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Modelling the load on a cantilever beam

Posted Apr 29, 2012, 9:44 a.m. EDT Modeling Tools & Definitions, Parameters, Variables, & Functions, Structural Mechanics Version 4.2a 2 Replies

Robin Persson
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Hi.

I'm quite new to COMSOL (4.2a) (only done some flow simulations before) and
I'm trying to model the load on a cantiliver beam. I think I going at the problem the
wrong way, but I can't seem to find any tutorials or guides for solid physics that
help me.

What I do is that I choose 2D (what I really would like to is to do it in 3D, but I think
it's better to get help with the 2D part and extrapolate that and learn how to do it I
3D that way), then solid mechanics, choose stationary, and create my beam. Under
material I choose aluminum. Under solid mechanics (solids) I add a rigid connector
(the beam is fixed and can't move in that end) Running a study at this point works
just fine, but gives an extremly boring result (0 stress everywhere).

So my questions are:
I think I have to add the gravity in someway, how do I do that?
I would like to have point load at the free end, how do I add one (none of the
avaible options seems appropriate)?
Am I doing it terrible wrong?
2 Replies Last Post May 1, 2012, 6:27 a.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Apr 29, 2012, 3:14 p.m. EDT
Hi

a agree always start simple, ie. first 2D the 3D etc

You are right in Solid you need to attach your beam somewhere you have a the fixed boundary condition, that is the same as the Prescribed displacement boundary condition with all displacements set to "0".

Then you can either add a "boundary load" on the opposite boundary to the fixed one, or you might add a gravity load by adding a domain body load and write -g_const*solid.rho as force density. g_const is COMSOL internal name for 9.81 m/s^2 earth gravity acceleration (check COMSOL list of predefined physics variables all names with the "_const" suffix.

You define the solid.rho because you give the density of each elements and COMSOL impliitely distributes this all over the domains. solid.rho is a generic name, if you have 2 materials in domain 1 and 2 respectively, COMSOL will take care to use the correct rho per domain. in fact solid.rho is not a variable but a field and is an implicit notation for solid.rho(x,y,z,t) where the spatial coordinates are defined by your domain or boundary selections.

If you are a newcomer to COMSOL, check carefully the naming conventions, and learn exactly what an "entity" means and what a "domain" and a "boundary" means, as the latters differs slightly for 3D and 2D, or 1D ...

--
Good luck
Ivar
Hi a agree always start simple, ie. first 2D the 3D etc You are right in Solid you need to attach your beam somewhere you have a the fixed boundary condition, that is the same as the Prescribed displacement boundary condition with all displacements set to "0". Then you can either add a "boundary load" on the opposite boundary to the fixed one, or you might add a gravity load by adding a domain body load and write -g_const*solid.rho as force density. g_const is COMSOL internal name for 9.81 m/s^2 earth gravity acceleration (check COMSOL list of predefined physics variables all names with the "_const" suffix. You define the solid.rho because you give the density of each elements and COMSOL impliitely distributes this all over the domains. solid.rho is a generic name, if you have 2 materials in domain 1 and 2 respectively, COMSOL will take care to use the correct rho per domain. in fact solid.rho is not a variable but a field and is an implicit notation for solid.rho(x,y,z,t) where the spatial coordinates are defined by your domain or boundary selections. If you are a newcomer to COMSOL, check carefully the naming conventions, and learn exactly what an "entity" means and what a "domain" and a "boundary" means, as the latters differs slightly for 3D and 2D, or 1D ... -- Good luck Ivar
Robin Persson
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Posted: 1 decade ago May 1, 2012, 6:27 a.m. EDT
Thank you Ivar, that did the trick.
Thank you Ivar, that did the trick.

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