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Screw fatigue in bending.
2

Screw fatigue in bending.

Screw fatigue in bending.

(OP)
I have a group of M8x30 High Tensile (12.9) cap head socket bolts which I am using in a vibratory system (in a vehicle). I have accelerometer data and want to calculate the fatigue life of the screw(s). I am missing two things:

1/ The S-N curve or similar for the material.

2/ The stress already existing within the screw before the oscillating bending forces are applied. - I can have a reasonable stab at calculating these but proper data would be good.

I seem to remember rolled threads giving a 1.4 stress raiser which also needs confirming.

Please don't point me in the direction of a bit of internet software, I'll never get it past our security and I've never had much success with them in the past anyway.

Anybody got any real data?

RE: Screw fatigue in bending.

If you're lucky your screws are used in a rigid connection.  In such cases, if you can get the preload high enough the screws won't see more than a little varying stress.  If non-rigid, you'll be busy.

RE: Screw fatigue in bending.

(OP)
I heard that said before: essentially that if you clamp it down hard enough there can't be a bending moment and it sorts itself out.

My concern with that was that by clamping it down very hard, the initial stress in the screw is raised further making the screw less able to deal with the vibration forces.

Maybe I'm wrong and this is actually the solution.

RE: Screw fatigue in bending.

Biggadike,

What you want to avoid is alternating the net stress in the bolt from tension to compression.  Think of your vibration load as a sine wave that does alternate from tension to compression with an amplitude of 10 lb (or N if you prefer).  If the bolt is tightened with an initial tension of 200 lb then the net stress in the bolt will vary from a max of 200+10 = 210 lb (divided by the cross sectional area of course) to a minimum of 200-10 = 190 lb (/area) all in tension.  If the bolt alternates between tension and compresion then the fatigue life is dramatically affected.  It is also more likely to loosen, which will only aggravate the situation.

- - -Dennyd, P.E.

RE: Screw fatigue in bending.

Biggadike,

It is true that using high fastener preload means there is less capacity for the fastener to take on additional force transferred from the joint.  However, there is a geometry effect as well.  The bolt doesn't absorb all of the force applied to the joint.  If the joint is designed well, then it will absorb little of the additional load.  Using properly sized fasteners tightened as much as possible is the correct procedure for fatigue-sensitive joints.  I recommend the following for bolted joint education:

FAQ725-600
FAQ725-536
http://www.boltscience.com

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Screw fatigue in bending.

you said your concern is that the clamping shall cause high pretension in the bolts. well, what are the loads (alternating and others) that each bolt sees, and what type of bolts are you using?

I guess you'll find out that the loads are rather small compared with the bolts yield point, and in that case you can preload them to a satisfying value (and that value is a function of the type of bolt you are using)

cheers,

guy

RE: Screw fatigue in bending.

Simply stated:

"If a screw or bolt is preloaded (tightened) beyond the working load encountered, changing stresses will not affect it"

Elton McBroom: Sports Car Magazine; January 1964

RE: Screw fatigue in bending.

(OP)
OK,

The problem I have with all this is the following:

Fatigue failure happens below yield stresses and it is the tensile forces which propagate the cracks. If the normal pre-load is 90% of the yield stress then fluctuating tensile stresses on top of that are certain to cause a fatigue failure at some number (N) of load cycles, especially as the thread profile gives a locally raised stress of 1.4 times nominal. This takes the root of the thread past yield point.

This screw is used in pairs to clamp 4 stainless steel mounting blocks to a stainless steel plate. The block is only 14.5mm thick, the mounting point is 37mm from the clamping face and is subjected to an extreme sequence of vibrations in a six hour test which equate to 25 years of sitting inside a military vehicle.

This being the case, I hope you can see why I'd like to see a fatigue performance curve for the screw material.

That said, the info posted so far has given me enough to get started and I'm now in the process of calculating the levels of stress the screw would see. So THANKS for that..

RE: Screw fatigue in bending.

Dennyd,
Yes, resistance to fatigue cracking depends largely on avoiding varying loads, esp. avoiding tension-comp-tension, but the applied loads are not added and subtracted per your example if the joint is rigid and the preload is high enough.

RE: Screw fatigue in bending.

(OP)
I've been working through the calculations and (because I keep having to do other things) I'm about half way through. I have established that the root of the thread is in a yield state which means that even in cyclic tension the root of the thread will work harden and crack.

I'm using an empirical equation based on Basquin's equation which equates the number of cycles to failure with the change in strain per cycle.

We shall see what it predicts.

RE: Screw fatigue in bending.

>"I have established that the root of the thread is in a yield state which means that even in cyclic tension the root of the thread will work harden and crack."<

Your conclusion may be wrong.  Many items are loaded beyond the "yield strength" and do NOT develop fatigue cracks.  Many others do develop cracks and the stresses are well below the YS and the PL.

RE: Screw fatigue in bending.

(OP)
Mmm, Maybe,

But I'm struggling to see how a material which work hardens can do anything but just that when cyclicly stressed in the yield state. If it does work harden, it must logically become brittle and crack. This would reduce the cross section and the process would repeat and increase in speed.

I know material science is tough to generalise but what am I missing in this case?

You are certainly right that you don't need to hit the Yp to get fatigue cracks, but having hit the Yp, cracks ahoy!

RE: Screw fatigue in bending.

Hmm, back to the basics.  Forget about trying to relate YS with fatigue cracking.  YS *does* relate fairly well to SCC and HE/HIC, but not fatigue.

Also, try to read the books by Bickford on bolting.  You will find that many structural bolts are intentionally tightened past their YS w/o problems.

RE: Screw fatigue in bending.

(OP)
I'm still not convinced. I'm not sure which basics you mean but my statement still stands - I have a case study sitting next to me where what I'm describing lead to failure so I'm not going to be so easily deterred. Also, Stress corrosion cracking (SCC as you say) is primarily related to tensile stress (often through process stresses) and corrosion at grain boundaries in materials which show no overriding alternative corrosion state (i.e. copper, aluminium). I see no relationship between SCC and yield strength except the stresses which working the material may give.

I haven't read Bickford and in the next few hours that really isn't an option but if you'd like to summarise what he says then I'd be interested to hear it.

RE: Screw fatigue in bending.

SCC does have a fairly close relationship with the YS of a material, but your concern is fatigue.  Fatigue crack resistance (resistance to cracking initiation) depends on many things, primarily the R ratio (ratio of max/min stresses).  *Correctly* preloaded fasteners in rigid connections keep the R ratio favorable--there is little change in the stresses on the fastener in service.

RE: Screw fatigue in bending.

agree w metalguy

RE: Screw fatigue in bending.

(OP)
I agreed with him too although my post didn't appear.

I am following those good design principals and my calculations give a correspondingly long life expectancy for my screws. All good stuff.

I am interested though in the idea that moving the screw into the yield area doesn't affect the fatigue life. Is that true? Or is it that experience shows that, when using good design practice, fatigue life is so long that it makes no difference.

RE: Screw fatigue in bending.

Here's another way to look at things.  It is generally assumed that as-welded welds and HAZ's have residual welding stresses approx. equal to the YS.  How many billions of such welds are in service, including varying-stress service?

Forget about the YS and concentrate on the important factors that *do* affect fatigue resistance.

RE: Screw fatigue in bending.

(OP)
Funny you should mention welds...

The reason for me putting these screws in is that the assembly used to be welded. The welds failed by cracking along the HAZ. As welds create homogenous material from assemblies, the crack was able to run through the assembly along the interconnected HAZ lines and one whole side fell off. As a rule, unannealed welds are poor in high fatigue situations regardless of how many there may be in the world.

I think anything which is over-engineered and therefore sees little changes in stress will deal well with fatigue. Stiff welds surrounded by flexible sheet can make use of the sheet as a spring and last longer.

I still say that a work hardening material must become brittle if flexed in the yield zone. It may not happen quickly, but it must happen.

RE: Screw fatigue in bending.

Biggadike,

Best source I have seen lately for axial fatigue strength in threads is the German spec VDI2230. The major consideration is if your threads are rolled before or after heat treat. If rolled after heat treat you get a higher fatigue strength.

For rolled before heat treat bolts 8.8-12.9 use:
  Stress Amplitude = 0.85 x (150/(bolt diameter + 45)

For rolled after heat treat bolt threads use:
  Stress Amplitude = 0.85 x (150/(bolt diameter + 45) x (2 - (mean stress / yield strength))

So, Axial Stress Amplitude = 54MPa for RBHT and ~67MPa for RAHT at a mean stress near 70% of yield.

For a conservative analysis, I recommend using the axial stress numbers. Bending fatigue strength for 12.9 high strength bolts is about 1.1x the axial fatigue strength. For 8.8 and 10.9 bolts I have seen 1.2-1.5x. (I hate to guess wrong, thats why I usually stick with the axial)

Now you need to figure out what stress range is in your bolt. Any of your buddies do FEA? They will want the loading. If you dont want to wait, try VDI2230, John Bickford's books, or search Eng-Tips for some good formulas and considerations for estimating the cyclic stress in the bolt.

-Its late - all numbers provided by me after 10pm should be checked, verified and rechecked...

Jeff

RE: Screw fatigue in bending.

>"I think anything which is over-engineered and therefore sees little changes in stress will deal well with fatigue."<

Good design (rigid bolted connections) and correctly installed fasteners are certainly not "over-engineered".  If you have  vibrating/shaking conditions, it is most important to use the above or use parts that can withstand whatever fatigue conditions you have, etc.

While little is known here about your particular conditions, so far it appears to me that "ordinary" good eng. practices would prevent failures.

RE: Screw fatigue in bending.

(OP)
Thanks Jeff,

That looks useful. I do know the stress profile across the core of the screw. I don't know whether the thread is rolled before or after heat treat but I can always assume worst case. The design is such to ensure the screw only sees axial stresses so no problems there. When I have calculated the stress amplitude as per your equations, is that the maximum the screw can see before fatigue becomes a problem?

Metalguy,
Whilst you are almost certainly right, I need to quantify the life that this design will give. I could just say that I've followed good design practices to avoid fatigue problems so all should be well. The thing is, this is our last shot at this so putting figures to the design is called for.

RE: Screw fatigue in bending.

Biggadike,

The stress amplitude calculated is for the endurance limit. Below the calculated stress amplitude, no failures should occur. You can run at higher stresses as long as you keep the number of cylces down. You can draw a SN curve on a log-log plot with the calculated endurance limit at 1,000,000 cycles and  the ultimate tensile strength at 1,000 cycles and check your cycles to failure at a known stress.

Have fun with the Dynamics Jeff

RE: Screw fatigue in bending.

(OP)
Thanks again, I'll check that now.

Chris

RE: Screw fatigue in bending.

If you will checkout the Website of my last post they have already done this for you.  
It's a database in the middle of the page.

RE: Screw fatigue in bending.

(OP)
Unclesyd,

I can't just whizz off and check web sites out because of security restrictions on access to e-mail sites. I can only see pre-approved sites which means I have to get new sites approved first....

In this case it was quicker just to do the calculations although I will have a look at all the sites reccomended in this thread when I get a chance to process the red tape.

Following Jeff's data, I'm at 1/13.5 the stress amplitude needed to reach the fatigue limit. This corresponds nicely with my earlier life calcs of 1.5x10^11 years.

Most of these results are down to the use of good design principals which have reduced the loads the screw sees.

Looks good.

RE: Screw fatigue in bending.

Heaven forbid the site is the ANSI Organization website.

RE: Screw fatigue in bending.

nashjp,

Do you have a link or copy of German spec VDI2230?

Thanks.

jetmaker

RE: Screw fatigue in bending.

jetmaker,

VDI 2230 is available from the German publishing company Beuth at:

http://www.beuth.de

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Screw fatigue in bending.

(OP)
From our source VDI2230-1 was £269.77. (Index house - Berks).

Unclesyd, - aren't the ANSI Organisation part of the axis of evil?

RE: Screw fatigue in bending.

Just a thought, if you have access to a lab could you not test several your assembly on a shaker table at both a low cycle fatigue until failure and then at a high cycle fatigue. From this you could then create your own SN curve for the assembly using some statistical equations.

BJP

RE: Screw fatigue in bending.

(OP)
BJP,

We have our own 'shaker' (I think of it as a giant loud-speaker) but as is always the way, only have time and resource to test the final complete assembly, hence the need for figures.

Given how many times people have generated the S-N curves for HT bolts, it suprising how elusive the figures are. There's probably a massive book somewhere we could buy for £200.

Never mind, we seem to have cobbled together enough data to be confident.

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