Environmental fatigue of an Al-Li-Cu alloy: part I. Intrinsic crack propagation kinetics in hydrogenous environments

Effect of water vapor pressure on fatigue crack growth in Al–Zn–Cu–Mg over wide-range stress intensity factor loading

“…Al [39]. 4 The upper dashed line in Figure 16 represents an upper-bound of environmentally 3 Upon reloading, the initial crack length was 30.93 mm, with the increase due to a compliance inaccuracy which caused the shift to slightly higher K as shown in Figure 15.…”

“…For a wide range of K, R impacts: (a) crack closure, (b) transport of water vapor molecules within the fatigue crack [47], and (c) tensile stress-plastic strain range sensitive crack tip damage [2,4,35]. These deficiencies were recently addressed for an Al-Zn-Mg-Cu alloy [40], as well as several Al-Cu-Mg/Li alloys [48], and limited results were reported for the P H2O /f dependence of da/dN at high R and low K for Al-Li-Cu and Al-Zn-Mg-Cu alloys in several purified gas environments [39]. Generally, however, systematic measurement and modeling of the effect of environmental exposure on FCP rate is lacking for the lower-Paris to near-threshold regime (da/dN < 10 -5 mm/cycle).…”

“…Typical experimental determinations of the P H2O dependence of da/dN employed rising K at constant R [11,21,24,25,47] or constant K at constant R [17,39]. The results of these approaches have not been compared to assess the impact of loading format and history on environment-sensitive da/dN.…”

“…The interesting threshold transition region in the present study occurs at growth rates well below this level, supporting the H embrittlement framework. Furthermore, testing of peak aged Al-Li-Cu alloy stressed over a wide range of K in high purity O 2 , N 2 , Ar and ultra-high vacuum (~0.25 Pa residual pressure)[39] produced identical da/dN, suggesting that a hard-surface film does not impact crack tip plasticity damage. However, a film-based mechanism may have been responsible for a mild deleterious effect of pure O 2 (ultra-high vacuum conditions where used to minimize H 2 0 contamination) on FCP in Al-Zn-Mg-Cu[11,39,66].…”

“…Modern work recognized the importance of controlling water vapor purity and pressure through an ultra-high vacuum (UHV) system which is capable of reducing impurity partial pressures to less than 1 Pa [6,11,17,24,[38][39][40]. Scientific studies, which focused on the effect of P H2O /f were largely limited to K levels in the mid-Paris regime (above about 6 MPam and 4 x 10 -5 mm/cycle) for Al-Zn-Mg-Cu [10, 11, 24-27, 29, 38, 41, 42], Al-Cu [20,21], Al-Cu-Mg [10,11,17], Al-Cu-Li [17], and Al-Mg [11] alloys.…”