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

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

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

(OP)
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?

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

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

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

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.

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

Leveling nuts under the base plate ?

Like this, but without the grout?
https://ascelibrary.org/cms/attachment/cbabef68-9b...

https://laurelleaffarm.com/item-photos/vintage-van...

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.

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

(OP)
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)

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

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.

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

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

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

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

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

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.

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

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).

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

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.

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

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.

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

Quote (Tmoose, 29 Aug 19 21:21)

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.

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

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.

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

Quote (HotRod10, 4 Sep 19 14:36)

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

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

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.

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

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.

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

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?

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

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.

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

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.

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

Quote (Hotrod)

"atomic dislocations" are not what causes fatigue failure

Not sure I follow/agree though on this particular point, atomic level dislocations are the cause of the initial crack initiation. Wikipedia

Surface of material dislocates (intrusions/extrusions) on a slip plane, results in stress concentrations, material strain hardens, material eventually fractures on a microscopic level initiating a crack, crack propagates under further loading, eventually leads to fatigue failure.

Don't get me wrong, intuitively tension makes sense, but its not backed by some of the research that's out there.

I'm guessing at an explanation here but given you'll get the same dislocations and stress concentrations under compression, and same strain hardening. If you consider some steel compressed and yield it locally in areas of these stress concentrations, if you release it there might be some sort of 'effective' tension state created as the material around it that is still elastic springs back, cycle the load enough and the crack might propagate.

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

Out of interest I note Eurocode has rules for dealing with fatigue and compression loading for non-welded and stress relieved details. If I'm interpreting it correctly, it allows you to effectively reduces the part of the stress range in compression to presumably allow for the improved fatigue life when all or part of the total stress range is in compression.





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

I'm not familiar with the Eurocode, so I won't comment on that, beyond pointing out that the figure posted shows a stress range that includes tension (stress reversal), not one strictly in compression, so I don't find that in itself to be convincing. What I quoted was from the 8th Edition of the AASHTO LRFD code published last year. AASHTO usually keeps up to date with and incorporates recent research findings, so either what you were reading is very recent, or the eggheads who propose changes to the code provisions didn't find it relevant or didn't find it convincing.

I'm not saying it isn't possible for some cracking to occur due to strain hardening in compression followed by stress relief in the component. I'm just having a difficult time envisioning how the crack would continue to propagate to failure. I also don't see how any portion of a component limited to the yield stress for design reaches strain hardening for cyclic loading.

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

Just to clarify the point you are reading into the Eurocode provision, the reduction for compressive stresses applies specifically if part or all of the stress range is compressive, its not reserved to stress reversal cases that also go into tension.

Quote (hotrod)

I also don't see how any portion of a component limited to the yield stress for design reaches strain hardening for cyclic loading.
You could say exactly the same for tension loading though, its minute stress raisers at dislocations initiating the cracks, often at much lower average stresses than yield. Depending on the detail that is why there are all these different detailing categories for fatigue, some details are worst at concentrating the stresses to a point where they are an issue and you are only evaluating against average stress in many cases (no idea if AASHTO is based on a similar method or not as Eurocode or NZS3404 as I'm simply 100% unfamiliar with it). But just giving an opposing stance from several other standards in how compressive loads are treated for actual design for fatigue.

For all I know because these standards are older than AASHTO, the current best practice knowledge may be reflected in the latest version of AASHTO (i.e. ignore compressive loads). But I would imagine it's always been that way possibly?

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

AASHTO has not included any fatigue stress limits for components not subjected to tension from before I started in bridge design, so for at least the last 20 years.

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

(OP)
A closer look at the recent DNV-RP-C203 -publication shows that fully compressed stress range with no residual stresses does not lead to fatigue:





If the thread of the bolt is rolled, there exists residual stresses and the DNV reduction factor is not zero. If the compression load is big enough, there exists local yielding in the thread, which lead to residual (tensile?) stresses and the DNV reduction factor is not zero. Thus, the compressive load range should be 60% effective.

So, according to the Eurocode and DNV, a fully compressed bolt has an effective stress range which is only 60% of a fully tensile case. I didn't find any fatigue test data of this kind of case by Googling, so I think, I will settle with this result.

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

Let's get to the basics. No matter the bolt is axial tension or axial compression, the material will eventually fail by shear due to fatigue. Steel is a ductile material. Under repeated cyclic loading, the crystals will slip in the shear plane in a microscope scale, and eventually fail on the shear plane. When the bolt is in "pure" axial compression, it has a maximum shear stress in the plane 45 degree to the axis of the bolt. Therefore fatigue failure can still take place at the plane. However, the presence of compressive stress tends to squeeze the slip plane, increasing friction of the slippage, effective increasing fatigue life as opposed to tensile stress. It is hard to explain. I found the website fatigue-life.com is helpful in understanding the basics. But in summary, bolt in "pure" compression does fail in fatigue, but not as quickly as bolt in tension. The codes and standards address this difference by "mean stress factor" as pointed out in the posts above.

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