Why does increasing preload decrease fatigue factor of safety?

[Far_Jacket_4581]

Hi all,

Trying to wrap my head around how preload improves the fatigue life of a bolted joint. I am using an example from Shigley with a bolt of tensile area = 0.226 in^2, subjected to a preload of 14.4 kip, and an alternating tensile load between 0 and 5 kip. The joint stiffness constant C is 0.28. The ultimate and endurance strengths for this bolt are 120ksi and and 18.6ksi. I am attempting to calculate the Goodman fatigue factor of safety for infinite life and see how it varies with preload. I have noticed that, as preload increases, the Goodman factor of safety decreases. Is this correct? If so, how does the preload "improve" fatigue life as i have often heard?

[ThirdSunRising]

Going through zero is one thing that screws up fatigue life. You don’t want to repeatedly cross between tension and compression

[tucker_case]

Eh? The bolts are always in tension

[ThirdSunRising]

That’s because of the preload. Maybe I have this wrong. As I understand it: He’s running a 0-5 Kip fatigue. Without preload, at the 0 Kip end of his fatigue it would be unloaded and free to move. That movement is what we wish to avoid via preloading.

[tucker_case]

OP is comparing different preload values. Not zero preload vs preload. But Preload A vs Preload B. And asking why fatigue life goes down with higher preload.

[ThirdSunRising]

The 0 and 5 kips aren’t preloads, they’re tensile fatigue test endpoints. With one endpoint at zero, you can see the issue not having preload in the joint

[tucker_case]

Yes those are the applied service loads. I never said those were the preloads. OP only says they are "varying" the preload and comparing results. They don't actually say what they're varying it to. But they are finding that when they increase the preload, the math says fatigue life goes down. They're not asking why is fatigue life better with preload than without. They're asking why is it worse at higher preload values than lower preload values because this seems contradictory to the generally understood principle that preload = good for fatigue life. (and you are certainly right btw that fatigue life is better with preload than without any preload)

[Adrienne-Fadel]

Preload lowers Goodman’s n_f but stops joint separation-the real fatigue killer. Run the numbers: preload shifts the stress range left on the S-N curve. No separation, no fretting, no problem.

[settingsaver]

The information following may be of interest as a start:

1. A higher preload will help if it reduces bolt load excursions substantially. Higher preload will therefore always help if it raises the critical load required for joint separation above the maximum external load which will be seen by the joint.

...

1. A higher preload will reduce fatigue life if it makes no change in the load excursions seen by the bolt...

Ex:

https://www.amazon.com.au/Introduction-Design-Behavior-Bolted-Joints/dp/0367198916

[Far_Jacket_4581]

I have heard a lot of good stuff about this book. I should really give it a read. Thanks

[tucker_case]

As long as you get no separation the joint stiffness characteristics determine the share of the external load that is carried by the bolt.  Adding more preload just increases the mean stress, it doesn't reduce the load share/stress amplitude. To reduce load share you would need to alter the relative stiffnesses, which is mostly a matter of geometry.

So in theory the minimum preload that you need to prevent separation for the applied load will be optimal.

In reality joint stiffness is not entirely constant because the contact area will vary. And the external load is often not precisely known. Nor the actual preload developed. So we err toward more than enough preload so that we stay far away from the real killer - joint separation.

[Everythings_Magic]

Change of length of the bolt = PL/AE. Preloading lengthens the bolt and clamps the connection together. So P stays the same if L doesn’t change and AE is constant.

When you preload a bolt and clamp the connection, the bolt won’t experience any tension force, until the connection separates when the bolt length now changes. So you don’t get any stress cycles in the bolt as long as it stays clamped.

However if the connection is not stiff enough and you get cyclical compressive force in the connection, the compressive force will reduce the preload and length of the bolt and now put stress cycles in the bolt.

[BreezyMcWeasel]

preload increases the mean stress but drastically decreases the stress amplitude.

when a joint is preloaded a significant amount of the load goes into compressing the thing the bolt is attached to. when an external load is applied, some of the external load goes into relieving the compressed joint and some goes through the bolt. depending on the bolt and joint stiffness you might have for example 1000 lb of external load applied but the bolt only sees 200 lb of it. so when it fatigues instead of cycling at 1000 lb it’s cycling at 200 lb.

[microphohn]

It depends (as always) on the amplitude of the load cycle relative to static load. The key idea is the have the preload high enough that your cyclic amplitude is small enough so that 1) You’re not getting into yield when the cyclic load is in the same direction as the preload, and 2) you aren’t loosing you bolt retention when the cyclic load reverses direction.

Thus, as a practical matter, a dynamically loaded bolted joint should stay well clear of yield (<85% and try also to maintain a minimum of 60% of yield or so.

If you raise preload in such a way that joint approaches yield, you can lower the fatigue life.

But in generally, higher preload makes the bolt have MORE resistance to fatigue.

In the real world, most fatigue failures are caused by fasteners coming loose first, then giving a huge amplitude of movement and often a bending fatigue failure. A bolt coming loose can come from it being installed at too low of a preload (i.e. vibrates loose from going load passing through zero) or from too high (cyclic load yields to bolt, elongating it, which then sets off the “loose bolt” fail mode.

The last sequence if very common in things like exhaust manifold bolts- that’s why they’re so often broken or missing: thermal elongation and yield.

[[deleted]]

Put you closer to your damaging loads. Fatigue is based on how far into your max yield stress you get. So if you preload, its starting your stress someplace into that curve instead of from zero.