PSI - Issue 16

Grzegorz Lesiuk et al. / Procedia Structural Integrity 16 (2019) 51–58 Grzegorz Lesiuk et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 6. Mixed mode (I+II) fatigue crack growth rate diagram for P355NL1 steel (R=0.05; 0.5) a) based on  K approach, b) based on new energy  S* parameter, Lesiuk (2019a).

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Fig. 7. Mixed mode (I+III) fatigue crack growth rate diagram for 18G2Asteel (R=0, -0.5, -1); a) based on  J approach, Rozumek and Macha (2006), b) based on a new energy  S* parameter by Lesiuk (2019a).

It is worth to note (as it follows to numerical analysis) that the highest mixity level for each test configuration is achieved in initial part of test. Consequently, the mixed loading is also significant for fracture features on the microlevel as it was shown by Student et al. (1999). In Figs. 8 – 9 are presented the fracture surfaces of the mixed mode fatigue crack path (I+II) for P355NL1 steel. This part of crack paths, is strongly affected by numerous of shear mode fatigue crack growth mechanism – Fig, 8. This fact is also reflected in numerical/analytical analysis, while K II tends to 0 as the crack length under mixed mode is enough long. As the crack increase, the tensile mode is dominant – Fig 9. For stable growing crack, the higher K I /K II ratio is expected due to change the crack plane in relation to the main load force F. In this case, a noticeable feature is secondary cracks – Fig. 9 – typical for tensile mode loading. Close to final, fast fatigue fracture region – (approx. 2mm from the end of crack length), the increasing number of secondary cracks is observed. In this case, the dominant tensile mode of fatigue crack growth is observed. For higher magnifications (Fig. 9b) fatigue striations typical of mode I crack growth mechanism is observed. Similar observations were confirmed for remaining specimens. According to the obtained experimental, numerical results and based on microscopic observations it worth to note that the proposed approach, based on strain energy density parameter, can be successfully implemented in general description of fatigue crack growth rate under mode I, and after extensions for mixed mode. Due to a nature of crack propagation mechanism, it is more likely that the elaborated approaches for mode I are enough good for mixed mode, but it requires a proper equivalent mixity crack driving force. In this case, the energy approach seems to be predestinated for such descriptions.