Fastener dia Vs fastener load

[JetLife]

Will fastener dia increase (oversize or nominal)from blueprint increase fastener load? For example, in Wing shear tie foot to skin joint there are usually 3 or 4 fasteners. If one of them is changed to next oversize (1st or 2nd)or next nominal will the fastener load change? From stiffness perspective, it should change. The question is how much? Is it D_new/D_bp, D_new^2/D_bp^2 or by Huth criteria? How will it be different for shear critical or bearing critical joint? Thank you

 

[LiftDivergence]

Oversizing a fastener will have a number of effects on a joint, including the percentage of load transferred as you mentioned. Since the fastener is stiffer it will increase the R/P a bit. I would not simply use the ratio of diameters to predict the change, since most models for flexibility take the effect of the adjacent plate elements into account as well. From my experimentation with different flexibility formulae in joint load transfer I can say generally the Swift model is the stiffest and the Grumman equation is the least stiff, in the range of thickness typically seen in aircraft structure.

So if you are working from a repair standpoint, which you may be if you are oversizing, the Swift model would be most conservative. I would simply do a comparative load transfer analysis to figure out the difference.

As for other effects, you may want to consider:
1. Change in the bypass and bearing stresses on the hole due to the increased diameter / bearing surface / new R/P. This will have an effect on the macroscopic crack growth.
2. Changes (if only slight) to the interfastener ligament. If you have to O/S several fasteners this is more of a concern.
3. The increase in diameter may actually reduce the hole severity factor (all other variables held constant) however:
4. Is the fastener countersunk? If so this would also affect No. 4

Keep em' Flying
//Fight Corrosion!

 

[rb1957]

"Will fastener dia increase (oversize or nominal)from blueprint increase fastener load?" … the total load on the joint will be the same (to about the 4th decimal place). The load distribution will change (slightly), like LD goes into above. The total static load capacity of the joint should increase.

There are a bunch of other things to consider, like edge distance, spacing, …

An important thing to note is avoid mixing fastener materials and types.


another day in paradise, or is paradise one day closer ?

 

[JetLife]

Thank you. Now on the fatigue perspective how should we proceed? As you mentioned Larger dia will reduce the bearing stress. However, increased pin load will diminish it. Bypass load will change if short E/D persists. Inner ribs have protruding head and outer ribs are countersunk.

 

[rb1957]

from the fatigue perspective LD has given good guidance. You need to redo the fastener load distribution within the joint using whatever stiffness model. With different dia/stiffness fasteners you may need to do more than one fastener.

another day in paradise, or is paradise one day closer ?

 

[LiftDivergence]

JetLife,

In addition to rb1957's comments I would say this:

Your first step is to look at the SRM and figure out if this is actually FCBS. Since you say the "foot" to skin joint, I assume we are dealing with lower wing skin, so it probably is.

What is the context of this action? Is this a deviation from an approved repair?

As far as analysis, what we have described above figuring out the new load transfer is probably necessary either way, but you have a couple options. You can do an absolute analysis which will require you to conservatively estimate the stress in this piece of structure (not necessarily simple in this scenario).

Or you can do a comparative analysis with only a relative stress. So, if there is a Cat B SRM repair you could put here, or this is just a deviation from some approved configuration you could do a relative analysis.

Please also watch out for any SSI's or ALI's in the area. You haven't mentioned the A/C model so I'm not sure if there would be any.

Keep em' Flying
//Fight Corrosion!

 

[JetLife]

Thank you all for inputs. I discovered some helpful tips searching for swift model on the web.
Can anybody shed some light on how non-deburred condition affect fatigue life of a joint?
If fastener installation calls for clecos or temporary fasteners, can burr be formed between plates?

 

[rb1957]

"Can anybody shed some light on how non-deburred condition affect fatigue life of a joint?" … umm, it destroys it !

plenty of aircraft accidents have been caused by damage initiated by a burr.

you're not, I hope, working on your own; so there should be a "typical practices" book around … your standard manufacturing processes, your standard engineering design practice.
FAA AC43-13 is not a bad place to start. I'm sure there's lots of stuff out there (like "repair design" engineering courses) that'll let you.

"good" places will size up a doubler, temporarily install with clecos, then full size the rivet holes, remove the doubler, deburr, install dblr with fay seal and clecos, and rivet away.

But there are also plenty of places that'll fit with clecos, remove dblr, install with fay seal and clecos, and then full size the holes and install rivets.

No one can say either procedure is Right or Wrong.

What sort of structure are you dealing with ? 'cause if it's secondary structure, then it's game on !

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

I can give an entire course on the 'simple subject' of manufacturing burrs.

IT IS MIND BOGGLING the degrading effect that retained burrs can have on aircraft structure.

Regards, Wil Taylor

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