due to fatigue, tension perpendicular to the axial compression is irrelevant because it doesn't result in fracture of the bolt.

 

[chez311]

Okay, well thats a fair assessment if we're talking about only the shank/body of the bolt, or a column which has the load applied from either end.

If we're talking about a threaded fastener I'm not sure how such a condition would exist in the threads as well. It seems to me the threads would always be in bending/net tension unless you had an interference fit thread - even then this could be overcome depending on the magnitude of the compressive force applied along the axis of the bolt. I'm not sure how much interference would be necessary - but I'd imagine that for most high load applications it would be enough that it would not be practical to assemble.

 

[Agent666]

From what I've (admittedly briefly) read about, the same atomic level dislocations occur under cyclic compression, and will eventually lead to fatigue cracks similar to cyclic tension or cyclic tension/compression. However it does take more cycles to get to the same failure state than cyclic tensile loads. Not sure of the underlying reasons for this, perhaps a mechanical/materials engineer can comment.

Why some codes ignore this effect I have no idea?

 

[BridgeSmith]

Agent666, "atomic dislocations" are not what causes fatigue failure. When there is sufficient tension at a cross section to initiate a crack, the stresses at the root of the crack are higher (I think it's what's referred to as a stress riser), so with each subsequent cycle where the tension stress is high enough, the crack deepens until the cross section is too small for the tension load and it fractures completely.

It's simple mechanics, people! Steel doesn't crack under compression stress. Cracking or fracture is a tension failure.

 

[BridgeSmith]

From Commentary in the AASHTO Bridge design spec (C6.6.1.2.1):

"Tensile stresses propagate fatigue cracks. Material subjected to a cyclical loading at or near an initial flaw will be subject to a fully effective stress cycle in tension, even in cases of stress reversal, because the superposition of the tensile residual stress elevates the entire cycle into the tensile stress region. These provisions shall be applied only to details subjected to a net applied tensile stress... Fatigue design criteria need only be considered for components or details subject to effective stress cycles in tension and/or stress reversal...If the tensile component of the stress range does not exceed the compressive stress due to the unfactored permanent loads there is no net tensile stress. In this case, the stress cycle is compression—compression and a fatigue crack will not propogate beyond a heat-affected zone."

Seems fairly straightforward to me.