Discussion Forum

Hi

I was always told, by my professors: "there are no stupid question, only stupid answers", so I'll try to not to be too stupid here ;)

first learn the new v4 naming convention, each predefined "physics" have an identifier "solid" is the one for structural analysis in 1D, 2D or 3D. To distinguish the variables used in the "solid" physics, these are prefixed by "solid." to make them unique so you can also access them from a linked "HT" or other physics.

Chaining simulation are an interesting way to decompose a loading case, but it's not always trivial. By default if you link in 2-3 or more stationary solvers one after the other and ask for the default COMSOL solver set-up, you will see that COMSOL stores the result after each solver sequence and uses the next solver initial conditions from the stored previous solved step. Well at lqast that is what you should use to link your models. When you hit the dependent variable node in the solver sequence you can change the settings for the initial variables, from the default to the current solution to the stored solution ...

Remains one difficulty, how to change the BC (boundary conditions) to step through your loading case, as i.e if you load a bar with a twist (moment) then remove the moment and compress it, the bar will release it's twist strain and return to its initial condition (except if you keep the moment load). Then today you cannot turn on and off BC from the solver nodes, these are not linked, so such a model you must run manually (right click the study node compute to selected) each node, then change manually the BC run next node etc.

Again there is an exception, you use Matlab to script the sequence and perform the intermediate changes to your BCs

I hope I have given you a few inputs, to test out. Do it on siiiimple problems so you really manage to see the different implication of each solver variable, that is what I find the trickiest

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Good luck
Ivar

Dear Ivar,

Thank you so much for your detailed reply. You have always answered peoples questions with a lot of detail and I must tell you that I very very much appreciate your help with this question.
The case which I suggested above was very much an arbitrary one! But you are right the BC's would need to be implemented with care. The main origin of this problem is looking to perform frequency response analysis on a pre-stressed structure. I did however wish to find a more general case regarding how Comsol implements multi-step problems.

But I fully agree with you, very very simple models are called for to fully get a handle on the "language" of Comsol.

Again, thank you very much Ivar, your reply has been printed off and firmly stuck in my lab book.

Rob

Hi
there are not that many examples I find, but there are a few since 4.2. The one I used to learn was the built-in solid buckling, as it relies on two solving steps in sequence, including a pre-load and linearisation for the eigenmode analysis

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Good luck
Ivar

Hi Ivar,

I am not able to find which example you are referring to. Is this the elbow-bracket example.

Rob

Hi again

try a search with "buckling" on the model library

as coupled cases you have the biased_resonator_2d_modes in the MEMS module but you need the MEMS toolbox


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Good luck
Ivar

Thank you Ivar, I have now found them!

I am looking forward to getting my hands on these examples!

Rob

Dear Ivar,

I have been having a look at the models, but I am having a problem still.

Would you mind having a very quick look, I dont think it will take you more than 5 minutes. I have attached the model (v4.2 using structural mech).

The model has two steps;

the first is a tension applied to the end of a rod
the second is a shear applied at the top of the rod

I know what you could model this as a single step, but I am trying to get a handle of how Comsol works so have split it into two steps.

For the second step I have given the rod initial stresses as computed from the first step. The problem however is that the rod is sheared in the second step but without the tension being applied. It looks like the rod returns back to its original shape.

Do you happen to know where I have gone wrong?

I would really appreciate your help on this.

Many thanks,

Rob

Hi

my first comment: if you want to displace the 10 meter bar by 2m you are in large deformation mode, so turn them on (solver first tick box)

I'm sure if you replace the 2m by 2cm you get something more reasonable (also with large deformations on

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Good luck
Ivar

Hi Guys,

Was this problem resolved? I am working on a similar model and have adjusted the displacements in the current problem as a reference. Comsol runs successfully, but the results to the second step does not show the initial displacement in the z direction.