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Can I prevent prescribed displacements

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Hi,

When I apply forces (in my case by boundary loads) I have add things that keep the object in place. Currently I tried to use prescribed displacement for this. However it seems that an object that is under prescribed displacement cannot be deformed (correct me if I am wrong) even in directions that aren't fixed. Now from the additional options (Symetry, Roller,... I didn't understand that much from the description). In the simple case of a cube under shear forces, what can I add that will keep it in place (solver find a solution) without impacting it's deformation in any way.

1 Reply Last Post 9 mar 2012, 15:06 GMT-5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

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Posted: 1 decade ago 9 mar 2012, 15:06 GMT-5
Hi

you should think "how do I hold an object in place ?

Mostly you would "fix" it by welding or gluing one face of your cube to something "very rigid".
In fact "fixed" = prescribed displacement => (0,0,0)= (u,v,w), where u,v,w are the displacment along x,y,z

So either you should fix one face or set it to prescribed displacement "0".

Another way is to model only 1/2, 1/4, or 1/8 th of the full model by applying 1,2 resp. 3 symmetry planes. this corresponds to fixng the vertex at the 3 sym. planes intersection and blocking all other 6 dof's along these sides by pairs. But such a design assumes you apply 1:2, 1:4 or 1:8 symmetric loads to your device, which might not e your desire.

A last way, is to fix one vertex=Point in prescribed dislacement all 3 direction to 0, then two other vertex of the same plane, each along 2 directions i.e. x,z and y,z + adding a "roller" condition on the boundary belonging to these 3 vertex's.
But "point loads" might give you singularities and excessively high stress concentrations, so I recommend to stay, as far as possible, with only boundary loads, and fixing

--
Good luck
Ivar
Hi you should think "how do I hold an object in place ? Mostly you would "fix" it by welding or gluing one face of your cube to something "very rigid". In fact "fixed" = prescribed displacement => (0,0,0)= (u,v,w), where u,v,w are the displacment along x,y,z So either you should fix one face or set it to prescribed displacement "0". Another way is to model only 1/2, 1/4, or 1/8 th of the full model by applying 1,2 resp. 3 symmetry planes. this corresponds to fixng the vertex at the 3 sym. planes intersection and blocking all other 6 dof's along these sides by pairs. But such a design assumes you apply 1:2, 1:4 or 1:8 symmetric loads to your device, which might not e your desire. A last way, is to fix one vertex=Point in prescribed dislacement all 3 direction to 0, then two other vertex of the same plane, each along 2 directions i.e. x,z and y,z + adding a "roller" condition on the boundary belonging to these 3 vertex's. But "point loads" might give you singularities and excessively high stress concentrations, so I recommend to stay, as far as possible, with only boundary loads, and fixing -- Good luck Ivar

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