# Some question about sensitivity analysis by c3d20 element

**URL:** <https://calculix.discourse.group/t/some-question-about-sensitivity-analysis-by-c3d20-element/1458>\
**Category:** Uncategorized\
**Created:** [January 26, 2023, 2:49pm UTC](https://calculix.discourse.group/t/some-question-about-sensitivity-analysis-by-c3d20-element/1458 "2023-01-26T14:49:23Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![Gong1999](https://avatars.discourse-cdn.com/v4/letter/g/ecd19e/32.png) [@Gong1999](https://calculix.discourse.group/u/Gong1999)\
**Post date:** [January 26, 2023, 2:49pm UTC](https://calculix.discourse.group/t/some-question-about-sensitivity-analysis-by-c3d20-element/1458/1 "2023-01-26T14:49:23Z")

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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.

![fafe37ecec4e432f3cdf833b80dc5ac](https://global.discourse-cdn.com/free1/uploads/calculix/original/2X/5/58b402751386206f4184c2b7220fa3e8fd0374c1.png)

![image](https://global.discourse-cdn.com/free1/uploads/calculix/original/2X/5/538221cd1e84af6ad8ca281da625f81fd0b86c5c.jpeg)

Happy Chinese New Year！

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**Author:** ![Disla](https://yyz2.discourse-cdn.com/free1/user_avatar/calculix.discourse.group/disla/32/3548_2.png) [@Disla](https://calculix.discourse.group/u/Disla)\
**Post date:** [January 26, 2023, 3:31pm UTC](https://calculix.discourse.group/t/some-question-about-sensitivity-analysis-by-c3d20-element/1458/2 "2023-01-26T15:31:39Z")

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Could you post your file?

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**Author:** ![Gong1999](https://avatars.discourse-cdn.com/v4/letter/g/ecd19e/32.png) [@Gong1999](https://calculix.discourse.group/u/Gong1999)\
**Post date:** [January 26, 2023, 4:06pm UTC](https://calculix.discourse.group/t/some-question-about-sensitivity-analysis-by-c3d20-element/1458/4 "2023-01-26T16:06:49Z")

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hello this is my input file

```auto

*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

```

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<div class="post-metadata">

**Author:** ![fgr](https://avatars.discourse-cdn.com/v4/letter/f/f4b2a3/32.png) [@fgr](https://calculix.discourse.group/u/fgr)\
**Post date:** [January 30, 2023, 7:38am UTC](https://calculix.discourse.group/t/some-question-about-sensitivity-analysis-by-c3d20-element/1458/5 "2023-01-30T07:38:12Z")

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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

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**Author:** ![Gong1999](https://avatars.discourse-cdn.com/v4/letter/g/ecd19e/32.png) [@Gong1999](https://calculix.discourse.group/u/Gong1999)\
**Post date:** [February 3, 2023, 5:13am UTC](https://calculix.discourse.group/t/some-question-about-sensitivity-analysis-by-c3d20-element/1458/6 "2023-02-03T05:13:19Z")

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Thanks!I will try it.

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<div class="post-metadata">

**Author:** ![Gong1999](https://avatars.discourse-cdn.com/v4/letter/g/ecd19e/32.png) [@Gong1999](https://calculix.discourse.group/u/Gong1999)\
**Post date:** [November 22, 2023, 1:02pm UTC](https://calculix.discourse.group/t/some-question-about-sensitivity-analysis-by-c3d20-element/1458/7 "2023-11-22T13:02:52Z")

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