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Odd SPC force output for topology optimization analysis

Rahul_P1

Hi,

I wanted to check the SPC forces in my model and found something strange. In different iterations with nearly the same mass and the same load case (GRAV card) there were different SPC forces.

My load case looks like: (20g in X and Y / 25g in Z; Unit: mm/s^2)

$HMNAME LOADCOL 3'LC_+X+Y+Z'
$HWCOLOR LOADCOL 3 5
$
GRAV 3 01.0 196200.0196200.0245250.0

The *.out file (just a part) looks like:

Design Parameters Summary :
---------------------------

Total # of topology design elements : 258636
Total # of topology cards : 1

Total Volume of Design Material : 3.8620E+06
Total Mass of Design Material : 1.1586E-02
Volume of Non-Design Material : 1.5629E+04
Mass of Non-Design Material : 8.2989E-04

ITERATION 0

Residual energy ratio for static load case 2 = -4.201099E-09
--------------------------------------------------------------------------
Label x-force y-force z-force x-moment y-moment z-moment
--------------------------------------------------------------------------
Sum-App. 4.264E+02 4.264E+02 5.330E+02 -2.312E+05 9.687E+04 1.075E+05
Sum-SPCF -4.264E+02 -4.264E+02 -5.330E+02 2.312E+05 -9.687E+04 -1.075E+05
--------------------------------------------------------------------------

Objective Function (Minimize WCOMP) = 5.03774E+03
Maximum Constraint Violation % = 0.31156E+02
Design Volume Fraction = 3.57023E-01 Mass = 4.96639E-03

ITERATION 1

Residual energy ratio for static load case 2 = -2.439880E-09
--------------------------------------------------------------------------
Label x-force y-force z-force x-moment y-moment z-moment
--------------------------------------------------------------------------
Sum-App. 5.995E+02 5.995E+02 7.494E+02 -2.361E+05 9.898E+04 1.097E+05
Sum-SPCF -5.995E+02 -5.995E+02 -7.494E+02 2.361E+05 -9.898E+04 -1.097E+05
--------------------------------------------------------------------------

Objective Function (Minimize WCOMP) = 4.29319E+03 % change = -14.78
Maximum Constraint Violation % = 0.41841E+02
Design Volume Fraction = 3.57024E-01 Mass = 4.96640E-03

And if i calculate the SPC force for iteration 1 (F=m*a) i get 974 N instead of 599.5 N.
Can anybody explain this?

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Rahul_P1

Linus,

This is due to the fact that the mass used to calculate the force is different from the mass displayed in the out file,

The mass displayed in the out file is just the mass based on the upper bound you have specified in the constraint.

Try this, remove any manufacturing constraints, minimum member size and also add DOPTPRM,DISCRETE,0 and see if you still have a discrepancy.

Rahul_P1

Hey,

It's quite a long time since your post, but now i'm back
I understand now what the mass displayed in the out-file is. Unfortunately this doesn't solve my problem. I want to calculate the mass of my optimized structure. Is there a way (e.g. in the post processing) where i can set a density value and calculate the mass (or volume) of the elements which are above this value?

Rahul_P1

The concept designs are different as you change the density threshold values. Hence, it is not useful to require the mass of the concept design. After extracting the geometry of the concept design, you can get the mass value from the CAD design software. Or you can remesh it and import the new design into HM and get the mass value from Tool->mass calc panel.

Perhaps A rough estimate of the mass of the final structure can be computed from the Volume Fraction at the final iteration from the *.out file. The final mass is less than or equal to the value indicated by the Volume Frac. and varies depending upon the design requirements. The mass can be computed as follows:
Mass = (Volume Frac. at the final iteration) * (Design volume at the beginning of the optimization i.e. iteration 0 in the *.out file) * Material density.