Using Kt in Limit Stress Analysis

[feajob]

We usually do the Stress Analysis with Ultimate Load Conditions. Now, I would like to know about your approach for the Stress Analysis with Limit Load Conditions. If we have a small hole in a cylinder then there will be an important stress concentration in this area. I believe that we have to apply a Kt on the stress obtained by hand calculation in this region. FYI, I have prepared a FEA model and I obtained my Kt from FEA. Well, I could get the theoretical value of Kt from other references (such as Peterson). I appreciate to know your replies.

Thanks,
AAY

 

[jetmaker]

feajob,

Agreed that you must include the Kt in the limit analysis. However, in a limit analysis, one is typically looking for deformation, and a small hole on a large part will produce only localized deformation. It is the designer/stress analyst responsibilty to assess if this deformation poses a structural/functional problem. So if the hole deforms under limit load such that a safety pin could not be reinstalled, then it is important. If the hole is a drain hole, then I'd say it is unimportant.

Regards,

jetmaker

 

[feajob]

Thanks jetmaker. Do you know any solid theoretical reference for using Kt in limit Margin of Safety calculation? I need a good reference for my boss. He disagrees with me on this issue.

AAY

 

[SWComposites]

AAY,

What is the purpose of the Limit Load analysis that you are doing? Is it for Damage Tolerance (residual strength with damage) or to show no detrimental deformation or something else? If it is the first, then you definitely need to include the Kt, just as you need to do when doing an Ultimate Load analysis for static strength.

What is the material of the part that you are analyzing? If it is a metal with large plastic capability, then you may be able to use plasticity arguements to ignore the Kt (this is waht jetmaker was getting at). If it is a composite material then you cannot ignore the Kt as these material have no appreciable yielding behavior.

What is the part and what is the detail that causes the stress concentration? Is it primary structure?

All these kind of details are needed in order to decide on the proper analysis.

Regards,

Steve

 

[feajob]

Hi Steve,
The purpose of my limit load stress analysis is to show no detrimental deformation. It will be used for Static Stress Report. My material is steel 300M. I got a fatigue problem in one of my FEA spots, that's why I would like to revise the hand calculation. FYI, I do Fatigue Analysis with MSC.Fatigue by using FEA models.

Thanks,
AAY

 

[SWComposites]

Analysis to show "no detrimental deformation" is generally concerned with significant yielding of the structure. Generally localized yielding around small holes (e.g., fastener holes) is ignored; however, yielding around larger holes and cutouts is usually considered. Fatigue analsyis is different from the limit load analysis to show "no detrimental deformation" - to preict crack initiation for fatigue analysis the Kt at a stress concentration must be considered. And just because you show "no detrimental deformation" at Limit Load does not mean that you won't have a fatigue problem; it all depends on the repeated load spectrum. Hope this helps.

 

[jetmaker]

feajob,

The report I would cite would be the FAR's regarding the detrimental deformation section. If the deformation will be detrimental to the safety of flight, then make sure the kt is included. The discrtion there is whether it is a safety of flight issue, and not just a maintenance/functionality issue.

SWComposite makes a very good point about the type of material being used.

I have worked alot with 300M in landing gear components, and know that they are very fatigue sensitive, which is why they are often designed to safe-life requirements, and fully tested.

How does the fatigue problem correlate back to the static strength report?

Regards,

jetmaker

 

[feajob]

jetmaker,

My limit stress analysis shows a very small Margin of Safety without including any Kt. But, my Strain-Life fatigue analyis shows fatigue failure for the same hot spot. I am trying to refine my fatigue analysis and get the more precise results.

I am comletely agree with both of you. As SWComposite mentions, if there is "no detrimental deformation" at Limit Load then it does not mean that I won't have a fatigue problem; it all depends on the repeated load spectrum.

However, I think that the hot spots with small Margin of Safeties with limit loads are excellent candidates for fatigue analysis.

Regards,
AAY