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Pressure in Earth crust

Posted Mar 13, 2014, 9:40 a.m. EDT Modeling Tools & Definitions, Parameters, Variables, & Functions, Structural Mechanics Version 4.3b 2 Replies

Alexis Hrysiewicz
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Hello,

I am trying to model the Earth crust. My model is 50 km * 30 km and in 2D (plane strain). I am using the solid-mechanics physics with linear elastic material. Fora first test, I chose E = 40*10^9 Pa, poisson = 0.25, density = 2800 kg/m^3. The left and right boundaries are roller, and the lower boundary is fixed (no displacement). Only the top boundary is free.

There aren't any inital values (no displacement, no velocity). Finally, I have a body load (the pressure; force per unit volume) : -rho*g for value y.

After calculation with Comsol, my problem is:
- The crust isn't static, there is a displacement. Why is that so?
- The stress distribution due to gravity calculed by Comsol differs from the pressure rho*g*y: there is a non-zero deviatoric stress. Why is there a deviatoric stress since the stress distribution should be lithostatic?

Thank you for your help!

Alexis
2 Replies Last Post Mar 16, 2014, 5:55 p.m. EDT
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Mar 14, 2014, 10:40 a.m. EDT
Hi Alexis,

There is a displacement because you have an unbalanced force applied on the Earth crust model. You are modeling the “deformed” geometry of the crust as if it is the initial stress-free geometry. There is an easy workaround though. You should apply an initial stress field in the X, Y and Z directions that increases linearly with depth. That will give you internal forces that will be in equilibrium with the body load, and therefore zero displacements.

This should also eliminate the deviatoric stress. Right now you are getting a deviatoric stress component because it is the correct solution to the model that you are currently solving!

Nagi Elabbasi
Veryst Engineering
Hi Alexis, There is a displacement because you have an unbalanced force applied on the Earth crust model. You are modeling the “deformed” geometry of the crust as if it is the initial stress-free geometry. There is an easy workaround though. You should apply an initial stress field in the X, Y and Z directions that increases linearly with depth. That will give you internal forces that will be in equilibrium with the body load, and therefore zero displacements. This should also eliminate the deviatoric stress. Right now you are getting a deviatoric stress component because it is the correct solution to the model that you are currently solving! Nagi Elabbasi Veryst Engineering
Alexis Hrysiewicz
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Posted: 1 decade ago Mar 16, 2014, 5:55 p.m. EDT
Thank you for all the information.

We calculated a simple analytical model starting from the initial conditions that we placed in Comsol. And after calculation, we found the results of COMSOL. It is a mistake of reason from us...

Thank you again

Alexis
Thank you for all the information. We calculated a simple analytical model starting from the initial conditions that we placed in Comsol. And after calculation, we found the results of COMSOL. It is a mistake of reason from us... Thank you again Alexis

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