PSI - Issue 71

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

192

Fig.4 . Creep strain and strain rate curves for IN-RAFM steel at 200-220 MPa and 823 K.

3.2 Creep rupture and damage The variation of creep rupture life with applied stress of IN-RAFM steel at 823 K is shown in Fig. 6. It follows the equation in the form of = [ ℎ ℎ ( ) ] −1 where B, χ, ( ) are material constants which must be greater than zero. The CDM based damage variable ( ) serves three functions: modelling the tertiary creep regime, tracking the evolution of creep damage and predicting the rupture. To achieve these goals, the damage variable, is coupled to the function [h(ω)= exp(λω 3/2 )]; where = ( ̇ | ̇ ) was incorporated into the creep strain rate equation to define the creep strain rate [equation (1)] of the Sinh model. The material constant controls the sigmoidal bend in creep curves at secondary to tertiary transition at all the stress levels. The elastic, creep and damage constants of Sinh model are given in Table.1 . The material constant q in equation (2) was calibrated by comparing the CDM-coupled FE analysis results with experimental creep strain data. The value of q at each stress level is therefore based on minimum deviation between the simulated and experimental creep strain versus time data. The material constant q controls the damage trajectory. The evaluated material constant, q value is at lower side for higher stress levels and vice versa. The range of estimated q values is 4-8 for the investigated stress levels.

Table.1 The values of the constants in Sinh model for the IN-RAFM steel creep tested at 823 K.

Young’s Modulus, E(MPa) 160,000

Poisson’s ratio, υ

A (h -1 )

σ

B (h -1 )

χ

σ

q

λ

s (MPa)

t (MPa)

0.3

4.83×10 -14

10.65

0.01357 19.82 257.07

4-8

3-6

The material constant, λ, is determined directly from the experimental data. The constant λ is the natural logarithm of the quotient of final and minimum creep strain rate. As the creep strain rate is dependent on stress and temperature, a decrease in str ess therefore increases the value of the constant λ.