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"Contact Pressure" vs. "Pressure"?

Posted Mar 13, 2015, 10:47 a.m. EDT Modeling Tools & Definitions, Parameters, Variables, & Functions, Results & Visualization, Studies & Solvers, Structural Mechanics Version 4.4 4 Replies

Daniel Kaiser
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When I compute "contact pressure" and "pressure" on the contact boundary and export the data, I get different answers (attached).

Does anyone know how does COMSOL caluclates "contact pressure" differently than "pressure"?

Thanks!

Daniel
4 Replies Last Post Mar 17, 2015, 11:06 a.m. EDT
Henrik Sönnerlind COMSOL Employee

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Posted: 10 years ago Mar 15, 2015, 3:54 p.m. EDT
Hi,

These are completely different quantities. The contact pressure is the normal component of the traction on the contact surface (positive in compression, though).

The pressure variable is the mean stress; p = -trace(stress_tensor)/3

Regards,
Henrik
Hi, These are completely different quantities. The contact pressure is the normal component of the traction on the contact surface (positive in compression, though). The pressure variable is the mean stress; p = -trace(stress_tensor)/3 Regards, Henrik
Daniel Kaiser
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Posted: 10 years ago Mar 16, 2015, 1:45 p.m. EDT
Henrik,

Thank you for the clarification. Can you tell me the formula that COMSOL uses to calculate the contact pressure? I tried n dot Sigma dot n on the contact surface, but that was not the correct formula (sigma is the stress tensor).

Thanks!

Daniel
Henrik, Thank you for the clarification. Can you tell me the formula that COMSOL uses to calculate the contact pressure? I tried n dot Sigma dot n on the contact surface, but that was not the correct formula (sigma is the stress tensor). Thanks! Daniel
Henrik Sönnerlind COMSOL Employee

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Posted: 10 years ago Mar 17, 2015, 2:58 a.m. EDT
Hi,

The contact pressure is a degree of freedom which is solved for (assuming that you are using the default augmented Lagrangian method). Please consult the theory section of the documentation for details.

Using 'n dot Sigma dot n' will in principle give the same quantity, but with significantly less accuracy since it is derived from the stresses in the element. Also, note that the 'n dot Sigma dot n' formulation must be used with the appropriate combination of stress tensor and normal vector if the deformations are not infinitesimal. The contact pressure variable is a '1st Piola-Kirchhoff type' quantity.

Regards,
Henrik
Hi, The contact pressure is a degree of freedom which is solved for (assuming that you are using the default augmented Lagrangian method). Please consult the theory section of the documentation for details. Using 'n dot Sigma dot n' will in principle give the same quantity, but with significantly less accuracy since it is derived from the stresses in the element. Also, note that the 'n dot Sigma dot n' formulation must be used with the appropriate combination of stress tensor and normal vector if the deformations are not infinitesimal. The contact pressure variable is a '1st Piola-Kirchhoff type' quantity. Regards, Henrik
Daniel Kaiser
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Posted: 10 years ago Mar 17, 2015, 11:06 a.m. EDT
Henrik,

Thanks, and I appreciate it!

Best,

Daniel
Henrik, Thanks, and I appreciate it! Best, Daniel

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