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constrain element/dof at the load point- Psolid-plane stress

Builder
Builder

Hi

So, at the body edge where the load applied in X direction, there is a displacement in the Y direction. I want to restrain this dof at this loaded nodes. In some FE package, you can restrict the dof to specific elements. How can we do this in Nx?

in other words, if I am correct, The 3D elements Hex 8,10 are plane strain elements. could I edit them to be plane stress elements?

lastly, if I used mixed order mesh, i.e. Hex 20 beside hex8? is it acceptable, please?

Thanks

3 REPLIES

Re: constrain element/dof at the load point- Psolid-plane stress

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

Hello!,

Aclarations:

  • "3D elements HEX 8, 10 ....", this statement is not correct, with NX NASTRAN we have solid CHEXA 8-node linear elements or high-order 20-nodes. A 3-D solid 10-nodes element is named CTETRA and is a tetrahedral element. 3-D Solid elements have only translational degrees of freedom. No rotational DOF are used to define the solid elements. Solid elements contain stiffness only in the translation degrees of freedom at each grid point.
  • "The 3D elements Hex 8,10 are plane strain elements", this statement is wrong. Please read the NX NASTRAN Elemen Library Referncehttps://docs.plm.automation.siemens.com/tdoc/nxnastran/11/help/#uid:index
  • For you to know: PLANE STRESS & PLANE STRAIN are 2-D SOLID elements, not 3-D, OK?. Probably you are confused with the statement that "TET4 solid elements are constant stress & constant stress elements", yes, but this is not applied to CTETRA 10-nodes tetrahedral elements.
  • Plane stress elements are 2-D elements with membrane only stiffness and in-plane loading, they have only 2 DOF per node. The plane stress element idealization has zero stress in the thickness direction. These elements represent structures that are thin relative to their lateral dimensions and planar. The NX NASTRAN entries CPLSTS4, CPLSTS8, CPLSTS3, and CPLSTS8 are plane stress elements.
  • Plane strain elements are 2-D elements with membrane only stiffness and in-plane loading. The plane strain element idealization has zero strain in the thickness direction. These elements represent structures that are very thick relative to their lateral dimensions. They are named CPLSTN4, CPLSTN8, CPLSTN3, and CPLSTN6.

In summary, you need to select & use the correct element from the NX NASTRAN Library, if you want to perform a 2-D plane stress analysis then use CPLSTS4 elements.

Finally, regarding the last question, mixing low order HEX8 with high-order HEX20 element is not correct at all, don't do it. If required, then use GLUE surface-to-surface to joint both meshes.

Best regards,
Blas.

Blas Molero Hidalgo, Ingeniero Industrial, Director
IBERISA • 48004 BILBAO (SPAIN)
WEB: http://www.iberisa.com
Blog Femap-NX Nastran: http://iberisa.wordpress.com/

Re: constrain element/dof at the load point- Psolid-plane stress

Builder
Builder

Hi Blas,
Yes you got what I meant,
I hope you have the answer to my 1St question. If I want restraint a dof. Of nodes on the surface is using the user-defined constraint is enough. see attached, please
The case is in plane load applied at the model edge. Due to asymmetry, there is out if plane displacement z at this edge. I want to restrain this movement just as actually test -uniaxial test- no out of plane movements at grips/supports.
I saw FE users either apply spring with small stiffness to constrain this movement or the time they apply load. Tick out all dof, apart from that in load direction so at the end this edge is constrained in all dof apart from the load direction.
How to do that, in NX please?
I got a warning when I applied the load in X, and putting BC in the same 3D body edge for all dof apart from X. Also I have BC in the other edge.

the warning is 

WARNING
Load "Force(1)" is defined on an object that also has a constraint applied to it. This  load may be ignored during analysis if it is in conflict with the constraint. SUGGESTION: Make sure that the load and the constraint are not in conflict.

 


Regards the 2nd question,
Thanks for your answer. totally agree with you.
Thanks

Re: constrain element/dof at the load point- Psolid-plane stress

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

Dear Wazy,

Sorry, I don't understand your problem, but basically if you constraint movement in a direction then not possible to apply any load in that direction, this is useless, then the warning message of the preprocessor. Only in the case of non-zero enforced motion loading is where NX NASTRAN require to constraint the node in the direction of the enforced motion.

I suggest to revise your loads & BCs to make all reasonable. Also, you can post your nx nastran input file (*.nas) together with a drawing  explaining the loads & BCs to understand the problem.

Best regards,
Blas.

Blas Molero Hidalgo, Ingeniero Industrial, Director
IBERISA • 48004 BILBAO (SPAIN)
WEB: http://www.iberisa.com
Blog Femap-NX Nastran: http://iberisa.wordpress.com/