====== ECHRA ======
===== Description =====
Constitutive law for the heat exchange by mutual radiation between surface elements of solids (for element [[elements:rayon|RAYON]])

==== The model ====
This law is only  used for non linear thermal analysis of solids.\\
This constitutive law allows to consider heat exchange by radiation between elements pertaining to the surface of a solid. 


==== Files ====
Prepro: LECHRA.F \\
Lagamine: ECHRA.F
===== Availability =====
|Plane stress state| YES |
|Plane strain state| YES |
|Axisymmetric state| YES |
|3D state| NO |
|Generalized plane state| YES |
===== Input file =====
==== Parameters defining the type of constitutive law ====
^ Line 1 (2I5, 60A1)^^
|IL|Law number|
|ITYPE| 126 |
|COMMENT| Any comment (up to 60 characters) that will be reproduced on the output listing|
==== Integer parameters ====
None
==== Real parameters ====
^ Line 1 (G10.0) ^^
|PLANCK|PLANCK's constant (given in the appropriate system of units)|

===== Stresses =====
==== Number of stresses ====
2
==== Meaning (numerical, not physical) ====
|SIG(1)|heat flow exchanged by radiation neglecting the view factor between the surface elements|
|SIG(2)|same as above but taking account of the view factor between surface elements multiplied by the area|

===== State variables =====
==== Number of state variables ====
3
==== List of state variables ====
|Q(1)| temperature of the first radiating point $(=T_1)$ |
|Q(2)| temperature of the second radiating $(=T_2)$ |
|Q(3)| $\sigma* \left( T_1^3 + T_1^2 T_2 + T_1 T_2^2 + T_2^3\right)$ |