Short Edge Margin

[harrai]

Hello All,
I have short edge margin condition of 1.36D on one of the part.Actually 3 parts of aluminum are joined together by bolts and one part has short edge margin.

All the parts have sufficient bearing strength.So if I understand right bearing failure occurs before shear-tear-out-failure.So if this is right then is 1.36D okay to prevent shear-out as there is no bearing failure due to sufficient bearing strength?

Thanks

 

[harrai]

Hi
If somebody is confused I just want to clear that my bearing allowable is much higher than shear tear out so I think I shouldnt be worried for shear-tear out failure even though it is 1.3 D<2D.

Am I right in this?

And is shear tear out always occur after bearing failure?

Thanks

 

[SWComposites]

NO, NO, NO,

shear out failures will occur before bearing failures when the edge distance is short enough.

at the typical 2D - 2.5D used for aluminum joint designs, bearing typically is the critical (lowest) failure mode (though there are probably a few exceptions).

at 1.3D, I would suspect that shear out failure may be critical. you need to find some short edge margin allowables for your material.

there is a section on short edge margins in Niu's, Airfame Stress Analysis and Sizing book, page 700.

 

[rb1957]

just a thought, since bearing allowables are a function of eD, and since most tables bottom out at eD = 1.5; how did you deterine the bearing allowable ?

the key question is how much load is directed towards the free edge ? or is the majority of the load axial ?

in any case shear tear-out is easy enough to calc so why not ?

 

[jetmaker]

If your loading direction is parallel to the short edge distance, then you actually have a short end margin. You will take a hit both in bearing and shear/tearout. Likely tearout will govern if the material is thick enough to develop full bearing initially. In this condition, static loading is critical.

If your loading in perpendicular to the short edge distance, then the same reduction in static bearing strength will apply, but shearout becomes less critical. However, do a hoop stress check for tearing across the short section. For this condition, fatigue issues become more of a concern. Peterson has a curve which shows the effect on Kt when the edge distance is reduced.

jetmaker