.......and it is important not to confuse ASME Section III, class 1, "type" stresses with B31 "type" stresses. Note that the Section III multipliers are nearly twice those of B31.

As John Luf pointed out it is useful to go back and touch base with the original ARC Markl papers on occasion (e.g., ASME Paper 53-A-51, "Piping-Flexibility Analysis" by A.R.C. Markl). The Tube Turns team did not really measure stresses; they back calculated them (they had a known displacement, a known "lever arm" and a known number of cycles to the "failure"). Our colleague, Ron Haupt, has a nice summary here:

http://www.sstusa.com/00julsep.htm#sep00

and here:

http://www.ppea.net/Articles/Comments%20on%20allowable%20stresses%20_Jun-04_.pdf

And John Luf's illustrated paper can be found here (and other places):

http://www.coade.com/newsletters/jun00.pdf

There are some interesting things to study in the Markl papers. The data for the several types of tested piping components were plotted on S/N plots and the data was "statistically smoothed" such that it resulted in a curve that was the best fit of the data. There is a fair amount of "off-the curve scatter" on those plots. Pipe elbows were rather straight forward but branch connections required some creative rationalization (the team decided that branch connections were to a degree analogous to bends/elbows). It is also interesting to see that although testing shows a direct connection between SIF's and component flexibility, the Codes use a flexibility factor of 1.0 at branch connections (a little conservatism is not a bad thing). Ev Rodabaugh (who was on the Markl team) has subsequently developed WRC Bulletin 329 "Accuracy of Stress Intensification Factors for Branch Connections" which is also a "must read". Another "must read" from WRC is WRC Bulletin 392, "Standardized Method for Developing Stress Intensification Factors for Piping Components” by Ev Rodabaugh and Glynn Woods. There also have been many (maybe 8 - 10) ASME papers on the subject presented at ASME PVP Conferences through the years.

Of course if developing Stress Intensification Factors by the FEA method, you are calculating stresses and to get the SIF you must do the stress ratio comparison as describe (in many fewer words) above by Chuck.


Regards, John.