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

[WhatUPSbox?]

Bruce, great work! That mesh looks really good.
I wonder about the axial load. It would not affect the hoop stress in the middle of the tube but I'm not sure if it affects the corner stress. Even a simple constraint at the thread end may be OK by the time it gets to the corner.

Marko, would it be possible to add strain gages to the tube for the destructive test? 

[WobblyHand]

Thanks, Stan.  Still trying to figure out how to set up a non linear simulation in FreeCAD.  It's kind of silly, but I don't understand the input data format.  I have to enter the stress strain curve for the material, but the format shown in an example FEM FreeCAD file is unknown to me.  Just an example, but no words describing the details.  Details that are undoubtedly known by practitioners of the art.  Probably obvious to someone with strong mechanical background, but as you can tell, I just starting to learn this.

[WobblyHand]

Well, got the non-linear formatting sorted out.  I had it all wrong.  
The data format for the non-linear is kind of strange (to me) but, it is a plot of the breakpoints of the stress strain curve, but the first breakpoint strain value is set to 0.
Trying again.  I have 8 cores running on the problem now.  Maybe this time the sim might converge.  Taking it's sweet time, it's been over 27 minutes...

[WobblyHand]

Foo, hit timeout before I could finish editing!

Edit: Finally finished, (about 1/2 hour) but I'm trying to interpret the results.  Quite a few of the simulations did not converge, so this took a while.  The sim generated 13 unique data sets!  The model/data file is now 435 MB.  Prior to this non-linear sim, it was a mere 19 MB.  Is the idea to sift through the data to determine if the Equivalent Plastic Strain greater than some value?  (3%?)  This is the cylinder with 0.4mm radius, at 75 MPa, using two points for non-linear 2024T3.  My figure shows the equivalent plastic strain.
I used the following points:
344.74, 0
358.53, 0.005
[MPa, unit less]
Got the material data from: https://icme.hpc.msstate.edu/mediawiki/index.php/Aluminum_2024-T3_Stress-Strain_and_Fatigue_Life_Data.html  figure 10, for tension.

[WhatUPSbox?]

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.

[WobblyHand]

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.

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]

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.

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