Why there is difference in stress and displcaments in a solid between optistruct and nastran ?

arthur blaise_20814

Hi,

I have done a comparison bewteen Optistruct and Nastran

I have two times more stress and displacements in optistruct than in nastran

The difference is in a fitting modelised by tetra, and the rest of my model is cquad4. 

In an other model without solid I have no differences between optistruct and nastran. 

Do you know where this difference come from ?

I a pretty sure that the problem is from solid. If you have article or something explaining how solid are made in Optistruc, I am interested.

Sincerly,

Arthur

Adriano_Koga

Could you double check what params are you using in Nastran?

Nastran uses a different quad element, specially considering the fof6 stiffness. In many cases, artificial drilling stiffness is added by using PARAM,K6ROT, that can be tuned using different values.

Nastran usually recommends using CQUADR elements as these have enhanced stiffness calculation for the dof6.

OptiStruct CQUAD4 element default is more similar to CQUADR in Nastran.

The fact that you're connecting your quads to tetra, might be related to this difference, so you could at least try running Nastran model with CQUADR, instead of CQUAD4 (just open the .bdf file and replace all CQUAD4 entries, by CQUADR and run again.

 

If results still don't match, maybe something else could be affecting.

Adding some pictures, specially of your connection, might be helpful.

 

 

https://simulatemore.mscsoftware.com/choosing-the-right-finite-element-msc-nastran/

"The membrane properties for the CQUADR and CTRIAR elements are less sensitive to the element shape than the CQUAD4 and CTRIA3. The CQUADR and CTRIAR elements can be used in conjunction with each other, but not with other elements.

An increase in accuracy can be expected when using the QUADR element vs the QUAD4 element, particularly in areas of the model that are more coarsely meshed.

Existing QUAD4 element models can easily be converted to QUADR elements by setting System Cell (QRMETH (370)) in the NASTRAN Statement. QRMETH=5 will convert all QUAD4/TRIA3 elements in the model to QUADR/TRIAR.To show the difference in accuracy between the QUADR and QUAD4 elements, four mesh densities for a simple “T-Section” test model were run using MSC Nastran. von Mises stress results, taken at a central position in the “T-Section”, were compared and they showed the QUADR was more consistant and accurate as the density was reduced. See Figure 9-5."

arthur blaise_20814

Adriano A. Koga_21884 said:

Could you double check what params are you using in Nastran?

Nastran uses a different quad element, specially considering the fof6 stiffness. In many cases, artificial drilling stiffness is added by using PARAM,K6ROT, that can be tuned using different values.

Nastran usually recommends using CQUADR elements as these have enhanced stiffness calculation for the dof6.

OptiStruct CQUAD4 element default is more similar to CQUADR in Nastran.

The fact that you're connecting your quads to tetra, might be related to this difference, so you could at least try running Nastran model with CQUADR, instead of CQUAD4 (just open the .bdf file and replace all CQUAD4 entries, by CQUADR and run again.

 

If results still don't match, maybe something else could be affecting.

Adding some pictures, specially of your connection, might be helpful.

 

 

https://simulatemore.mscsoftware.com/choosing-the-right-finite-element-msc-nastran/

"The membrane properties for the CQUADR and CTRIAR elements are less sensitive to the element shape than the CQUAD4 and CTRIA3. The CQUADR and CTRIAR elements can be used in conjunction with each other, but not with other elements.

An increase in accuracy can be expected when using the QUADR element vs the QUAD4 element, particularly in areas of the model that are more coarsely meshed.

Existing QUAD4 element models can easily be converted to QUADR elements by setting System Cell (QRMETH (370)) in the NASTRAN Statement. QRMETH=5 will convert all QUAD4/TRIA3 elements in the model to QUADR/TRIAR.To show the difference in accuracy between the QUADR and QUAD4 elements, four mesh densities for a simple “T-Section” test model were run using MSC Nastran. von Mises stress results, taken at a central position in the “T-Section”, were compared and they showed the QUADR was more consistant and accurate as the density was reduced. See Figure 9-5."

Thanks you for your answer,

Sorry I made a mistake, I don't use CQUAD in this area, the solid elements are TETRA10 and shells are CTRIA6. 

Shell and solid elements are coinciding. The PSHELL associate have a thickness of 0.001mm. The shell is here for post-traiting. 

I run the nastran model with PARAM,K6ROT = 100

I don't know why, but when I run the optistruct model with AUTOSPC,YES I have same results than in Nastran with PARAM,K6ROT = 100 and PARAM, AUTOSPC,NO

I work on the model of someone else, so I will ask if I can share picture.