Real constants and sections
===========================

Some elements need geometric data the mesh doesn't carry — a shell
thickness, a truss cross-section area, a beam inertia tensor, a
point mass.  femorph-solver stamps that data on the model via
:meth:`Model.assign(..., real=...) <femorph_solver.Model.assign>`,
which writes a 1-D array under a ``real_id`` that the element
kernel later indexes.

Native API
----------

.. code-block:: python

    import femorph_solver as fs

    model = fs.Model.from_grid(grid)
    model.assign(
        fs.ELEMENTS.QUAD4_SHELL,
        {"EX": 2.1e11, "PRXY": 0.30, "DENS": 7850.0},
        real=[0.005],   # 5 mm shell thickness, written under real_id=1
    )

The element kernel reads the array as a flat positional buffer and
extracts the named slots via the table below.

Per-element-type layouts
------------------------

.. list-table::
    :widths: 18 25 57
    :header-rows: 1

    * - Element
      - Positional layout
      - Notes
    * - TRUSS2
      - ``r = [AREA, ADDMAS, ISTRN]``
      - Only ``AREA`` is mandatory.  ``ADDMAS`` and ``ISTRN`` are
        parsed but not yet used.
    * - SPRING
      - ``r = [K, CV1, CV2, IL]``
      - ``K`` is the spring constant (mandatory).  ``CV1`` /
        ``CV2`` damping and ``IL`` initial length are parsed but
        not used in the current :math:`K_e` path.
    * - POINT_MASS
      - ``r = [MASSX, MASSY, MASSZ]``
      - ``MASSX`` is mandatory; ``MASSY`` / ``MASSZ`` default to
        ``MASSX`` when omitted.
    * - BEAM2
      - ``r = [AREA, IZZ, IYY, J]``
      - ``AREA`` cross-sectional area; ``IZZ`` / ``IYY`` the two
        bending inertias about the element's local axes; ``J`` the
        Saint-Venant torsion constant.
    * - QUAD4_PLANE
      - ``r = [THK]``
      - Out-of-plane thickness; default 1.0.
    * - QUAD4_SHELL
      - ``r = [THICKNESS]``
      - Through-thickness dimension.  Mandatory.
    * - HEX8 / HEX20 / TET10 / WEDGE15 / PYR13
      - (none)
      - Solid elements carry no real constants; geometry is fully
        determined by the nodes.

Foreign-deck readers (CDB / BDF / INP / FEM) round-trip the
positional layout exactly — an element carrying ``REAL,2`` in the
input deck ends up with ``model.real_constants[2]`` populated in
the correct order.

Introspection
-------------

.. code-block:: python

    model.real_constants
    # {1: array([0.005])}              # QUAD4_SHELL thickness
    # {2: array([1.0e-4, 8.3e-9, 8.3e-9, 1.7e-8])}   # BEAM2 section

    # Which real id does an element use?
    model.grid.cell_data["ansys_real_constant"]

Sections (future)
-----------------

Beam cross-section description blocks (e.g. MAPDL's
``SECTYPE`` / ``SECDATA``, NASTRAN's ``PBARL`` / ``PBEAML``,
Abaqus's ``*BEAM SECTION``) are not yet consumed by femorph-solver.
A foreign deck with only the equivalent of ``R`` data works today;
a deck describing a beam through a cross-section catalogue does
not yet translate into the equivalent BEAM2 real constants.  All
three formats will eventually share a common intermediate
representation.

See also
--------

- :doc:`materials` — material properties (``EX``, ``PRXY``,
  ``DENS`` etc.).
- :doc:`/user-guide/pre-processing/elements/index` — per-element
  documentation including the exact real-constant layout.