Permanent Set Criterion in Lug Analysis 2

[Palpatine]

I would like some help with the term 'permanent set'.

The permanent set criterion is used as one of the allowables to be used for lug analysis in addition to net section failure and shear tearout/bearing.

Can anyone help me with the following questions:

- Is permanent set any different from material plasticity (yielding) or maybe material bearing?
- Why is permanent set an important criterion for lug analysis? Is it related to fatigue or the functioning of the design in combination with the bearing?
- If fatigue is not an issue and a damage tolerance analysis has been done, can the permanent set criterion be neglected in lug analysis?

Any information would be greatly appreciated.

 

[gwolf]

The 'permanent set' criterion is used in association with limit loads and is related to the continued functioning of the component. Permanent set is allowed at limit as long as it does not prevent the component from functioning properly. In the context of a lug, this could well be the fit of a bearing, and permanent set might prevent the bearing from operating correctly.

Small highly localised areas of plasticity around stress concentration features are allowed at limit load. The effect of these small stress concentration features is to reduce fatigue life not limit or ultimate loads. They should be dealt with using classical fatigue and/or damage tolerance calculations.

- If fatigue is not an issue and a damage tolerance analysis has been done, can the permanent set criterion be neglected in lug analysis?

You would have to consult your industry rules on this, I would say yes as long as it doesn't cause the lug to jam on anything, but I don't write the rules. Look in the JAR or mil spec books. Perhaps someone else can post some references.

 

[rb1957]

i agree (for whatever that's worth) with the above post; tho' i find the OP's terminology a bit confusing (hopefully it's the OP who's confused, and not me !).

"permanent set" referes to inelastic deformation, typically expressed as a strain.

the yield point for steels, for example, is usually referred to as 0.2% permanent set.

the rules (and as we're all aero. users we'll assume you're dealing with an airplane) allow permanent deformation with ultimate load, and "without detrimental" (usually expressed as none) deformation for limit load; see FAR25.301 and subs.

the reason for these various criteria is exactly because of your specific case ! it looks like your lug is critical for static loading, it sounds like the fatigue loading is not critical. because your lug is static critical, then the limit/ultimate static loads are going to define the critical loading; had your lug been fatigue critical then i think you'd have found that satisfying the fatigue requirements would have also satisfied the static requirements.

that being said, i'm a bit surprised that you're worried about limit load (implied by being concerned about the onset of plastic deformation). most aerospace materials have a yield strength > 67% of ultimate, so ultimate strength is critical.

btw, please don't include the stress concentration for the lug in static analysis. i've worked for some people who require a positive margin for the stress concentration peak under ultimate load. this is (in my popinion) too conservative ... the stress concentration peak is highly localised and any deformation would be constrained by the bulk material around the stress concentration geometry, and so (again, in my opinion) this deformation would be very unlikely not to be "non-detrimental").

good luck

 

[Palpatine]

Thank you both, gwolf and rb1957! It is clear to me now. There is no difference between permanent set and yielding other then the extent of yielding.

Your information is most helpful. We are doing static lug analyses for a spacecraft where the specified safety factors are j=1.1 at yield and j=1.25 at ultimate level.

The handbook formulas we use have a built-in aircraft safety factor of j=1.5 at ultimate level, which causes confusion.

Concerning the stress concentration at ultimate level mentioned by rb1957: yes, I agree that it is way too conservative to apply a Kt at ultimate level as a redistribution of load has taken place at ultimate level and the entire section has yielded, causing the stress concentration to disappear.

 

[rb1957]

i think you need to be a little careful with the factors.

airplanes use ultimate load = 1.5*limit; sometimes satellites use specific limit and ultimate loads (rather than factoring ultimate from limit). this 1.5 is sometimes called "a factor of safety" (FAR25.303).

i think your spec means that the yield strength of the lug has to be greater than 1.1*limit load and the ultimate strength greater than 1.25*limit load. i think this makes the limit condition critical.

i'm abit confused by "The handbook formulas we use have a built-in aircraft safety factor of j=1.5 at ultimate level, which causes confusion." if you're using a calculation for ultimate strength that includes the airplane 1.5 factor explicitly, say something like Ftu*At/1.5 (which would give you yhe ultimate strength as a load comparable to limit), then factor this allowable by 1.5/1.25 for a load comparable to limit. but then your yield allowable is going to be something like Fty*At/1.1 (normally airplanes don't apply this 1.1 factor).

alternatively, you can say the ultimate strength of the lug is Ftu*At, your applied load is limit, and your RF is greater than 1.25 (satisfying your spec).

good luck