4 Replies Latest reply on Feb 26, 2015 11:24 PM by Mike Pogue

    Simple Cantilever Validation Prediction Problem!

    David Flemming
    David Flemming Feb 26, 2015 8:56 AM

      Hello everyone,

       

      I have a question w.r.t. a simple cantilever beam stress study. I am doing a validation prediction using classic beam theory which involves using a Free body diagram, a Shear Force diagram then a Bending Moment Diagram.

      When i use hand calculations to determine the maximum bending moment and then the maximum stress (using, max stress = Mc/I), I get a value that is much different from the SW simulation value of the maximum stress. I get 87.69 psi while SW Simulation says 118psi. I have tried every possible configuration of boundary conditions for this. Alternatively however, I used the prediction for a simply supported beam and the results were correct. Can anyone help me with this.

       

      The beam is 600 inches long and the details of the study are in the attached images.

       

      Much Thanks,

      David

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        • Re: Simple Cantilever Validation Prediction Problem!
          Mike Pogue
          Mike Pogue Feb 26, 2015 11:00 AM (in response to David Flemming)

          I'd say there may be a stress concentration at the fixture, since a layer of nodes that cannot move is connected to a layer of nodes that has to move. Refine the mesh, and you should see the large stress get larger and larger, but become packed into a smaller and smaller area. If you extract the stress from the spot closest to the fixture where it is converged (where it stops getting higher when you change the mesh size), then it ought to be closer.

           

          In general, you are not going to be able to get an accurate stress at the boundary of your model, because the boundary conditions are so phony. You have to take your model out past the area you are analyzing.

           

          BTW, if you run this as a beam mesh, I'll bet it agrees with you hand calcs to about 5 decimal places.

            • Re: Simple Cantilever Validation Prediction Problem!
              David Flemming
              David Flemming Feb 26, 2015 8:52 PM (in response to Mike Pogue)

              Thanks guys...I tried the problem again using the 'Treat as Beam' setting and it worked out exactly as my hand calculations did. I've been trying to understand SolidWorks Simulation by reading 'Finite Element Concepts via SolidWorks' by J.Ed Akin but it seems difficult to follow. Maybe it's because I'm a beginner. I previously read 'Building Better Products with Finite Element Analysis' by Adams and Askenazi and that helped a lot with the theory behind FEA.

              Do any of you by chance have any recommendations for good books on FEA using Solidworks? I see that Dassault Systems themselves don't have any at the online SW book store.

               

              Much Thanks,

              David

            • Re: Simple Cantilever Validation Prediction Problem!
              Jared Conway
              Jared Conway Feb 26, 2015 11:51 AM (in response to David Flemming)

              this or simply supported beams comes up about once every 3 months

              i would recommend taking a look at the displacement first and then the stress

              and then also compare beams and solids like mike suggested

              the displacements will be great and stress will be great in locations where there aren't stress concentrations/singularities

               

              something else to note, solidworks has many of these in the help and also the afnor document