Shear doubler field rivet spacing 1

[digidocs]

I'm looking at shear doubler which is intended to make an existing skin panel shear-resistant. To reach the desired non-buckling shear capacity, the doubler and the existing skin will need to act as one monolithic piece. How does one generally calculate the field rivet spacing required to achieve this effect? I'm not finding any help in my usual resources (Bruhn, Niu, Flabel, etc.).

My current thinking is to look at the diagonal compression created by the shear load and then make the rivet spacing such that this compression does not cause the thinner layer to buckle between the field rivets. Are there other conceptual approaches that are commonly used to analyze this situation?

Thanks,
David

 

[rb1957]

inter-rivet buckling of a single ply ?

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

 

[Sparweb]

Shear buckling isn't really the same as compression, though it looks like it... sort-of.
You need "Shear flow". While it isn't your answer, it's the tool that will get you there.

The shear in the panels enters through rivet joints. Hence, the strength of the rivets define the maximum shear load that is possible, provided that buckling doesn't limit it.
Since it sounds like your existing panel will buckle, then that actually is the limit... but when you reinforce it using the same rivet joints, and show it won't buckle with the new thickness, then rivet shear becomes the limit again.

Finding the maximum possible shear load that can be applied to the panel is simple: shear strength of rivets divided by their pitch. Notice that I didn't divide by panel thickness. I am expressing shear as Lb/inch or (N/m if you're metric). This is shear flow. It eliminates the thickness from the analysis of stress - specifically for the purpose of dealing with loads and geometry of the panels without getting stuck on how thick or thin each panel is. Handy when aircraft skins have chem-milled patches of varying thicknesses, etc. And handy when choosing a panel thickness at the design stage, where you're at. Once you find the shear flow, you can then pick a panel thickness that won't buckle.

Oops - gotta finish this later. Hope this gets you started. I will check in tomorrow.

STF

 

[Burner2k]

Hi,
Since you have access to Flabel, please check out one of the worked out examples (Example 8-2) in the chapter Shear-Resistant Beams. I think the problem involves determining field rivet spacing on a doubler in a web to make the web shear-resistant. Like SparWeb has mentioned, you need shear flow information.

In the example, he has already provided shear flow values in the web. He then determines what needs to be new thickness to make the web section shear resistant (Positive MS). Based on this new thickness, the doubler thickness is determined. He then determines how much shear flow is taken by the doubler. And he uses again a plate buckling formulation (between 4 rivets representing the 4 corners of the plate; a = b = assumed pitch) and calculates MS until satisfactory. Very interesting solution...

Further, I assume you have tried the standard pitch of 4D-6D and checked its validity?

 

[digidocs]

rb1957, SparWeb, and Burner2K thanks for the replies.

I had not seen the Flabel example before, but it directly addresses my question. I appreciate you bringing it to my attention. I need to confirm, but I think Flabel uses the shear buckling equation for a simply supported plate---this is interesting because the field rivets in the corners of each "square" seem to be the only source of restraint in this situation.

David

 

[Burner2k]

Yep,
I remember reading in Flabel about most riveted connections, especially single row connections, are considered as offering only simply supported edge condition. It is not common to find more than 2 rows of rivets in panel edges. However for edges where 2 rows of rivets are used, I don't think assuming a simply supported condition is valid anymore.

 

[rb1957]

OP is talking about riveting two skins (or skin and dblr) together ... spacing between field rivets to permit assuming both skins work as one (much higher buckling allowable).

how about working backwards ... start with the buckling stress of the combined panels, then what pitch would allow this stress as inter-rivet bucking.

I know shear buckling is different to compression buckling, but if the panel is stable in compression then it should be stable in shear.

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