12 Replies Latest reply on May 30, 2018 1:55 PM by Paul Millhouse

    How do I find the volume change of a cavity in a part?

    Paul Millhouse

      I would like to run FEA on a single part. The part contains a cavity within it. There is a force applied along the top of the part. I would simply like to see the Volume or Δvolume of the cavity within the resulting deformed part after the load is applied. Is this possible?

       

      I know how to view various output variables in the Simulation such as stress or displacement but I don't know how to set a custom output variable (i.e. volume).

       

      EDIT: Can this be done with a sensor? I've never used sensors.

        • Re: How do I find the volume change of a cavity in a part?
          Andrew Cuttle

          Few questions to see what you're working with:

          Regular shape or irregular? (Screen shot would be ideal)
          Material?
          Large or small displacement analysis?

            • Re: How do I find the volume change of a cavity in a part?
              Paul Millhouse

              The part is quite simple really. The shape is "regular" I suppose. It's symmetrical in two planes. It's basically just an extruded trapezoid with a rectangular cavity near the short side (if that makes sense). I can't attach a screenshot of the exact part for proprietary reasons but I can provide an image if my simplified description above is difficult to visualize. We will be doing a similar simulation on another part in the future though.

               

              The material choice is one of the reasons we'd like to run the simulation, but initially this would involve PEEK, acrylic (PMMA, aka Plexiglass) and a flexible thermoplastic polymer such as polycarbonate urethane (PCU).

               

              I'm not exactly sure what you mean by "large or small" but I imagine it's small displacement.

                • Re: How do I find the volume change of a cavity in a part?
                  Andrew Cuttle

                  Large displacements is a mode for FEA which covers significant changes in geometry, it adds complexity.

                   

                  I'd agree with Ian, the easiest way of getting volumes out is probably to bake out deformed geometry post run. Then use a Boolean intersection to create a volume to measure. Alternatively if your geometry is regular enough you could section the deformed geometry and then calculate the volume off the sections.

                   

                  It depends on your application is but I'd definitely take your analysis results with a pinch of salt. FEA with plastics can be a bit (or a lot) iffy, particularly as the results you are after (volume change) are on the non-conservative side of the analysis. You'll be running a non-linear analysis and your results will be very dependant on the quality of the curves you are using for your materials, particularly the specific material batch you are testing. If you need any accuracy then you'd want to produce those curves specifically for the material batch you are planning to use (or at least to validate the common curves don't demonstrate any significant deviation).

                    • Re: How do I find the volume change of a cavity in a part?
                      Paul Millhouse

                      Great thanks for the responses.

                       

                      I'd agree with Ian, the easiest way of getting volumes out is probably to bake out deformed geometry post run. Then use a Boolean intersection to create a volume to measure. Alternatively if your geometry is regular enough you could section the deformed geometry and then calculate the volume off the sections.

                       

                      ...You'll be running a non-linear analysis and your results will be very dependant on the quality of the curves you are using for your materials, particularly the specific material batch you are testing.

                      What do you mean by "the quality of the curves you are using for your materials"?

                      Can you explain the Boolean intersection method in a bit more detail?

                       

                      The cross-section of the cavity is simple enough - it's just a rectangle with fillets on the corners. When the cavity is deformed (compressed under load) an equivalent volume of fluid within it will be displaced into an adjoining channel. We are trying to determine if we can correlate the distance the fluid travels down the capillary with the compression force applied. We can assume the material is isotropic and the fluid is Newtonian, or just ignore viscous stress as the fluid will be moving slowly.

                       

                      I can't really figure out where to begin. I tried saving a result body and finding the volume of the cavity but the results were essentially 0, much lower than what we're getting in real life with various materials. Also, when I reduced the flexural modulus and changed other properties (to something more in the range of a material we'll be using) the results didn't seem to change at all. In fact reducing the flexural modulus dramatically didn't seem to do anything, which made me wonder why it's even there.

                       

                      How do you do a non-linear analysis? Can you step me through? I don't even see that option in Simulation.

                       

                      I understand the results may not be very accurate, but we would at least like to generate some pretty pictures to say that we've done FEA. Of course mechanical testing will be necessary to really determine the response with a given material.

                • Re: How do I find the volume change of a cavity in a part?
                  Iain McEwen

                  Hi Paul,

                   

                  I don't think you'd be able to use a sensor for volume in this case. You'd need to have a body that deformed with zero resistance, where you could see the updated enclosed volume.

                   

                  What you could do is export the deformed result as a body? then you could create a body which would give you that volume of the deformed shape?

                  • Re: How do I find the volume change of a cavity in a part?
                    Chris Saller

                    Measure the volume before and after the cavity feature.