[Getfem-users] Plasticity brick

[logari81]

Hi all,

I have recently worked with the elastoplasticity brick and I have one 
relevant comment.

As a scalar measure of stress one can use either the euclidean norm of 
deviatoric sigma or the Von Mises stress. The difference is an sqrt(3/2) 
factor:

$\sigma_{vm} = \sqrt{3/2} |\sigma^{dev}|$

In getfem, |\sigma^{dev}| is used through the whole brick implementation 
except one point. Only in the stresses calculated by 
compute_elastoplasticity_Von_Mises_or_Tresca the result is multiplied by 
sqrt(3/2).

The stresses calculated by "compute_plastic_part" and the input 
parameter "datathreshold" do not include the sqrt(3/2) factor. So the 
user of the brick has to divide the tensile yield strength \sigma_y by 
sqrt(3/2) before assigning it to "datathreshold" and he also has to 
multiply the results of "compute_plastic_part" with the same factor in 
order to receive stresses comparable to the Von Mises definition.

Is this behavior intentional? Wouldn't it be more consistent to either 
remove the factor from "compute_elastoplasticity_Von_Mises_or_Tresca" or 
add it to "compute_plastic_part" and "datathreshold"?

Best regards

Kostas