buckling analysis under L.L. for aircraft composite structure 1

[minny]

Hello
I'm working on the sizing of the aircraft structure in composite, I'm in the phase of preliminary load.
I did a first size at Ultimate Load considering the strain check and the buckling check, all the MS>0: The post-buckling isn't allowed under Ultimate Load.
Now I'm considering the Fail Safe under Limit Load. Surely I have to do the strain check (MS>0).
What about the buckling check, do I check buckling also (as I did for Ultimate Load Cases)?

I think that the answer is in the FAR and JAR ...

May you suggest me in which FAR/JAR I can read about sizing guideline in Failure Load Case (L.L.)?

Thanks in advance.

 

[rb1957]

i'd be surprised if the answer is in the FARs, just possibly it might be an AC but usually this is a company decision. if you're not allowed post-buckling under limit load, then you've got the same riteria for limit, you're basing your allowables on elastic buckling (the onset of buckling).

 

[SWComposites]

This sort of criteria is not defined in the FARs/JARs/ACs. It should/would be defined in the company/program design criteria document.

If you are designing to be stable at ultimate, I would suspect that you should design to be stable to limit for fail safe load cases. Otherwise, you are going to need test data to validate post-buckled capability with damage.

What specific structure are you sizing, and what specific damage cases are you considering for Fail Safe.

 

[RPstress]

Agree.

Post-another-part's-failure, a failsafe part checked for ultimate failure at limit load (and/or onset of buckling), usually doesn't have any further proof-criteria: you don't have to check for metallic yield and analagous composite effects at less than limit load. However, there should be clear design sizing requirements documented.