Previous posts discuss the use of a 2x adjustment for evaluating the number of cycles for a stress intensity (3DMSI) amplitude calculated for as-welded pipe when comparing to the S-N curves of ASME B&PV Code, Sec. VIII, Div. 2, Appendix 5 which are based on polished bar specimens. This adjustment or factor of safety is based on the ratio of the Sec. III, Subs. NB indices to the B31 SIFs.

One source (Rodabaugh/Pickett, TID-25553) states that “In the failure cycle range of 1000 to 100,000 cycles, the pipe shows about the same fatigue strengths as the polished bars; i.e., the surface effect was negligible. At lower cycles, the pipe fatigue strength was higher than that of the polished base; probably because the pipe tests were deflection controlled whereas the polished bar tests were load controlled. At higher cycles, the pipe fatigue strength is lower than that of the polished bars; i.e., as normally the case, the surface finish is significant for a large number of cyles.”

1. It seems that if I am going to determine the number of cycles for a specific component, I could calculate the C2K2/i ratio to get a better estimate for the component than 2x, but in the low cycle range, the source above seems to indicate there is some additional fatigue strength and the C2K2/i ratio would be too high an adjustment…am I trying to split hairs where the 2x adjusted is generally accepted as good practice?

2. I am particularly interested in very low cycle fatigue, below 200 cycles. Would WRC 474 Master S-N Curve Method… be helpful to understand this better?

2. I am particularly interested in very low cycle fatigue, below 200 cycles. Would WRC 474 Master S-N Curve Method… be helpful to understand this better?

When you start looking at low N high S design I start to cringe because of the shotgun pattern assoiciated with the raw data. The WRC paper is a good read but its kind of like the Dirty Harry quote.... "Do you feel lucky punk?"!!!!

Thanks, John:

I understood the Markl data is very scattered at low cycle, particularly below 100 cycles. But I haven't seen raw data on the Appendix 5 curves (don't know where to access that), but I understand you to say that the fact that smooth polished bars were used for those tests, didn't improve the data at low cycle.

Yes still a lot of scatter.... you strt getting into terra incognito... I am never lucky therefore I tend to be "conservative" I guess....