6 Replies Latest reply on Apr 28, 2015 8:31 AM by Alan Thomason

    Simulation using distance instead of force

    Alan Thomason

      I am trying to perform an impact test on a bicycle frame (ISO 4210-6).  I have the following problems:

       

      1. Forcing a load doesn't seem to make sense, since the maximum load might occur before the impact energy is absorbed.  Therefore, I think I need to simulate a displacement.  My end goal is to simulate this in a non-linear study, but to start I just want to set the model up to simulate a fixed displacement well below the yield strength.  The technique that I am using gives two different stresses though even when the load is set up to produce the same displacement:

       

      Using a fixed load:

       

       

       

      Using a fixed displacement:

       

       

      To summarize,

           275N load on left most surface                    2.1mm displacement               3.1E7 N/m^2          0.25J Total strain energy

           2mm displacement, left-most surface          2.1mm displacement               3.2E8 N/m^2          0.29J Total strain Energy

       

      So, not a good match. I have never done a fixed displacement set-up before, so very possible I'm doing something silly.

       

      2. The color gradient does not work on the fixed displacement.

       

      The assembly file (2015) is attached, but I had to strip the results out to be able to post this at a reasonable size.  Please note that the original frame has been replaced by a very, very simplistic entity just for the sake of portability and so that confidentiality is not breached.

       

      Any observations would be appreciated.

        • Re: Simulation using distance instead of force
          Jared Conway

          are you looking only at the max stress?

           

          the displacements agree, that is the important part, stress could be any number if there is a singularity

           

          have you read the reaction force on your "fixed displacement"? does it match the 275 you input in your other study?

          • Re: Simulation using distance instead of force
            Glenn Whyte

            Hi Alan,

             

            I'm not sure I can completely explain why, but the pusher part that you have set as "rigid" doesn't seem to be helping here. Set that to "deformable" and assign it a fairly stiff material and you should see better correlation between the two approaches.

             

            Or better yet, eliminate the "pusher" part and apply the loads/displacement directly to the end of the head tube, which will eliminate a contact pair, and make the analysis run faster.

              • Re: Simulation using distance instead of force
                Alan Thomason

                The reason I put the 'pusher' in place was to make sure that the frame does not rotate about the rear axle.  Any suggestions on how to ensure that does not happen, without putting a moment on the front axle? 

                  • Re: Simulation using distance instead of force
                    Glenn Whyte

                    Your pusher part is currently preventing motion of that end in X and Y. You can apply that same restriction directly to the end of the head tube using restraints (for example, you could set the x and y displacements to zero in your second "use reference geometr"y feature and apply it directly to the end of the head tube).

                     

                    It will be slightly different - the pusher + contact approach would allow local rotation of the end of the head tube, which the direct approach will not, but it will solve much faster.

                     

                    However, The source of the issue with your original model looks to be that the "rigid" component was introducing unnecessary stiffness - if you change it to deformable and assign it a material, you should be able to persist with the "pusher" approach, if you prefer.

                      • Re: Simulation using distance instead of force
                        Alan Thomason

                        Thanks Glenn, that was very helpful.  It is still applying a torque on the part, which is logical.  If you hold every point on the face of a cylinder to the same Y, but translate every point identically the same amount on a part that is connected elsewhere, the result will be a torque on the part.  Nonetheless, your advice sorted me out since that part is a fixture and I can ignore it.  Thanks so much!

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