Dorin,

You are 100 % correct. It is amazing that this also happens to be the topic of an article in COADE's July 2000 newsletter (about to be published) - "WRC 107: Elastic Analysis v/s Fatigue Analysis". Main points are listed here,

WRC 107 module can be used to perform elastic and/or Fatigue analysis. Users should decide if they require Fatigue analysis or if elastic analysis will suffice (using the guidelines of section AD-160 Section VIII Div. 2). If only elastic analysis is required then:

• Set up different types of load cases (Sustained, Expansion and Occasional).
• Include Pressure Thrust if needed.
• Do not include Pressure Stress Indices (they intensify the internal pressure stress, used for computing peak stress due to internal pressure).
• Do not include stress concentration factors Kn and Kb (this is done by omitting the fillet radius between the vessel and nozzle).
• Use stress summation to compare the actual stress to the allowable stress.

If fatigue analysis is required then an elastic analysis is still required. Additionally, the following procedures should be followed:

• Set up the range pair and load cycles (e.g. - Installed to operating, pressure fluctuations) for the fatigue loading.
• Evaluate each load range using the WRC 107 module one by one. Enter each cyclic load as a sustained load and leave the other types of loads as blank.
• Include Pressure Thrust if needed.
• Include stress concentration factors Kn and Kb, by entering the fillet radius between the vessel and nozzle. This will gives peak stress due to applied loads.
• The easiest way to combine the stresses due to external loads with the ones due to internal pressure is to include the Pressure Stress Indices and perform stress summations. Use the results for Stress Intensity (Pm + Pl + Q + F) (though in the current version it reads as Pm + Pl + Q, but will be corrected in future) and ignore the results for first two equations (Pm and Pm+Pl) and ignore the comparison to the allowable stresses.
• Then take that stress intensity (thermal or pressure cycles result in the range of stress) and refer to the rules of Appendix 5 in Section VIII Div. 2 for performing a fatigue analysis.

This topic is discussed in more detail in the WRC 107 article appearing in the next newsletter. This edition of the newsletter will also be available next month on our website, www.coade.com/newsletters/jun00.pdf (Note: this has not been posted at this time).

Your suggestion about combining the intensified stresses from the internal pressure under the category Q loads is very interesting, we will investigate it further.

CAESAR II is a tool, which can be used incorrectly. In areas where this possibility is identified, we make every effort to document the proper procedure, or modify the software. The WRC107 analysis is one area where future refinement is planned.

Regards,
Mandeep Singh
COADE, Inc.



[This message has been edited by rich_ay (edited July 28, 2000).]