PSI - Issue 71

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

190

Creep tests were performed under constant load condition at 823 K and at 200 MPa – 260 MPa as per the ASTM standard E-139 and the test temperature was controlled within + 2 K.

2.2 Finite Element (FE) analysis coupled with CDM (Sinh creep damage model)

The governing equation used for calculating the evolution of creep strain rate and damage rate through FE- analysis for uniaxial creep data is given below, as per CDM-based Sinh creep-damage constitutive model (Stewart, 2013). The creep strain rate ( ̇ ) is given by: ̇ = ℎ ℎ ( ∕ ) ( 3∕2 ) (1) Where A, are material constants, and , ( ) are damage parameter and the secondary creep mechanism transition stress respectively. (2) Where B, , ( ) are the material constants. The damage variable’ ω’ varies from zero to unity. The material constants B, χ, q and ( ) are estimated based on experimental creep rupture life versus stress plot fitted with equation, = [ ℎ ℎ ( ) ] −1 . The details of evaluating the different parameters and their physical significance has been described in detail by Mohammad S Haque et al. (Haque and Stewart, 2019). The Sinh Hyperbolic model typically includes nonlinear terms requiring more complex regression techniques. Parameters may not generalize well across different heats or batches of IN-RAFM steel. Moreover, isotropic creep damage formulations were inadequate for capturing anisotropic microstructural features. In the present study, 2 axisymmetric analysis was carried out in FE analysis due to geometrical and loading symmetry using quadrilateral structured elements type of CAX4R employing ABAQUS finite element solver. Global element size of 0.1 mm was employed in FE analysis. The FE analysis was carried out under elastic-creep conditions with the creep rate equation of Sinh model [eq (1))] with ω=0. VUMAT subroutine was implemented in the ABAQUS finite element solver to calculate the strain evolution. The simulation geometry consists of a planar section with gauge length of 25 mm and gauge diameter of 2.5 mm as mentioned in Fig.2 along with the appropriate boundary conditions. The damage rate ( ̇ ) is given by: ̇ = [ 1− ( − ) ] ℎ ( ) ( )

Fig. 2. Geometry of plain specimen used in FE-analysis along with boundary conditions.

3. RESULTS AND DISCUSSION 3.1 Creep Deformation Behavior

Creep tests were conducted on the IN-RAFM steel at 823 K over a wide range of stresses. Fig.3 and Fig.4 show the variations of creep strain and strain rate with creep exposure time. Creep deformation curve comprised of small instantaneous strain on loading, a short primary region, and extended tertiary region.