Discussion Forum

February 9, 2012 2:57pm UTC

How to apply boundary loads

So far I only used comsol for meshning. Now I wanted to solve a structure. I can do It with prescribed displacement, now I wanted to do it with boundary loads. But for some reason I cannot solve my structure. Can somebody explain me where my problem lies? (As I have little experience, the problem is most likely trivial)

Reply  |  Reply with Quote  |  Send private message  |  Report Abuse

February 13, 2012 11:21am UTC in response to Samuel Andermatt

Re: How to apply boundary loads

Hi

first of all in your model, your Material 2 has probably a wrong Young modulus, I suspect you are missing 1E9 as it looks like aluminium spheres (but I might be wrong ;)

If you apply a pressure it's always perpendicular to the surface. but you can change from pressure, to Total force (in Newton, COMSOl looks after Area calculations, and Surface loads in N/m^2 (basically Pascal) but independently for all 3 directions

Then its better to fix one boundary and appply the load on the other part. if you use symmetry, take care to block all free DoF otherwise your solver will have problems finding a solution

--
Good luck
Ivar

Reply  |  Reply with Quote  |  Send private message  |  Report Abuse

February 13, 2012 1:10pm UTC in response to Samuel Andermatt

Re: How to apply boundary loads

Hi

it depends what you are looking for, a constant pressure with a material mix like you have make deformed shapes, you can also apply a prescribed displacement but that implies variable pressure load.

in y<our case you have a nice symmetry so by applying a 1/8 symmetry you can fix all 6 dsof and use only one pressure or Force (equivalent up to the surface value) or a prescribed displacement

--
Good luck
Ivar

Reply  |  Reply with Quote  |  Send private message  |  Report Abuse

February 13, 2012 1:41pm UTC in response to Samuel Andermatt

Re: How to apply boundary loads

Hi
If you look at modes you must check all combinations of symmetry and anti symmetry boundaries, but if its only symmetric load use symmetry.

Then ensure the mesh is fine enough, or even perhaps apply a boundary mesh on the " stiff side" to better resolve the stress gradients at high values

You also have "roller load" this does not block all DoF so you must add something

And do not forget that pressure is applied onto the surface, note the sign

And check your strain, you are outside normal "linear range"

--
Good luck
Ivar

Reply  |  Reply with Quote  |  Send private message  |  Report Abuse

February 13, 2012 2:51pm UTC in response to Samuel Andermatt

Re: How to apply boundary loads

Hi

It's more the validity of the equations for large deformation, I would rather use the Solver incliude geoemtry nonlinearity. Or better lower the load so one remains within resonable limits.

If you apply a compression on two sides of your cube, you have a symmetry plane in the middle, and it's as if you model just 1/2, still if you leave your volume free in the 2 other directions the solver will eventually diverge, so you need at least to apply some "soft" load to keep it in place, or add more symmetry, or some "soft" spring load

The issue comes from any nuemrical instabilities and assymetry in the mesh FEM is an approximation, no "real" continuity model

--
Good luck
Ivar

Reply  |  Reply with Quote  |  Send private message  |  Report Abuse