FEA of pressure vessel

[Anonymous]

Hi,

I am currently in the process of modelling a simple pressure vessel, to represent a SCUBA cylinder. 

I have created the model with no problems and applied the pressure to the internal wall, however I am not sure on the accuracy of my results when I run the stress analysis. 

I have calculated the hoop and axial stress by hand, and cannot see what Inventor classes these stresses as, with no figures seeming to relate. Are they the 1st Principal stress, or for example the YY stress (if the cylinders length is in the Y axis). 

After reading the forums it seems my problem may be in the constraint, I have currently constrained the bottom face of the cylinder using the 'fixed' option. There was mention of creating three points on the base and constraining these in a certain way, however when I tried this it just created a large stress raiser at the base.

Attached is a screenshot with the model as it currently stands. (The small stress raiser on the face is intentional and is to model impact damage, this was added well after the problem arose, I assume this does not affect the rest of the model drastically?). 

Thanks

Sean

[JDMather]

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@Anonymous wrote:

...plastically deforming the cylinder.

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The material had to significantly yield (deform) to go from a cylindrical plug to thin wall tank.

Given that you are interested in plastic (non-uniform) deformation caused by damage (like a dent - plastic deformation) or corrosion, Inventor might not be the correct analysis tool.

Inventor FEA is limited to the linear static stress (elastic) range of the curve.

You might be able to pictorially show a stress riser due to corrosion or damage, but it is not going to tell you anything about failure.

[Anonymous]

I believe I mispoke on my previous info of anisotropic vs. isotropic material.  I was thinking of orthotropic vs. isotropic.  I apoligize for any confusion.

Is the resistance to create a surface model, then analyze the surface model, because Inventor's FEA does not have that capability?

Is this an academic assignment of any type?  Not to be rude.  I only ask becaue I think a more suitable FEA package should be considered if this is a project for industry.  I use Inventor's FEA a little.  But only as a tool during initial design to get things close, which I think is what it is intended to be used for.  In my opinion, I don't think it's intended to be used a replacment for dedicated FEA software.

[Anonymous]

I am not too interested in modelling the failure itself, it is purely to demonstrate a localised increase in stress relative to size of defect. I can then just state the stress in MPa and that it has exceeded the Yield/UTS and would cause deformation/failure.

Would these results still be accurate, or once it exceeds the materials figures does the analysis begin to fail? If so can I select an extremely high strength material, as to obtain accurate figures based upon geometry? And then just compare these results to the data sheet of the actual material?

Edit: Apologies I have just understood what you mean. The impact damage as plastic deformation and thus increase in stress and the accompanying strain. Thinking now of my method of cutting a small section of material I assume it will create a rather different stress raiser to a true plastic deformation with no lack of material.