PSI - Issue 2_A

Donka Angelova et al. / Procedia Structural Integrity 2 (2016) 2726–2733 Author name / Structural Integrity Procedia 00 (2016) 000–000

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corresponding numbers of cycles N are plotted as: - functions “Crack length, a – Number of cycles, N ”, { a-N } , in log-log scale, shown in Fig. 3a; - functions “Number of cycles, N – Crack length, a ”, { N-a } , in semi-log scale, N in log scale and a in ordinary scale; they are presented in Figs 3b, 4, 5, 6, 7a; - functions “Crack growth rate da/dN – Crack length a ”, { da/dN -a } , in semi-log scale, N in log scale and a in ordinary scale; they are presented in Figs 3c, 3d, and Figs. 4, 5, 6, 7b. Propagation of those short cracks through the microstructure is presented in Figs 2e, 2f, 4, 5, 7a. The first analysis begins with the notched specimen from Group C . This specimen shows appearance of 8 cracks started from the notches, Figs 2e, 2f, Davidkov and Pippan (2006). Our main interest is focused on cracks 1, 2, 3, 4 which fracture the specimen, Figs 2f, 3a. The longest crack is crack 2, which originates from crack 1 as its branch at 15000 cycles. At this moment crack 1 has a length of 107,73 µm. After the bifurcation (point A in Fig. 2f) the crack 2 grows faster than the other cracks during the rest of fatigue lifetime (Fig. 3c) and causes the specimen failure, merging with crack 3 which starts from the central notch 1. Even showing high growth rate during its propagation, Figs 3c, 3d, finally the crack 1 becomes a non-propagating crack that stops at 27000 cycles with a final length of 367,36 µm. The central notch 1 is machined in the ferrite band, so that its right side is very close to the next pearlite band, Figs. 2a and 2f. Thus the crack 3 starts its growing through the pearlite band at the very early loading cycles without significant retardation. On the other side of notch 1 crack 4 begins its growing into the ferrite band; in the beginning its growth is slower than that of the crack 3. Afterwards both cracks continue to grow faster and faster through the microstructure having higher growth rate than those of the other central-notches cracks, Figs 3c, 3d. It can be clearly seen in Figs 3b, 3c, 3d, 7b that short fatigue crack growth data exhibits considerable fluctuations and a b

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Fig. 3. Plots { a-N } for cracks from Groups A, B, C – T-T, RB, PB – (a); { N-a } for cracks 1, 2, 3, 4 from a Group C specimen – PB – (b); { da/dN -a } for all 8 cracks from a Group C specimen – PB – (c); { da/dN -a } for cracks 1, 2, 3, 4 from a Group C specimen – PB – (d); plots from (b), (c) and (d) show a coordinate system with N -axis located in the middle of the central notches specifying left and right crack propagation