Cbeam in tension and Cquad4 in compression? 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

 

[truckcab]

If I have understood the geometry correctly then the compression in the beam is due to bending. Your shells and the beams interact as a composite (which is really the whole idea with a stiffened skin). Internal pressure in the fuselage will cause the "composite" section to bend, thereby causing "global" bending stresses that are compressive due to an internal pressure. These stresses can be greater to the "pure" axial stresses caused by the elongation of the beam (dist between node A and B increases) thereby causing compressive stress in the beam.

I attach to you a figure in which i try to explain what I really mean! Please check it out!


And by the way! I know I have an artistic touch )

Live Long and Prosper