11 Replies Latest reply on Apr 4, 2013 11:49 AM by James Pare

    Displacement in a fatigue study?

    James Pare

      I have a model where I am seeing .005" in displacement when a force is applied

      My customer has asked how much displacement will there be after 220000 cyles per year after 5 years

       

      Please help, is this possible to predict?

        • Re: Displacement in a fatigue study?
          James Pare

          any suggestions?

          • Re: Displacement in a fatigue study?
            Jared Conway

            fatigue study takes your alternating stress and compares it to the SN curve. then outputs how many cycles to failure or if the alternating stress is below the endurance limit tells you that it will survive forever.

             

            or if you enter how many cycles it will see, it will tell you the damage.

             

            If you didn't have a simulation tool, how would you try predicting what they are asking for?

              • Re: Displacement in a fatigue study?
                Bill McEachern

                The question is a bit tricky. Would there be a stiffness loss with fatigue damage - maybe. Thinking might go something like this:Fatigue cracks start on the surface. Assume damge accumulation scales linearly (or some other function) with crack depth, Reduce section depth based on damage accumulation, Recompute stiffness.

              • Re: Displacement in a fatigue study?
                Mike Pogue

                If the part is metal, the answer is zero. Fatigue happens in the elastic range pretty much by definition. This is why fatigue is such a huge problem, and why the tops still rip off of airplanes every five years or so. Fatigue is difficult to diagnose and difficult to model. This is also why fatigue analysis must include huge safety factors.

                 

                If the part is plastic, this is a really tricky question.

                  • Re: Displacement in a fatigue study?
                    James Pare

                    It's a metal part, I am seeing about 10000 pis of stress

                    So I am thinking since I am way below yield the deformation should be constant throughout it's cycle life???

                      • Re: Displacement in a fatigue study?
                        Bill McEachern

                        It comes down to how big a crack you will have in five years and where is it. Put that in and see what that does to the stiffness. If the crack is really small then zero might be a good approximation depending on how tight an estimate they are looking at. That is what Mike is essentially saying. On the other hand try and get a feel for how big a crack you are likely to get split the model to reduce the section and see what it changes in terms of stiffness.......you probaby need to look a bit deeper into these thigns to figure this out and get an estiamte that will bound up the problem.
                        The other way tocondier the problem is to take the maximum reduction in stiffness they can tolerate and see what crack size will get you there and then see if it looks like something that might occur.

                        • Re: Displacement in a fatigue study?
                          Jared Conway

                          James, I think you have to look at this from a materials and fracture/failure perspective like Bill is suggesting. Either you're going to get more displacement because cracks (even micro cracks) are going to start forming or something is going to happen to the material structure overtime. This will likely be due to the material chosen, the way it is manufactured (think about heat treating, cold working..etc), the way the part is manufactured (think about surface finish..etc) and probably how fast or how slow the part is cycled. None of the tools in SolidWorks can predict this. I don't think I've seen a tool out there that can predict this either.

                           

                          In SolidWorks simulation you really have 2 options: fatigue for high cycle fatigue problems, calculates alternating stress and then predicts life and damage, displacement would be the same or if you're creating damage, you'd have to use Bill's technique of figuring out how it affects the material and rerun your simulation to see how it affects the displacement. In nonlinear, if you go past yield, you can look at permanent set and the effect on the material, but trying to do that with a high cycle fatigue problem would be difficult and time intensive.

                           

                          A third option in nonlinear is creep, but that doesn't sound like your situation here. Dynamic analysis might be another one but you'd have to look at nonlinear dynamic and still, your material will be assumed to stay the same or follow the rules of a nonlinear material.

                            • Re: Displacement in a fatigue study?
                              Mike Pogue

                              I should clarify that I was talking about permanent deformation. I think trying to find the effect on stiffness would be an empirical exercise, not a modeling exercise. I don't know of any mathematical model of that effect, though if anyone does know, please point me to it.

                               

                              If you can't find a model and you have to create one, I recommend contacting a university before you start. You are definitely going to want to get your PhD out of it.

                               

                              10000psi is just below the fatigue stress of Aluminum 6061. This means that 6061 will never fail from fatigue at this stress, which in turn means that there are no cracks at a scale that could affect material properties. What is your material? Design so that you are below the fatigue stress, eliminate any fatigue aggravators like notches and interior corners, and I believe you can stop there.