5 Replies Latest reply on Sep 22, 2010 11:42 AM by Peter Holmi

    Stray elements with excessive values?

    Peter Holmi

      Does anyone know why SolidWorks sometimes gives "stray" elements with excessive values?  Over the last two weeks of multiple simulations, I have only one model with a maximum Von Mises stress of 8,584,023 psi while the real peak stress areas are running at about 60,000 psi. The peak stress occurrs at a small point on a face awash in stress values <1000 psi.  This is with a standard mesh size of 0.6 in and automatic transition.  The same thing happened with a mesh size of 0.7 in when the peak stress was 6 Mpsi.

      SW 2010, V4

      Analysis type Linear dynamic analysis (Random Vibration)
      Mesh type Solid Mesh
      Number of modes  10
      Solver Type FFEPlus
      Soft Spring Off
      Improve accuracy for contacting surfaces with incompatible mesh (slower) On
      Low Frequency Limit 5 Hz
      Upper Frequency Limit 505 Hz
      No. of output frequencies 10
      Correlation Fully correlated


      Mesh type Solid Mesh
      Mesher Used Standard mesh
      Automatic Transition Off
      Include Mesh Auto Loops Off
      Jacobian points 4 points
      Element size 0.6 in
      Tolerance 0.03 in
      Mesh quality High
      Total nodes 146338
      Total elements 84375
      Maximum Aspect Ratio  19.259
      Percentage of elements
      with Aspect Ratio < 3  80.4
      Percentage of elements
      with Aspect Ratio > 10  0.0332
      % of distorted elements
      (Jacobian)  0
      Remesh failed parts with incompatible mesh Off
      Time to complete mesh(hh:mm:ss) 00:00:20

        • Re: Stray elements with excessive values?
          Ryan Werner

          Hi Peter,

           

          This is a very common occurance.  Simulation sees the stress there as being infinite or close to it so refining the mesh actually causes the reported stress to climb.  A coarser mesh will most likely give smaller stresses in that area but they will probably still not be totally accurate.  Without seeing you model or the mesh I cannot really say if there is a way that you can easily get around this but it is a meshing issue.  Have you tried the Curvature Based Mesher?  Since it meshes the part based on curved geometry sometimes the mesh will be cleaner and give better results.

           

          Ryan W.  

          • Re: Stray elements with excessive values?
            Bill McEachern

            Looks like bad elements. You should plot aspect ratio and jacobian and see if red shows up  in the same location. Looks like it will. You just need to make smaller less distorted elements. Try a mesh control.

             

            The geometry does look right for a stress singularity (accelerated increasing stress with smaller mesh size)

            • Re: Stray elements with excessive values?
              Roland Schwarz

              Lots of possible reasons: overconstrained boundaries, bad mesh, high aspect ration, element size w.r.t. neighboring elements.  Use mesh controls to ensure good mesh in key areas.

              • Re: Stray elements with excessive values?
                Anthony Botting

                Hi Peter: Prob just a "bad" set of elements, as others have remarked. You can plot "Element" stress to show the averaged value of the stress throughout the elements and probably identify the exact element(s) causing the issue - put mesh controls there. Also create a mesh plot of the elements' Aspect Ratio, and another of "Jacobian". You really do not want elements AR > 5 (in areas of interest). The Jacobian shows you how "folded-over" the element might be (numbers < 40 are supposed to be acceptable). Here are a couple screen shots showing AR and Jacobian from a course I developed.

                • Re: Stray elements with excessive values?
                  Peter Holmi

                  Thanks for the follow up.  I really appreciate the thoughts and different ways to approach the issue.  I took another path, I suppressed the feature from the model and re-ran the simulation.  It ran normally and the results are reasonable.

                  Peter