Use in Code_Aster
===========================

In Code\ _\ *Aster*, the definition of orthotropic elastic characteristics that are constants or functions of temperature are defined by the command DEFI_MATERIAU, keywords ELAS_ORTH, ELAS_ISTR, ELAS_ISTR_FOou ELAS_ORTH_FOpour shell elements and isoparametric massive elements or the constituent layers of a composite (see command DEFI_COMPOSITE).

To define the orthotropy coordinate system :math:`(L,T,N)` linked to the elements, we can refer to the documentation [:external:ref:`U4.42.03 <U4.42.03>`] DEFI_COMPOSITE and [:external:ref:`U4.42.01 <U4.42.01>`] AFFE_CARA_ELEM.


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    /ELAS_ORTH = _F (♦ E_L = ygl = ygl Longitudinal Young's Modulus.

♦ E_T = ygt transversal Young's modulus.

◊ E_N = ygn normal Young's modulus.

♦ GL_T = gltShear module in the :math:`\text{LT}` plane.

◊ G_TN = GTNshear module in plane :math:`\mathrm{TN}`.

◊ G_LN = GLNshear module in plane :math:`\text{LN}`.

♦ NU_LT = zero Poisson's ratio in the :math:`\text{LT}` plane.

◊ NU_TN = nutn Poisson's Ratio in the :math:`\mathrm{TN}` plan.

◊ NU_LN = zero Poisson's ratio in the :math:`\text{LN}` plane.
◊ ALPHA_L = diln Longitudinal mean thermal expansion coefficient.
◊ ALPHA_T = said Transverse Mean Thermal Expansion Coefficient.
ALPHA_N = DIN Normal mean thermal expansion coefficient.

**Important note:**


.. csv-table::

    "*The presentation of this reference note is based on the convention of the books by J.L.Batoz and D.Gay. The documentation of* *DEFI_MATERIAU* [:external:ref:`U4.43.01 <U4.43.01>`] *describes these choices, and the coefficient* * *NU_LT* * *is interpreted as follows in* *Code_* *Aster:*
    *if we exert traction along the axis* :math:`L` *giving rise to a deformation along this axis equal to* :math:`{\epsilon }_{L}=\frac{{\sigma }_{L}}{\text{ygl}}` *, we have a deformation along the axis* :math:`T` *equal to:* :math:`{\epsilon }_{T}=-{\nu }_{\text{LT}}\cdot \frac{{\sigma }_{L}}{\text{ygl}}` *.*"