Pipe or thin walled vessel collapse calculations

[beta69]

I am interested in know what calculations are used to calculate the collapse stress of cylindrical thin walled tubes. E-funda indicated the hoop stress calculation and that it would yield a negative stress if external pressures are applied, but I am not sure this is correct. Does anyone have a different calculation?

 

Hello,

Stress (per se) is not THE determining failure criteria for negative (external) pressure, buckling is the problem. This is a structural instability issue. The reference that you mention regarding hoop stress is VERY wrong.

The piping B31 Codes refer you to the ASME B&PV Code, paragraph UG-28 et. al. You will see rules in these paragraphs with table (the Code also gives you the data in the curves as a set of tables. You will notice that the B&PV Code rules are worked out so that you do not calculate stresses.

Best regards, John.

 

[beta69]

I understand what you are saying, however I do not have access to the ASME references that you specified. Are you referring to the formula ts=d((Pc*(1-ucul))/2Ec)^1/3 for pipe collapse under external pressure? I am actually an engineer that works with medical devices and I am trying to understand the collapse of tubing from the pressurization of a balloon attached to the tubing.

 

[JohnBreen]

Hi,

Please give me an e-mail address. I may have something to help you.

Regards, John

 

[beta69]

My e-mail address is dder@guidant.com. There is another formula by Timoshenko that is being looked at as well. I understand that these are the formulas for calculating the force at which buckling occurs, but isn't hoop stress still a fairly valid indicator as well? For instance, if a pipe collapses at a know pressure during testing and the wall thickness and diameter are known, then isn't the applied stress at failure still an indicator of the pipe strentgh? Thanks for taking the time to reply to this.

 

[JohnBreen]

Hello,

I tried the e-mail address twice and my message bounced both times. Can you give me a different e-mail address?

Regards, John.

 

[beta69]

John,

Actually, I recieved both of your e-mails. I'll take a look at the spreadsheet and let you know how it works. Thanks again.

Daniel Der

 

[poetix99]

The mechanics of compliant tubing has been analyzed as related to the flow of fluids within such tubes subjected to external pressures which cause the tubes to collapse. (Blood vessels under a sphygnomanometer cuff are one example.)

The references that you have been given above might be fine, especially if your tubing is relatively stiff. If it is instead rather flexible and/or you are not actually so interested in the onset of collapse as you are interested in the obstruction of flow that is associated with the nearly complete collapse, then I suggest you read on:

Check some fluid mechanics and biomedical engineering journals. I know that Ascher Shapiro (emeritus professor - MIT) and some of his colleagues and students (S. Lichter is now both at Northwestern U.) published many papers on this subject beginning about 25 year ago (late 70's/ early 80's).

 

[beta69]

Thanks for the information. The actual application has to do with the locking of a catheter around a guidewire that is run through the internal lumen of a flexible tube. When a balloon catheter is inflated to pressures of 300 psi, the inner tubing tends to collapse around the guidewire and prevents removal of the catheter without removing the guidewire along with it. This is complicated further by the fact that the tubing is in a tortuous anatomy when it is inflated which adds to the collapse. Even though the tubing is produced with an elastomer, the collapse is at times non-reversing. I will look at some of the sources you sited.