Some question about sensitivity analysis by c3d20 element

1

Hello,
I have used a c3d20 element to test the function of sensitivity analysis in Calculix.The problem is that when I applyed symmetrical load in this element,the result is not symmetrical ,which is counter-intuitive.The response function is K-S stress,which can estimate the maxnium stress of a structure and design variable is surface nodes coordinate(x,y,z).The contour expose the gradient respect to each node’s coordinate along the surface normal direction.I would be pleased if you could help.

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Happy Chinese New Year！

2

Could you post your file?

4

hello this is my input file


*NODE, NSET=Nall
         1,  0.5            ,  0.0            ,  0.5            
         2,  -0.5           ,  0.0            ,  0.5            
         3,  -0.5           ,  -1.0           ,  0.5            
         4,  0.5            ,  -1.0           ,  0.5            
         5,  0.5            ,  0.0            ,  -0.5           
         6,  -0.5           ,  0.0            ,  -0.5           
         7,  -0.5           ,  -1.0           ,  -0.5           
         8,  0.5            ,  -1.0           ,  -0.5           
         9,  0.0            ,  0.0            ,  0.5            
        10,  0.5            ,  -0.5           ,  0.5            
        11,  0.5            ,  0.0            ,  0.0            
        12,  -0.5           ,  -0.5           ,  0.5            
        13,  -0.5           ,  0.0            ,  0.0            
        14,  0.0            ,  -1.0           ,  0.5            
        15,  -0.5           ,  -1.0           ,  0.0            
        16,  0.5            ,  -1.0           ,  0.0            
        17,  0.0            ,  0.0            ,  -0.5           
        18,  0.5            ,  -0.5           ,  -0.5           
        19,  -0.5           ,  -0.5           ,  -0.5           
        20,  0.0            ,  -1.0           ,  -0.5           
*ELEMENT, TYPE=C3D20, ELSET=Eall
         1,         1,         4,         3,         2,         5,         8,         7,         6,
        10,        14,        12,         9,        18,        20,        19,
        17,        11,        16,        15,        13
*BOUNDARY
2,1,3,
3,1,3,
6,1,3,
7,1,3,
12,1,3,
13,1,3,
15,1,3,
19,1,3,
*****

*NSET,NSET=DESIGNNODES,GENERATE
1,20,1      
*DESIGNVARIABLES,TYPE=COORDINATE
DESIGNNODES


** MATERIAL PROPERTIES
**
*MATERIAL, NAME=TI
*ELASTIC, TYPE=ISOTROPIC
1.09e11, 0.303
*DENSITY
4440

*****
*SOLID SECTION,MATERIAL=TI,ELSET=Eall


*STEP
*STATIC
*CLOAD
1,1,10.0           
4,1,10.0           
5,1,10.0           
8,1,10.0             
*NODE FILE
U
*EL FILE
S
*NODE PRINT, NSET=Nall
U
*EL PRINT, ELSET=Eall
S
*END STEP
*STEP
*SENSITIVITY
*DESIGN RESPONSE,NAME=STRESS_RESP
STRESS,DESIGNNODES,12.,825e5
***DESIGN RESPONSE, NAME=MASS_RESP
**MASS,Eall
*FILTER,TYPE=LINEAR,EDGE PRESERVATION=YES,DIRECTION WEIGHTING=YES
1
*NODE FILE
SEN
*END STEP
*STEP
*FEASIBLE DIRECTION
*OBJECTIVE,TARGET=MIN
STRESS_RESP
***CONSTRAINT
**MASS_RESP,LE,1.,
*NODE FILE
SEN
*END STEP
1 Like
5

It seems you’re not quite familiar with basic element behavior. You are loading the C3D20 as a linear element but C3D20 is a parabolic element capable of holding a parabolic stress distribution along the element edge and for that reason a linear load distribution requires a parabolic load.

For C3D20 corner nodes should hold -F/12 and middle nodes F/3 as described in section 6.11 loading

3 Likes
6

Thanks!I will try it.

7

I have solved this problem, just reset the variable dispmin to smaller number(1e-12) in objectivemain_se.c.