Progress of our comprehensive experimental research on fatigue performance of niobium stabilized (type 347) steel X6CrNiNb1810mod has been regularly reported since PVP2009. In addition to the revision of design curves in the German KTA Program of Standards, this work has contributed to fundamental understanding of cyclic performance of austenitic stainless steels. The quantitative results apply for type 347 steel, which is generally applied as primary piping in the German NPP’s, but the studied phenomena seem to be generic for most austenitic steel grades.

Strain controlled LCF tests at room and elevated temperatures were continued up to 10 million cycles resulting to run-outs beyond one million cycles. The results reveal particular features in cyclic response of austenitic steels, which are able to withstand notable amounts of reversible plastic strain in HCF regime. Combined with secondary cyclic hardening, the reversibility of inelastic strain retards initiation and/or growth of micro-cracks. Fracture of a long running specimen is effectively prevented and the strain life curve turns abruptly horizontal at the endurance limit. The endurance limit depends on temperature, but the knee point is constantly reached by about one million cycles, i.e. within the range of the original reference and design curves for the 1963 issue of ASME III. In other words, extrapolation of the regression models used by Langer (for ASME III) and Chopra et al (for NRC) beyond million cycles deviate from the laboratory data and underestimate the endurance limit.

Furthermore, fatigue tests interrupted by holds aiming to simulate PWR normal operation between the fatigue relevant transients have shown that the endurance limit increases in correlation with time and temperature. This temperature effect counteracts with the effect of straining temperature. Re-testing of run-out specimens and variable amplitude straining in HCF regime confirmed that cycles below the endurance limit did not contribute to cumulative fatigue usage. Rather the opposite, the sub-limit cycles either extended the fatigue life or increased the endurance limit applicable for these non-standard laboratory tests.

All the above suggests that austenitic stainless steels have an ‘enforced endurance limit’ tendency, which bends the reference curve horizontal, increases the endurance limit and reduces the scatter in grade and temperature specific HCF data. The modified Miner rule with S-N –curves extrapolated to low amplitudes is generally applicable for carbon steels, but not for stainless. From a ‘design by analysis’ point of view, this makes the current ASME III stainless steel design curve questionable. In particular, the extrapolation of the curve beyond a million cycles may be overly conservative. Furthermore, if the reference curve were based on relevant test materials and test methods, the experimental scatter would approach zero (in direction of strain amplitude or stress intensity).

This paper reports experimental results and analyses, which justify reconsideration of the reference curve and vertical transferability margin in HCF regime.


Fatigue Performance of Austenitic Stainless Steel - Enforced Endurance Limit and Questionable Design Curve

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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