Negligible pressure boundary loads are giving non-physical displacements

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Hi all,

I am a new comsol user, and am getting started with a very simple solid mechanics modeling problem. Here is the basic setup:

  • Rotationally symmetric steel shell with 1.0 m and 0.5 m outer and inner radii respectively. The Young's modulus is 200 GPa, and the Poisson ratio is 0.3.
  • The boundary load on the outside boundary is P_1 = 10 Pa.
  • The boundary load on the inside boundary is P_2 = 1 Pa.
  • Solve for the equilibrium stress tensor and displacements.

I am attaching the .mph file for this problem. This particular problem is analytically solvable. For such a small pressure difference, the displacement of the outer boundary should be tiny. When I solve, I see this is actually the case. One of the attached plots is the calculated displacement along the radial direction. The reported displacement is negligible.

displacement.png

However, when I display the 2D and 3D models of the stress, it looks like the whole model has been compressed by 20 cm in the radial direction (see attached figure). When I set P_1 = P_2 = 0, there is no compression (but also no stress field), however any nonzero pressure difference is giving me these crazy nonphysical distortions of the body.

stress.png

What am I doing wrong here!



1 Reply Last Post Aug 30, 2024, 10:31 a.m. EDT
Jeff Hiller COMSOL Employee

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Posted: 2 months ago Aug 30, 2024, 10:31 a.m. EDT
Updated: 2 months ago Aug 30, 2024, 11:01 a.m. EDT

Hello True,

Good news: You are not doing anything wrong. The deformations are scaled in that plot; the scaling factor is chosen automatically such that the deformations are visible to our naked eye. Change that scaling factor to 1 if you want them represented at their real magnitude - but of course you will no longer see them because they are very very very small.

Jeff

-------------------
Jeff Hiller
Hello True, Good news: You are not doing anything wrong. The deformations are scaled in that plot; the scaling factor is chosen automatically such that the deformations are visible to our naked eye. Change that scaling factor to 1 if you want them represented at their real magnitude - but of course you will no longer see them because they are very very very small. Jeff

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