Effect of pitch for Composite joints

[cyt4]

I was trying to think of the effect of large pitch distances for single lap composite parts. Normally the pitch distance between the rows is low. What would happen if you were to increase that to lets say 10-15 in, making the joint a very long joint. Would the first row have to be sized for the entire load(conservatively), rendering it ineffective? I can't really imagine that there is no difference in the way you share the load between two rows if you increased the pitch.

 

[rb1957]

sound like a very "odd" joint, 10-15 inches between rows ?? Is the joint always in tension ?

I suspect that the load distribution between the two rows would be 50/50, since I imagine the two skins are similar in thickness.

fay sealant is your friend ... in real life it would act as a bond, but not something you'd include in your analysis.

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

 

[SWComposites]

If the joint straps are identical then the joint length has no effect on load distribution. If the straps are not identical, then length has an effect. Look at joint flexibility equations that include the strap stiffness terms. Or run some FEM test models.

But 15" between fastener would be a very odd (absurd) joint.

 

[aerostress82]

Depending on the behavior of your structure under loads (ie. bending/shear/tension/compression + direction of these behaviors), you will have fitting factors of 1.5 coming into effect in your joint calculations at these rows of fasteners with specific directions of loading (as mentioned in the previous parenthesis).

But as composites "always" rely on experimental data, after your simulations, your actual structural tests will asses if you were too conservative/unconservative depending on the direction/type of your loading.

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer

 

[rb1957]

"1.5" fitting factor ?? 1.15 maybe ?

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

 

[aerostress82]

oops
1.15 is about right. Thanks for the correction Sir!

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer

 

[SWComposites]

The fitting factor is just an arbitrary uncertainty factor. It has nothing to do with actual load distribution or analysis.

 

[aerostress82]

But it is bound by 1.15 so far, right? Have you ever witnessed a higher fitting factor for composite joints?

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer

 

[rb1957]

composites get hit with such high factors already (the knock-down factors, like 50%) that FF = 1.15 should be fine. SWC is right too, that this is a "catch-all" factor, to protect against analysis only situations; I think it is meant to work as a "single loadpath" factor, so that if you don't test a fitting which is one a a limited number of attachments then add the FF.

for composite joints, the thing I know is e/D needs to be >3 (ie higher than Al).

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

 

[SWComposites]

The fitting factor is a regulatory proscribed value. Has nothing to do with the materials. And only applies to certain joints, and only if they are not tested.

e/d while usually good practice to be 3 or more can be whatever you want as long as you have static and fatigue test data to cover it.

 

[aerostress82]

Thank you both for your comments gentlemen. I sincerely appreciate all the insight you provided on this.

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer

 

[cyt4]

Thanks for the feedback and commments