Discussion Forum

Hi

have you tried to plot it with the expression for the x-axis as "x" (and then later X) ? (or Y if appropriate)
this will most probably give you different results, I assume the arc length is referrred to the material frame (before deformations)

--
Good luck
Ivar

Hi Ivar,

As ever, thanks for the reply.

I have tried both x and X as an expression.

I think that you are correct that the expression line is made with reference to the structure before deformation.

I do not however see the deformation at all in the plot. The disc, taken from the arc length appears to be the same size as before deformation.

This is however wrong, as the deformation of the disc comes AFTER the tensioning of the membrane.

So I am a little puzzled. I guess the question is...

How can I probe values in a structure once the structure has been deformed? I think this would apply to all structural mechanics problems where there is deformation.

Thanks for your help :-)

Rob

Hi

I believe the arc is defined w.r.t. the material frame, while in structural x and X are related to the Spatial, respectively Material frame, but I do now believe x=X+u only when you apply the "include geometric non-linearities" (this has changes a few times over 4.0 to 4.2 but I beleive it's stable now ;)

You need to apply a deformation sub node to your plots, this is not automatic (except for a few default plots initiated by the solver the first time it runs)

you have the deformations vector _u_ = (u,v,w) use that in stead of the disp=sqrt(u^2+v^2+w^2)

--
Good luck
Ivar

Hi Ivar,

You have managed to solve my problem yet again!

By plotting u, the deformation is taken into account. The arc line method, can be a little misleading as it does not state that it is w.r.t. the material frame.

But I guess if it was easy we would not enjoy it :-)

If there is a Comsol Award, you should be first in line.

Thanks so much.

Hi Ivar,

I may have spoken too soon.

Could you perhaps see what I have done wrong here.

Again, I have the membrane and I stretch it by a percentage of its original radius. I have 2 different stretch factors.

- 0.05% and 100% [for illustration purposes more than anything]

For both of these, the membrane is stretched past its original shape. As shown in Image 1. So I know that the model is being stretched, but am having difficulties extracting stress values.

Then, when I wish to plot the Mises stress through the middle of the membrane, shown in Image 1. And I plot them, I get the problem shown in Image 2 (the x-axis shows that the deformation is not 5% and 100%).

The diameter of the membrane is 2mm, so I would expect the x-axis "Displacement field" to be -2mm +2mm for 100% stretch and -1.05mm +1.05mm for the 5% stretch.

In figure 2, I have plotted 'u' in the x axis, as the expression. Plotting x or X, is shown in Figure 3. Figure 3 is the most confusing for me as it shows the stress within the membrane but to me it looks like its suggesting that both membranes are the same size (which from figure 1 is obvious that they are not).


This is quite confusing. Any suggestions?

Rob

Hi

No no suggestions, but I tried a simple test for me the logic seems coherent but for such large deformation I use the "geometrical non linearity" solver setting (nece my x=X+u)

--
Good luck
Ivar