PSI - Issue 60

Md Rakim et al. / Procedia Structural Integrity 60 (2024) 298–310

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Md Rakim et al. / Structural Integrity Procedia 00 (2023) 000 – 000

Fig. 2: Schematic diagram of one-quarter CT specimen with all the dimensions used in the fracture toughness testing.

For Fracture toughness testing, pre-cracked CT specimens have been used. Pre-cracking has been carried out with an initial a/W ratio of 0.45 at room temperature (27 °C) and a load ratio ( R ) of 0.1 for obtaining a sharp fatigue crack in front of the notch tip. The process of pre-cracking has been conducted at decreasing Δ K (stress intensity factor range) mode as per ASTM E 647-15. Normalized crack gradient ( C ) can be defined as the change in stress intensity factor ranges and that can be expressed as 1 . The value of ‘ C ’ has been set to -0.08 as per protocols described in ASTM E1820-13. = 0.4 02 (2 + 0 ) ℎ , = ( + 2 ) (1) At the time of ΔK decreasing test for any crack length, the necessary computation of the force range ( ΔP ) by the test system has been computed by the following relation as per ASTM E 1820-13: = √ (2+ (1− ) ) 1.5 (0.886 + 4.64 − 13.32 2 + 14.72 3 −5.64 4 ) (2) where, = The test specimens have been made side-grooved to 20% to restrain the crack growth along the plane of symmetry for the minimization of short crack growth [ASTM E 1820-13]. Fracture toughness tests have been conducted at 27 °C, 650 °C, 710 °C and 750 °C with different aged durations of a maximum of up to 20000 hours using monotonic loading conditions with a constant stroke rate of 1 mm/s in an automatically controlled furnace attached to servo hydraulic UTM. The load-line displacement (LLD) has been recorded with the help of high-resolution COD gauges. The method of direct current potential drop (DCPD) has been used to measure the crack propagation. The voltage vs. crack length for the DCPD system has been calibrated initially using specimens of known and accurate crack lengths. The data acquisition software has been used to acquire data that specifies load ( P ), load line displacement (LLD) and crack length ( a ) through test application.