Environment-exposure-dependent fatigue crack growth kinetics for Al–Cu–Mg/Li

Environmental Fatigue-Crack Surface Crystallography for Al-Zn-Cu-Mg-Mn/Zr

“…This is in contrast to fatigue-crack growth in vacuum, where slipdeformation character strongly affects crack-surface morphology and crystallography. [11,42] For example, the flat crack surfaces for the overaged Al-Zn-Cu-MgZr/Mn alloys stressed in vacuum ( Figure 6) are very different from the tortuous, faceted surfaces of Al-CuMg/Li alloys fatigued in ultra-high vacuum (Figures 1 and 2 [17] ). Such differences are understood based on homogenous vs heterogeneous slip at the crack tip due to the relative shearability of the strengthening precipitates, leading to deformation-band localization.…”

“…(c) What mechanistic implications are inferred from these comparisons, building on discussion of facet formation in Al-Cu-Mg/Li ascribed to environment-exposure-dependent hydrogen embrittlement? [8,17,42,43] A. Comparison with Al-Cu-Mg/Li…”

“…These fatigue-crack path characteristics are consistent with the fact that crack-growth rates are much more sensitive to aluminum-alloy composition and aging condition for cracking in inert vs moist environments. [42,44] For each Al-Zn-Mg-Cu alloy considered, as well as the Al-Cu-based alloys in Table III, the environmentalfatigue crack is characterized by two transgranular facetlike morphologies, broad-flat and repeating stepped. Each feature exhibits a range of orientations well removed from {111}, and sometimes involving low-index planes ({100} and {110}) as reported in the literature (Table I).…”

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

“…For precipitation hardened aluminum alloys, loading in moist-gaseous environments enhances da/dN relative to crack growth in ultra-high vacuum, with growth rate increase correlated to the ratio of water vapor pressure (P H2O ) to loading frequency (f ) [6][7][8][9]11]. For this class of alloys, da/dN depends on the interaction of K, K max and P H2O /f; the resulting complex crack growth rate law is central to accurate damage tolerant component prognosis [12][13][14] and alloy development [15][16][17].…”

“…Specific models have been developed for Al alloys with da/dN limited by: (a) transport of water vapor molecules from the bulk environment to the occluded crack tip as governed by impeded Knudsen flow such that da/dN is directly proportional to P H2O /f [20][21][22][23][24][25][26][27][28][29], (b) the rate of the Al-H 2 O surface reaction to produce a surface layer of adsorbed-atomic H, where da/dN is independent of P H2O /f [20,21,27] and (c) H diffusion in the FPZ with da/dN proportional to a complex function of P H2O /f [17,[30][31][32]. The fundamental mechanism by which H in the FPZ interacts with local plastic strain range and stress is controversial [33,34], but recent results demonstrate a physical basis for H-assisted damage evolution within the crack tip FPZ [35].…”