PSI - Issue 60

P.K. Sharma et al. / Procedia Structural Integrity 60 (2024) 335–344 P.K. Sharma/ StructuralIntegrity Procedia 00 (2023) 000 – 000

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4. Evaluation of fracture resistance in terms of J-R curve and estimation of crack initiation toughness Using the load-displacement and crack growth data, the fracture behavior in terms of J-R curve is evaluated following the procedure discussed in section 2.3. It can be observed from Fig. 5 that the fracture resistance initially reduces from room temperature to 700°C however, it further increases at 800°C. Lowest fracture resistance is observed at 700°C. Crack initiation toughness is evaluated as discussed in section 2.4. For evaluating the crack initiation toughness, the tensile tests have carried out at different temperature to determine the value of yield and ultimate tensile strength of Alloy 690 material. The yield strength value obtained for the test carried out at 25, 700, 800 and 1000 o C are 340, 213, 175 and 45 MPa respectively. The ultimate tensile strength obtained for the test carried out at 25, 700, 800 and 1000 o C are 700, 365, 190 and 55 MPa respectively. These values have been used for determination of the blunting line equation as described in Eqs. (11) to (16) respectively. The intersection between the blunting line and JR curve at 700°C is shown in Fig. 6(a). The value of crack initiation toughness comes out to be 55 kJ/m 2 . Variation of crack initiation toughness with temperature is shown in Fig. 6(b). It was observed that it reduces from 140 kJ/m 2 at room temperature to 55 kJ/m 2 at 700°C and further increases to 105 kJ/m 2 with subsequent increase in temperature to 800°C. This is in line with the nature of load-displacement and crack growth data discussed in the previous section. Reason for this peculiar nature in the fracture behavior is discussed in subsequent section.

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Crack growth (mm)

Fig. 5: Comparison of fracture resistance curves of SENT specimens for Alloy 690 material at different temperature.

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JR curve (700deg.C) Blunting line

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J i,0.2 = 55 kJ/m 2

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Fig. 6: (a) Evaluation of crack initiation toughness at 700°C (b) Variation of crack initiation toughness (Ji) at different temperatures.