Lug analysis as a cantilever beam

[franc11]

Hi,

I'm reading a single lug stress analysis. The theory being used is not familiar to me. I was expecting an analysis based on what we can find in books like Bruhn and Niu. When using this method, the margins of safety were very negative. So what they used is an analysis considering the lug like a cantilever beam with a load transfer around the lug and calculating a maximum moment at a section 45 degrees from transverse direction.

Does it ring a bell to anyone? I never heard of all this and since it's the only way the lug could have a positive margin of safety, I would like to know if it is relevant.

Thanks you very much!

 

[vonlueke]

franc11: Yes, for a transversely-loaded lug, in no case should the ultimate bending stress margin be negative when the lug is fractured at 0 deg and the lug bottom ligament is analyzed as a cantilever. Let's say the lug axial direction (0 deg) is horizontal, and the transverse load is downward. Then note that the maximum bending stress on the bottom ligament, analyzed as a cantilever, won't necessarily occur at 135 deg; the maximum bending stress location depends on the particular lug geometry (namely, edge distance ratio at hole centerline, and taper angle of lug edges relative to lug axial direction).

 

[dooron]

For Lug analysis, Bruhn and Nui are very conservative, in fact Nue shows the Lug fails (UTS) before it yields. This is obviousely an error. The more comprehensive Lug analysis is in ESDU. Treating the centre of the Lug as a beam with point load in bending is common and the formulaes and correction factors are in Roark 7th edn. sec. 8.10 "beams of relatively great depth"

 

[franc11]

Thanks for those advice. I have access to ESDU, so I'll look it up more closely. I read them quickly a couple of time age and thought is was basically the same as in Niu or Bruhn. I'll also look in Roark which I only have the fifth edition, but I guess it's mostly the same.

 

[KENAT]

Depending on your lug it may be more effort than it's worth but...

For a simple lug you should be able to do a simplistic analysis of it from first principles to compare.

You've got the tear-out, the Brunelling and fracture across the hole.

If I recall this is summarised in a work sheet from I think it was Folland aviation, probably not something you have to hand.

 

[franc11]

Dooron,

I've just read ESDU 91008 : Lug strength and their approach is exactly the same as in Niu or Bruhn. Maybe there's another ESDU I didn't find.

But Niu and Bruhn propose to analyse the lug as a cantilever beam if Ktru or Ktry fall under an approxiamte cantilever strenght curve, but they do not show how to make this analysis. But this seems to be a very rare case.