Simple load distribution in fasteners 1

[Benbarca7]

Hello Folks,

I am currently doing basic Aircraft Repair training.
The section I am currently studying is the load distribution in two plates fastened together (Loads in the elastic domain).
It is the first steps to calculate the load distribution in X (number) rows of fasteners using compatibility equations.

Please have a look at the uploaded file.

http://files.engineering.com/getfil...9340-4431-a3a2-e5a30fdfdd50&file=example.docx

There is an expression in the geometry and properties that I dont really understand:
Fs:= L/ (Es*ts*W)
Unit is In/Lbf

I understands the terms of the equation, but I dont understand what is Fs.
Is it like the flexibility (in opposition to the stiffness) of each plate ?
You can see that the two plates are made of the same material but have different thicknesses.

I would like to understand what this term corresponds to.

And in a second time, I have this question:

Which plate is stiffer? the thicker one?

The load will stay in the stiffer plate as long as possible, so I should find by calculation that the load in the last fastener will be higher than in the other ones (load will stay in the primary plate Tp=0.020in).

I just got confused a little bit with the terms employed in the training and maybe it's because I'm not english but If someone could confirm/infirm what I said, I could move on without having this stupid cunfusion in my mind.

Thank you and best regards,

Ben

 

[rb1957]

arrhhh, mathcad ...

F is the compliance of the skin/plate.

you use this with the fastener flexibility to match the deflection is the skin and plate (compliance compatability).

Niu has something on this in his books.

terminology ... it doesn't matter ... define it for yourself, picture it in your reports (to save confusion ... possibly your own if you have to re-read your stuff later).

stiffer loadpaths ... you got 1/2 the message ... the end fasteners (the first and the last) see more load transfer than the middle ones.

 

[Benbarca7]

Please dont hate on my Mathcad! I love it ! lol

Thanks for the response, I will then check in the Niu book.

I agree with you concerning the middle fastener and the first and last fasteners.

But if I take two rows (or three rows) of fasteners.

I will have more load in the last fastener than in the first one as my first plate is stiffer (thicker), so the load will want to stay as much as time in the stiffer load path. Do you confirm that?

I appreciated your response thanks a lot

 

[rb1957]

different thickness between the rows will create different rivet loads (due to the different compliance).

once you've got the calc laid out, try a bunch of different geometries. from my experience you have to have a very extreme geometry before you have a serious variation in rivet shear. this has (IMHO) only been given much attention 'cause some guys in the past Really effed their repair, and other people wanted to make an example of them.

you'll find more variation in rivet load from changing the rivet flexibility equation (huth is pretty common these days) than in any sensible doubler geometry.

 

[Benbarca7]

Ok yeah you are right.
Usually we consider in repair that the holes taking more loads will plastify and the extra load will be redistributed in the other fastener to have an equal distribution.
I dont think I will use it in most of the repair I'll be doing, but it still good to understand how it works. It depends on what level of analysis I may have to provide.

Thanks rb1957 I valuated your post.

Cheers,

Ben

 

[ESPcomposites]

Ben,

For the static analysis, what you say is generally true, provided it is bearing critical (and not fastener shear critical). The difference will should up in fatigue scenario where the elastic response is relevant.

Brian

http://www.espcomposites.com

 

[Compositepro]

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[Benbarca7]

Brian,

you're right and I need to be careful when I say things, I really depends on the analysis I'm doing, and thank you for the precision.

Compositepro,

Yep, I did it: rb1957, you've been officially valuated! lol

thank a lot guys
Now let's go back training!

cheers