Loads on Shear Cleats (fuselage frames)

[gokmavi]

My question is about loads acting on shear cleat attaching a typical fuselage frame to the skin.
It is a Airbus design policy to use L- shape shear cleats to attach frames to the skin & stringers. In this case frames have no mouse-holes,where stringers pass through. Mouse-holes are on shear cleats.
As far as I know,load on the fasteners attaching shear cleat to the skin, shear only. Is it the case also for deltaP load cases? I mean, for internal pressure load cases, might there be any tensile load on these fasteners i.e. does shear cleat becomes tension cleat

If you can help me I will be glad.
Thanks in advance.

 

[rb1957]

I think you're correct, under pressure the fuselage skin is going to "breathe" outwards and the only connection between the frames and the skin is the shear clips, so they'll carry some tension. Mind you it'll be pretty small, I'd expect less than 100 lbs/rivet, but the best way to find out is a non-linear FE, maybe a simple linear model will get you close enough.

 

[gokmavi]

Thanks rb1957.

Probably, since the magnitude is small, it is ignored for metallic designs. Therefore, in my short carrier, I have never seen that they have ever been taken into account.
But in composite design, it seems, even this magnitude, that it can cause a problem because of pull-through phenomenon.

Cheers.

 

[Klompder]

I agree with the previous replies and like to add that the hard points in the stringer stiffened skin are at the stringer - frame junctions. At these stiffer locations the highest restraint will occur and one can see that the considered cleats are connected to the stringers as well as the frame and the outward load due to delta P can be transferred as shear thru these connections. Also note that one can also see floating frame designs where only stringer - frame joints are made with cleats and no connection exists between skin and frame.

 

[SWComposites]

Gokmavi - why don't you just get the loads from the airplane internal loads FE model and use those loads to design the shear clips?

 

[gokmavi]

SWComposites -
Question was whether there can be any tension load on the shear cleats due to internal pressure. In metallic fuselage structure I have never seen anybody who is trying to calculate such load acting on the cleats.
Question was not about shear loads.

Regards.

 

[SWComposites]

Yes, there can be tensile loads, with magnitude depending on whether there are also clips between the stringers and frame. Does your internal loads FEM not calculate the loads in the shear clips? or is the model idealized such that only shear loads are reacted in the shear clips.

 

[gokmavi]

In global FEM, there is no shear cleat.
Shear loads on them are calculated by simply taking the differences of normal forces (calculated from grid point force balance) at two ends of each BAR element idealizing the frame.

 

[rb1957]

that's pretty odd ! ... not modeling the shear clips would have a significant effect on the loads in the frames (and the skins and the clips, too).

if you're stressing hte clips you always try "no analysis required, as these aren't primary structure (not modelled in the fuselage model)". my next tack would be "OEM, because you didn't model them I'll have to do ,000s of hours of work to determine the effect (basically to model them !), so you pay me for that" seriosuly, trying to actuately account for their effect is going to be little short of adding them into the model; you might (just, might) get somewhere by modelling a section of the fuselage with and without shear clips, applying a unit shear load into the skin to see how it redistributes with the new load path.

good luck !

 

[gokmavi]

Dear rb1957,
I am not in global model business. But as far as I know,in the company that I work for, they model frames as BAR elements and skins as membrane elements and nodes are located at the outer loft. I did not see any shear cleat idealization in their global models. If I do the fuselage global model, I would model them with elements like NASTRAN CSHEAR elements lying on the plans perpendicular to the fuselage skin. Anyway, as I said, that's not my business and not my responsibility and I am not the decision maker.
I have got my answer about order of magnitude of possible tensile loads from you. It is as I expected. Thanks again.

 

[rb1957]

actually that may be ok. your structure, for a frame, has an inner cap, a web attached to the skin, and the outer boom of the frame is pretty much the effective skin of the fuselage; there may (or may not) be an outer cap, continuous, floating on the stringers. The global model sounds reasonable ... fuselage skin panels, broken by stringers and frame caps (well your shear clips), frame webs, an inner boom and an outer boom (again probably not much more than effective skin).

from this model i think you could say that the radial tension stresses in the frame webs (as modelled) is the tension that the shear clip has to carry, in addition to the shear introduced into the frame web.