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# Fatigue on a threaded bolt

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## Fatigue on a threaded bolt

(OP)
I feel like I'm missing something simple here. I'm doing a simple fatigue analysis on a bolt. The loading scenario is a simple repeated load. The part that I am struggling to figure out is how to consider the bolt preload. I've prescribed a torque spec and can find a nominal stress due to tightening the bolt. When treating this scenario as a fluctuating (never = 0) load, my mean stresses are way too high. I'm putting my numbers against an existing setup we already have and our expected life is something like 2 cycles. Am I supposed to only consider the alternating component of stress?

FWIW, I am looking an A581/A582 bolt and plotting on an S-N diagram. If there is a better method, I am open to feedback.

Thanks!

### RE: Fatigue on a threaded bolt

kDiqq,

If your bolt is sufficiently preloaded, there is no alternating stress, and no metal fatigue.

Are you are adding the external load to your preload? You should not be.

--
JHG

### RE: Fatigue on a threaded bolt

kDiqq
There are plenty of texts around giving explanation as to calculating preloaded bolted connections. I prefer VDI 2230 (Both in German and English). It is always a matter of optimizing the stiffness between bolts and the clamped parts. As stated by drawoh, the higher the preload, the less fluctuating load in the bolt. You should always use bolt materials with high ductility, to prevent crack opening in critical areas.

### RE: Fatigue on a threaded bolt

(OP)
Sorry, the bolt is preloaded (tightened) then the pieces it holds together are subjected to an axial load in tension.

The part that confuses me is that several places people have discussed that as long as the axial load doesn't exceed the bolt preload force, there is no alternating component. But in my head I see the preload force in the same direction as the tensile axial force. Shouldn't those forces be able to 'combine'?

I tried to find VDI 2230 a while back, but my company wouldn't pay the ~\$200 for the text. They said something about hiring an engineer instead :P

### RE: Fatigue on a threaded bolt

kDiqq,

This one has confused me in the past.

Look into your machine design textbook. Carefully draw a free body diagram.

You clamp your bolt down a preload of 1000lb. The force between your bolt head to your part is 1000lb. The force between your part and your base is 1000lb. Apply 500lb tensile force to your part. The force between your part and the base now is 500lb. There is no strain on your bolt beyond what you applied when you pre-loaded it. The bolt's force still is 1000lb. You can alternate your external force between 0 and 500lb, and there is no effect on the bolt, and no fatigue. This is a good thing. The bolt almost certainly is the weak point of your structure.

Apply 1500lb external force to your part. Now, you have exceeded your bolt pre-load. Your part will separate from your base, and your bolt will see the 1500lb external force. You have a force that alternates between 1000 and 1500lb, and you have metal fatigue.

The Handbook of Bolts and Bolted Joints by John H. Bickford and Sayed Nassar, has a chapter on VDI joint design procedures, as well as a chapter on fatigue. If your company does not want to buy it, you can buy it yourself and add it to your library. There is all sorts of useful stuff for you to know, even if the rest of your company does not want to know it.

--
JHG

### RE: Fatigue on a threaded bolt

I may be out of my depth, but here's what I think...

I think in the case of alternating external force between 0 and 500 pounds discussed above, there is some change in tension of bolt but it is far less than 500 pounds, something like (500 lbf) *Kb/(Kb+Kj) where Kb is bolt stiffness and Kj is joint stiffness. In the case of infinite Kj (or Kj>>Kb) there is no change (or no significant change) in tension but for non-infinite Kj I would think there is some change in tension that could contribute to fatigue.

=====================================
(2B)+(2B)' ?

### RE: Fatigue on a threaded bolt

Actually the bolt does see alternating loads. But it is only a fraction of the applied alternating load. The amount that the bolt sees is:

(bolt stiffness)/(parts stiffness) * the applied load.

If the part stiffness is 3x stiffer than the bolt then the bolt sees only 1/3 of the applied load.

### RE: Fatigue on a threaded bolt

Hi

try this website it should explain all:-

http://www.boltscience.com/pages/basics4.htm

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

### RE: Fatigue on a threaded bolt

(OP)

So I threw together a rough model of what I'm looking at. What I'm trying to look at is how much tension 'T' is in the bolt. We have a force due to preload 'P' and then an alternating force 'F'. Based on my observations, I'd feel like the tension would be P+F. So my mean stress component would include both P and F while the alternating component would only be F. But looking at it this way yields unrealistic results.

### RE: Fatigue on a threaded bolt

(OP)
And thank you all for your responses. This has been overwhelmingly helpful.

### RE: Fatigue on a threaded bolt

Hi kDiqq

The tension the bolt will see if it's preloaded correctly is its preload + (kb/(kb+kj))*F.
The kb and kj are the bolt and joint stiffness respectively, with F being the externally applied load.

How the joint works is when the preload is applied to the joint; the joint faces compress slightly generating a compressive stress and a compressive strain, conversely the bolt is tensioned and under goes a tensile stress and tensile strain.
If you now apply a external load trying to separate the joint,what happensis is that some of external force is used up in reducing the initial compressive stress generated by the preload and consequently reducing the compressive strain. In order for this to happen the bolt or screw must elongate by the same amount as the joint faces did when the compressive stress is reduced by the external load.
In order to find the fluctuating force in the bolt you have to estimate the bolt and joint stiffness as indicated in the previous paragraph. The link below might help:-
http://portal.ku.edu.tr/~cbasdogan/Courses/MDesign...

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

### RE: Fatigue on a threaded bolt

(OP)
desertfox,

Thank you for the information. I still have the problem of my mean stress component being very high while the alternating component ( (kb/(kb+kj))*F in this case ) is very low. Not so much an issue except the fact that the mean component causes me to get unrealistic results (something like 1.5 cycles). How do I consider the stress due to preload when using an S-N diagram to estimate life?

### RE: Fatigue on a threaded bolt

If your mean stress is too high, the bolted connection needs to be fitted with stronger bolts (increaed diameter, higher strength or more bolts) Again VDI 2230 gives you a straightforward recipie for this.

### RE: Fatigue on a threaded bolt

Hi

The mean stress isn't a problem the fatigue stress in the bolt should only be considered on the cycling stress unless your cycling stress when added to the preload of the bolt takes it beyond the elastic limit, in the latter case then as izax1 stated you need to alter your joint.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

### RE: Fatigue on a threaded bolt

Have a look at this link start around page 54 and scroll down

http://www.unbrako.com/docs/engguide.pdf

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

### RE: Fatigue on a threaded bolt

Hi

In order for you to do a fatigue calculation on the bolt you need the proportion of the external load seen by the bolt during operation, the mean stress is the minimum stress (in this case your bolt preload)which then should be added to the maximum stress the bolt sees in operation ( the proportion of the load due to bolt stiffness) and finally divided by two. The alternating stress is the difference between the above two stress figures divided by two.

See this link

https://materion.com/~/media/Files/PDFs/Alloy/News...

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

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