PSI - Issue 60

Md Rakim et al. / Procedia Structural Integrity 60 (2024) 136–148 Md Rakim et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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All FCG tests have been carried out at 27 °C along with temperatures of 650 °C, 710 °C and 750 °C by a test system, i.e., a servo-hydraulic machine whose capacity is 100 kN. The input attributes of the experiment are frequency, temperature, waveform, and R for investigating FCG that has been given into the machine with the help of software installed interfaced through a controller window in the system. A furnace that is ceramic-lined is attached to the test machine which helps in achieving the desired temperature for the conduction of the tests. To achieve the desired temperature uniformly, 30 minutes of soaking time has been provided and ±2 °C temperature is sustained throughout the testing period as per ASTM E 647-15. The experiments have been repeated twice and conducted in all conditions in accordance with the uncertainty of the results that occurred during the FCG test. Also, the decrease in experiment has been carried out on well polished surface of a CT specimen with R of 0.1 in Equation (1) and a frequency of 15 Hz. For the loading cycle, the waveform that has been selected is sinusoidal. = =0.1 (1) ASTM E 647-15 is the standard guideline for FCG test. To reduce notch radius effect, pre-cracking has to be conducted according to ASTM E 647-15 protocol. The pre-cracking has to occur for ensuring that the final value of does not surpass the initial value for actual testing. For this criterion fulfilment, the procedure of decreasing has to be selected for the pre-cracking. In this approach, there has been a gradual reduction in the load range ( = − ) by the test control system to decrease the as there comprises an increasing crack length. The rate of change in concerning crack length resembles normalized K- gradient and it can be derived through the expression 1 . The normalized K- gradient value should be considered as − 0.08. At the time of decreasing test, for different crack lengths, the needed has been operated using the test system by the following relationship as per ASTM E 647-15: = √ (2+ ) (1− ) 1.5 (0.886 + 4.64 − 13.32 2 + 14.72 3 −5.64 4 ) (2) where, = / For a particular R , the initial value of of the actual FCG test has to be calculated using the following expression: = (1 − ) (3) As the FCG test performance proceeds by decreasing method, the instantaneous value has also been derived using equations (2) and (3). The load-line displacement (LLD) has been monitored with the help of high resolution COD gauges. The direct current potential drop (DCPD) approach has been used in the measurement of crack growth. Here, under cyclic loading, the input magnitude of the current has to remain constant for the satisfaction of Ohm’s law, which shows an increasing value of both the electrical resistance and the potential drop due to the pre-existing crack propagation at the notch root in a CT specimen [according to ASTM E 647-15]. A constant current of 5A has been applied using a current lead and the output value of the voltage drop during crack extension has to be extracted with the help of a voltage lead connected to the specimen. So, the crack length versus voltage relationship (equation 4) has been established using a cubic order polynomial and obtained constants are