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#101 - 06/07/00 04:40 AM cyclic pressure loading
jameshampson Offline

Registered: 06/06/00
Posts: 3
Loc: Stockton, Cleveland, England
Thank for the reply to my recent question, the info. was very useful.
I have read the copy of COADE mechanical engineering news and found an article on 'code requirements for fatigue loading'. This article refers to thermal cyclic loading. Do you have any info. on cyclic pressure loading?.
Also is it possible to recieve further copies of COADE mechanical engineering news?.
James Hampson

#102 - 06/08/00 06:11 AM Re: cyclic pressure loading
Ohliger Offline

Registered: 12/16/99
Posts: 246
Loc: Mannheim,Germany
You can use the equation for the pressure stress calculation and take pressure difference dP instead of pressure P.

#103 - 06/08/00 08:32 AM Re: cyclic pressure loading
Richard Ay Offline

Registered: 12/13/99
Posts: 6049
Loc: Houston, Texas, USA
Fatigue analysis is no different for cyclic pressure or thermal loadings. The example in the newsletter actually did include a pressure variation.

If you have two different pressures, you could set the load cases up as follows:

  • W+P1+T1+D1 (OPE)
  • W+P2+T1+D1 (OPE)
  • W+P1 (SUS)
  • W+P2 (SUS)
  • DS1-DS3 (EXP)
  • DS2-DS1 (FAT) (Cyclic pressure only)

Or (as has been mentioned above), you could specify your dP as P2 and set the load cases up as follows:

  • W+P1+T1+D1 (OPE)
  • W+P1+P2+T1+D1 (OPE)
  • W+P1 (SUS)
  • W+P1+P2 (SUS)
  • P2 (FAT) (Cyclic pressure only)
  • DS1-DS3 (EXP)

This second set of load cases should yield identical results to the first set, <font color="0000ff">assuming there are no expansion joints or bourdon effects in the model</font>. If so, use the first approach.

With regards to the newsletter, please e-mail me your current contact information. I will see that you get on the list (if you are not on it already).

Richard Ay (COADE, Inc.)
Richard Ay

#104 - 06/12/00 06:31 PM Re: cyclic pressure loading


Beam Element models (Such as Caesar II) are wonderful tools. However none the less pressure and pressure cycling will not have a large impact on properly supported beam element models results!

However, the previous statement not withsatnding pressure cycling may indeed cause COMPONENT fatiuge failures!

However note the key word (COMPONENT). The simplified rules of ASME b31.1 or B31.3 provide no specific directin on component (three dimensional) analysis or should I say "design by analysis (Ref ASME Sect VIII Div2)". The closest code that incorporates some of the concept of pressure within a beam element model is Section III NC.

So if you have a pressure cyclic problem where pressure is a large component of the loads look at more advanced methods of analysis.

Best Regards,

John C. Luf


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