Issue 50

A. Sarkar et alii, Frattura ed Integrità Strutturale, 50 (2019) 86-97; DOI: 10.3221/IGF-ESIS.50.09 87 service life of such components [1-2]. Prediction of fatigue life under LCF-HCF interaction is thus highly essential to ensure the integrity of the components. In this regard, 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 [3-4]. LDR is further refined by modeling attempts, leading to “Damage Curve Approach” (DCA) [5]. However, a major limitation of DCA is its semi-empirical nature which does not account for some intrinsic factors like crack length. Such disadvantages will become more prominent at extreme conditions such as elevated temperature LCF- HCF interactions. This necessitates the development of alternate life-prediction models based on crack propagation with periodic measurement of crack length. In view of this, the block-loading experiments including combinations of both LCF and HCF are specially designed so that crack-growth based life-prediction models can be developed based on the same. E XPERIMENTAL METHODOLOGY ylindrical fatigue specimens with a semi-circular initial surface notch of 100 µm diameter and 50 µm depth (Fig. 1) were chosen to study fatigue crack growth behavior under block-loading (as per Fig. 2) at 573 K. Cumulative fatigue damage in terms of LCF-HCF interaction occurs in sodium-cooled fast reactor (SFRs). A closer simulation of the actual reactor conditions can be realized through a loading pattern involving repeated blocks consisting of LCF as well as HCF cycles. Since LCF stress/strain for every start-up and shut-down operation actually results in one cycle and HCF stress/strain are caused by in-service vibrations, the loading pattern in a block shown in Fig. 2 consists of one fully reversed LCF cycle followed by a specific number of HCF cycles superimposed on the LCF cycle which introduces mean stress/strain during HCF cycling. Short fatigue crack growth rate was reported to increase with increase in notch-tip radius [6]. Similar effect was found to follow in case of notch-sensitivity [7]. In the present investigation, the main reason for using a small surface notch was to enable LCF-HCF interaction under block-loading at higher temperatures avoiding extensive intergranularity or tendency towards rupture which is accounted for by creep, ratcheting and their interactions with LCF or HCF. The block-loading experiments are hence redesigned to ensure that specimen fails through initiation and propagation of cracks rather than rupture, so that crack based life-prediction models can be applied to them. The combined cycling experiments on notched specimens were designed in two steps on similar lines as carried out on smooth specimens in earlier investigations [8]. In Step-I, the specimens were initially subjected to strain controlled LCF loading up to a specific number of cycles corresponding to stabilization of the cyclic stress response (CSR). However, unlike smooth specimen testing, in the present case, the LCF cycling in Step-II was carried out under strain control (  t /2 LCF : ±0.6%) and not under stress-control to prevent tendencies of rupture through strain accumulation induced by creep and creep assisted ratcheting. HCF cycles of three different B s (1, 10 and 200) with strain amplitude (  t /2 HCF ) of ±0.1% (~ σ HCF of 150 MPa, using elastic modulus) were introduced at the maximum LCF strain (Fig. 2), once the CSR reaches stabilization. B s indicates the ratio of HCF cycles to LCF cycles in a particular block. The notch was considered as the initial crack and propagation of the crack from the notch was studied by taking replicas of the specimen surface after interruption at regular intervals, particularly in the short crack growth domain. In the latter stages of crack growth, travelling microscope was used to measure the crack length. In the present study carried out under strain-controlled mode, significant mean strain will act on the specimen which will lead to plastic ratcheting ahead of the crack tip. This phenomenon was considered for developing the life-prediction model. The model was further refined by carrying out block-loading experiments at higher temperatures, in the range from 573 to 923 K at a high B s of 5000. Figure 1 : Geometry of the specimens used for crack growth experiment C