Limit Loads & SCFs vs Ultimate Loads 1

[Noor]

Hello all,

It is well known that the stress concentration factor at a hole in a tension coupon can reach 3 times the average stress, while the stresses are still within the linear elastic range for the material.

When the material begins to plastically deform, then the stress field begins to flatten out and become more uniform across the part cross-section.

My question: is it common practice to size tension-side beam chords with fastener holes (say for a wing spar) for limit load using net section area & SCF... and not just for ultimate loads on the net section (and assuming uniform distribution)?

If no local yeilding around a fastener hole is allowed, then the limit load case with SCF included will be the critical case for sizing.

In my limitted experience, I didn't see this approach used in the aircraft industry, nor documented in the texts. I would appreciate it if anyone can share how they approach this problem and what their views/justifications are.

Thank you.

 

[rb1957]

FAR25.305 ...
"(a) The structure must be able to support limit loads without any detrimental permanent deformation. At any load up to limit loads the deformation may not interfere with safe operation."
the rule does not say "thou shalt not yield at limit", limited, not significant, yielding (such as would be confined to the local edge of a hole) is (IMHO) permissible.

i believe it is impractical (= ultra conservative) to design to Kt*limit stress(net area) < Fty.

 

[Noor]

Thanks for the quick reply. That answer hit the nail right on the head. I agree that sizing to limit*kt is ultra conservative. I just needed to clear that up with a sound second opinion.

 

[TVP]

I agree with rb1957, especially because residual stresses from shot peening, hole cold working, etc. are not taken into account with Kt*limit stress < Fy.

 

[jetmaker]

Be careful tho, because the ignoring of SCF at ultimate load is conditional upon the material being ductile, of large net section, and static loading.

Another note is that designing to limit load is not typically the critical condition for most materials used in aerospace as the ratio of yield to ultimate is often less than the 1.5 ultimate factor and thus ultimate load cases are usually critical over the yield. I know that is badly phrased, but hope you get the point.

jetmaker

 

[Noor]

jetmaker,

I understand your point about most aerospace matallic materials being "ultimate critical". If you were to design for no yielding at limit loads with a SCF=3 around a bolt hole, then you will find out very quickly that the this would be the critical design case... since SCF > 1.5 (factor from limit to ultimate).

 

[jetmaker]

Noor,

Do not disagree with you, however, that is a local failure and in fact, does not truely constitute a failure in my mind. Bolt hole yielding is not critical unless the joint is a critical single path, limited bolt number joint.

In fact, it is often important to have yielding around bolt holes in joints as it helps ensure "uniform" fastener loading in a complex joint pattern. Without this yielding, joint failure would be more common.

So if someone told me to design to limit... I would be concentrating on net sections rather than specific details like fastener holes.

jetmaker

 

[Compositepro]

Don't open hole tensile and compression tests give designers the data they need without having to use stress concentration factors?

 

[plasgears]

Recommend we rise above SCF and focus on finite element and fracture analysis, you know, assuming an initial defect in the stress field.

 

[Sparweb]

Look for the Advisory Circulars on this topic. They tell you the FAA policy, and offer some examples.

Steven Fahey, CET

 

[sigmatau]

What advisory circular dicusses SCF's?