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How to Find the Expression of Plastic Strain in Solid Mechanics

Posted May 10, 2015, 7:04 p.m. EDT Parameters, Variables, & Functions, Structural Mechanics Version 4.4 2 Replies

Rong Xu
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Hi,

In solid mechanics module, I want to take derivatives of the plastic train with respect to the coordinates. The plastic strain curve is like Fig.epl11.
However, I found when I use the operation d(solid.epl11, X), the result is zero, like Fig.depl11. It seems like the plastic strain is not a function of the coordinates, no matter x or X.
I also check the 'Equation View' of Plasticity module. I found there is not expression for the plastic strain solid.epl11, as shown in Fig.Equation View.
Can anyone tell me:
1.How can I find the expression for plastic strain in solid mechanics module?
2.what is its independent variable?
3.How can I take derivatives of the plastic train with respect to the coordinates?
Thanks so much!
2 Replies Last Post May 11, 2015, 3:19 p.m. EDT
Henrik Sönnerlind COMSOL Employee

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Posted: 10 years ago May 11, 2015, 10:30 a.m. EDT
Hi,

The reason that you cannot see an expression for the plastic strain in equation view is that the plastic strains are computed by an algorithm, rather than by an explicit expression.

The plastic strains are state variables computed and stored in the integration points (Gauss points). They are not described by any spatial shape functions and are not assumed to have any specific degree of spatial continuity. This is why you cannot compute spatial derivatives.

If you really need these derivatives, you can map the plastic strains (in a weak sense) to a continuous field of your own choice. One way of doing this is to add a Weak Contribution as shown in the model library example stress_depdendent_elasticity (in that case a stress quantity is mapped).

Regards,
Henrik
Hi, The reason that you cannot see an expression for the plastic strain in equation view is that the plastic strains are computed by an algorithm, rather than by an explicit expression. The plastic strains are state variables computed and stored in the integration points (Gauss points). They are not described by any spatial shape functions and are not assumed to have any specific degree of spatial continuity. This is why you cannot compute spatial derivatives. If you really need these derivatives, you can map the plastic strains (in a weak sense) to a continuous field of your own choice. One way of doing this is to add a Weak Contribution as shown in the model library example stress_depdendent_elasticity (in that case a stress quantity is mapped). Regards, Henrik
Rong Xu
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Posted: 10 years ago May 11, 2015, 3:19 p.m. EDT
Hi Henrik,

Thanks for your suggestion. I will try it!

Best,
Rong
Hi Henrik, Thanks for your suggestion. I will try it! Best, Rong

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