# Kaihes

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1. ## Size Optimization doesn't lower desvar value

Thank you all for your replies. I understand the problem now. It's more complex than I thought to optimize the cross sections of >80 beams based on a catalogue with >300 cross section possibilities. Best regards, Kai
2. ## Size Optimization doesn't lower desvar value

Hi @Jan Grasmannsdorf, thank you for your reply. But why does it work, with my own linear function? I attached the file. The DGlobal function works well for this small example, but unfortunately my real optimization is much bigger, which make it necessary to use a high number of starting points. Best regards, Kai example1_linearfunction.hm
3. ## Size Optimization doesn't lower desvar value

So it seems that the problem is based on the equations, but I cannot find the problem. The solutions of the equations are correct, but somehow the optimization cannot lower the desvar value...
4. ## Size Optimization doesn't lower desvar value

Hi, I'm working on a complicated size optimization with many different beams. This script https://connect.altair.com/CP/kb-view.html?kb=41128 seems to be really useful for the problem. Unfortunately, the values of the design variables doesn't reduce during the iterations, which leads to the soft convergence. If I change the initial value near the lower bound value, the optimization works. Of course this isn't an acceptable solution for bigger models. I attached the example file. The initial values are set equal to the upper bound values, so that the problem becomes clear. Could you help me with this problem? Best regards, Kai example1.hm
5. ## Wrong results with negative compliance

Thank you Rahul, but I think I made a mistake in the model setup.
6. ## Wrong results with negative compliance

Hello all, I'm trying to simulate some load cases of a car chassis. It should later be used for a topology optimization, so that I start with the design space. I started with one side, mirrored it and delete the coincident nodes. The blue beams are stiff (very high elastic modulus) and connected with ball joints (coincident nodes). Those beams are connected to the chassis with RBE3 elements (free around the x-axis). I realise that the mesh is not the best, but it's just for testing the suspension. i got the following warnings (using HM 2017 and optistruct): *** WARNING # 5629 In static load case 2, The magnitude of Residual Energy Ratio, 1.83121e+006, is large (larger than 10%). The results for this subcase may not be correct. *** WARNING # 312 In static load case 2 the compliance is negative or large -316.526. Optimization/buckling analysis cannot be performed. due to possible rigid body mode. Subcase: 2 -------------------------------------------------------------------------- Label x-force y-force z-force x-moment y-moment z-moment -------------------------------------------------------------------------- Sum-App. 0.000E+00 0.000E+00 1.000E+04 1.416E+07 -1.743E+07 0.000E+00 Sum-SPCF 3.630E+04 -2.914E+04 4.500E+03 -2.370E+07 -1.162E+07 -1.056E+08 I don't understand, why I get a the following displacements (scale factor 1000). Moreover, the displacements are not symmetrical, although the forces are applied symmetrically. @Prakash Pagadala: I uploaded the file "Suspension_Beams": I would really appreciate your help.
7. ## Output of optional responses

Ok so I entered DREPORT,99,1 in the BULK_UNSUPPORTED_CARD. This leads to an Warning in the .out file: *** WARNING 1931: DREPORT SID= 99 is not referenced by Case Control. Where should I reference the DREPORT SID? Thanks in advance, Kai
8. ## Output of optional responses

Hi @Prakash Pagadala, unfortunately this just seems to work with responses, which are referenced either as an objective or a constraint. My response isn't referenced by any of that. Is there another solution? best regards, Kai
9. ## Output of optional responses

Hi @Prakash PagadalaI have the same question: I want to display an optional response (displacement of a specific node) in the output file. Could you give us a short explanation? I couldn't find a tutorial. Thanks in advance, Kai
10. ## Error #153 modeling suspension

Okay thank you all. I reduced the values of the spring stiffness and the elastic modulus in the following example and it finally works. Thank you all for your help ;-) @Prakash Pagadala Is there a highest value for the elastic modulus or a specific value which is assumed to be "stiff"? Best regards, Kai
11. ## Error #153 modeling suspension

Thank you gopal_rathore, but I think my problem is due to the limits of a linear static simulation. Maybe I have an incorrect understanding of it? I placed RBE2 Elements (DOF: 1,2,3) between two coincident nodes (see the blue circles), which should allow rotations. Do you see a reason why this model results in an error? *** ERROR # 723 *** An invalid rigid element. This RBE2 is not connected with any structural element. RBE2 element id = 8 independent grid id = 4 Note: If this rigid element is also connected with other rigid elements, then this error means that there is rigid body mode or mechanism remained in this rigid element chain due to lack of connected structural elements.
12. ## Error #153 modeling suspension

Thank you Prakash. Unfortunately Joints just seem to work with MBS. I sent another example with the same error. The RBE2 Elements transfer no rotations around z, so that they should work like a revolute joint. Could you explain me, why I keep getting the #153 error?
13. ## Error #153 modeling suspension

Hi Prakash, thank you for your reply. MCHECK is on to get a result for the displacement. Otherwise I would just get the error message. OK, so I get transverse load, which does not work with ROD Elements, because of the shared nodes between the ROD-Elements and the RBE-Elements. But actually the shared nodes should work like a ball joint, like the one in the picture. To my knowledge this ball joint should prevent any transverse loads, so that I could work with the CONROD Elements. Do you have an idea how I should model this type of connection? Best regards, Kai
14. ## Error #153 modeling suspension

Good morning, at the moment I'm doing some tests for an upcoming optimization. I use HM 14.0 with Optistruct. I want to simulate a simplified suspension.If I model the suspension rods with CONROD-Elements, the following error message appears: *** ERROR # 153 *** Exactly zero pivoting encountered during Numerical Factorization; the model may have rigid body mode. Solver error no. = -503 spc set id = 1 index = 1 Possible reasons are: 1) insufficiently constrained model, 2) having rigid body mechanisms within the model, 3) extremely ill-conditioned rigid element sets, 4) extremely thin shells (as used for skinning) that have MID2/MID3, 5) gap elements with extremely high stiffness (KA, especially KT or MU). Check the model and rerun the problem. (MECHCHECK may be used to find the rigid body modes. To do so, change the input to be an eigenvalue analysis and add MECHCHECK.) (WARNING: results obtained with MECHCHECK cannot be used because the model is changed internally.) This error was detected in subroutine bcsmtxfct. If I turn MECHCHECK on, the max. displacement is 0.008061 mm. When I replace the CONROD Elements with nearly stiff CBEAM Elements, the error message disappears. The max. displacement is 0.00769227 mm. To my knowledge, the suspension rods are just tension and compression rods. Could you help me, why I get the error message? I attached the file. Best regards, Kai
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