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All Springer/NP/PCP Air Gun Discussion General > Engineering- Research & Development
Request for FEA Analysis
rsterne:
That model looks perfect, it is exactly what I want.... I am primarily interested in two thing.... How the stress on the ends compares to the hoop stress, and if a radius equal to the wall thickness reduces the stress below the hoop stress.... If the stress on the ends is greater than the hoop stress, we will have to try one with thicker ends.... ;)
I tried to download that file, and all I got was a .zip folder with a bunch of files in it.... I could not get either of the .xml files to open in my FreeCad....
Bob
WobblyHand:
Yes, we all have been tripped up on units! Myself, more than once.
Yes, the previous FEM sim of the other tube, failed to converge on the nonlinear sim at pressures over 75 MPa, but did converge at 70 MPa.
Next step is to add the FEA stuff, the model is complete. So we select the FEM Workbench. This is selected from the drop down menu where Parts Designer is normally displayed. If FEM is not installed you can go to Tools > Addon Manager and install it. Once in FEM Workbench, we add an Analysis Container, by selecting the yellow capital A. On my display it is the first icon on the left, in the second row.
Then add the material, the icon for this is a big yellow circle. If you hover over the icon, it says "Material for Solid". Clicking this gets us an FEM material. Unfortunately, there is no "material card" for 2024T3. I had to create one. I call it "Aluminum-2024T6". I made a mistake in the name! I used the generic aluminum card and added in the material properties that I found at https://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA2024T3 overwriting the generic values. So I changed density, Youngs Modulus, and a few other properties. I saved this material card into the same directory as where the other materials are.
After that, one needs to add in some constraints to the model. We put in some displacement constraints on the quarter sections, so it behaves as if it were a full cylinder. We also must pressurize the inside faces. In our case, lets start out with something like, say 60 MPa. We can increase this later! So first lets do pressure. Click the pressure constraint. Edit the pressure to 60 MPa. Then click Add, and control click faces that are under pressure. Make sure we get all of them. Of course, check that the pressure is in the right direction!
The displacement constraints are a little weird, but all we are trying to do is make sure the 1/4 section behaves exactly like a full section would. Basically we line up the faces to the axes and figure out which dimension is pointing out of the screen and fix that.
Finally we need to mesh the solid. We select the solid, in my case, called CavityGroove then click the icon Gmsh. I typically give it a maximum element size, so the mesh makes sense for the part. We'd like to have 3 tetrahedrons fit in the minimum wall thickness. So I pick a number around 1mm. Element dimension: From Shape, 2nd order element. Click apply. If we are lucky, we get a good mesh. Sometimes we don't. If it says "Clean run of Gmsh" we have been successful. Sometimes playing with the max and min element sizes helps. Then click ok. For this mesh I used max size = 1mm, min size = 10um. Not sure I like the mesh behaviour, but that is what I tried.
One last thing, before getting to the solver. There's an incorrect setting in FreeCAD, that will utterly screw up your simulations. We can fix that! Took me a couple of days to find this... At the top menu bar, select Tools > Edit Parameters. Navigate the BaseApp to select Preferences > Mod > Fem > General. Click on General in the group. On the other pane, at the top is a parameter called "AnalysisGroupMeshing". It will be set to true. This is wrong, it must be set to false. Double click true, and a pop up window will appear. Change the value to false, then click ok. Then click save to disk. And then close. Can't tell you how much grief this caused me...
Right click Analysis, in the Combo View and Activate Analysis. Then go to the top menu and select Solve > CalculiX standard. This adds a solver to the Analysis container. Double click the solver (in the Tree in the Combo View). The window says Analysis type = Static. Click "Write .inp file". If successful, it will state Write completed. Then click "Run Calculix".
Then you get to explore all the linear simulation data at 60 MPa. For what it is worth at 60 MPa, the maximum sheer stress in the inside corner is 340.9 MPa. The von Mises stress is 600 MPa! It appears the whole tube surface is very near to or at yield.
Once the file is meshed, the file grows a lot! Put in your simulation, and it gets even bigger. Just doing this exercise, has the model size now at 42.8 MB. To reduce model size, I believe one needs to clear the simulation results and the mesh. I did that, and the model is back down to 25KB again.
For anyone following along, the model with the linear problem set up for FEA analysis is attached as a zip. You will need to have FEM workbench set up and you MUST do the Edit Parameters thing. Other than that, just follow the instructions above, and you might find some interesting things to explore.
For the record, I am using Version 0.21.0 of FreeCAD. I believe one can get a prebuilt AppImage to run on any platform. I'm not a professional CAD person, I only learned FreeCAD in January of 2022. Prior to that, I had no 3d CAD experience. Likewise, I am not a professional licensed engineer. Just a person with some interest in the field...
Prior to oh, three weeks ago, I had no idea how to do anything with FEM. So I am telling you that I am not an expert, far from it... Just trying to add to my skill set...
If you still want to play along, here is the set up file. Change the extension from zip back to FCStd. You need to do the mesh. It is set up, but you need to Apply to remesh. Likewise, you need to recreate the input file for the simulator and then run the simulator per the instructions above.
Hope no one is bored by all this. If you are, well, there's lots of other gates and threads to explore on GTA!
Bob, saw your message while I was typing this up. You should only have to do a file extension name change. A .FCStd file is really a zip file. So you should simply be able to open the whatevericalledthefile.FCStd file from FreeCAD. I must ask, what version of FreeCAD are you using?
WobblyHand:
--- Quote from: rsterne on May 15, 2023, 04:50:29 PM ---That model looks perfect, it is exactly what I want.... I am primarily interested in two thing.... How the stress on the ends compares to the hoop stress, and if a radius equal to the wall thickness reduces the stress below the hoop stress.... If the stress on the ends is greater than the hoop stress, we will have to try one with thicker ends.... ;)
I tried to download that file, and all I got was a .zip folder with a bunch of files in it.... I could not get either of the .xml files to open in my FreeCad....
Bob
--- End quote ---
Rename the file extension, from xxxxxxy1.zip to xxxxxxy1.FCStd and open the main file in FreeCAD. Don't try to look inside the file, it makes no sense to normal people! What version of FreeCAD are you using?
At least on my computer, I changed the extension from zip back to FCStd. I then used the File > Open to open the file. Worked perfectly for me. Hope you can get this part to work. The version information can be found by Help > About. It even has a button to copy to clipboard. Copy it and paste in a reply message.
WobblyHand:
--- Quote from: rsterne on May 15, 2023, 04:49:03 PM ---You had me at "L" vs "LT", so I had to look it up.... The Longitudinal (L) direction is the direction the material is rolled or extruded in.... The Long Transverse (LT) direction is across the width of the plate (and I would assume around the circumference of a tube).... The Short Transverse (ST) direction is across the thickness of the plate (radially, across the wall of a tube?).... It the part is made in such a way you cannot tell the direction (eg. a round rod), then they just use "T" for Transverse.... Here is a chart on 2024-T3 showing the stress/strain relationships at different temperature.... I would only use the room temperature data....
https://icme.hpc.msstate.edu/mediawiki/index.php/Aluminum_2024-T3_Stress-Strain_and_Fatigue_Life_Data.html
Bob
--- End quote ---
Thanks for interpreting that. As far as that stuff, I am admittedly out of my league.
WhatUPSbox?:
Bob, I think you will need to pick a radius for the sharp corner. See paragraph "A Common Case of Ignoring Convergence " and many similar discussions for FEM analysis of sharp corners
https://www.nafems.org/publications/knowledge-base/the-importance-of-mesh-convergence-part-1/
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