Matrial Bearing vs Compressive Yield Stress

[bojanche007]

Hi all,

I am doing a study of a structure which is basically a lug joint which I know, through both test and analysis to fail in bearing mode (i.e. plastic deformation of the lug hole). I need to come up with some analytical safe load recommendations for this structure.

Through FEM and manual calculation I have come up with stresses at the critical point for a given load.

My question is, to calculate the safe load for bearing failure, I'd like to confirm that I need to use the bearing yield properties for the material, and not the compressive (or tension yield).

I have the material properties from MIL-HDBK-5H, and if I use the bearing yield it gives me a much less conservative result for the safe load. So I'd just like some second thought before making recommendations.

 

[CoryPad]

Bearing yield stress from MIL-HDBK-5 should be applicable. The actual value can vary based on geometry (pin diameter, hole diameter, lug outer diameter, lug thickness), but the bearing number is better than a uniform compression/tension value.

 

[graemew]

You are on the correct path, but please remeber that the values in MIL-HDBK-5H are "dry-pin" values which normally require a knock-down factor in the real world.
Also the fit of the pin in the hole will impact upon your bearing allowable as well.
The larger the hole/pin clearance, the lower the bearing allowable.

 

[jetmaker]

I would have though compressive yield would be more conservative than bearing yield as bearing allowables are often higher than straight compressive yield.

As to your question tho, if weight is not an issue, then I would be conservative as many other factors can play a role in your final lug failure. But if the failure is always bearing, then a bearing allowable would seem appropriate as the refeerence.

jetmaker

 

[bojanche007]

The stress calculations with compressive yield failure corresponded to the test results well, and it was also the more conservative option, so I've just gone with that. Thanks all!

 

[rb1957]

just semantics, but if compressive yield co-relates well with the test results, why do you think that's "the conservative option" ?

 

[bojanche007]

Because if there were no test results on the face of it I could have had the option to use either. I guess when you look at it that way there were no "options" really. Just one choice.

 

[rb1957]

either could represent the observed phenonemon, one does a better job (so, yeah, i guess, no choice) ... surprising, though, given your original post saying "both test and analysis to fail in bearing mode" ...

 

[CoryPad]

I would guess the factors in my previous post are narrowing the spread between uniaxial compression yield and bearing yield behaviors.

 

[WKTaylor]

bojanche07... and all..

a few added points...

Suggest reading MIL-HDBK-5J or MMPDS-03 (my preferred) Chapter 1. Also, review static lug analysis in various texts (Bruhn, USAF Stress Analysis Manual etc).

By the time Your mind is tired, You'll realize there is a vast difference in thin sheet metal tear-out and thick stout lug tear-out [failures].

For instance, You have NOT mentioned 3D lug dimensional parameters [Shape ratios, edge treatments, shot-peening and/or hole-cold working, etc] and if there is any misalignment of the lug-load-center to the ajoining part strain/load center, bolt patterns, etc.

Also: is there is bushing installed [hard alloy, high strength and stiffness... or medium hardness and stiffness with lubrication for limited rotation wear-resistance, etc]? Is the bushing in high or low interference; and is it thick or thin wall?

Is/are Your and pin and attaching bolts high strength/and stiffness or softer/springier titanium? What is the fit of the pin to the hole [or bushed-hole]; and what is the fit of the attaching bolts [if any]?

I/we are assuming this is a single lug problem... or is this issue common to multiple parallel lugs?

What kind of material and configuration are the mating lugs (I presume multiple)?

What finishes (inorganic, organic and sealers) and lubricants will be used... or could be used?

What in-service damage (stress concentrations) are You anticipating? Corrosion, scratches, gouges, fretting, etc. Do You have an established NDI plan and damage limits?

CAUTION: some acft I have worked on (shall go nameless) have such low MS, that ANY damage was critical. Also some alloys were SCC and exfoliation corrosion critical. Repairs were very limited and lug replacement was a major effort. Design for damage tolerance often changes/confuses the design "picture" radically. One of the major problems with this design was that some lugged-assemblies were occasionally being removed to facilitate maintenance, using the good-old hammer and bullet-pin methods... Gaaaaaaa.


Regards, Wil Taylor