Evolution of damage under combined low and high cycle fatigue loading in a type 316LN stainless steel at different temperatures Aritra Sarkar a,⇑ , A. Nagesha a , P. Parameswaran b , R. Sandhya a , K. Laha a , M. Okazaki c a Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu 603102, India b Material Synthesis and Structural Characterization Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu 603102, India c Department of Mechanical Engineering, Nagaoka University of Technology, 9402188, Japan article info Article history: Received 20 March 2017 Received in revised form 16 May 2017 Accepted 17 May 2017 Available online 20 May 2017 Keywords: LCF HCF Creep DSA Ratcheting 316LN SS abstract The present study investigates the effect of different damage modes like low cycle fatigue (LCF), high cycle fatigue (HCF), creep and ratcheting during combined cycling at various temperatures ranging from ambient to 923 K, in a type 316LN austenitic stainless steel. The experiments were designed with multi- step load sequences where specific number of small amplitude HCF cycles (referred to as blocks) were introduced at the stabilized cyclic load under LCF for a given strain amplitude and repeated until failure. Cyclic life was found to decrease with increase in temperature as well as block-size. The decrease in cyclic life with block-size is more significant at 923 K where multiple damage modes like creep and ratcheting are activated. Dynamic strain aging (DSA) was found to operate in the temperature range, 823–873 K where the decrease in cyclic life with block-size gets saturated. Typically, transgranular fatigue fracture, intergranular creep fracture or dimpled rupture was identified when failure was dictated by LCF, creep and ratcheting respectively. However, synergistic interaction between the above damage modes leading to a mixed mode fracture carrying signatures of fatigue striations, intergranular facets and dimples occurred at specific combinations of block size and temperature. HCF damage played an important role for some specific loading conditions by acting as a link between intergranular (creep) cracks, thus facil- itating the crack propagation and final failure. The regimes of dominant failure modes and interactions among them were suitably mapped as a function of temperature and block size. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction Investigations pertaining to the fatigue behavior of structural materials are dedicated mostly to pure low cycle fatigue (LCF) or high cycle fatigue (HCF) loading, in spite of the fact that engineer- ing components experience varying load histories throughout their service life. This is a significant issue in sodium-cooled fast reactors (SFRs), where components of the primary sodium circuit are prone to damage induced by LCF as well as HCF which can lead to a sig- nificant reduction in service life of such components [1,2]. In SFRs, LCF damage arises due to temperature gradient induced thermal stresses which are cyclic in nature as a result of startups, shut- downs and transients [3,4]. Thermal striping, stratification and flow induced vibrations in the sodium free level in the main vessel of SFRs cause stress fluctuations leading to HCF damage during steady state operation [5]. The damage processes resulting from HCF get superimposed on the LCF damage arising out of thermal cycling, leading to strong LCF-HCF interactions [1,2]. Prior crack initiated under LCF strain cycling can advance further during the superimposed HCF cycles even at very low stresses, thus reducing the lifetime of the components significantly [6]. This calls for a thorough investigation into the potentially deleterious synergistic interactions between LCF and HCF. It is more important to carry out such investigations over a wide range of temperature with par- ticular emphasis in the range, 823–923 K that encompasses the operating temperature of SFRs. Similar investigations using both sequential (prior LCF exposure under strain control for specific life fractions followed by HCF cycling under stress control) and block loading pattern (separate loading blocks consisting of LCF as well as HCF cycles) have been carried out earlier by the authors at 923 K in a type 316LN austenitic stainless steel, which is the main structural material for the in-vessel components of SFRs [7–9]. The investigations showed that prior LCF cycling significantly curtails the HCF life under sequential loading, depending on the LCF strain amplitude and the degree of prior exposure to LCF (fraction of LCF life) [8,9]. The above investigations yielded a comprehensive understanding about the interaction effects, resulting in http://dx.doi.org/10.1016/j.ijfatigue.2017.05.012 0142-1123/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: aritra@igcar.gov.in (A. Sarkar). International Journal of Fatigue 103 (2017) 28–38 Contents lists available at ScienceDirect International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue