PSI - Issue 42

Hasan Saeed et al. / Procedia Structural Integrity 42 (2022) 967–976 Hasan et al./ Structural Integrity Procedia 00 (2022) 000 – 000

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Successful applications of this technique have been reported for the analysis of crack growth in small scale fracture toughness tests as well as full scale tests (Van Minnebruggen et al., 2017). Specifically, two reference probes are positioned remote from the region of interest. The potential between the reference probes, denoted as , and the potential across the region of interest, referred to as , are measured. Under theoretical conditions, the normalized potential drop / is independent of current leakage in the setup and uniform temperature changes. To convert this normalized potential drop to a physical crack size, an analytical expression described by Johnson in 1965 (Johnson, 1965; Schwalbe and Hellmann, 1981) is used: = −1 ( ℎ ( 0 ) ℎ ( ℎ −1 ( ℎ ( 0 ) ( 0 ) )) ) ( 3 ) Where, / represents the normalized potential drop, 0 is the distance between potential drop leads, and 0 is the initial crack length. 4. Results and discussion 4.1. Validation of strain compliance based on DCPD DCPD data was recorded throughout the fatigue tests. Care was taken to ensure that the current passing at the probes had fully stabilised before the PD output was measured and a noise-cancelling filter was applied within the DCPD system. The PD values are normalized to remove any environmental effects and are plotted against the number of load cycles in Fig. 8 for a representative fatigue test.

Fig. 8 Change in normalized potential drop during fatigue testing. The crack length to width ratio / is calculated from the normalized potential drop using Johnson’s equation Error! Reference source not found. , and compared to the crack length to width ratio / calculated using the extended back-face strain compliance relation eq. ( 2 ), in Fig. 9 Comparison of crack length measurement (in terms of a/W) using back-face