Multiple Fastener Row Tension Distribution

[aerohead56]

I am looking for a set of equations that explain the loading distribution on a single-lap joint that only contains axial load. This condition is mentioned on Page 307 of "Aircraft Structures" by Peery, 1st edition, but no equations are given. References would be appreciated.

 

[jetmaker]

aerohead,

Are you talking about fatigue or ultimate loading? The equations are different, and so are th assumptions that get made with each.

jetmaker

 

[aerohead56]

Ultimate.

 

[jetmaker]

aerohead,

For ultimate loading the general approach is to assume that all bolts carry and equal amount of load. This assumes that the fastener fit, type, and plate thickness for each fastener row are the same. Also, this assumes ductile material.

In theory, the first and last bolt in the joint are the first to load up, but they eventually reach a yield point where they start to shed load to the other fasteners. Eventually, the overall deformation of the joint is such that the fastners are carrying all the same load at the time of failure.

Therefore, for ultimate load distribution on a single-lap joint, Pbolt = Total Load / number of fasteners.

Hope this is what you were looking for. If not... provide more details and we'll see where it goes.

Later,
jetmaker

 

[aerohead56]

I have used the even distribution method in the past for analyzing this type of joint. However, we are trying to understand the effects in the linear region of deformation, ie before plastic deformation of fasteners, or bearing into the hole edges starts occuring. The page I previously referenced in Peery shows an excellent example of the type of fastener loads of which I am speaking. However, it gives no equation. My lead says that he has seen loading equations for this condition but cannot find his reference material.

 

[jetmaker]

aerohead,

Get the following paper, it hopefully will help you.

T.Swift, "Repairs to Damage Tolerant Aircraft", FAA-AIR-90-01, March 19,1990. This paper was presented to International Symposium on Structural Integrity of Aging Airplanes, Atlanta, Georgia, March 20-22, 1990.

There is a really good summary of lap joints, and some graphical data in it. It may be too theoretical, but only you will be able to evaluate if it is appropriate to your needs.

regards,

jetmaker

 

[aerohead56]

Thanks, I think that exactly answers my question!

 

[edbgtr]

Aerohead56

The above report can be downloaded from:

http://www1.faa.gov/certification/aircraft/FAA-AIR-90-01.pdf

Ed

 

[koopas]

Hello Ed, Jetmaker,

In a similar post, I had asked some questions about the details of Swift's "compatibility analysis". I found Swift's report to be an excellent "high level" paper that introduced useful damage tolerant principles in repairs. However, I found the analytical part incomplete, with lots of steps skipped. I guess I was looking for more details and examples in the actual calculation of the fastener loads themselves.

Ed, you suggested that I read some Niu refs. as well as obtain the paper "Analytical Design Methods for Aircraft Structural Joints" by McCombs, McQueen and Perry, Technical Report AFFDL-TR-67-184.

Does that paper walk you, step by step, into calculating operational (not limit) fasteners loads in aircraft metallic joints? Does it require some programming or FEA software?

Can you suggest any other work that addresses my interest? Are there even short courses on the subject?

Thanks,
Alex

 

[edbgtr]

Alex

Bill McCombs' report develops an analytical solution to the fastener load distribution, albeit for the limited case of uniform stiffness through the joint. If you want to do a variable stiffness joint,FEM is a possible way to go. I have seen some simultaneous equation solutions for a limited number of fasteners in the joint. Usually in-house stress office programs.
And yes, the solution is in the elastic range, otherwise the fasteners load up evenly as Jetmaker says.

Regards,

Ed.

 

[koopas]

Ed,

I sent the $ to Mr. McCombs and patiently await the report.

Have you tried to model a joint using FEM? It it difficult?

I did some FEM at the university, but have forgotten most of the concepts. FEMAP is somewhere in one of the hundred boxes in the garage

Is there some FEM program that's specifically tailored to modelling and obtaining fastener loads in a joint? Hopefully, this will be something other than a full blown finite element program...more of the likes of a small FEM application designed for liaison/repair engineers (with built in fastener types, materials, etc.)

Worst comes to worst, I can try to find my FEMAP CD, but I'll need to brush up on the theory. Do you know of any benign novice guide/literature/book on the subject? The more "layman", the better

Oh, one last thing, does it exist a short FEM course in the US that has a good reputation?

Thanks,
Alex

 

[edbgtr]

Alex

Try one of the freeware sites on the internet. For straight inline joints you only need a package that can do endload elements and springs. If no springs, then you can model the fasteners as endload members as well, you'll just have to model it a bit more carefully. For the infrequent need I have to do FEM, I have an old MSC-PAL2 package. Runs under DOS (don't laugh) with hand-driven input, no fancy mesh generators and post processing, but with a bit of careful modelling, a lot can be achieved. It's useless without the manuals, so if you get one of these things, make sure you get the instructions!!

Good luck.

Ed.

 

[koopas]

Good morning Ed,

Should one be able to concoct an FEM procedure after reading Mr. McCombs' report? I am the kind of guy that needs be shown once

Alex

 

[edbgtr]

Alex

You should be able to set up a set of equilibrium equations at each fastener and then solve the set of simultaneous equations, for Fi at each fastener (that is FEM!).

For a non-stepped joint you can also estimate the worst fastener load from one of the derived formulas.

Ed.