PSI - Issue 14

M S Nandana et al. / Procedia Structural Integrity 14 (2019) 314–321 M S Nandana/ Structural Integrity Procedia 00 (2018) 000–000

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re-ageing step of RRA treatment. This makes the alloy to regain the strength levels equal to that of T6 condition. At the same time retrogression makes the grain boundary precipitates to resistance to the alloy, similar to observations demonstrated by arrange discontinuously, imparting good SCC Li et al. (2009) and Rout et al. (2014).

Fig. 4. Bright-field TEM micrographs of

AA7010 in different ageing conditions: a) T6, b) RRA

Fig. 5. Stress vs strain plot for AA7010 in RRA and T6 conditions

3.3. Fatigue crack growth behavior

Figure 6 illustrates the FCGR curves (da/dN vs ΔK) for the alloy in T6 treated condition compared with that of the RRA condition. The results show that RRA treated samples exhibit lower crack growth rate from near regime to major portion of the Paris regime when compared to the T6 treated sa behavior (Fig. 6b) indicates the increased ΔK th by about a unit value in RRA treated sample when compared to the T6 sample. The ΔK th evaluated in RRA and T6 treated samples were 9.33 and 8.25 MPa instance, the growth rate at ΔK = 9.6 MPa√m, is 1.08 x 10 lower i.e., 3.78 x 10 -5 mm/cycle observed for T6 treated sample. The microstructure dependent fatigue crack growth rate in the near-threshold regime, up to mid Paris regime is evident in the RRA treated alloy. At high crack growth rate is completely independent of microstructure and hence the final failure occurs at lower -threshold mple. The near-threshold FCGR √m respectively. For -5 mm/cycle in RRA treated sample, which is 3 times ∆K regime, the ∆K in