Fatigue life calculation

[MrHiLok]

I am working on a project which deals with A330 cockpit cross beams. the cross beams are fatigue critical structure. i am going to put an external weight (18 kg) on these cross beams and would like to understand how the fatigue life of these cross beams will be effected (decrease) due to the increase in weight (load)? the cross beams will have a fatigue life, i want to understand how to calculate the fatigue life of this structure, before and after addition of the weight. i will highly appreciate any suggestions/ideas.

 

[rb1957]

1) are you really doing fatigue analysis or damage tolerance ?

2) you are increasing the load on the beams, so increasing the stress (probably), so reducing the fatigue life (or the crack growth life or the inspection intervals).

3) do you have access to Airbus data ? then you'd know the pre-mod stress in the beam and access the impact.

4) without OEM data, I'd suggest you either ...
a) create a story that the impact is small/negligible, or
b) if you believe the impact is significant, then add structure to the beam to carry your load.

another day in paradise, or is paradise one day closer ?

 

[LiftDivergence]

Finding a fatigue life is typically based on S-N curves for the material at hand. I'm assuming your stress level is not under the endurance limit, so changing the stress will decrease your life.

You can look at the material data curves in MMPDS to figure out what your fatigue life might be. Keep in mind though, that that is not the whole story. If you are attaching a mass somehow I'm assuming you have fixtures of some kind. Which means you have added a stress concentration. Your Kt needs to be accounted for in the fatigue life calculation. A good reference for doing this is NASA report CR-2586.

As RB says, if you don't have airbus data you will need to engineer a conservative stress. If you are operating under FAA jurisdiction the FARs are your rulebook. Coming up with a stress for the cockpit cross beams probably won't be simple.

Also keep in mind that if you are modifying FCBS under 25.571, your damage tolerance evaluation must include more than a fatigue check. If you are not an OEM, a threshold for inspection of a structural repair or modification will be based on a fraction of the fatigue life, or a fraction of the crack growth life (which is usually critical).

How is the external weight mounted to the cross beams? Are you making new holes? If you are altering the Airbus FCBS you probably need to do some crack growth, unluess you have some existing inspections of this structure which you can leverage and knock down.

Keep em' Flying
//Fight Corrosion!

 

[MrHiLok]

Thank you for the response.
To keep the design simple and avoid drilling holes in primary structure. I am attaching the brackets to the composite floor board. floor board attachments are same as original.
I have qualified the crash criteria, the brackets from static point of view. Working out if DTA is required or not. if required then find out a simple and easy way to calculate it.

 

[rb1957]

are the floor boards part of FCBS ? (see FAR part 26) ... probably not

sounds "odd" ... to mount onto the floor boards.

how do the floor boards feel about doing this job ? have you analyzed them (or only the attachments) ?
how do you know what the attachments are good for ? what sort of floor board attachment are you applying ??

another day in paradise, or is paradise one day closer ?

 

[aerostress82]

There is no easy simple way to assess the fatigue life in such structure on such a large aircraft.

Your fatigue life isn't only the static loads that undergo the concentration factors, but is also vibrational effects. I've done both for some time, they are totally 2 different things.

If you go to that domain, you'll come to realize that you are hurting the structure by adding load. And drilling some holes to attach it there is getting it even worse.

You can't go cheap on this kind of change if that's what you are looking for really. You need to find an SRM that qualifies what you are doing. And you can only get it from the OEM which is Airbus or its supplier in this case. If they don't have an SRM, they will re-do "every analysis" that was previously made on the structure. Especially if it is not below the endurance limit, you will have some serious problems in the future. If you are below the endurance limit, you will most definitely still end up shortening the life of the structure significantly.

Easy - Fast - Cheap are the main origin of most commercial aerospace accidents in history. Consult with the OEM and get this approved with an official analysis. Otherwise, I don't think the level of your question resembles your completion of these analyses successfully.

I strictly recommend you to consult with the OEM. Primary structure (you said you won't attach it there) - even floor structure. Everything in the aircraft is sized for a purpose. You can't really blindly change it as you see fit without having any data about it. Like if it were a bolt you were oversizing, and you did significant amount of work on this repair before - yeah then maybe. But not this one. Please try to find some official direction rather than here alone.

Wherever you attach the brackets you mentioned, that structure alone has its own problems in its lifetime. You will end up overstressing things without knowing where you can actually stress. Hope my point is clear.

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer