Attempt to edit into a more specific question as suggested by moderator:
I would like define the shape and material properties of a 3D thin-walled shell/membrane-like structure (for instance a baloon). Then I would like to approximate the pressure volume-relation (PVR) of this structure - what pressure will we obtain when increasing the internal volume?
I'm planning to create a mesh of 2D triangular elements for a 3D shell with volume V0 and internal pressure zero. After defining the stiffness matrix for the material I should be able to compute the nodal forces, geometric configuration and internal volume for a certain internal pressure (the plan is to use an iterative approach searching for a configuration where all resulting nodal forces equals zero, that is where expanding forces arising from the internal pressure equals constricting forces arising from displacement).
To do this I need to calculate the nodal forces created by 1) displacement from the original V0 configuration and 2) from the internal pressure. I think I can figure out the first one, but I do not know how to transform the internal pressure (acting perpendicular on the triangular elements) into nodal forces. Any ideas?