PSI - Issue 71

K.M.K. Chowdary et al. / Procedia Structural Integrity 71 (2025) 188–195

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~823 K. The IN-RAFM steel is being considered for the Lead-Lithium Ceramic Breeder (LLCB) blanket application in the DEMO fusion reactor. RAFM steels typically belong to the family of 9–12% Cr ferritic martensitic steels, with modifications in alloying elements aimed at reducing the induced radioactivity after service life. For instance, elements such as Niobium (Nb) and Molybdenum (Mo) are used in place of Tantalum (Ta) and Tungsten (W) to lower the material's long-term radioactivity (Laha et al., 2013). In ferritic-martensitic steels, creep damage generally initiates from microstructural degradation, eventually leading to the nucleation and growth of cavities or cracks (Zhao et al., 2023). Therefore, it is crucial to investigate the creep behavior of IN-RAFM steel and model its minimum creep strain rate, deformation, damage progression, and rupture characteristics. Cauvin and Testa (Cauvin and R.B. Testa, 1999) categorized damage across three scales: micro-, meso-, and macro-scales. At the microscale, damage manifests as atomic voids, while at the macroscale it appears as visibly detectable cracks and cavities. The mesoscale serves as the foundational level in continuum mechanics, where a representative volume element averages the effects of microscale damage features such as voids and cracks. The concept of a homogeneous constitutive relationship describing damage evolution forms the basis of the continuum damage mechanics (CDM) framework (A. Cauvin and R.B. Testa, 1999). This framework was first introduced by Kachanov, who proposed a scalar phenomenological approach to damage evolution. Rabotnov later extended this work by formulating a damage evolution equation coupled with the power-law creep strain rate, giving rise to the Kachanov-Rabotnov (KR) creep-damage model (L. M. Kachanov, 1967; Rabotnov YN, 1969). The KR creep damage model can lead to numerical instability due to near-infinite damage rates (Wen et al., 2014; Y.Liu and S.Murakami, 1998). To address this issue, Stewart (Stewart, 2013) introduced a set of coupled creep damage equations incorporating a sine-hyperbolic (Sinh) function to enhance numerical stability and predictive capability. This paper presents the creep properties of IN-RAFM steel and a corresponding Finite Element (FE) analysis. The FE analysis is integrated with a CDM-based formulation using the Sinh-hyperbolic creep damage model to evaluate the creep deformation, damage accumulation, and rupture behavior of IN-RAFM steel over a range of stress levels at 823 K.

2. EXPERIMENTAL 2.1 Materials and Testing

The IN-RAFM steel plates (of 25 mm thick) were produced using vacuum induction melting and vacuum arc refining at M/s. Mishra Dhatu Nigam Limited (MIDHANI), Hyderabad, India by using virgin charge of very high purity. The chemical composition (in wt. %) of the steel is Fe-9.03Cr-0.126C-1.38W-0.24V-0.06Ta 0.56Mn. The plates were received in normalized (at 1253 K/30 min) and tempered (at 1033 K/90 min) condition. The creep specimens were extracted (in the rolling direction) from the plates. The fabricated creep specimens have 50 mm gauge length and 5 mm gauge diameter with overall specimen length of 120 mm as shown in Fig.1.

Fig. 1. Smooth specimen used for uniaxial creep test.