PSI - Issue 71

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

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across different temperature and stress conditions. The desired data (displacement, creep strain, time etc.) were saved and further post-processing of the data was carried out. The specimen before and after tests are shown in Fig. 2(b) (smooth specimen) and Fig.2(c) (notched specimen). 3. Creep deformation behavior obtained for different specimens The creep tests were carried out on smooth and notched specimen as per the test matrix shown in section 2.2. The creep strain and displacement data with time is plotted and the results for smooth and notched specimen here. 1.4. Smooth specimen The comparison of creep strain curve at different temperature and stress levels is shown in Fig. 3(a). It was observed that negligible primary creep occurs for all the specimens. Alloy 690 material has finely dispersed carbides that hinder dislocation movement preventing rapid initial strain. It has a high strain hardening rate that limits further deformation reducing the initial creep rate. The material also transitions swiftly from primary to secondary creep reflecting its design for excellent creep resistance. The steady state creep rate increases with an increase in stress levels. The material stays in secondary region for most of the deformation region. There is a large increase in creep strain with increase in temperature. At 40 MPa stress level, the specimen has less than 5% strain at 800 °C however, the specimen fails within 6 hrs with the increase in temperature to 900 °C. Steady state creep rate was obtained for all temperature and stress levels as shown in Fig. 3(b). Using this data, the parameters of Norton’s equation (Eq.(1)) i.e., hardening exponent (n) and activation energy (Q) were evaluated. ̇ = . . − (1) Where, ̇ is the minimum creep rate, A is the Norton’s constant, T is the test temperature.

(a)

(b)

(c)

Fig. 2: (a) Machine used for the tests; (b) Typical smooth specimen before and after test (c) Typical notched specimen before and after test.

The values of hardening exponent (n) were evaluated by taking the logarithm of Eq. (1) at constant temperature and the slope shall give the value of hardening exponent as shown in Fig. 3(c). The value of n is found out to be 3.6, 2.8 and 3.1 at temperature value of 800 °C, 900 °C and 1000 °C resp ectively. Average value of ‘n’ in this temperature range is around 3.15. The value of activation energy is obtained by taking logarithm of Eq.(1) at constant stress. The average activation energy is evaluated as 345 kJ/mol-K in this temperature region. These values were further used for simulation of notched and smooth specimen using finite element analysis for determination of stress triaxiality.