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Elastoplastic material for vibration isolator.

Posted 17 gen 2011, 11:48 GMT-5 Acoustics & Vibrations, Geometry, Materials, Parameters, Variables, & Functions, Structural Mechanics Version 3.5a, Version 4.0 2 Replies

Chad Van der Woude
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Hello all,

I am working in COMSOL 3.5a; I do have access to version 4.0 but I am not yet well-versed in its use.

I am using the Structural Mechanics Module to simulate a structure that is composed entirely of beam elements. I would like to incorporate a non-linear vibration isolator into the structure (not at the base) which has bilinear stiffness, similar to the Elasto-Plastic material model with kinematic hardening. I initially intended to include a beam element, constrained for lateral motion only, with elasto-plastic material, to achieve the desired isolator stiffness, but it appears I can only use a linear elastic material for my beam element.

Can anyone suggest a method of modelling such an isolator simply? I had the idea of "exploding" the model and creating a 2-D body which could approximate the desired stiffness properties of the isolator, but ideally I would like to use a beam element for simplicity - the physical details of the isolator itself are not of concern in this simulation, only the characteristic stiffnesses and yield displacement.

Thanks in advance.
Chad
2 Replies Last Post 8 apr 2011, 13:19 GMT-4
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Hello Chad Van der Woude

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Alexander Robertsson
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Posted: 1 decade ago 8 apr 2011, 03:47 GMT-4
I have almost the same scenario. Have you solved it yet?
I have almost the same scenario. Have you solved it yet?
Chad Van der Woude
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Posted: 1 decade ago 8 apr 2011, 13:19 GMT-4
Hello Alexander,

What I ended up doing was dropping the idea of trying to model the isolator by a physical element and instead coupled the top and bottom of the isolator explicitly using equations.

I built the portions of the structure on either side of the isolator separately, defined coupling variables for the displacements at the top and bottom points of the isolator and defined global expressions for displacements across the isolator (e.g. isoDispX = topDispX - botDispX, meaning isolator displacement in x-direction = top displacement in x-direction - bottom displacement in x-direction). Then knowing the displacement (and velocity if damping is used) across the isolator, I could use the force-displacement relationship to compute the forces across the isolator, to be applied in opposite directions to the top and bottom points. (I should note that this formulation is very simple and doesn't consider that the isolator has physical height.)

For the force-displacement relationship of the isolator, I used a Bouc-Wen model. References abound for the mathematical formulation, but basically it involves solving a first-order non-linear differential equation and using the solution as well as the physical isolator displacement to determine the total force across the isolator. The non-linear DE can be input directly into COMSOL, and its parameters can be varied to give a variety of force-displacement behaviours, including ones that approximate a bilinear stiffness.

I hope this helps you - perhaps a little bit more complicated than I had originally hoped, but still not too difficult to implement.

Chad
Hello Alexander, What I ended up doing was dropping the idea of trying to model the isolator by a physical element and instead coupled the top and bottom of the isolator explicitly using equations. I built the portions of the structure on either side of the isolator separately, defined coupling variables for the displacements at the top and bottom points of the isolator and defined global expressions for displacements across the isolator (e.g. isoDispX = topDispX - botDispX, meaning isolator displacement in x-direction = top displacement in x-direction - bottom displacement in x-direction). Then knowing the displacement (and velocity if damping is used) across the isolator, I could use the force-displacement relationship to compute the forces across the isolator, to be applied in opposite directions to the top and bottom points. (I should note that this formulation is very simple and doesn't consider that the isolator has physical height.) For the force-displacement relationship of the isolator, I used a Bouc-Wen model. References abound for the mathematical formulation, but basically it involves solving a first-order non-linear differential equation and using the solution as well as the physical isolator displacement to determine the total force across the isolator. The non-linear DE can be input directly into COMSOL, and its parameters can be varied to give a variety of force-displacement behaviours, including ones that approximate a bilinear stiffness. I hope this helps you - perhaps a little bit more complicated than I had originally hoped, but still not too difficult to implement. Chad

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