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Question about Truss Module and plasticity

Posted 07.06.2013, 13:49 GMT-4 Materials, Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers, Structural Mechanics Version 4.3a 3 Replies

Byron
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I am modeling a reinforced concrete beam using almost an identical model to the reinforced concrete beam that is provided in the COMSOL tutorials/models. I need to model the rebar as elastic-perfectly plastic, i.e. it is elastic until it reaches its yield strength, at which point the strain increases with no increase or decrease in stress. The rebar I am modeling will yield once the strain reaches 0.002. In the Truss module, there is only an option for an elastic analysis. Is there a way of incorporating elastic-perfectly plastic behavior into the truss module or is there another way I can simulate elastic-perfectly plastic conditions in the rebar?

Thanks in Advance
3 Replies Last Post 24.10.2013, 06:48 GMT-4
Henrik Sönnerlind COMSOL Employee

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Posted: 1 decade ago 10.06.2013, 03:43 GMT-4
Hi,

There is no built-in elastoplastic material for the truss elements. There are however ways in which you can perform such a simulation anyway.

As long as you are only increasing the external load, there is no difference between an elastoplastic material and a nonlinear elastic material. You can for example then enter Young's modulus as

E0*((abs(truss.een)<=eps_y) + eps_y/(truss.een+eps)*((truss.een>eps_y)-(truss.een<(-eps_y))))

Here E0 is modulus of elasticity in the elastic regime, and eps_y is the strain at yield. truss.een is the axial strain.

The expression is based on that stress = E * strain so that

stress = E0*truss.een when -eps_y<=truss.een<=eps_y
stress = E0*eps_y = sig_y when truss.een>eps_y
stress = -E0*eps_y = -sig_y when truss.een<-eps_y

If you have a situation where the load is not monotonously increasing, the situation is a bit more complex. The you will need to add a 'state' variable to keep track of the plastic strain, and the expression above needs a modification.

Regards,
Henrik
Hi, There is no built-in elastoplastic material for the truss elements. There are however ways in which you can perform such a simulation anyway. As long as you are only increasing the external load, there is no difference between an elastoplastic material and a nonlinear elastic material. You can for example then enter Young's modulus as E0*((abs(truss.een)eps_y)-(truss.een
Byron
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Posted: 1 decade ago 11.06.2013, 11:30 GMT-4
I was thinking a solution similar to this might be possible. I very appreciate you spelling it out for me. I will give this a shot later today. Thanks again!

Byron
I was thinking a solution similar to this might be possible. I very appreciate you spelling it out for me. I will give this a shot later today. Thanks again! Byron
Arturo Pascuzzo
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Posted: 1 decade ago 24.10.2013, 06:48 GMT-4
Hi,

I read the discussion and I found it very interesting.

I'm trying to replicate the elasto-plastic behavior of a cable by truss element.

The cable has a tension-only behavior.

Can you give me an example of a state variable to keep track of the plastic strain? (eg. plastic work..)

How can I implement an incremental elastic-plastic problem in COMSOL?

Is there any example?

Hi, I read the discussion and I found it very interesting. I'm trying to replicate the elasto-plastic behavior of a cable by truss element. The cable has a tension-only behavior. Can you give me an example of a state variable to keep track of the plastic strain? (eg. plastic work..) How can I implement an incremental elastic-plastic problem in COMSOL? Is there any example?

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