Loads
===========

The keyword factor EXCIT makes it possible to describe, at each occurrence, a load (solicitations and
boundary conditions), and possibly a multiplying coefficient and/or a type of load.

The nonlinear calculation is solved by an incremental method, so it is necessary to
**setup** the calculation.
So we apply the loads incrementally also, for example, by applying
the force progressively (a ramp).
The basic principle is to separate the *spatial* part of the load and the part
*(nickname) -temporal*.

.. math::
\ underline {L} _ {ext}\ left ({\ underline {u}, t}\ right)
    =
    g\ left (t\ right)
    \ underline {L} _ {ext}\ left (\ underline {u}\ right)

The spatial part of the load
:math:`\underline{L}_{ext} \left ( \underline{u} \right )` is given by the keyword
CHARGE (possibly including the evolution of a temperature field).

notes
    * In a thermo-mechanical calculation, if the initial temperature is different from the temperature
      reference (given in [:ref:`U4.43.01 <U4.43.01>`]), the deformation field associated with
      the initial moment may be incompatible and therefore lead to a state of constraints and
      associated non-zero internal variables. If one uses an incremental behavior relationship
      What if we do not explicitly define an initial state of constraints and internal variables
      (associated with an initial temperature field different from the reference temperature),
      the field of constraints and internal variables calculated at the first increment will not take into account
      only from the only variation in temperature between the initial moment and the first moment, and no
      any compatibility constraints associated with the initial temperature. To take
      Once this initial state is taken into account, it must be given explicitly, for example thanks
      to the keywords in ETAT_INIT. To avoid such situations that may lead to
      calculation errors, it is better to start a calculation by considering that you have to start from a
      pristine condition.
    * If an axisymmetric calculation is performed and nodal forces are imposed, these forces
      must be divided by :math:`2\pi` (we are working on a sector of one radian) compared to
      to the actual loads. Likewise, if we want to calculate the resultant of the efforts, the
      The result must be multiplied by :math:`2\pi` to get the total resultant for the structure
      complete. Likewise in plane stresses or in plane deformation, we work on a
      unit thickness: the forces (on the thickness) applied must be divided by the thickness,
      the real forces are obtained by multiplying the calculation efforts by the thickness.

The time part of the load :math:`g \left ( t \right )`
is given by the keyword FONC_MULT.
If the multiplier function is not specified, a step function is applied.

It is possible to define directly
:math:`\underline{L}_{ext} \left ( {\underline{u}, t} \right )` in the CHARGE keyword
if you use the AFFE_CHAR_MECA_F command (time function). In this case, you have to be careful:
Indeed, if we use FONC_MULT at the same time, we multiply the load again by
a function of time.