PSI - Issue 52

Valery Shlyannikov et al. / Procedia Structural Integrity 52 (2024) 214–223 V.Shlyannikov,A.Sulamanidze,D.Kosov/ Structural Integrity Procedia 00 (2023) 000 – 000

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Table 2. Mechanical properties for nickel- based alloy ХН73М. Temperature [°C] E [MPa] σ 0 [MPa]

G [MPa]

n

α th [1/°C] 1.14*10 -5 1.36*10 -5 1.46*10 -5 1.57*10 -5 1.62*10 -5

23

215775 203780 206180 189236 188098

904.04 829.95 836.42 868.10 896.58

89.99 78.37 79.30 72.78 72.34

0.244 0.222 0.202 0.135 0.087

400 550 650 700

In Table 2: E is Young's modulus, σ 0 is yield stress, G is tangential modulus, n is strain hardening exponent, α th is thermal expansion coefficient.

3. Crack tip fields and stress intensity factors The purpose of the present study is to assess the fatigue crack growth rate when the SENT specimen subjected to isothermal and thermo-mechanical loading. During these tests the crack length as a function of number of cycles was determined using the optical method in accordance with conventional investigations involving the same or similar specimen geometry. It should be noted that the differences in the applied nominal stress level, types of mechanical loading (pure fatigue, creep-fatigue interaction, thermo mechanical fatigue) as well as temperature variations (isothermal and non-isothermal) in the implemented test program predetermined the necessity to calculate the stress intensity factors for each tested specimen. To this end was employed a numerical method based on the relationship between the crack length and the crack tip stress field in the considered specimen configuration that are acquired from finite element simulations. In order to simulate the conditions of isothermal and thermo-mechanical loading conditions in accordance with the tests performed on SENT samples composed of the heat-resistant nickel-based superalloy XH73M, FE computations were performed for pure fatigue, creep-fatigue interaction and both the IP and OOP TMF cycles, to determine the local thermo-mechanical stress – strain rate and displacement fields. The conventional elastic crack tip fields for pure fatigue and creep-fatigue interaction under the isothermal loading conditions were determined by setting discrete values of the modulus of elasticity as a function of test temperature 23°C, 400°C and 650°C. To study the effects of the TMF conditions induced by heating and cooling cycles during mechanical loading on the stress – strain state of the ХН73М SENT sp ecimen fully coupled thermal-stress analyses were performed using the commercial FE software ANSYS 2021R1 (2021). The entire simulation process was divided into three steps. First, basic electromagnetic parameters and a temperature field were applied to the specimen. Second, velocity-inlet and pressure-outlet boundary conditions, as well as symmetry condition were set for the nozzles and fluid domain boundary to calculate the temperature field of the specimen, taking into account heat losses and convective heat transfer processes. Third, the elastic stress changes during a full IP or OOP TMF cycle were calculated through transient structural analyses. The electromagnetic and heat transfer characteristics of a fluid medium and the mechanical properties of the nickel- based alloy ХН73М were represented via a fitting procedure using piecewise linear approximation functions for the considered temperature range. 3.1. Crack tip stress distributions ANSYS FE code was used to generate the temperature field in the test specimen at the characteristic points of each cycle, such as the start and intermediate end sequences. Subsequently, a structural analysis was conducted using the elastic material constitutive model. To verify the sensitivity of the simulation results to the mesh density, normal stress fields of the isothermal and non-isothermal thermo-cycles were computed using different mesh sizes, ranging from 0.0005 m to 3.33·10 -6 m. The Maxwell3D module model contained 49367 tetrahedral elements; there were five mesh layers inside the skin layer, and the thickness of the first layer of elements, from the sample surface, was 0.05 mm. For the CFD analyses, the near-wall mesh in the specimen volume had a first cell height of 0.005 mm and 20 layers over a total boundary layer height of 1 mm. There were 1634717 solid high-order elements, namely Hex20, Tet10, and Wedge15. The transient structural module model of the SENT specimen contained 132646 20-