PSI - Issue 71

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

70

0.012

100

2 Test temperature 800 o C 900 o C 1000 o C ln(min creep strain rate) in s -1 3 4 -14 -12 -10

Test Temperature 900 0 C

Stress (MPa)

80

40 (800 o C) 60 (800 o C) 40 (900 o C) 20 (1000 o C)

0.009

60

0.006

40 MPa

40

0.003

20

30 MPa

Creep strain (%)

Creep strain rate (/s)

0.000

0

5

0

2

4

6

8

0

2

4

6

8

ln(Stress) in MPa

Creep time (Hrs)

Creep time (Hrs)

(c)

(b)

(a)

Fig. 3: (a) Typical creep strain vs time curve for smooth specimen tested at different stress and temperature; (b) Creep strain rate variation with time at 900°C (c) logarithmic plot of minimum creep rate and stress at different temperature for determination of hardening exponent.

1.5. Notched specimen The notched specimen of radius 0.5 mm, 1 mm and 2 mm were tested at different temperature and stress levels as per the test matrix given in section 2.2. The stress at minimum cross-section (new section stress) were kept same as that of smooth specimen. The results obtained for different specimen are discussed below. Creep deformation behavior of notched specimen at different temperature and stress levels The displacement response of alloy 690 material at different temperature and stress levels are shown in Fig. 4. It was observed that the steady state displacement rate increases with increase in stress levels. The specimen fails at a lower stress value with increasing temperature. There is some primary creep observed for most of the notched specimens as opposite to smooth specimen where negligible primary creep was observed. This is due to its higher stress triaxiality, localized deformation and stress concentration at the notch root. These conditions accelerate dislocation motion, void nucleation and creep mechanisms. Stress redistribution around the notch leads to localized strain amplifying the initial creep response as the material adjusts to the elevated stress. Higher rupture displacement were observed for lower stress levels at all the temperatures.

5

1.0

Test Temperature - 1000 0 C (Notch radius = 1 mm) Stress (MPa)

Test Temperature - 800 0 C (Notch radius = 0.5 mm) Stress (MPa)

4

0.8

25 30 35

50 70 90

3

0.6

2

0.4

1

0.2

Displacement (mm)

Displacement (mm)

0

0.0

0

2

4

6

8

0

2

4

6

8

Creep time (Hrs) Creep time (Hrs) Fig. 4: Displacement of notched specimen at different stress levels and notch radius tested at temperature (a) 800°C (b) 1000°C. Effect of notch radius on creep deformation behavior of alloy 690 material The comparison of various notched specimen with smooth specimen at different temperature and stress levels is shown in Fig. 5. At a particular net section stress value of 70 MPa, test temperature of 800 °C and time value of 4 hrs, the displacement of specimen changes from 0.2, 0.34, 0.5 and 2.25 mm respectively for notch radius of 0.5 mm, 1 mm, 2 mm and smooth specimen respectively. Similar behavior is observed at 900 °C and stress levels of 50 MPa (Fig. 5(b)). At other stress levels and temperature, a similar nature of displacement response is captured. This indicates that there is significant effect of multiaxial stresses on the displacement behavior response of alloy 690 material. Since the amount of deformation is more for smooth specimen than notched specimen at a particular time value, it can be said that the material exhibit notch strengthening behavior. Hence, the material shall be safe in presence of any discontinuities having triaxiality values in the range of the tested notched specimens. The material is seen to be strengthening in presence of notches due to very high ductility at this temperature interval. (a) (b)