PSI - Issue 60

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

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Table 3: Crack initiation toughness Ji at different temperatures as obtained from testing of SENT specimens of Alloy 690 material. Temperature (°C) 25 700 800 1000 Crack initiation toughness J i (kJ/m 2 ) 140 55 105 90 5. Reason for peculiar behavior in fracture toughness using SEM analysis Scanning electron microscope examination of the fracture surface of the specimens tested in the range of 25°C - 800°C were carried out for understanding the reason for lower fracture toughness observed in temperature range of 650°C - 750°C. It was observed from Fig. 7 that at 25°C, the fracture surface contains a large number of voids and dimples indicating the nucleation, growth and coalescence of voids in the matrix representing ductile crack growth process during the fracture test. However, in the temperatures range of 650-750°C, the fracture surface shows presence of prominent brittle phases in the matrix from where cracks initiate during loading and their propagation through the matrix leads to reduced fracture toughness of the material. However, when the temperature is increased to 800°C, these brittle phases start dissolving and the matrix softens leading to improved ductility and fracture toughness at higher temperatures.

(a) (c) Fig. 7: SEM images of fracture surface of Inconel 690 material tested at (a) 25°C (b) 700°C and (c) 800°C. (b)

6. Conclusions In order to understand the high temperature fracture behaviour of Alloy 690 material, several tests were carried out on SENT specimen in the temperature range of 25°C to 1000°C. Following conclusions can be drawn from this study.  Rate of decrease in load from the maximum load value is highest at temperature of 700°C indicating higher crack growth values in this range.  Fracture toughness was lowest at 700°C temperature.  Crack initiation toughness 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.  At 700°C temperature, fracture surface shows presence of prominent brittle phases in the matrix from where cracks initiate during loading and their propagation through the matrix leads to reduced fracture toughness of the material.  When the temperature is increased beyond 700°C, these brittle phases start dissolving and the matrix softens leading to improved fracture toughness at higher temperatures.