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All Springer/NP/PCP Air Gun Discussion General > Engineering- Research & Development

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

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WobblyHand:

--- Quote from: WhatUPSbox? on May 14, 2023, 01:41:16 PM ---Good job getting Freecad to run non linear solutions. I think with a .4mm flaw size, this ends up being a fracture mechanics research project. Just reading the Wiki page should show how much fun that would be.
I can't understand why the manufacturer would try to defend using material yield to meet safety margins instead of the simple 3 mm fillet that solves the problem with straight forward linear analysis. At some point, maybe they should just stop digging the hole.
Great work.

--- End quote ---
Thanks, some of this stuff (documentation) takes a while to ferret out.  Like it's nowhere in the docs.  Had to point blank ask the question about the data formatting.  A gentleman on the FreeCAD forum told me that at 75 MPa, my simulation wasn't really converging.  He suggested a 70 MPa value, saying that at 75 MPa, it's perhaps this thing would have blown up.  Honestly, don't know if that is true, or he was just trying to have me be extra cautious.  He did say plastic material analysis is forbidden (in the EU) for pressurized vessels over the whole structure (for good reasons).  That I believe. 

Here's a non-linear simulation at 70 MPa, showing the cylinder at yield stress along nearly the whole HPA area, with the stress above UTS in the radius.  Pretty scary, at least to my ignorant self.  Perhaps, this model isn't constrained correctly, or incorrectly implemented, but it is an eye opener.  This took 3 minutes, with two point nonlinearity. 
When you do the problem correctly, the file size seems to decrease, thankfully.  Now at 230MB...

WhatUPSbox?:
It may be interesting to run the 30MPa basic fill pressure case and see how much is above yield.
I think your constraints are OK for what you are doing. This is an internal loads case. Constraining the model with a 6 degree of freedom connection to ground is sufficient. I think you have that on the right side of the o-ring block. As you have it, there is an artificial compression force on the o-ring block. That is not the high stress area. To be more representative you could add a cap disk to the threaded end, join it to your model as mentioned earlier and then ground your model there. That would add the axial load to the tube and remove the compression on the o-ring block. This may change the stress in the fillet.

WobblyHand:

--- Quote from: WhatUPSbox? on May 15, 2023, 12:18:48 PM ---It may be interesting to run the 30MPa basic fill pressure case and see how much is above yield.
I think your constraints are OK for what you are doing. This is an internal loads case. Constraining the model with a 6 degree of freedom connection to ground is sufficient. I think you have that on the right side of the o-ring block. As you have it, there is an artificial compression force on the o-ring block. That is not the high stress area. To be more representative you could add a cap disk to the threaded end, join it to your model as mentioned earlier and then ground your model there. That would add the axial load to the tube and remove the compression on the o-ring block. This may change the stress in the fillet.

--- End quote ---
Good stuff.  I will try that out in a couple of days. 

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