One formulation for non-linear material is implemented. https://forum.freecadweb.org/viewtopic.php?f=18&t=23101&hilit=plastic+material
For applying non-linear material
- select existing material object
- then add nonlinear properties
Great! I am going to check the implemntation.
Just for the record, the nonlinear properties option is under ‘Model’ menu when you are in the FEM workbench. It took me a while to found it 
hello,
Is there a way to feed material properties (Young modulus, thermal conductivity) varying with temperature 
?
Can it be done by editing the .FCMat file? if yes how?
Thanks in advance.
Kazu
You can add temperatures directly to the CalculiX input file before running thermomechanical analysis.
Search for *MATERIAL card and modify it from
*MATERIAL, NAME=SolidMaterial
*ELASTIC
210000, 0.300
*CONDUCTIVITY
43.000
*EXPANSION
1.200e-05
*SPECIFIC HEAT
5.900e+08
where each of these entries can be temperature depended - if you add temperature to 3rd position in the line with numbers, e.g. elastic modulus (210000 MPa, 200000 MPa) varying with temperature (300K, 500K):
*ELASTIC
210000, 0.300, 300.
200000, 0.300, 500.
you can check details in the ccx manual
http://www.feacluster.com/CalculiX/ccx_2.13/doc/ccx/node244.html
http://www.feacluster.com/CalculiX/ccx_2.13/doc/ccx/node220.html