7 Replies Latest reply on May 13, 2010 9:39 AM by Joe Grant

    Can I use non-linear analysis to solve this problem?

    Joe Grant

      I would like to be able to predict when a part will fail. I define a failure for this case as a complete fracture of the part which allows the component to separate into two pieces. I have looked at some of the non-linear static examples and set up a simple problem but don't really see how to get there from here.

       

      If I model a cylindrical hollow aluminum tube that is 48" long x 4" dia. X .25" thick I can restrain one end and apply a load to the other end. I can also depict this problem on paper and solve for the bending stress and displacement which matches the SolidWorks solution very closely. That's the good news. How do I use the simulation to predict when the part will actually fracture? Can I do that? Obviously stress in excess of the tensile strength is a problem but I would like to quantify what value is required to fail the part.

       

      When i vary loads on the example described above I seem to continue to get linear results even as the stress in the part goes well above the tensile strength. Shouldn't the displacement and stress be non-linear by definition?

       

      I picture is attached. Any insight would be appreciated.

      pic.JPG

        • Re: Can I use non-linear analysis to solve this problem?
          Bill McEachern

          Well, I am going to assume the part is made out a ductile metal. If we imagine what happens when this thing i s loaded we get the following sequence of events:

          1. the linear range - up to either material yield or the end starts to deflect over half the tube diameter in the direction of the load or the tube collapses due to an instability.
          2. IF the material goes NL first the elements that go post yield & turn to mush - lose their stiffnesses, deflections go up, load redistributes more elements turn to mush and process continues and eventually the deflections go NL, then the member starts to line up with the load and if things work out the tube lies flat against the wall with enough material remaining and it just hangs there or somewhere along this sequence the the area is reduced and the thing lets go.
          3. If the tube collapses somewhere along its length as the compressive stresses produce zero stiffness it buckles goes flat at that location and the thing bends over as a shorter cantilever, the section squashes flat the and it hangs and then maybe the inboard sections do as described above.

          A NL static analysis can probably take you a ways along either of these paths but all the way down either is quite tricky for any code. Ideally you would need a cohesive failure model for your elements to have them "fracture" SWX Sim Premium does not have this capability but some codes do. It maybe able to go post buckling, if that happens but how far would probably be dependent on element distortion levels and incremental strain levels - my guess is not too far along but probaly past the initial buckling load.

           

          Usually the magnitude of the onset of one of the above failure condition is sufficient to estimate ultimate strength. Any of those work for you?

          • Re: Can I use non-linear analysis to solve this problem?
            John Greiner

            Bill is correct in that SolidWorks Simulation cannot tell you when the part will fracture, in fact no FEA code I know of can give you the exact point at which the part will fracture.  (Remember that finite element code cannot predict material flaws.)  What simulation will tell you is what the approximate magnitude of the loads will be before the part will fail.  From here you need to build a few samples and take them out and test them.

             

            In my opinion using the Non-Linear solver on this model is a waste of time.  You can get 90% of your answer from a static analysis in a fraction of the time and use that information to design your real life tests on the part.

             

            Good luck and I would love to see your actual failure results and testing data.

            • Re: Can I use non-linear analysis to solve this problem?
              Joe Grant

              Thank you for your responses. I did a poor job of communicating my questions. Let me provide some more information and answer some of your questions to me.

               

              1. The material is ductile, stated as aluminum in the first post.

              2. I understand that the code will not depict a "fracture" lets forget I said that

              3. There will be no testing of this hardware

              4. Yes, this is a simple case. It is academic. The reason for the simple case is so I can generate hand calculations. My intent is to understand how the software works. I do not intend to use a product unless I understand how it works.

               

              O.K. back to some specific questions and results. My expectation was that I would get non-linear results when running a non-linear analysis. That doesn't seem to be the case. I have run analyses in the static and non-linear tool and get virtually the same results. The material is aluminum 6061-T6 at the dimensions stated in the first post. I have run the following analyses in both linear and non-linear and gotten the same results. I ran a case with a load close to yield strength, a case at tensile strength, a case at 1.35 times tensile strength and a case at 1.75 times tensile strength. I get the same displacements and stresses. My expectation was that the results say for displacement would increase in the non-linear study above the tensile strength and "track" the stress/strain curve.

               

              Again any comments are appreciated. Attached are two graphs showing stress and displacement for the two analyses. It's difficult to see there are two sets of data as they are right on top of one another.

              • Re: Can I use non-linear analysis to solve this problem?
                John Greiner

                Joe,

                 

                Can you post your SolidWorks file so we can take a look at it, don't us all the results files, just your SolidWorks part file with the studies defined.  As you know, there are a ton of settings that could be messing with your results.

                • Re: Can I use non-linear analysis to solve this problem?
                  Joe Grant

                  I think I figured out what was going on. It appears that one can select a non-linear static study and then execute the study without selecting non-linear material properties which is what I did. Creating a custom material seems to do the trick.