Designing a fuselage repair to 15 ksi

[koopas]

Hello all,

Is designing a 767 rectangular [pressurized] fuselage cutout repair to carry 15 ksi in the hoop direction and half of that in the longitudinal direction acceptable, given the fuselage radius, local skin thickness, and pressurization level?

Indeed, this is much less than designing to the ultimate strength of the skin of 60 ksi (2024-T3)

The lack of conservatism also lies in designing to only operating pressurization levels.

I expect most everyone to find such a repair generally unsafe.

My second question would be: In such an unfit repair scenario, can one omit the presence of:

1. additional axial stresses due to fuselage bending?
2. shear stresses due to bending and torsion? Is there much fuselage torsion? Would shear stresses be present?


What are your $0.02?

Alex

 

[jetmaker]

koopas,

That is a very general question, and it really depends on where you are in the aircraft (i.e. forward of the wings, in the crown, towards the tail).

I would not design to pressure cases only, unless you were certain that that was the critical condition. Look for similar structure such as lap and butt splices, or stinger/skin attachment, and then calculate the capability of those details. Use those details to obtain minimum capability.

Good luck,

jetmaker

 

[johnchance]

Koopas,

Hoop and axial tension acting alone might be the critical load case for your panel if there were no additional out of plane bending, or in plane shear forces acting on the panel. The walls of a cylindrical pressure vessel, far from the endcaps, approaches this condition. I imagine one or more of the following exist in your structure, causing the hoop tension to be a small part of the critical load:

1- Bending moments caused by the proximity of frames, intercostals, skin padups, or other structure attached to the skin. These structures restrict the skins deformation in the radial direction, inducing bending moments in the skin.

2- Axial stresses in the effective skin adjacent to longerons and intercostals due to fuselage bending, along with the shear from these loads and any reacted torsion .

3- Net section effects due to the area out of fasteners

4- Fatigue/damage tolerance/ fail safe concerns due to stress risers at fasteners, repair doublers, and padup fillets.

By the way, the old Atlas and Centaur rockets were pressure stabilized, without any support structure away from the bulkheads. Much of the structure was hoop tension critical.

http://www.astronautix.com/lvs/atlasa.htm

 

[inertia4u]

I think you are treading on very thin ice. I would *never* design a repair to only fuselage pressurization - you don't know all the loads in the area. Both jetmaker and johnchance offer good guidance though. Look at any joints in the area.

Your best bet is to assuem a *minimum* 3 rows up/down and fwd/aft using structural rivets (No. 6 and above). Anything less and (IMHO) you are asking for trouble. Most repairs that I've designed for 737 were min 3row, most 757 were 4 row using NO. 6 rivets.

Cheers,
Nert

-----
Nert

 

[AeroE]

It's all about the design of the joint and the load transfer in the fasteners, not just the overall stress level.

In fact, if you were to assume 15 ksi to develop loads for developing joint loads, your guess would be sadly undersized.

On the other hand, adding a repair that would lower the overall local stress to 15 ksi would probably be good, but that case implies that you know the design loads.

As others pointed out above, you need to have some basic understanding of the dominant stress state - longitudinal compression in the belly during flight, for example.

Is the skin really 2024-T3? I have doubts - it's probably heat treated to some other condition after forming. -T3 is a work hardened condition that must be produced by the stock manufacturer.