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free finite prescribed displacement

Posted 16.01.2012, 17:43 GMT-5 Modeling Tools & Definitions, Parameters, Variables, & Functions, Structural Mechanics Version 4.2a, Version 4.4 7 Replies

George Rosala
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Evening everyone,

Is there a way of modelling a finite free displacement along a direction, as in the case of components attached via bolts with axial clearance?

For example, to allow a max 0.001m free displacement in the "x" direction I've tried to apply a prescribed displacement u0 of 0.001*(u>=0.001) but it appears not to work.

Many thanks for any suggestion.
George
7 Replies Last Post 04.05.2016, 06:10 GMT-4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 16.01.2012, 18:06 GMT-5
Hi

in structural, by default in geometry "union" mode you have continuity between two adjacent domains, sharing a common boundary. So if you displace one domain, the other will follow as if it was "glued".

You need either to tweak the default continuity conditions/equations along the desired axis, or define a geometry "assembly" mode = dedoubling the boundary and define contact "pair" and sliding boundary conditions

check the doc

--
Good luck
Ivar
Hi in structural, by default in geometry "union" mode you have continuity between two adjacent domains, sharing a common boundary. So if you displace one domain, the other will follow as if it was "glued". You need either to tweak the default continuity conditions/equations along the desired axis, or define a geometry "assembly" mode = dedoubling the boundary and define contact "pair" and sliding boundary conditions check the doc -- Good luck Ivar
George Rosala
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Posted: 1 decade ago 17.01.2012, 03:56 GMT-5
Hi Ivar,

The point is, I'm trying to avoid modelling with contact; so I didn't model the bolts, I would just like to model their effect on my component.

The component should deform freely until the axial clearance (gap) in the bolts closes, and than be held fixed in that direction. Any thoughts?

Thanks,
George
Hi Ivar, The point is, I'm trying to avoid modelling with contact; so I didn't model the bolts, I would just like to model their effect on my component. The component should deform freely until the axial clearance (gap) in the bolts closes, and than be held fixed in that direction. Any thoughts? Thanks, George
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 17.01.2012, 04:23 GMT-5
Hi

you could try a non linear spring, but in some sense that is a "contact" model ;)

--
Good luck
Ivar
Hi you could try a non linear spring, but in some sense that is a "contact" model ;) -- Good luck Ivar
George Rosala
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Posted: 1 decade ago 17.01.2012, 05:24 GMT-5
Ivar,

Is there a non-linear spring capability in Comsol? Where?

George
Ivar, Is there a non-linear spring capability in Comsol? Where? George
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 17.01.2012, 05:51 GMT-5
Hi

not directly but you must play with either "thin elastic pairs" or a combination of "spring foundations" (the latter attaches a spring to a fixed frame, the former between two domains or via "pairs". Some equation tweaking is required. You can also implement this fully by your own as weak constraints. The non linearity is depending on the spring constant values (expressions) you use.

it's worth to study the doc on the two contact implementations as these are very close to the special spring case

--
Good luck
Ivar
Hi not directly but you must play with either "thin elastic pairs" or a combination of "spring foundations" (the latter attaches a spring to a fixed frame, the former between two domains or via "pairs". Some equation tweaking is required. You can also implement this fully by your own as weak constraints. The non linearity is depending on the spring constant values (expressions) you use. it's worth to study the doc on the two contact implementations as these are very close to the special spring case -- Good luck Ivar
Maria Rey
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Posted: 1 decade ago 30.06.2015, 11:32 GMT-4
Hi I have a similar problem, I have to design (3d) a spring between two points, but the k constant change when the distance (between the points) gets a specified value. Ive use pprobe, I use piecewise function to define k but the spring function in the model doesnt accept the k function, also I dont know if Im using pprobe correctly. I have to use two points, but the variable is the distance, I dont know how to set this newvariable, Thanks
Hi I have a similar problem, I have to design (3d) a spring between two points, but the k constant change when the distance (between the points) gets a specified value. Ive use pprobe, I use piecewise function to define k but the spring function in the model doesnt accept the k function, also I dont know if Im using pprobe correctly. I have to use two points, but the variable is the distance, I dont know how to set this newvariable, Thanks
F. Xavier Borras
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Posted: 10 years ago 04.05.2016, 06:10 GMT-4
Hello,

I am also trying to avoid modeling 2 bodies for simulating a contact pair.

I understood the implementaiton of a piecewise function spring constant so when a certain position is overcome a high load is applied simulating the contact.

- Wouldnt it be easier to set a Maximum/Minimum Prescribed Displacement?

maximum(u) = (x<x_lim)*(x_lim-x)

Comsol allows to easily define; u = (x<x_lim)*(x_lim-x) however this is wrong since the Prescribed Displacement on the nodes where (x>x_lim) will then be set to zero. We require a Prescribed Disaplcement which only affects the nodes overcoming x_lim so no Prescribed Displacement is applied on the rest of them.

How can a Maximum/Minimum Prescribed Displacement be implemented?

Thanks in advance
Hello, I am also trying to avoid modeling 2 bodies for simulating a contact pair. I understood the implementaiton of a piecewise function spring constant so when a certain position is overcome a high load is applied simulating the contact. - Wouldnt it be easier to set a Maximum/Minimum Prescribed Displacement? maximum(u) = (x

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