Stress analysis for a lug under interference fitting

[feajob]

Hi Guys,

We do lug analysis under oblique load as per BRUHN Method § D1.11 (classic approach). We also consider rework in our dimensions. Now, I would like to know that do you combine press-fit effect too? or not?

I believe that we may be too much conservative by adding our load with the radial load due to press-fit effect. Because, by loading our lug we are actually relieving the load due to press-fit at load application direction (at least). What do you think?

By the way, there is good presentation at:

http://www.agingaircraftconference.org/all_files/43/43c/99_ppt.pdf

Thanks,
A.A.Y

 

[RPstress]

Generally at ultimate load a metallic lug is yielded and any pre-stress due to shrinking in a bush or similar is removed. Having said that, I have never seen it proven by experiment. However, I suspect even high elastic loads effectively relieve the stresses due to shrink fit, etc. That might be something to bear in mind if the fit-up stresses are being taken advantage of for a fatigue analysis.

 

[rb1957]

the press-fit strains the lug wall, no? the nett section thruogh the bore CL is reacting some load due to the press-fit. i can't see why the press-fit would increase the allowable load in the lug.

i agree (for whatever that's worth) with superimposing the applied load on top of the press-fit.

 

[rb1957]

looking at the slides on fatigue ... the forcemate installation shows tension stresses at the edge of the lug, 'cause it is installed with much higher interfernce than press-fit. if i shot peened the surface to remove these stresses, i'd expect to remove some of the beneficial compression stresses at the bore.

from the presentation, i think it's dangerous to generalise about lugs. This is a specific lug geometry, with steel bushes. The residual stress field due to interference is highly dependent on the local geometry. the local stress field due to loading is (obviously) highly dependent on loading direction. the interaction between geometry and loading is pretty unique for each lug.

I thought it was "bad" practice to repair bores in Al with Steel bushes. i thought part of the point of a bush was to be a removable, an easy way to repair a damaged bore. if the parent material is Al and the bush is Steel, then the bush will bear into the parent material; if the bush is Al then the bore will bear into the bush, and replaceing the bush should repair the hole (without affecting the parent lug).