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FEA for HPA - it's not out of reach for us

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WobblyHand:
The recent thread on a ruptured pressure vessel, reminded me that HPA can be dangerous.  I was wondering if there were tools that could help us analyze things like this, and could be used by folks that hadn't done this kind of work for a living.  I'm thinking in terms of people that want to make or machine items for airguns.

Many CAD packages have some level of finite element analysis tools.  Ones that have these FEA packages include Fusion360, SolidWorks, FreeCAD, and I'm sure others.  The question I had was how hard is it to actually do this analysis.  Turns out, it's not too hard, at least in FreeCAD to do FEA.  FreeCAD being non-commercial, isn't quite as slick as the other tools, but, it seems to work ok, once you figure out a few quirks.  Now I am not a mechanical engineer, nor have I done this professionally, but I have had exposure to FEA for solving electromagnetic problems.

The basic steps are as follows

* Create a model of the structure under test
* Add any fixed constraints
* Apply pressure constraints
* Apply the material
* Mesh the model
* Set up the Solver and run the Solver
* View the resultsHere's a simplified model, hey we have to start somewhere.  It's 38.5mm OD with 3mm walls.  It has a horrible sharp inside corner, which we know is not good for a pressure vessel.  Lets say it is made of 6061-T6 and we charge it to 300 MPa.  To the right side of the line is pressurized.  Then we put some constraints.  We mesh the model.  At least in FreeCAD, it's a good idea to constrain the maximum element size.  Since this model is roughly 150 mm long, I chose a max element size of 1.5 mm.  The meshing takes a little bit. Then one runs the solver.  At least on my PC, it uses all the CPU cores for solving.  On an i7 it took 30 seconds.
We can show various results including von Mises Stress, Max Shear Stress, and displacement.

What I am trying to learn still is where and how to constrain the model.  This took me a couple of days to figure out so far.  Have a lot more to learn.  I can post the model if anyone is interested.  My original model had the vessel threads in it, but I stripped them out, since they were not in the pressure region.




WobblyHand:
As you practice a bit, it gets a little easier.  (Instead of being clueless, I have progressed to slightly befuddled  ;) )Here's the interior view of a pressure cylinder made of 7075-T6 running at 105 MPa, 3.5 x its working load.  You can see the stress induced by the sharp interior edge.  Had to play with some of the defaults in FreeCAD to get this to render correctly.  The default is no anti-aliasing, which is a poor choice.  I set the anti-aliasing setting to MSAA-4x, and the formerly gray interior now seems to be rendered better.  In a while, I will radius that inside corner, because we all know that is the right way to do it.

Been interesting doing this.  Makes me leery of using aluminum pressure cylinders...  Wonder what titanium would be like?  (A devil to machine!) 

sb327:
Iíve no input here but enjoy your explanations. Very interesting subject for sure. Keep us updated.

Dave

Privateer:
Interesting thread for the software your using.
Can't say that I really care for the programs being used by Marco or Ed.
And I have to wonder IF both are using the exact same 3D mesh and how accurate the mesh is to the final produced part.


WobblyHand:

--- Quote from: Privateer on May 03, 2023, 02:47:21 PM ---Interesting thread for the software your using.
Can't say that I really care for the programs being used by Marco or Ed.
And I have to wonder IF both are using the exact same 3D mesh and how accurate the mesh is to the final produced part.

--- End quote ---
I think Marko uses Fusion360, not sure about Ed.  Unlikely they are using the same mesh.  Ed, unlike Marko has the original drawings.  Marko is measuring "used equipment".  Not sure if that is good or bad.  Perhaps you could say Marko is using "as built".

The meshing on FreeCAD is sort of ok.  I simply controlled the maximum cell size and let it figure it out from there.  Fusion360 and all the other packages may have smarter routines.  All meshers will fill the model.  There will not be any unmeshed areas - that would trigger a meshing error.  It's possible to zoom in on the mesh and verify full coverage.

I'm so far from being expert at this...  But it has been interesting working on FEA.  I'm now trying a cylinder with a rounded bottom.  Having trouble with FreeCAD meshing and constraining, due to me not understanding some key point.  I'll figure it out eventually. 

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