PSI - Issue 71

P.K. Sharma et al. / Procedia Structural Integrity 71 (2025) 66–73

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develop in these localized areas. Although these components are typically designed using uniaxial creep data, a more realistic evaluation of their service life requires determining creep deformation and rupture life under the multiaxial state of stresses (Ando et al. (2014)). To simulate multiaxial stress states, notched specimens are commonly used. These geometries introduce varying stress gradients around the notch root, resulting in complex multiaxial stress states that significantly influence creep deformation behavior and rupture life (Yu et al. (2009); Gong et al. (2021)). The representative stress in notched specimen can be evaluated through various techniques using the relative contribution of different stress components i.e., hydrostatic stress (govern the cavity growth), maximum principal stress (governs diffusion controlled intergranular cavity growth (Nix et al. (1989)) and von-Mises stress (governs creep cavity nucleation and subsequent deformation). (Hayhurst et al. (1977)) utilized skeletal point stress to predict rupture life of the notched components. During stress redistribution under creep, there is a point at the minimum cross-section of notched specimen where the stress remains nearly constant for different hardening exponents. This point is termed as skeleton point stresses. Skeletal point stress has been employed to describe the material deformation and failure behavior under multiaxial creep conditions. This model is particularly effective for notched round bar specimens, as the skeletal point can be identified along the radial direction. However, when the specimen's cross-section is rectangular or irregular, as in real components, locating the skeletal point becomes more challenging (Ji et al. (2022) ). Hayhurst et al. (1977) defines the representative stress as the algebraic sum of the maximum principal stress and the von-Mises stress while, (Cane (1979)) defines the representative stress as the product of the maximum principal stress and the von-Mises stress, incorporating a material-dependent parameter that influences how each stress contributes. Based on these different methods, an equivalent formula was proposed by (Zhang et al. (2024)) for circumferentially notched tension specimens that is capable of generating equivalent predictions of multiaxial rupture life when the skeletal point stresses are available. Nomenclature A Norton’s equation constant σ m Hydrostatic stress ̇ Steady state creep strain rate σ r Representative stress n Hardening exponent Q Activation energy η c Stress triaxiality at central plane of notch R Notch root radius η e Stress triaxiality at ends of notch T Temperature σ e von-Mises equivalent stress W Specimen width The notch weakening or the strengthening phenomena may occur when comparing the creep rupture behavior of smooth and notched specimen at the same net section stress (stresses at the minimum cross-section of notched specimen). A decrease in net section stress may cause the material to change from notch strengthening to weakening as demonstrated on Nimonic 80A material by (Eggler et al. (1993)). The notch strengthening or weakening also depends on the notch shape, acuity ratio and the material ductility. In ductile material, the stress redistributes quickly below the applied stresses causing the notch strengthening however, notch weakening may occur for brittle material where crack may generate due to high stresses near the notch tip. This behavior is studied extensively in various material like Nimonic 80A (Dyson et al. (1981)), 2.25%Cr-1%Mo steel (Al-Faddagh (1984)), 0.5Cr-0.5Mo-0.25V steel (Ng et al. (1980)), P91 steel (Eggeler et al. (1991)) etc. The effect of U-notch angle on creep deformation and rupture behavior of 304HCu SS was studied by (Sahoo et al. (2024)). The material should exhibit notch strengthening effect in all the service conditions that should be ensured during design and manufacturing of the components. Hence, the investigations of the effects of notches on creep deformation behavior of the material are very important. In this work, a systematic investigation has been carried out to evaluate the effect of multiaxial stresses on creep deformation behavior of U- type notched specimen in the temperature range of 800 ˚C to 1000 ˚C. The notch root radius has been varied from 0.5 mm to 2 mm that changes the value of stress triaxiality. Variation of the creep deformation and rupture with stress triaxiality have been determined. 2. Experimental details Creep tests were carried out in the temperature range of 800 ˚C to 1000 ˚C for both smooth and notched specimens. The design and fabrication of specimens, test matrix and experimental procedures to conduct creep tests under various stress triaxiality conditions are discussed in subsequent sections.