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# Learning pressure vessel DBA using ANSYS MECHANICAL

## Learning pressure vessel DBA using ANSYS MECHANICAL

(OP)
Dear all,

I am new in the forum, very interesting place I have found :)

I am learning how to do a DBA using ASME BVPC Sect.VIII code. I created my own vessel to work with, and after a few analyses I have a couple of questions:

1. I realised that at least 4 failure modes are identified by the code. Based on screening criteria, my vessel dimensions and characteristics excluded it for a fatigue calculation. In any case, ratchening is mandatory. So I carried out a static analysis in ANSYS Mechanical, with a load-unload condition (t=1-Pressure=0, t=2-Pressure=Pmax, t=3-Pressure=0, t=4-Pressure=Pmax....and so on). I have very low plastic strain (so I guess this criteria is not fulfiled according with that is said in the code). A chart of max.stress, max.deformation, volume....exhibits a constant trend after a couple of cycles(I mean, for example, t=2-max.stress=1e+08, t=4-max.stress=1,001e+08....t=10-max.stress=1,01e+08...). Is this enough to guarantee the bessel is safe against ratchening? If not, what criteria should I follow for determine properties are not varying with cycles? Sorry if I am not explaining this well.

2. I used a static structural analysis as a pre-stress condition for Eigenvalue Buckling, in order to determinate the load multipliers. I found all of them with negative values. I think I undersatnd the physical meaning, but, how is this translatred to the norm criteria? Should I choose absolute values? Or should I deactivate the "Include negative multiplier" option in Anaysis Settings? I choose 4 modes to find, is this enough? Is there any rule of thumb to guess this number of modes? I guess the deformation contour plot has no physical meaning, is that correct?

Many thanks in advance and I apologise if my english is not good enough...I am glad to become a aparto of this interesting community!

Kind regards,

Charas

### RE: Learning pressure vessel DBA using ANSYS MECHANICAL

Charas - for me, anyways, we are getting into the realm of going beyond what can be provided, for free, over the anonymous internet.

Do you have mentors? Do you have the ability to take a training course on this topic?

### RE: Learning pressure vessel DBA using ANSYS MECHANICAL

(OP)
Thanks for comment.

Unfortunately, no, I don't (at least at the moment)

If you have a recommendation for training course, please let me know. I will save it for the future.

### RE: Learning pressure vessel DBA using ANSYS MECHANICAL

Any of the top four options when you do a google search for "asme section viii division 2 part 5 training" would be good choices.

Regarding ratcheting - remember that it is a failure mechanism about dimensions/displacements.

Regarding buckling - take a look through these forums for discussions on buckling. Regardless - you need the positive eigenvalues to get the mode-shapes. I, personally, would never use an eigenvalue buckling analysis solely, because I have been burned too many times. Most pressure vessel buckling failures are elastic-plastic buckling, which cannot be detected by elastic-buckling alone.

### RE: Learning pressure vessel DBA using ANSYS MECHANICAL

(OP)
Thanks you Mr.Seipp for your kind suggestions to follow your courses. I will consider them for the near future (I cannot pay 2000 dollars now).

And also thank for your comments. I thought my questions were direct and clear (some of them could be answered with "yes" or "no"), anyway I will try to make them more clear the next time :)

I have a couple of papers that can help to infer the answer to my questions. If anybody is interested, please let me know and I will include here the links.

Regards

Charas

### RE: Learning pressure vessel DBA using ANSYS MECHANICAL

I would be thankful if you will be able to share what you have found.

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