7 Replies Latest reply on Dec 31, 2013 12:39 PM by Mike Pogue

    Inertial relief application

    Mike Pogue

      I have a study of a non-code pressure vessel. The external pressure is 10,000 psi. Projected area in the z-axis (right to left) is about 10 in^2, so the resultant force is about 100,000 lbf. Of this, 165 lbf (0.2%) remains unbalanced after all of my boundary conditions are applied, because of limitations in the geometry I can pick on. The symmetry balances the y-axis forces, and the axisymmetric simplification accounts for the x-axis forces, which are otherwise ignored.

       

      To handle the 165 lbf, I’ve turned on inertial relief. It’s my (weak) understanding that this is exactly what inertial relief is for, to balance slightly unbalanced loads in free floating parts by adding a body force. The knowledge base warns that inertial relief will give seemingly good results when improperly used. The results are matching hand-calcs within reason (20%).

       

      I’m wondering specifically whether I’m using inertial relief intelligently, and generally what are the pitfalls that would leads to bogus results.inertial relief.PNG

        • Re: Inertial relief application
          Jared Conway

          best way to check, is there a reasonable way to restrain the problem? overall i prefer to avoid using soft springs and inertial relief unless i back it up with a known solution or that trends match my expectations

            • Re: Inertial relief application
              Mike Pogue

              Well, there was a factor of sqrt(2) error in my hand calculation, so the inertial relief solution was off by about that much. When I put the right most surface on a slider and took off inertial relief, the solution popped right in within a percent.

               

              I'm a little disappointed, though. Here it makes sense to fix that surface, because it is a separate part which is almost rigid. But frequently there is no rigid surface to fix and I wind up with big phony stress concentrations around the fixtures.

               

              A lot of the vessels we design tend to crumple like oil cans. Any substantial fixture in the analysis really interferes with that. And insubstantial fixtures are not enough to stop the large unbalanced loads (like the one in my original post) from throwing the assembly into large displacement mode.

               

              I had hoped that inertial relief or soft springs was going to be the solution, but they don't seem to work very well. Soft spring pretty much crash every time I turn them on. And inertial relief seems pretty unreliable.

            • Re: Inertial relief application
              Shaun Densberger

              It depends on how the inertial relief in SW is being carried out. For example, Creo Simulate (formerly Pro/Mechanica) uses a new coordinate system and soft springs in a 3-2-1 constraint setup. This doesn't effect the stress results, but does effect the displacement results. In MSC Nastran, I believe they calculate the rigid body acceleration and apply a counter-acceleration, then does a displacement enforcement to 0 (but don't take my word for it).

               

              At the end of the day, regardless of the method, your stress results are typically good, but your displacement results are usually questionable.

                • Re: Inertial relief application
                  Mike Pogue

                  Thanks Shaun,

                   

                  According to the knowledge base, the soft springs option is a spring attached to every node. Inertial relief is a body force. To me, it sounds as if these should give exactly the same result, though with different rigid body displacement. But the KB seems to think the soft springs give more realistic results. That doesn't help me, though, because checking the soft springs box is the same as checking the box that says just crash without attempting to solve this problem.

                   

                  My results with inertial relief don't seem to be good. It seems to me they are underestimating the stress. Though there are more tests I can do.

                    • Re: Inertial relief application
                      Jared Conway

                      mike on the 2d soft springs crash, have you tried a pie slice or similar? it could just be the 2d that is causing it. i don't think i've seen someone try 2d with soft springs, i'm not sure it is supported.

                        • Re: Inertial relief application
                          Mike Pogue

                          This is a good point.

                           

                          So, here are some experiment results based on a highly simplified geometry. Basically, everything works fine except 2d with inertial relief and 3d with soft springs. I had long given up on soft springs, but they worked pretty good in the axisymmetric 2d.

                           

                          The unbalanced load warning that I kept associating with the z-axis reaction listed in the results was actually in the x-axis (the ignored dimension in the 2d simulation). So even though SW will let you turn on inertial relief in 2d, there seems to be a calculation problem.

                           

                          Closed end tube

                          ID: 3.125

                          OD: 3.656

                          L: 7.00

                          External pressure: 10 ksi

                           

                          Max stress at inside radius:

                           

                          Roarke thin wall: 62 ksi

                          Roarke thick wall: 75 ksi

                           

                          3d with fixed external ring at end cap: 64 ksi

                          3d with inertial relief: 64 ksi

                          3d with soft springs: crash

                           

                          2d with fixed external ring at end cap: 65 ksi

                          2d with inertial relief: 34 ksi (warning large unbalanced load in x axis i.e., normal to screen)

                          2d with soft springs: 65 ksi (warning large displacement, though expected)