Understanding racheting term (SL)

Hi dave/richard

Can u pls elaborate the below phrases that i copied in one of the discussion regarding App-P specifically ratcheting term.

(What we have always been doing in CAESAR II with the typical "L1-L2 (EXP)" is just a type of range calculation. Before Appx. P, we assumed the highest SL in setting the (EXP) limit in (1b). That ratcheting check in (1b) (the "-SL" term) is replaced by the (OPE) check found in Appx. P's (P1a).)

thanks

Yes, Appendix P removes the issue of what SL to use in setting (1b).
Yielding caused by strain can be accommodated by the piping system. In fact, the calculated elastic stress range (from, say, cold to hot) can be as large as two times yield to avoid low cycle fatigue (the 1.25*(Sc+Sh) in B31.3). High cycle fatigue is considered by the cyclic factor f in equations (1a) & (1b). The problem is, the existing state of stress due to force-based loads (gravity & pressure) reduces the amount of strain you can apply before yield occurs. This is that ratcheting term. Essentially, a high sustained load will reduce the available range for safe shakedown. This is the "-SL" term in (1b).
A simple way to look at it is to take the "-SL" term to the other side of the evaluation: SE<SA using the SA of (1b) and you get SE+SL<1.5(Sc+Sh). Appendix P goes on to say that SE+SL is approximated by the state of stress in operation.
The proper application of Appendix P requires more work than the base Code. The base Code checks SL<Sh and SE<SA while Appendix P replaces SE<SA with SE<1.25f((Sc+Sh) and Sope<1.5(Sc+Sh), but you must still check SL<Sh.
Appendix P will not appear in B31.3 - 2014.

Dear Dave,

Thanks for your reply.

However, need further clarity on your below quotes

Quote:

1.The problem is, the existing state of stress due to force-based loads (gravity & pressure) reduces the amount of strain you can apply before yield occurs. This is that ratcheting term. Essentially, a high sustained load will reduce the available range for safe shakedown.Why?

2.Appendix P will not appear in B31.3 - 2014.Why?

Unquote

1 - Try Googling "Fatigue Ratchet Definition". You will find many interesting approaches to the subject. "Bree diagram" would also be a path to take. I find this reference interesting with it's example: http://rickbradford.co.uk/T73S04TutorialNotes38A.pdf.
2 - Appendix P was misunderstood as an "alternative method". It could also be not conservative. S(ope) used in Appendix P cannot always replace SE+SL. (As I said above: Appendix P goes on to say that SE+SL is approximated by the state of stress in operation.) A positive moment in SE combined with a negative moment in SL will give a lower moment producing S(ope) so the sum of the expansion and sustained stresses (unsigned stresses) may be larger than the stress from the sum of the moments (the operating stress state).

Dave,

"but you must still check SL