Rethink beam materials in core and 4C

[isteinbrecher]

Thanks again for the new functionality, I really enjoy using the get_four_c_beam function - it makes it so easy to quickly switch the entire discretization without importing anything else!

#430 (comment) it would be nice if we can also do this for materials and not just for the beams. Currently, if one switches between beam types, e.g., Kirchhoff, Reissner, ... one has to switch the Material (with imports) as well. So it would be nice to have a similar functionality.

Alternatively, we can also rethink how beam materials are handled in FourC. A few things that come to mind:

• Maybe it makes sense to generally unify this and not have individual classes for different types. How does this work if I have different beam types referencing the same material?
• Add a basic support for different cross-section types, e.g., circle, rectangle, ... that can then also be customised and extended by the user.

Refactoring the material has been on my mind for a long time, so let's use this issue to gather thoughts and see if and how we can do this.

[davidrudlstorfer]

Sounds really good!

One question: do we even have the capabilities in 4C to use other cross sections than a circle?

[isteinbrecher]

The beam formulations in 4C are somewhat agnostic w.r.t. the cross-section shape. We can provide the stiffness values for arbitrary non-symmetric cross sections:

  - MAT: 1
    MAT_BeamReissnerElastHyper:
      CROSSAREA: 0.007853981633974483
      DENS: 0.0
      MOMIN2: 4.9087385212340526e-06
      MOMIN3: 4.9087385212340526e-06
      MOMINPOL: 9.817477042468105e-06
      POISSONRATIO: 0.0
      SHEARCORR: 1
      YOUNG: -1.0

The values CROSSAREA, MOMIN2, and so on are not required to follow any specific formula for a cross-section shape.

Now the "however", most of the beam-interaction stuff in 4C is build on the assumption of circular-cross sections.

[davidrudlstorfer]

Thanks for the explanation :) I thought I missed something in 4C and it would be possible to somehow define arbitrary cross sections everywhere 😅