Fuselage structure under pressure 1

[Chris31]

Quick question for anyone that has any insight. Fuselage stringer is modelled as a beam and the skin as a plate. The beams have offsets. When I run a pressure case only, the skin goes into tension and the beam goes into compression. Anyone have any ideas as to how this can happen? GPF balance shows a net tension, which is expected as the pressure bulkhead expands and the skin bulges out due to pressure. I'm trying to explain why the beams see compression. When I look at displacements, end a of the beam/skin (fwd grid) expands in the hoop direction, but end b (aft grid) contracts - im guessing because the pressure bulkhead is moving aft as this beam element is just fwd of the bulkhead. However, the net displacement between grids shows tension (moving away from each other), so how can the beam be in compression?


Thanks in advance

 

[rb1957]

"The fuselage sees area ruling at this location as well ..."

we're trying to help, but when we don't have the complete question, it becomes pretty futile

vent over, off soap box

 

[Chris31]

rb1957,

I appreciate all your help. I didn't hold back information on purpose. The stringers do not see this compression for non -pressure cases, which is why I didn't think of this earlier. However, I did mention the high bending moments previously - I just didn't think (or understand) how the bending could cause such high compression values in the beams. ieaz123 explains that this is due to the pressure fwd of the bulkhead, and lack of pressure aft - creating bending. I simply stated that the area ruling aft of the bulkhead could also be adding to the bending moments - even though compression is not seen in the beams for non-pressure cases.

Thanks for the help everyone

 

[Aerodesign]

is the structure behaving with ETB? If its going outside ETB a non-linear analysis may be needed. We found this when we runa soem FE simulations with inconsistent pressure units. It wasn't until we looked at th deflections that it became clear the displacements were absurd.

So a checklist:
Modelling unit of length?
Loading, consistent units.
Constraints - realistic, consistent with a test (or use inertia relief for flight cases).
Material linearity.
Geometry linearity.

I guess if you are using sensible structural bays and stiffener sizes you will get tension in most locations, maybe some local compression where structure is tied.