3 Replies Latest reply on May 6, 2011 9:49 AM by Anthony Botting

    Stiffness matrix diagonal coefficient is zero at equation: 20407

    Daniel Hernly

      Hello Everyone,

       

      I am new to Simulation and I am trying to create a full assembly to do some simulation on.  When I added a part I meshed it and when I run it I get the following error message "Stiffness matric diagonal coeficient is zero at equation: 20407.  If I remove the part everything is fine.  I don't see why I am getting this error. 

       

      Please Help.

       

      Thanks

        • Re: Stiffness matrix diagonal coefficient is zero at equation: 20407
          Anthony Botting

          Hi Daniel:

          That "zero in the main diagonal" just means the solver cannot invert the stiffness matrix, so it gets stuck. Well, that's the math explanation, but it translates to physically mean there is a component loose in the assembly (they call that a "rigid body mode").

          Try checking for bonded contact between the part you removed and other parts. There could be a gap and the nodes did not "bond" across the junction.

          ..or the part is just not restrained enough

          ...or the math got messed up when the solver generated the stiffness matrix. You could try a different mesh density (and even try the other mesher - not sure which one you used first, but try the other one) and see if it solves.

          There is also a "Soft spring" option in the properties of the study. That will insert a very small value of stiffness to prevent the zero in the main diagonal, which is not supposed to affect the answer very much. There is also an "inertial effect" switch in there that is designed to offset any imbalance between applied loads, reactions, and inertia (happens sometimes in the nth digit on dynamic problems) - you can switch that on, too.

          If it only runs with those switches on, make sure to plot displacements, undeformed, and observe colors. The red-colored areas should reveal components that are 'loose'.

           

          Hope that helps a little.

          Tony