Menu

Discussion Forum

New Discussion
Filter

Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

Transient thermal stresses-unexpected behaviour

Posted Oct 4, 2011, 11:25 a.m. EDT Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.2 6 Replies

Ali Ahmad
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

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 Oct 13, 2011, 10:20 a.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Oct 4, 2011, 2:20 p.m. EDT
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
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Oct 6, 2011, 12:40 p.m. EDT
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
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Oct 9, 2011, 7:26 a.m. EDT
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
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Oct 10, 2011, 1:12 a.m. EDT
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
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Oct 12, 2011, 4:58 a.m. EDT
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
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Oct 13, 2011, 10:20 a.m. EDT

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.

Suggested Content