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

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

**WobblyHand**:

Was having a little trouble with meshing the cylinder, and found I had an old version of the mesher. Updated the mesher and it easily did the cylinder with rounded inside corners. The solver had fewer issues with the mesh and was quicker as well, so that's a win win. The stresses are a bit more spread out over a larger area, primarily over the fillet area. Material: 7075-T6, 3mm wall thickness, 105 MPa. (3.5X operating pressure) Peak von Mises Stress is still high at 639 MPa. I used a 1.200 mm largest element size for the mesh. Curiously a 1.250 mm mesh had errors in it. Running at a 1mm mesh gave the same peak stress, to within 0.1%, but took more than twice as long to run. Not going to lie, the application tends to favor server grade computation and memory sizes. Peak memory consumption was around 28 GB, for the 1mm mesh, so it was getting close to the limit on my laptop with only 32 GB RAM. But running at finer and finer mesh only seems to refine the answer by a minuscule amount - and is usually only used as a sanity check. Most of the time a coarser mesh is perfectly acceptable. Or simplifying the model. I put in threads in the model, which strictly speaking, is not required, since they are out of the HPA zone. Lots of ways to make the model fit, if required.

**WobblyHand**:

Turns out the meshing was a little coarse in the example above. Went to a finer mesh, which does stress memory constraints, on my 32GB system. Went to a 0.990mm mesh, which guaranteed there were at least 3 elements in the thickness of the pressure tube. Probably should make the cell smaller, but that's pushing it on my laptop. But now we can see the stresses in the cylinder, which seem to make some sense. In the curved region (5mm radius) the stresses are moderate. The peak stresses are on the inner surface where the HPA resides. There's clearly an art to this - as well as the science. First picture is with 50% transparency, so you can see through the part. Second picture is 0% transparent, but the cylinder has been cut in half lengthwise so you can see inside. The second picture shows the maximum stress is on the inner surface.

**WobblyHand**:

At 105 MPa, (3.5x operating pressure) the von Mises stress (638 MPa) is well over the typical yield strength of 7075-T6 (503 MPa). The von Mises stress is also greater than the ultimate strength of 572 MPa. Doesn't seem like a safe area to operate...

At 90 MPa (3x operating pressure) the von Mises stress is 547 MPa, which is still higher than the typical yield strength of 7075-T6 and somewhat close to the typical ultimate strength of 572 MPa. Still not quite a happy place to be, if I am understanding correctly. I think this means there will be permanent deformation of the cylinder at 90 MPa, but I'm just a dabbler at this.

At 75 MPa (2.5x operating pressure) the maximum von Mises stress is 456 MPa, which is under both the typical yield strength and the ultimate strength. So not likely to have an issue, he naively thinks. So roughly speaking, this design has about a 2.5x safety factor.

Please don't design to this and think it is safe - this is merely an engineering exercise - make sure you verify any of your designs to make sure they are safe! I am not a professional mechanical engineer, I do not have access to the minimum values of the material properties, nor have I learned all the rules of thumb, so take all my comments above with a grain of salt. Merely the ramblings of an amateur.

**Privateer**:

How accurate is the scale of your Model? I mean is it to scale of the Real World part?

Not trying to start a problem just want to understand.

**WobblyHand**:

--- Quote from: Privateer on May 06, 2023, 01:35:20 PM ---How accurate is the scale of your Model? I mean is it to scale of the Real World part?

Not trying to start a problem just want to understand.

--- End quote ---

Don't honestly know Jeff. I looked at the photos in the epic thread, and estimated values based on a couple of measured values that Marko showed, and the proportions relative to those measurements. So it's a WAG, but not too far off. The model above I used a 5mm radius on the end, rather than a sharp inside corner. Within the guide lines Marko mentioned for a 3mm wall. As far as I know, Marko didn't publish that many dimensions, perhaps not to publicly show the dimensions (as a courtesy?). If I had better dimensions, of course the model and results would be better.

I simulated 7075-T6 above. That is what I am guessing that has 2.5x safety factor. Not 2024 (D16T) which is the material for the actual tube.

2024-T4 is not as strong... Yield strength of 2024-T4 is 324 MPa, but 7075-T6 is 503 MPa. Ultimate strength 2024-T4 469 MPa vs 7075-T6 572 MPa. https://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA2024T4

https://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA7075T6

I'm not a professional ME. Just dabbling in this, because it really caught my attention.

BTW, I agree, Ansys is a professional grade simulator. We used HFSS (an Ansys product) for RF simulations for quite a while. We found it didn't scale well for the really huge problems we wanted to simulate for millimeter wave radar, so we transitioned to a different technology using GPU's. This was because the new technology could use/exploit massive parallelism and use GPU's for computation. We achieved 30-100x faster solutions than using HFSS, which didn't exploit parallelism. This meant simulations that took days the old way now took under an hour. The new method called Finite Difference Time Domain, allowed us to see electromagnetic waves as they were propagating and how they interacted with the structure. HFSS in contrast was a steady state solution. We were able to massively increase our understanding and throughput using the new system. I don't know if HFSS has been updated to use GPU's. It was a rough transition at our company, with the HFSS guys having to learn new ways... We also had to install huge fast disk farms - if I recall correctly, we had 1 Exabyte capacity. It was a major problem just shuttling data around, due to the puny 10GbE networks we had, because moving 10TB files took a significant amount of time! These are typical problems with high performance computing - there are bottlenecks in the most unforeseen places.

Anyways, this is a mechanical FEA thread, not electromagnetic one, sorry for the digression.

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