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Transient thermal stresses-unexpected behaviour

Posted 4 oct. 2011, 11:25 UTC−4 Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.2 6 Replies

Ali Ahmad
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Dear Comsol experts/users

I started using Comsol just recently. I am interested in doing a basic thermo-mechanical stress analysis for validation purposes before moving to a more detailed study.

I actually have a question regarding the modelling of thermal expansion in comsol. When the temperature changes the thermal (von mises) stress will have to increase due to thermal expansion. After certain time you would expect the thermal stress to decrease due to relaxation. In the few examples I run, that wasn't the case in comsol. The stress always increases to a certain constant value and never relaxes.

I would be grateful if anyone could tell me if am I missing something or there is a way to account for stress relaxation over time.

Many thanks in advance,

Ali
6 Replies Last Post 13 oct. 2011, 10:20 UTC−4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 4 oct. 2011, 14:20 UTC−4
Hi

if you use linear material models I do not believe you have "relaxation" that is a different non-linear phenomena, you need to model using another physics settings and material properties. The linear model is correct for "small" temperature changes (even if these can be for a human quite "large"

--
Good luck
Ivar
Hi if you use linear material models I do not believe you have "relaxation" that is a different non-linear phenomena, you need to model using another physics settings and material properties. The linear model is correct for "small" temperature changes (even if these can be for a human quite "large" -- Good luck Ivar
Ali Ahmad
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Posted: 1 decade ago 6 oct. 2011, 12:40 UTC−4
Hi Ivar,

Thank you so much for your reply. I've changed my physics setting to thermal viscoelastic now. The linear model seems to be fairly simple, however, I'm still having the same problem.i.e the stress never relaxes to zero, even at very short relaxation time (1 s) . According to Maxwell model, you would expect the stress to decrease exponentially with time. so why comsol still asks for a strain reference temperature which in my opinion is the reason why the stresses in my test case never relax. Apologies if I'm asking naive questions, it's just I've just started looking at this sort of problems.

Many thanks,

Ali
Hi Ivar, Thank you so much for your reply. I've changed my physics setting to thermal viscoelastic now. The linear model seems to be fairly simple, however, I'm still having the same problem.i.e the stress never relaxes to zero, even at very short relaxation time (1 s) . According to Maxwell model, you would expect the stress to decrease exponentially with time. so why comsol still asks for a strain reference temperature which in my opinion is the reason why the stresses in my test case never relax. Apologies if I'm asking naive questions, it's just I've just started looking at this sort of problems. Many thanks, Ali
Kelly Funke
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Posted: 1 decade ago 9 oct. 2011, 07:26 UTC−4
Ali,

You are correct in your description of the phenomenon of thermal stress relaxation with time, temperature, and stress. The physics you describe needs to be represented in your model. The relaxation arises from time dependent deformation due to the material behavior of creep, which can be incorporated by adding a general form PDE to your model. To do this, you need to know the creep behavior of your material. See the example model and documentation at the link below.

www.comsol.com/showroom/documentation/model/207/

Regards,
K. Funke
Ali, You are correct in your description of the phenomenon of thermal stress relaxation with time, temperature, and stress. The physics you describe needs to be represented in your model. The relaxation arises from time dependent deformation due to the material behavior of creep, which can be incorporated by adding a general form PDE to your model. To do this, you need to know the creep behavior of your material. See the example model and documentation at the link below. http://www.comsol.com/showroom/documentation/model/207/ Regards, K. Funke
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago 10 oct. 2011, 01:12 UTC−4
Check your viscoelastic constants. To approach a basic Maxwell model the long term shear modulus in the Generalized Maxwell Model must be zero. That should get the stresses relaxing to zero regardless of the reference temperature.

Nagi Elabbasi
Veryst Engineering
Check your viscoelastic constants. To approach a basic Maxwell model the long term shear modulus in the Generalized Maxwell Model must be zero. That should get the stresses relaxing to zero regardless of the reference temperature. Nagi Elabbasi Veryst Engineering
Ali Ahmad
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Posted: 1 decade ago 12 oct. 2011, 04:58 UTC−4
Many thanks, it works well now by adding a creep physics as you suggested.

Best,

Ali
Many thanks, it works well now by adding a creep physics as you suggested. Best, Ali
Fragrance Paramoure
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Posted: 1 decade ago 13 oct. 2011, 10:20 UTC−4

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