General Mechanical

General Mechanical

Problem about SHELL181

    • ceHumaNait
      Subscriber

      Dear all,


      Recently, I am trying to simulate a orthotropic plane in ANSYS using SHELL181. The APDL input is as follows:


       


      finish


      /clear


      /prep7


      *afun,rad


      et,1,shell181


      sectype,1,gens,,bandanyuan,


      sspa,420626,126188,,4.24713e8,,,


      sspd,43.6662,13.0999,,48446.7,,30.6256,


      sspe,,8.15102e7,


       


      k,1,0,0,0


      k,2,2.31,0,0


       


      l,1,2


      cm,tra,line


       


      k,3,0,0,-12.164


      l,1,3


      adrag,tra,,,,,,2


       


      allsel,all


      mshape,0


      mshkey,1


      aatt,1,1,1,,1


      amesh,all


       


      !D11


      allsel,all


      lsel,s,loc,x,0


      dl,all,,ux


      dl,all,,uy


      dl,all,,uz


      dl,all,,rotx


      dl,all,,roty


      dl,all,,rotz


      allsel,all


       


      allsel,all


      lsel,s,loc,x,2.31


      dl,all,,ux


      dl,all,,uy


      dl,all,,uz


      dl,all,,rotx


      dl,all,,roty


      dl,all,,rotz,1/50


      allsel,all


       


      When I ran the code, I got the following error messages:


       


       *** NOTE ***                            CP =      10.374   TIME= 15:46:05


       Predictor is ON by default for structural elements with rotational


       degrees of freedom.  Use the PRED,OFF command to turn the predictor


       OFF if it adversely affects the convergence.


       


       


       *** ERROR ***                           CP =      10.374   TIME= 15:46:05


       Membrane stiffness of Element 10 (integration point 1) at temperature 0


       is not positive definite.


       


       *** ERROR ***                           CP =      10.374   TIME= 15:46:05


       Shear stiffness of Element 10 (integration point 1) at temperature 0 is


       not positive definite.


       


       *** ERROR ***                           CP =      10.374   TIME= 15:46:05


       Membrane stiffness of Element 12 (integration point 1) at temperature 0


       is not positive definite.


       


       *** ERROR ***                           CP =      10.390   TIME= 15:46:05


       Shear stiffness of Element 12 (integration point 1) at temperature 0 is


       not positive definite.


       


      It seems that some material stiffness was not well defined. Can anyone give some suggestions about how to solve this problem?


      Thank you very much!


       


      Kind regards,


      ceHumaNait


       

    • Rohith Patchigolla
      Ansys Employee

      Hello ceHumaNait, 


      Can you also include the material property definition. I don't see it in your script. 


      Best regards,


      Rohith

    • sk_cheah
      Subscriber

      Looks like your stiffness matrix is not positive definite. 


      Kind Regards,
      Jason

    • ceHumaNait
      Subscriber

      Hello rgpatchi,


      In our simulation, we intended to specify a preintegrated membrane stiffness for shell sections instead of specifying isotropic linear elastic material properties. We first calculate the stiffness properties of the shell and then apply them using the "SSPA" "SSPD" and "SSPE" commands, which is corresponding to command lines:


      sectype,1,gens,,bandanyuan,


      sspa,420626,126188,,4.24713e8,,,


      sspd,43.6662,13.0999,,48446.7,,30.6256,


      sspe,,8.15102e7,


      However, the commands do not work well. Do we make any mistakes using these commands?


      Thank you very much!


       


      Best regards,


      ceHumaNait

    • ceHumaNait
      Subscriber

      Hello Jason,


      Thank you for your reply. Do you have some suggestions to make the stiffness matrix positive definite? As this is the first time we use preintegrated stiffness for shell elements, we find it difficult to solve this problem.


      Best regards,


      ceHumaNait

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