Investigation of Cumulative Fatigue Damage Through Sequential Low Cycle Fatigue and High Cycle Fatigue Cycling at High T
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MOST of the current investigations pertaining to the fatigue behavior of structural materials are dedicated to either low cycle fatigue (LCF) or high cycle fatigue (HCF) loading even though it is a well-known fact that engineering components experience a varying load history throughout their service life. To secure long-term reliability in engineering components, effect of random fatigue and multi-level loading on mechanical behavior is fast becoming an important field of research. Currently, this is an important issue in sodium-cooled fast reactors (SFR) where components of the primary sodium circuit are prone to damage induced by LCF as well as HCF which can lead to a significant reduction in service life of such components.[1,2] In SFRs, LCF is caused by sharp thermal gradients from the surface to the core during reactor start-up, shut down, and power ARITRA SARKAR, A. NAGESHA, R. SANDHYA, and K. LAHA are with the Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu 603102, India. Contact e-mail: [email protected] P. PARAMESWARAN is with the Material Synthesis and Structural Characterization Division, Indira Gandhi Centre for Atomic Research. M. OKAZAKI is with the Department of Mechanical Engineering, Nagaoka University of Technology, Nagaoka 940-2188 Japan. Manuscript submitted May 17, 2016. Article published online January 4, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
transients.[3,4] Thermal striping, stratification and oscillations in the sodium-free level in the main vessel of SFRs cause stress fluctuations leading to HCF damage during steady-state condition.[5] The damage processes resulting from HCF get superimposed with the LCF damage arising out of thermal cycling, leading to strong LCF–HCF interactions[1,2] which call for a thorough investigation into the associated mechanism for better structural design of SFR components. Towards this objective, a preliminary investigation was initiated by the authors on high-temperature LCF–HCF interaction in a type 316LN austenitic stainless steel (the main structural material for the in-vessel components of SFRs)[6] which is further advanced in the present paper through a much detailed and in-depth study. The investigation was carried out at 923 K (650 C) which encompasses the operating temperature as well as that of power transients in SFRs. Prediction of fatigue life under such complex loadings to ensure the integrity of the components is also another key issue. The well-accepted Miner’s linear damage summation rule (LDR) poses serious non-conservatism in terms of huge deviations from linearity due to the large difference in lives between LCF and HCF.[7–10] This drawback can be resolved by the use of suitable non-linear cumulative fatigue damage models which can cater to such extreme conditions. Over the last few decades, a number of non-linear damage models VOLUME 48A, MARCH 2017—953
have evolved,[11–15] but have not been used extensively for structural design or life prediction due to the requirement of a large n
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