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.  In MAPDL that data goes into a *real-constant* set, identified
by a ``REAL`` id and attached to each element through its
``REAL`` stamp.  femorph-solver reproduces the convention with
:meth:`femorph_solver.Model.r` and :meth:`~femorph_solver.Model.real`.

R and REAL
----------

.. code-block:: python

    m = femorph_solver.Model()
    m.et(1, "SHELL181")
    m.mat(1)
    m.mp("EX", 1, 2.1e11)
    m.mp("PRXY", 1, 0.30)
    m.mp("DENS", 1, 7850.0)

    m.real(1)             # REAL,1 — stamp the id for subsequent E,...
    m.r(1, 0.005)         # R,1,0.005 — 5 mm thickness

    # ... n(), e() ...

:meth:`Model.r` stores the positional values on the grid; the element
kernel pulls them out as a 1-D array and indexes them by the layout
documented in its theory page.

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

.. list-table::
    :widths: 15 25 60
    :header-rows: 1

    * - Element
      - Positional layout
      - Notes
    * - LINK180
      - ``r = [AREA, ADDMAS, ISTRN]``
      - Only ``AREA`` is mandatory.  ``ADDMAS`` and ``ISTRN`` are parsed
        but not yet used.
    * - COMBIN14
      - ``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.
    * - MASS21
      - ``r = [MASSX, MASSY, MASSZ]``
      - ``MASSX`` is mandatory; ``MASSY`` / ``MASSZ`` default to
        ``MASSX`` when omitted.
    * - BEAM188
      - ``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.
    * - PLANE182
      - ``r = [THK]``
      - Out-of-plane thickness; default 1.0.
    * - SHELL181
      - ``r = [THICKNESS]``
      - Through-thickness dimension.  Mandatory.
    * - SOLID185 / SOLID186 / SOLID187
      - (none)
      - Solid elements carry no real constants; geometry is fully
        determined by the nodes.

CDB decks round-trip the positional layout exactly — an element
carrying ``REAL,2`` in the input deck ends up with
``m.real_constants[2]`` populated in the correct order.

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

.. code-block:: python

    m.real_constants
    # {1: array([0.005])}              # SHELL181 thickness
    # {2: array([1.0e-4, 8.3e-9, 8.3e-9, 1.7e-8])}   # BEAM188 section

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

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

MAPDL's ``SECTYPE`` / ``SECDATA`` commands for beam cross-sections are
not yet consumed by femorph-solver.  A CDB with only ``R`` data works today;
a CDB using ``SECTYPE,...,BEAM,RECT`` + ``SECDATA`` for a beam
section does not (yet) translate into BEAM188 real constants.  Both
paths will eventually share a common intermediate representation.

See also
--------

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