PSI - Issue 71

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

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Table 3: Value of stress triaxiality at 900 °C obtained for different specimen at different points along the notch. S. No Type of specimen Stress triaxiality ( η c ) (at central plane at notch) Stress triaxiality ( η e ) (at notch ends) 1 Smooth specimen 0.33 0.33 2 Notch radius - 2 mm 0.388 0.35 3 Notch radius - 1 mm 0.482 0.37 4 Notch radius - 0.5 mm 0.89 0.58

5. Results and discussions To evaluate the effect of multiaxial stresses, triaxiality values were plotted against the displacement rate for different specimen types and stress levels tested at 900 °C as shown in Fig. 8(a). It is observed that at a given stress triaxiality, the displacement rate increases with higher stress levels with the rate of change becoming more rapid as stress levels rise. The displacement rate decreases with increasing stress triaxiality indicating that notched specimens with a notch radius of 0.5 mm exhibit lower creep deformation rates as compared to specimens with larger notches. A significant drop in displacement rates is observed when comparing smooth to notched specimens as notched specimens experience slower creep damage accumulation such as cavitation or void growth. Moreover, the material surrounding the notch confines the deformation reducing effective creep strain and delaying the progression of creep damage resulting in slower displacement rates for notched specimens. The change in displacement rate between notch radius of 2 mm and 0.5 mm is minimal compared to the difference between smooth and notched specimens. These results were also evaluated at different temperatures and stress levels, and the data can be directly applied to components made of Alloy 690 material with stress triaxialities within the calculated range. To better understand the deformation behavior of notched specimens, rupture data for smooth specimens were calculated at different stress levels at 900 °C. A total of 7 tests were performed at 900 °C to establish the relationship between rupture time and stress levels. Similar tests were conducted at other temperatures to determine whether the notch had a strengthening or weakening effect. The results show a linear dependence between rupture data and stress levels when plotted on a log-log scale, as shown in Fig. 8(b). The representative stress for the notched specimen can be directly derived from this graph. The representative stress (σ r ) is the applied stress on the smooth specimen that produces the same creep deformation or rupture lifetime as the notched specimen. A straight line representing the rupture time of a specific notched specimen is plotted, and the stress where this line intersects the plot is considered the representative stress for the notched specimen.

Rupture data at 900 0 C Linear fitting (log scale)

100

40 55 70 85

3.0

Test Temperature - 900 0 C Stress (MPa)

2.4

40 45 50

1.8

Representative Stress = 42.5 MPa

25

1.2

Stress (MPa)

Rupture life of notched specimen (R = 2 mm, Net Stress= 50 MPa)

0.6

10

0.0

0.60.8

1

2

4 6 8

10

20

40

0.4 Displacement rate (mm/hr) triaxiality (η) 0.6

0.8

1.0

Rupture time (hr)

(a)

(b)

Fig. 8: (a) Variation of displacement rate with stress triaxiality for different stress levels for specimen tested at 900°C; (b) Evaluation of representative stress for a given notch acuity ratio (2 mm notch radius) for alloy690 material.

For a notched specimen (R = 2 mm) tested at 900 °C as shown in Fig. 6(b), the rupture life was approximately 6.5 hours. The corresponding representative stress for this rupture time was found to be 42.5 MPa which is lower than the net section stress of 50 MPa for this specimen. This suggests that Alloy 690 exhibits notch strengthening behavior at different stress triaxialities. This behavior is due to the lower average saturated von-Mises and principal stresses at the notch root compared to the applied nominal stresses due to stresses redistribution. Hence, the notch strengthening results from stress redistribution at the notch, triaxial stress effects and strain hardening which together reduce localized creep rates and improve the material's resistance to deformation.