PSI - Issue 58

K N Pandey et al. / Procedia Structural Integrity 58 (2024) 122–129 K. N. Pandey and G. Singh / Structural Integrity Procedia 00 (2019) 000–000

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3. Results and Discussions 3.1 Crack growth investigation of AA-6061 with the help of COD gauge The results of constant amplitude and block loading conditions are shown in Table 4 for AA6061 aluminium alloy. S � , S � ,and S � are the three samples tested for each loading condition. Table 4. Life till fracture under constant amplitude and block loading conditions for AA-6061 aluminum alloy Maximum Loads No of cycles till failure Avg. Std. C. Level � � � CAL-1 21660 20450 21205 21105 611 1518 CAL-2 14025 15260 15010 14765 652 1622 Results show that the life of a component is influenced by the type and sequence of loading. Life was maximum for CAL – 1 loading (maximum load of 4.0 kN) in comparison to the CAL – 2 (maximum load of 4.5 kN). Similarly for block loading conditions, life is more for Block Hi-Lo in comparison to the Block Lo-Hi load sequence. This is due to crack growth retardation for the case of Block Hi-Lo. The observations are in line with the results reported in the literature. 3.2 Crack growth investigation of Al-6061 with the help of EMI signature The real part of admittance signature (EMI) was used as a damage quantifier due to its higher sensitivity as compared to imaginary part of admittance signature (Soh et al., 2012). In this study the frequency range selected for crack detection is 50 kHz to 180 kHz to minimize effects of bonding and other environmental conditions. The conductance versus frequency plots for the loading conditions shown in Table 3 and for the crack lengths of 12 mm, 13.5 mm, 15.5 mm, 18 mm, 21 mm and 24 mm are shown in Figs. 3, 4, 5 and 6. From the active conductance signature it was observed that resonant frequency (RF) shifts leftward with the increase in the damage i.e. with increase in crack length. From these signatures, the shift in resonant frequency  RF for 13.5mm, 15.5mm, 18mm, 21mm and 24mm crack lengths was obtained with respect to the initial sage i.e. crack length of 12 mm. From Table 4, percentage of life was determined. Cycle corresponding to life till fracture was considered as 100% life. Thereafter a graph was plotted between  RF and percentage of life which is shown in Fig. 7(a). This figure was compared with the conventional crack length versus number of cycles as shown in Fig. 7(b). Both the figures present a similar trend. Further, a graph between crack growth rate da/dN versus shift in resonant frequency was plotted for the loading conditions shown in Table 3 (Fig. 8a) and compared with the conventional da/dN versus range of stress intensity factor,  K (Fig. 8b). These figures are matching with each other. In Fig. 7(a) the shift in RF are plotted against the life cycles (%) for 4kN, 4.5kN, L-H, and H-L block loading conditions. The curves presented in Fig. 7(a) show that shift in RF with percent of life cycles has similar trend as crack length with number of cycles as shown in Fig. 7(b). Shift in RF depends upon the loading amplitude and sequence of loading. In case of L-H loading sequence, an increase in resonant frequency shift (RF) was observed as life cycle progress. For the H-L loading sequence, a decrease in resonant frequency shift (RF) was observed as life cycles progress. Block - Lo-Hi Block - Hi-Lo 18600 19400 19100 20100 17850 19600 18516 19700 629 360 1563 896