Learning Forum

[navidMnzh]

Hi!

I'm working on a project (which I got help about it a lot in here?).

I'm aiming to achieve thermal stress and strain. Ansys workbench has a Thermal Strain option by default.

The thermal strain result is:

[Erik Kostson]

Hi

First we should say that there is no thermal stress, with that meaning that temperatures do not causes stresses (they cause strains, thermal strains), hence why there is no such result output. To show this, say a body is not fixed and hence completely free to expand we will expand (thermal expansion) and develop thermal strains (alpha_thermal_expansion*Delta_Temperature - this is what we plot only in ansys with thermal strains results) but of course no mechanical stresses will be present. We get mechanical stresses due to thermal loads when we have constraints and other surrounding effects prohibiting free thermal expansion of a part. Say we have a 1D stress (say a 1D rod) that is restrained at both ends, and then we apply a temperature to it. In this case the mechanical strain is equal and opposite in sign to the thermal strain and we then get of course a compressive stress in the rod. You can try and study this in ansys and compare to hand calculations, that is very good exercise.

I would also suggest to go through this course that will explain this:
So one can choose to look at VM stress or minimum principal stress, or normal stress (say radial direction),etc, and that will stress caused by thermal loads/strains.

You have also asked a similar question on your other post so please be mindful not to open multiple post with similar questions.

All the best

Erik

[navidMnzh]

Thanks a lot for your great help sir I asked this question in one of the other discussions, but didn't get complete answer. so I asked it again, any way I will try to ask questions with less similarity I have a central fixed constraint on my spheres, and have thermal loads on both spheres.
This condition will cause thermal strain to appear, and based on loads, we are going to have stress too.
The strain is mentioned before, but the stress is like below:
as we now, based on hooke's law, we have the following equation:
STRESS = (modulus of elasticity) * STRAIN
So we should have a stress, which is maximum in the center, and gets lower and lower as it moves to the surface. Exactly like the thermal strain results.
but none of the stress result outputs show the desired result.
The question is whether the stress result is normal or not, or I'm missing something in here.

[Erik Kostson]

Hi

Please read through what I wrote and see that course I suggested - it will help you understand this.

"So we should have a stress, which is maximum in the center, and gets lower and lower as it moves to the surface. Exactly like the thermal strain results."
Look at the example I gave above which is for a simple free part case (not restrains or other parts with different expansion to provide any resistance to its movement). There the mechanical stresses/strains will be zero, but thermal strains are not and are just alpha_thermal_expansion*Delta_Temperature and constant at their maximum (since we assume a constant temperature increase across the free body)- this is what we plot in ansys with thermal strains results, but of course as we said we do not have any stresses since it is free to expand, and hence zero stress is observed. So what you are saying is not observed, and the mechanical stress clearly does not need to follow the thermal strains plot as explained here.

Finally in order to check these results you will need to have some form of hand calculation or another numerical way to obtain the results and then compare that to the results obtained with Ansys .

All the best

Erik

[navidMnzh]

Sorry for the late reply I read about Thermal strains from the useful source that you shared and also wikipedia, and yes, we shouldn't expect any thermal stress to occur, if there is nothing that prevents the body from expansion.
Here we got a different case, we have a small sphere with the alpha of 1.2E-5 and a large sphere, surrounding the smaller one, with alpha of 2.3E-5.
As we can observe, the alpha of larger sphere is about two times bigger, so the large sphere will expand at a higher rate in comparison to the smaller sphere. So the smaller sphere will face no obstacle in its expansion process and hence there is no stress.
But I also solved the reversed condition, giving the higher alpha to smaller sphere, and the lower one to larger sphere. In this case, larger sphere must put some force on the smaller one and prevent it to expand, which results in stress that is related to thermal strain, as wikipedia says:
---------------------------------------------------------
If the body is constrained so that it cannot expand, then internal stress will be caused (or changed) by a change in temperature. This stress can be calculated by considering the strain that would occur if the body were free to expand and the stress required to reduce that strain to zero, through the stress/strain relationship characterised by the elastic orYoung's modulus
---------------------------------------------------------
Results didn't change that much in reversed condition, and I still got the blue color in small sphere.
I thought there is something wrong with the connection between two surfaces, but dear helped me a lot with it, so the modeling seems fine.
So the question is:
If the modeling is fine, so we must have some kind of thermal stress (caused by difference in alpha) in smaller sphere. But there is no stress. what is the problem here?

[peteroznewman]

Erik may have read my comments in your other thread where the modeling issues were fixed.
/forum/discussion/34916/strange-results-in-thermal-structural-analysis-2
In that model, the small sphere has a reference temperature of 0C and a temperature load on the body of 10C while the large sphere has a reference temperature of 10C and a temperature load on the surface of 0C so the small sphere is getting hotter and the large sphere is getting colder. Thermal strain is alpha*dT not just alpha. dT for the small sphere is +10C, while dT for the large sphere varies from 0 to -10C.
"the alpha of larger sphere is about two times bigger, so the large sphere will expand at a higher rate in comparison to the smaller sphere"
That is only true if both bodies have the same dT, but in your model, you are imposing opposite values of dT on the two spheres.
"So the smaller sphere will face no obstacle in its expansion process and hence there is no stress."
Even if the dT was the same, this is only true if the small sphere is not bonded to the large sphere so a gap can open up, but you have the two spheres bonded, so that is not true.