Negative or Large Compliance

Altair Forum User

Hello,

I am attempting to run an optimization using Optistruct but the solver keeps outputting the following error:

 

            *** ERROR # 312 ***
            In static load case 1
            the compliance is negative or large 1.7638e+015.
            Optimization/buckling analysis cannot be performed.
            due to possible rigid body mode.
            INFEASIBLE DESIGN (AT LEAST ONE CONSTRAINT VIOLATED).

 

From what I understand, this error could be from a model that is not fully constrained or from poorly defined materials properties. My model is constrained with 2 RBE3 elements so I believe my constraints are not the issue and I don't understand what could be wrong with my material inputs. 

 

I have attached my model for reference. Any help would be appreciated

 

Thanks

Unable to find an attachment - read this blog

Altair Forum User

Hi,

 

There are no constraints on the model and that is why you see infinite displacement or negligible stiffness. 

 

 

Apply SPs and check if you are able to run the model. 

Altair Forum User

Hello @Prakash Pagadala,

 

I was under the impression that the rbe3 elements constrained the model. 

 

If not, I'd like to constrain the model at the two holes shown in the airfoil. Can this be done by adding an SPC at the dependent node of the rbe3 elements?

 

Thanks,

Malcolm

Altair Forum User

Hi @malcolmdinovitzer

 

Instead of RBE3 use RBE2 to constraint at the center of element. 

Altair Forum User

Hello @Prakash Pagadala,

 

RBE2 elements results in the same error.

 

Is it correct for me to have the RBE2/RBE3 elements defined in the nondesign component?

Altair Forum User

Hi @malcolmdinovitzer

i have gone through the model again. 

 

The reason for high displacements is because SPCs are not considered as they are not referenced in loadstep at all. 

 

Update the loadstep with SPCs and try again.