Fatigue of NPP components simulated by non-uniformly strained stainless steel specimens

Comprehensive experimental research on fatigue performance of niobium stabilized (type 347) steel has revealed beneficial effects of hot holds aimed to simulate normal operation of NPP between the fatigue relevant transients. Reduction of plastic strain, extension of life and increase of endurance limit has been demonstrated in strain controlled HCF tests. Our latest results indicate moderate, but still measurable 'hold effects' even without any stop of straining and loading, when blocks of low rate cycles are applied between normal frequency straining at constant 325°C. A new lab testing approach was developed to simulate the 'component behavior' in moderate strain concentrations within the NPP primary circuit. Strain concentrations in range of 1.5 = K_e = 2 are simulated through displacement controlled straining of standard and modified geometry LCF specimens. New results confirm the earlier results and introduce another consequence of holds. Cyclic softening promotes localization of strain, but hold hardening reverses this trend. The holds retard strain localization not only within the material microstructure, but also in geometric strain concentrations. We conclude that the geometric delocalization of strain can amplify beneficial hold effects for components. The local strains may reduce below the endurance limit resulting to run-out tests beyond millions of cycles, even though notable values of fatigue usage (CUF) had been accumulated during earlier phases of the tests. Applicability of the transferability factor introduced in 2013 to the German KTA standard No. 3201.2 is supported. Exact quantification of the factor is not easy, but in all considered cases Fhold = 1. This means that the fatigue usages are overestimated without this factor.