How to make a rigid body? 2

[mdama]

I created a part and gave it a very high value for elastic modulus to consider it as rigid, but in the result I see the part has some stress contour which is not supposed to be. So, Do anyone know why and is there any other way to create rigid body without any problem in the assembly!
Thanks

 

[FEA way]

There's no need to artificially increase Young's modulus if you want to have rigid part in Abaqus. Just chang its type to discrete rigid.

 

[mdama]

The problem is the assembly, It says you should have to have shell which I do not want to. I can not do assembly.

 

[FEA way]

Discrete rigid body must be a shell. If it's solid you should convert it (its faces) to shell. Another option is to use rigid body constraint. This way you can make an instance rigid even though the part is made as deformable.

 

[Mustaine3]

Keep the body deformable and use a Rigid Body constraint in the Interaction-module.

 

[mdama]

Thanks, In the interaction module, I use rigid body constraint for the part I want to be rigid, It asks me a reference point region, How should I deal with this? What does this reference point mean?

 

[FEA way]

When using rigid body constraint you must specify the region to be treated as rigid (whole part in your case) and reference point. Just add datum point and assign RP to it (available in the Interaction module toolbar). Then select it in the constraint. Reference point is necessary for all rigid parts so that you can apply boundary conditions or mass to it.

 

[mdama]

Makes sense, I applied boundary conditions (displacement) to the whole body of rigid part, is that fine? or I must apply boundary condition on the reference point only?

 

[mdama]

I have also another question; I use elastic-perfectly plastic material properties suppose using yield strength as 500MPa, The output result I get for stress contours I see stresses higher than 500MPa, How is that possible? and in order to see stress contours, should it be in Quilt mode or banded mode?

Thanks

 

[FEA way]

Apply BCs to reference point only. Motion of the whole rigid body is constrained to the motion of this point.

This is normal that stresses exceed yield point. It means that there are points where your material is yielding. Plot plastic strain (PEEQ) to see the extent of this process. Quilt mode along with averaging set to 0 will give you results in each element without influence of results from other elements. This is useful for example when you want to verify the correctness of high stress concentrationa.

 

[mdama]

"This is normal that stresses exceed yield point", even when we have perfectly plastic behavior (No hardening)?
Where is "averaging set to zero"?

Thanks

 

[Mustaine3]

To be clear, the stress will not exceed the final (or only) yield stress. At the integration point you will see the truth. But in a normal contour plot you will see values that extrapolated to the nodes and then usually averaged. You can change the averaging settings, but not the extrapolation. The extrapolation is usually the reason for the higher values at certain nodes. It is caused by a stress gradient within the element. Use Tools->Probe Value to look at the values at the integration points.

Btw. you should never reach the final yield stress. When this happens the results becomes questionable. And often the convergence gets bad.

 

[FEA way]

Oh, right, I forgot it's perfect plasticity ...

You can turn averaging off by setting it to 0 in Results --> Options menu.

 

[mdama]

I have a structure attached here under compression in a displacement mode. I use alstic,perfectly plastic behvaior as you see here (

https://www.dropbox.com/s/kuzmpbrfujzhdzv/perfectplastic.PNG?dl=0)

and when I see von Mises contour in banded mode (

https://www.dropbox.com/s/n81szuho6xjh8il/Banded.PNG?dl=0)

vs quilt mode (

https://www.dropbox.com/s/jqnxwxnbra3bhxz/quilt.PNG?dl=0).

In both cases, stresses are higher than the yield strength (I used perfectly plastic, stress is constant after yield), With Tools>Prob I get all values not higher than 503 MPa, which is the yield strength, but in stress contours using bith banded abd quilt give higher stresses, (quilt is lower than band). So my question here is that If I want to show the stress contour for the structure, what should I show?
Thanks

 

[mdama]

I used quilt and went to Results>Options and put "Averaging threshold" to 0 but still higher values in stresses. which might be due to the extrapolation which Mustaine mentioned about. But what should I show as a stress contour?

 

[FEA way]

To specify elastic perfectly plastic behavior you should only give yield stress and corresponding 0 plastic strain in this table.

 

[mdama]

I think I have tried this before, the result doesn't change. But I will try again to make sure.

 

[Mustaine3]

1. Abaqus automatically uses perfect plasticity after the last yield stress. So as mentioned, you just need the first line of your table.

2. You still see higher values, since, as I mentioned, it is mainly caused by the extraploation, not the averaging. A Quilt Plot just uses the extrapolated nodal values and averages them to one value for each face of each element. It is not solving your problem.

3. When extrapolation generates such differences, then it is a clear indication that your mesh is too coarse. You have high gradients within elements and this causes the large overshoot in the extrapolation. Refine the mesh and this effect will go down. And read again what I said about perfect plasticity.
You can also plot the discontinities (Result->Options) in the results or request variable SJP to get indicators for mesh convergence. It is the duty of the user to define a mesh that matches the expected result quality. Or use the help of the Adaptive Remeshing method in A/CAE.